Signal Processing in Radar
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cvairwerks
Staff Sergeant
Not required for a simple radar. It's all based on single pulse measurements until you get into frequency agile systems and ones that use variable pulse rates and powers.
Here is a link to a good basic primer on radar. https://faculty.nps.edu/jenn/Seminars/RadarFundamentals.pdf
Here is a link to a good basic primer on radar. https://faculty.nps.edu/jenn/Seminars/RadarFundamentals.pdf
Token
Senior Airman
Not always, no. As C cvairwerks pointed out, it depends on the radar and how it process the returns.
Think about how a radar works at the most basic level, it emits a pulse and looks for that pulse after it has been reflected off things. From that single pulse return you can get everything you need (as long as your needs are not complex). The range is determined (in radar working at unambiguous ranges) by the amount of time the pulse took to return. The direction is determined by where the antenna is pointed at the time the pulse was emitted. etc.
Tracking radars, radars with an ability to auto track a target, may require more processing. And they may require comparing conditions across multiple pulses. But be careful with the "data" and "memory" terms. Remember, tracking radars existed long before a digital computer was included in any radar design. So sometimes the "memory" is as simple as a sample-and-hold circuit, or the persistence of a CRT.
But your question sounds like you had something specific in mind?
T!
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Token
Senior Airman
A sample-and-hold is an analog circuit that takes a sample of a voltage and holds it for later use. So in a radar you could sample a voltage level during a short duration event and use it for control after the event time period has expired.
For example, to create an auto tracking range gate you might have the range gate (centered on the target in time/range), an early gate (which happens before the center of the target range/time), and a late gate (which happens after the center of the target range/time). All three of these sample gates happen at different times but known in relationship to the others, typically the range gate overlaps both the early and late gates. By using a sample and hold in each gate, and comparing the target amplitude in each (detected video voltage level) you know if the target is centered in the gate, on the inside edge of the gate, or on the outside edge of the gate. As the target drifts or moves out of the gate you know which direction it is going, and so you (the radar circuitry) knows which direction to move the range gate to keep the target centered.
T!
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So, it's like a circuit that acts like a (very) short-term memory?
Looking at the diagram: It appears to basically retain the voltage from two pulses in this case, combine them together to produce a modified reading.
notes: Amended from initial post
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It can be as simple as a capacitor that holds a voltage value (signal strength) until it's "read" by a comparator circuit.
In early radar, the time base of the screen is synchronised with the transmitter and receiver. As each pulse is transmitted the time base starts to track across the screen returning echoes are therefore directly proportional to the time taken and time taken is directly proportional to distance x 2. The operator sees a stationary line on the screen with "blips" where signals are returned but it actually cycling continuously at the PRF of the transmitter, however like a television your eyes make it just one line.
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Token
Senior Airman
Absolutely. But in most of the early radars I have seen it was more typically a high impedance op-amp type of circuit. The WW II stuff was a tube based op amp design (specifically I saw that in the SCR-584 and the SCR-702, I assume it was used in others of the same time period), later discrete solid state components, and finally the IC based op-amps.
T!
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Token
Senior Airman
This is still the way it is done today in radars that do not present the operator synthetic video or that present a combination of raw and synthetic video. There are still modern radars that present raw video on either PPI or A/AR/B/C-scope kinds of displays. I have even seen a relatively modern radar that includes a J-scope, one of the oldest style sweeps used in some of the very earliest radars.
There are 5 basic parts to any pulsed radar, the Synchronizer, the Transmitter, the Receiver, the Antenna and the Display. In this case "Display" is a generic term describing the end use of the radar data, obviously there are some systems that have no human interface display, but rather apply/transfer their track results to something else. The important take away is that the Synchronizer talks to most of the parts, it tells the Transmitter when to transmit, tells the Receiver when to listen (and when to not listen, such as firing any active ATRs during the transmit pulse), and it tells the Display when to sample the Receivers data. On some systems it may tell the Antenna to form or shift its beam, but on many radars there is no Synchronizer involvement with the Antenna.
T!
