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DTUsat-1: Radio
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Radio


If the satellite is unable to communicate, is could as well be made of wood - the density is about right. Our radio transceiver is capable of radio amateur compatible half duplex 2400 bps AFSK communication in the 70 cm band.

The radio transceiver is built around commercial chips intended for the ISM bands. It transmits a short beacon signal every 2 minutes in failsafe mode, but every 5th of these are replaced with a morse code signal giving the satellite amateur call sign followed by the name.

We recognize that the AMSAT frequencies are valuable natural resources, and would hereby like to thank the amateur community for letting us use these for DTUsat. The satellite is very small and has very little power, and will not be able to support communication services as do e.g. the pacsats. Instead, we plan to return the favor after commissioning by periodically transmitting a test signal with calibrated power steps. This should make it possible to assess the sensitivity of a rig by noting the last signal received and computing the path loss.

The uplink and downlink frequencies for all communication are 437.475 MHz. Data are modulated using 2400 bps AFSK. The frequencies are 1200 Hz for 1, 2400 Hz for 0. Our modems are constructed using the CMX469 integrated circuit, using the reference schematic found in the data sheet. The UART on board the satellite demands start- and stop bits when running in synchronous mode, which makes it possible to use a MAX232 or similar circuit to directly interface between the modem chip and a serial port. Alternately, it should be possible to tweak one of the soundcard modems currently available to accommodate the encoding as described above.

Due to lack of manpower in our software group, the satellite will run in safe mode the first weeks. This means that the satellite will transmit a simple beacon signal every two minutes. The beacon contains basic telemetry battery voltage, currents and temperature, encoded in a slightly modified AX.25 packet format. The modifications are primarily that start- and stopbits for the onboard UART are included in the bitstream. The beacon signal sounds like this. Every 5th of the beacon signals (i.e. every 10 minutes) are transmitted using morse code at 13 wpm, on-off keying modulated on the FM carrier and with a 1200 Hz modulation. The transmitted message is "OZ2DTU DTUSAT". This beacon signal sounds like this. (1.1 Mb wav). Later in the mission, the beacon will change to a more advanced format containing more information. Also, a test transmitter will be brought on-air, generating a calibrated test signal which is repeated several times at amplitudes from approximately 25 dBm EIRP down to 0 dBm.

Details on both packet data formats will be uploaded here shortly, but until then, every 5th beacon should be easily understood :-)

To listen to DTUsat, you will need a 70 cm rig and maybe a modem. Your antenna system will probably have to be directional and capable of elevation, as the maximum EIRP of DTUsat is less than 400 mW, about 25-26 dBm. The satellite used circular polerization but direction depends on satellite attitude. DTUsat is hard to acquire without a good idea about where to look for it, as it transmits beacons only every 2 minutes and in short bursts. The first update from NORAD says that it is likely to be around the following two-line element:

DTUSAT                  
1 27842U 03031C   03209.47176581  .00000166  00000-0  97626-4 0   238
2 27842  98.7261 216.0304 0008734 201.5048 158.5762 14.20499763  3941

The current orbital parameters are posted and updated here (Heavens Above):

DTUSAT DEC03
1 27842U 03031C   03337.52439911 +.00000344 +00000-0 +18013-3 0 01180
2 27842 098.7268 342.4493 0008833 192.9441 167.1552 14.20572845022128
And they still are:
DTUSAT FEB06
1 27842U 03031C   06056.01968681  .00000009  00000-0  24426-4 0  6972
2 27842 098.7225 066.5130 0009961 013.2878 346.8553 14.20799018137773

Obsolete: Please report any received transmissions from DTUsat, decoded or not, with your position and the time of the reception to dtusat@dtusat.dtu.dk. If you include information on your position, we will calculate the absolute signal strength and return it to you per e-mail as a virtual QSO-card.