I have an APM1 with Mediatek gps connect via standard cable. I have had many flights in the past and seemed that when gps blue led locked on the red APM lite followed. I now loaded the new B6 firmware and the APM seemed like there was no gps - no flashing red, or solid red after the gps blue lock. I remembered the new sw was pickier about home lock (a good idea) but the gps was well locked.
I then unplugged the battery and ran from usb (no esc).. The gps locked, and apm locked every time. My scope showed fairly clean +5. I then used the battery so esc were powered (I have a BEC running the apm, gps, etc).and the apm no longer saw gps lock. The scope showed a lot more noise on the +5 line, maybe from esc.
I finally just put a 470uf cap across Mediatek pins 1-2 and the noise disappeared. The gps - apm lock works perfect no matter how it is powered. Just like the sonar, filtering power after a cable run might be a good idea - these are high impedance devices and we do have a very noisy electrical environment.
I only got one flight in after this mod, but it locked great and there was no doubt it knew where it wanted to loiter.
Replies
Sounds like your BEC is buggy. What kind is it? It may have lost its' filter capacitor for some reason. Try re-flowing the solder on the cap or the inductor may have a bad connection. It never hurts to have a filter cap near the load, but something happened to the BEC. Switching BEC's can get very noisy if the output is not filtered as opposed to a regulator. They just get hot and waste your amp-hours.