Questions about Sensors (MgT v2 & HG)

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GarethL
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Questions about Sensors (MgT v2 & HG)

Postby GarethL » Wed Sep 21, 2016 5:53 am

Hi all,

1/ re MgT v2, pg.150 Sensor Target chart, What is the difference between EM and passive RADAR/LIDAR sensors? These seem like the same thing (observing for EM sources)? If they differ, should there be an entry for passive sensors on the Sensor Detail chart if the distinction is relevant?

2/ Traveller aims to be generally hard-scifi, but I note that active sensors are longer ranged than passive sensors? In reality the opposite is generally true (the radio waves have twice as far to go with active sensors for twice the lag and twice the scatter, the catch is the reduction in detail afforded by passive sensors),

3/ HG, pg.24, this grants two extra range bands - what is the detail afforded at these ranges? Can all sensors be used at this range? (some sensor types don't even reach the end of the regular chart, so it seems possible that some fall short here too!)
AnotherDilbert
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Re: Questions about Sensors (MgT v2 & HG)

Postby AnotherDilbert » Wed Sep 21, 2016 7:48 am

1) Radar is "radio waves", i.e. a limited spectrum (frequency range) of electromagnetic emissions. Lidar is laser, hence light, hence another limited spectrum. EM sensors is presumably a wide spectrum sensor for all EM emissions. In MT is was defined as: "An EMS Active Sensor Array combines radar, all-weather radar, ladar, radar jammer, radio jammer, active IR, and image enhancement into one Integrated and optimized sensor array."


2) Active sensors should be better? On a dark night, do you see further with a torch (active) or without (passive)? A passive sensor has to rely on the emissions originated in the target or background (the sun), which works well for an IR or visual sensor, but a passive radar is rather useless (except to detect an enemy with active radar). Active radar lets us measure the difference between the origin pulse and the returned pulse giving us additional information such as relative speed.


3) Undefined. We can only assume that the sensor detail is the same as Distant Range?
GarethL
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Re: Questions about Sensors (MgT v2 & HG)

Postby GarethL » Wed Sep 21, 2016 12:20 pm

AnotherDilbert wrote:2) Active sensors should be better? On a dark night, do you see further with a torch (active) or without (passive)? A passive sensor has to rely on the emissions originated in the target or background (the sun), which works well for an IR or visual sensor, but a passive radar is rather useless (except to detect an enemy with active radar). Active radar lets us measure the difference between the origin pulse and the returned pulse giving us additional information such as relative speed.
Just to be absolutely clear, I never said better, I said longer-ranged, and I can easily demonstrate it both practically and with numbers,

For starters, you can easily assess that passive sensors have a longer range because with active sensors the radiation has twice as far to go (from you to the target and then back again) - effectively doubling the range...

Using your example of the torch on a dark night:
Active sensors - I hold a torch (the "emitter") and use my eyes (the "sensor") to spot you. Maybe I can see you perhaps 50 meters away being generous and depending upon the power of the torch.
Passive sensors - you hold the torch (the "emitter") and I use my eyes - depending upon the terrain, etc I could probably see you several hundred meters away if not a lot further.

Using numbers, radiation intensity from a point source has an inverse-square relationship (Intensity α (1 / [Range²])). Consequently if the range double the intensity is reduced to one fourth. If your sensor has an intensity threshold (below which any return signal is "too weak to detect") then that sets the range for your sensor.
Since the active sensor has to travel twice as far as the passive signal, its signal will have 1/4 the strength - this means that it will have apx. 1/4 the maximum range.

i.e.:
At a range of 1km, the passive sensors signal intensity is α (1 / [1²]) = 1.
At the same range, the active sensor has a signal intensity of α (1 / [2²]) = 0.25.

Now, the information gathered will be different - with the active flashlight I might be able to make out your features well enough for identification, etc. The passive sensor grants a lot less information - perhaps a bearing and if I have really good sensory equipment the brand of lightbulb in your torch. Of course, identifying the bulb (or RADAR emitter) might give me some clues as to your identity (Imperial Crysanthumums are equipped with RRD-172a RADARs).

So, as I said above, longer ranged but more limited info.

