Satellite Telemetry
The word is derived from Greek Roots
roots tele = remote, and metron = measure.
Telemetry 'literally - measurement at a distance' is the means used to gather information on what is happening inside the satellite as it goes about its business. The onboard telemetry system gathers information from sensors in the satellite and codes it in a consistent format for transmission to the ground, usually by sending the data as a signal superimposed on the beacon frequency. The beacon is therefore serving two useful functions-
1) Providing a 'signal strength' reference to measure your transmitted signal against, and
2) Providing feedback to the satellite's control station (and interested users) about the internal health of the onboard systems.
Two main types of telemetry systems are used in amateur satellites -
1) Hardwired telemetry systems - used in the RS series to code the onboard measurements for transmission as CW data superimposed on the beacon signal. One advantage of CW based telemetry systems is that you only need a pair of ears to copy it!
2) Software based (computer controlled) systems - as used on UoSAT2, DOVE, LUSAT, PACSAT etc. and on the forthcoming P3D satellite.
The downside of software based telemetry (TLM) a based system is that you need a computer and some kind of 'decoder' to copy the data.
A plus point of course is that many more channels may be accommodated than with simple 'hardwired' systems.
The inclusion of onboard computers in satellites permits greater flexibility in the telemetry systems used.
How can I copy satellitetelemetry?
UoSAT2
If you've never tried this before, UoSAT2 is an easy place to start.
UoSAT2 is really a scientific 'data collection platform in the sky' but it transmits bulletins of interest to radio amateurs. The satellite transmits on 145.826MHz FM and can easily be copied when in range on a handheld radio fitted with a 'rubber duck' type antenna. The data is coded as AFSK at 1200 bauds in a standard ASCII format so it is possible to copy the bulletins by using an old 'BBC' computer and a suitable 'dumb terminal' program. The bulletin data is transmitted on the 145.826MHz beacon in sequence with raw telemetry 'frames' and 'whole orbit data'. The different data formats have characteristic sounds on the Downlink and with a little practice, you will be able to identify what is being transmitted by ear!
The information transmitted by UoSAT2 may be fed by a 'homebrew' data cable connected between your radio's 'speaker out' connection and the 'cassette input' DIN connection on the computer. This is possible due to the choice of audio tones used by the BBC computer's audio-cassette recording (and playback) system - 'Kansas City CUTS' format. The audio tones used (1200/2400hz) match those used by UoSAT2 and so the internal tape circuitry is happy to accept the satellite tones as valid 'data'.
These computers are now obsolete but can often be found in junk sales and sometimes are disposed of by schools and colleges upgrading to PC based systems. It's always worth asking! Alternatively, UoSAT2 data may be decoded by a modem that supports the 'BELL 202' data standard.
Jim Miller G3RUH designed a 1200 bauds AFSK decoder system for the UoSats, which was sold by AMSAT as an unpopulated PCB. This used readily available parts to produce an RS232 compatible data stream to suit most personal computer systems. Although no longer in production, these units still turn up for sale occasionally on the second-hand market.
UoSAT2 has been transmitting telemetry and bulletin information - with the odd technical hitch - since launch in 1984. An impressive record indeed!
Because of the ease with which UoSAT2 data may be copied, it makes an ideal satellite for the beginner to telemetry to 'cut their teeth' on.
UoSAT-2 transmits its telemetry as a 'frame' of data with a 'header block' identifying the satellite followed by the date and time in coded form.
The 'header block' is followed by seven rows of numeric data.
Each row contains ten channels of data.
Examine the following raw unprocessed data frame and identify the following information -
UOSAT-2 9805174112923
00380013700266103481040590504606027070560804009033
10512113571200013089140001500016000174871846719572
20523210612265923000240002500026104274832860029544
30435310403228333000340003537836434374603852939539
40855410004267443000441714500146000475284854149509
50569510735270453296549905500056000575355853059538
60790615BC6280063024640006500066000670006800069000
The header block - Identifies the satellite along with the date and time of the measurements.
Seven rows of coded data -the first 2 digits are the 'channel identifier'. the next 3 digits are the 'channel value'.
This sequence repeats until the end of the first line (channels 0-9) and the next line continues with channels 10 - 19 etc.
Breaking down the first line of telemetry into individual channels gives -
00380 01370 02661 03481 04059 05046 06027 07056 08040 09033
| | | |
| | | Value 370
| | Channel 1
| Value 380
Channel 0
Each channel monitors a different parameter within the satellite.
Channel 0 is a measure of the solar array current.
Channel 1 is the X axis magnetometer value.
