From FM 44-1-2 ADA Referennce Handbook, 15 June 1984, see page 21 "Rings of Supersonic Steel"
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This information is grouped into the following sections:
"RADAR" is a short form of the name
"RAdio Detection And Ranging".
(Also from Doyle Piland )
Chirp is another technology which is also used with pulsed radar. The
general idea is to take advantage of the high power contained in a long
pulse and still retain the range resolution of a short pulse. There are
also other side advantages which makes it harder to use ECM against. Chirp
simply uses a swept frequency, long pulse of up to 120 microseconds. I
think the convention is the beginning of the pulse is the lower frequency
and the high frequency is at the end of the pulse. When the return is
received, the receiver delays different frequencies differently (called
de-chirp). Thus, the energy contained in the return is compressed into a
very short period of time, as if it were a short pulse. The
Sentinel/Safeguard system made extensive use of "Chirp" techniques.
As Nike radars (except HIPAR - which I do not know) used "pulse radar", we will discuss "pulse radar".
Short Powerful Pulses The "pulse" radar set sends out a very short (0.2 micro second to 1 microsecond) very high energy pulse of radar waves - then listens for echoes of this energy pulse. The pulse of radar waves needs to be as strong as practical so that the echo can be detected as far as practical. For Nike radars, the peak energy rate in the pulses ranged from 250 kilowatts for tracking radars to over 1 megawatt for the LOPAR acquisition radars. The average transmitted energy is much less because most of the time the wave is not being transmitted. (The average transmitted energy from most radars is about the same as a home microwave oven.)
Some search radars such as the Nike HIPAR, and the airborne AWACS search radar use much higher average and peak powers. The AWACS uses klystrons rated at over 50 megawatt peak power and over 50 kilowatt average power. HIPAR is reputed to have used similar technology.
Radar Echos are caused by many things, birds, planes, ocean waves, metal buildings, metal ships, some weather conditions, and so on. The pulses are short so that the echoes are short so you can more easily tell when the echo starts and stops from a particular object. If the pulse is long, and there are many objects, several objects could be echoing at the same time from the same pulse, making resolution between objects difficult or impossible.
Make a beam - like a long range flash light
Radar usually uses the shortest practical radio waves because short radio waves can be focused into a narrow "beam" with a smaller antenna than long radio waves. This is especially important in ship and airborne radars, but still important in all practical movable, steerable radars.
"Short" radio waves for radar usually are between 1 meter (300 million waves per second or 300 megahertz) and 3 centimeters (10,000 million waves per second or 10,000 megahertz or 10 gigahertz). Longer wave lengths than 1 meter require inconvenient sized antennas for anti-aircraft sites, and wave lengths shorter than 3 centimeter are increasingly hampered by weather and moisture in the air.
The focusing ability of a lens or mirror type antenna is directly related to its width in wavelengths of the radar wave. The wider the antenna is in wavelengths the smaller the angle of the beam that contains 50% of the radiated energy. The smaller the angle of the beam, the farther the radar can see the target and the more precisely the angle of the target can be known. Nike tracking radars had an effective antenna width of about 150 wavelengths. (The antennas were actually physically a little larger, but there are edge effects which decrease the focusing effect of the edge areas.)
Nike tracking radars focused more than 50% of the radar pulse into a beam less than 1 degree wide, both horizontally and vertically. The acquisition radar beam was about 1 degree wide horizontally, but spread out vertically into a fan shape to see aircraft both near the horizon and also higher up.
Radar Range to any echoing object is measured by determining the delay between the transmitted pulse and the echo. The speed of a radar wave in air is about 300 meters per microsecond. (It varies very little with normal ranges of altitude and weather.) The round trip time for a radar pulse from transmitter to echo object to receiver is about 150 meters (164 yards). With electronics, measurement of the echo time to with in 5 meters is no technical challenge.
|
One Antenna
is used for both transmitting and receiving. This is
actually rather tricky, as the transmitter sends a pulse of energy to
the antenna sufficient to cook or spark most receiving components, then
with in a few micro seconds, the transmitter must be electrically
disconnected from the antenna and the receiver connected.
This is microsecond switching function is
performed in the radar "wave guide" by a "duplexer" circuit usually using
a "TR" tube (transmit/receive tube). Basically, the powerful radar
pulse causes an arc in the TR tube (in the wave guide), and the arc,
being a conductor reflects most the pulse away from the receiver connection,
keeping the pulse from the delicate receiver components. |
 Download photo of a British TR tube |
Before 1939, radar waves were created using rather standard vacuum tubes.
The tube shapes were changed to permit shorter wires (higher frequencies) but
even the best technology was limited to pulses of about 2,000 watts at about
700 megahertz (700,000,000 waves per second). There was great desire to
get higher frequency (for tighter beams with smaller antennas) and higher
power (for longer range).