As an aside, daylight vision is something of a red herring - effectively you have a third party (the sun) with a massive radiation emitter shining light on you and your foe.
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Re: Questions about Sensors (MgT v2 & HG)

Postby AnotherDilbert » Wed Sep 21, 2016 1:58 pm

GarethL wrote:Just to be absolutely clear, I never said better, I said longer-ranged, and I can easily demonstrate it both practically and with numbers,
Sorry, that was sloppy of me. "Better" is not well defined.
GarethL wrote:For starters, you can easily assess that passive sensors have a longer range because with active sensors the radiation has twice as far to go (from you to the target and then back again) - effectively doubling the range...
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.
GarethL wrote:Using your example of the torch on a dark night:
Active sensors - I hold a torch (the "emitter") and use my eyes (the "sensor") to spot you. Maybe I can see you perhaps 50 meters away being generous and depending upon the power of the torch.
Passive sensors - you hold the torch (the "emitter") and I use my eyes - depending upon the terrain, etc I could probably see you several hundred meters away if not a lot further.
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?
GarethL wrote:Using numbers, radiation intensity from a point source has an inverse-square relationship (Intensity α (1 / [Range²])). Consequently if the range double the intensity is reduced to one fourth. If your sensor has an intensity threshold (below which any return signal is "too weak to detect") then that sets the range for your sensor.
Since the active sensor has to travel twice as far as the passive signal, its signal will have 1/4 the strength - this means that it will have apx. 1/4 the maximum range.
There is no reason to assume the intensity in my emitter is equal to the intensity of your natural emission or reflection.

Even if we are both using the same active sensor only a fraction of the emitted radiation would be reflected back?
GarethL wrote:As an aside, daylight vision is something of a red herring - effectively you have a third party (the sun) with a massive radiation emitter shining light on you and your foe.
That is one standard case for passive sensors? The other main case would be IR, sound, and Neutrino that the target naturally emits.


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.


We would have different cases for Visual, Radar, Lidar, IR, or Neutrino sensors.
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Re: Questions about Sensors (MgT v2 & HG)

Postby GarethL » Wed Sep 21, 2016 4:47 pm

AnotherDilbert wrote: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 wrote: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 wrote: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 wrote: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 wrote: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 wrote: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?
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Re: Questions about Sensors (MgT v2 & HG)

Postby AnotherDilbert » Wed Sep 21, 2016 6:25 pm

GarethL wrote:
AnotherDilbert wrote: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?
That is the basic question. As soon as I go active I have given my position away. At a guess that is generally undesirable.

Depends on the scenario? If I am an intruder raiding an enemy position I really do not want to give my presence away, I must go in passive. If I am defending a position I have to see the enemy before they can start shooting, I cannot stay at action stations forever.

If I am a BatRon with a bustling screen I will be seen anyway, so active sensors are probably a good idea.

Either way I would not let my ships go active as long as I have fixed sensors or sensor drones available.
collins355
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Re: Questions about Sensors (MgT v2 & HG)

Postby collins355 » Wed Sep 21, 2016 6:42 pm

I think the issue is the model MGT have chosen for the sensors doesn't very effectively reflect the fact that if you are using your active sensors then (all other things being equal) I'll detect you at a much longer range with my passive sensors.

MGT gives me a die roll modifier on the roll to detect if the adversary is using active sensors - but this does nothing about the shorter range hardwired in for passives. They will never detect you at ranges of Long or more no matter what. You have a chance to detect me.

They should have gone with a different system IMHO.
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Re: Questions about Sensors (MgT v2 & HG)

Postby GarethL » Thu Sep 22, 2016 3:30 pm

AnotherDilbert wrote:That is the basic question. As soon as I go active I have given my position away. At a guess that is generally undesirable.

Depends on the scenario? If I am an intruder raiding an enemy position I really do not want to give my presence away, I must go in passive. If I am defending a position I have to see the enemy before they can start shooting, I cannot stay at action stations forever.

If I am a BatRon with a bustling screen I will be seen anyway, so active sensors are probably a good idea.

Either way I would not let my ships go active as long as I have fixed sensors or sensor drones available.
Oh, absolutely - it is very situational, I quite agree, if you're a big obvious target then generally you may as well go active,

But if you have drones, fighters, scouts or whatever, then you're going to depend upon those for your information because even if your location is obvious, using your sensors is potentially giving your foe additional information about your force composition, etc.

Another obvious counter-situation is if you suddenly detect missiles out of "thin space" - you're attacked, your foe clearly knows where you are but you can't locate him so you may as well go active because you're definitely screwed if you don't,
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Re: Questions about Sensors (MgT v2 & HG)

Postby GarethL » Thu Sep 22, 2016 4:33 pm

collins355 wrote:I think the issue is the model MGT have chosen for the sensors doesn't very effectively reflect the fact that if you are using your active sensors then (all other things being equal) I'll detect you at a much longer range with my passive sensors.

MGT gives me a die roll modifier on the roll to detect if the adversary is using active sensors - but this does nothing about the shorter range hardwired in for passives. They will never detect you at ranges of Long or more no matter what. You have a chance to detect me.

They should have gone with a different system IMHO.
I tend to agree - they've made what could be an interesting decision for players into something that's a no-brainer in most situations,

As an aside - optical sensors are awesome, but as they don't seem to be fitted to any of the standard sensor suites I am guessing that they represent "looking out the window", possibly with some magnification? They really make the other passive options look weak...

Thanks guys - you've given me food for thought, I might look at rewriting the sensor rules for my own edification,

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