Radar questions

Does a longer wavelength require a larger antenna for emission or reception or both?

Radar uses specular reflection. Why not also diffuse reflection?
How come stealthy aircraft like the F-35 still have round surfaces underneath? I understand that round shapes are generally avoided because they contain so many angles that at least one of them is bound to be just right for a specular reflection back at the emitting radar.
How effective are differently-placed emitters and receivers at countering stealth aircraft and stealth ships?
How does the limited range of a radar manifest itself? What happens outside it range that doesn’t happen outside of it and vice versa?
Is it like low light in a camera? When light is insufficient in a camera, there’s things you don’t see or see in low detail. To remedy that, you can crank up the sensitivity but that can get you false positives in the form of noise.

When using low frequency waves at ranges of 100-200km, you get low-quality information, correct? How does that manifest itself? Do you get a very wide range of possible true values for size, location and speed of objects?

How can Home-On-Jammer guidance be countered? From what I understand, some missiles will use normal radar illumination but when they get jammed, they switch to HOJ guidance. What can the target do then?

Does a longer wavelength require a larger antenna for emission or reception or both?

The wavelength is tied to the antenna size. A shorter wavelength (higher frequency) allows you to use a smaller antenna and still get the same amount of gain out of it. Using a longer wavelength with the same size antenna will result in less gain, meaning that the antenna won’t be as sensitive to the signals.

For things like airborne radar, you end up with a whole bunch of things that you need to factor in, and antenna size is only one of them. Other important factors are things like the absorption of your desired frequency by water vapor in the air. Longer wavelengths tend to propagate better through the atmosphere. Shorter wavelengths are also scattered more easily, meaning that longer wavelengths can often detect objects that shorter wavelengths will sometimes scatter off of.

In other words, it’s all very complex.

**Radar uses specular reflection. Why not also diffuse reflection? **

You get the strongest return from the specular reflections. Diffuse reflections by their nature involve scattering and radiating the energy level out in various directions.

A good clear specular reflection also allows you to measure doppler shift to determine speed as well as location.

How come stealthy aircraft like the F-35 still have round surfaces underneath? I understand that round shapes are generally avoided because they contain so many angles that at least one of them is bound to be just right for a specular reflection back at the emitting radar.

The F-117 used very angular surfaces, which are great for channeling radar returns along certain paths so that you hopefully steer them away from the targeting antenna. Angular surfaces aren’t so hot for aerodynamics, though. The F-117’s nickname among its pilots is the “wobbly goblin” which should give you some indication of how well it flies.

The F-117, despite its F designation, is more of a light ground attack aircraft than a real fighter. The F-35 on the other hand is a fighter, and must handle and perform like a fighter.

Between the development of the F-117 and the B-2, a lot of the math was worked out so that signals could be reflected and channeled away as desired but you could still have an aircraft with less angular and more aerodynamic surfaces. This has allowed folks to build stealth aircraft that don’t tend to have the word “wobbly” in their nicknames.

Ironically, one of the things that really helped this effort was some Russian publications that were in the public domain.

How effective are differently-placed emitters and receivers at countering stealth aircraft and stealth ships?

I suspect that only people with fairly high security clearances can answer this one. They don’t tend to release data like this, for fairly obvious reasons.

How does the limited range of a radar manifest itself? What happens outside it range that doesn’t happen outside of it and vice versa?
Is it like low light in a camera? When light is insufficient in a camera, there’s things you don’t see or see in low detail. To remedy that, you can crank up the sensitivity but that can get you false positives in the form of noise.

Radar doesn’t work like a camera. It’s more like marco polo. You yell marco and wait for the target to polo back. What happens when the return signals get weak is that they get lost in the background clutter. Simple radars like weather radars will often show all kinds of clutter. Sometimes what you are looking at on the map isn’t weather near the antenna, it’s ground clutter.

Aircraft radar is often more sophisticated and uses all kinds of filtering techniques to differentiate between clutter and real returns.

When using low frequency waves at ranges of 100-200km, you get low-quality information, correct? How does that manifest itself? Do you get a very wide range of possible true values for size, location and speed of objects?

Long distance radars like that tend to be lower resolution due to the wavelength. You can’t really determine size from a radar signal like that. All you can go by is the strength of the return. Generally speaking, larger aircraft return larger signals than smaller ones. The F-117 stealth fighter, on the other hand, returns a signal roughly equivalent to that of a bird.

