radar detection infantry movement
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Big_Zucchini
Well-Known Member
Hi,
The short answer is no.
The long answer begins with them simple question of "why?".
Some radio and radar basics to make this as simple to understand as possible.
Disclaimer: I have experience in radio tech, but only started working on radars weeks ago, so I'm not too keen on nuances of radars, but the physical principles are identical.
So let's go with the best case scenario of a 50MHz radar (we want as high a frequency as possible). Frequency has an inverse linear relation to wavelength. Lower the frequency = higher the wavelength. At 50MHz we have a wavelength of ~6m.
Why is wavelength important? Because it means our radio waves will be reflected by objects that are related to the wavelength.
The rule of thumb is a quarter of wavelength, or in our case 1.5 meters.
Right off the bat, with an ideal but most simple radio system, we can theoretically detect a human with a height of 1.5m, assuming we are presented with that size. Meaning, if he's prone but perpendicular to us, we'll still see him. But if he's prone but facing us, he will be invisible to us.
But merely detecting him is not enough. How do we know that's really a person and not a bush, tree, or an animal? Against a cluster of other objects, said person will again turn invisible on us.
Since physical properties have put a limit in front of us, we have to use advanced methods to improve our resolution and detect finer objects.
The straightforward methods are:
1. More antennas - by adding more antennas, we increase the aperture of our radar. Think of it as if you are looking at an object in the distance with your eyes, and the longer you look at it the better you understand that object.
2. SAR (Synthetic Aperture Radar) - moving our antennas relative to the object allows us to grab multiple takes and thus analyze it better. Same as us observing multiple photos to gain more knowledge.
3. ISAR (inverted SAR) - moving the object relative to the static antenna, and not the other way around.
All of these essentially require a complex system, especially on the side of hardware. And that's where another big problem comes in - radio hardware size, much like wavelength, has an inverse ratio to the frequency. So if we look back to our rule of thumb, the minimal size of an antenna would be 1.5m (in one dimension). The standard would be twice that, so 3m. And we need multiple. By multiple I mean usually a dozen at the minimum. Civilian radars have mere dozens of antennas. Military radars for long ranges can have thousands, depending on frequency. Distance between antennas must also be kept, and it's also half of wavelength, or in our case also 3m.
HF radio systems for military tend not to have many antennas and yet are quite enormous. Certainly not something you can add to an AFV or any mobile asset. It would be a static.
If you're building a huge radar station, would you really want to dedicate it to an extremely inefficient search of humans?
Large stations in the VHF and UHF range serve as early warning against aircraft which are obviously much larger than humans, and even then, early warning means "look, there's something in that direction, go check it out", rather than "enemy fighter over there!".
A more practical radar would be close to 100GHz. Automotive radars are 77GHz as a standard, and even they need advanced algorithms to properly recognize humans.
Hope this answers your question.
The short answer is no.
The long answer begins with them simple question of "why?".
Some radio and radar basics to make this as simple to understand as possible.
Disclaimer: I have experience in radio tech, but only started working on radars weeks ago, so I'm not too keen on nuances of radars, but the physical principles are identical.
So let's go with the best case scenario of a 50MHz radar (we want as high a frequency as possible). Frequency has an inverse linear relation to wavelength. Lower the frequency = higher the wavelength. At 50MHz we have a wavelength of ~6m.
Why is wavelength important? Because it means our radio waves will be reflected by objects that are related to the wavelength.
The rule of thumb is a quarter of wavelength, or in our case 1.5 meters.
Right off the bat, with an ideal but most simple radio system, we can theoretically detect a human with a height of 1.5m, assuming we are presented with that size. Meaning, if he's prone but perpendicular to us, we'll still see him. But if he's prone but facing us, he will be invisible to us.
But merely detecting him is not enough. How do we know that's really a person and not a bush, tree, or an animal? Against a cluster of other objects, said person will again turn invisible on us.
Since physical properties have put a limit in front of us, we have to use advanced methods to improve our resolution and detect finer objects.
The straightforward methods are:
1. More antennas - by adding more antennas, we increase the aperture of our radar. Think of it as if you are looking at an object in the distance with your eyes, and the longer you look at it the better you understand that object.
2. SAR (Synthetic Aperture Radar) - moving our antennas relative to the object allows us to grab multiple takes and thus analyze it better. Same as us observing multiple photos to gain more knowledge.
3. ISAR (inverted SAR) - moving the object relative to the static antenna, and not the other way around.
All of these essentially require a complex system, especially on the side of hardware. And that's where another big problem comes in - radio hardware size, much like wavelength, has an inverse ratio to the frequency. So if we look back to our rule of thumb, the minimal size of an antenna would be 1.5m (in one dimension). The standard would be twice that, so 3m. And we need multiple. By multiple I mean usually a dozen at the minimum. Civilian radars have mere dozens of antennas. Military radars for long ranges can have thousands, depending on frequency. Distance between antennas must also be kept, and it's also half of wavelength, or in our case also 3m.
HF radio systems for military tend not to have many antennas and yet are quite enormous. Certainly not something you can add to an AFV or any mobile asset. It would be a static.
