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Mro On Approach, TCM-3 not required
Airbag
post Feb 19 2006, 07:26 PM
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Another interesting tidbit - MRO uses the same X-band uplink/downlink channel (32) as Spirit...an unexpected problem relating to Spirit's longevity. Expect to see much more UHF commanding of Spirit via Odyssey in the near future. Opportunity's channel (29) is shared with Deep Impact, but that has not been an issue so far because of their spatial separation.

Airbag.
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Bob Shaw
post Feb 19 2006, 09:41 PM
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QUOTE (Airbag @ Feb 19 2006, 07:26 PM) *
Another interesting tidbit - MRO uses the same X-band uplink/downlink channel (32) as Spirit...an unexpected problem relating to Spirit's longevity. Expect to see much more UHF commanding of Spirit via Odyssey in the near future. Opportunity's channel (29) is shared with Deep Impact, but that has not been an issue so far because of their spatial separation.

Airbag.


Am I alone in thinking that this is, er, silly? Or was it too expensive to change the ground station gear?

Bob Shaw


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Guest_Myran_*
post Feb 19 2006, 11:03 PM
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QUOTE
Bob Shaw said Am I alone in thinking that this is, er, silly?


It doesnt sound like good planning to me. Not that I am as singleminded to think that any command from Earth intended for MRO will make Oppertunity to attempt spinning its nonexistant flywheels, or MRO to start evasive manuvers to avoid dangerously soft dunes. tongue.gif

Yet there could be a potential problem if any or both are programmed to stop and recieve when they catch a signal from Earth, and so might stop whatever work they do when the commands actually are for the other one. Are this a risk or do they really go completely 'mute' when they execute the set of commands they have gotten previously?
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djellison
post Feb 19 2006, 11:07 PM
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I imagine that a spacecraft that notices a signal on it's own channel, but can't understand it ( which is quite likely given the different bit rates the two spacecraft would be commanded with ) would probably go into a safe mode under the assumption that there is something wrong.


Doug
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SteveM
post Feb 19 2006, 11:27 PM
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QUOTE (djellison @ Feb 19 2006, 06:07 PM) *
I imagine that a spacecraft that notices a signal on it's own channel, but can't understand it ( which is quite likely given the different bit rates the two spacecraft would be commanded with ) would probably go into a safe mode under the assumption that there is something wrong.
Doug

Assuming intelligent design among the engineers at JPL (the pun was unintentional, but irresistable) can't we assume that they have a way to turn off Sprit's receiver when they plan to send signals to MRO and vice versa.

Steve
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Bob Shaw
post Feb 19 2006, 11:40 PM
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QUOTE (Steve @ Feb 19 2006, 11:27 PM) *
Assuming intelligent design among the engineers at JPL (the pun was unintentional, but irresistable) can't we assume that they have a way to turn off Sprit's receiver when they plan to send signals to MRO and vice versa.

Steve


Steve:

Please don't put that sort of idea into their heads!

Bob Shaw


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djellison
post Feb 19 2006, 11:42 PM
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I was thinking that, I imagine they can sequence things whereby the two dont listen in on one another, but there's still scope for cross talk at some point. For UHF commanding, as I understand it, you have quite a big latency, as they don't uplink to Odyssey as often as they would want to uplink to Spirit.

Doug
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belleraphon1
post Feb 20 2006, 12:51 AM
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QUOTE (Airbag @ Feb 19 2006, 02:26 PM) *
Another interesting tidbit - MRO uses the same X-band uplink/downlink channel (32) as Spirit...an unexpected problem relating to Spirit's longevity. Expect to see much more UHF commanding of Spirit via Odyssey in the near future. Opportunity's channel (29) is shared with Deep Impact, but that has not been an issue so far because of their spatial separation.

Airbag.



Also.... these engineering decisions are made long before launch.... it is possible that no one anticipated Spirit still being functional in 2006.

Craig
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edstrick
post Feb 20 2006, 06:08 AM
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On it's second lunar day, JPL tried to get Surveyor 6 to wake up and transmit. Surveyor's 1 and 5 had operated successfully on the second (and for 5, later) lunar days, despite cold damage.

Surveyor 6 transmitted signal briefly, then went silent. While they were attempting various commands, switching systems in the blind, Surveyor 1 (which had the same receiver frequency) woke up instead on it's 7'th lunar day and briefly transmitted. I don't recall for sure, but I don't think they got interpretable telemetry from either of them during the attempts.

Surveyor 7 also was successfully operated during it's second lunar day.

Lunar Orbiter 1 was deliberately crashed at the end of it's extended mission to clear up a communications frequency.
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ugordan
post Feb 20 2006, 08:27 AM
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Don't the DSN uplink commands have a sort of digital signature as well as intended receiver "address" in their packet headers so that Spirit/MRO know the transmission is directed towards them specifically?
That would sure seem a logical and sensible thing to do. What would otherwise prevent, say, ESA's New Norcia DSN station to "accidentally" transmit garbage on channel 32 and screw up both probes at once?


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edstrick
post Feb 20 2006, 08:46 AM
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I think we're dealing with paleolithic technology standards, maybe from the 70's or 80's.
<grin>
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Airbag
post Feb 20 2006, 07:22 PM
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QUOTE (edstrick @ Feb 20 2006, 03:46 AM) *
I think we're dealing with paleolithic technology standards, maybe from the 70's or 80's.
<grin>


Perhaps, but the Small Deep Space Transponders are new technology and used by many current missions. There are only so many X-band channels and they are simply all used up. This is all from the DESCANSO MER telecoms report (very interesting reading I thought).

The spacecraft do have their own IDs, but best not to tempt fate with multiple bit errors etc. and perhaps accidentally accept and decode a command not intended for that spacecraft. And yes, that can happen - Opportunity accidentally went into safe mode during a solar opposition experiment in which a bunch of NOPs was changed into something else as multiple bit errors slipped by the error checking etc.

There are several techniques (other than the obvious one of using Mars itself as a "blocker") that can be used to make sure X-band signals intended for MRO (and that is the important one during aerobraking) only get to MRO. From memory, I can recall these (there were others too):

- Change Opportunity's antennas' polarization (so that it is opposite to that of MRO)
- Doppler adjustment for specific target
- Reduce transmit power level (MRO has more gain)

Airbag

PS BTW, receivers are typically *never* turned off (except during Opportunity's Deep Sleep).
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Bob Shaw
post Feb 20 2006, 10:18 PM
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Can I patent interplanetary MAC addresses - they could be stored digitally in a small solid-state pod, perhaps?

We could call, them, oh, iMACs and iPods...

Seriously, if a $10 network card can have a unique identifier burned in, why don't spacecraft?

Bob Shaw


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Guest_Myran_*
post Feb 20 2006, 10:40 PM
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Airbag said: Change Opportunity's antennas' polarization


Of course! If they can do that relatively easy its a good solution. And delegate the problems to the communication engineers then who are more qualified in the first place.
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helvick
post Feb 21 2006, 12:08 AM
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QUOTE (Bob Shaw @ Feb 20 2006, 10:18 PM) *
Seriously, if a $10 network card can have a unique identifier burned in, why don't spacecraft?

I had a long boring waffle about bit rate, harmonics, the history of MAC addresses, Ethernet's emergence in the late 1980's and collisions on ARCNET networks in the early 1990's here but re-reading it made my eyes glaze over so I thought it would be better to simply point to the CCSDS website and in particular the Space Packet Protocol Blue Book Specification as an example of just hom much thought has to go into doing this sort of thing properly.

If only it was as simple as tagging each packet with a 12 byte target identifier.
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