In 1939, the British developed a remarkably simple method of generating an intense pulse of radar waves. This was the multi-cavity magnetron The arrival of the secret working British prototype magnetron into the U.S. caused great hope and excitement. The British prototype could deliver 10,000 watt pulses at 3,000 megahertz. This was 5 times the power (great!) at 4 times the frequency (wonderful!) of the best current technology. And the current technology seemed just about at its maximum (the components and systems had been pushed and tweaked extensively). And the newly developed magnetron from the British was just a research prototype - there could be room for big improvements.
This was a stunning "breakthrough". See "A History of Engineering and Science in the Bell System: National Service in War and Peace (1925-1975)" for further details. The British prototype was certainly improved in the U.S. for much higher power, manufacturability, stability, frequency adjustability and range, and other factors, but the impact of this basic invention on the successful Allied radar development was very great. It turns out that the mass manufacture of high performance magnetrons is much more tricky than first imagined. There were whole new worlds of large glass/metal seals, permanent high vacuum evacuation of machined metal castings, manufacturing tolerance of the cavity size and shape, cathode resistance to back bombardment, etc to be solved. Bell Labs and Western Electric made more than 100,000 magnetrons of various frequencies and powers for World War II.
This is an Allaire interview that describes a bit of the additional development work. This Bainbridge interview describes other radar related adventures of the times.
This "tube" helped guide the British fighter planes in the "Battle of Britain" bombings and gave the British (and the Americans) an advantage in the radar race until the Germans also developed one (from a downed British bomber?).
The 3,000 megahertz magnetron perfected from the British prototype had
a very large (30 pound, 14 kg) magnet with a metal and glass "tube" about the size
of a hockey puck (small can of tuna). (Higher frequency, shorter wavelength magnetrons
and magnets are smaller and lighter.) It had a peak power of 1,000,000 watts (an
improvement by a factor of 100). It was rugged and reliable.
The above is a diagram of a magnetron (with out output loop)
Download photo of a British magnetron
Notes about the photo - The large glass part is to help prevent the high voltage from the cathode circuit from arcing to ground. You may notice the two contacts on the top the the glass section. These allow for cathode heater current (remember vacuum tubes had a hot part called the cathode to "boil off" electrons into the vacuum?). A special 5ish volt transformer was used which permitted the whole cathode to be at 18,000 volts during the short (about 1 microsecond) time the magnetron "fired". The black magnet and other parts stayed at "ground" (zero volts).
There were many interesting effects in the magnetrons (as in most of the other radar components). For instance, after the cathode was heated by the filament current, and the magnetron was pulsed with the high voltage pulses, there were so many electrons that would gain energy then come crashing back to the cathode that the cathode would over heat unless the cathode heater current was reduce or eliminated.
For a more detailed description of how a magnetron works, see The Magnetron Tube, Structure and Operation . This describes a microwave oven magnetron, which does not have the pulse width and tunability requirements of a radar magnetron, but the principle is identical.
For a detailed description of how a radar magnetron works, see Magnetrons.
The most noticeable component of the LOPAR modulator was the hydrogen thyratron tube. This tube was about 24 inches tall and about 5 inches in diameter. This was the tube that switched the 18,000 volt 100 amp current mentioned above on very quickly, about 0.05 microsecond. The hydrogen gas in this big tube glowed violet when it was working. A "delay line" circuit was used to help limit the length of the pulse.
This modulator tube took about 15 minutes to warm up properly. (Every thing else in the Nike system warmed up adequately in 5 minutes or less.) A 15 minute timer prevented the tube from being used during this warm up period. (There was a timer over-ride circuit so that it could be used sooner in a "battle emergency".) One night during the beginning of a routine alert, the captain got impatient waiting for this timer and activated the over-ride switch after about 10 minutes. The tube seemed to work just fine, the radar worked fine, nothing bad seemed to happen.
The Nike tracking radars had physically smaller (higher frequency) components with about 1/4 of the peak power (250,000 watts) and 1/5 the pulse width (0.18 microseconds).
the Klystron another microwave source
During World War II, the klystrons were primarily the reflex type and were used primarily as low power (milliwatt) oscillators in test equipment and radar and microwave receivers.
By the 1950's, there was a considerable demand for high power (kilowatt average power) microwaves, but with more precise control than could be generated by magnetrons. The customers were communications, medicine, science including particle accelerators, and radar. The Varian brothers, with the patents and the skills, did very well. Soon klystrons with average powers of 50 kilowatts and peak powers of 50 megawatts were available. To achieve the high current electron beam densities at these powers, powerful magnets (usually electromagnets) surround the tube. To get the most power from each electron in the beam, very high (100,000) voltages are typical.
These powers were impractical with magnetrons. The klystrons could deliver both the higher powers and also could amplify low level precise signals to these high powers. The klystrons were much larger (up to 2 meters long) and with their magnetic solenoids quite heavy (500 kilograms) and more expensive ($50,000), and more troublesome to keep running (required vacuum pumps) but they could be much more powerful and precise than magnetrons.
