[amsat-bb] Re: General Telemetry Question
jim at coloradosatellite.com
Mon Dec 9 16:49:27 PST 2013
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
- 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
- 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
- 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
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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