I would have a hard time expecting that a problem from the power lines or a transformer is affecting the front end of a GPS receiver at 1.5ghz??? but anything is possible???
Hold the bus here. I just caught that we are talking microwave radio frequencies. Power lines are not likely to affect any high frequency communications. Possible but highly unlikely. If it is power line interference you can still find it with a AM radio, but I have doubts it is power lines.
I have a hunch I might know what this is, and it has been happening a lot as of lately. Look around for a nearby Cell Tower, and if there is one nearby find out who the operator is. If it is LightSqaured, you likely found your source of interference. LightSquared operates on a band very close in frequency to GPS L1 band of 1525-1559 MHz. GPS L band operates at 1575.42 MHz. That is only a 160 Khz difference and if the GPS receiver front end is loose, the cell tower is over driving the front end of the GPS RX.
Another source could very well be associated with an airport, golf course, or other area needing precision GPS. In which case there maybe what is called Differential Correction GPS which is a land based transmitter. If near an ocean there is another correction system called SBAS and for FAA operations WAAS.
With Microwave to locate the esource will take a directional antenna, Spectrum Analyzer, and a trained operator to find it. .