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On the earths surface, radar is pretty sweet. It apparently penetrates the atmosphere well and bounces back and can be detected. I think this has to do with the wavelengths absorbed by our atmosphere.

But what about space? For detecting things at a distance (for example - rock on a collision course) does radar offer any advantage over visible light?

I am trying to justify an interstellar ship tricked out in klieg lights, looking for asteroids like Londoners looking for bombers during the blitz.

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    $\begingroup$ Your suspicion is correct. We use radar because atmosphere and phenomena such as smoke, clouds etc. would make using light impractical. Radar also has the useful Earth property of bouncing better from metal than other materials. In space, LIDAR is probably a better choice. Yes, there are materials that reflect light poorly, but there are also those that reflect radar poorly (e.g. ice). Maybe using both could be a thing. $\endgroup$ – LSerni May 9 '20 at 19:21
  • $\begingroup$ To detect things at the greatest distance you'd need to be observing gamma rays. The Hubble has seen the farthest, but it wasn't looking at anything (The Ultra Deep Field). The Swift actually looks at things with its "Burst Alert Telescope (BAT), the X-ray telescope (XRT), and the UV/Optical telescope (UVOT)." – NASA. I'd presume that 'detectability' follows exactly the EM spectrum in descending order of frequency. It depends on what your telescope can detect and what you want to look at. $\endgroup$ – Mazura May 9 '20 at 23:23
  • $\begingroup$ You'd have to justify why there's a part of the EMS that a spaceship can't detect.... or, how you're making emissions that block all sensor readings that haven't irradiated (you, and) the subject to certain death yet. $\endgroup$ – Mazura May 9 '20 at 23:25
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    $\begingroup$ Why limit yourself to one spectrum? If you have the tech to travel through space with practicality, why wouldn't you have the capacity to detect along a vast spectra using both active (emitted frequencies and perceived frequencies) and passive (only perceived frequencies) analysis? Every spectrum has its pros and cons. I'd think you want all the pros possible. Heck, why not tow a drone at a considerable distance with a highly coherent EM lock for gravity wave detection? $\endgroup$ – JBH May 10 '20 at 1:58
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There are several nice things about light compared to radar. Maybe the most important for this question is that you can have a diffraction limited beam. This is because the wavelength of light is much smaller than that of radio waves.

The formula for $\theta$, the half angle of how the beam is expanding, is

$$\theta=\frac{\lambda}{\pi w}$$

where $w$ is what is called the beam waist of the beam, and is often the aperture size of the laser, although one can also put the beam waist somewhere in front of the laser depending on the lens design.

This means that the laser beam is much much more directional than the radar, but far from the laser you can figure out what the radius of the spot size is by taking the angle in radians and multiplying it by the distance. So for the big distances of deep space that probably determines how many photons hit your target per laser pulse.

Another nice thing about light is that you can have detectors that can detect single photons. The amount of energy in a visible photon is a couple of electron volts, whereas a radio wavelength photons is a few milli electron volts.
So, you don't absolutely have to have a lot of light return especially if you have some other information to tell that it is your photons and not some other source of photons. There are a bunch of games that can be played to discriminate you photons from others since you know the wavelength, polarization, timing of the signal and other things about your photons.

An emerging real technology is the use of quantum mechanics to have entangled photons. When the photons are "entangled" when something happens to one photon, it can be observed by measuring the other photon. This is hard to explain in a couple of sentences, but this can be used for communications, quantum cryptography, and with some creative license can be applied in a hard science kind of way for your application of sensing other spacecraft or other applications.

As an aside, many of the LEO satellites and probably others that are being networked together are probably going to be communication via light as an alternative to radio waves. This is partly because of spectrum allocation but light can have higher bandwidth, and rather than large antennas can be smaller packages.

In general for either light or radio wave radars and communication systems the size of transmitters/receivers telescopes or antenna that ultimately determine the amount of energy that hits the target and then how much gets received. With radio waves, since the wavelength is longer and the signal typically has more coherence it is easier to make arrays of antenna dishes and end up with an effective larger aperture. However, you can do the same thing with light it is just harder engineering. For radar you can do beam steering by controlling the relative phase between antenna elements. The more elements you have typically the more directional you can make the beam. You can also do the same thing with light, but it is harder, to coherently beam steer the light from coupled laser emitters is an active area of research.

Optical systems are typically more expensive than RF systems, but there are many applications in space, including ranging to satellites to very accurately get their position and velocity that is being done right now. So it is probably reasonable to extrapolate to deep space applications.

