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The Question about the blockade of en entire star system makes me wonder:

How would, if possible, a space "radar" work? Or how would you scan for Ships further away than your Optical sensors can see.

The big space makes is semi impossible to look for the reflected radar rays. 0-100 Eyeball also does not work because of the hugeness of Space, same as all other optical installments.

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    $\begingroup$ @MichaelKjörling detection at all would be a first step - to know there is something is more important than to know on when it was there, orbit prediction and linear prediction should be no problem after the ship is noticed. $\endgroup$ – Fulli Mar 12 '15 at 13:45
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    $\begingroup$ From someone with knowledge how radiowaves work, this question is absurdely broad. $\endgroup$ – Jorge Aldo Apr 23 '15 at 0:35
  • $\begingroup$ Do you wish to account for relativity? what-if.xkcd.com/imgs/a/140/general.png $\endgroup$ – Draco18s Feb 10 '16 at 20:43
  • $\begingroup$ I wonder if laser distancing would be an option. You could send out beams in all directions and anything within the effective range of the lasers could be measured for distance. I would assume from your world laser technology would be better than current so it could be possible to have a beam maintain it's intensity enough to measure over a very large distance. Covering and measuring a response over every possible angle would be a feat though, so the smaller a vessel the closer it could potentially get to the sensor without detection. $\endgroup$ – アキオ Apr 25 '16 at 10:56
  • $\begingroup$ The Arecibo Observatory is essentially a massive radar system which has been used to map the surface of Venus through the cloud layer from Earth. The issue isn't emitter power, but rather the sensitivity of the receiver. Since we can still receive the Voyager probe signals from the edge of the solar system despite having an effective power similar to a 5 watt lightbulb, ordinary radar works just fine with a large and sensitive enough receiver. $\endgroup$ – Thucydides Aug 1 '17 at 16:15
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In the article "Detection" on the website Project Rho says, "There Ain't No Stealth In Space". Any spacecraft will create some kind of emission which can be detected. The most common is infra-red radiation because any space ship will generate waste-heat. That's an unavoidable side-effect when any form of energy is consumes on-board.

With a common infra-red detector you might be able to detect that there is a source of infra-red light, but you might not be able to pinpoint its exact direction. But when you have multiple detectors, you can use triangulation to pinpoint the exact position of the source.

When you know the position of the source but want more information about it than can be detected from passive infra-red, you can then illuminate it with active sensors like a laser- or radar-beam aimed exactly at the source.

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    $\begingroup$ "But when you have multiple detectors, you can use triangulation to pinpoint the exact position of the source." At some earlier point in time, yes. Unless you're talking about detection at the level of a few lightminutes away at the very most. $\endgroup$ – a CVn Mar 12 '15 at 13:24
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    $\begingroup$ @MichaelKjörling In a hard science-fiction universe FTL detection is just as impossible as FTL travel or FTL communication. $\endgroup$ – Philipp Mar 12 '15 at 13:32
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    $\begingroup$ I was thinking it would be possible to run cold (no thrusters) and mask your heat, but I'm guessing this is one of those inconvenient "nothing is 100% efficient, therefore waste heat" deals? What about radiating your waste heat in a specific direction? $\endgroup$ – Dan Smolinske Mar 12 '15 at 13:33
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    $\begingroup$ @DanSmolinske I also definitely recall seeing something about radiating waste heat in a specific direction as a means of cloaking here on the site. Bottom line there: you don't know which direction to radiate into, and you probably can't radiate in a narrow enough cone anyway. $\endgroup$ – a CVn Mar 12 '15 at 13:37
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    $\begingroup$ @Philipp Project Rho consistently, severely overestimates the capabilities of sensors. Unless you're using really long exposure (which fails with moving targets), even relatively big objects at room temperature do not emit enough blackbody radiation to trip a pixel on a sensor chip over Moon-Earth distance. $\endgroup$ – Mike L. Mar 12 '15 at 14:48
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looking for reflections of sunlight IS radar. Just with receiver (your sensors) and transmitter (the sun) at different positions.

Using a different transmitter would not gain much though looking beyond the visible spectrum would, just about every energy using object will emit black-body radiation. You can look for that.

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It's Complicated

There's no stealth in space - you can't be invisible. But that's not quite the same as saying your enemies can't hide. The most obvious tactic would be a large-scale decoy attack.

