For a while now I've played ToS:WS and I haven't been able to shake the feeling that some important concept has been missing from the whole "realistic" Mecha combat angle. I was thinking about how to convert a Mekton battle when it stuck me. Active Sensors! That's what's missing. As it stands, the only time ATs can engage, is when at least partial LoS has been established. Whenever I lose a target in the woods at 2-3 hex range I keep thinking, how can you be hiding 40 - 200 tons of metal in a forest less than a mile away? Shouldn't my sensors be able to pickup something through the trees? Sure, 600+ meters of trees would block most EM signals an AT is likely to be emitting, but this is the future™ haven't ground penetrating radars shrunk down to less than say 10 tons?
First, since an Active Radar (AR) is a type of scanner, there's no need to purchase it separately. The rating of an ATs AR would simply be same as the ATs Scanner.
Second, AR and Sensor Range. The AR is supplemental to the current sensor array (which behaves as a Passive Radar), so to maintain play balance the range of passive sensors would need to be scaled down slightly*, and AR would have a range slightly greater than the current sensors**.
* Passive vs. Active Sensors. If one AT is using AR and another is using passive sensors, the AT with passive sensors can detect the AT using AR at extended range (i.e. a range = passive sensors range + 1/2 targets AR range).
** AR and Extended Range. This extended range applies only to ATs, not terrain. An opponent could hide behind a mountain and evade an AT using AR. Furthermore, AR would reduce the effects of cover for purposes of during whether or not a target can be detected.
Third, AR and weapons. As GME are designed to target radiation sources, they gain a bonus for targeting an opponent with AR engaged (though if they get a bonus for having any scanners on that bonus would be reduced/eliminated for passive sensors). In addition, if coding would allow it, a new weapon would be created for the GM family, the GMR. The GMR would require that the launching AT have AR engaged, and have range, heat, and damage determined by whatever the GM family is missing the most. ECM would act to deflect GMRs in the same manner as flares deflect GMHs.
Fourth, AR and Hiding. Hiding with AR on is impossible as the AT is broadcasting its position. Against a hiding, using AR would reduce the cover modifier either directly (i.e. by rating/10) or by some factor (i.e. 1/(1 + rating/10)). I don't know enough about hiding to wager a guess as to which method is better.
Fifth, AR and targeting/scanning. Painting a target with AR makes establishing a lock quick, but is easily countered by ECM. The time to lock would by reduced by (scanner rating - ECM rating)/2 (or 3, 4 ...) seconds where the attacker can't establish a lock if the target has a higher ECM rating until the target enters passive sensor/visual range. Scanning would work the same way, AR (well imaging radar/ladar in this case) decreasing the time to scan and increasing the accuracy based on ECM and scanner ratings. Conversely, locking on a target that is using AR would be easier as well. The time to lock would be reduced by (scanner rating - ECM rating/2)/2 (or 3, 4 ...) seconds where both the scanner rating and ECM rating of the target are used. I'm not sure how scanning would be affected, but at the very least, AR should reduce the effectiveness of ECM.
Sixth, AR and buttons. While AR, passive, and off, should probably have its own 3-position toggle, the simplest place to put the selection would be as a submenu of the toggle scanner button. The menu would be similar to how the shields are handled.
There you have it, active radar in a nutshell. I know that implementing all that jazz now that the game has been released is well ... less than feasible, but its something to think about for a future release, maybe an expansion pack or sequel. Before anyone starts worrying about balance, all you have to do is increase the power requirements and heat generation from using AR to the point that recons running around with AR aren't broken, and an Assault has to think twice before flipping on AR while sporting a shield, ECM and running.