AnotherDilbert said:
I would say this is a false comparison, there is no reason to assume that the amount of radiation is the same in the two cases. One important property of the active sensor is that we are adding more radiation to the system.
True, but if your emitter is lower powered it already has a range disadvantage, unless...
Your sensor has a lower intensity threshold for detection (and appropriate software, etc for scrubbing noise from the system - 3rd party sources of radiation aren't always helpful for detection) - but then that is what I would called a "better tech level advantage",
AnotherDilbert said:
I would say a more accurate comparison is Active (I hold a torch) and Passive (there is no torch). We cannot rely on the target being helpful enough to illuminate himself?
Yes and no,
If he isn't illuminating himself then he's also relying on passive sensors for detection - but that is really an aside, my real comment here is that:
This is the nature of passive sensors - if the target is not emitting in the part of the EM spectrum my sensor operates on then my sensor is effectively blind to that foe.
With that said,
active sensors have exactly the same problem - if your foe is not emitting in the part of the spectrum your active sensor operates on then your sensor is blind to that target. This may occur because your target simply absorbs the energy your emitter directs at it, or because it is transparent to the energy your emitter uses.
So really your argument simply demonstrates one of the weaknesses of passive sensors. That said, I imagine most civilian craft will make heavy use of navigation radar, so passive sensors will certainly have some utility in that circumstance.
(And I should point out that IR detection in space is virtually guaranteed, but I also accept that stealth makes for better RPGs, so I'll live with that)
AnotherDilbert said:
There is no reason to assume the intensity in my emitter is equal to the intensity of your natural emission or reflection.
True - the emitter probably has a
much higher intensity than reflections or similar (you want long-range detection, and the emitter is designed to emit whilst a reflection is an incidental side-effect to you being there, and will vary according to the strength and distance to the ultimate source),
AnotherDilbert said:
Even if we are both using the same active sensor only a fraction of the emitted radiation would be reflected back?
True, but I fail to see the relevance - and would point out that this is taken into account with the inverse square relationship (if your ship covers 4% of the sky picture at 1km it will cover 1% of the sky picture at 2km, resulting in a 75% reduction in reflected energy).
AnotherDilbert said:
That is one standard case for passive sensors? The other main case would be IR, sound, and Neutrino that the target naturally emits.
The other cases are all interesting - I would also add LIDAR to the list.
LIDAR is still covered by inverse square (laser beams do gradually diverge, but only very slowly assuming it is well collimated), but is very directional, so you'll only get a passive LIDAR fix if the sensor is pointed at you or very near to you. This means that passive detection will still have a longer range (assuming the same sensor), but the foe must be looking your way to pick it up.
IR is tricky because it doesn't really work in most scifi settings. You emit pretty much all of the time (assuming you don't want the crew to cook), and unless you take great care to make it otherwise, you emit in all directions. Active IR has exactly the same issues as using a torch in the visible spectrum however.
Sound doesn't work in space, but submarine warfare is a very interesting example of active and passive sensor application (you don't "go active" unless in dire circumstances, you instead listen very carefully for your foe with massive passive sonar arrays).
Neutrinos are an interesting exception. They are subatomic particles (rather than EM radiation emitted in all directions), in this case most likely released by on-board reactors. The Inverse-square law still applies, but in a probabilistic sense. If a neutrino is released in a random direction and my neutrino sensor (at, say 1km) takes up 4% of the sky picture as seen from the source then it has a 4% chance of being detected. If the same sensor is at 2km it will instead take up 1% of the sky picture and the detection chance drops to 1%. So detection is almost guaranteed (neutrinos pass through most matter easily, so if they go in the right direction they will hit the sensor) - it's just a matter of how long it takes for a neutrino to go the right way - and that's down to probability and the rate of neutrino releases.
AnotherDilbert said:
I think we have to be more specific about what type of sensor and what scenario we are discussing. I suspect you are assuming two ships, both using active radars. That would somewhat like two cars approaching each other at night with low beams on. Yes, you would see the other cars lights long before your lights illuminated the other car, but you would get very little information. You would see the direction to the other car, but not distance, speed, or size. In this case passive has longer reach.
As soon as the other car turned off his lights (went passive) you would stop seeing him, until you get close enough for your lights to illuminate his car. In other words if he is not emitting, active sensors have longer reach.
Four situations:
1/ Both cars have lights on - both detected at a good range - a tie
2/ I have lights on, he doesn't - He sees me a long ways off, I spot him when he enters the range of my headlights - advantage him
3/ I have lights off, his are on - I see him a long way off, he spots me when I get into the beam of his lights - advantage me.
4/ Both have lights off - we both fumble in the dark until we make detection at very close range - a tie and a messy knife-fight
In cases 2 and 3 passive sensors have a longer range.
In cases 1 and 4 neither side has an advantage.
You're on the bridge - do you turn the active sensors on?
