The Anritsu MT8222A Spectrogram with my prior message looks pixelated due to changes in the signal's amplitude with time and due to multi-path reception. I thought that I would prompt that color change by rotating my receive antenna. So, today's Spectrogram attachment shows reception with the antenna first pointed south, then southeast, next east-northeast and finally west-southwest. Sometimes the color change is due to my antenna's directivity with the same station while at other times a different station is being received. Channel 23 shows an example of at different headings, three different stations were being received.
At the lower edge of each signal, you can see a pilot stronger in amplitude than the main signal. Some stations have this pilot's frequency controlled by the GPS satellite. When that is the case the pilot will be precisely 309.440559 kHz above the lower edge of a station's 6 MHz bandwidth. An example of where that is the case for a local station on channel 22 is shown on the second attachment. With the spectrum analyzer set for 100 Hz bandwidth and both a RBW and VBW of 1 Hz, you can see that pilot just below 518.309441 MHz on my GPS locked MT8222A. It demonstrates how broadcast stations with GPS receivers connected to their exciters can be used as frequency references. But remember, not all stations have GPS receivers in use.