US 20070099565 A1 Abstract A system includes a remote station and a local station having a receiver. The receiver operates in an unlocked state corresponding to its best lock frequency (BLF). The local station derives data indicative of a ratio of the BLF to a reference frequency of the receiver, and telemeters the data to the remote station. The remote station estimates the BLF based on (i) the telemetered data, and (ii) a predetermined estimate of the reference frequency.
Claims(16) 1. In a system including a local station and a remote station, the local station including a receiver capable of operating in an unlocked state when not tracking an uplink signal, the unlocked state corresponding to a best lock frequency (BLF) of the receiver, a method of remotely monitoring the BLF, comprising:
(a) in the local station, operating the receiver in the unlocked state corresponding to the BLF; concurrently (b) in the local station, deriving data indicative of a ratio of the BLF to a reference frequency of the receiver; (c) telemetering the data indicative of the ratio from the local station to the remote station; and (d) in the remote station, estimating the BLF based on (i) the telemetered data indicative of the ratio, and (ii) a predetermined estimate of the reference frequency, to produce an estimated BLF. 2. The method of (e) transmitting, from the remote station to the local station, an uplink signal having an uplink frequency that will be received at the local station at the estimated BLF; and (f) in the local station, locking the receiver to the uplink frequency. 3. The method of (i) the estimated BLF, or (ii) offset from the estimated BLF by an amount substantially equal to a Doppler shift arising from relative motion between the remote station and the local station. 4. The method of 5. The method of 6. The method of producing a first linear combination frequency as a first linear combination of the BLF and the reference frequency; producing a second linear combination frequency as a second linear combination of the BLF and the reference frequency; repeatedly counting cycles of the first linear combination frequency to produce successive first count values; and repeatedly counting cycles of the second linear combination frequency to produce successive second count values corresponding to the successive first count values, wherein the successive first and second count values represent the data indicative of the ratio. 7. The method of step (c) comprises telemetering the successive first and second count values from the local station to the ground station; and step (d) comprises estimating the BLF based on (i) the telemetered successive first and second count values, and (ii) the predetermined estimate of the reference frequency, to produce the estimated uplink frequency. 8. The method of subtracting the telemetered successive first count values from each other to produce a first difference value; subtracting the telemetered successive second count values from each other to produce a second difference value corresponding to the first difference value; and estimating the BLF based on the first and second difference values and the predetermined reference frequency estimate. 9. The method of the first linear combination frequency is equal to A·f _{u}+B·f_{o}; the second linear combination frequency is equal to C·f _{u}+D·f_{o}; and said estimated BLF ({tilde over (f)} _{u}) is given by {tilde over (f)}_{u}=f_{o}(B−Dr)/(Cr−A), where
f
_{u}, f_{o}, and {tilde over (f)}_{o }respectively denote the BLF, the reference frequency and the predetermined reference frequency, and r represents a ratio of the first difference value (ΔN
1) to the second difference value (ΔN2). 10. The method of 11. (canceled) 12. In a system including a local station and a remote station, the local station including a receiver, a method of remotely monitoring an unlink frequency of an uplink signal at the local station, comprising:
(a) in the local station, operating the receiver in the locked state when tracking the uplink frequency; concurrently (b) in the local station, deriving data indicative of a ratio of the uplink frequency and a reference frequency of the receiver; (c) telemetering the data indicative of the ratio from the local station to the remote station; and (e) in the remote station, estimating the uplink frequency based on the telemetered data and a predetermined estimate of the reference frequency, to produce an estimated uplink frequency, wherein step (b) comprises: producing a first linear combination frequency as a first linear combination of the uplink frequency and the reference frequency; producing a second linear combination frequency as a second linear combination of the uplink frequency and the reference frequency; repeatedly counting cycles of the first linear combination frequency to produce successive first count values; and repeatedly counting cycles of the second linear combination frequency to produce successive second count values corresponding to the successive first count values, wherein the successive first and second count values represent the data indicative of the ratio. 13. The method of step (c) comprises telemetering the successive first and second count values from the local station to the ground station; and step (d) comprises estimating the uplink frequency based on (i) the telemetered successive first and second count values, and (ii) the predetermined estimate of the reference frequency, to produce the estimated uplink frequency. 14. The method of subtracting the telemetered successive first count values from each other to produce a first difference value; subtracting the telemetered successive second count values from each other to produce a second difference value corresponding to the first difference value; and estimating the uplink frequency based on the first and second difference values and the predetermined reference frequency estimate. 15. The method of the first linear combination frequency is equal to A·f _{u}+B·f_{o}; the second linear combination frequency is equal to C·f _{u}+D·f_{o}; and said estimated uplink frequency ({tilde over (f)} _{u}) is given by {tilde over (f)}_{u}={tilde over (f)}_{o}(B−Dr)/(Cr−A), where
f
_{u}, f_{o}, and {tilde over (f)}_{o }respectively denote the uplink frequency, the reference frequency and the predetermined reference frequency, and r represents a ratio of the first difference value (ΔN
