The Secret History of Silicon Valley (56 minutes) Google Tech Talks, Dec 18 2007 by Steve Blank is a very interesting overview of Electronic Warfare and how Stanford professor Terman helped develop the west coast infrastructure. Highly recommended. The cost in human lives is has a noticable impact on Steve.
Hidden in Plain Sight (1:02:45) at the Computer Museum, by Steve Blank, Nov 20, 2008 - a very similar talk
The PDF version is as it appeared originally and contains all the illustrations, some of which may be missing or distorted in the html version.
Moon Bounce Elint - PDF - a CIA paper was SECRET NO FOREIGN, declassified 2 July 96 - why the Stanford 150 foot dish is one of the best for Moon Bounce ELINT
QUALITY ELINT - PDF - a Feb 1968 CIA paper was SECRET NO FOREIGN, declassified 2 July 96 - looking at antenna patterns along with actual power levels.
An Elint Vigil, Unmanned - PDF - a CIA paper was SECRET NO FOREIGN, declassified 2 July 96 - about the SA-2 SAM system (one of the systems the Limiter Detector was developed) a proposed automatic system (was it deployed?).
Communist Defense Against Aerial Surveillance in Southeast Asia - a CIA paper was SECRET NO FOREIGN, declassified 2 July 96 - all illistrations at end of paper
THE OXCART STORY - a CIA paper was SECRET, declassified 2 July 96 - about the A-12, aka SR-71
SCIENTIFIC AND TECHNICAL INTELLIGENCE ANALYSIS - a CIA paper was SECRET, declassified 2 July 96 - includes mention of SA-2 & SA-6
ELINT a Scientific Intelligence System - PDF - 12 page overview formerly SECRET declassified 22 SEPT 93
The U.S. Hunt for Axis Agent Radios - PDF - Official Use Only declassified 18 SEPT 95 -
The carrier based aircraft operating near Vietnam was supposed to turn a manual switch on their Radar Warning Receiver (RWR) to short out the receiver before they left the plane. They were also not supposed to activate any of their radar's while on deck. If both of these rules were broken the result would be that the RWR with the switch in the receive position would have it's front end burned out. The pilot would not know this until he was attacked with no warning from the RWR.
The soviet block surface to air weapons were the SA-x missiles and the ZU-23 gun.
At the time I was working at Aertech Microwave and we had just started to make microwave modules. These were patterned after the HP comb generator and PIN diode switch modules that were cylinders about 1/4" in diameter with glass to metal seals in each end. The HP design sealed the outer sleeve to the module using a welding process that left the end rough. This unit had a metal strap forming just under 1 turn that induced current in the sleeve and module bringing them to solder meting temperature within seconds. To get a good joint HP had a recess that was the mating surface for microwave contact. This was a difficult thing to make without adding an extra part and so was more expensive than our way of doing it.
I designed our module so that the mating surface was proud of the rest of the module on each end and used a solder process to seal the module.
Rather than use an iron to solder we used a Seven Associates single turn inductive heater with the module held in vertical position so you could see the end under a microscope. A solder pre form was placed in the groove designed for this purpose and the heater start button pressed. In a few seconds the solder sealed the module. The module would then be leak tested using Helium. Helium is the smallest molecule available for leak testing, Hydrogen is a diatomic gas and has a module that's almost twice as big as Helium. Helium leaks out of balloons much easier than hydrogen, that's why mylar helium balloons stay aloft much longer than rubber ones.
Rucker & Kolls and Micromanulipator were the common analytical probe stations that we used a lot. These have a level horseshoe ring that moves up and down relative to the chuck. A stereo zoom microscope and an illuminator would complete the station, plus the probes and test equipment. The micrometer head sets the down height of the horseshoe. The black knobs on either side raise or lower the horseshoe. You can see a double sided socket for a plug-in PCB below the horseshoe and there were (are?) companies that would make up probe cards with the tips where you wanted them so you could use either a plug-in card or individual probes mounted on magnetic mounts, each with it's own positioner. The knurled knobs in the front are the X-Y stage adjustments. The black knob at the very front is for stage rotation.
We had a number of different modules in our product line including switches, comb generators, limiters and detectors. A need came up for a combined limiter detector for a classified military program related to Vietnam. This was for a Quick Reaction Contract (QRC). These contracts typically carried a government priority rating of DX-A7. This meant that we could get our orders delivered before any civilian got his parts and it also meant that the program was watched very closely.