Power klystrons, such as described above have power gains (output signal/input signal) of over 10,000. As a comparison, typical power transistor in your stereo has a power gain of 20.
The Nike HIPAR radar transmitter used a
powerful klystron. 57 K Bytes.
Image from Rolf Goerigk
This one is about 5 feet tall
18 inches in diameter (including a focusing magnet - solenoid), and could output
10.4 megawatts peak pulse power - average power was 26 kilowatts.
To help get that peak pulse power, 210,000 volts were used. This voltage gives more powerful
X-rays that your doctor's office machine - yes - the tube was surrounded by a lead shield.
The cooling system included 60 gallons of mixture of ethylene glycol and water (anti-freeze).
This class of tube does not sit happily in a glass tube and run unattended for years. The vacuum needed to be very high, and needed to be attached to a very good vacuum pump while in operation.
Because of possible rapid and precise changes of the frequency, amplitude, and phase of the output radar waves, very interesting receiver options are available to increase receiver efficiency (detect less reflective or further targets) and also to help suppress the effects of jamming (ECM).
Reflex klystrons were used as local low power ( 0.1 watt) microwave oscillators in many of the Nike radar receivers and test instruments.
Wave guides (some times called "radar plumbing") are simple and complex at the same time. Radio waves can travel inside of a conductive (copper) pipe as long as the inside circumference of the pipe is longer than 1/2 of the wavelength of the radio wave. (Low frequency radars require larger wave guides.)
Radar waves can go through the convenient coaxial cables (similar to your cable TV lines). However there are several problems:
To provide better control of the various internal transmission modes, wave guides are usually constructed with a rectangular cross section. This limits some of the undesirable electrical modes possible in circular cross section wave guides.
All of the radars in the Nike system used wave guides. Almost all of the radars you have ever seen use wave guides. (The little radar receivers used to detect police speed radar "guns" use other methods.)
Most large acquisition radars have the magnetron in a fixed location.
How do you get the radar waves from the fixed wave guide to the rotating wave guide
if the radar "dish" is going round and round, and the magnetron is sitting in a fixed
place?
A very practical question. The answer is a rotating microwave joint. At the center
of rotation, the rectangular wave guide merges into a circular wave guide. The circular
wave guide forms the center of the rotation. There is a trick used so that the
copper of the rotating part does not need to touch the copper of the fixed part.
Up in the rotating part of the antenna, the circular waveguide converts again to
a rectangular wave guide and on to the feed horn (the part that lets the radar waves
out into the air - or back again into the wave guide).
For various reasons, Nike tracking radars have the magnetrons and receivers in the rotating part of the antenna. Later when we discuss "How The Tracking Radar Points at an Object", these rectangular wave guides will split the energy from the feed horns, rotate the waves, combine the waves in a subtractive way, do some more electronic tricks, and get antenna pointing error information. Just like magic.
Most radios (including your AM/FM, TV, cellular phone, and radar set) convert the received radio waves to a fixed frequency for amplification. This conversion is actually much simpler than trying to tune about 5 high gain stages through the desired frequency range. This group of about 5 high gain amplifier stages is formally called the Intermediate Frequency Amplifier or more commonly called the IF Amplifier. This technique saves many "big time" amplifier design, fabrication, and adjustment headaches.
The usual frequency in most radar sets for the IF Amplifier is about is about 30 megahertz (give or take 10 megahertz). To convert an example input radar signal of say 5,000 megahertz down to say 30 megahertz for the IF Amplifier you generate a signal 30 megahertz away from the input radar signal (in this case 5,030 megahertz is fine). The unit to make this extra frequency is called the "local oscillator" or "beat frequency oscillator".
Put this "local oscillator" signal, and the input radar signal together into a "mixer". The output of the mixer will contain all of the input frequencies plus the sum of the input frequencies (10,030 megahertz, which is not used) and the difference of the input frequencies (30 megahertz) which is amplified by the IF Amplifier.
The local oscillator at radar frequencies is usually a little
reflex klystron .
The mixer can be a radio tube below about 1,000 megahertz, but above this frequency the radio tube is too inefficient and noisy. A "crystal" mixer was used in almost all of the radar sets during the 1940s and 1950s, (and is still in common use today in many commercial radar sets). (During the 1960s, a "traveling wave" tube was developed which could be made to have even lower noise than the crystal mixer. This is used in some demanding military, space, and research receivers, and was used in the Nike HIPAR radar receiver.)
So - in 1939 the invention of the magnetron permitted reasonable radar above 1,000 megahertz, and reliable, rugged crystal mixers were developed as low noise mixers to handle this higher frequency range. The research at Bell Labs that helped create those crystal mixers led directly to the invention by Bell Labs of the transistor a few years after the war and to the continuing semiconductor revolution and to your computer.
