[Spaceships] Atmospheric Sensor and Comm Range?

[vicky_molokh]

Greetings, all!

Does anybody know what would be the plausible multipliers for passive/active sensor ranges in atmosphere? Particularly things like Over-The-Horizon RADAR, emissions detection, LIDAR, and optics? Same goes for radio and beam comms.

Thanks in advance!

[Celti]

Originally Posted by vicky_molokh

Greetings, all!

Does anybody know what would be the plausible multipliers for passive/active sensor ranges in atmosphere? Particularly things like Over-The-Horizon RADAR, emissions detection, LIDAR, and optics? Same goes for radio and beam comms.

Thanks in advance!

Any plausible spacecraft C/S array is going to have range clear out to the horizon for all active sensors and comms even in fairly dense atmospheres. Optics are optics in space or air, you've got whatever Telescopic Vision rating your array gets you. For things like over-the-horizon radar I would require a special system and/or design switch for funky sensors, or maybe let an Enhanced array do it; not being currently aware of the capabilities of such systems, I cannot offer any guidelines as to a multiplier.

[vicky_molokh]

The issue has come up again. My party is planning to buy an SM+3 adventure-oriented jeep, and even an SM+3 TL9 Control Room seems to provide a Sensor Rating 1, which is equivalent to 0.1 LS base range of active sensors and a 0.1a.u. base range of comms, for the low low cost of $6k. Yes, I know that SM+3 craft are a result of highly optional and custom rules, but even at SM+4, the numbers look suspiciously high compared to what equivalent-TL standalone RADARs offer.
Dividing range by 10 in atmosphere is my kneejerk reaction, but I'm not sure that's the right reduction of range.

Anyone got better benchmarks of what the multiplier should be?

[weby]

In 3rd edition you had two applicable modifiers, only one of them(the less relevant) has survived into 4th edition.

The one that has not made it: In space, non-FTL radars, ladars and AESAs get 10× range (and thus +6 Scan).

The one that can still be found in characters scanning sense: Not silhouetted against sky:-4

Thus I would divide the the air range of a space radar by 10 and the ground detection range by a further 5 for a total of 1/50 range(or just use the 1/10 and the -4)

But really most civilian vehicles would have some sort of cheap sensor option in addition as they do not need such long ranges.

[Anaraxes]

Originally Posted by Celti

For things like over-the-horizon radar I would require a special system

They are indeed unlike line-of-sight microwave radar. The wavelengths are lower (thus with less resolution). And they're much more dependent on the properties of the planet (the ionosphere for most such, or ground-wave propagation along the planetary surface). The ionosphere bounce and backscatter also tends to require huge antenna arrays, much bigger than most spaceships would have as a minor component rather than the entire purpose and "spinal mount" of the ship. (Typically these antenna arrays are hundreds of meters to kilometers.)

I don't know how portable the physics of the ground-wave systems are to other planetary surfaces. Moisture content of the soil matters, and terrain, but I don't know what else is involved, or how different the system would have to be to work on completely different surfaces like Mars or Venus (much less gas giants). There are some such that work only over Earthly oceans, as they need the (salty, conductive, and mostly flat) ocean surface to carry the signal. Higher frequencies get attenuated much more than lower ones, so these tends to be the lowest-resolution systems, which affects both size of objects detected and accuracy of range/speed estimates.

The sky-wave ones would be quite problematic for interplanetary portability. They're hard enough to build even with just one ionosphere to worry about, as the ionosphere itself keeps moving up and down, changing angles of reflection and propagation delays. Current systems use active monitoring of the ionosphere as well as the radar itself. Higher tech signal processing and more planetary science and experience would let the systems handle a wider range of environments. But then, you have to have an ionosphere at all for these to work. Those airless worlds, for instance, mean the system is useless, as there's nothing to reflect the signals and returns.

[Anthony]

The problem has less to do with range than with processing power, though this isn't something GURPS handles well. In general you should divide range by 100-1,000 if the target isn't silhouetted against space.

[Anaraxes]

Also doctrine, or design philosophy. After all, the only reason to build OTH radar or HF comms is because there's a horizon in the way. And when you're a spaceship, there's usually not one. If there's some reason your ship has to operate bound to a fixed point on the surface, what does an OTH radar on the ship do that an LOS radar satellite (or two or three) or remote with satcomm links doesn't do better? If you're deploying spaceships, then space tech is cheap.

[Flyndaran]

Even 60s era Star Trek shot out probes to check around things they didn't want the ship proper to get near. Probes also allows specializing of sensors so you don't need to make your ship the Swiss Army Knife of abilities.

I think that there are tricks that can be done with UT processing power and numerous carefully spaced sensors to somewhat act like one giant sensor.
Interferometry and synthetic apertures make my head spin.

[vicky_molokh]

Originally Posted by Anthony

The problem has less to do with range than with processing power, though this isn't something GURPS handles well. In general you should divide range by 100-1,000 if the target isn't silhouetted against space.

Silhouetting against space provides a +24 to the check, which is equivalent to a ×1,000 increase in detection range. Edit: oh wait, I think it's the passive section. So yeah, seems to make sense at first sight.

[Flyndaran]

Originally Posted by vicky_molokh

Silhouetting against space provides a +24 to the check, which is equivalent to a ×1,000 increase in detection range.

Shouldn't that also fit somewhat when the object is emitting a signal that the surrounding area does not at all?

Would the lone powerful radio emitter on Mars be that much harder to detect than if it were in orbit?