brainwagon

The legendary Phil Karn, KA9Q is apparently the brains behind the digital telemetry modem that will be used aboard the ARISSat-1, a satellite designed to be tossed off the ISS sometime next year. From his paper:

ARISSat-1 will carry a new telemetry modulation and coding scheme, BPSK1000, designed to handle the severe fading often encountered with low orbit satellites without attitude control. Its performance and the link budgets for the ARISSat-1 spacecraft are such that reliable reception should require only a simple whip or ground plane antenna, a conventional 2m SSB receiver, and a reasonably modern personal computer with audio A/D input.

BPSK1000 uses differential binary phase shift keying (DBPSK) at a channel symbol rate of 1 kHz in a SSB bandwidth. With constraint length 7, rate ½ forward error correction (FEC), the user data rate is about 500 bits/sec. HDLC framing provides application flexibility (including the ability to carry AX.25 in other applications) and a deep (16 second) convolutional interleaver provides strong protection against fading.

The BPSK1000 Telemetry Modem for ARISSat-1

I wasn’t able to find any links to the software, but I’m not panicking, since the launch is several months away, but worth looking into.

While googling, I found that KE9V had mentioned this a couple of days ago. One of his comments asked “Why not just use AFSK/FM like all the traditional packet satellites?” The reason is simple: it doesn’t work as well. The way these things are usually evaluated is in terms of Eb/N0, which is the “energy per bit, per unit of spectral noise density”, usually defined for a given error rate (10^-5 seems typical). For AFSK/FM at 1200 baud, Eb/N0 works out to 24db. For Karn’s proposed technique, it works out to a mere 6.7db. In practical terms, this means the satellite can operate at much lower power levels (it’s going to only be sending out 100mw) and the signal can still be reliably detected by omnidirectional antennas.

Very cool.