To decode the actual values, a series of equations (published by the University of Surrey) are applied to each value in turn to provide a 'calibrated' value. This of course would be very tedious to do manually, so the reader is advised to use 'What's Up' (see above) in conjunction with a PC and suitable decoder unit.
As an example-
channel 0 (Solar Array Current -Y) uses the equation .... I=1.9*(516-N) ma.
N is the telemetry value and I is the 'calibrated' answer in milliamps.
UoSAT-2 transmits 60 channels (0 - 59) of ANALOGUE information followed by a number of binary 'flags' relating to internal 'switch' settings.
A full description of UoSAT-2 telemetry is beyond the scope of this booklet but we hope our brief introduction will lead the reader to experiment with the data from this interesting scientific 'flying laboratory'.
For BBC computer owners, the following short program written in BASIC by Trevor Stockhill G4GPQ will display (but not decode) incoming telemetry from UoSAT2. In addition, it will display the 'plain text' information bulletins from this fascinating satellite.
10 MODE3
20 ON ERROR GOTO 340
30 DIM CODE%100
40 FOR PASS% = 0 TO 2
STEP 2
50 P%=CODE%
60 [OPT PASS%
70 .start JSR &FFE0
80 BCC chaok
90 CMP #&1B
100 BEQ error
110 JMP start
120.chaok AND #&7F
130 CMP #&0D
140 BEQ print
150 CMP #&0A
160 BEQ exit
170 CMP #&1F
180 BPL print
190 LDA #&7C
200 .print JSR &FFE3
210 .exit JMP start
220 .error LDA #&7E
230 JMP &FFE4
240 ]
250 NEXT PASS%
260 *FX205,64
270 *FX7,3
280 *FX156,3,252
290 *FX156,2,252
300 *M. 1
310 *FX2,1
320 *FX156,1,252
330 CALL start
340 *FXFX2,0
350 *M. 0
360 *FX156,2,252
370 *FX205,0
Connect your 2m receiver output (on 145.825MHz) to the BBC computer cassette input and run the program below.
Type the program in exactly as below taking care with commas and the '#'
And '&' characters. You should save the program on cassette or disk before
attempting to run it. Take care during your typing to differentiate between -
0 = ZERO and O = Letter 'O'
When the satellite signal is audible, run the program and adjust the volume on your receiver for best copy of the received signal.
The program displays the incoming data from UoSAT-2 directly on the screen in real-time. It is basically a 'dumb terminal' program.
We leave it to the reader as an experiment to modify this simple program to 'capture' the incoming data to disk.
An ASCII file reader or word processor could then view the data.
By suitable modification, this program could form the basis of an automatic telemetry collection system.
Phase3 satellites use PSK coding at 400 bits/second which means a G3RUH modem or similar must be used to convert the incoming 'burble' to serial RS232 data for input to your computer.
The PACKET radio microsats use standard AX25 protocol data that can be decoded by a packet TNC and passed to a suitable program such as 'What's Up'. Note that many of the packet satellites transmit at 9600 bauds and decoding the data requires a suitable radio - taking the audio straight from the discriminator stage and using a suitable (G3RUH) 9600 bauds modem.
Modifications have been published - packet network and CD-ROM - describing how to modify a range of amateur radio gear to handle 9600 bauds packet.
Doing the modifications to allow access to the discriminator stage of your radio is not for the faint-hearted. If you are not sure of what you are doing - seek some expert help - satellite Elmer or maybe your local radio emporium.
You must keep the lead lengths as short as possible and make the modifications ‘exactly’ as described in the relevant text.
What use is telemetry to me?
Satellite telemetry if collected on a regular basis provides a valuable insight into conditions on board the satellites. By monitoring the data, the solar panel illumination, battery condition, transponder performance and other information may be graphed against time to reveal the changing conditions under which the satellite operates.
The telemetry provides vital
information to the groundstation controllers who use it to ensure the 'health'
of their satellites.
Various programs are available to decode satellite telemetry. Some are included on CD-ROMS intended for radio amateurs and others are available via the Internet. 'What's Up', written by Joe Kasser is a good example of a telemetry decoding program for the PC. It can decode telemetry from a variety of satellites by loading a 'configuration' file for the satellite of interest.
The AMSAT website – www.amsat.org
also carries a selection of downloadable telemetry decode software.
Another site worthy of mention is that run by Clive Wallis G3CVW.
This site is dedicated to the UoSAT2 satellite.
There you will find archived telemetry from UoSAT2 and free software to decode the data. Also available on this site are plans to build suitable decoders for UoSAT2.
Point your Internet browser at
– HTTP://WWW.USERS.ZETNET.CO.UK/CLIVEW/