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APRS ThermalSAT Launched



We just launched a ThermalSAT (onto the roof of our engineering building)
which contains 3 Thermisters on 3 panels, WHITE, ALUMINUM, and BLACK.
Anyone can access the telemetry by telnet to 199.227.86.221 port 23.

Look for packets from W3ADO to APRTLM in the format shown below.

We used the APRS telemetry format, since it was the easiest way to get
telemetry from these student experimental payloads to the students via the
worldwide APRS telnet site (APRServe)  We use a number of these
"Babysats" in our ASTRO labs to give the students hands on experience
with space hardware.  Since the data is  so easy to access, we thought we
would post this to the SIG so that anyone else can access it just as
easily.  The satellite is using the APRS MIM telemetry chip which is a
complete 5 channel telemetry system and AX.25 TNC transmitter on a chip.
Driving a 1 watt xmtr on 2 meters.

Being on the earth has lots of problems, though, so dont expect the same
readings you would get in real space:

1) The solar flux has been filtered by the atmosphere (40 deg EL @noon)
2) The panels are not exposed to the 4 degree coldness of space, but
     the ambient temperature in Annapolis.
3) Due to the light box, the panels are only exposed between about noon
   to 1330.  The solar current will tell you once all panels are
   exposed, but will not tell you when the WHITE panel goes into shadow
4) This experiment will only last a few days or until it rains...

Here is the LAB assignment that we handed out to the students:

THERMAL SATLAB

    The ThermalSAT has 4 experimental thermal panels in a row, each
panel is about 2 inches square for a total of about 8 inches wide by 2
inches high.  These are angled at the sun but completely enclosed in a
black walled light box.  On the south side of this box, which is tilted 
towards the sun is a 5 inch by one foot horizontal slot that permits the
sun to shine on the ThermalSAT panels around noon. 

    The panels should acheive differing temperatures relative to their
absorbtance from the sun and their emittance in the IR to the black panels
which are at ambient temperature.  The thermal panels are arranged in the
following order from West to East.  White, Aluminum, Black and Solar
Panel.  Thus, when the sun aproaches the noon hour, the solar panel will
be the last panel illuminated.  Hopefully, this will give the thermal
panels time to stabilize before you take your readings.

    APRS Telemetry Packets will look like this:

    W3ADO>APRTLM,RELAY,WIDE,WIDE:T#nnn,111,222,333,444,555,1000010

    Where W3ADO is the callsign of our Thermal Babysat
          APRTLM is the packet identifier as being Telemetry
          RELAY,WIDE,WIDE is the path via many hops (not important here)
          nnn is a serial number (once every 15 seconds)
          111 is the first channel which is the White panel thermister
          222 is the 2nd channel which is showing solar array current
          333 is the 3rd channel showing the Aluminum Panel thermister
          444 is the 4th channel (Not used here.  Curently about 69)
          555 is the 5th channel showing the Black panel thermister

All values are decimal values between 000 to 255 which represents a
voltage between 0-5 volts.  The solar panel current reading is actually
the voltage drop across a 50 ohm resistor from our solar panel.  If this 
reading is not above about 130, then it is not a clear day and the sun is
not shining... When you take your readings, make sure to record this value
as a relative measurement of solar flux.   Do NOT take data when cloudy or
sunlight is variable.

To get your data, telnet to APRServe 199.227.86.221 port 23 which is at
the Miami Science center.  APRServe maintains a constant connection
to a receiver in our Satellite lab which is listening to all APRS packet
activity in our area on 144.39 MHz.  For the noontime  durations of this
experiment, Bob Bruninga will re-tune the Rickover receiver to the 145.79
MHz frequency of our ThermalSAT.  (If you telnet to APRServe and do not
see one of our packets every 15 seconds, Bob is probably the failure
mode.  Call the satellite lab at 410-293-6417 and remind Bob to tune the
radio...)

The thermisters are 3k ohms at 25 deg C. and form the upper half of a
5 volt voltage divider with a 3K resistor at the bottom.

The equation for the Thermister Resistance is:
    
                      (9.93 - 3.48 LN(T) + 2957/T)
         RT = 3000 * e

Enjoy!

Bob Bruninga, WB4APR, US Naval Academy Satellite Lab


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