[amsat-bb] Re: General Telemetry Question

[Jim White]

To follow up on Bob's comment.  If you send the raw analog sensor data 
to the ground you can
- Fix mixed up channels if you got them wrong before launch.  This 
happened with the 1990 AMSAT Microsats and I've seen it since then in 
other birds.
- Change calibration values if found to be wrong after launch. I've seen 
this most often with ACS systems where the sign of a magnetometer or 
torq rod is backwards.
- Change cal equations if an analog sensor or its system partially 
fails.  Most recently I've seen this with a science mission cubesat that 
has been in orbit about 6 months and suffered a partial failure.  
Adjusting the equations on the ground allowed for a continued science 
mission.
- Save downlink characters, hence time.  You can get more data down in a 
shorter packet.  Example: To send human readable ASCII for a telemetry 
value like "A=3676 " takes 7 bytes.  If you just send the number and a 
space it's 5 bytes. The same value in binary is two bytes (long int in C 
language).
- Get all values in a single AX.25 frame with a single and common time 
stamp.  In binary you can get about 225 values in a frame (with a time 
stamp, ID, etc.).  In ASCII you can only fit about 50ish.  A typical 
cubesat has more than 50 TLM values (although some have less).  A 
typical microsat may have as many as 200.
- When downloading science or sensor data the amount you can get to the 
ground is often the limiting design factor.  With current technology you 
can usually store as much as you want in the sat. But to get it to the 
ground you need to be as efficient as possible. Binary is most often 
used, but that's not efficient enough for some missions and further 
compacting the data is needed - using one or more of several other 
techniques.

Jim

On 12/9/2013 7:44 AM, Robert Bruninga wrote:
> Answer: Engineering efficiency..
>
> There is far more computing power on the ground than the satellite.  Also,
> KISS principle.  Also, calibration can be done without modifying flight
> code.  And finally, it is far more compact to send binary or hex than
> human readable decimal.
>
> Bob, WB4aPR
>
> -----Original Message-----
>
>> why not provide the engineering values in the downlink without the extra
> step having to be done on the ground?  What is the logic of doing this?
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