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  • $\begingroup$ excellent comparison between radar and visible light. thank you $\endgroup$ – Willk May 10 '20 at 3:04
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The big problem with using radar in outer space is simply range. The received flux of a radar signal falls off as $1/r^4$ instead of the $1/r^2$dependence we're used to getting for signals emitted by a source far away. The $1/r^4$ arises from the fact that the signal has to travel from the transmitter to the object (a factor of $1/r^2$) and then back to the receiver (another factor of $1/r^2$).

This means that the signal gets extremely weak extremely quickly. We can use radar to detect spacecraft-sized objects at perhaps 10-20 times the distance to the Moon, but beyond that, we'd need transmitters much more powerful than any equipment we currently have. For bodies the size of comets or asteroids, the problem is slightly lessened because they have substantially larger cross-sections than spacecraft, but we still have to deal with the $1/r^4$ problem.

(We of course detect these bodies in the optical by looking at reflected light - this time from the Sun - but the source of that light is extremely powerful, so range is less of a problem. The Sun is obviously much stronger than any radar source we humans could produce!)

Another problem you have is that you would need to get reasonably lucky to detect an unknown object via radar. Space is, well, big, so angular sizes are small, and it would be quite easy to simply miss an object that's very far away. It's much easier to detect something if you have a good idea of where it is. Perhaps radar could be useful for measuring the trajectories of extremely dark, low-albedo asteroids which would reflect very little visible light (although perhaps they would also be poor reflectors of radio waves, too). Performing an all-sky survey, on the other hand, might be a job for optical telescopes.

On the third hand, in interstellar space, far from any light source, it might be easier to find an asteroid with radar because you would be able to generate your own signal. At the same time, of course, any such objects would be exceedingly rare, so the odds of finding one by any method are low.

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    $\begingroup$ Isn't radar light? Does intensity fall off in proportion to wavelength? $\endgroup$ – Willk May 9 '20 at 15:08
  • $\begingroup$ @Willk I should have made that clearer - let me edit. $\endgroup$ – HDE 226868 May 9 '20 at 15:13
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The short and sweet answer is 'no'. The reason, however, may not be what you want. Neither would be very good in deep space as active sensor systems. That is, radar would have no advantages over light, because neither would be particularly effective.

If radar were the preferred method of sensing things in space, we would have scanned Mars with very high energy radar from earth. Unfortunately, we had to get the source of the radar (the satellite) as close to the planet as we could, before usefully using it to scan the surface.

One problem with ANY EM signal, be it light, radar, infrared, long wavelength, short wavelength, is the fact that the time lag is doubled. From the time you send out the pulse and it arrives at the object, the object has moved. By the time the pulse is reflected back to you, the object has moved twice the distance. You have to repeatedly paint the object in order to get any kind of targeting information. So the problem? You can not paint the object at any frequency greater than the period of the time to get the signal to the object and back again, otherwise the return signal is conflated with every other signal. At long distances, this could be hours, or even days.

Another problem, as stated, is the degradation of the signal. Imagine looking at Mars through a telescope. Okay, the details are low resolution. But this image is from generally reflected light doing a one-way trip. Now imagine how much more degraded the signal would be if we had to SEND it to Mars in the first place?

A third problem, is the insignificance of any signal we could send out in the first place, compared to the background radiation noise. Why would we send out a proportionally weak signal to reflect back off the object, when the universe is saturating that object with its own illuminating radiation? Just use passive sensors to detect what the universe is providing anyway. Why shine a flashlight at Mars, when the Sun is a much bigger flashlight?

To be at all useful, whatever active sensor one uses, would have to be in a very directed coherent cohesive beam. I am thinking a laser beam, or lidar. To my knowledge, light is the only edit active sensor beam we can currently get into a coherent beam. But even lidar diffuses at great distances, so it might be possible to know that some sort of object was where you pointed the beam, but the signal would be so diffuse that getting any detail of the object would be impossible. This is information that you probably could have gotten just as effectively using passive sensors to detect radiation given off or incident radiation reflected by by the object itself.

I suggest that any active sensor system to be used in space would have to be something that paints the object with enough energy that the object itself is excited enough to emit its own radiation, in some method that your passive sensors could detect. That is, perhaps paint it with something like a concentrated directed beam of gamma rays and cause it to glow. Do klieg lights give off gamma rays?

