Back on the dead horse of sensors in space.
Most of our familiar sensors are next to useless in space because they're in a frequency optimized for punching through atmosphere. Long wavelengths are less bent by it so they work at longer ranges.
Even microwave is longer wavelength than visible and IR.
In space long range sensors are going to trend towards passive and because it's much less cluttered out there and computers are getting so much better it's going to become possible to use the visible spectrum, or even ultraviolet.
Resolution is STILL going to be an issue.
For example, the 2.4m diameter optical scope in the Hubble can't resolve something the size of a Type S at Luna distances. It can't see smaller than 0.000000193 rad (1.93x10^-7) and a 36.4m long Type S is 0.0000000947 (9.47x10^-8) rad that far out using the best case wavelength of 380nm (nearly UV violet).
For the record, you need on the order of a 5m aperture to see a Type S at lunar distances in violet and 10m in red... So a 10m scope to see a Type S in the full visible spectrum at 1.28 light seconds.
Radar is our normal go-to for long range sensing here on earth. That's normally X-band or 2.5-3.75 cm for wavelength. To resolve our Type S orbiting with the moon you need a staggeringly huge 330km diameter radar dish! As is mentioned in the comments, an interferometer is a means of creating a larger aperture with smaller individual units. This is great for fixed and orbital installations, but not so great for shipboard use. Such is taken to extremes with the canon Long Baseline Observation Window.
To see it at the maximum range of our missile command laser, though, you just need a 70km array. This is why LIDAR in visible or shorter wavelengths will supplant radar in Traveller. A green LIDAR only needs a 1.5m lens. GURPS:Traveler TL10 PESA goes into UV where just a 300 cm lens can see out that far. The AESA can see three times as far, out to 240,000km so a 1m aperture is required. The TL12 sensors use higher frequencies for their extended ranges.
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http://en.wikipedia.org/wiki/Astronomical_interferometer
ReplyDeleteThe sentence I'd intended on interferometers didn't make it out of my mind. Thanks for the addition.
DeleteI see that visible and IR interferometers are in service, they were merely a thought exercise when I first learned most of this.
I was just noting that, if brightness is our concern, we simply need to gather more photons. But if resolution is the problem and we're worried about needing a huge aperture to see any detail on the target, we can instead use a distributed network of smaller apertures and some fancy computation to solve the problem.
ReplyDeleteDistributing semi-redundant sensors across the ship has other benefits as well.
Heck, in a settled system, we may have access to an arbitrarily large array. Time lag will play havoc with using sensors too widely distributed, but let the computers figure that out. If I'm in communication with nearby friendlies, we make a much larger virtual aperture.
Or if I'm alone, I could send out drones to widen my observation network.
Hell, even ordinance on the way to the target can contribute.
Something I've been having a hard time getting a good handle on is how bright something has to be to be seen, even if it's below the minimum size to be resolved. It's obvious that there's an answer because we can see stars (and the ISS) which are much smaller than the minimum angle our eyes can resolve, but we can't tell much more than it's a dot of light.
DeleteThe aperture size is inflexible. There are ways, like interferometers, to make the effective aperture bigger, but the required value remains.
ReplyDeleteShipboard we're somewhat limited in how much interferometry we can do by the size of the ship. The longest distance on the aforementioned Type S is just 36.4m and 25m wide. Drones take up space on board, of course so might be less than ideal for small ships, like the Type S.
An intriguing idea is a capital ship and its escorts spreading out a bit and linking their individual sensor suites to form a larger aperture.
Traveller really dislikes autonomous missiles and drones that really are logical solutions to some of the problems. My kinetic abuse of the deceased equine stems from a complaint that Traveller ship combat ranges are unreasonably short and I am trying to show, without beating on the person who brought the complaint to my attention. Traveller ranges are actually rather reasonable, and not dissimilar to ranges from GURPS: Transhuman Space, a hard sci-fi setting rather than Traveller's self-consistent space opera. Transhuman Space uses distributed sensors, computer simultaneity correction and autonomous missiles/drones; and still doesn't really extend the ranges from Traveller much.
In addition, I am constantly impressed by what I've learned about the universe from a game.