CA2509707A1 - Apparatus and method for tracing a path travelled by an entity or object, and tag for use therewith - Google Patents

Abstract

Apparatus and method for tracing a geographical journey taken by an entity, for example an article (shipment), asset or person, on a worldwide neap uses a miniature, passive, low power, radio receiver tag that is attached to the entity and collects multiple FM
spectra during the travels of the entity. When the tag is retrieved at the end of the journey, the FM spectra are downloaded and compared against a collection of constructed spectra constructed from antenna transmission and location data derived. Correlating each of the spectra collected by the receiver tag with the corresponding spectra in the database allows the regions traversed by the receiver tag, and hence the entity, during its travels to be plotted.

Description

APPARATU AND ME'~HOD FOR TRACING A PATH TRAVELLED B'Y AN ENTTTY OR
OBJECT. AND TAG FOR USE'TP1EREWIT~T
BACKGROUND OF THE INVBN'TrON

The invention relates to a method and apparatus for tracing a path t7raversed by an entity ox object, for example an article, e.g. a container shipment, asset or person, such as a persozt under surveillance, potentially worldwide. The invention also relates to a tag for use with the method and a method of providing a historical database of frequency spectra extant at different X 5 geographical locations.
BACKGROUND,ART
The popularity and availability of the Global Positioning System (CPS) and low cost radio frequency identification {Rk'1D) systems have led to significant increases in the use of 2.0 systems fox tracking articles or people. Thus, CPS is becoming a commodity product for the transport industry and RF (Radio Frequency) tags are also becoming widely used for indoor tracking of articles and for access control.
,A, l~ey feature a;Ftl~ese tracl~izxg systezn,s is their ability to provide far accurate real-time location of the article under observation. To do so, ]~owever, they employ devices which must 25 either be active {transmit RF signals) or rely upon other RF transmitters (e.g. G15S or '1)fAN
system) with known positions.
For example, U.S. patent No. 6,014,p40 and, more recently, U.S. patents Nos.
6,891,469 and 6,$89,135, disclose tracking systems in which an active radio transmitter associated with the article or person being tracked is used to set up a known radiation field, an,d multiple tuned receivers are used to detect the presence ofthe radio transmitter, the location ofthe transmitter being determined using triangulation. Disadvaxrtages of such active 1tF
systems include short operating times for transmitters or transceivers that, as a general rule, will be battery operated, and the fact that the transmlssiozls may be detected easily, making them unsuitable for covert operatiozr.
Some lrnown systems utilize existing navigational systems, such as LQRAN and Global Positioning Systems (GPS) to determine the exact location ofthe vehicle, in terms ofgeographic coordinates {longitude and latitude), either during its travel or at the locations of specific operations such as loading or unloadizrg. Examples are disclosed in U.S, patent No. 504Q240 )E O (Keegan) and U.S. patent No, 5,014,206 (Scribner et al.).
U.S. patents Nos. 6,606,556 and 5,223,844 disclose security systems based upon GPS
technology, the former for tracking a person, and the latter for tracking a stolen vehicle. A
major disadvantage of such GPS-based traclcirrg systems is that they require a line-of sight between the GPS antenna and the satellite(s). lrx additiotr, the receiver circuitry is too large to 1 S zr~eet most size requirements of covert systems and power consumption may limit battery life.
U.S. patent No. 6,774,797 discloses a system for tracking movements of an individual by means of a tracking tag which uses GPS andlor cellular telephone geolocation methods to determine its location and transmits the location information to a monitoring center.
US patent No. 5,648,763 discloses a security system for controlling access to cargo 20 containers which has a receiver for receiving LORAN-C or GPS signals and using them to produce a position signal corresponding to the current geographic position of the container.
This position signal is compared with a preset signal representing a predetermined geograplue position and, when the two correspozld, an enabling signal permits the security system to gain access to the container. Again, the use ofGPS introduces line-of siglxt limitations, LORAN-C
25 is limited to maritime applications, and only the one location of the container is of interest or consedueace, 'fkrere is also a growing need for tracing systems which differ from tracking systems because, at least in the context of this specification, a tracking system is defined as a system that allows deterrzliz~atxon of the current geographical location of the article being tracked, whereas a tracing system is defined as a system that allows determination of a geographical path traversed by tlxe article after it has completed its travels.
Someknown systems, specifically systems using active RF techniques, provide tracking and/or tracing capabilities but, as cozztpared with passive RF techniques, have limitations in that they do not provide, for example, (i} very low DC power consumption, meaning longer operational life; (ii) simple electro~tie circuitry, leading to miniature datnexasions and very low cost; and (iii) inaccessible real-time data, meaning better compliance with human rights legislation_ LT. S. patent No. 6,480,108 discloses a system for tracing the path taken by a tag through 1 O several debited geographic areas. The tag includes a transmitter and a receiver. Tag readers in the different geographic areas transmit RF signals and, in response, the tag transmits RF signals to the tag-readers. Knowing the areas in'uvhiclt floe tag readers axe located, the system can trace the path taken by the tag. This is an active system and so suffers from at Ieast some of the disadvantages of active tracking systems, as descxi~bed above.
It is also known to use signals from cellular telephone base stations to track vehicles. For exat~aple, U.S. patent No. 5,85,436 discloses a system for tracking a vehicle using cellular telephozte signals which are detected by a cellular transceiver installed in the vehicle. Such cellular telephone txacking systems suffer from lack of ground coverage and dead spots, especially when the vehicle is indooxs. Moreover, life GPS receives, cellular telephone xeceivexs consume substantially more DC power in coznpaxison to )~'M
xeceivexs, especially if the FM receiver is used without any audio output.
It is also known to use electronic identification tags to track the movement a~ liv8stack.
Fox example, U.S. patent No. 6,577,241 discloses a system is which a manufacturer creates a plurality ofidentification tags each having a unique identification number that is associated with a corresponding "prerecord" in a central database. loch customer purchasing a tag is given access to the database to enter or read data in the corresponding prerecord, Although the system allows the movement of the animal to be traced, the tag is not neither a radio transmitter nor a radio receiver. The system requires data to be entered and updated by tkte customer.