If you're building a huge radar station, would you really want to dedicate it to an extremely inefficient search of humans?
Large stations in the VHF and UHF range serve as early warning against aircraft which are obviously much larger than humans, and even then, early warning means "look, there's something in that direction, go check it out", rather than "enemy fighter over there!".
A more practical radar would be close to 100GHz. Automotive radars are 77GHz as a standard, and even they need advanced algorithms to properly recognize humans.
Hope this answers your question.
Existing military radar systems for infantry detection work in X-Band (8-12 GHz) and can in principle detect a walking human at 4-5 km distance. I say in principle because the azimuth accuracy of most such systems over such distances tends to be rather atrocious, in particular compared to electrooptical systems with some sort of pattern recognition analysis behind it.
I would assume OP's question about HF is due to him having read somewhere about over-the-horizon propagation by groundwave or skywave in that frequency band.
If I put "hf band radar military" into Google, the first hit i get is a 20-page document written by an Australian for NATO's research agency titled "Target Classification, Recognition and Identification with HF Radar". Mind you they're talking about targets with somewhere on a scale of 1000 times the RCS of a human.
What's your take on the use of systems like the Fara surface to surface radar? It's claimed to detect people from iirc 3 kms away and it certainly doesn't look like a large and complex system.
Big_Zucchini
Well-Known Member
Human detecting radars certainly don't need to be large or look complex. They can be something you can hold in one hand.
Cheap commercial ones can go for a few dollars only. Maybe $20. At least in fabrication costs.
Military grade ones will just look large because you want to harden the antennas, add some tripod, manual control options, and you bring your own power source.
The Fara-VR definitely looks legitimate. Its specs are nothing to boast about. Definitely old tech or very minimalistic algorithms. But for its intended job, it doesn't really lack anything.
Sure, modern radars can track not 3 but 300 targets at a longer range and with better angle and range accuracy, better beam forming (alteration of the radiation pattern), lower false alarms, and even scanning and tracking simultaneously (MIMO radars).
But the Fara, just by existing, brings 90% of the value.
Extra points for mounting on HMGs and AGLs.
That's been my impression, it detects some human movement in a location, and you then have area suppression weapons ready to fire on that location, especially your AGLs. I'm wondering if the Soviets were right when they thought that surface-surface radar will be standard for armored vehicles in the future.
Big_Zucchini
Well-Known Member
They definitely have a place in current and future AFVs.
If not for detecting enemies then for autonomous driving, active protection, weapon cueing, ballistic correction/calibration (NLOS munitions), BDA, and generally augmentation of other sensors (sensor fusion - more than sum of sensors).
There will however be a parallel process to reduce reliance on radars and Lidars, as they are emitting detectable radiation.
So in addition to radars I believe we'll see passive radar technology implemented on AFVs.
For those who don't know what passive radars/sensors are - passive means non-emissive. Or in other words, it relies on emissions from other sources. It allows getting indicative information.
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I like to replay regarding antenna size, I'm not playing with radar, more with SDR. I bought airspy HF+ dual port, specially made for HF frequencies. When I plan to buy antenna, personally have active loop MLA 30+ antenna, but also I read lots of article, and one of them also mention antenna size with generally rule 1/4 of wave length, one person modified dipol antenna which came with rtl-sdr V3, he simply move ground and connect 1/4 of wave length coaxial cable 50 ohm, and he simply wrapped cable in circle. In that way he made perfect antenna just for one frequency, but it's also much better for other frequency with in 0-30MHz range than telescopic antenna (example) with size of 0.9m. You also have metamaterials, in that way you bypass length antenna gap, so hardware don't actually need to be that big. This person when they plan jammer for FM, he made coil and with coil and modulated capacitor he actually define frequency for FM band, antenna is not that big, FM is from 88-108 MHz, not so far from 0-30MHz wave length.
Do you know about correlations between distance from radar and human, whether the "HF radar" can map smaller objects like human, if we cut distance between radar and human.
Big_Zucchini
Well-Known Member
You mentioned a lot of niche tech with which I am not particularly familiar, certainly not metamaterials.
But what is important is not really the antenna itself, but rather what you transmit, how it reacts to the objects at hand, and how it is later processed.
Even if you somehow managed to make antennas below 1/4 of wavelength, and thus create a small array of many antennas so you can beamform, you're still stuck with the 50MHz EM wave, which would simply not bounce back from a person in most scenarios, and even then would be near impossible to discern even with modern algorithms.
We pick wavalengths sometimes by limitations of SWaP (Size Weight and Power), but in the vast majority of cases, we pick them according to our needs. If we wish to detect large objects, we won't bother with high frequencies. If we wish to detect fine objects, we won't even think of low frequencies. And if it's communication, then according to desired bandwidth.
If you already have the equipment, try experimenting by yourself. And if you manage to track a human while crouching or prone and with good reliability, then I'll be happy to hear that. Until then, physics are your enemy.
As for your question on relation between distance and resolution of size, then there is no effect by changing distance, unfortunately.
There are parameters that determine a radar's maximum range (beyond which objects' distance becomes ambiguous, i.e unknown), but they do not help us here. You are strictly limited by wavelength.
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