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  • $\begingroup$ Passive radar, +1. "In Article 1, Section 1 of the Radio Regulations, radio astronomy is defined as astronomy based on the reception of radio waves of cosmic origin." – www.itu.int (PDF). If the OP is looking for "justification" where visible light does not suffice, then they need 'Radio Regulations'. $\endgroup$ – Mazura May 10 '20 at 0:02
  • $\begingroup$ I think for the tracking problem you may be able to sent a series of closely spaced pulses, or have the single pulsed changing its frequency and watch the changes in the return series of pulses, or changes in frequency to get velocity and direction information. Some dolphins and whales do these kind of things acoustically. But you are certainly correct about the information only telling about things that happened in the past. $\endgroup$ – UVphoton May 10 '20 at 2:11
  • $\begingroup$ @UVphoton The problem with radar to determine distance, is that you send out a pulse and count how long it takes to return. Thus, you need to know which pulse you are receiving. If you send out multiple pulses, you have no idea which one is returning to you. If you have the luxury of several units in different locations, you can triangulate the multiple pulses to get location based on direction, not on time delay, but in space you would need several spaceships spread out. Dolphins traveling in groups probably do this. Each dolphin sends a different frequency to identify where it came from. $\endgroup$ – Justin Thyme the Second May 10 '20 at 3:26
  • $\begingroup$ @JustinThymetheSecond Since you sent out the pulse and the pulse sequence/shape you have more control over the situation than you might first think. Especially if you have an approximate range and can range gate the object you are examining. Modern radar systems do this in sophisticated ways. Modern jamming systems try to overpower the returning pulses or confuse the receiver by messing with the pulse sequences, or send out streams of slight modified pulses. So most radar designs are more than just time delay. A single dolphin can also range gate and frequency shift. It is pretty amazing. $\endgroup$ – UVphoton May 14 '20 at 13:23
  • $\begingroup$ @UVphoton Are they echo-locating or communicating? $\endgroup$ – Justin Thyme the Second May 14 '20 at 19:58
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The best detection of another ship in space is HEAT. A vessel must radiate heat, lest it cooked its crew. All that sunshine's energy has to go somewhere, and no conduction to carry the heat away, nor convection. The only way to get rid of excess heat is to RADIATE HEAT. You may think it's easy to do so in space, but it's not.

For example, ISS has two folding radiator arrays if it requires emergency cooling.

https://www.nasa.gov/pdf/473486main_iss_atcs_overview.pdf

With both radiators deployed on max cooling, ISS can vent about 70 kilowatts/hour. For comparison, that's ONE charge of a Tesla S (when it was first launched, it has since gotten 100 KWh batteries)

So it seems IR detection may be more useful than LIDAR or RADAR, as IR detection is purely passive and generates no signature.

Which also implies that "stealth" designs may be possible, but which direction do you radiate toward? Murphy's law suggests that is the direction enemy will see you from. :D

Do keep in mind that these are all limited by lightspeed. So their effectiveness depends on your ship's travel speed.

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  • $\begingroup$ Isn't heat detected in the infrared region of the EM spectrum (which spectrum includes visible light and radar, among others)? How have you answered the OP's question? He didn't ask, "what's the best EM spectrum?" He asked "is radar better then visible EM?" This could be an answer if it were a frame challenge, but you don't challenge the frame ("the benefits of radar over visible EM are so bad because... that you should consider....) As such, this is a non-answer. $\endgroup$ – JBH May 10 '20 at 2:03
  • $\begingroup$ Heat detection is passive. Radar and LIDAR are active measures. In a way, heat detection is very much like operating a telescope. So I guess I'm saying the same: radar has less range than passive observation. $\endgroup$ – Kasey Chang May 10 '20 at 2:17
  • $\begingroup$ Technically, you do not radiate 'heat', you radiate some form of energy that you have converted heat into. There is no such thing as 'heat', we only measure it by its effects. Usually, when we 'feel' radiated heat, we feel this radiation. When 'heat' is 'conducted', what is transferred from one object to another is 'movement', not 'heat'. What humans refer to as 'temperature', 'cold' and 'hot', is nothing more than an artifact of our sensory system - a sensation our mind creates, like pain. $\endgroup$ – Justin Thyme the Second May 10 '20 at 3:42
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Radar has some advantages...

Earth-based visible light telescopes can typically see approaching asteroids like this: 1999 JM8 OBSERVATORIO ATLANTE http://atlante.org.es/asteroides/53319_1999_JM8.htm

...while the earth-based radar can sometimes give these images: (of the same asteroid) Radar images and computer models of (53319) 1999 JM8 https://en.wikipedia.org/wiki/(53319)_1999_JM8#/media/File:Radar_images_and_computer_model_of_asteroid_1999_JM8.jpg

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Sensing light from an object is passive sensing. Using Radar is active sensing. Passive sensing does not make you more visible. Active sensing means you are transmitting which makes your location easy to detect.