U.S. patent No 5,939,982 discloses a system for detecting opening of a shipping container which houses, along with the goods-in-transit, at least the antenzaa of axe 1~'M receiver that monitors FM broadcast signals. If the receiver detects an increase in the received signal strength, evidencing at least partial or opening ofthe contaizter, itproduces an. aiazxxx signal. '>'he receiver may also scan and store the FM spectrum pertaining at the time the container was open.
Comparison of the stored FM spectrum with charts of FM spectra for various cities alXows any investigator to identify the approximate location of the container when the intrusion occurred.
While this arrangement maybe acceptable inthe context ofintrusion detection, thereceiver only scans and stores location information if the container i5 opened, and the system does not provide 1 O for tracing a path taken by the container. Also, the comparison is not automatic.
SCTMMA1~Y OF TIE INVENTION
The present invention seeks to eliminate, or at least mitigate, th.e disadvantages of the prior art and has for its object to provide a system for tracing a geographical path previously 1 ~ taken by an entity or object, such as a person or a shipping container. To this end, A Otacing system comprising:
a radio receiver associated with the entity or object and operable to scan a predetermined radio band at preset times as the entity or object traverses the path, and capture the extant frequency spectrum, and storage zz~eazzs for storing said captured frequency spectrum and means for retrieving a plurality of captured frequency spectra from the tag and correlating said captured frequency spectra with known antent~.
txansrrxissions arid location data to determine the location of the tag when each captured 25 &equeney spectrum was captured and deriving the path fxozn the successive locations.
The computer means may be operable following retrieval of the captured frequency spectra to (i) receive user-supplied information identifying at least approximately a first search area traversed by the tag, to retrieve antenna transmission and location data to construct a first frequency spectrum comprising a set of signal frequencies of signals receivable in said search area at the time the tag was in S said first search area;
(ii) correlate the first constructed frequency spectrum with each captured frequency spectrum in turn to determine the captured frequency spectrum having the highest correlation therewith and store coordinates of the first search area as th;e location of the tag when said highest-correlation frequency spectrum rocs 1 O captured, (iii) define a second search area encompassing an area around or neighboring the first search area;
(iv) construct a second constructed frequency spectrum for the second search area comprising a set of signal frequencies of signals receivable izt said second search area at the time the tag was in said second search area, (v) correlate the second constructed frequency spectrum with at least those captured frequency spectra captured at times close to the time of capture of the first captured frequency spectrum to determine a second captured frequency spectrum having the highest correlation therewith;
2O (vi) store coordinates of the second search area as the location of the tag when said second highest-correlation frequency spectrum was captured, and repeat the process for other captured frequency spectra to obtain coordinates of successive regions traversed by the tag.
Additionally or alternatively, the receiver may demodulate, franc each transmission, signal information (e.g. ItDS) identifying the source, and store said inforrllation in the corresponding record for that preset time, the identification information beiztg used, when the tag has traversed the path, to determine the locations of the receiver at the preset times and hence the path traversed by the tag and entity or abj ect with which it travelled.

Preferably, the computer means is operable, prior to correlating a particular captured spectrum with a constructed spectrum, to compare signal strengths of signals in said captured spectrum with a predetermined threshold signal strength level and select only those signals whose strengths exceed the threshold for correlation with the constructed spectrum.
The computer means may be operable to derive from the location coordinates of the series of regions a reap of regions with the path depicted as an overlay thereupon.
Tlae database preferably comprises historical collections of said reference spectra allowing comparisons with reference spectra that were valid or pertaining at the times and locations at which the tag scanned and stored the frequency spectra.
>< O
According to a second aspect of the invention, there is provided a method of tracing a path previously taken by an entity or object comprising the steps of:
associating with the entity or object a radio receiver operable to scarf a predetermined radio band at preset times as the entity or object traverses tine path, and capture the extant frequency spectrum, and store said captured frequency spectrum;
and when the tag and entitylobject have traversed the path, retrieving a plurality of captured frequency spectra from the tag and correlating said captured frequency spectra with known antenna transmissions and location data to determine the location of the tag when each captured freduency specter was captured and deriving the path from the successive locations.
The step of retrieving and correlating the captured frequency spectra may comprise the steps of (i) using user..supplied information identifying at least approximately a first search area traversed by the tag, retrieving antenna transmission and location data to construct a first frequency spectxuzn comprising a set of signal frequencies of signals zeceivable in said seazclx area at the ti~a~e the tag was in said first search area;
(ii) correlating the first constructed frequency spectrum with each captured >='requericy spectrum in tur~1 to deterz'niz~e the captured frequency spectrum having the highest correlation therewith and store coordinates of the first search area as the loeataon of the tag when said highest-correlation frequency spectrum was captured, (iii) defining a second search area enco:e7passing an area around or neighboring the &rst search area;
1 O (iv) constructing a second constructed frequency spectrum for the second seazch area comprising a set of signal frequencies of signals receivable in said second search area at the time the tag was in said second search area, (v) correlating the second constructed frequency spectrum with at least those captured frequency spectra captured at times close to the time of capture of the first captured frequency spectrum to determine a second captured frequency spectrum having the highest correlatxozr therewith;
(vi) storing coordinates of the second search area as the location of the tag when said secoztd highest-correlation freduency spectzum was captured, and repeating steps (iii} to (vi) for other captured frequency spectra to obtain coordinates of successive regions traversed by the tag.
The correlation step may comprise the step of, prioz to correlating a particular captured spectrum with a constructed spectrum, comparing signal strengths of signals in said captured spectrum with a predetermined threshold signal strength level and select only those signals whose strengths exceed the threshold far correlation with the Constructed spectrum.
t~ccording to a third aspect of the inwexition, there is pzovided a radio receiver tag suitable for use in tracing a path taken by an entity or object with while accompanied by the radio receiver tag, comprising:
a radio receiver operable to scan a predetermined xadio frequency band and capture data representing a frequency spectm~xi thereof;

storage means for storing a plurality of data records each corresponding to a respective one of a plurality of the frequency spectra;
control means fox controlling the radio receiver to scan the radio frequency band at preset times, while the entity or object is traversing said path, and store the resulting Frequency spectra data in said storage rraeans; and output means for outputting said frequency spectra data.
Preferably, the antenna transmission and location iztformation are available via a public data network, such as the Interrnet, perhaps on a subscription basis.
The predetermined radio frequency band may be selected according to the path beinglto 1 O be traversed. For example, the receiver may scan the broadcast FM radio bands) used in the countries througlx which the tag will travel. The FM broadcast frequency band of about 88 to 108 MHz is preferred because it is virtually a worldwide standard. In some Asian countries, however, the broadcast FM radio band is about 86 to 104 MIdz. In order to accommodate both, therefore, the receiver could be preset to scan from about $6 M~Iz. to 108 MHz. Additionally 1 S or alternatively, the receiver may be designed to scan other radio frequency bands, such as television bands, cellular telephony bands and maritime bands (e.g, LOkAN-G).
The preset times may be set according to tJae path being traversed and the resolution requixed. For example, where the entity or object is to travel by sea, the intervals between the preset times may be longer than the intervals between preset times when the entity or object is 20 to travel byroad.
The analysis of the stored frequency records to determine the geographical locations of the corresponding antennas may also take into account un~xque identification data incorporated into their transmissions by the radio broadcasters. The use of such additional information may greatly improve determination of the geographical area within which the set of transmissions 25 were detected by tlxe tag and reduce mis-correlation to negligible levels.
For some applications, the data record createdby the tagmayinclude etlvironmental data acquired from ozze or more sensors in or associated with the tag, for example, motion, temperature, lig$t level, and so on. Such environmental data may enhance determination of the tag locations at the preset times.

various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description, in conjunction with the accompanying drawings, of a preferred embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic representation of a first embod~e~nt ofthe invention, specifically a system for tracing paths taken by entities or objects by means ova radio receiver tag travelling with the entity or obj ect;
Figure 2 is a simplified scheimatic diagram of the radio receiver tag showing a radio 1 O receiver unit thereiz~y Figure 3 illustrates the assignment of frequencies used by an FM
transrrtitter;
Figure 4 illustrates exposure o~thc tag to different sets ofradio signals during its travels;
Figures SA, SB and SC are respective sections of a flowchart depicting operation of the system, specifically a correlation process which compares frequency spectrum data records stored in the radio receivertagv~rith frequency spectra constructed frorc~
historical records ofFM
fiequencies transmitted by antenna towers and stored in a database; and Figure 6 illustrates a typical radio frequency spectrum as scanned by the radio receiver o~the tag.
2.0 DESCRIPTION Op' THE PREFERRED EMBODIMENT
The present invention is predicated upon the fact that, typically, within a specific geographical area, a radio receiver will r~eive a wide range ofznan-made radio frequency (R.F~
signals including the broadcast commercial (AM radio, FM radio, T'V, etc.) signals and private (communication, cellular, satellite, ere.) signals. Such Rh' signals form a frequency Spectrum ~5 or Rh signature that is unique to tlxat specific geographical area.
Databases are available that contain geographical locations (latitude, longitude) oftransmission antennas associated with the frequencies ofthe signals transmitted from each such transmission antenna.
Consequently, it is possible to construct a RF spectrum for a particular geographical area by identifying all of the antennas in the area and corn~bining their transmission frequencies as provided by the database.