How can Home-On-Jammer guidance be countered? From what I understand, some missiles will use normal radar illumination but when they get jammed, they switch to HOJ guidance. What can the target do then?

I have no idea what current technologies and strategies are used for electronic countermeasures these days. Googling the phrase “electronic countermeasures” might give you some insights.

ECG,

Thank you for your answers.

About the first question, I meant to ask if the wavelength is tied to antenna size when said antenna is emitting or when it’s receiving. In other words, if the emitting and receiving antenna are different and I want to use long wavelengths, which antenna has to be large?

I work in this field and I can give you some non-specific answers that I hope will satisfy you.

How effective are differently-placed emitters and receivers at countering stealth aircraft and stealth ships?
Generally quite effective; stealth can only be optimized for certain conditions such as frequency band or aspect which means you have to give up something somewhere else. A stealth fighter would be usually be optimized for airborne radars and so will not be as effective in other bands as it would for these and mostly from the front as, hopefully, if you are turning tail you are going home with empty missile racks.
How does the limited range of a radar manifest itself? What happens outside it range that doesn’t happen outside of it and vice versa?
Radar range is a function of receiver sensitivity; once the signal is buried in the noise floor you won’t have a coherent signal, much like your camera analogy below.

Is it like low light in a camera? When light is insufficient in a camera, there’s things you don’t see or see in low detail. To remedy that, you can crank up the sensitivity but that can get you false positives in the form of noise.

When using low frequency waves at ranges of 100-200km, you get low-quality information, correct? How does that manifest itself? Do you get a very wide range of possible true values for size, location and speed of objects?

This is dependent on more than just distance. You also have to consider other factors like how often you’re “pinging” the target, how you ping, and your receiver and processor capability.
Imagine you are in a dark gym or warehouse with a blinking flashlight and no other light source. You start spinning in a slow circle, looking around when you see something on the far wall. You still want to make sure there is nothing else around so you spin faster to see it more often or you can focus on a specific area but that means the rest stays dark. The blink rate is your repetition interval, the spin is your sweep rate, and focusing in one spot is a sector scan. By manipulating these and other intervals you can tailor what you want to find out.
About the first question, I meant to ask if the wavelength is tied to antenna size when said antenna is emitting or when it’s receiving. In other words, if the emitting and receiving antenna are different and I want to use long wavelengths, which antenna has to be large?
Both antennas will usually be the same size as that “tunes” the receiver and lets in the right frequencies, in simple terms.

How can Home-On-Jammer guidance be countered? From what I understand, some missiles will use normal radar illumination but when they get jammed, they switch to HOJ guidance. What can the target do then?

Turn off or modify the jamming technique, use manuevers, terrain or some other means of defeating the missile radar.

In the early days of radars it was fairly common to have a separate transmitter and receiver. These days the same antenna is usually used for both transmitting and receiving. There are things like RF circulators that make this fairly easy to do.

There’s also a common saying about RF antennas that says “you give what you get”. If a particular antenna design is more directional, it has that same characteristic for both transmitting and receiving. The same goes for frequency. If a particular antenna is tuned to a particular frequency, it both transmits and receives better on that frequency.

Generally speaking, antenna sizes and spacings and such are tied to the desired frequency, or some multiple of that frequency (or a division - there are things like half wave and quarter wave antennas). So as the wavelength goes up, the antenna size goes up proportionally.

As for your specific question about whether the transmitter antenna size or receiver antenna size is more important (assuming you’ve got some kind of oddball case where they are two separate antennas), it’s kinda difficult to answer. It’s the total round trip that matters. So you have to be able to transmit enough power to get a decent reflection back, then your receiver antenna has to be sensitive enough to pick it up as does your receiver circuitry. Reduce the gain anywhere along that path and the range of radar detection suffers. You can compensate for a poor antenna design on the transmitter by cranking up the power. Cranking up the receiver sensitivity gets to be a bit more difficult because that is often limited by things like background RF noise and thermal noise in the electronics of the receiver, and those are harder to eliminate.

The reason I asked about different emitter and receiver antennas was because of semi-active guidance: Semi-active radar homing - Wikipedia
The aircraft/ground emitter sends radiation toward the target and the missile receives the radiation that bounces off. I imagine that in most cases, the missile’s antenna is considerably smaller than the aircraft/ground antenna.

