Countering Free-Electron Lidar?

Question

In my hard sci-fi setting, there is a technology called free-electron lidar, usually deployed in space. Edit: Essentially, it is a lidar with the ability to change its laser frequency.

One free-electron lidar station can scan the entire celestial sphere with a laser beam in five ten minutes, and it does the scan repeatedly, using a random laser wavelength between extreme-UV and far-IR for each scan. Since no known material is good at absorbing all of these wavelengths, reducing your observability in front of a free-electron lidar would be very difficult.

And my question is: How would you reduce the observability of your spacecraft in front of a free-electron lidar? Any progress on reducing observability would help.

Assume the following:

• Your spacecraft has nearly no heat signature or exhaust. (You can think of it as an RTG-powered spy satellite or the like.) Furthermore, it dumps the heat of incident solar radiation by vaporizing liquid helium, so its hull keeps at 3K.
• Your spacecraft weighs a few tons. (It has no organic living crew.)
• The beam power of the lidar station trying to detect your spacecraft is 1MW 1GW.
• Moving away from the lidar station is not a valid answer. Your spacecraft's mission requires it to spend at least an hour within half a million 25 000 kilometres of the lidar station. Edit: I.e. the design of the spacecraft should be such that the lidar can NOT detect it at a distance greater than 25 000 kilometres. And the closer it can get before getting detected, the better.
• Similarly, your spacecraft can not hide in front of or behind a celestial body for a prolonged period.

Last but not least, I would like answers that conform to the laws of physics we know today. Thank you.

Answer 1

We've already solved this problem: absorb or deflect the photons

1. There's nothing magic about lasers or Lidar. (And there's really nothing magic about free-electron lasers, that's just a method for causing light to lase. Lased light is lased light.)

2. Also, there's nothing magic about light or radio waves — they're all photons at different wavelengths in the EM spectrum. Photons be photons whether you're watching TV (Lidar) or listening to your favorite radio station (Radar).

3. Finally, Lidar and Radar both work on a simple principle: send out a pulse and measure how long it takes to return. If it never returns, there was nothing there to reflect it, right? Increasing the resolution of the image means increasing the number of pulses-per-second during the sweep.

So the easiest solution to the problem is one that humanity is employing right now to be stealthy in the face of Lidar's cousin, Radar.

• Use appropriate EM spectrum absorbing materials to keep the light from bouncing back to the Lidar detector.

• Use angled panels on the surface of your ship to deflect what you can't absorb. Remember, the goal is to keep the light from bouncing back to the receiver. If it never returns, you look like empty space.

One last thing...

Since you're writing hard science, keep in mind that no information moves faster than the speed of light. If you emit a pulse from a satellite in Earth orbit with the hope of seeing something in Mars orbit, then at its closest you need 6 minutes (3 minutes each way) for the pulse to get to Mars and back. At its furthest, you need 44 minutes. With just a 5 minute limit your "celestial sphere" has a radius of only 2.5 light-minutes, which isn't even the shortest distance from Earth to Mars. You're also limited by how quickly you can rotate the emitter/receiver assembly or assemblies. There are ways you can speed that up, but they come with a cost. The further away something is, the harder it is to detect because it would take a WHOMPING LOT of pulses at the emitter to drop one pulse per radial kilometer at the 2.5 light-minute extent of your sweep.

This also means you only know where the object was in the past. 3-22 minutes into the past in our example. Multiple readings means you can track where the object was and possibly predict where it will be. But you'll never know where it is.

Which brings us to our last method...

Keep your distance...

The further away you are from the emitter, the more likely the size of your ship will fall between pulses and never be detected at all.

Granted, that last step violates your rule that the ship must come within a half-million km of the station, but it's still a valid point and it still requires a LOT of pulses even at that short distance to predictably detect something as small as ship with a mass of only a few tons. That's one-hundredth of the size of the Ecstasea luxury yacht, which weighs 585 metric tons, and is only 11.5x86 meters. That makes your ship 5 metric tons and 1x10 meters.

You probably could make it out of mirrored material and would have a good chance of never detecting it.

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Fun and Giggles...

You don't indicate what the maximum detection distance is, so let's assume it's the maximum distance from Earth to Mars: 22 light-minutes. What density of Lidar pulses must be emitted to guarantee the detection of our previously described ship? That means you need a grid that's at least, for convenience, 0.9 meters maximum width at maximum distance.