Such a database is owned by RadioSo$ Corporation, of Demorest, Georgia 3053 5, USA
and accessible to subscribers via the Internet. Embodiments of the invention trace the journey taken by an object or entity of interest by capturing such unique frequency spectra or R.F
signatures at different locations during the course of the object's travels and then correlating 5 them with a collection o~previously obtained reference frequenc~r spectra or 1ZF signatures to determine where the object was when each RF signature was captured.
As illustrated in Figure 1, embodiments of the invention employ a tracer tag containing a miniature RF receiver which accompanies the entity or object being traced, in this case a shipping container 12 carried by a vehicle 14.
1 O boring the travels with the object/entity 12, the tracer tag 10 will encounter a variety of different broadcast radio signals. Thus, at the location shown in Figuxe 1, the tag's radio receiver will receive signals from three transmission towers A1, A2 arid A3.
Usually, several FM stations will transmit from the same tower. Thus, tower A1 transmits signals of FM stations A, B and C, tower A2 transmits the signals of FM stations I and r, and tower A3 transmits the >< 5 signals of FM stations ~C, Y and Z. The signals of stations I and Z
include RI)S data (to be explained later).
As also illustrated in Figure 4, at sample point 1 in city area A, the signal strengths of signals F,,b,t, F;~~~sy, and F,,,,,~.~~s~ received from transmission towers Al, A2, and A3, respectively, may far exceed those of other stations as a result of a combination of proximity and transmitted sigrial strengths at their respective broadcast frequencies.
As the tracer tag 10 travels along the desired path, idexxtlfied by broken line 50 in Figure 4, the signal strengths of the received station signals will change and, eventually, some of the signals will no longer be detected, being replaced by signals from other stations iri closer proximity to the tracer tag 10, Thus, at sample point 7, signal F;,~,,~ ~, ~t+~s)," from transmission tower $1 mig>xt dominate the scanned frequency spectrum.
Typically, the receiver will scan the predetez~mined FM hand at 200 kliz or ineremezits and encode the signal strength, at every ineremezit, with a, resolution of, say, 12 bits, equivalent to a dynamic range of 72 dB. Hence, if the scan increment is 200 kl3z over a 20 MHz band, each frequency spectrum data record, as stored by ttie tag, will comprise 100 frequency m samples which, with 12 bit conversion requires only 1200 bits or 150 bytes of data. The size of the memory unit 20 and the sampling intervals will determine how many data records can be stored. Where sampling is infrequent, the tag 10 might have enough capacity for years of operation, li~united, perhaps, by battery life.
As illustrated in Figure 2, the receiver tracer tag 10 comprises a radio receiver unit 16 coupled to miniature antenna 1$, which receives commercial 1tF signals. The r~eiver unit 16 scans the predetermined frequency band incremetally and, at each step, extracts the signal strength. Each signal strength readirxg is digitized by the said receiver and stored in permanent non-volatile memory 20. Far embodiments of the present invendion, it is preferred to use a 1 D receiver which scans the public FM broadcast frequency band of about 88 MHz to 108 MFIz because it is a substantially worldwide standard. However, in some Asian Countries the FM
band is from about 86 to 104 MHz, so, to accommodate both bands, it may be desirable for the receiver to scan from about 86 to 108 IVIfIz.
In most countries, the public bx-oadcasters transmit at fircquencles set to 87.7 -r (m x 0.2}
1 S MHz., where m is an xuteger, i.e., the tuning increments or steps are 200 kHz. In some European countries, however, for example rtaly, same broadcasters transmit at li~uencies set to 57.7 +
(m x 0.1 ) M~Tz, wlrere m is azt integer, meaning that the receiver step size should be 100 kHz rather than 200 kHz.
In the receiver unit 16, the radio frequency signal received from the associated antenna 20 18 is coupled to a baz~dpass filter 22 which restricts the radio signal to the FM spectrum from about 86 M'ETz. ox 88 MHz, to 10$ MHz. and passes it to a low noise amplifier ?.4. The amplified signal from amplifier 24 is down,-converted to an intermediate frequency (1F) signal of 10.7 MbTz. by a mixer 26 which derives its local oscillator signal (LQ) from a phase-locked loop oscillator (PLO) 28. 'The PLO 28 is controlled, via bus 30, by a microconfroller 32 which, at 25 preset times tlxraughout the jourrxey by the object ar entity (and tag), activates the receiver unit and causes the local oscillator frequency to scan the spectrum in steps of 200 kHz, which, as explained above, is the usual spacing between FM radio stations in most of the ruorld. (Of course, #ze steps could be 100 lcHz, if preferred.) lior each frequency Step, the down-converted If signal from mixer 26 is filtered by a second bandpass filter 34 having a bandwidth of 300 kHz, centered upon the 1F
frequency. The magnitude of the output from second bandpass filter 34 is measured using a logarithmic amplifier 36. ~'he analog signal froze the logarithmic amplifier 36 represents the amplitude of the radio frequency signal for a selected station and is filtered by a low pass filter 38 having a cut-offof 80 ~-Iz. The filtered signal Ar from low pass filter 38 is converted to a digital signal by a twelve-bit analog-to-digital (A-to-D) converter 40 within the microcontroller 32.
The digital signal firm A-to-D converter 40 is processed by a signal processor 42 of the microcontroller 32. D.C. po'~srer for the receiver unit's components and mexoory unit 20 is 1 O provided from the internal battery (not shown). The microcontroller 32 is programmed to put the receiver unit 1 G into an e~zergy-saving mode when not in use, e.g. when not actually capturing a frequency spectrum, so as to reduce power consumption.
Qnce the frequency spectrum has been digitized, the microcontroller 32 associates with it the date aad time of capture (derived from au internal real-time clock), and stores this information, as a data record, in a non-volatile memory unit 20. As will be desenibed later, the receiver unit 16 :may also incorporate a FM digital demodulator (not shown) to demodulate each RDS and read each public broadcast station's call sign and associate it with.
the frequency spectrum in the data record, for use in facilitating the comparison with the reference database spectrum. Tv save merxiory space, the real-time, time and date need not be recorded. Instead, the time aand date information could be computed zetroactively for each captured frequency spectrum, based upon the time and date the data was downloaded froze the tag and with, knowledge of the time interval between the preset times at which the fi~equency spectra were captured.
As shown in Figure 3, which shows the assignment of frequencies used in art FM
transmitter, at 57 kETz there is a RDS subcaxrier which may be used to carry iddntificatian information, such as the commercial broadcast station's call sign and information about the programme content. Although use of the RDS feature is not yet worldwide, it is becoming common practice among industrialized countries. Consequently, whenever possible, the receiver unit 16 demodulates the RDS subcarrier, decodes it to obtain the station's call sigh, and records the latter along with its sigtxal strength.
The microcontroller 32 has an u0 interface 44 which, by mean of an R,S 232 port 46 enables the captured datareco~tds to be transferred from mernoryunit 20, convenientlybymeans of az~ infra-red coupling to enable the data to be downloaded to an end-user's personal computer 4$ (Figure 1) once the tracer tag 10 has been retrieved at the end of its jourxxey with the object/entity, The tracer tag 10 may then have its battery recharged (if necessary) and the present time interval reset by way of the xztfr~ red IIO interface, before the tag l, p is re-used.
Hefore use, the tracer tag 10 battery must first be charged, the microccontrvller 32 set I O to record PM spectra at desired preset time intervals and the tracer tag 10 associated with the entity or object whose path is to be traced, for example hidden upon it for later retrieval. The tag 14 receiver unit 16 may be activated by any suitable means; fox example by tapping tile tag a number of times within a certain time interval, if it contains a motion-activated on/off switch.
Once the tag 10 has been activated, the microcontroller 3Z will track t~,e time and, at each preset interval, taro, on the receiver unit 16, which will detect the radio frequency signals to which it is exposed. The microcozttmIler 32 will control the receiver unit 16 to scan and sample the radio frequency spectrum, measuring the signal strength at each increment of the frequency spectnuu and storing the data in associated memory device 20.
The personal computer 4$ is used to upload the recorded FM spectrum. data via the Intem,et to a sewer 52 having access to a database 54 of historical reference FM transmitter antenna tower activities which, as described above, are used to construct reference FM
frequency spectra in order to extract the geographical trace.
The server 52 has a web-based RF spectrum correlation engine program (not shown) which compares the uploaded fredueocy spectra with constructed frequency spectra from ttxe database to determine where the tracer tag X 0, and hence the entity ox obj ect 12, was when each individual spectrum was captured.
Thus, the historical database of records of lrnown FM transmission antenna towers is used to construct the frequency spectra of the geographical locations within the search area, In the database 54, each FM transmission anteauua tower data record contains not only the radiated Il~
power level for each of the transmitted frequencies but also the coordinates (latitude, longitude, elevation) of the geographic location needed to construct the spectrum, together with the applicable date and time and, where applicable, station call signs. Typically, publicly available databases ofFM transmission spectra (FCC, EU equiv., CITT, CRTC branch oflndustry Canada " rC " in Canada) are updated frequently, say every six hours, as it is well larow that FM
broadcasters go off the air for a few hours, normally during the night, two or three times a year for antetlna maintenance proposes. Also, some 1~'M stations close pernaanentIy and others commence operations. Consequently, each FM spectrum taken at a particular location is dynamic as it changes over time.
1 O The frequency spectrum measured by the tag 10 at a parkiculax location, and heztce at a particular time, must be compared with constructed freduency spectra comprising signals that would have been transmitting at that particular preset time. Consequently, the reference database is historical at~d stores transmission activities of 1'M broadcast antenna towers taken over $ period of tome that is long enough to ensure tlxat the reduired spectra are available when Z 5 the data is being retrieved and compared.
Hence, the worldwide historical reference database 54 also txray be updated every six hours by downloading, via the Internet, the latest transmitting activities of broadcast antenna tower data either dixectlyfrom a gowemment web site sex-ver 56, such as the FCC server, ar &am the website server 58 of an intermediary, e,g. a private organization such as Radiosoft whose 20 website Radio$o;ft.com is lmown as the most popular private web site that supplies duasi real-time FM signal strength contours for every registered bz~oadcaster, worldwide.
Iu order to facilitate comparison of each captured frequency spectrum data record with the appropriate historical data, when obtaining each frequency spectrum, the receiver unit 16 also collects the dateltime information for subsequent use in matching the measured frequency 25 spectrum with the records in the historical database for the same time and date.
The web~based spectrum processing engine 52' on server 52 constructs the regional RF
spectrum for each geographical location based upon the activity information of each FM
broadcast antenna tower in the search area. The area is defined in terms of a radius around a specific pair of survey coordinates. Time and date information is also processed for each 1~
sample, i.e_, captured spectrum. For example if the city area A of Figure 4 were the same as the search area, all of the signals from towers A1, A2 and A3 would be identified as within tl~e search area. This group of signals constitutes what will be called a "coztstructed" frequency spectxum for area A.
Before seeking to correlate this constructed frequency spectrum with those captured by the #ag 10, the correlation engine 52' processes the tag's raw captured spectra to screen out noise, including weak stations. Figure 6 illustrates a typical raw captured frequency spectrum in which a group 71 of only five broadcast signals exceed a threshold 70 at a signal strength T.
Oxtce the engine 52' has processed all of the captured freduency spectra to eliminate the 1 C noise and weaker signals 72, it proceeds to contipare them a turn with the constructed frequency spectrum for the search area. Again, if city area A is the search area, the correlation engine should find two captured spectra (sample 1 and sample 2} which correlate.
This process is repeated until all of the captured spectra have been correlated with one or more tt~ansmission towers along the path travelled by the tag 10 and entity/obj ect 12.
At this point, the correlation engine has a series of survey coordinates which identi fy the locations ofthe transmission towers along the path ofthe tag 10. This information, typically as latitude anal longitude pairs, could be supplied to the end-user pC 48 to enable the end-user to trace the path, for example by reference to a suitable set of maps.
~iowever, as illustrated in Figure I, once the overall regional information is obtained, the correlation engine 52' may send each pair of latitude/longitude coordinates to a web-based geomatic mapping server 60 and download therefrom a geographical map showing the tag's path overlaid upon it. This is analogous to a driver, for example, obtaining a route from a geomatic database similar to that accessible at the website wvrw.mapquest.com ofll~apquest Inc.
Having obtairxed the xnap and overlaid trace from the geomatic server 60, the Correlation engine 52' transmits the information back to the personal computer 48, to be displayed by the PC monitor.
The~s, the infozmation from the correlation engine 5z' could be provided to the end-user as sixnpIy a list of coordinates ofpoints along the path travelled, or a graphical representation similar to that shown in Figure l, or even a series of place names.

