Here are the FFTDSP spectrograms of MGS for the initial CW Mode at 1413 on Nov 25, 1996:
MGS Spectrogram at 1413 CW Mode Turn-on
MGS Spectrogram at 1528z as Rotation Brings Back Antenna Gain
Conclusion from Spectrogram Data:
Several interesting observations can be made from the spectrograms:
- The Receiver was set so that 800 Hz on the FFTDSP screen corresponded to 437.09551
Mhz which was estimated to be the receive frequency at the start of CW mode. MGS was
detected 180 Hz lower at 980 Hz.
(the receiver was set in CW mode which will shift the FFTDSP trace to the right as the receive signal moves down).
- The CW mode was turned on during the middle of the high gain portion of the 100 minute Spacecraft rotation period.
- The signal level dropped below detection for 45 minutes during the worse gain period of rotation.
Here is a plot of the NASA published gain variation due to Spacecraft rotation and the corresponding period of detection in the spectrogram:
MGS Ant Gain Plot from JPL
- By matching the 45 minute dead period to the above gain chart, it can be concluded that the MGS signal was 5 dB above the minimum detection level for the two 5 foot Helix Antennas.
- As the antenna rotates back toward full gain, there appears to be an additional frequency shift before full gain is reached. Could this be due to the heating effect as the antenna rotates out of shade back to full sunlight?
- Earth rotation doppler is clearly seen in the spectrogram at -30 Hz from 1413z to
1443z and -20 Hz from 1528z to 1608z
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