The common RWR at this time was called a "Crystal Video" microwave receiver because it had no RF amplification ahead of the detector and no mixers or Local Oscillators. An early patent for a Microwave Filter and Detector filed in 1958 and granted 1960 is US 2954468. You can see that the "filter" grew into a multi band device.
This Detector (no limiter) has a meandered ceramic matching section. I made up a special housing to hold our separate limiter and detector (LD) in a long tube. On one end was the microwave input SMA(m) connector and on the other end was the connector for the detector output.
I took this prototype up to the Applied Technology Inc. building on a hill in the Stanford Industrial Park. It had a great view of the Palo Alto bay area. Inside there was a room with walls formed by chain link fencing that went all the way to the ceiling. The gate was open and there were men inside carrying snub nose 38 revolvers.
We tested the prototype by applying radar level power levels (accounting for the path loss across a carrier deck) then checked to see of the detector was fried. It passed.
Limiter-Detecton in a single package. I bent the leads to get the module to stay upside down on the scanner. The two black dots to the left are the shunt limiter diodes, then a 1/4 wave ceramic transformer with a Schottky diode mounted at the right end, then a ceramic capacitor. I next designed a way to package a combined LD in a single longer module and add a housing at the back to hold the factory select bias resistor and blocking capacitor, DC bias terminal and Video output terminal and have the mounting holes and RF connector be in the same place as the original detector. This was a form, fit and function replacement that included both Limiter and Detector functions..
This is a reject unit without the rear (right end) cover installed. The back end housing was made from a piece of square Aluminum stock with a single round cavity (easy to make with a milling machine or screw machine). The printed circuit board (PCB) that went into the housing was circular in shape with a diameter that matched a punch that was already in our machine shop. The PCB could be made up in advance with a range of the common resistor values used for setting the detector bias and once the operators had determined the correct bias the correct box would be mated to the limiter detector.
The square box contains the bias circuit and positions the DC input and video output connectors in the same location with respect to the mounting holes as the original ATI detector. The back of the box was bored in a milling machine with a single cylindrical hole. The PCB was punched using a Rotex so it would be circular and fit the milled hole.
The ALR-xx systems that used these LDs covered a very wide frequency range. This was handled by using a triplexer (or quadraplexer) to split the input frequency band into narrower bands. The exact frequencies were classified. We built the LDs in various bands to match the requirements for each system.
Another version was the Switched Limiter Detector where the limiter DC return was brought out on a connector. That way you could apply a bias to the limiter diodes turning them on as PIN switches. This allows the detection of a CW signal. This is a prototype unit. An earlier version applied the limiter diode bias from the back end of the module, but that did not work because there was cross talk between the diode drive signal and the detector output.
The modules were tuned in a clean room using various test setups. Early on we used the Systron Donner small sweep oscillator that had a box full of signal generator heads and switched between them to get a wide band sweep. The setup included either an HP Scalar Network Analyzer (SNA) or shortly later the 8410 Vector Network Analyzer (VNA). There were some simple things that could be done to determine how to improve the VSWR by the use of the VNA that were not possible with the SNA.
A good VSWR that was obtained by good matching was far superior to a good VSWR that was obtained by loss. This made our LDs more sensitive and at the same time they had the limiter to protect from carrier zapping.
The Wild Weasel (Wiki) project is documented in a short movie produced by the Association of Old Crows called "First In, Last Out" that chronicles the early days of the Wild Weasel program.
Crystal Video - Aperiodic receivers
A big advantage of the crystal video receiver is it's simplicity (no local oscillator, no mixer, no IF amp) which also means lower cost than a hetrodyne type receiver. The down side is that it is not as sensitive. But for some applications sensitivity is not the key parameter.
A Crystal Video receiver consists of an antenna, detector and video output. It covers a wide input band and there's no tuning required. The crystal video detectors on this page were built for specific (often classified) microwave frequency bands (that corresponded with various Surface to Air Missiles or guns). These typically include a multiband filter (tri-plexer, Quadra-plexer) with a detector optimized for each filter output (see the AM-6536/ALR-54 for an example).