1s As noted above, when constructing the constructed spectrum, the correlation engine 52' does not differentiate spatiallybetween the signals from the different towers ixA tlxe group ox take into account the terrain in the search area. However, the typical government database 56 will usually have additional data, e.g, tower elevation, vvhick~ private database suppliers, such as RadioSoft, process to provide detailed FM mapping contours, for example as provided on~ theju~
web site.
It is envisaged, therefore, that the embodiment described above could be modified to allow the spatial and elevational information to be used to enhance the correlation process and obtain better resolution as to the location of the tag 14 when a particular frequency spectrum was 1 O captured.
Thus, in such a case, once the correlation engine 52' has obtained tk~e survey coordinates for the towers in the search area, it will send that information to a >?M
cOtttour mapping server ~ 8 which will use tb~e information to access its own data and generate a FM
contour map for that area and return it to the correlation engine 52'. At this point, the correlation engizte will compare the raw captured frequency spectrum in question with the FM contour ztlap data to determine more closely the location ofthe tag 10 relative to the towers within the search area when the raw frequency spectrum in question was captured. Fence, the correlation engine performs a coarse analysis using the constructed spectra and follows that with a flrier analysis using tlae FM
mapping contour data.
Constmction of the historical database 54 and the correlation process carried out by the correlation engitle will now be explained more fully with reference, where appropriate to the flowchart of Figures SA, 5B and SC.
al Consfiuctina the "historical FM station" database from Goverri ent aeenci_es FM stations are known to be extremely reliable. Indeed, within their Ir~co~cporation Act, broadcast services must remain operation even during major countrywide disasters to allow government authority maintaining contact with its citizen. FM station radiating more than 100 1cW must be equipped with UPS azxd self power generation to maintain power autonomy with na duration licuitation. However, in reality, some 1~M broadcasters cease commercial operation and others new ones arrive on the market. Extensive regulation of the FM band allows government control of the spectrum. Hardware maintenance activities that required temporally shutdown are z~otxb,ally conducted twice a year and during low audience hours, e.g. 3:00 am.
Although they appear very reliable and stable, l~M spectra have some level of dynamic.
In order to compare tire k'M spectrum captured by the tracer tag 10 without any chance of introducing database ezz-or, it is mandatory that the correlation be done between data that belong to same time period. Far example, if the tracer tag 10 happens to scan and store a FM
spectrum while FM brnadcast at frequency XY.Z in city ABC is temporally off air, it should be expected that correlation will be done with a FM database that excludes frequency ~Y.Z in city ABC. To do so, a "historical" FM station database must be constructed.
1 p The construction of the historical FM station database is a novel aspect of the invention.
Being heavily regulated by govertament agencies (i.e. FCC in USA, IC in Canada), a worldwide FM database of FM broadcast transmitter activities is constmcted based on sub-daily (i.e, FCC
refreslx rate is four time per day) information available from government agencies worldwide.
The database includes a collection ofworldwide broadcast atrtenna including;
survey coordinate, 1. S elevation, frequency, antenna radiation pattern, radiation power and RDS
(optional) qualified by ON-O)~p' air. .Also, it should be noted that some broadcasters reduce radiation power at night to save power consumption. Such information is also collected.
b Esc actin the "historical FM s ectrun;a" from th "historic M station"
database FM broadcasts, operating in the VHF band 8$ to 10$MI-Ia, present a well known 20 radiation behavior that is almost independent of environmental and climatical coxiditions. At the opposite end, AM broadcasts are known to have a longer propagation distance at night, versus day light conditions, due to stratospheric reflection and creation of "waveguide" effects caused by cold-trot air high altitude conditions. To that effect, radiation patterns, forming FM
mapping (likewise altitude mapping contours) of constant FM broadcast receiving power is 25 mathematicall~r werypredictable. Sorne companies (e.g. I~adioSoft) specialize in computing and supplyiztg FM mapping contours worldwide via the web. Since zone collect "historical" data, the combination of the "historical FM station data" with the "FM mapping contour" will allow the "historical FM spectrum" to be constructed for the period {date-time) corresponding to the ~$
period during which the FM spectnizn data was captured by the tracer tag 10 10 in the vicinity of the known geographical area.
cl Extracting tlxe tag 10 "path" from the "historical FM transmitter tower"
database and FM
data stored in the FM tea.
The operation of the web-based correlation engine in comparing the captured frequency spectra, with the historical database of reference spectra, and determining the path travelled by the tag, is depicted in Figures SA, 5B and SC. The technique used to extract the "path" assumes that the end-user is capable of identifying a minimum of ane known area (or city), most likely the "end" or the "start" ordnance survey coordinates or any other iulterim coordinates. In the 1 Q event that the extd-user does not know, he will be invited to guess to the best of his knowledge.
It should be noted that, in embodiments of this invention, the proposed technique (correlation process) will converge even ifthe guessed coordinates are tatallywrong, Precision and accuracy in the known coordinates will allow extraction of the "path" with less processing power, and hence more quickly.
1 S Referrixxg to Figure SA, the FM spectrum and, where applicable, the RDS
for every sample permanently stored in the tracer tag l0 are read (step 5.1), via the infrared (wireless) interface, by the PC (48) of the end-user that has retrieved the tracer tag 10 with the intention of extracting the "path" data. The read data is dawnloade~l to the "GeoPath"
server 52 for further data analysis. The "GeoPath" server 52 prompts the end-user to enter, in Step 5.2, "kzaawn"
20 survey coordinates, e.g. survey coordinates, city name, etc. to define one or more points known to be on the path, such as Start, Finish or an intezmediate "iix transit"
location, followed by a predicted area (general region) through which the tracer tag 10 leas travelled. If RDS data is available, that may be entered. The "minimum correlation Ievel" then is entered to define the level of minimum correlation factor that must be maintained for each data sample; otherwise the 2S data sample is rejected. The survey (latitude, longitude) coordinates axe obtained from the web-.
based geoznatic server G0. The retard date and time are attached to the survey cooxdixlates.
In step 5.3, a "historical ,FM spectrum" is constructed from. the "historical FM station"
database ire line with the recorded period, based on the FM stations surrounding the given survey coordinates. In step 5.4, the constructed frequency spectrum 1s compared and correlated, in turn, 1. 9 with each of the captured FM spectrum data records frown tlae tracer tag 10 to identify the captured frequency spectrum that most closely matches (i.e. has the highest correlation factor) the said constructed FM spectrum. $efore doing so, however, the e~ugine 52' compares each captured spectrum with the threshold T, as previously described, to eliminate noise axtd weaker S signals.
The highest correlation factor is compared (step 5.6) against the entered xninimtun correlation level and prompts an error message (step 5.7) if it is less or equal. Zf it is greater, the sample that presented the highest autocorrelation factor is identified as the "start point" sample atxd saved 8 with the ordnance survey coordinates.
1 O ThE next captured data record 9 is obtained. The search area associated with the "start point" is extended (10) until at least one more FM broadcast antenna is added, then a revised map of FM spectrum and RDS is constructed . The extended search area xs compared (11 ) with the entered predicted area (5.2) and pronrxpts a warning message (5.12) if exceeded_ The end-user then is asked (step 5.13) to decide whether to "skip" or "end" the data analysis. If the 1 S end-user chooses to "eizd", then creation of the path (5.20) is comztlenced. Tf the end-user decides to"skip, then the next sample or captured data record (5,9) is obtained and process step 5.10 is repeated. If it does not exceed 11, then correlation (5.14) is conducted between the revised map of1~M spectrum present in the search area and the FM spectrum associated with the said next tracer tag 10 data record, The correlation (x.14) is done by comparxz~g the said next 20 Tag 10 FM spectrum record with each one of the FM spectra records available within the map ofp'M spectra and the ozte withthe highest correlation factor is selected. As before, such highest correlation factor is compared (5. T 5) against the entered minimum correlation level (5.2). If this highest correlation factor dies not exceed the minimum, then the search area is extended (5.10) and the process steps (5.x0) to (5.14) are repeated until a correlation factor exceeds the 2 5 minimuzu correlation factor (5.15) or the search area exceeds the predicted area 11. If the said highest correlation factor do es exceed the minimum, then the survey coordinates associated with the highest correlation factor are saved (5.16) with the said next x 0 sample data record. The search area is updatedby replacing (5.17) the center coordinates of the search area with the latest coordinates, i.e., ofthe sample having the highest correlation factor. The engine then checks, by moving the search area, if new broadcast antenna need to be added or old need to be deleted, thenreconstructs the revised map of FM spectrum and IDS. Ifthe said next Tag 10 sample data is the last sample (5.18) then the same process is repeated starting from the said "start point"
sax~,ple, going into the opposite direction. If it is not the Iast sample, the engine gets the next sample (5.19) and repeats the correlation (5.14) within tlxe same search area.
~~splaying the Taa 10 "yath"
Step 5.20 groups all Tag ZO data with corresponding survey coordinates. Fox a "start point" within the "path", the two sub-paths are grouped together. (Lf the "start" point is actually the path start or end, the path is obtain in one piece, But if the "start" is anywhere along the x O path, the process described here will generate 2 path pieces or sub-paths that acre joined by the start point). If some sample data got skipped 13, the skipped data is interpolated. Information pertizient to other sensor data array then be added. In step 21, the exagine then uses tl~e coordinates to "call" for map information from any suitable web-based map provider 60 (e.g, similar to MapQuest) that matches the "path" survey coordinates tp "box in"
the usual square ~ S map shape. The engine then generates a display on the xuonitor 4$M of the PC of the end-user (23) showing the "path" travelled by the tracer tag Z 0 overlaid upon the map obtained from the map provider. When finished, the correlation engine prompts the ertd-user for any future mission of the tag 10 and revises the preset times, if needed. The engine also proaalpts the end-user to erase the tag 10 data.
20 Thus, at the end of each journey, or after a number of journeys, the RF
trace tracer tag can be initialized and set with a different sampling sequence interval so that it is ready for its next tracing mission.
It will be appreciated that correlation of the raw captuxed frequency spectra with the reference database spectra would be sinupler and less expensive, in terms ofprocessing time and 2 S power, if the correlation process wexe limited, at least initially, to a sub-group of the reference spectra. T'lxis restriction could be based upon ixaformation supplied by the subscriber when upIoadirxg the captured data to the ~F' correlation engine 52, Additionally or alternatively, conrelatiou could be simpli~,ed and/or enhanced by also using the station's call sign, which would allow extraction of the exact geographical location of each record point along the geographical path or trajectory. In fact, if sufficient broadcasters include RDS data xn their transmissions, then tb.at data alone could be used to identify the required location data.
Although the above-described receiver tracer tag 10 scaas the FM broadcast frequency band, for some applications it might be desirable to scan a different band, or several bands, such as AM, FM and VHF TV connznercial broadcast signals. The low fiequency Rl~
signals are very strong (coverirAg more than 90% of earth ground) and propagate reasonably well through 1 O buildings and other man made constructions. Low frequency broadcast signals also brave the advantage of "over the horizon" propagation, which contributes to the quasi-full I"tF earth coverage. On the other hand, the higher frequency signals provide better spatial resolution, so a combination may be preferred. Indeed, future digital radio and T'V systems that operate at much higher frequencies Will allow this technique to give better location accuracy.
For maritime applications, the predetermined band may be the LORAN-C band.
Since a trajectory is composed of multiple sample locations, the known geographical information can be used to obtain a realistic trajectory. Example: for a trace tracer tag 10 10 travellxz~g by car from New Xork city to Washington bC, if one sample spectrum data is identified as being somewhere in Europe, such sample data should be rejected, especially if the time interval made such traveling distance impossible.
The correlation factor or accuracy figure is a functioxa of the number of samples, number o~ rejected samples, number of known set of data records based on broadcaster IP (i.e. its station's callsign), etc. ,A, correlation factor can 6e extracted and displayed for each of the captured frequency spectra forming the trace. Such correlation factors can be useful when the trace information is part of (or used as) evidence in a court case. It should be noted that a correlation factor of 100% could be obtained where the captured data record consisted of a collection of at least one station's call sign 1D recorded for every sampling of the captured frequency spectrum.