Now, it's hard to create decoys for moving ships, because engines and fuel are expensive, and the energy of the thrust correlates to mass as a general rule. So if you want to create a decoy for a 20-thousand ton ship, you need a 20-thousand ton decoy. You can strap an engine to an asteroid (and do some outside cosmetic work to make it look like a ship), but in general this will be too costly for common use. But that doesn't mean we can ignore it as a possibility - it only takes a few attacks to get through to ruin your day, so that means that it could still be cost-effective.

Ballistic ships, on the other hand, are relatively easy to decoy. You just need a shell that looks like a ship, and some systems to mimic the radiation signature your normal ships give out. The downside is that once the attackers are forced to maneuver, the decoys stop being, well, decoys - it becomes obvious which ships are real and which are fake (unless your enemy is tricky and has some real ships pretend to be decoys, of course...).

So. Space Radar.

So given the above, what you want is something that fits the following criteria:

  1. Test for as many frequencies as possible. If you just watch one thing (radar, background radiation, infrared) that makes it easy on your enemies to create useful decoys. Watch for as many things as possible, and your enemies have to spoof all of those to fool you.
  2. Multi-tier. You want to detect enemies as early as possible, so you need to have installations spirally outward in all directions. Unfortunately this quickly becomes a scaling problem, and one that's not in your favor - your defense has to grow in three dimensions, but your enemies will still only attack at the point they choose. Doubling your effective detection range involves an 8-fold increase in cost for you, and most of that will be wasted on any particular attack.
  3. Don't be predictable. Unless you have overwhelming force, predictable in warfare is often synonymous with "dead". It might make sense to have a perfectly optimized defense setup to maximize coverage, but optimized is also predictable, and... well, you know what that means. And once your detection is taken out (or at least degraded significantly) then you're in trouble. Thankfully you can also use decoys to make it harder for your enemies to pick out your detection installations and take them out. You should also move them periodically to prevent an enemy from mapping your setup.

Nothing's Perfect

In the end though, you need to accept that if your enemy really wants you dead, then you're going to be dead. There's no such thing as a perfect system - you will never stop 100% of the attacks against you. There's a reason systems aim for things like 99.999% - there's always the possibility that a few things will get through. And unfortunately, in space and with relativistic speeds, your enemy really only needs one thing to get through. Throw enough big rocks at an immobile target, at high enough speed, and it's just a matter of time before your target dies.

Hopefully your enemy wants your planet intact and isn't willing to just bombard you to death. In that case you're ok as long as you keep your ships and installations periodically moving.

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A bunch of people have given great answers revolving around how there is "no stealth in space," but I feel like they might be slightly missing the core of the question.

How would, if possible, a space "radar" work?

Pretty much the same as on Earth. RAdio Detection And Ranging (aka radar) is a system that bounces radio waves off distant objects to see them. Radio waves, being just a specific slice of the light (EM) spectrum, can travel through space just fine. Actually, light travels slightly faster in space than in air.

Or how would you scan for Ships further away than your Optical sensors can see.

As I mentioned above, radio waves are light (outside the "visible" range for human eyes, but still light). Assuming you mean ships that are too far because not enough light will make it into your detector, then the answer probably is "you can't scan for them." Light is a pretty good way to see things. It travels at the cosmic speed limit. It's a wave that's also its own medium (so it goes through open space without a problem). The right frequencies of light easily interacts with most things (it's reflected or deflected by most things), so it's great for seeing things. And, it's easy to detect across a broad range of frequencies.

There really aren't any other mediums of detection as good as light. So if light isn't good enough for seeing something, you probably can't see it. For example:

  • W and Z bosons are force carrier particles, like the photon. Maybe that means they could play in the same weightclass as light ...if their range wasn't so limited.
  • Neutrinos, have a very long range, travel at about the speed of light, and pass through miles/kilometres of rock like it's nothing. Does this mean we can use neutrinos for some super penetrating form of sight? Nope. It passes through things so thoroughly, the Super-K neutrino detection experiment had to be built 1 km underground, as a 40 m by 40 m cylindrical stainless steel tank holding 50000 tons of ultra-pure water, etc. Here's a picture. It takes the analysis of super computers around the world to determine if any single neutrino particles have been detected.
  • Gravity waves, a far reaching disturbance in the very fabric of space, is nearly impossible to detect, especially when caused by small objects. It took building multiple 4 km by 4 km observitories just to detect the gravity waves from colliding black holes. Those waves caused disturbances shorter than the diameter of an atomic nucleon.

