Waterfall plot showing several dozen marine radiobeacons (vertical lines), which transmit differential GPS corrections as part of the U.S. Coast Guard’s Nationwide Differential GPS (NDGPS) System. There are 86 such transmitters around the U.S. Although allocated 285-325 kHz, the lowest frequency beacon is presently 286 kHz. Other signals outside the range 285-325 kHz are aeronautical radiobeacons and local interference. This observation was obtained in western Virginia (38.9N, 78.6W) in February 2013. Color represents signal strength: blue is weakest, red is strongest.