Talk:Low probability of intercept

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One of the major characteristics of an LPI radar is that is has small side lobes?

Is this really correct? However big the side lobes are, the main lobe should always be several dBs stronger, which makes it the most probable part of the radar signal to detect. Could someone please explain the sentence above?

europrobe 17:44, 2005 Feb 13 (UTC)

My understanding is:

• Minimising side-lobes allows the use of lower total power for the same sensitivity. In addition, signal processing can become somewhat easier.
• Larger side-lobes increase the chance of detection by objects not being tracked in the main part of the beam. You can't avoid radiation reaching the target(s) being tracked but you can hopefully avoid it being detectable from other locations which may contain sensitive instruments.
• Side-lobes are effectively wasted energy and only contribute to the detectable signature, not the resolution or detection capabilities of the radar.

This is not to say you must have small side-lobes to create an LPI radar, but rather that the minimisation of side-lobes contributes to making a radar more difficult to detect. I don't have a degree in electrical engineering though so if you do you almost certainly know better than I do. Nvinen 23:51, 13 Feb 2005 (UTC)

"If the sidelobe clutter is effectively a much larger scattering center than the mainlobe airborne target, then the detection process could easily produce a type II error and the threat target will go unreported. One approach to rejecting clutter competing with mainlobe targets is to lower the sidelobes of the antenna to the point where unwanted interference is suppressed to below the thermal noise floor of the receiver. Then only mainlobe clutter will remain, and these unwanted returns can be rejected by Doppler processing." http://www.prometheus-inc.com/asi/algebra2003/papers/wicks.pdf

This suggests that if you can cut sidelobes you can reject clutter better, which to me suggests that you may be able to get away with using lower power levels for scanning, for example. It seems that from what I'm reading, the main reason for reducing side-lobes is to prevent radar characterisation for jamming.

Here's a quote, although it's pretty non-specific and sounds like a sales pitch:

"[The Master-S radar's] advanced design features including a low-peak-power distributed solid-state transmitter and an ultra-low-sidelobes antenna, make Master-S a highly survivable radar – as hard to jam as it is to detect." http://www.armedforces-int.com/category.asp?pubID=15&catID=874

Possibly the wording of the article is bad. Maybe it should say "modern LPI radars also tend to have small side-lobes"? Nvinen 00:03, 14 Feb 2005 (UTC)

Yeah, I think a rephrasing something like the one above would be appropriate. Most often, interception of radars is aimed at search radars, where the main lobe will (nearly) always be the highest strength signal to look for. True enough, radar designs tend to minimize side and back lobes, but this is primarily to reduce clutter and vulnerability to jamming, and to lower the power requirements. Interception of fire control radars is usually only done to determine if you are being tracked or not, and if you are, it's the main lobe that's pointing at you anyway.

europrobe 18:35, 2005 Feb 14 (UTC)

I think you're mostly correct but I feel that one of the important advantages of an LPI radar for a fighter aircraft is to make it harder for people to notice that you're looking for them. I imagine fighter pilots spend a fair bit of time in track-while-scan modes (I know I do in flight simulators), always on the lookout for new threats and targets. Your radar typically scans a sort of distorted cone in front of you, anyone in that cone is swept with your radar as you search that space. However, I believe it is still possible for people outside that cone to detect your radar and know that you are there, and they may even be able to determine the type of radar and therefore the type of aircraft you are flying, by your radar characteristics. This is invaluable information to anyone who may be threatened by you. What's more, people who are not in your frontal cone may threaten you seriously because they could approach with their radar off, homing in on your radiation, and attack you at close range without you noticing. This is the main reason that I was suggesting that reducing side-lobes is helpful from an LPI point of view - in order to virtually guarantee that if anyone is detecting your radar, you are also detecting them. Granted, side-lobes are usually a lot lower strength than the main part of the beam, but since you have to get returns off the targets to detect them (and thus get strength dropoff with something like distance ^ 4), they only have to pick up and analyse your energy after a one-way trip (something like distance ^ 2), then theoretically despite the lower power your sidelobes should be detectable at moderate ranges.

As I said, I'm not an EE, this is some conjecture based upon my knowledge of how fire control radars work and some high-school/college physics understanding, so I suppose we shouldn't make the article sound like I'm an absolute authority on this subject. Nvinen 22:55, 14 Feb 2005 (UTC)

[edit] But what is the definition of Low Probablity of Intercept?

When is a radar LPI? How low must the antenna sidelobes be to be LPI. How low must the transmit power be? How wide must the frequency spectrum be? How much pulse compression must be used? How sophisticated must the waveform be? How many of the techniques must be used to be considered LPI? Can "LPI-ness" be measured?Is there a standard definition of Low Probability of Intercept?

Similar questions for Low Probabilty of Detection (LPD). --199.73.74.5 17:37, 10 April 2007 (UTC)lkremer