The tracer tag 10 does not have any means o~tzansmitting its recorded FM
spectz'a data in real-tir~xe; hence the tracer tag 10 mast be retrieved to extract the data.
This limitation is applied voluntarily ixa order to avoid infringing individual privacy rights when the tracer tag 10 is hidden on a person.
As indicated above, if RI~S is used, the coozdinates of the transmission towers can be deter,~iz~ed from it and used in conjunction with FM mapping contours far the area to give the tag's enact location.
As shown in Figure 2, the receiver unit 16 may have an input 52 for inputtzztg via the IIQ
interface 44 information frozen environmental sensors, for sensing, for example, temperature, 1 O acceleration, light level, atmospheric pressure, etc. This sensor infot~aation can he integrated with the spectrum information for display with the trace. As an example, an tracer tag 10 hidden in a piece of luggage of a pexson taking a flight usually will show a sudden absence of FM signals when the aircraft Xs izt transit between two cities. It is laiown that atmospheric pressure inside the airplane cabin could be around 80% lower than ground atmospheric pressure.
Consequently, if the data supplied to the correlation engine 52' included readings from an atmospheric pressure sensor iz~ the tracer tag 10 , the coxrelatar engine 52' could use it to conclude that the tracer tag 10 was in an aircraft in flight between two airports and hence display such additional information along with the trace. This information could be depicted (say in broken lines) in the map showing the trace of the path.
2Q Similarly, accelerometerreadxn~gs would allow arx indxcatio~n as to whether tlxe tracer tag 10 was in zmotion ox at a standstill at vaxious locations along the trace.
Readings fxoxn biorhythm sensors would allow correlation of body conditians with the location information, which could be of interest in cases where the tracer tag 10 must be worn at all times by an entity whose mor~enxents are being tracecL
Application of the present invention is very wide. Without restricting application ofthe invention, the following applications are given by way of example:
a) Private or legal investigation of people conducting business oz pzivate tFavelix~g. The tracer tag 10 could be hidden in the luggage of a suspect person and, upon returning home, the tracer tag I O retrieved and the path t0.kez~, city by city, since departure, plotted.

