I believe radar is obvious it's just a quick ping to let you know something is out there and it's approximate location/speed etc.
Next comes Ladar, which when within range can "image" a target for it's outline. I assume once range is known and an image is gained, a computer can then determine it's dimensions; which are then compared against a database. I've seen ladar on Discovery and it could make a pretty decent scan at our own tech level, in an eyeblink. At Traveller tech levels the analyzation and comparison should be workable.
Other types of sensors, include neutrino detectors. This can tell if a ship is under power (unless of course it's got a massive plume of heat venting out the back already as a pretty good clue). I'm more used to GURPS Traveller which uses reactionless thrusters. They don't shoot out jets, but still have heat issues.
IR can also detect the same. Active IR can see in the dark and even let you read writing on things. You can google for active IR and I've got a link around someplace. Active IR will let you see thru fog, rain and snow at distances of kilometers. Again at Traveller TLs and in space or on a planet it should be a routine sensor action.
My take on sensors in general is that if you don't have military experience or science experience, you simpy have to dig on your own for explanations.
Once again I'm more used to GURPS Traveller which gets a bit more grainy
and they define the sensors into two categories: AESA and PESA or active and passive Electromagnetic Sensor Arrays. Despite the names laser/ladar is included since they function in the EM spectrum but are different than radar.
As part of all this comes Millimeter Wave Radar which can image thru some materials (but not metal). It too is active an passive and it's used in measuring things in a precise manner. Some militaries use it to image thru brick/stone (non-conductive) to see if anyone's inside a building in urban warfare. Apparently it's so fine resolution that you can tell if a body lying there is breathing or not. It's also used to image people to see if they're carrying weapons, despite wearing a shirt/pants etc. It resembles a thermograph. This could get long winded, but there's some nice stuff on the web to give you more info.
I believe in GURPS "regular" radar can also image, but range is reduced drastically.
So, for me, radar is used to detect things, then the ship would decide whether it needs to pursue further looks at something using it's other sensors. Once you're in close, you can use all your other sensors to piece together an idea of what's going on with what you've found.
Radio can be handy too
Radscanner in GURPS is a passive radar/radio detector or geiger counter. You might want to go over to e23 and see if any of the GURPS Traveller books (down to $9.95) some in PDF interest you.
This one is $9.95 and gets heavily into all types of systems for vehicles
and even has scenarios for sensor use in the back.
http://www.sjgames.com/gurps/books/Vehicles/
In the Table of Contents PDF (which is free) Chapter 13 has a whole section on Sighting and Detection as well as breaking that down further. So not only do you know "something is out there" but depending on your roll and equipment, you can get more information from it.
I'm a layman, so don't take everything I've said as gospel. Make sure you dig on your own and make your own conclusions. Next is UV which I haven't done much research on.
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