We adjusted the local oscillator power going into the crystal to give a crystal current of about 2 milliamperes. Too little power gave lower mixer efficiency, too much power gave more local oscillator generated noise.
Radar receivers are very similar to your usual TV receiver, and in many ways simpler because we don't have to play such interesting games processing the audio and color video. So we will just consider the TV "front end" through to the beginning of the audio and video (throwing out about 3/4 of the TV electronics.
All of the components are similar in function, and most are almost interchangeable with a radar set. The big difference is the front end where the incoming frequency is much higher. We will see that we quickly reduce the frequency to TV IF (intermediate frequency) and any TV repair person can take it from there.
| Component Name | typical TV | typical radar | ... Comment |
| Tuned Circuit | 60-500MHz | 9000-10,000MHz | reduce undesired frequencies |
| Mixer | a tube or transistor | a crystal | output difference between signal and oscillator |
| Oscillator | 87-527MHz | 9030-10,030MHz | produce "beat frequency" for mixer, could be a klystron |
| Auto Freq Control | same | same | (AFC) controls frequency of oscillator |
| AFC gate | tracks sync pulse | tracks magnetron pulse | track only transmitted signal |
| IF strip | 27 MHz | . | increase signal to desired voltage using single frequency |
| . | . | 30 MHz | (acquisition radar) this lower frequency reduces noise |
| . | . | 60 MHz | (tracking radar) higher frequency to increase range resolution |
| Auto Gain Control | same | same | control gain of IF strip |
| Gain Gate | tracks sync pulse | tracks target | control gain of desired object |
| Detector | same | same | convert intermediate freq to video |
More correctly, military radar receivers are somewhat different from your TV in internal details to increase ruggedness, testability, maintainability, and to reduce the effects of various forms of enemy jamming. The field is large and complex and is beyond our scope here.
We have described many of the interesting radar components above. If we could visit a
radar component supermarket (close out sale today), we could select components
and build our own radar. Actually millions of people assemble components for
their IBM clone computers and survive about the same complexity. There is a
big difference in size, weight, voltage and powerful microwave radiation hazard.
The general schematic would be:
The components and their uses are:
| Component | Comments
| Start_Pulse_Generator
| One pulse a few microseconds before each radar pulse | Power Supply
| 18,000 volts, otherwise a bit boring | Modulator
| Sends a powerful 1 micro second pulse to the magnetron | Magnetron
| Makes powerful radar waves (for 1 micro second) | TR Tube
| Keeps most of powerful radar waves out of the receiver | Receiver
| Amplifies the returned radar waves, giving video
| Display Tube
| Shows the return radar waves/video to the operator
(PPI)
(A scope)
| Tracking Unit
| Helps follow the selected signal in range, azimuth, elevation | |
I must apologize to designers of military radars who add many small enhancements to reduce the effects of enemy jamming (and accidental friendly jamming). These enhancements may include:
Or you can WOW your neighbors and buy major Nike system components, see How to get Nike Parts? .
Acquisition Radar (the wide eyes)
The Acquisition Radar is a most interesting looking radar. It is large and has motion, going round and round. It is often called a "surveillance" radar, providing the slant range and azimuth (direction) of all the radar visible objects in the area 5 to 10 times a minute.
Usually surveillance radars have a longer wavelength than the tracking radars,
as minimum beam width is less important. In the case of Nike, the LOPAR
surveillance radar had a wave length of 10 centimeters (about 3,000 megahertz).
(The Nike tracking radars had a wavelength of 3 centimeters or less.)
The HIPAR surveillance
radar had a wave length of 23 centimeters (L-band, about 1,300 megahertz) and an
effective range against large high-flying non-stealth aircraft of about 200 miles.
The HIPAR radar had a large control building.
There was very sophisticated pulse generation, and multi-channel receivers
with unique moving target indicators (MTI) and great deal of anti-jamming capability.
It was made by General Electric.
