Best air->air radar WWII

kitplane01

Airman 1st Class
105
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Apr 23, 2020
What was the best air to air radar of WWII, and how good was it, and what airframe carried it?

I did find this resource for just US radars on night fighters, and none of them are able to find a fighter at beyond 8,500 yards which is 4.8 miles. Maybe some of the ASW radars could do better when aimed at fighter altitudes? Or maybe the british/germans had better?

 

pbehn

Lieutenant Colonel
11,881
8,359
Oct 30, 2013
What was the best air to air radar of WWII, and how good was it, and what airframe carried it?

I did find this resource for just US radars on night fighters, and none of them are able to find a fighter at beyond 8,500 yards which is 4.8 miles. Maybe some of the ASW radars could do better when aimed at fighter altitudes? Or maybe the british/germans had better?

Others are much more expert than me here but from what I know. As per your link British RADAR (RDF at the time) was part of the Tizzard mission, from 1940 RADAR was developed jointly between the UK and USA much of it produced in the USA so I cant see the British being more advanced. 4.8 miles is actually a long way just not considered so today because of many advances in the technology. The Chain Home system used huge amounts of power flooding the whole area with radio waves. This is because the attenuation (loss of signal strength) is huge. Only a fraction of the power used becomes a signal, that signal disperses with distance in three dimensions, only a fraction of the signal hitting an aeroplane is returned to the transmitter and a fighter is a small "target". This weakened signal the attenuated further by distance and only a fraction of this returned RADAR signal is turned into an electrical signal in the plane for its amplifiers to work on.
 
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Token

Senior Airman
349
205
Nov 7, 2011
Mojave Desert, California
What was the best air to air radar of WWII, and how good was it, and what airframe carried it?

I did find this resource for just US radars on night fighters, and none of them are able to find a fighter at beyond 8,500 yards which is 4.8 miles. Maybe some of the ASW radars could do better when aimed at fighter altitudes? Or maybe the british/germans had better?


Regardless of who answers, or what the answer is, this is probably going to end up being an opinion based answer.

What is your definition of "best"?

Longest range? Range is not everything, and range will depend on the target and specific conditions during detection. Most accurate? Accuracy is going to be a huge factor in a targeting system, less so in a detection / early warning system. Most robust and failure resistant? Even if not very accurate, or long ranged, a system that works dependably might be better than a hyper accurate / long range system that never works. Most used? There is definitely a quality to quantity.

The AN/APG-16 (an X band modification of the AN/APG-2, the APG-2 was originally meant for the P-61) was probably one of the best radars of the war, developed just before the end of the war, but since it was never used in any numbers at all during the war that one obviously should not be in consideration. Similarly, the APG-1 and APG-2 were VERY good, but never got used (regularly) in combat as their capability was less important than the large numbers of less capable radars already in use. A case of more units less capable outperforming a few units more capable.

T!
 

wuzak

Captain
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Jun 5, 2011
Hobart Tasmania
Regardless of who answers, or what the answer is, this is probably going to end up being an opinion based answer.

What is your definition of "best"?

Longest range? Range is not everything, and range will depend on the target and specific conditions during detection. Most accurate? Accuracy is going to be a huge factor in a targeting system, less so in a detection / early warning system. Most robust and failure resistant? Even if not very accurate, or long ranged, a system that works dependably might be better than a hyper accurate / long range system that never works. Most used? There is definitely a quality to quantity.

The AN/APG-16 (an X band modification of the AN/APG-2, the APG-2 was originally meant for the P-61) was probably one of the best radars of the war, developed just before the end of the war, but since it was never used in any numbers at all during the war that one obviously should not be in consideration. Similarly, the APG-1 and APG-2 were VERY good, but never got used (regularly) in combat as their capability was less important than the large numbers of less capable radars already in use. A case of more units less capable outperforming a few units more capable.

T!

Was the AN/APG-2 radar a gun laying radar for the turret?

The main radar still being the SCR-720?
 

kitplane01

Airman 1st Class
105
22
Apr 23, 2020
Regardless of who answers, or what the answer is, this is probably going to end up being an opinion based answer.

What is your definition of "best"?