The military units I worked on were for detecting pulse modulated RADAR signals so the input had modulation. A closely related crystal video receiver was the Fuzz Buster (Wiki) for detecting the CW signal from a police speed RADAR. It used a couple of W.W.II surplus microwave diodes (1N23?) in a waveguide (Wiki). One end of the waveguide was the horn antenna. Just behind the antenna the first diode was modulated by a square wave signal at an audio frequency (1 kHz?). The audio frequency will not propagate in the wave guide so the second detector diode only responded to the change in strength of the RADAR signal. A narrow band audio amplifier followed the detector diode. Note a high Q narrow band audio amplifier followed by a meter is the HP 415 meter. Although made for VSWR (Wiki) measurements, it's also great for doing lab work on something like a Fuzz Buster. This is because the narrower the bandwidth the weaker a signal can be detected.An improvement on using a square wave, like in the Fuzz Buster would be to use a pseudorandom (Wiki: PR) on/off modulation and a correlation detector. This is also called a Lock-In Amplifier (Wiki). I worked with an engineer from England who used this technique to measure the step response of a steam powered electrical generating plant. It would be impossible to make a direct measurement since changing the input steam pressure from zero to full blast all at once would destroy the turbine. So he used a piston on the input steam line to make a very small change to the input pressure. The piston was modulated with a very long pseudorandom code and a correlator sensed the output. Analog Devices makes Synchronous Detectors for doing this and AFAICR some of these also have a PR source. I did not see the part I remembered, maybe it was the ADA2200 or AD630 ? These can see signals 100dB below the noise level.
The common Boy Scout crystal radio is very similar and uses a tuned tank circuit to select the desired station.
I have a Wi-Fi detector that is a crystal video receiver with an LED bar graph display. It consists of a patch antenna, a bandpass filter, an RF amplifier and a detector followed by an analog circuit driving a LED bargraph display.
Tunnel (really back diodes) make excellent microwave crystal video detectors because the diode impedance is near 50 Ohms so not only provides very good input microwave frequency matching but also provides a very wide bandwidth output signal source from near 50 Ohms so great for seeing the true shape of narrow pulses. The common crystal microwave detector that harks back to the 1N23 point contact diode (Wiki) has a high output impedance and so is not good for wide bandwidth signals, like RADAR pulses.Mike Valentine makes the Valentine 1 radar detector (factory, my cars page). Rather than use the old fashioned crystal video detection method he uses a hetrodyne type receiver (Wiki) and processes the Intermediate Frequency (Wiki) in a way similar to a spectrum analyzer. That's to say the receiver can recognize multiple simultaneous signals and so warn the driver of a number of parameters: is the radar gun in front, at the side or behind you (based on multiple antennas), what frequency band is the radar (based on the design of the LO and IF frequencies), and how many radars (based on counting the peaks in the spectrum analyzer display). This is the ultimate in police radar detectors.
Related non tuned wide band receiversThe Bug Duster by Wenzel has a Sinitsa (Crypto Museum) mode of operation. Some possible improvements: The 1N5711 is a guard ring Schottky diode and so has much more capacitance than a similar diode without the guard ring so might work at higher frequencies.
During W.W. II the aperiodic receiver was invented (see patent 2513384 below). The SSR-201 (Wireless for the Warrior Vol 4) will receive any frequency between 50 kHz and 60 MHz without tuning, i.e. it's a wide open front end. Also see the free on line book The History of the Radio Intelligence Division Before and During World War II. For example one of these was at the Japanese interment camp at Thule Lake, California (Wiki) and caught the man who had built a transmitter used to talk to other inmates who had regular AM radios. They used radio transmissions to coordinate a escape attempt.
Sweep GeneratorsIn the beginning Aertech used Alfred Electronics BWO sweep generators, these literally could be used for boat anchors. They were rack panel width and a couple of feet tall. Later the HP 690 series sweepers with the plug-in BWO and snap in plastic frequency dial were used. It was possible to put a number (3?) of plug-ins in one rack and the master in another in order to sweep more than one standard band (standard bands were AFAICR, L = 1 to 2, S = 2 to 4, C = 4 to 8, X = 8 to 12.4, K = 12.4 to 18, Ku = 18 to 26 GHz). Then the Kruse Stork 5000 solid state sweeper came out and it too had a combiner for multiband sweeps. Then the HP 8350 sweepers came out with a single plug-in for multi-band sweeps and later multi octave band plug-ins. These had poor phase noise, but for most microwave components work worked well. You could tie two together for mixer work. Then there were the synthesized sweep generators with excellent phase noise that were required for precision mixer work.
