Sorry i couldn’t reply earlier, this board is, unfortunatly, very slow for me early in the day, so I couldn’t get here to post a follow-up.
Yes, active sensors are generally a “bad thing” that gives away your possition. That’s something I already know Normal ship-based sensors center around thermal/IR-band passive sensors, used to detect the heat radiating from a ship, or at much longer ranges, the near star-hot jet of vented reaction material (These ships use fussion process to super-heat a reaction mass of water into plasma, at high pressure, and vent it out the rear of the ship for propulsion at extremely high velocity… Gives good thrust, but it’s about as stealthy as broadcasting your possitions over the radio). They also have a full spectrum of other sensors, usefull for detecting, among other things, trace radiation or EM fields emitted from opperating electronics or powerplants. That’s the primary way of detecting a ship. Active sensors are a ‘backup’, and usually used an already-known contacts to get exact data on them, such as range (Passive sensors only give signal strength, not range, unless you have another observation point to triangulate from).
I havn’t read that book, I’m afraid. The closest work of fiction I can think of is the depictions of space combat in Albedo, which is the only realistic portrayal of space combat that I’ve seen before…
Basically, the best comparison I can think of is modern-day sub-vs-sub combat. Passive sensors (In this example, hydrophones) watch carefully for an opponent, while trying to stay stealthy enough to not be seen. Sometimes, active sensors have to be used, though. A ship armed with -only- passive sensors would find itself at a fair disadvantage if the other ship had both, much as a sub without active sonar could be, if it finds a time it needs it.
In this case, though, the delay between the signal going out and returning is still very, very short compared to travel times. Combat in this case is measured not on a scale of minutes or even hours, but days and weeks. A ship generally “jumps” in-system at aproximatly 40-50 AU out, and then accelerates toward its objective… Usually, it will spend 1-5 days accelerating (No way to counter the force of inertia, such as in ST, so it has to be kept down to crew sustainable levels), then shut down and coast most of the way (Hopefully finding and destroying any targets before the ship has to turn around and decelerate, unless it’s just a hit-and-run mission). Normally, it takes a couple weeks for a ship to get from jump-in to the biozone of a star, or even longer if the ship needs to conserve fuel (They have a fairly limited supply). At that scale, manuvering to give ‘false’ active sensor reports is rather futile. The time delay is taken into account, and even then, while an incomming ship may be able to approach at speeds up to a few thousand kilometers per second, light (And radiation) travels many times faster; At 5 AU, a ship could add a mere 7KPS to its velocity in one direction (Assuming a sustained 1.5 gee acceleration, which is rather tough on the crew) before the signal arrived at the other ship, or a “mere” 3750 kilometers distance. A normal targeted “ping” pattern would therefor account for this, and scan at least that wide of an arc… However, in doing so, the other ship would be venting -plasma- out to one side (As seen by the other ship); Not only did the other ship get a precice range with the “ping”, as well as a velocity if it was a long ping or a couple rapid ones, or it’s a dopler-style system, but the other ship is now lit up brighter than any possible background emission. The emitting ship now has all possible data it needs to get an extremely precice firing solution, while the other ship only has a bearing and signal strength/type to go by (Unless, of course, it has another ship or outboard sensor to triangulate with, or in the process of changing its vector, triangulates from regular updates on the passive track – coincidentally, the exact same thing done in submarine combat)
So while they’re a backup system, they can be quite usefull at times, especially if the ship you’re facing is particularly stealthy. It’s not something they’re going to go pinging away with just for the heck of it, though, since its detection abilities are rather low compared to the passive sensors. Also, when a ship decides to go active, they are likely to follow up the initial ping with a few more, if it doesn’t see the ship emitting (Manuvering) after that, just to make sure. The risk of attack from “pinging” is also low; At such range, energy-based attacks will have dispersed much too much to do damage. The main ship-to-ship weapons are long-range kinetic-kill missiles (Which at the multiple-thousand KPS speed of an incomming ship, PLUS the speed of the missile, is just about a guaranteed kill on anything it hits). Those missiles have both passive and active sensors, as well.
Trust me, I’ve gone through a lot of it, comparing all the tactical strengths and weaknesses already; When they’d be used, what they would be capable of, ways to counter them, etc. I’ve already got the story down, in rough form, and I’m just trying to figure out some of the more technical (And less important) aspects at this point… Like what wavelength to use