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A Nike HIPAR radar antenna with out protective radome, image is 74 K Bytes (Photo credit - page 303 "Jane's Radar and Electronic Warfare Systems", 18th edition.) There is information that this picture does not include: 1) an anti-jamming antenna at the top of the main antenna, 2) two small antennas on each side, 3) an IFF antenna. |
| A Nike HIPAR FAN radar antenna , image is 53 K bytes. (Photo credit Rolf Goerigk |
| A Nike HIPAR radar antenna with protective radom,
image is 60 K Bytes (Photo credit - adapted from Bill Benson , benson@efdata.com) Note that the HIPAR antenna is high on a pedestal. There are 2 main reasons, 1) have the high power beam safely above any near by personnel 2) to gain a little range over the curvature of the earth. |
| Drawing of HIPAR Station image is 71 K Bytes Image from Rolf Goerigk |
From Rolf Goerigk, Specification for the HIPAR include:
| Polarization | horizontal |
| Antenna Gain | CSC2 antenna = 34.8 dB = 3020
("CSC2" stands for co-secant squared,
a vertical pattern optimized for aircraft detection
at low and high altitudes and ranges) FAN antenna = 29 dB = 790 |
| Beamwidth | 1.2 deg Azimuth at 3 dB 1.3 to 7.1 in elevation FAN 1.35 deg at 3 dB |
| Vertical Coverage | 0 to 60 deg., 46 km height, 425 km length |
| Antenna Speed | 6.6 and 10 RPM (Revolutions Per Minute) |
| Azimuth Accuracy | > 0.25 deg. |
| Noise max. | 6.5 dB (1005 deg. Kelvin) |
| Reflector Dimensions | height 6.3 m (20.6 ft.), width 13.11 m (43.0 ft.), 82.6 sq. m (900.9 sq. ft.) - BIG |
| Pulse Width | 6 microseconds |
| RF Freq Range | 1350 - 1450 MHz (10 channels) |
| RF Power: | nominal: 10.4 MW / 26 kW average |
The HIPAR radar was very expensive, and was only used at selected Hercules sites. The other Hercules sites had "Alternate Battery Acquisition Radar"(ABAR) radar which was not so sophisticated, not so long range, and not so expensive. There were three models called "ABAR", the models were identified as AN/FPS-69, 71, and 75.
A catalog description of the AN/FPS-71 included the following phrases:
Peter DeMarco wrote about the AN/FPS-75
The ABAR we had in Alaska was the AN/FPS-75. I can't compare it to HIPAR but it had a range of about 200 miles and the ECCM equipment on it was very sophisticated. There were 6 different radar presentations I could view at the same time. Lots of buttons and lights.
| LOPAR Radar Photo credit, the Greek magazine "Modern Air Force & Navy", April 1998, Leonidas Blaveris (Nike articles by Pericles Zorzovilis) (49 K bytes) |
The LOPAR radar was very much like the original Nike Ajax (and M-33 gun) acquisition radar but with reduced pulses per second to match the longer range HIPAR radar or the ABAR radars mentioned above. It provided another "eye to the sky" and another problem for enemy jammers.
From Rolf Goerigk, Specification for the LOPAR include:
| Polarization | horizontal |
| Antenna Gain | CSC2 = 32 dB = 1600 ("CSC2" stands for co-secant squared, a vertical pattern optimized for aircraft detection at low and high altitudes and ranges) |
| Beam Angle | 1.4 deg Azimuth 2 to 22 deg. Elevation |
| Antenna Speed | 5, 10 and 15 RPM (Revolutions Per Minute) |
| Azimuth Accuracy | 1 deg. |
| Noise | 7.5 dB (1341 deg. Kelvin) |
| Overall Noise | 8 - 9 dB (1540-2014 deg. Kelvin) |
| RF Freq. Range: | 3.1 - 3.4 GHz |
| RF Power | average 650 W, peak 1 MW |
| Pulse Width | 1.3 microseconds |
| Band Width | IF = 4 MHz, Video=2 MHz |
| Reflector Dimensions | height 1.32 m, width 4.57 m, 5.6 sq. m |
From Rolf Goerigk "As I first worked on site (1961). I was able to change the elevation by operating the ELEVATION switch on the ACQ control console and some hydraulic control under the ACQ-RADOM. During the early 60s the control was modified to electromechanic. It was possible to change elevation between 0 to 391 mils and to change the elevation mode too. Actually controlled was the point in mils when the cosecant-rods were driven in or out the lower part of the reflector, i.e. changing from pencil beam (long range) to cosecant (great height)."
Acquisition Radar Displays and Identification Friend or Foe (IFF)
The "IFF" (Identification Friend or Foe) is basicly a series of coded pulses sent
out at the same time and same direction as the surveillance radar pulse.
It was sent out by separate IFF equipment. A "friendly" unit was equiped to respond to
the coded pulses with another set of coded pulses. These were received by the
IFF equipment, and usually displayed with the surveillance radar display.
The system resembles the methods FAA aircraft controllers use to track commercial
and other suitably equiped aircraft.
Enemy or accidental jamming can/will cause many other interesting
displays on the tracking scopes. Go to jamming for
more information.
The tube that turned on the pulse of current to the magnetron in the
LOPAR Acquisition Radar Modulator (see above) took up to 15 minutes to warm up
to operate reliably and not risk damage. There was a 15 minute timer to
prevent operation (with a switch to override the timer in case of emergency).
The operator has a number of controls, the following are of special interest:
"Also in your information on IFF, the Aradcom had two types of IFF. The
standard IFF had a limited set of codes and you used a code of the day.
Generally all freindlies used the same code. In 1963 we also adopted
SIF/IFF which allowed enough codes for individual identification of aircraft
or flights of aircraft. The IFF sent a pulse out a few microseconds after
the radar. The aircraft responded with its pulse code. This difference in
delay is why a second bar was painted above the aircraft on the screen. The
IFF/and SIF had a particular code sequence for emergencies. When the pilot
switched to that code, it automatically painted four bars which was Mayday."