• r = 3.957e+11 meters.
• Surface of a sphere: 4πr² = 1.96762e+24 square meters.
• Grid: 0.9x0.9 = 0.81 square meters.
• Number of pulses in one complete spherical sweep: 2.42916e+24 pulses
• Complete sweep in 5 minutes means 8.09720e+21 pulses-per-second or 8.0972e+12 per nano-second.

Your three-dimensional array spinning at considerable velocity must have enormous power and billions of optics to make this happen. And that doesn't count the detectors, which must be in exactly the right place at the right time to determine the timing of each individual pulse. The sphere's navigational capabilities are impressive....

This is going to strain the credulity of a hard science story. Unless you're trying to detect something much, much, much closer. You might consider limiting the effective range of your detection system to a handful (maybe ten) of light-seconds.

However, it might be uber-cool in your story to discuss the weaknesses of using pulsed light to detect anything in space. Remember what Douglas Adams said, space is big... really, really, really big....

Answer 2

Probably the best tactic is shape.

Just as even the WWII Horten 209 (flying wing jet fighter that never flew) had reduced observability simply due to its shape reflecting very little radar energy back to the source, if your intruder knows the FELidar is there (and how could they miss a periodic beacon of huge power level, even if it's frequency-agile?), they need merely point a needle-shaped ship directly at the FELidar transmitter. The very steeply sloping sides of the ship will reflect energy away from the FELidar installation, not back to it, making detection very difficult.

This can actually be improved by giving the ship a mirror polish over as wide a range of wavelengths as possible (plating with gold overlaid with vacuum coated aluminum might be a start). Even close up and by eye, it would be hard to tell what such a ship actually is; all you'd see if the reflections of the universe (and yourself and your own ship, of course).

A larger problem might be the helium cooling exhaust; that will have a non-zero and unmaskable reflection and absorption signature -- and after a bit, will be far larger than the ship itself, while still millions of times as dense as the interstellar medium (which is mostly hydrogen ions, with very different spectral characteristics).

Answer 3

Blind the lidar

Just like a camera has a range of wavelengths in which it operates, so does a lidar. A camera can be temporarily or permanently be defeated by shining certain strong wavelengths at it, overpowering the sensor(s). This is responded to in certain ways. Covering the camera to prevent further overexposure that might cause damage. Reducing the sensitivity of the sensor. Turn it away from the source. Each can still damage the sensor if overwhelmed.

The free electron lidar has to have some sensors to catch the reflections. But it is expected to only sense the reflections of an insignificant amount of the 1 MW. This is because per distance away from the station the power is exponentially reduced.

Put a free electron lidar on your spaceship. Make it directional. Overwhelm the sensor(s) all at once. Even if they know your initial location they will be blind after. Hopefully enough to damage or destroy the sensor, or otherwise blind them for some time. There can be strategies where you can keep blinding your opponent every time the sensors turn back on. If the time between the sensors turning on is long enough and you can move well enough you can be gone from any potential attack.

The drawback is that they know something is there, unless you can mask it as a natual phenomena, but as the ship seems invisible to basically any other form of detection you should be fine.

Answer 4

The tiny target is not detectable at half a million kilometers. They don't need to hide from the radar, because the radar cannot see them.

The problem is that radar is grossly underpowered. It has no hope of detecting the tiny target at 500,000 kilometers given the small available power and the short 5 minutes to scan the entire sphere.

Being generous, we will assume that 1 MW is the average power.

The question constrains the radar to scan the entire sphere in five minutes. Since the radar is supposed to be able to detect stuff at 500,000 kilometers, and in order for it to detect stuff it must wait for the echo to come back, it must wait at least 2 × 500,000 / 300,000 = 3.3 seconds between impulses. There are 300 seconds in 5 minutes, so that it has only 91 impulses to scan the entire sphere. Since the average power is 1 MW and there are 0.3 impulses per second, it follows that it puts 3.3 MW in each impulse, covering 1/91 of the sphere.

^{In practice such a radar would have multiple independently steered subunits. This changes nothing, because we are speaking of overall averages.}

A few tons of spacecraft is not a large spacecraft. Let's be optimistic and assume that it has an effective radar cross-section of 100 square meters, because the manufacturer did not care about radar.

Since each impulse covers 1/91 of a sphere with a radius of 500,000 km, and the target is 100 square meters, and each impulse is 3.3 MW, we compute that the target will receive about 0.96 nanowatts of power. Since we are extremely optimistic, we assume that it reflects all that power back into a solid angle of 1 steradian.

How large is the receiving antenna of the radar? Don't know, and the question doesn't say, but let's continue being generous and assume 1 square kilometer. (That is a really really large antenna.) Applying the same considerations, the power of the echo received back by the radar antenna is about 38 attowatt, 3.8E-20 W.