HP 415E SWR MeterThis is really a very narrow band AC voltmeter centered on 1 kHz. Maybe a few Hz bandwidth. When working with weak microwave signals you can 100% AM modulate the RF (either using the internal 1 kHz modulation feature of the sig gen or an external PIN diode modulator) then feed the output from a detector to the 415 meter. Originally these were used for making slotted line VSWR measurements, but can be used for other applications.
HP 3400 True RMS AC VoltmeterThis is the only meter that I was aware of that could make such a wide band true RMS measurement. Used for making noise measurements in a number of applications.
Also see my Microwave Test Equipment and Military Test Equipment pages for scalar and network analyzers, noise figure meters, &Etc.
1966 vector receiver (the small CRT showed the relative bearing to the threat as the angle from center screen, and the distance from center screen was a relative distance to the threat, made by Itek used to detect:
S/X/C-Band Radar Detection and Homing Set; manufactured by Itek; part of AN/ALQ-27; used in A-7E, U-8, U-21, OV-1D, B-52G, RA-5C, A-6E, F-4, F-14, F-100, F-105, C-123, C-130
- S-band emissions from SA-2s
- early warning ground-control radars
- C-band radiations from the improved SA-2 radars
- X-band characteristics of airborne intercept radars
crystal video Radar Warning Receiver made by Itek Used to sense power-level changes in the L-band command guidance radars of the SAM i.e. a launch detector.
SAM launch warning system
SAM Launch Warning Set; manufactured by Itek; used with AN/APR-25; used in F-100, F-4, U-8, U-21
Homing and Warning ECM Receiver (improved AN/APR-25); manufactured by Itek; used in F-105, EF-4E, A-7, B-52, F-5E/F
IP-957/APR-36 - CRT display showing relative bearing to and type of threat by Applied Technology Inc (ATI) another view - Label -
"early A-10's used the IP-957/APR-36 azimuth indicator as part of the ALR-46 system and the indicator could display alphanumeric symbols as opposed to simple strobes."
Can anyone define for me what the "BG06" SAM signal was and how the EWs "played" with it? I'm a retired Lt. Col. who flew Ds as a Nav on CAR E-57 during LB II from Andersen. B... B,,,,,,
Re: ECM Signal
BG06 referred to the guidance channel for the SA-2 missile. The missile was tracked by the targeting radar and corrections to the missile were up linked to the missile via the BG06.
Since the BG06 antenna was "looking" back at the launch site, the only way to jam it was to be between the missile and the site. To do this, either you were below the missile doing support jamming or you were lucky and the missile missed you.
The radar warning receiver had a launch light to indicate the BG06 was active. My experience was that the light was inaccurate. I looked for the signal on my ECM receiver.
I an unaware of any way to play with it.
Radar Homing And Warning System; used in F-4G; replaced by AN/APR-47
TM11-5841-283-12 Operation & Maint Manual is on line at ETM as PIN 053495.pdf
IP-1150 CRT eBay photo
Made by Texas Instruments. There were a number of different seeker heads that could be installed, each for a different target (different frequency band).
We made a set of 4 matched detectors for one of these heads. The one shown is band IX. After properly torquing the SMA nut on the RF (bottom) end a tube with a flange was slide down and the O-ring seated on the tube. The flange was bolted to the guidance section to support the detector in the high vibration environment found on an aircraft wing. A computerized test system was used to measure hundreds of detectors at a time and output the serial numbers of the matched sets. We could match much better than the spec using this system and it improved our yield. (Another version of this test method was later used by ST Microwave to match detectors fro a satellite program to extremely tight tolerances.)
Since the frequency band was determined by which guidance section was installed on the missile, the target needed to be known before take off.
The Shrike has the steering wings located at the center of mass of the missile. This causes the missile to move sideways rather than to rotate.
The four detectors are 17, 17', 18 & 18' 3712228 Target marker warhead
China Lake, CA,Jan 23, 1973
replaces the warhead with a marker
making it easier for others to bomb.
Made by Texas Instruments. Instead of using detectors tuned to specific threat frequency ranges the HARM has a wide band mixer that can be electronically tuned to the desired frequency. It's a Superheterodyne receiver (Wiki) not a crystal video receiver.