b) Cargo port inspection. It is well know that only 5% of maritime cargo containers are plxysxcahy inspected at major seaport. The point of departure, point of temporary disembarkation and point of final destination could form the basis for a port authority to decide whether or not to conduct an inspection of incoming shipments. A radio receiver tracer tag 10 could be permanently mounted on every maritime cargo container, reset at tlxe point of departure and interrogated at the point of arrival. Reliable worldwide tracing infoxrzxatio~, will allow port authorities to make better decisions. It should be noted that, for maritime applications, the LORAN'-C frequency band can replace or be added to the FM
band and any other bands scanned by the receiver.
1 O c) Rental car companies apply insurance according to countries being visited. This reality is even more pronounced where the vehicle crosses a border between countries with significant economic differences (e.g. USA and Mexico, Western and Eastern Eumpe, African countries, etc,). In this special case, the existing vehicle's on-board radio receiver could be used substantially "as is", with minor modifications tv interface to it the storage and reader circuitry required to provide the functiozzs of the present inventiozt. When the vehicle was returned, the rental company vcrould be able to download the captured frequency spectrum data and use it to determine where the vehicle had travelled during the rental period.
d) Livestock animals and bio-alimentation. Increasingly in order to control atximals spreading disease (e, g, mad-cow disease), international regulations demand traceabilxty fvr all such livestock.
In embodiments of the present iztvention, a tracer tag could be installed on each animal with preset time intervals set to, say, I week or longer. The tracer tag 10 data could be ~retxieved and stored, for example, when the animal was received at an abattoir, It is also envisaged that, where the tag 10 is worn by an anixztal or person, the tag 10 could be powered by means of a theraxlocouple source of energy, or other source such used in eurrerxt pacemakers.
e) Migration control. Since the tzacer tag 10 is so small and of low weight, it could be installed on a bird's leg or foot (like a ring tag I O as used hitherto). A
biologist or other expert then could retrieve the tag 10 periodically, in the usual way, and retrieve the data to determine where the bird had been since the tag 10 was installed or previously checlGed.
This would e~aable more accurate tracing of migration activity thaw existing technitlues.
Of course, use of the invention to track migration and so on is not limited to birds but could also be applied to tlxe tracking of other living creatures.
S fj Irnmigratioxa control. Temporary immigrants will be mandated to carry a temporary passport equipped with a tracer tag. Tlae immigrant is to report himself periodically to authorities. During such reporting, the data loaded in the tag hidden in the passport will be used to trace the immigrant's travels between each visit. The immigration authorities will then have valuable information, without the new immigrant knowing it.
O g) Release of prison inmates on parole. Inmates on parole must stay within a pre-determined geo&raphical area. A tracer tag could be hidden within their ID
card, used to trace information belrreen every report or visit to the parole officer. In the event that someone had a court order that restricted thetas to stay at pre-determined distance away from someone's residence (often the case on judgment against a violent ex-spouse) a low-power FM transmitter 15 with specific RDS can be installed at the restricted residence area. (Such low-power FM
transmitters are commonly used today to broadcast dominical mess to the paroisian on Sunday morning, used to traztsxnit the '"house for sale" ixd'o;rmation where the broadcast FM frequency is shown on the "house for sale" sign) The person that is under a court restriction must carry their ID card at all times, and xzxust report to authorities periodically.
20 h) Postal service quality control. livery postal service uses multiple point of data entry, all networked to a centralize database. However, there is no rrAeans of double-checking that such in-house tracking system is working and safe from human mistake and manipulation. The quality control department will then insert a tracer tag on selected andlor random packages.
Upon axxrival of the package at its destination, the tag is retrieved and tracing information is 25 compared with an internal tracking system. Disparities could then be addressed and corrected.
i) Dangerous Hazard materiai (HazMat) in transit. Regulations impose detailed tracking requirements to follow the HazMat at all times and provide a "paper trail".
Transpaztation companies must obey regulations but at present compliance is not strictly monitored by the authorities. Crovernment trazzspoxt authorities may require the use of a receiver tracer' tag 10 on ~. $
every HazMat container. The transport authority would then be able to trace the path taken by the HazMat, allowing them to enforce the regulations. A particular case might apply to tunnels and bridges. Most HazMat are not allowed to use such critical and vulnerable transport infrastructure. Nowadays, most tunnels have private repeaters to radiate broadcast signals within, the tunnel. The tunnel information could then be transmitted via the RDS and recorded by the tracer tag 10 . It should be noted that, for I~azMat tracability, size and battery dimensions are not important. In such a case the tracer tag 10 could be set to record the radio spectrum at z o second intervals when in transit. When equipped with a motion sensitive accelerator, the tag can record "moving" versus "statxdstill" information.
IO Embodiments of the invention adrrantageously provide ~or coarse tracing of the geographical, potentially worldwide, displacertrent of an entity, e.g., an object, article, person under surveillance at very low cost, in eonttast to previously known techniques based on GPS
tracking or cellular phone tracking which require expensive and large receivers that operate for only a few days when powered by battery. In embodiments of the inventiotr, the "coarseness"
of the tracing can be tailored to the application, allowing global tracing with resolution equivalent to sub-cities ox local tracing with resolution equivalent to, say, 100m. Such local tracing might apply to the above-described use to monitor a violent person approaching an ex-spouses' home.
It is envisaged that the tracer tag 10 may comprise a micro FM receiver costing a few dollars, having dimensions smaller thant a 10-cent coin and power consumption so snrrall that it could be supplied by a watch battery for a period exceeding one year.
Although the receiver o~the above-described system monitors the FM frequency band, a hybrid system operating on multiple broadcast bands (AM, FM & T~ will present the best of both world in terms of wide coverage and fine resolutions. As mentioned hereinbefore, AM
signals are known to cover a wider geographical area due to their ability to radiate "over the horizon", especially at night. When used with AM only, the tracer tag 1.0 must have a real-time clock in order to add time information with the collected AM spectrum, which could be used to compensate for the fact that AM propagation during daytime is less than during nighttime.

