[amsat-bb] Re: General Telemetry Question (PITFALL!)
Robert Bruninga
bruninga at usna.edu
Wed Dec 11 10:43:48 PST 2013
> To follow up on Bob's comment. If you send the raw analog sensor
data...
> Change calibration values if found to be wrong after launch...
We did on PCSAT!
Caution to Satellite Builders: Be careful when using an EXCEL TREND LINE
equation for doing Engineering Unit conversion back to original units. It
was a big lesson for us back on PCSAT in 2001.
The problem is, generally, EXCEL displays trend line equations in a nice
GENERAL human readable form. For example, for our thermistors, the 3rd
order trend line equation to convert from telemetry count back to degrees
C was displayed by EXCEL as something like this: 2E-7 X^3 - 2E-4 X^2 +
1.804E-1 X + 2.379E2.
One would think one is getting a very precise to 4-significant digit
equation. WRONG. Notice the Cubed and Squared terms (which can be very
big at warmer temperatures) are only represented to a single decimal
digit(+/- 10%)!!! (2 and 2)...
When this trend line is used (as displayed), to give back our
temperatures from the incoming COUNT, the temperatures were way off!
The key is to make sure the trend line equation is displayed in SCIENTIFIC
format before you write it down and then try to use it. Then the first
two terms above are properly displayed by EXCEL as 1.544E-7 X^3 and
-2.069E-4 X^2. (instead of 2E-7 and 2E-4).
We catch this error in a lot of student's work...
Bob, WB4APR
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