As far as I know, when emission and reception are split, it’s the launching platform that sends radiation and the missile that receives it. Might it be possible to do it the other way around? That would alert the target that it’s being fired at by a missile but it would keep the aircraft/ground radar safer since it would only receive and not send.

How do radar warning receivers detect AESA? Active electronically scanned array - Wikipedia

Given the range of frequencies and the hoping, I don’t see how a radar warning could know it’s being lit up.

Also, what’s the cost range for radars? I know that it varies very much but I’m only asking for particular examples to give me an idea. For example, how much do US destroyer AESA cost? How about the AESA of F-35 and F-22?

I haven’t seen the antennas in question here, but I would expect the missile antenna to be as small as possible. There’s a tradeoff here. A larger antenna would be able to pick up a weaker signal, but also adds weight and size to the missile. Since it’s not the missile that is painting the target, I imagine they just blast away with a decent amount of power and use that to compensate for the missile’s small receive antenna. Missiles are also relatively close range. They may not need all that sensitive of a receiver for the thing to work in this case.

If you are steering the missile, it’s the missile that needs to know precisely where the target is. The aircraft or ground station painting the target only needs to know approximately where it is so that it can blast the target with enough of a signal that the missile can see it.

I don’t see any way for that to work the other way around.

No idea. This stuff was still highly classified when I got out of the defense business.

When I worked on the F-16 radar in the late 1980s I think the cost of each unit was somewhere around $2 million.

Raytheon got a contract last year to provide 15 units for F/A-18 E/F Super Hornets for a total of $41 million, or roughly $2.7 million per unit.

A ground based radar like a typical airport might have is probably somewhere in the $10 to $15 million range.

Low observable (LO) strategies encompass a variety of technologies. One of them, as you mentioned, is shape. Another is radar cross-section. And another is the use of radar-absorbent materials.

And then there are… special techniques:

One common practice when being illuminated by a targeting radar (search radars have different frequencies, a sweep pattern rather than steady transmission, etc…) is ejecting aluminium chaff to confuse incoming missiles. Hopefully, the missiles will target the chaff cloud and the aircraft will have manuvered out of the intercept envelope.

Doesn’t the horizon limit radar range more than simple signal loss? Signal loss, you can always increase signal intensity (assuming the radar is mounted on a ship or aircraft) until the reflections are above the noise floor. Horizon, well, not much you can do. Some wavelengths will go over the horizon but they are very long wavelengths and thus low resolution.

The horizon does limit the range. Cranking up the power does help with range, but you also end up with more clutter as a result, especially since you’ll get stronger returns out of your sidelobes (Sidelobes - Wikipedia for those who don’t know what sidelobes are). Also, doubling the power doesn’t double the range. You have to really crank up the power a lot more than that to double your range, and more powerful radars tend to have bigger and heavier components, so again you run into a tradeoff when you are talking about airborne systems.

More sensitive receivers also increase the range, but their greater sensitivity also tends to make them more susceptible to jamming.

One of the ways to get better resolution of your target is to send out pulses faster. The shorter the delay between pulses, the closer you can track your target. This leads to another distance limit in that once the pulses get too close together, a return from far enough away comes in after the next pulse has been generated, making it appear to be a return from a much closer object than it really came from.

It should also be mentioned that lowering the noise temperature of the LNAs is (pretty much) equivalent to increasing the power output of the radar.

Indeed. You need 16 times the power to double the range, since the returned power goes as the inverse fourth power of the range.

Yikes. Makes perfect sense, they are just coupled 1/r^2 events.

This is starting to explain why detecting stealth aircraft is hard. Well, sort of. In the Tom Clancy novels - which admittedly are probably totally wrong - they could detect stealth by using extremely high power phased array radars and then calculating the doppler shift of the tiny amount of energy reflected off of a stealth aircraft.

Since a stealth aircraft is traveling at hundreds of knots, this works because while the radar is also reflecting off of flocks of birds, the ground, maybe even currents in the air, as well as picking up it’s own sidelobes, none of these things have a frequency shift.

In an old video game that tried to simulate stealth bomber flight, you had to not fly directly at or towards a doppler radar for this reason.

Using Doppler shift to pick targets out of clutter is standard stuff dating from the 1960s, having nothing specifically to do with counter-stealth.

