QUOTE (Gerald @ Apr 25 2013, 05:14 AM)
Don't they add CRC32 or CRC64 checksums to the code blocks to be shure that only correctly transmitted commands will be executed, with a fallback strategy in case the command sequence is corrupted?
If you look at the Descanso website I linked to above, you will find all the details you could want regarding DSN protocols.
However- even error checking/correction does not solve the problem. You radiate a command load - the spacecraft identifies it as 'bad'. That command load could have been very important - it could have been something to prevent the spacecraft doing something to harm itself in someway . If the spacecraft doesn't accept it - you are quite probably in a worse situation. You radiate commands because you want the spacecraft to do something. Imagine the MSL side-B swop, or MER-B flash anomaly - but with the added complexity of unreliable communications.
There is a less obvious benefit of this strategy - conjunction offers up an opportunity for spacecraft operations teams to have a little down time. It's very very hard for a lot of flight-ops people to find time for vacations etc. Projects also can not afford enough personel to staff up enough to have redundancy for every job required on the mission. Conjunction offers a chance for people to catch up on other responsibilities, or take a well earned break.