surface. As shown, the received signal 42 is mixed with plished through generating a compensating control the originally transmitted signal 4 in the detector 31, voltage wave form utilizing digital information stored generating a difference signal 48 (ref. FIG. 4c). Differ- in the central processor. Under control of the central ence signals are stored in sample windows 43, having a processor, linearizing signals are applied to the voltage period 44 identical to and synchronized with that of the 5 controlled oscillator portion of the variable frequency frequency sweep portion of the transmitted signal 40. oscillator 27.
Operation of the circulator 35 will be well known to Novel aspects of the discovery disclosed herein can those skilled in the microwave arts and therefore not best be described through the following exemplary and discussed in further detail. In operation, portions of the nonlimiting example. For the case typically disclosed transmitted signal are also directly sent to the detector. 10 where the tank ullage range or distance of 40 meters is Detector inputs now consist of the initially transmitted desired, the following well known relationship between signal and the received signal delayed in time as indi- return difference frequency, and other system
cated above. The mixing or detecting operation, also parameters is representative;
well known to those skilled in the radio frequency arts,
provides an output frequency equal to the difference of 15 /^the return difference frequency)-=45/!/rc
the transmitted and returned signals. In the case of the
invention disclosed herein, the mixed or difference sig- where;
nal 48 is in the audio frequency range characteristically B=the return signal frequency band width, i.e. the
0.250 to 9.6 kilohertz for reflected signals traveling the sweep frequency range of the oscillator 27 under sweep
distance 19 of 1 to 40 meters. A graphic representation 20 control 36 from the sweep linearizer 10.
of these return signals 48 are shown in FIG. 4d R=40 meters, the desired measurement range.
In keeping with the novel and inventive aspects of the T=0.025 seconds corresponding to the sweep freapplicants' invention, applicants have discovered that quency period of oscillator 27 at 19 sweeps per second, samples of the difference signal 48, taken in synchroni- it shou]d ^ noted that the swecp freqUency is essenzation with the period of the transmitted sweep signal 25 tially an arbitrary quantity chosen to provide adequate 41, are stored for periods identical, i.e. having the same measurement taking into account the maximum rate of initial and terminal times, to that of the sweep portion of change of the ullage 19.
the transmitted signal. This synchronous processing of c=3 X 10» meters/seconds, the speed of light and the difference signals, when transformed into the frequency transmitted signal.
domain, eliminates the phenomena known as frequency 30 If the above relationship is rearranged to determine leakage. Non-synchronous sampling would result in ^e range R; introduction of additional frequency components, introducing frequency leakage errors into the signal repre- R^Tcfj/AB sentative of the tank ullage 19, in the mixed signal 48.
Applicants have further discovered that application 35 Substituting the above constant provides the followof on-line direct Fourier transforms of the stored differ- jng; ence signals during the intervals Ts', indicated as 46 on FIG. 4c, provides a frequency domain representation 50 R=fd/2so (reference FIG. 4e) of the measured and stored time
domain return frequency 48, containing essentially all of 40 Since the difference frequency is measured to the the frequency components necessary to accurately de- accuracy of the quantity Sf, then the corresponding termine the tank ullage 19 contained in the returned accuracy of the range measurement SR is; signal 42.
Further signal processing in the frequency domain in BR**6fj/iso keeping with applicants' inventive discovery, proceeds 45
with determination of a centroid of the synchronous Since, in the system disclosed, the sweep or sample frequency distribution of the return signal 48. With frequency is approximately 18.75 hertz, the observaparticular reference to FIG. 5, the now transformed tional period is approximated by; signal 55 of the returned mixed signal 48 is shown in a
SINx/x form. As those skilled in the art of transform 50 , - -L 1 = 53 3 x 10-3 seconds
mathematics will readily see, the SINx/x representation 1 ~ fi '8-75 hz ~ ■
55 shows as vertical components along the frequency
axis here typified as 57 and 58, frequency components of In the frequency domain then, the return difference the returned and mixed signal 48. As transformed, the frequency represents a measured range increment 6R components of the returned signal include extraneous 55 equal to 1/250.X53.3X 10-3=0.075 meters, frequency components returned to the antenna 15 The above figure, i.e. 0.075 meters represents the through oblique reflection, noise, and non-direct radio basic distance which can be resolved through convenfrequency signals from other unknown sources. As tional conversion of the returned time domain signal fd these are not of interest in measuring the tank ullage, in the frequency domain. Also, return signal errors in applicants have discovered that the frequency domain 60 the above measurement are introduced by conventional representation establishes a dominant frequency shown instrumentation and measurement inaccuracies, in FIG. 5 as component 56. An additional and substan- Most importantly, errors due to corruption of the tial error is also introduced due to the relative non- return signal spectrum in the frequency domain as linearity of the frequency sweep over its range of 9.5 to would be transformed by prior art approaches, by inclu10.5 gigahertz. In keeping with the invention disclosed, 65 sion of energy in the form of extraneous signals due to applicants have utilized techniques for linearizing noise, uncontrollable, spurious reflections from inside sweep frequency voltages as are typically available the measuring tank or container, and other random from state-of-the-art equipment. Linearization is accom- energy measured as signals, are incorporated in the