The big space makes is semi impossible to look for the reflected radar rays. 0-100 Eyeball also does not work because of the hugeness of Space, same as all other optical installments.

You've just intuitively stumbled over the inverse square law. As Wikipedia puts it, "a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cause for this can be understood as geometric dilution corresponding to point-source radiation into three-dimensional space."

For light sources, this means means visible intensity rapidly drops off. This is why stars far brighter than the Sun are only specks in the night sky (at best). The principle behind this law is also why, as you noticed, it becomes very difficult (very fast) to survey everything within a 3 dimensional space as you expand the spherical area you're interested in looking at.

The inverse square law also means things can, in fact, hide in space, but only at great distances (several AUs at the very least). This is why we might have a proper ninth planet which no one ever noticed. Light traveling out to those distances and back would be extraordinarily diluted. Even the heat generated by a super-Earth to Neptune-sized planets at those distances would not be detected. This is why it'll be years before we confirm or falsify Planet Nine.

So, what does space-based detection look like?

  • Radar (and other light-based detection systems) works in space but decreases in usefulness with distance.
  • Active omnidirectional systems are only good for one's immediate area. Precisely how immediate/large that area is depends on the power output of the antennae and the sensitivity of the detectors. (I.e. you'd need to come up with numbers before someone could calculate the distances.) Whatever the case, we are still talking about how many km.
  • Passive omnidirectional systems would be pretty good at seeing fairly distant ships if the sensors are designed to pick up heat. Even with the inverse square law, a computer-based detection system could spot above average heat spots moving through space. This means it's nearly impossible to sneak up on someone unless there is a large object between them (e.g. a planet or large moon). However, the inverse square law does mean you probably won't passively detect ships more than a few AUs out. As before, the exact distance will vary based on detector sensitivity.
  • Directional systems (active and passive) will have far greater ranges, but you need to know where to look for them to be useful. (Note: active directional systems only work to distances in the thousands of km. Even the best Earth-based lasers are multiple km wide by the time the get to the Moon.)
  • Given that most people in even the most advanced spacefaring society will live on or around planets, moons, and space stations. Directional detection systems will probably be useful for distant areas known to be populated. In that case, ships would have a difficult time sneaking out from home since a directional system could keep an eye on them as the move out into open space.
  • Ships could try to (somewhat) counteract heat based detection by coasting as cold as they can through areas they're likely to be detected. They could also have a design that attempts to direct a good deal of their excess heat in a single direction (one where a detection system is not). Like conventional stealth, neither of these techniques would completely hide a ship.
  • Conventional stealth would still be useful for things like radar. This is more important in close range, when active detection becomes an issue.
  • At any significant distance, light travel time becomes an issue. Mars, for example, is many light minutes away. This means keeping an eye on ships a few planetary orbits out could mean you're looking dozens of minutes to several hours in the past. (Correspondingly, things take months to travel any significant distance.)
  • Detection at great distances and behind obstacles like planets could be dealt with by putting orbital radar systems in place. (That wont help the transmission time, of course.)
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  • $\begingroup$ I think this is the most complete answer. +1 for gravity waves. $\endgroup$ – ESL Sep 6 at 18:20
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For a civilian applications, everyone would "squawk" their own position for everyone else on a known channel. At sea, today, this is done using AIS - and it includes position, speed, direction of motion, and intended destination. Civilian aircraft use a similar system.

For a military application, you would most likely use a passive sensor. It could be thermal (detects heat/black body radiation), neutrino (detects tell-tails of fission/fusion power sources), EM (detects radio frequency leakage from 60Hz / 50Hz electrical power supply, intra-ship wireless comms, etc).

Probably it would be a combination of all the above, plus whatever else works.

These sensors will give you a bearing to your target, but do not provide any range information - without a time a flight you cannot calculate range based on the known speed of light. In order to obtain range, you would have to make an educated guess based on a couple of factors:

1) Relative intensity: particularly if you have observed this vessel or vessels of the same "class" before, you might have information along the lines of "when we are this far away, I expect a signal strength of X." This will mostly serve as amplifying information to the next indicator, which is...

