Seeing that blog (hi Kulthea, I'm part of that spike) reminded me that I'd been meaning to start one of my own, so I did:
https://whosoncomms.blogspot.com/
I've also picked up the Harrier supplement as an eBook and, apart from concluding that I will need to get High Guard as well to properly understand some of the features of the ship, looking at the description of the ship systems combined with writing up my first session report on the blog prompted me to work through my understanding around handling stealth and detection in my game. The rules for this are a bit scattered.
To start with there's the section on sensors in the Spacecraft Operations chapter (p150-1) - which states that a successful Electronics (Sensors) check is what you need and then lays out what level of detail this gets you depending upon the range to the target and the sensor packages you have. This is backed up by some passages of text at the start of the Space Combat chapter:
Most hostile encounters in space will start at Very Long or Distant ranges, when the combatants first detect one another [...] However, in some circumstances ships might get a lot closer before hostilities begin, perhaps getting as near as Close range if a pirate successfully pretends to be an honest merchant, for example [...] surprise is very difficult to achieve in space, as there are few places to hide behind. However, damaged sensors or inattentive sensor operators can mean a ship can approach another without being spotted. (p155)
However there's also the quality of the sensor package, which is introduced in the Common Spacecraft chapter - where ships can have Basic (-4), Civilian (-2) or Military (+0) grade sensor suites plus there was the question of what qualifies as 'advanced' and 'very advanced' for including NAS and Densitometers in the suite, cross-referencing to my 1ed copy of the rules cleared that one up for me (these are the super-duper military packages).
So pulling all that together, I reviewed the first interception my players did - they have Stealth coating on the hull (-4) and are sneaking up on a civilian freighter (-2). The civvy is running for safety, so they are only using passive thermal/radar/lidar* but I reckoned they had a +2 modifier because they've got a bridge crew who are nervous about being out where pirates roam, many hours from the nearest patrol vessel and they've put their best sensor operator on the scopes. Overall then, they have a -4 which means they only succeed on a throw of 12. The question of whether they register a bogey on their screens before my PCs can fire a shot across their bow from Medium range then hinges on how many detection attempts they get in the time it takes for the Harrier to set up their shot - total thrust of 90 is required to close from Distant to Medium and the Harrier has a thrust of 6 - so it took them 15 space combat rounds (or 90 minutes) to close.
35/36^15 works out to the freighter having a roughly 1-in-3 chance of making a detection in that 90 minute window assuming they make a scan attempt every round (as implied by the write-up for the 'Make A Detailed Scan' task under Electronics (Sensors) - routine with a 10 minute increment drops down to standard difficulty with a one minute increment). The ability to close on prey undetected drops off very sharply if the target has better sensors or better operators of course (that ^15 really starts to bite) - a -3 on the roll means that the Harrier only gets into range undetected about 1-in-4 times, while for a -2 this chance drops to roughly 1-in-20 (moral of the story - don't try to sneak up on military vessels).
I didn't work all this out at the table of course - I wasn't going to spend 20 minutes scrolling through a PDF cross-referencing stuff and then make 15 sensor check rolls while everybody else watched me and yawned - I saw that the PCs had some hefty modifiers in their favour, made a single throw (which was came up as something like a 3) and ruled that the PCs got complete surprise. Still it's nice to work through the numbers afterwards and see that the outcome I narrated at the table was a reasonable one.
Regards
Luke
[*] Here's a question though, what sensor detail do you get for passive radar/lidar? There isn't a column for that in the table on p150. My assumption is that the column heading on that table should just read 'Radar/Lidar', with the distinction between active and passive determining the range at which different levels of detail are achieved.