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Huygens AGC test extended report

Huygens AGC test extended report

The Huygens Probe's Automatic Gain Control (AGC) signal level measured during the two in-orbit checkouts (F1 and F2) was observed to be depressed compared to the pre-flight values and also to be strongly modulated with the variation of the HGA pointing at the Sun. It was suspected that the AGC drop was caused by solar radio noise.

The overall results of all three in-flight tests are summarised in the following two plots:

Figure 1

Figure 2

The Probe AGC signal level measured during the two in-orbit checkouts (F1 on 23 October 1997, 1 AU), (F2 on 27 March 1998 0.6 AU) are plotted as a function of time. The F1 data are plotted in Figure 1 and the F2 data are plotted in Figure 2. Each figure consists of three panels.

Top panel: shows the variation of the pointing of the HGA (boresight and -3dB values) on the solar disk which is represented by +/- 0.2° band (for F1) and a +/- 0.42° band (for F2).

Middle panel: the same information as in panel 1, presented in a different way. The projection of the solar disk on the HGA beam is pictured every 10-15 minutes during the checkout. An artificial vertical displacement of + or - 5° has been applied on every other plots to enhance the clarity of the figure.

Bottom panel: the time variation of the AGC value for both chains is plotted. The AGC varies between -100.5 dB and -102 dB for F1 and between -102 and -108 dB for F2. The AGC variation is clearly correlated with the HGA pointing variations. The variation for F2 is larger, mainly because the solar noise input was larger due to the closer distance to the Sun. Different levels of Solar activity may also partly explain the differences between F1 and F2. This aspect is still being analysed. The differences between the two channels seen during F2 is explained by a different behaviour of both receivers at high noise level.  This difference was reproduced on the test bed at HPOC/ESOC, using the Probe Engineering Model. On the same time scale, the range of AGC values obtained during the ground testing activities is also plotted (variation between -95.5 and -100.5 dB). Finally we have also plotted the AGC value (very similar value for chain A and chain B at about -94.5 dB) obtained during the Off-Sun test. The value obtained during the Off-Sun is about 1 dB higher than the best value obtained on ground. The results of this test confirmed that the Probe hardware is healthy and that the depressed AGC values during F1 and F2 were the result of solar noise.

Last Update: 1 September 2019
21-Sep-2021 23:08 UT

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