Determining the position of spacecraft with passive sensors.

[adam_grif]

Going through the AR detection page again for kicks, and I noticed the following passage:

When the enemy spots your ship by the exhaust plume, it not only knows that a ship is there, it also knows the ship's exhaust velocity, engine mass flow, engine power, thrust, acceleration, ship's mass and ship's course. Not only can it tell a warship from a cargo freighter with all that information, but it can also tell the class of warship, and maybe make a good stab at determining which particular member of that class it is.

In more detail: as mentioned above, propulsion system's exhaust velocity is revealed by the doppler shift in the emission lines, mass flow is revealed by the plume's luminosity, the thrust is exhaust velocity times mass flow, acceleration is revealed by watching how fast the plume origin changes position, ship's mass is thrust divided by acceleration, and ship's course is revealed by plotting the vector of the plume origin.

How exactly do they determine the ship's course? How did they work out the ship's relative position to the observer without using active sensors?

[Vendetta]

Going through the AR detection page again for kicks, and I noticed the following passage:

When the enemy spots your ship by the exhaust plume, it not only knows that a ship is there, it also knows the ship's exhaust velocity, engine mass flow, engine power, thrust, acceleration, ship's mass and ship's course. Not only can it tell a warship from a cargo freighter with all that information, but it can also tell the class of warship, and maybe make a good stab at determining which particular member of that class it is.

In more detail: as mentioned above, propulsion system's exhaust velocity is revealed by the doppler shift in the emission lines, mass flow is revealed by the plume's luminosity, the thrust is exhaust velocity times mass flow, acceleration is revealed by watching how fast the plume origin changes position, ship's mass is thrust divided by acceleration, and ship's course is revealed by plotting the vector of the plume origin.

How exactly do they determine the ship's course? How did they work out the ship's relative position to the observer without using active sensors?

Infrared telescope or radiotelescope. If you're close enough to do anything interesting, you're close enough that your emissions give a reliable indication of where you are.

[adam_grif]

Cool. Can you explain that any further? How accurate will the data be?

Assume you're ~1 light second away, if you need exact distances for that question.

[GrandMasterTerwynn]

Going through the AR detection page again for kicks, and I noticed the following passage:

When the enemy spots your ship by the exhaust plume, it not only knows that a ship is there, it also knows the ship's exhaust velocity, engine mass flow, engine power, thrust, acceleration, ship's mass and ship's course. Not only can it tell a warship from a cargo freighter with all that information, but it can also tell the class of warship, and maybe make a good stab at determining which particular member of that class it is.

In more detail: as mentioned above, propulsion system's exhaust velocity is revealed by the doppler shift in the emission lines, mass flow is revealed by the plume's luminosity, the thrust is exhaust velocity times mass flow, acceleration is revealed by watching how fast the plume origin changes position, ship's mass is thrust divided by acceleration, and ship's course is revealed by plotting the vector of the plume origin.

How exactly do they determine the ship's course? How did they work out the ship's relative position to the observer without using active sensors?

You've answered your own question. If you know the exhaust velocity and mass flow, then you know the distance from you to the thrusting spacecraft. You derive this simply by taking the intensity of the detected signal and solving for the inverse-square law. From there, you can refine the position estimate through continued observation. Or possibly drop off a second passive sensor platform or dial up your patrol-mate and do a triangulation that way.

[Guardsman Bass]

Going through the AR detection page again for kicks, and I noticed the following passage:

When the enemy spots your ship by the exhaust plume, it not only knows that a ship is there, it also knows the ship's exhaust velocity, engine mass flow, engine power, thrust, acceleration, ship's mass and ship's course. Not only can it tell a warship from a cargo freighter with all that information, but it can also tell the class of warship, and maybe make a good stab at determining which particular member of that class it is.

In more detail: as mentioned above, propulsion system's exhaust velocity is revealed by the doppler shift in the emission lines, mass flow is revealed by the plume's luminosity, the thrust is exhaust velocity times mass flow, acceleration is revealed by watching how fast the plume origin changes position, ship's mass is thrust divided by acceleration, and ship's course is revealed by plotting the vector of the plume origin.

How exactly do they determine the ship's course? How did they work out the ship's relative position to the observer without using active sensors?

You've answered your own question. If you know the exhaust velocity and mass flow, then you know the distance from you to the thrusting spacecraft. You derive this simply by taking the intensity of the detected signal and solving for the inverse-square law. From there, you can refine the position estimate through continued observation. Or possibly drop off a second passive sensor platform or dial up your patrol-mate and do a triangulation that way.

That might give you an idea of where something is (assuming it isn't moving and far-enough away that it has completely changed position by the time your sensors pick up on its IR emissions), but I don't entirely buy Atomic Rocket's argument that you could determine the ship's mass off it just by looking at how bright the plume is. That came up in the huge "sensors" thread that's now in the library, and one of the things that came up was that beyond a certain distance, all you'll get is a glowing dot unless you have some ridiculously large lens. You can't tell whether that dot is a decoy emitter drone, a warship, a freighter, or the like.

[Coalition]

If I understand the page correctly, you get:

Exhaust velocity - gotten by comparing the spectrograph of the exhaust plume against known materials. You can measure the Doppler shift, and calculate the relative velocity of the exhaust.

Measuring the exhaust plume's brightness gives you mass flow. (If there is no plume, the ship is drifting, and you can take your time.)

With mass flow and Exhaust velocity you can get the thrust.

Side motion is observed by the telescope or other gizmo you have watching it, so you know the actual acceleration. Thrust divided by observed acceleration gives you the mass (good way to tell strategic decoys from starships).

So you would know that it was a 5 kiloton ship accelerating at .1G, using a H-H fusion drive. You don't know if it is a five kiloton freighter or warship. Sending a message to the ship might work, along with an expendable drone if you are feeling suspicious. If the ship popped off a 50 kilogram decoy you would see a second plume with a much lower brightness, and your systems would ignore the decoy as athreat, and log it as a navigational hazard.

Overall heat signature tells you distance and power (but not either one individually). A closer object that is cool might appear the same as a farther hotter object to an infrared sensor. A second sensor (or lots of math) would reveal the distance, allowing you to plot heat.

That is what I understand you can figure out from watching a starship, and running math.

[Crossroads Inc.]

We actually did thuis a while ago, maybe this will be of help:
http://bbs.stardestroyer.net/viewtopic. ... ilit=space