Communications: directive radio wave systems and devices (e.g. – Testing or calibrating of radar system – By simulation
Reexamination Certificate
2001-06-07
2003-03-04
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Testing or calibrating of radar system
By simulation
C342S041000, C342S042000, C342S043000, C342S044000, C342S051000, C342S169000, C342S173000, C342S174000
Reexamination Certificate
active
06529156
ABSTRACT:
I. FIELD OF INVENTION
This invention relates generally to an apparatus for determining an inherent delay in a transponder, wherein the delay corresponds to various physical quantities. More particularly, the invention concerns a method and apparatus for determining the inherent delay of a transponder on a pulse-by-pulse basis such that variations in the delay, which occur over the course of operation due to variations in various physical quantities such as temperature, pressure, stress, strain and the like, are immediately known. In addition, the method and apparatus of the present invention allow for determination of the transponder delay without interruption of the normal operation of the transponder device.
II. BACKGROUND OF THE INVENTION
At the most fundamental level, radar systems depend on measuring the time delay between a transmitted and a subsequently received signal. The necessary accuracy for the measurement of this delay can be in the sub-microsecond range, and direct measurement of such time intervals is a substantial technical challenge.
In the past the standard procedure of measuring the range accuracy of shipboard radar equipment was to provide a target consisting of a passive reflector situated on a known surveyed point, which is tracked by the ship's radar. This procedure is typically limited to the lower range scales. In order to test higher range scales, an active transponder is used to produce an artificial range, which is added onto the actual separation between the ship's radar and the transponder unit. The resultant sum of actual and artificial ranges is then compared to the range indicated on the shipboard equipment.
Such prior art transponders typically consist of three basic units, a receiver, a clock timer, and a transmitter. The receiver and transmitter are tuned to the frequency of the radar under test. Upon receiving the pulse transmitted by the ship's radar, the clock counts out a specified period of time and triggers the transmitter, which sends an artificial echo return pulse back to the ship's radar. This clock time is directly proportional to the desired artificial range. This artificial range is to be used as the standard against which to compare the radar unit under test, so it must be known to a high degree of accuracy. Since the range is directly related to the time delay, the problem becomes that of determining precisely the elapsed time between reception at the transponder of the pulse transmitted by the ship's radar, and the transmission of the return pulse by the transponder unit. The precision required necessitates that such factors as receiver and transmitter response times and propagation delays in the waveguides and coaxial cables be considered along with the clock time.
In order to obtain the required precision, the total delay time for a signal to complete the signal path through the transponder must be known. In the past a variety of methods have been used to measure this delay time, most of which involved the use of an oscilloscope for the actual measurement. Measurements performed by these different techniques have yielded results, which disagree. These methods involve artificially introducing a triggering signal and watching for the transmitted pulse on an oscilloscope. The time between the trigger signal and generation of the transmit pulse is taken as the total delay time, T. This method requires the selection of appropriate points on the leading edge of each waveform to serve as the beginning and end points of the delay period. This is critical because the rise times of the pulses are on the order of 30 nanoseconds, which is equivalent to 5 yards in range. The pulses also exhibit about 60 nanoseconds of jitter, which at a typical sweep speed (e.g., 0.2:sec/cm) corresponds to a trace movement of 3 millimeters. This jitter must be visually averaged to an accuracy of 0.3 millimeters to meet the desired measurement accuracy capability of 1 yard. In addition, the scope sweep itself must be calibrated by observing the output waveform from a crystal oscillator of known frequency and adjusting the scope. This scope calibration is itself also subject to similar considerations regarding the necessary visual acuity. In order to achieve the time resolution necessary for the measurement, a sweep speed of at least 0.2 &mgr;s/cm would be necessary. This in turn means that the inherent delay can only be measured at limited ranges, for which T is small enough that both pulses can be displayed on the same sweep. Once the measurements are made, the transponder is considered calibrated for any indicated range set into the delay controls.
While modern oscilloscopes have overcome some of the above mentioned measurement difficulties, there remain problems associated with the described prior art delay measurement techniques. A first problem is that it is a static (one time) calibration as opposed to a continuous calibration. Accordingly, variations in the delay, which may be caused for reasons such as a change in temperature, are not accounted for by the system. A second problem is that because the counter circuits in the delay clock may not function perfectly at all ranges and the calibration is performed at a fixed range, a detected delay value may have a further unknown and undeterminable error. A third problem is that if the transponder delay is smaller than the pulse width of the test pulse, it is not possible to determine the amount of the delay. A fourth problem is that normal operation of the transponder and receiver must be halted during the calibration process.
One prior art method for calibrating transponders for aircraft includes manual calibration with test signals. Specifically, in one method while the aircraft is grounded, a ground crew will manually test the transponders on board the aircraft to determine the inherent delay. With the grounded method, the inherent delay information must then be transferred to the active radar systems, wherein it may be subtracted from future detected radar signals. In another method, while the aircraft is in flight, a flight crew may manually test the transponder on board the aircraft to determine the inherent delay. With the in-flight method, the inherent delay information must then be transmitted back to the active radar systems, wherein it may be subtracted from future detected radar signals.
Manual calibration of transponders as described above has an inherent limitation that restricts any practical application. Specifically, any subsequent change in temperature, pressure, stress, strain and the like, will result in an unknown and undeterminable inherent delay in the transponder. As such, unless a transponder is constantly manually calibrated, the true, real-time, delay of the system cannot be determined. However, it is impractical to expend manpower to constantly manually calibrate the transponder. Furthermore, even when a transponder operator calibrates the system in flight, the manual operation is difficult and the results are difficult to communicate to a receiving station in a timely manner.
U.S. Pat. No. 3,803,607 to Robinson, the entire disclosure of which is incorporated herein by reference, is an exemplary prior art ship board ring-around radar system calibrator. Robinson provides a system and method for calibration of a transponder, for use with ship board radar, by measuring the total delay time required from antenna to antenna. In the reference, stray signals from the transmitting antenna of a remote transponder are allowed to enter the receiver of the remote transponder and initiate a self-sustaining “ring-around” oscillation. That portion of the oscillation required for the pulse to travel internally through the transponder from the receiving antenna to the transmitting antenna is the total delay time.
Typically, ring-around is an undesirable phenomenon that creates interference with transponded signals, and/or creates confusing “ghost” echoes of the transponded signals. As such, Robinson is the only prior art transponder calibration system that uses, a
Diecke Dietrich W.
Kocian Donald J.
Morchel Herman G.
Gregory Bernarr E.
ITT Defense and Electronics
McDermott & Will & Emery
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