Longest range? Range is not everything, and range will depend on the target and specific conditions during detection. Most accurate? Accuracy is going to be a huge factor in a targeting system, less so in a detection / early warning system. Most robust and failure resistant? Even if not very accurate, or long ranged, a system that works dependably might be better than a hyper accurate / long range system that never works. Most used? There is definitely a quality to quantity.

The AN/APG-16 (an X band modification of the AN/APG-2, the APG-2 was originally meant for the P-61) was probably one of the best radars of the war, developed just before the end of the war, but since it was never used in any numbers at all during the war that one obviously should not be in consideration. Similarly, the APG-1 and APG-2 were VERY good, but never got used (regularly) in combat as their capability was less important than the large numbers of less capable radars already in use. A case of more units less capable outperforming a few units more capable.

T!
OK. I agree with everything you wrote.

How about range? I want the radar with the longest range. But if you have a "radar I think is the best by my criteria" that works too.

What ws the range of an APG-16 or APG-2?
 

EwenS

Staff Sergeant
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Oct 19, 2021
The “best” Allied AI radar by the end of WW2 was the SCR-720 or AI.Mk.X in British service. It reached squadron service with the RAF in Jan 1944 and went into Mosquito XVII/XIX/30/36 through into 1946. It also equipped the P-61.

The earlier AI Mk.VIII (introduced from Dec 1942) as fitted to later Beaufighters earlier marks of Mosquito remained in service until the end of the war.

It was 1948 before Britain introduced the AI Mk.IX on the Mosquito NF.38. Development of that set had begun in WW2.
 

pbehn

Lieutenant Colonel
11,881
8,359
Oct 30, 2013
OK. I agree with everything you wrote.

How about range? I want the radar with the longest range. But if you have a "radar I think is the best by my criteria" that works too.

What ws the range of an APG-16 or APG-2?
Both UK and Germany first developed airborne RADAR for defence and to be used in conjunction with ground based RADAR. Both the UK Chain Home and later GCI and German Freya systems could guide an interceptor to within 5 miles of a target. It became a different issue outside of ground control.
 

kitplane01

Airman 1st Class
105
22
Apr 23, 2020
The “best” Allied AI radar by the end of WW2 was the SCR-720 or AI.Mk.X in British service. It reached squadron service with the RAF in Jan 1944 and went into Mosquito XVII/XIX/30/36 through into 1946. It also equipped the P-61.

The earlier AI Mk.VIII (introduced from Dec 1942) as fitted to later Beaufighters earlier marks of Mosquito remained in service until the end of the war.

It was 1948 before Britain introduced the AI Mk.IX on the Mosquito NF.38. Development of that set had begun in WW2.
Googling doesn't show me any data about the SCR-720. Can you tell me more about it.
 

EwenS

Staff Sergeant
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1,941
Oct 19, 2021
Here you go.

Plenty of links at the foot of the Wiki page to good sources.

While the AI equipped night fighters operated under the control of GCI radar stations in a defensive role, from mid-1943 radar equipped night fighters began to be used offensively in the bomber support and intruder roles from mid-1943 IIRC. Centimetric AI.Mk.VIII as not cleared for use over enemy territory until April 1944. NF squadrons in 100 Group and later ADGB/Fighter Command provided escort to the Bomber Command bomber streams in. I've never heard of radar range being criticised in these operation. After all, unless the target was detected the NF crew didn't know the target was there in the first place!

You might find this site of interest as it has data on various British ground radars including GCI sets.
 

pbehn

Lieutenant Colonel
11,881
8,359
Oct 30, 2013
That was super interesting. Much thanks
It is not so much what was "state of the art" at the time it was the state of research development. When you are using civilian technical specialists to operate the equipment you are some years from a production system that can be operated by technicians. The Wellington had many uses because it had useful space inside, that system and technology was years of development away from being installed in any type of fighter.
 

Token

Senior Airman
349
205
Nov 7, 2011
Mojave Desert, California
Was the AN/APG-2 radar a gun laying radar for the turret?

The main radar still being the SCR-720?

No, only one radar would have been on the aircraft.

