I tested this in a controlled environment with a friend of mine in 0.3.2. We used Osiris as the basic mech with no relevant equipment, Puma as the GECM mech, and Raven variants as BAP/BHP mech. Can't remember the exact procedure, and we had to redo some tests because of the C3-strangeness detailed below. This is how I believe it works(or worked, although the XML files seem to have no changes):
Base detection range is 1000m. Going passive lowers this range to 250m, while being active and having GECM lowers it to 500m. Curiously, being passive and having C3 is less effective than just being passive; with passive+C3 your detection range is lowered to 500m, which is the same as with active+GECM. This seems to mean that there is no reason to go passive if you have both C3 and GECM. This feature of C3 is reflected in the XML files, where C3Radar.xml has passiveRadarModifier = "-500", while Radar.xml has passiveRadarModifier = "-750". This seems to be the case with 0.4.3 XML files too.
Adding BAP/BHP to the mix is very straightforward, since they just seem to raise the final detection range by 200m/400m. For battle armor, the base detection range is 100m, and BAP/BHP raises that by 100m/200m.
Please make corrections if these results are wrong. I would also be very interested in getting some hard facts about AECM; we just couldn't get that to affect detection ranges in any way in our tests. The Atlas and the mech in it's field didn't seem to have any kind of ECM, because their base detection range was 1000m. I haven't tested this in 0.4.3 yet, but I stopped trusting the AECM fields completely in 0.3.2 after our tests.
EDIT: I'll just clarify that all the ranges above are for an active detecting asset. If you are passive, according to these test results you can only detect other active ECM-less assets at 300m, and BA at 100m. Everything else should be invisible.
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