2) Bearings over time: as both the sensor and the target move through space, the bearing your sensor observes will change. Your targeting system will crunch the numbers, and constrain range as you gather more information. If the target was stationary and your sensor was moving, then it would be simple to use two or more samples over time to triangulate target position. Since BOTH vessels will be moving, this will be more difficult, but fundamentally still achievable. If the sensing ship changes course it can greatly constrain the problem, and rapidly resolve range.

Passive sensing has the added bonus that you are not emitting "loud" radar signals that enable other ships to identify, localize, and attack you.

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How about using gravity instead of radio waves? Gravity has detectable effects over great distances, and we currently have sensors that can detect and map fluctuations in gravity, so if you had a detector that was sensitive enough you could send out some kind of graviton pulse and then register where it reacted to other gravity fields and/or objects.

Even without a gravity pulse, all objects have gravity, so passive detection would be possible, especially for very large ships, or ships with artificial gravity.
A stealth ship would be one that is small, has radio/radar and light absorbing material, active cooling of the outside surface to space ambient (maybe dump the heat into an internal heat sink which means the stealth could only be used short term before the heat would need to be dumped), and no artificial gravity when in stealth mode.
So, active stealth would be cramped, hot, and weightless.

There is also a theory that the effects of gravity travel faster than the speed of light*, so you'd potentially be able to detect something faster than relativity would allow. A normal radar signal would take about 20 minutes to cross the distance between Mars and Earth.

TL;DR, it would be a lot like sonar in modern navel warfare. Big capitol ships and planet/moon/asteroid based installations actively pinging, smaller stealthy ships sticking to gravity shadows when possible and trying to remain undetected while listening for other stealthy ships...

Gravity Mapping: http://en.wikipedia.org/wiki/Gravity_of_Earth

Edit: I'd like to point out that the theory of a graviton particle, if it could be harnessed, might allow for controlled gravitational waves.
Another way to generate gravitational waves would be to generate a pair of very small black holes orbiting each other.
As the waves propagate outward detectors could look for ripples in spacetime where no object should be, or for a change in the size of the ripples caused by known objects. If an asteroid is suddenly causing a larger ripple than normal, then it would indicate that the mass had changed and a ship is probably hiding behind it.

* This is based on Newtonian physics, and doesn't hold up with modern science.

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    $\begingroup$ Gravity waves wouldn't 'reflect' the way EMR does so active gravity detection wouldn't work, even if we had any idea how to make a "gravity pulse". The best you could manage would be to make a gravity wave so powerful it would 'jostle' the target to produce another gravity wave. The 'reflection would be absolutely minuscule. All masses may produce gravity, but it's very week, and gravity waves require the mass to be shaking. Detecting gravity is also rather hard, let alone using it to produce an image. You need to look at minute differences in the forces acting on widely spaced masses. $\endgroup$ – smithkm Aug 25 '15 at 4:19
  • $\begingroup$ Hmm, drive by down votes with no comments... that's useful. $\endgroup$ – AndyD273 Jun 22 '16 at 14:38
  • $\begingroup$ Gravity waves are what you get on water in the sea. You are thinking of gravitational waves, and they hace been detected. They are not contingent on particles. $\endgroup$ – JDługosz Jun 22 '16 at 14:52
  • $\begingroup$ There is no theory that the effects of gravity travel faster than light. "Just a theory" means "I don't know what words mean". $\endgroup$ – JDługosz Jun 22 '16 at 14:55
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    $\begingroup$ "Gravity waves" is from other comments. With your edit, your point makes sense. $\endgroup$ – JDługosz Jun 22 '16 at 15:43
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It works like this. See this video for a very informative presentation on just how radar is used to detect and explore bodies in out interplanetary neighborhood.

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Any "active" sensor you might wish to employ is simply going to take twice as long to give you the information you'd get from emitted radiation of the target, anything you can't see isn't a threat anyway unless you have FTL in which case no EM based detection system is going to do you any good anyway, because the attackers arrive before you can see them.

Have a look at Jack Campbell's Lost Fleet series for a really good hard science look at the issue of detection and signal lag in sub-light combat situations, also a pretty good read in and of itself. The main bit that's relevant here is from the first book where they point out the fact that radar has to go twice the distance, there and back again, that optical signal data does.

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