OK, information on some of this is lacking, and some of the following is informed supposition on my part. Getting / finding confirmation can be difficult. I have first hand experience with some of the radars I will discuss, but not any operational experience with the AN/APG-1 or -2. I have seen some APG-1/2 hardware (specifically the antenna), but I never saw the original system in operation. Basically the AN-APG 1 and APG-2 appear to have been airborne radars that used some of the technologies found in the SCR-584. They were not exactly air going versions of the -584, but they were close cousins.

The AN/APG-1 and -2 (similar radars, I think made by different companies) were both gun layers and AI radars, with search / AI using a Palmer scan and autotrack capability using con scan (SCR-720 used helical scan). From what I gather it appears the APG-1/2 were the original concept for the radars that were desired in the P-61 (an appropriate radar did not exist when the P-61 effort was contracted), however they took longer to develop (being more complex) and the SCR-520 / 720 were used in the mean time. I suspect (just me reading between the lines of the limited documentation I can find) that by the time the APG-1/2 were ready for field testing the mission requirements had shifted slightly, and the 520/720 were "good enough" and readily available, for the tasking as had been developed.

Think of the APG-1/2 as an SCR-720 that could also do gun laying.

6 (or 7) P-61 airframes were actually tested with the APG-1/2 radars, however I suspect that by then the tactics that had been developed around the 520/720 had partially negated the potential advantages of the -1/2. I have read that all of the airframes tested with the -1/2 were eventually retrofitted to the -720.

The APG-2 was later modified to X band, and became the APG-16. This modification would have (at a minimum) improved the angular tracking accuracy as well as the rejection of unwanted signals.

T!
 

Token

Senior Airman
349
205
Nov 7, 2011
Mojave Desert, California
OK. I agree with everything you wrote.

How about range? I want the radar with the longest range. But if you have a "radar I think is the best by my criteria" that works too.

What ws the range of an APG-16 or APG-2?

I find no testing data on the range of the APG-2 or APG-16 vs specific targets, but then there is VERY little information in general on these radars so that does not surprise me.

However, the factors of radar performance are not a mystery, and when you know a few details you can make educated guesses as to what the performance might be.

The performance of the SCR-720 is fairly well documented, and the system itself can be used as a yardstick, so lets compare the APG-1/2 to the SCR-720.

The SCR-720 and the APG-2 apparently used the same transmitter (not an uncommon practice), this would have made the frequency and peak power the same. The APG-2 appears to have included an additional selectable transmitter pulse mode, a shorter pulse, which would have improved range resolution when used, at the cost of reduced average power return and so probably reduced maximum range in that specific mode (but the same in shared modes).

The antennas were the same diameter between the two radars, meaning the antenna gains were probably identical. Dish antenna gain is directly tied to dish size, efficiency, and frequency. The same size dish, at the same frequency, with the same efficiency (no reason to think otherwise) yields the same gain.

The receiver technologies were probably near identical, if anything the APG-2, being a bit later, may have had a slight advantage there. In general radar receiver sensitivity increased, predictably, across the war effort, specifically, noise figures in microwave designs generally improved as the war progressed.

With all of that being considered, it is probable that the APG-2 and the SCR-720 had near identical range performance on a given target size, if there is any delta the advantage probably went to the APG-2, with its later war background. However the APG-2 absolutely had to have had the ability (with con scan) to indicate enemy aircraft position with a greater degree of accuracy. And the added short pulse mode probably enhanced this further.

I find almost nothing on APG-16 performance. Since this was used after the war it appears that it was treated like other post war radars, and the information protected to a greater level. But again, we can make some educated guesses on probable performance from basic radar theory.

The APG-16 was an APG-2 modified to X band (the APG-2 was S band). This, with no other modifications at all, would have probably increased track accuracy by virtue of a tighter beamwidth (the same size dish antenna at a higher frequency must yield a tighter beamwidth and correspondingly higher gain, assuming surface irregularities of less than 1/8 wavelength at the higher frequency). This tighter beamwidth would also have made the radar less susceptible to interference and jamming.

The increased gain of the antenna would have increased the ERP of the radar, however X band has a higher space loss than S band and the X band receiver might not have had the same MDS as the S band receiver. But the gain of RCS for a target (how much radar energy is reflected) would have been greater at X band than S band. So not sure what the end result of all of that would have been, but I don't think the APG-16 range would have been much less, if any.