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My knowledge of RADAR is from reading stuff anyone can read but mainly from working in ultrasonics which has the same principles and in most cases the same language. The earliest sets I have seen for ultrasonics were very similar to early RADAR sets and operated in a similar way, they used millivolts of attenuation at that time instead of dB as they do now. The guy who taught me ultrasonics was one of the main developers of it in UK, he had his own workshop and "museum" of ancient sets. However even by 1988 there were systems that took the analogue transmit and receive information and system setting and recorded the lot digitally. So you could run a scan in a few minutes and analyse the whole lot at leisure with the A scan display and B scan C scan map on screen at the same time. The system had been developed to scan the Space Shuttle boosters after the Challenger disaster.
Token
Senior Airman
I said it in another thread, but I will say it again, I came into the radar and counter radar / ESM / ELINT world at a very interesting time. WW II was only a bit over 30 years in the past, so I got to work first hand with people still in the field who had been involved in very, very early radar stuff, and there was still a lot of the old gear, and old gear documentation, hanging around. Other people I worked with had themselves worked directly with the pre-war pioneers of the field. I got to work with some WW II era radars either in their original form or lightly modified for new tasking.
Unfortunately, as is common, people did not necessarily think of the history involved. For example, during an upgrade we scrapped a 1944 production SCR-584 (actually, 4 of them) and were happy to see them go. Today it would be a blast to have one of those things in a petting zoo. The same thing with an SCR-720 and SCR-702. By the end they were just clutter hanging around, taking up space, so they got scrapped. We had a 35 acre bone yard that was full of WW II and later stuff, including things like a full, new in crates, never installed, AN/BPS-1 radar, and a complete, as removed, SJ radar. We had a 40 foot sea container absolutely floor to ceiling packed with airborne WW II ESM and radar jamming equipment. ARR-5, ARR-7, APR-1, APR-2, APR-4, all the kit (displays, antennas, etc) that goes with them. All of this got cleaned up after an admiral flew over in a helo during a dog and pony and asked what all the clutter down there was for.
And documentation. Paperwork that historians and researchers would love to have. We had a full set of cabinets of WW II radar and counter radar documents. One day it just all went away because the space was needed for a new office manager.
Hind sight is 20/20, and some of this stuff should have been archived or otherwise retained. Much of it is probably no longer available in any form. But at the time it was just old junk.
T!
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manta22
Banned
In the very early '60s I attended the Corporal Ground Guidance Electronics course at the Ordnance Guided Missile School at Redstone Arsenal in Huntsville, Alabama and became an instructor there until the Berlin Wall went up. The heart of its guidance system was an SCR-584 which tracked the missile after launch, determining its azimuth and slant range. The radar van was modified by removing its dish antenna from the mechanism that raised it up to the roof and placing it on its own towed trailer. The SCR-584 had minimal signal processing. It simply tracked the missile by receiving the return pulse from a conical scan system which drove the antenna motors through an amplidyne. (now there is a word I haven't used in years!). Some of the recycled SCR-584s reputedly had patched holes from WW II gunfire. The '584 was a really great advance in radar technology when it was originally deployed. Too bad those four units were not taken to a museum.
Token
Senior Airman
Interesting. I have run into a couple of the radar antenna trailers for the MGM-5 over the years in other applications, and I have seen the MGM-5 missile at the White Sands museum area. But I have never seen a complete battery.
While the SCR-584 would be rather light in what we currently call signal "processing", it was pretty complex for the day. The process of turning the raw RF into video to be displayed on the J scopes, developing and positioning the sample gates around the video of interest, and automatically driving the range and angles tracks based on those gates, all was revolutionary and new at the time. The SCR-584 was a major development effort for the US in WW II.
SCR-584, and later Nike, radar parts ended up being used for a lot of other purposes over the years. I have seen both -584 and Nike radar systems re-tasked as instrumentation systems. The pieces/parts of both of those systems show up in the oddest places. The 584 feed and scanner has been used for a lot of different applications. And you have probably seen Nike antennas used with weather chasers, several of the older mobile tornado radars (Doppler on Wheels trucks) are based on a Nike dish antenna mounted on a Hawk pedestal.
T!
manta22
Banned
yes, the
Yes, the MGM-5 was a complex system indeed! You should have seen us scramble when an Alert was called in the middle of the night in Germany. Surprisingly, our whole battalion could exit the kaserne and meet at an assembly point, and get ready to fire in an amazingly short time. It took a far greater effort and a much higher level of training that our solid-fuel replacement, the Sergeant missile.
