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Dust Storm
post Aug 17 2018, 03:59 AM
Post #196


Group: Chairman
Posts: 14082
Joined: 8-February 04
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Yeah - someone saw Opportunity in lock on eyes.nasa.gov/dsn and told the world the spacecraft was clearly alive, talking to us a 6 megabits per second.

This is the bulk of what I said to correct him........

A few background details....
Comm subsystems for both MER and MRO here : https://descanso.jpl.nasa.gov/DPSummary/summary.html
MER fault protection primer here : https://trs.jpl.nasa.gov/bitstream/handle/2...750/05-0557.pdf

Opportunity is in a low power fault or loss of clock fault - neither of these attempt to use the high gain antenna. Expect data rates of 10bps or so in this scenario. Even if it were somehow using the HGA - you would never ever see a data rate from it at more than 28,800 bps - never 6 megabits per second.

What does talk at 6 megabits per second is MRO with its 3m wide HGA.

I'm afraid what you saw was an antenna briefly locking up on MRO as its orbital motion put it's nominal downlink frequency on top of the expected MER frequency due to Doppler shift.

All the 8 Ghz X-Band frequencies for these spacecraft are pretty close together and so it's easy to see why a spacecraft in orbit that is sometimes coming 'towards' Earth in that orbit, and sometimes going 'away' from Earth in that orbit could very easily have it's carrier frequency pass through the expected MER frequency twice per orbit - and that's what you saw. The antenna will briefly acquire 'lock' on that passing signal and follow it until the receiver is commanded to drop lock and start looking back at the proper frequency for a signal to acquire.

If you watch DSN Now enough - you'll have seen this has been happening a lot over the past 2 months.

Sadly what you saw was NOT a signal from Opportunity

Someone still wasn't sure....so....

If the antenna is looking for MER-B, then the session on DSN Now says 'MER1'.

If MRO comes along and dumps it's enormous power downlink right where that antenna is looking - DSN Now will still report that it's looking for MER1 - but show there is a signal. That's what DaveS saw. It's been happening on an almost daily basis for two months.

The spacecraft names on DSN Now are essentially what's "booked" on that antenna at that time - it's not responsive to where a signal is coming from. You'll notice - it will happily put a label over an antenna when neither uplink nor downlink are occuring - it's because the antenna is booked for that spacecraft and is either setting up for it, cleaning up after a pass, or looking for a signal. At the time I'm posting this - that is true for OSIRIS-REX on DSS55, STEREO-A on DSS14, Voyager 2 on DSS43 and MRO on DSS46.
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