The APG-16 would probably have been a lighter radar than the APG-2. The transmitter physical size and all of the waveguide components, both transmitter and receiver, would have been smaller (higher frequency = shorter wavelengths = smaller RF parts). We all know lighter is better in an aircraft, every pound is potentially better aircraft performance.

T!
 

Token

Senior Airman
349
205
Nov 7, 2011
Mojave Desert, California
How about range? I want the radar with the longest range. But if you have a "radar I think is the best by my criteria" that works too.

I came back to this thread several times before I realized what was bothering me about this statement / question. Sorry, this is going to be a long discussion, feel free to skip it if you think I dive into mansplaining.

Radars, like most technical endeavors, are designed to a specification or a set of specifications. Bigger / faster / further does not automatically equal better, if those attributes are of no use in an application, and they may be detriments, depending on why the design specifications are the way they are.

When designing a radar (or almost anything) you work towards a set of specifications. When discussing range you don't say "what is the longest range I can design", but rather you say "what is the range required to do the task assigned". I will get into the whys later.

When thinking of an air-to-air radar, be it gun laying or AI, what range do you really need? If the weapons on the platform in question are good for something like 1200 meters, you don't need a gun layer working at a range much beyond that. Sure, a little further, to set up the shot, have it all ready to go, at max range. For an AI, in a WVR, 0.5 Mach, world, there is little need for a fighter to track targets at 200 km. Having too long a range increases the work load on the RIO / Pilot that is using the sensor, and tracking aircraft that are 20 minutes away at Buster (and yes, that term was used in WW II) is not of great use in a fight. And long range in the radar potentially costs you aircraft performance.

The maximum range of a radar, for a given state of technology, is determined by several factors. Key among these are the receiver, the transmitter, the antenna, and the selected radar parameters.

The sensitivity of the receiver. This is often defined by technical limits that are consistent for a given design source, i.e. most radar receivers, of a given frequency range, designed in the US / UK during 1944 would have similar performance with regards to sensitivity.

The peak (and average) power of the transmitter. Powerful transmitters are heavy, and consume lots of power. Aircraft don't want heavy things, and have a limited power capacity. So you end up designing the transmitter to have "enough" power to do the job at hand, while not costing too much in aircraft performance.

Higher gain antennas (generally better for radar applications) are physically larger. Physically large antennas are not conducive to use on aircraft. So you figure out the maximum footprint, and weight, the aircraft can carry, while not costing too much in aircraft performance, and you design to that limit. And antenna performance has not changed much since before WW II, i.e. a given size dish antenna (or antenna array) today has the same gain as that sized dish antenna did in 1941. It is a physics problem, and roughly stated, aperture = gain, or, simply, size matters.

So you have this box you are working in, with many factors beyond your control. You have this much size, that much weight, and X power available. All of which limits your transmitter power (size, weight, and available power), antenna gain (size and weight), and receiver performance (defined by the current state of technology, generally slowly moving).

But there is one more factor (set of factors) you can adjust and optimize, and that is the selected radar parameters.

In its most basic description, a pulsed radar (most air-to-air, gun laying and AI, radars in WW II) is a transmitter that sends out a pulse of energy (traveling at the speed of light, c) and then listens for that pulse to reflect off a target. By detecting the angle of arrival of the reflected pulse the radar can determine the direction of the target. By measuring the time the pulse takes to return, time based on when each pulse is transmitted, the radar can determine the range to the target.

Angle of arrival has nothing to do with the distance to the target, and is generally a factor of the radar antenna system and signal processing, and we have discussed how we may have limitations beyond our control in those regards. But, distance is time to a radar, and we can play with time for free. We can control how long the transmitter transmits, how often the transmitter transmits, and, inversely related, how long the receiver listens.

How long the transmitter transmits. The length of a radar pulse, how long the duration of the transmitted pulse, is called the Pulse Width (PW), and determines the minimum range a radar can be used at (you can't see a target until your transmitter stops transmitting), the range resolution of the radar (this is the ability to resolve individual targets that might be close to each other in range), and is a major factor in PD, Probability of Detection, (longer pulses result in higher average power on return, increasing the PD). So we select our pulses with this in mind. Longer pulses are better for PD, but we need a short pulse to work at close ranges and to improve range resolution of targets at any range.