Cellular phone technology presents similar characteristics. Txadeed, the cellular tower presents a unique RF signature like broadcast signals. However, the information about transmitted signals is proprietary, hence not available to the public, and may require a licensing agreement. A key advantage of using cellular phone freduencies comes from the fact that a regional area is covered with multiple transmitters resulting in finer traceability resolution. On the other hand, cellular frequency signals are not present in remote areas whereas broadcast signals are. Also, cellular systems often have dead spots. An FM broadcast signal, being very wide (20% bazzdwidth) is equivalent to spread4spectrum. Thus, a dead spot may happen at one specific frequency, but not to all others frequencies, hence the feature of spread-spectrum.
1 O It is expected that, in a few years, all-digital radio and TV broadcast will become available, New frequency bands will then be allocated. When such new digital broadcast becomes available, the techniques described in this patent will remain the same with the exception of operating at different frequencies. However, being all digital, it is expected that broadcasters will transmit much more identification data, i.e. tizxle, date, antenna azimuth and elevation, signal strength. All such inforxn.ation is useful for fuithex improving the performance of embodiments of this ixtvention.
In the event that a broadcaster transmits its anteniz~a azimuth coordinates and elevation, there will no longer being any need to construct the reference spectrum and conduct spectrum correlation with the captured spectra.
Sinee the late 1990's, many broadcasters have been transmitting their own identification data string making the RF signature I00% unique with zero chance of miseorrelation.
In the US, the transmission of extra broadcasting material via the I 9 kHz subearrier has been known as SCA (Subsidiary Communications Authorization). The diagram in Figure 3 shows the assigzrment of frequencies used in an FM transmitter. The m,aixx channel is the program heard with a mozraural F'M radio. These frequencies extend from below 25 Hz to 15 kHz. At 19 kbIz, is the stereo pilot. The pilot is used to extract the stereo programming, which e~cists from 23 to 53 kHz. At 57 klTz is the RDS subcarrier used for data to carry such information as the station's callsign, and the name of the selectiozr currently playing. From 60 to 74 kHz is the 67 kHz subcarrier and from 85 to 99 kHz is the 92 kHz subcarrier.

The technical nomenclature for SCA (Subsidiary Coxtun.uni.catians Authorisation) txansrnissions xs; k'M subCamier on FM carrier, frequency division multiplexing.
In Europe the general concept has been referred to for several years as RDS
(Radio Data System). It allows transmission of a station ID, program info, local travel info including traffic reports and other information. 1t will continuously probably evolved into much more. As digital broadcasting takes over in the future, even more uses rx~ill be found.
Previously known techniques based oz~ GPS tracking or cellular phone tracking demand expensive and large size receiver that operate for only a few days when powered by battery. 13y eznployiztg coarse tracing, embodiments of this invention provide global tracing with resolution 1 O equivalent to sub-cities. For a tag comprising a micro FM receiver, the manufacturing cost can be less than $3.50 USD each, while its dimensions may be smaller than a dime and battery operation time may exceed 1 year.
This ~ system provides a worldwide coarse traceability passive, miniature and low-powered apparatus and method at a significantly lower cost than the active systenx described 1. 5 in the prior art.
It b.as been proposed for public broadcasters to incorporate in their signals not only the station identifier but also the current time, In that event, receivers embodying the present invention need not generate the time stamp for the frequency spectra but rather will extract the current time fro xt oz~e of the received signals. Moreover, it is envisaged that the broadcaster 20 could also transmit the station's geographical coordinates, e,g,, as ordnance survey rt~ap coordinates, in which case the receivers eaxlbodying the present invention could capture that information in addition to, ar errentually instead of, the frequency spectra.
Although an embodiment ofthe invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration arid example only and not 25 to be taken by way of the limitation, the spirit and scope of the present in'~ention being limited only by the appended claims.