I know about SIF/IFF because during a simulation with the Air
Force in 1963, we accidentaly engaged a flight of USAF planes. This caused
a stir when it was found that we had SIF installed but were still using the
old IFF. So I was sent with two other technicians from Fort Heath to a USAF
Radar Site in North Truro, Mass where USAF personnel trained us on the SIF
equipment.
Return to beginning of Nike Radars
This site shows further
radar history Spotted by
Donald E. Bender.
Return to beginning of Nike Radar
Although the higher frequencies permit much smaller antennas to get the
same beam width, the higher frequencies suffer increasingly from
from moisture in the air absorbing the radar waves. And also rain drops
reflect them more giving an effect similar to chaff. The choice of
radar frequency range for a particular application is a complicated compromise
involving many factors.
Radio Frequency Naming Convention since 1969
from Rolf Goerigk
As a point of reference, the stealth fighter/bombers used in the Iraq conflict
are said by TV documentaries to have the radar reflectivity -"cross-section"- of a pigeon.
That seems an interesting accomplishment, as even one wheel of the aircraft
must have a much larger radar cross-section than a pigeon.
Accidental Jamming - Pulse Radars (used in Nike)
There are many friendly sources of radar interference.
Most folks worry about other friendly radars. However, these
are not a big problem at a typical Nike site, or even a
group of Nike sites very close together such as a Nike range at Red Canyon or McGreggor.
How can this be? Radars "shouting" all over the place,
and no problem? Unbelievable! But oddly enough, there is no big problem.
Here is why.
The attempt is not perfect, but usually about 95 % of the energy
gets directed as desired.
Nike radar magnatrons tune over a range of +- 5% from their center
frequency. And the receivers are tuned to be sensitive to roughly 0.1 % of that range.
In effect, about 500 radars in one band (+- 5% ) could be tracking one target
and not interfer with eachother with respect to frequency.
Except - Nike site SF-59 was reported to be
jammed by the TV Channel 2 transmitter a half mile away.
Radar Jamming, Electronic Counter Measures (ECM)
The target may not wish to be observed, and/or may wish to reduce the
effectiveness of the radar attempting to observe it.
One way to reduce the effective range of the radar is to reduce the
reflectivity (ratio of energy reflected back) of the target.
This is called "stealth"
and is for aircraft designers, not us.
"Jamming" or "Electronic Counter Measures" (ECM) is a term used to describe
active means of trying to prevent the radar system from working as well as
intended. And of course radar designers actively try to defeat the ECM.
It is a great (but deadly) game of radar counter measures, counter-counter measures,
counter-counter-counter measures, played with very serious intent.
We will very briefly mention a few popular forms of
jamming:
Some of the nomenclature found by Don Bender include:
Impulsive noise, that can shock-excite the " narrow-band " radar receiver
and cause it to ring, can be reduced with the Lamb noise-silencing circuit,"
or Dicke fix." This consists of a wideband IF filter in cascade with a limiter,
followed by the normal IF matched filter. The wideband filter is designed
to include most of the spectrum of the interfering signal. Its purpose is to
preserve the short duration of the narrow impulsive spikes. These spikes
are then clipped by the limiter to remove a considerable portion of their energy.
If the large noise spikes are not limited and are allowed to pass they would
shock-excite the narrowband IF amplifier and produce an output pulse much
wider in duration than the input pulse. Therefore the interference would be in
the receiver for a much longer time and at a higher energy level than
when limited before narrowbanding. Desired signals which appear
simultaneously with the noise spike might not be detected, but the circuit
does not allow the noise to influence the receiver for a time longer than the
duration of a noise spike. This device depends on the use of a limiter. Limiters,
however, can generate undesired spurious responses and small-signal suppression,
and reduce the improvement factor that can be achieved in MTI
processors. It should therefore be used with caution as an ECCM device. 1f
incorporated in a radar, provision should be included for switching it
out of the receiver when it does more harm than good.
... Furthermore, at the higher frequencies the antenna sidelobe levels can be lower, making it
more difficult for sidelobe jamming. However, the advantages of operating against jammers at the higher
frequencies are balanced in part by the disadvantages of the higher frequencies, especially above L band,
for long-range air-surveillance radar.
The noise that enters the radar via the antenna sidelobes can be reduced by
coherent sidelobe cancelers. This consists of one or more omnidirectional antennas and cancelation circuitry
used in conjunction with the signal from the main radar antenna. Jamming noise in the omnidirectional
antennas is made to cancel the jamming noise entering the sidelobes of the main antenna." An antenna can also be
designed to have very low sidelobe levels to reduce the effect of sidelobe jamming. Low sidelobe antennas require
unobstructed siting if reflections from nearby objects are not to degrade the sidelobe levels.