How often the transmitter transmits, and how long the receiver listens. How often the transmitter transmits is called the Pulse Repetition Interval (PRI) or its inverse, the Pulse Repetition Frequency (PRF). Simply put, PRI is the time from the leading edge (start) of one transmitted pulse until the leading edge (start) of the next transmitted pulse. A radar with a PRI of 1 millisecond (0.001 second) transmits a pulse every 0.001 second. It is also said to have a PRF of 1000 Hz, i.e. something that happens repetitively every 0.001 second happens 1000 times per second.

To increase our PD we want to pulse the transmitter as often as we can. More pulses per second is better for PD.

So the radar pulse is transmitted, goes out to the target, is reflected, and comes back to the radar. But, to be unambiguous in range (for us to be able to determine the range with confidence) it must do this round trip in under the PRI, in our example in under 0.001 second. So this PRI sets, determines, the maximum unambiguous range of the radar. And this range is calculated based on (c*PRI)/2, that is, the speed of light (how fast the radar pulse travels) multiplied by the time allowed for that pulse to travel, all divided by 2, since we must account for both the trip to the target and the trip back from the target.

Our radar, with a 0.001 second PRI, has a maximum unambiguous range of 150 km, regardless of how much power the transmitter transmits, how sensitive the receiver is, or how large (determining how much gain) the antenna is. Of course, this is the maximum possible range, and the usable range might be less, depending on transmitter power, target reflectivity, receiver sensitivity, and antenna gain.

So we will set our transmitter PRI and PW combinations to meet our desired radar performance characteristics. For long range we want a long PRI and a long PW. But we need (to increase the PD) as short a PRI as we can get away with. And we want (to improve range resolution) as short a PW as we can have and still keep a good PD.

Lets lay out the PW and PRI / PRF requirements for a notional radar combining gun laying and AI functions, with a maximum desired AI range of 100 km, the ability to resolve targets flying within 500 meters of each other at that 100 km, and a gun laying function with a maximum range of 5 km and a minimum usable range of 100 meters.

For a maximum range of 100 km, and the ability to resolve targets as close as 500 meters to each other, we might not use a PRI of 0.001 second (1000 Hz), instead we might select something more like 0.00067 second (1492 Hz). This limits us to our spec of 100 km, but the advantage is that we do not waste time on range we don't want and we increase the pulse rate, increasing the PD. The Range Resolve spec will drive the transmitter pulse width, in this case, to range resolve targets within 500 meters of each other, we would select a pulse width of less than 3.34 microsecond (0.0000034 second) length. To maximize PD we would maximize (use the highest possible) PRF and maximize (use the longest possible) PW. We will also select our transmitter power level / receiver sensitivity / antenna size to be optimized for this range vs a specific target size, instead of wasting weight and power by supporting too large / heavy / powerful a radar.

We could add other user selectable PWs that would allow breaking out targets that were closer together, and yet other user selectable PRIs that would decrease maximum range while increasing PD. For example, the late SCR-720 had 4 different range settings, 3 of them bringing up different PW and PRF combinations, as well as changing things like antenna rotation rate and indicator scaling. Note that the 100 mile setting for this radar was intended to be used in radar beacon mode only, RACON.

If another design spec is a gun laying function in the same radar, with a max required tracking range of 5 km and a minimum usable range of 100 meters, we would select a set of parameters optimized for this task. To track as close as 100 meters we must select a pulse width that is short. To meet this spec, this pulse must be less than about 650 nanoseconds in length. To accommodate receiver recovery this is probably more like 500 nanoseconds or shorter. And the max gun laying range of 5 km would drive us towards a desired PRI of something like 34 microseconds. However, a 34 usec PRI and 500 nsec PW would make the duty cycle (DC) just short of 1.5%, pretty high for some magnetron based radars, and so we might be forced to lengthen the PRI to achieve a DC compatible with our magnetron limitations.

And so we see, it is all a compromise. The "longest range" only counts if long range was the goal. The best air-to-air radar is going to depend on what specific factor is your goal post, and simple long range is not going to be a realistic measurement of "best".

T!
 

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