This is the system where we used the HP 8566B spectrum analyzer to directly measure the spurious mixer products very quickly. This system was in an access controlled room and had a number of security features. The test time was reduced dramatically compared to manual testing. The HARM could be tuned to any desired frequency so one missile could be used for whatever target came up.
Replaced by the ALR-67 (Wiki).
2-18 GHz Radar Warning Receiver; manufactured by Litton; used in F-8, F-14, A-4, RA-5C, A-6, EA-6B, A-7, F-100, F-4, F/A-18, CF-104 (Canada)
I think this is the one that we made all the LDs for?
Control - Close Up - Label - 3 buttons for enabling or bypassing the lo, med and high frequency bands, also 3 buttons for Built-In-Test of the three bands and these double as AAA, AAA/AI or AI selectors?
Digital Warning Receiver; manufactured by Litton; used in B-52H, A-7D, A-10, C-130, F-104, F-105G, F-111, F-4, F-5E/F, RU-21H, OV-1, OV-10, HH-53
long-range homing receiver
Used on the F-111
0.5-18 GHz Multiband ECM Receiver (improved AN/ALR-59); manufactured by Litton; used in E-2C
(also called the LR-100), a lightweight radar signal receiver designed in-house by Litton Amecom using COTS (commercial-off-the-shelf) components. It can serve as a radar warning receiver (RWR) and also provides precision emitter location and identification (ESM/ELINT) as an electronic support measures (ESM) system.
This microwave component is composed of a combination of power spliters and couplers and contains 4 detectors. One pair of detectors provides either an X value and the other pair provides a Y value for a vector. The magnitude of the vector is proportional to the amplitude of the incoming signal and the phase angle is proportional to the frequency of the input signal. In order to get higher frequency resolution you can combine a number of these using delay lines that are increasingly longer so that the frequency is read using a gas meter approach. In this way you can make an instantaneous frequency measuring (IFM) receiver. Note that this receiver can determine the frequency of an incoming signal on only one pulse. We made a number of different models of this device. they have a frequency range that's no more than an octave, else there would be a frequency ambiguity.
Wide Bnad Systems - makes these
WJ App Note: High Probability of Intercept Receivers in an EW Environment -
Not really RWR, but this is the closest web page.
A RADAR countermeasure developed during W.W.II was then called Window by the British, but Chaff (Wiki) by the U.S. This consisted of thin strips of Aluminum foil cut to be a half wavelength long. The trouble was that the strands tended to stick together. The solution was to use small diameter glass fibers with a metallic coating. These do not stick together and you can get a lot more of them in the same dispenser volume.
2907626 Metal coating of glass fibers at high speeds, John B Eisen, Nachtman John Simon, Bjorksten Johan, Lawrence A Roe, Bjorksten Res Lab Inc, Oct 6, 1959, 65/446, 65/475, 65/468, 118/DIG.220, 264/DIG.190, 65/453
3120689 Fiber winding and fabricating method and machine, Drummond Warren Wendell, Bjorksten Res Lab Inc, Aug 20, 1959, Feb 11, 1964, 156/181, 156/433, 156/166, 28/290, 156/425, 28/289
The Hunter Killers: The Extraordinary Story of the First Wild Weasels, the Band of Maverick Aviators who Flew the Most Dangerous Missions of the Vietnam War, Dan Hampton,2015
2513384 Aperiodic radio receiver, Hoffert William J, Veatch James P, Feb 14, 1947, 375/339 - since no wide band (500 kHz - 80 MHz) RF amplifiers existed then they used an RF detector on the input followed by audio amplification.
1464322 Radio receiving method and apparatus - tuned
1593837 Radio signaling system - tuned RF amp then detector
1615636 Signaling system - infrasound made so it can be heard with LO
2127525 Radio receiving system - superheterodyne radio
2383126 Spaced wave keying - teletype mark and space signals each has it's own channel
2400133 Double modulation radio receiver - VHF receiver suitable for CW
Radar warning system,Williams Everard M, Oct 26, 1945, Sep 19, 1950, 342/20, 327/37, 327/20
A method to make it easier to differentiate high pulse repetition rate signals (tracking radar) from slower rate search radar signals using vacuum tubes.
Early warning detector,Edgar Koontz Clarence, Page Irving H, (Sec of the Army), Nov 17, 1942, Jul 8, 1952,
342/13, 324/76.39, 375/224, 342/20, 327/20
A variable inductor is in the input circuit that includes a RADAR antenna and is followed by a number of vacuum tubes.