To put this in a better light, 3.8E-20W is about 0.2 electron-volts per second. Good luck detecting such a signal.

Take home message: The power of the radar echo goes down with the fourth power of the distance between the radar and the target. Half a million kilometers is far away. With a radar with the average power of 1 MW, scanning not the entire sphere but just a tiny tiny solid angle, you can detect a planet at that distance. Not a small spacecraft.

Answer 5

Frameshift: As many people have pointed out your emitter is both impractical and very easy to defeat. The impractical part I can't help you with, but there's a solution to make it much harder to defeat: It's just an emitter. The receivers are at a substantial distance from it and are purely receivers, they broadcast nothing.

Stealth shapes are based on reflecting the energy away so as to ensure it doesn't get sent back to the transmitter. However, it gets bounced somewhere, all stealth shapes can be detected by receivers in the right location that know exactly what the transmitter is doing. Detection gets even more problematic for them as they will only see something where the beam from the transmitter intersects the cone the listener is looking at. It's not impossible, though.

As for how to counter that--you'll need a hull that can reshape itself to some degree. (Not insane in a spacecraft--it can be just thin panels outside the true hull. Think a blocky, angular thing like the F-117, not the smooth curves of a B-2. The F-117 is a lot easier to model and in a spacecraft there's no penalty for such a shape. The F-117 was built that way because the computers of the time simply couldn't handle crunching the numbers for smooth curves.) They will have to know where all the receivers are and ensure no hull panel will bounce the incoming beam towards any detector. (Note that the smooth curves of a B-2 fare far better against an unknown separate detector, but the blocky approach fares better against known detectors. The smooth curves leak a little bit to every receiver not with the transmitter, the blocky approach leaks either nothing or a whole bunch.)

Answer 6

Fake signal, masking presence

A lidar is blind until it received the sent signal back. But what if... Your ship also had a receiver, managed to decrypt the lidar signal, and with the help of an AI, you can try to alter the sent signal by sending another signal and making them combine.

Detect the signal, Decrypt its contents, get the waveform data etc etc. Then, fit it inside an AI and find the potential reflected signal, and then send another signal to distort the original signal just enough to make yourself look like another celestial object, or maybe a meteorite. Considering the timing of the signal, ship may send some probes, check for the cycles of lidar and emitting angle, then calculate the signal and send the distruptive signal back with the right timing, messing up the lidar readings and reflected signal back.

What shape?

Make your ship's shape closer to a meteorite. This way, even if the ship was detected, if you can arrange your orbit and move around planets in a believable way, you can trick the Lidar. This is a large celestial body. Meteorites are everywhere. Mimic one of them. Or just outright strap a cluster of meteorites around your ship. Why not?

Comet cloud

You may not "hide" behind a celestial object, but you may try to "make" or "mimic" a celestial object's properties. Basically, lidar will hit and reflect. Right? Comets has a special tail that may be seen in lidar (I am not sure. Add a bit suspension of disbelief maybe. This is sci-fi.) If it is possible, your ship may emit a comet like dust cloud around it and arrange its movements by taking into account of nearby stars, effectively blocking most of the ship details and acting like a celestial object. If you can, try to use properties of other celestial objects as well.

Just distrupt the signal, confuse the receiver

The first idea, but forget the AI and calculation. Constantly emit a signal in different waveforms, or just detect the waveform cycle if it has any, then send bunch of signals to make the reflected signal unreadable. Or send so many information to overload the sensor, maybe the circuits or the algorithm. You see, algorithm design is a constant battle between optimization and advanced calculation. You need to have just enough processing capability while palcing as little burden on hardware as possible. If some signals were, too out of algorithm's scope, then it may not even detect it or give errors. Send some probes to expand the "error" zone and they have to figure out what is happening. "Quick! A planet sized error zone is detected! And it is expanding! Fast!" You may not hide behind an object for long, but you may hide probes and devices temporarily to start building up this error zone.

Lights refract.

Make a transparent cover around your ship that will refract the light and "bend it around" the ship, then make the signal go to ad infinatum. Make sure to not stay in front of a celestial object, or make the refraction both ways. So the refracted light bounding off objects behind, can refract back around the ship and to the receiver.

Kaboom

Just shoot the lidar with a long distance energy weapon if you have. Energy isn't reflective and if it is like an IR or ultraviolet, detection is the same as getting burned by it. No one can see you if no one is there to see. An EMP wave could also work. Frying electronics.