1) to the second difference value (ΔN2). 16. The method of Description This application claims the benefit U.S. Provisional Application No. 60/434,259, filed on Dec. 18, 2002, the contents of which are incorporated herein by reference in their entirety. 1. Field of the Invention The present invention relates generally to systems including a control station and a remote receiver, and more specifically, to such a system wherein the control station remotely monitors an operating frequency of the receiver using telemetry. 2. Discussion of the Related Art In a typical satellite system, a ground station transmits an uplink signal to a satellite and the satellite transmits a downlink signal to the ground station. The satellite includes a receiver configured to phase and/or frequency track the uplink signal. That is, in the presence of the uplink signal, the receiver locks onto and tracks the uplink signal frequency (referred to as the uplink frequency) and/or phase. However, in the absence of the uplink signal, the receiver is unlocked, i.e., free-running, and thus settles to a rest frequency, also referred to as best lock frequency (BLF). The BLF corresponds to the uplink frequency that, if present, would cause the receiver to transition from the unlocked state to the locked state in a minimum amount of time and with a minimum amount of frequency pull-in. The BLF can be considered a natural frequency of the receiver. The receiver may have a relatively narrow uplink frequency pull-in range typically encompassing the BLF. However, the BLF is typically uncertain, i.e., not precisely known. Thus, in a known technique for uplink frequency acquisition, the ground station sweeps the uplink frequency over a relatively wide frequency range anticipated to include the uncertain BLF. When the swept uplink frequency moves near to the BLF and is within the frequency pull-in range, the receiver captures or locks onto and tracks the uplink frequency, and is said to have acquired the uplink signal. Causes of BLF uncertainty include, for example, receiver temperature variations and component (e.g., oscillator) aging, or other physical effects. The larger the BLF uncertainty, the larger the uplink frequency sweep range, and disadvantageously, the larger the uplink signal acquisition time. It is desirable to minimize or eliminate the BLF uncertainty, and correspondingly narrow the uplink signal sweep range, so as to minimize the uplink signal acquisition time. Therefore, it would be advantageous to be able to accurately determine the BLF at the ground station. In other words, it would be advantageous to be able to monitor at the ground station the BLF of the spacecraft receiver. After the uplink frequency is acquired, the ground station performs two-way Doppler tracking of the uplink and downlink signals (i.e., frequencies). The accuracy of the Doppler tracking depends on an accuracy with which the ground station can determine the uplink frequency as received at the satellite. Thus, there is a need to be able to accurately determine at the ground station the uplink frequency as received at the satellite. In other words, there is a need to monitor at the ground station the uplink frequency at the satellite. These and other embodiments of the present invention will become apparent from the ensuing description. An embodiment of the present invention includes a method of remotely monitoring a BLF of a local receiver. Another embodiment of the present invention includes a method of remotely monitoring an uplink frequency received at a local station. A system in which the embodiments may operate includes a remote station, such as a ground station, and a local station, such as a spacecraft. The local station includes a receiver having first and second digital counters. The first counter is clocked at a relatively low rate related to a down-converted and scaled frequency that is input to the receiver, which may represent either (i) the BLF of the receiver in the absence of an uplink frequency, or (ii) an uplink frequency input to the receiver when the uplink signal is present. The second counter is clocked at a high rate related to a frequency of a local reference frequency of the receiver. The receiver includes latch logic configured to simultaneously latch first and second counter output values of the first and second counters, respectively, responsive to a trigger signal in the receiver. The latch logic operates repeatedly to generate successive pairs of first and second counter values. The successive pairs of counter values are indicative of a ratio of either (i) the uplink frequency, or (ii) the BLF, to the reference frequency. The local station telemeters the successive pairs of counter values to the remote station. The remote station determines either (i) the uplink frequency, or (ii) the BLF, based on the successive pairs of telemetered counter values and a predetermined estimate of the reference frequency available at the remote station. In the absence of the uplink signal, the present invention enables a user at the remote station (e.g., ground station) to remotely determine the BLF of the receiver. The BLF represents the frequency that, if injected into the input of the receiver, will place the least stress on an uplink signal tracking system in the receiver and permit the tracking system to quickly establish receiver lock using an efficient frequency sweep strategy. This advantageously improves the use of costly ground station time. In the presence of the uplink signal, the present invention enables the user at the ground station to remotely determine the uplink frequency at the receiver, to maintain the uplink frequency at a desired frequency in the presence of Doppler shifts, and roughly estimate the Doppler velocity without the use of a ground-based Doppler measurement. In another application, the ground station combines the telemetered successive counter values with a telemetered phase-locked loop static phase error (produced by the receiver tracking system) in order to monitor variations in the BLF over time. This allows the ground station to maintain the uplink frequency near the BLF and to ascertain what uplink frequency will quickly lock-up the receiver if the uplink signal link, between the ground station and spacecraft, is broken and must be re-established. The above and other features and advantages will become more readily apparent from the detailed description of the invention accompanied by the following drawings, in which: Definition The term “receiver frequency” used herein denotes a frequency f System Ground station Spacecraft In the presence of uplink signal The first operational arrangement of system The BLF corresponds to the uplink signal frequency f In the second operational arrangement (uplink signal not present), receiver In addition to satellite systems, such as system Frequency Converter and Tracker module Telemetry system Counter and Latch module Receiver The above-described operations repeat continuously over time to produce a series of recovered count values N When uplink signal Method Flow Charts Methods of remotely monitoring (i.e., determining or estimating) a local receiver frequency that may be performed in system Remote Estimation of Local Receiver BLF In a first step In a next step (i) telemetry system (iii) Counter and Latch module Step In a next step In a next step In a next step (i) differencer module (ii) differencer module (iii) estimator Estimator where r=ΔN In this manner, both ratio r (i.e., ΔN Uplink Signal Acquisition First step In a next step In a next step In a next step In a next step Remote Estimation of Uplink Frequency at Local Receiver In a first step In a next step In the next step In a next step In the next step In a next step It will be understood that various modifications may be made to the embodiments disclosed herein. Therefore, the above description should not be construed as limiting the scope of the invention, but merely as exemplifications of the preferred embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto. Classifications
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