Claims (20)

1. A tracing system comprising:
a radio receiver associated with the entity or object and operable to scan a predetermined radio band at preset times as the entity or object traverses the path, and capture the extant frequency spectrum, and storage means for storing said captured frequency spectrum and means for retrieving a plurality of captured frequency spectra from the tag and correlating said captured frequency spectra with known antenna transmissions and location data to determine the location of the tag when each captured frequency spectrum was captured and deriving the path from the successive locations.

2. A tracing system according to claim 1, wherein the computer means is operable following retrieval of the captured frequency spectra to (i) receive user-supplied information identifying at least approximately a first search area traversed by the tag, to retrieve antenna transmission and location data to construct a first frequency spectrum comprising a set of signal frequencies of signals receivable in said search area at the time the tag was in said first search area;
(ii) correlate the first constructed frequency spectrum with each captured frequency spectrum in turn to determine the captured frequency spectrum having the highest correlation therewith and store coordinates of the first search area as the location of the tag when said highest-correlation frequency spectrum was captured, (iii) define a second search area encompassing an area around or neighboring the first search area;
(iv) construct a second constructed frequency spectrum for the second search area comprising a set of signal frequencies of signals receivable in said second search area at the time the tag was in said second search area, (v) correlate the second constructed frequency spectrum with at least those captured frequency spectra captured at times close to the time of capture of the first captured frequency spectrum to determine a second captured frequency spectrum having the highest correlation therewith;
(vi) stare coordinates of the second search area as the location of the tag when said second highest-correlation frequency spectrum was captured, and repeat the process for other captured frequency spectra to obtain coordinates of successive regions traversed by the tag.

3. A system according to claim 2, wherein the computer means is operable, prior to correlating a particular captured spectrum with a constructed spectrum, to compare signal strengths of signals in said captured spectrum with a predetermined threshold signal strength level and select only those signals whose strengths exceed the threshold for correlation with the constructed spectrum.

4. A system according to claim 1, wherein, the computer means is operable to derive from the location coordinates of the series of regions a map of regions with the path depicted as an overlay thereupon.

5. A system according to claim 1, wherein the receiver demodulates from each transmission signal information (e.g. RDS) identifying the source, and stores said information in the corresponding record for that preset time, the identification information being used, when the tag has traversed the path, to determine the locations of the receiver at the preset times and hence the path traversed by the tag and entity or object with which it travelled.

6. A system according to claim 1, wherein the database comprises historical collections of said reference spectra allowing comparisons with reference spectra that were valid or pertaining at the times and locations at which the tag scanned and stored the frequency spectra.

7. A system according to claim 1, wherein the radio frequency band is selected according to path being/to be traversed.

8. A system according to claim 1, wherein the predetermined radio band is a broadcast radio band(s) used in geographical areas to be traversed by the entity or object, for example a radio hand selected from the group consisting of the FM broadcast frequency band of 88 to 108 MHz., the FM broadcast frequency band of 86 MHz. to 108 MHz., broadcast television bands and cellular telephony bands.

9. A system according to claim 1, wherein the preset times are selected according to the path being or to be traversed and the resolution required for the mission of the tag.

10. A method of tracing a path previously taken by an entity or object comprising the steps of:
associating with the entity or object a radio receiver operable to scan a predetermined radio band at preset times as the entity or object traverses the path, and capture the extant frequency spectrum, and store said captured frequency spectrum;
and when the tag and entity/object have traversed the path, retrieving a plurality of captured frequency spectra from the tag and correlating said captured frequency spectra with known antenna transmissions and location data to determine the location of the tag when each captured frequency spectrum was captured and deriving the path from the successive locations,

11. A method according to claim 10, wherein the step of retrieving and correlating the captured frequency spectra comprises the steps of (i) using user-supplied information identifying at least approximately a first search area traversed by the tag, retrieving antenna transmission and location data to construct a first frequency spectrum comprising a set of signal frequencies of signals receivable in said search area, at the time the tag was in said first search area;
(ii) correlating the first constructed frequency spectrum with each captured frequency spectrum in turn to determine the captured frequency spectrum having the highest correlation therewith and store coordinates of the first search area as the location of the tag when said highest-correlation frequency spectrum was captured, (iii) defining a second search area encompassing an area around or neighboring the first search area;
(iv) constructing a second constructed frequency spectrum for the second search area comprising a set of signal frequencies of signals receivable in said second search area at the time the tag was in said second search area, (v) correlating the second constructed frequency spectrum with at least those captured frequency spectra captured at times close to the time of capture of the first captured frequency spectrum to determine a second captured frequency spectrum having the highest correlation therewith;
(vi) storing coordinates of the second search area as the location of the tag when said second highest-correlation frequency spectrum was captured, and repeating steps (iii) to (vi) for other captured frequency spectra to obtain coordinates of successive regions traversed by the tag.

12. A method according to Claim 11, wherein the computer means is operable, prior to correlating a particular captured spectrum with a constructed spectrum, to compare signal strengths of signals in said captured spectrum with a predetermined threshold signal strength level and select only those signals whose strengths exceed the threshold for correlation with the constructed spectrum.

13. A method according to claim 10, wherein the correlation step includes the step of deriving from the location coordinates for the series of regions a map of regions with the path depicted as an overlay thereupon.

14. A method according to claim 10, wherein the receiver demodulates from each transmission signal information (e.g. RDS) identifying the source, and stores sand information in the corresponding record for that preset time, the identification information being used, when the tag has traversed the path, to determine the locations of the receiver at the preset times and hence the path traversed by the tag and entity or object with which it travelled.

15. A method according to claim 10, wherein the correlation step uses a database comprising historical collections of said reference spectra allowing comparisons with reference spectra that were valid or pertaining at the times and locations at which the tag scanned and stored the frequency spectra.

16. A method according to claim 10, wherein the predetermined radio frequency band is pre-selected according to path being/to be traversed.

17. A method according to claim 10, wherein the predetermined radio band is a broadcast radio band(s) used in geographical areas to be traversed by the entity or object, for example a radio band selected from the group consisting of the FM broadcast frequency band of 88 to 108 MHz., the FM broadcast frequency band of 86 MHz. to 108 MHz., broadcast television bands and cellular telephony bands.

18. A method according to claim 10, wherein the preset times are selected according to the path being or to be traversed and the resolution required.

19. A radio receiver tag suitable for use in tracing a path taken by an entity or object while accompanied by the radio receiver tag, comprising:

a radio receiver operable to scan a predetermined radio frequency band and capture data representing a frequency spectrum thereof;
storage means for staring a plurality of data records each corresponding to a respective one of a plurality of the frequency spectra;
control means for controlling the radio receiver to scan the radio frequency band at preset times, while the entity or object is traversing said path, and store the resulting frequency spectra data in said storage means; and output means for outputting said frequency spectra data.

20. A system according to claim 1, wherein the known antenna transmissions and location data are stored on a network element of a public data network, such as the Internet and accessed from a computer means used to retrieve said captured frequency spectra.