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Mro On Approach, TCM-3 not required
Bob Shaw
post Feb 21 2006, 11:16 AM
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QUOTE (helvick @ Feb 21 2006, 12:08 AM) *
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.


I followed the links, and yes, there's a lot of work been put into the definition of the systems and architecture pertaining to Space Packets and wholly laudable commitments to international and interplanetary interoperability. But nowhere do I see such a thing as an analogue of a MAC address, which has historically provided a more-or-less fail-safe identifier for almost any bit of hardware. I still can't see why not...

...am I missing something?

Bob Shaw


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helvick
post Feb 21 2006, 02:19 PM
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QUOTE (Bob Shaw @ Feb 21 2006, 11:16 AM) *
But nowhere do I see such a thing as an analogue of a MAC address, which has historically provided a more-or-less fail-safe identifier for almost any bit of hardware. I still can't see why not...
...am I missing something?

Ah I get your point now - why not just have a unique hardware based ID per probe and leave it at that? Use something like a MAC address (12 bytes) so you can be pretty sure you'll never get a collision.

I don't think that the identity is the problem and that's all a MAC type idea will give you - an assured unique identity to enable specific devices to extract their data from a shared transmission medium. The spacecraft all have adequately unique identifiers at the moment in any case so the two main problems are firstly whether they are included in the command data packets that are transmitted at the moment and secondly if they are then what sort of error handling is in place to ensure that cross talk doesn't happen? Multiple bit flip errors seem to have made it possible for one asset to incorrectly intepret a signal intended for a different asset in the past so it would seem that the error codes being used by the MER's are not 100% perfect.

Another problem is just overall congestion of the system. If there is some need for complicated command sequencing on MRO and MER at the same time then one or other will have to back off in order to allow the other to finish.
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mcaplinger
post Feb 21 2006, 02:39 PM
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QUOTE (Bob Shaw @ Feb 21 2006, 03:16 AM) *
...am I missing something?


See the Global Spacecraft ID document: http://public.ccsds.org/publications/archive/320x0b1s.pdf

Of course, these are only 8 or 10 bits long.

But has been already pointed out, just relying on an ID field to reject commands would be foolish in the presence of bit errors. If you were designing a mission-critical system based on Ethernet (which probably wouldn't be a good idea anyway because of single point failures in the shared transmission medium) you would hopefully not rely on MAC uniqueness to keep the wrong receiver from doing something. It might be unlikely for a burst of errors to corrupt one MAC address into another in such a way to get past CRC checking, but it's not "fail-safe". Even with your home LAN, there are acknowledgement protocols in TCP/IP to keep such problems from happening.


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djellison
post Feb 21 2006, 03:27 PM
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As I understand it - spacecraft are all moving very quickly in different directions, so there is dopler to worry about, and they are scheduled to listen at particular upilnk rates, and so on and so forth.

If MRO's listening to it's uplink at X bps, and it's being sent at Y Hz off the specified frequency, it may be that Spirit interprets that as a bad uplink as it was expecting W bps and is moving in a way that is Z Hz of specified wavelength.

I'm fairly sure they'll be able to sequence things intelligently to sort this problem out.

Doug
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Toma B
post Feb 24 2006, 07:47 AM
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Is there any planed imaging with HiRISE on approach to Mars?
If not, why not?


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ugordan
post Feb 24 2006, 08:06 AM
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There's always a good reason for not performing observations shortly before orbit insertion. The latter is such a critical maneuver that the spacecraft team doesn't want to take any chances that might jeopardize the entire mission. When you use the instruments on board, they can always report a fault state due to various reasons and cause the S/C to go into safe mode which will then obviously just sit around and wait to hear from Earth.
Obviously, if a safing event happened shortly before orbit insertion the whole mission could go down the drain. That's why we typically get imaging after the maneuver and only on some missions a very distant image of the target well before the arrival.


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djellison
post Feb 24 2006, 08:27 AM
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cool.gif--><div class='quotetop'>QUOTE(Toma B @ Feb 24 2006, 07:47 AM) *</div><div class='quotemain'><!--quotec-->
Is there any planed imaging with HiRISE on approach to Mars?
If not, why not?
[/quote]

Not that I know of - because the optical navigation camera is not boresighted with HiRISE and needs to be aimed in that direction.

(ONC is the silver thing on the near side of the spacecraft - http://mars.jpl.nasa.gov/mro/gallery/space..._pre-launch.jpg )

Doug
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Guest_Sunspot_*
post Feb 24 2006, 09:38 AM
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Guests






The same was done with Cassini during the week or so before orbit insertion.
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Guest_AlexBlackwell_*
post Feb 24 2006, 04:35 PM
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The MRO arrival press kit is now up:

Mars Reconnaissance Orbiter Arrival at Mars
posted Feb. 24, 2006 (984 Kb)
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djellison
post Feb 24 2006, 04:53 PM
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And the press conf is in an hour and a bit as well

Doug
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jmknapp
post Feb 25 2006, 03:29 PM
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QUOTE (djellison @ Feb 24 2006, 11:53 AM) *
And the press conf is in an hour and a bit as well


One item from the press conference: other than the test images on March 14, there will be no images during the entire aerobraking phase--the instruments will be on "survival heaters" and the electronics will not be exercised at all.


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mcaplinger
post Feb 26 2006, 06:06 PM
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QUOTE (AlexBlackwell @ Feb 24 2006, 08:35 AM) *
The MRO arrival press kit is now up...


"The three cameras from the science payload -- the High Resolution Imaging Science Experiment, the Context
Camera, and the Mars Color Imager -- will take their first test images of Mars as the orbiter passes low over the southern hemisphere near the end of the third orbit, on March 14."

Now that the press kit has let this out of the bag, I'll note that the timing and location moved by ~2 hours and ~30 degrees of longitude between the mid-January predict and the most recent one -- so the imaged areas will be in the southern hemisphere, but we probably won't know exactly where until after MOI.

I don't know yet when we can expect to have the image data back.


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RNeuhaus
post Feb 26 2006, 06:27 PM
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The MRO spacecraft will remain out of contact for a half hour. Lots of time of suspense. There will be a TV NASA coverage during this MOI?

The 109 km, I think it is the outside of ending of Martian atmosphere. Isn't it?



Rodolfo
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edstrick
post Feb 27 2006, 07:10 AM
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With apollo, for both orbit insertion and trans-earth-insertion burns, there were 2 important times. The times of expected AOS <acquisition of signal>times: for 1.) if the engine burn did not occur, and for 2.) If the engine burn did occur.

On orbit insertion, a no-burn signal would be early.. an improper burn later, a nominal burn at a specific time.. and an overburn or kablooie if no signal shortly after the optimum burn time.

On TEI, a nominal burn gets you back into view early, as you've sped up in orbit.

To some extent, the same will apply here. If AOS is within a few tens of seconds of the expected time, it's likely we're in a good orbit.
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jmknapp
post Feb 27 2006, 03:32 PM
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Does anyone know which DSN stations and/or other facilities will be listening for MRO's signal as it emerges from behind Mars at 22:16UTC on March 10th? Which one will likely be the first to report the signal?


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