By employing some or all of the above techniques, the effect of the sidelobe noisejammer can be significantly
reduced. Some of the above techniques can also reduce the jamming that enters via the main beam. The effects
of main-beam jamming can be further reduced by employing a narrow beamwidth to limit the region over which the
jamming appears. If the main beam cannot be made narrow because of constraints on the antenna size, an
auxiliary antenna can be employed to create a notch in the main-beam radiation pattern in the direction of the
jammer. With adaptive circuitry similar to that of the sidelobe canceler, this main-beam notch can be
automatically adjusted to be maintained in the direction of the jammer.
A web site involved with jamming is
EW Tutorial, Table of Contents
A book recommended by Aidan Fabius
via newsgroup sci.military.naval and Patrick Tufts
recommends
For further information, see "An Illustrated Guide To The
Techniques And Equipment Of Electronic Warfare"
There is a T1 manual on-line at
T1 AN/MPQ-T1
(another site)(.zip -> .pdf files)(10 files totaling 6 Mbytes)
50,000 yards (about 28 miles)
150,000 yards (about 85 miles)
250,000 yards (about 140 miles)
350,000 yards (about 200 miles)
Although some very long range radars (such as Nike HIPAR) were fixed at about 7 RPM
From Robert Noakes March, 2001
A PPI display with planes and jamming image is 35 K bytes.
(Photo credit Rolf Goerigk
There are various unverified stories that in practice combat between the
Air Force with their jamming equipment, and the Nike with their anti-jamming
equipment, that the Nike successfully tracked the Air Force planes and would
have had successful intercepts with the Hercules missiles.
This was reputed to be true even when the Air Force used their best jamming equipment
to try to confuse the tracking.
Radar History
(C) 1977 ARTECH HOUSE, INC. ISBN 0-89006-021-5
1886-1888 Heinrich Hertz demonstrates generation, reception,
and scattering of electromagnetic waves
1903-1904 Christian Hulmeyer develops and patents a primitive
form of collision avoidance radar for ships
1922 M.G. Marconi (in acceptance speech for an honor)
an angle-only radar for ship collision avoidance
1925 First short pulse echoes from the ionosphere are
observed on CRT by G. Breit and M. Tuve of
Johns Hopkins University
1934 First photo of short pulse echo from aircraft made
by R.M. Page of Navel Research Lab.
1935 First demonstration of short pulse range
measurements of aircraft targets, by British and Germans
1937 First operational radar built - the Chain Home in
Britain, designed by Sir Robert Watson-Watt
1938 Signal Corps SCR-268 becomes first operational
anti-aircraft fire control radar; 3100 sets eventually produced,
Range, > 100 mi.; freq, 200 MHz
1938 First operational shipboard radar, the XAF, aboard the
battleship USS New York, 12 mi ships, 85 mi aircraft
1941-Dec By this date, 100 SCR-270/271 Signal Corps early
warning radars have been produced. One of these radars,
located (near) Honolulu, detects Japanese planes approaching Pearl Harbor ...
Radar Frequency Bands
Wavelength in cm = 30,000 / frequency in MHz
Band
Frequency(MHz)
Wave Length
Comments & common usage
VHF
214- 236
130 cm
?
P
300
100 cm
?
UHF
425- 610
60 cm
BMEWS (ballistic early warn)
L
1250-1380
23 cm
Airport Surveillance, Nike HIPAR acquisition
.
1,421
21.11 cm
Hydrogen (Radio Astronomy)
.
2,450
12 cm
(magnetron in your microwave oven)
S
2700-3900
10 cm
Sage, DEW line, Nike LOPAR acquisition
C
5300-5520
5 cm
Height Finder, Patriot
X
9230-9404
3 cm
Precision Approach, Nike Target Tracking (TTR)
Ku
16,000
18 mm
Nike Target Ranging Radar (TRR), Mortar-location
K
20,000
15 mm
?
.
22,000
13.6 mm
minor H2O absorption line
.
31,500
10.5 mm
(Cosmic Background - "Big Bang")
Ka
35,000
8.5 mm
nominal upper limit of traditional radar
Q
"millimeter"
40,000
7 mm
used in outer space
.