By calibrating in input tuning dial the input frequency can be determined and by means of the audio tone the pulse repetition rate.
Can detect CW signals by using a switch in front of the crystal video detector (the method used in Fuzz Busters).
Pulsed and continuous wave electromagnetic signal detectors,Rittenbach Otto E (US Army), Feb 9, 1967, Sep 2, 1969,
455/281, 342/20, 342/203, 455/337
Two paths one for pulsed and one for CW signals.
3061795 Flip-flop varies frequency of blocking oscillator
30946633408574 Portable radar warning receiver, Oliver G Currlin, Charles J Schmidt (Maxson Electronics Corp), Oct 29, 1968, 343/703, 324/95, 342/20
Microwave signal checker for continuous wave radiations, Siegel Vernon H, Radatron Res & Dev Corp, Aug 3, 1962, Jun 18, 1963,
455/324, 455/226.1, 375/338, 455/130, 342/20, 343/767, 330/10, 455/347, 455/325
Handheld device to detect CW radars like used for door openers and traffic lights.
battery powered flashlight sized housing, log amp, uses what may be X-band waveguide
3500401 Radar detection device, Elliott Denman R, Miller Gerald O, US Navy China Lake, Jul 15, 1968, Mar 10, 1970, 342/20, 375/339
A crystal video receiver feeds video pulses to a blocking monostable multivibrator with a 200 micro second recovery time so that the highest output frequency is 5,000 Hz. In front of the detector is a shutter which when closed protects the detector from being burned out by a close by radar (like on an aircraft carrier when another fighter plane pilot turns on his RADAR even though he is not supposed to do that). This is the case that triggered my development of the limiter-detector.
3550008 Radio frequency carrier wave signal detector, James A Bright, Dec 22, 1970, 375/339, 455/351, 342/20, 455/334, D10/104.1
X-Band police radar detector using 1N23 diode and solid state circuit.
Automatic radar detection device,Trochanowski Andrew J, Wicks Steven A (US Navy China Lake), Jun 20, 1972, 342/20
Shipboard RWR for fast PRF (tracking radar).
Portable radar-detecting receiver,Allan B. Hitterdal, Northrop Corporation, Jan 1, 1980, 342/20, 343/774 -
wave guide input to crystal video receiver acts as a 9 GHz high pass filter
Police Traffic Speed Radar
The first popular police speed CW Doppler (Wiki) radars operated at 10.525 GHz. This frequency is used for Doppler motion detection modules like for door openers. The Fuzzbuster was a crystal video receiver that consisted of a horn antenna followed by a wave-guide with a couple of W.W.II detector diodes. The first diode was driven with a square wave that added modulation to the incoming signal and the second diode detected the modulated signal. An AC coupled amplifier made the detected signal large enough to trip an alarm.
Even though these first generation police RWRs were not very sensitive they worked because of the RADAR range equation (Wiki), that's to say the power level at the RWR is inversely dependent on the range squared but the power available to the receiver in the radar gun is inversely dependent on the range to the fourth power, so the RWR is working with a much stronger signal that the radar gun.
Means for detecting presence and movement of bodies,Eastern Ind Inc, Oct 7, 1944, Aug 2, 1949, -
342/69, 246/30, 342/109, 340/936, 342/114, 340/935, 246/182.00A, 340/552, 342/128, 340/933
Radio echo apparatus for detecting and measuring the speed of moving objects,Barker John L, Eastern Ind Inc, Jun 13, 1946, Feb 24, 1953
342/104, 342/165, 246/182.00A, 340/936, 324/76.39, 246/30
Traffic speed monitor,Barker John L, Midlock Bernard J, Lab For Electronics Inc, Dec 2, 1958, Mar 6, 1962,
340/936, 340/441, 235/132.00E, 327/77, 235/99.00A, 324/161, 377/9
Radar warning receiver,Richard L. Grimsley, Michael D. Valentine (Cincinnati Microwave, Inc.), Apr 8, 1986 455/226.1, 340/902, 324/76.27, 342/20
Motor vehicle police radar detector for detecting multiple radar sources,Clarence R. Groth, Stephen R. Scholl, Michael D. Valentine (Valentine Research, Inc.),
Sep 8, 1992, 342/20 -
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