Here we show a "spectrogram" of a GPS occultation. The left axis indicates the passage of time since the start of the occultation (time=0 at the top). We can think of the left axis as a proxy for the height of the occultation, starting at the top of the atmosphere and moving downward with time. The bottom axis indicates the frequency of the observed signal, shown by the intense color in the center. More precisely, it shows the deviation of the observed frequency from the nominal expected frequency — hence the value is near zero. It is these small deviations from the nominal frequency that contain the information on the atmospheric state. Note also a prominent bright line cutting diagonally from the left, midway up, to the lower right. It turns out that this is caused by a reflected image of the occulting signal coming off the ocean near the horizon (see previous slide). It was by examining such spectrograms that scientists serendipitously discovered these reflections. Had the receiver been programmed to track the reflection, as it did the occultation, its intensity would be considerably greater.  (Figure by G. Hajj et al., JPL)