60,000
5 mm
major O2 absorption line, fog (clouds) becomes a major problem
Band
Frequency(MHz)
A 0 - 250
B 250 - 500
C 500 - 1000
D 1000 - 2000
E 2000 - 3000
F 3000 - 4000
G 4000 - 6000
H 6000 - 8000
I 8000 - 10000
J 10000 - 20000
K 20000 - 40000
L 40000 - 60000
M 60000 - 100000
Range vs. Stealth
(who wants his radar to spot aircraft as far away as practical)
Doubling the detectable range requires (all other things remaining the same)
increasing power by 16
target_radar_cross_section * radar_power * antenna_gain
/ range^2
reflected_power * antenna_gain / range^2
so
effective_radar_range ~ ((antenna_gain ^2) * radar_power) ^0.25
so
multiply original power by
increases range by
range, if original radar could see 64 miles
2 1.2 76
4 1.4 90
8 1.68 107
16 2.0 128
(who wants his aircraft to have low visibility to radar)
Pity the poor aircraft designer whose aircraft can be seen by a radar at 64 miles,
and is told to make the aircraft "invisible" a 4 miles.
or
reducing effective cross section by 16
So, to decrease the detection range of the aircraft by a factor of 16,
the designer/manufacturer has to decrease the reflectivity of the
aircraft by a factor of 65,536, likely to be a really major effort.
divide original reflectivity by
decreases range by
detection range, if originally 64 miles
2 1/1.2 53
4 1/1.4 46
8 1/1.68 38
16 1/2.0 32
256 1/4 16
4096 1/8 8
65536 1/16 4
Summation: Accidental jamming was not a significant problem in Nike sites,
even when located close together as in firing ranges.
And thanks to Robert Noakes for nudging me into
further reading and quoting the above book sections.
[From the book Introduction to Radar Systems by Merrill L. Skolnik, 1980,
pages 549-550]
[From the book Introduction to Radar Systems by Merrill L. Skolnik, 1980,
page 549]
Patrick Tufts then found an almost local
(for me) copy the book
via Advanced Book Exchange.
... excellent book on the subject called "An Illustrated Guide To The
Techniques And Equipment Of Electronic Warfare" by Doug Richardson (An ARCO
Military Book) which has tons of info on that kind of thing. My copy is
pretty dated (1985) but there's probably a newer version of it by now.
There's about 20 pages on different jamming techniques and a really good
introduction explaining how radar works and all the different types of
radar. The book has plenty of pictures and diagrams, and is really easy to
understand. I highly recommend it."
and if that is not enough see
"Radar Electronic Counter-Countermeasures"
should provide more than enough.
In principle the aircraft being illuminated by the radar permits one of the aircraft's ARM missiles to lock onto that radar, and the aircraft can then "launch and forget" the ARM missile. The illuminating radar may be able to see the very small "radar cross section" of the missile at closer ranges, especially if the missile is currently in the major lobe of the radar.
I understand that that ARMs were used successfully in the Gulf War, if not previously. Unfortunately, at the present time I know nothing about them.
And then Bill Cahill responded to the above:
|
The actual category of weapons is
called ARMs (Antiradiation Missile) while the HARM (High Speed
Antiradiation Missile) refers to a specific weapon, the AGM-88.
ARMS were first used in the Vietnam War in Mar 1966 by Wild Weasel 1
(F-100F) aircraft carrying the AGM-45 Shrike, the first ARM in the US
inventory.
The Shrike, built by Texas Instruments, had different variants designed to home in on different frequencies. If the radar went off the air, the missile went stupid. The Shrike was essentially an air launched version of a semi-active radar homing surface to air missile, homing in on the target radar instead of a target illuminated by a host radar. Shrikes were effectively used by the US in Vietnam and Israel in the Middle East. The AGM-78 Standard ARM augmented the Shrike in 1968, providing greater range and mission flexibility. The Standard was improved over the next 10 years, with many variants entering service with the USAF and USN. Unlike the Shrike, the Standards can 'remember' where a radar is even if it ceases transmission. The ultimate (to date) ARM, the HARM, entered service in the mid 1980s. This high speed, maneuverable missile is currently in inventory with the USAF and USN and has replaced the Shrike and Standard. |
And then Nicholas Maude responded to the above:
| I just read the update, at the bottom you assert that the HARM is the best ARM around ,that is only partly correct. You are correct if you saying it is the best American ARM around. The best ARM around is the british aerospace ALARM, not is it more modern, it is half the weight of a HARM and is flexible since if it does not immediately detect its target ,it climbs to 70000Ft and deploys a drogue shute. It can stay in this mode for 15 minutes, if detectsthe target during this period it cuts lose and literally drops in detonating when it is beside the target in a vertical dive. |
U.S. Artillery also uses measurements of historical significance. These units were in use 40 years ago.
| unit | S.I. (mks, metric) name | an "English" name | Artillery name |
| length | meter | yard | yard |
| volume | liter | gallon | gallon |
| force | newton(*) | pound | pound |
| mass | kilogram | slug(*) | . |
| plane angle | radian | degree or radian(*) | mil(**) (equals milliradian) |
| time | second | second | second |
| temperature | degree Celsius or Kelvin(*) | degree Fahrenheit | degree Fahrenheit |
In 1997, I asked Col. Moeller - MoellerS@ssdch-usassdc.army.mil - if these units were still in use. He kindly responded as follows:
and "We us standard US weather lingo, definitely F, not C."
http://www.bts.gov/NTL/DOCS/3910-3a.html
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Updated March 29, 2002