Jim White, WD0E, firstname.lastname@example.org
November 5, 1997
We are attempting to keep the DOVE S-band transmitter on for those who wish to test receive equipment on that band and will announce via amsat-bb if it is temporarily off. It is presently on.
The DOVE S-band transmitter puts out about .8 watts to a 1/4 wave whip mounted on the +Z surface about 4" in from one edge. The +Z is generally pointed away from the earth in the northern hemisphere and toward the earth in the southern. If you are listening from north of the equator you should hear deep fades about every 30 seconds or so as the satellite rotates about its Z axis and the S antenna is blocked by the body of the satellite. Below the equator the signal is pretty steady.
The data transmitted on S is the same as on 2 meters, telemetry and a short text broadcast about every 30 seconds to 1 minute depending on power management needs. All data is ASCII text. The modulation is 1200 baud PSK.
Decoding the data from this transmitter presents an interesting challenge because the modulation is about 20dB below the carrier. The carrier suppression failed at launch resulting in lots of carrier and low level modulation. Only a few packets have been successfully decoded from this transmitter - those by using a tracking DSP notch filter to suppress the carrier. That technique requires a receive system capable of achieving about 30dB signal to noise to provide modulation about 10dB above the noise, and a way to eliminate the carrier.
A good test of your S receive capability is to listen for the buzz of the flags which are continuously sent. If you can detect the pitch of the buzz change as data is sent your receive is better than 20dB SNR. If you can notch out the carrier, run the resulting signal into a PSK demod and decode a packet you have a pretty darn good S receive system.
Tracking the Doppler is also a challenge. Generally it is necessary to tune a radio 100 Hz per step to keep up with the Doppler at TCA. If the radio steps in the middle of a packet it will nearly always be corrupted.
Incidentally, in the near future we may have a sound card/Pentium software demod for this transmitter that might extract the data without the need for notching out the carrier. If that works it would provide an easier way to test the quality of an S receive system.
This article was originally posted to the AMSAT-BB mailing list on November 5, 1997, by Jim White, WD0E, email@example.com.