Passive sensors can be fooled or spoofed more easily. in space a dark hull that reflects little light would be hard to see, but it might be easier to see in infrared as it is warm body emitting heat. Possibly using some sort of heat sink might help at least until they are saturated.

Active sensors can also be spoofed, but it is harder. SO called stealth coatings could absorb rather then reflect radar. They might also scatter radar so the echo going back is smaller. You can also deceive radar by rebroadcasting the signal with time delays or frequency shifts.

active sensor usually provide better data when compared to passive sensors. A passive sensor will let you detect there is an object, but radar can also give you the range to the object and often its speed (Doppler shift).

Passive light based sensor could be used for detection and radar for attacking.

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Some of the answers to this question:

Spacewalking in 0.3c - is it feasible? [Generations novel]1

describe the dangers of travelling fast, even in interstellar space where the density of particles is much less than in interplanetary space.

If the "interstellar ship" is travelling fast in interstellar space, at a significant fraction of the speed of light or using a faster than light space drive, then the asteroids it would be looking for would the ones directly ahead of it.

A collision at such speeds would result in the total vaporization of the entire spaceship and everything and everyone in it.

In fact colliding with single molecules, atoms, and subatomic particles would also be bad for the entire ship or at the least for anyone who was in the way of one of those particles as it passed through the ship.

So possibly the interstellar ship would sent out a beam of energy ahead of itself to electrically charge particles ahead of it and then use a magnetic field to repeal them out of its way so it never collides with them.

But that would certainly not work for objects as large as pebbles or gains of sand, let alone for asteroids.

One way to reduce the danger of running into a grain of sand at relativistic speeds would be to make the ship very narrow. Thus there is strong reason to expect that many real interstellar ships would be very tall, narrow, many decked cylinders offering as as small a cross section to hit interstellar objects as possible.

And to detect object above and ahead of them they would use radar beams and brilliant lasers aimed straight above and ahead. In deep interstellar space objects would be lit only by distant starlight and so would be very dim, so a laser would be necessary to increase the illumination of objects by many millions of times to make them visible at a sufficient distance. a similarly an intensely strong radar beam would be necessary to detect objects ahead at a sufficient distance.

Once an object was detected ahead it would presumably be vaporized and ionized by intense microwave or laser blasts and then be repealed out of the way by the ships magnetic field.

Or if the object was too large the ship would have to change course to avoid it.

If the ship was travelling at relativistic speeds that would be many, many, many times as fast as the normal speeds of interstellar particles or objects. There would be some danger of being hit by objects coming from the side, but they would be travelling very, very slow compared to the objects ahead that the ship would be rushing toward at relativistic speeds. Thus the radar or bright lights used to detect dangers coming from the side would be much less intense since the distance those objects would need to be detected at would be so much less than for objects ahead of the ship.

And possibly someone might want to calculate what the danger of hitting things in interstellar space is, and what the necessary distance to detect those objects in time would be, and whether using lights or radar would be worth the energy used.

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  • $\begingroup$ A good overview but I am really looking for a comparison of radar vs visible light (like the lasers you mention). We use radar to look for distant things on Earth. Is there are reason to choose radar over lasers in space? $\endgroup$ – Willk May 9 '20 at 18:07
  • $\begingroup$ @Willk maybe against long range or space sniper or camouflage type of warfleet? $\endgroup$ – Li Jun May 10 '20 at 5:00
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Given the vast distances between luminescent objects in space, visible light is merely an illusion. When you the sun in the sky it's an 8 minute old image. Stars in the sky at night are hundreds, thousands of years old. Be it radar or light, All electromagnetic signals travel at light speed. Radar in space already exists. Radar based weather and observation satellites. Synthetic aperture radars for studying planets have been deployed for years to study and accurately geographic map Mars and Venus. Visible light cannot do that but can be used for high resolution visual maps. enter image description here

If you're talking about interstellar spacecraft, sure you could. The Problem with radar in space is Range and power. Most of the radars deployed on our spacecraft had limited power so the radar range was poor and the penetration limited. Assuming you have a vast power supply like a nuclear reactor or something else then a high power, high bandwidth, high data stream radar is doable. Also take into account power output, radar cross section of the receiver and what bandwidth. So yes Radar works, however the system would be rather large to be very powerful, for example the SBX-1 Naval radar is quite the enormous machine, so massive it fits on board a floating oil platform. the Radar dome is over 100 feet tall.

There's other sensors like LIDAR, which works like radar but instead uses lasers not radio waves.

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