It also works in reverse for airborne radar. An airborne radar will take an input from the nav system to know its speed and direction over the ground. Any return whose Doppler shift corresponds to that speed is assumed to be the ground, not a moving target.

When you’re trying to map the ground (e.g. aiming a bomb or navigating versus the terrain) you want to pay attention to the ground return and filter out the movers.

When you’re trying to track aircraft, ships, or ground vehicles you want to pay attention to the movers and filter out the ground return.

A side effect of Doppler processing is that targets moving tangential to the radar can be misread as stationary and hence filtered out in air-to-air mode or incorrectly included in air-to-ground mode. Likewise ground based Doppler radars can develop blindness to tangentially moving targets if the Doppler signal filtering is too extreme.

The only thing target stealth does is make the signal you’re looking for even harder to pick out of the noise.

Doppler processing also opens the door to countermeasures. If you know you’re being painted by a Doppler-type radar you can transmit a counter-pulse at a slightly higher or lower frequency to make the radar believe your velocity is different than it really is. Do that a few times and you can convince the radar’s predictive algorithms that you’re someplace you aren’t. Then stop emitting and the radar breaks lock since you aren’t where it’s now expecting to find you.

Or simply jitter the counter-pulse frequency to preclude the radar from developing useful knowledge of your velocity.

And for comparison sake, the expected unit cost of an AMDR radar (which is the successor to the Aegis SPY-1 radar on U.S. ships) is probably going to cost around $200 million each.

The relevant AMDR talks about it being able to do electronic attack. Aside from jamming, what kind of electronic attacks can be done?
What can the 200M$ AMDR do that the 2.7M$ F-18 radar can’t?

Are shipborne radars like the AMDR and the presently-used AESA limited to the radar horizon of a few dozen nm against surface targets?

How does radar lock work? LSL talks about using a different frequency to fool the Doppler radar. I get why given how the Doppler effect works. I get that the radar would not find you where it expects you to be so I get how a lock would be broken. But then, won’t it find you again on the next sweep?

From wiki: Although it was not an initial requirement, the AMDR may be capable of performing electronic attacks using its AESA antenna. Airborne AESA radar systems, like the APG-77 used on the F-22 Raptor, and the APG-81 and APG-79 used on the F-35 Lightning II, F/A-18 Super Hornet, and EA-18G Growler have demonstrated their capability to conduct electronic attack. The contenders for the Navy’s Next Generation Jammer all used Gallium Nitride-based (GaN) transmit-receiver modules for their EW systems, which enables the possibility that the high-power GaN-based AESA radar used on Flight III ships can perform the mission. Precise beam steering could attack air and surface threats with tightly directed beams of high-powered radio waves to electronically blind aircraft, ships, and missiles.

New radar has increased range and target discrimination capability. Compared to the aircraft radar; it can track many more targets (volume and types - cruise missiles, aircraft, ballistic missiles, drones, etc…) and can control multiple intercepts.

About jamming, based on this: Radar jamming and deception - Wikipedia

It seems that spot jamming wouldn’t be all that effective against military radars of even basic sophistication. Radar hopping must be easier than countering the hopping.

Sweep jamming: It would seem fairly straightforward to program a radar to discard radar images which look very much unlike the others.

Barrage jamming: Would this not be effective only when the jammer has much bigger antenna and power than the jammee? Like a destroyer or ground station jamming a plane.

What is base jamming? How is it done?

I don’t understand how pulse jamming works.

If you really want to know how all this stuff works I highly recommend EW101. It is pricy but you my be able to get it from the public library. We used it fairly extensively on my EW courses, and is a fairly easy read. EA and EPM ( Newer terms for ECM and ECCM ) has always been a cat and mouse game, and like sports, a good defense is always a little harder.

Base jamming is basically directed barrage jamming tailored to a specific radar type. Say your radar can operate from 3-4.5 GHz, you aim your jamming across that span at that specific set.

You are correct about barrage jamming.

Remember my analogy about the warehouse and the flashlight?

Imagine you are in a dark gym or warehouse with a blinking flashlight and no other light source. You start spinning in a slow circle, looking around when you see something on the far wall. You still want to make sure there is nothing else around so you spin faster to see it more often or you can focus on a specific area but that means the rest stays dark. The blink rate is your repetition interval, the spin is your sweep rate, and focusing in one spot is a sector scan. How much are you going to see if someone sets off a strobe in your eyes whenever you look toward them? That’s basic pulse jamming.