Ask Sail

GPS Harmony

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Dick Hansen of Norwalk, Connecticut, asks:

"My new chartplotter is just great and gives me aerial and radar overlays. But when I transmit on VHF Channel 68, a frequency we use a lot in my area, the GPS hiccups, the chartplotter freezes, and it can take about 30 seconds for the system to recover. Everything works fine when I transmit on other channels; it’s just 68 that is causing me the problem."

Gordon West replies:

What is happening is that the 10th harmonic of the VHF frequency is winding up on your GPS receive frequency band. Although spread spectrum GPS reception isn’t jammed easily, your radio likely has a spur impurity that freezes GPS reception when you transmit on that frequency.

This problem is not that common, although it does occur, and I have two suggestions. First, make sure your VHF antenna is nowhere near the GPS antenna. Second, replace your radio with a newer unit.

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