Hi Bill, Hans,
thanks for the replies. I was curious of your setup Bill, I expected you to say the opposite!
I also run my gps units from their own analogue supplies. I even built my own analogue ‘rf friendly’ psu so I knew what was in it.
I have disconnected my leds (no effect on interference), and put so much capacitance and series resistor filtering on the outputs that the U3s could not decode the data anymore (but still got the interference). The only thing that works is to disconnect the 9600 baud DATA line from the gps end and that gives maximum reduction in noise but rather defeats the issue. I have a switch now that disconnects that line to my U3S once CAL has been achieved whenever I want to receive with reduced background noise.
Strangely, I can run a marine NMEA receiver at 9600 baud with long leads, no special screening to a nearby PC with no interference issues at all, so I don’t understand why the gps modules give me so much grief.