Bulletins from the Pacific Packet Radio Society - page 130

One of the major problems with such a concept is A that packet radio represented new technology to most amateurs, although we see a marked rise in amateur interest in computers. In order to make a PACSAT be a viable concept, the current packet radio "experts" will have to devote considerable efforts to making reliable ground-station hardware and software available to AMSAT's user community, and they will have to embark on a concentrated educational program to explain these new concepts -- some of the attendees at the working group meeting noted that they were around when the SSB vs. AM "wars" were raging in the early 1950's.

One of the major technical problems that will have to be solved before a PACSAT (or the AMICON channel for Phase-3B) is viable involves modems. None of the standard commercial modems seem suitable for noisy channels with doppler shift. Several alternatives were discussed during the meetings. The modulation techniques mentioned included phase-shift keying (PSK) and minimum-shift keying (MSK). Discussions of data rates for up- and down-links ranged from 400 baud up to 56 kbaud; the technical constraints associated with acceptable bit error rates and practical constraints of implementation cost and difficulty plus available spectrum space led to a consensus that likely rates were in the 400-2400 baud range. Although the Phase-3B telecommand group have implemented state-of-the-art 400 baud PSK modems and W4RI has been working on MSK designs, the use of either technique will require considerable work in order to develop high reliability modems usable by the amateur community. For any digital usage of amateur satellites, it is clear that modems will be more complicated that the traditional frequency-shift keyed systems currently used for RTTY.

The strawman design Tom presented involved multiple uplinks and a single downlink (e.g. , one calling uplink channel and perhaps four working channels). This built upon typical amateur net experience with the satellite acting as "NCS" on a calling-and-answering frequency. After the user calls in and establishes that he has uplink traffic (or that the satellite has a message for him ), he would directed to QSY to a working message channel where the spacecraft's computer would "poll" the user until both he and the spacecraft are QRU. This design is based on the use of ALOHA-type protocols where the multiple users cannot hear each other, leading to possible "collisions" on the uplink channels; this , combined with Tom's assumption that "what goes up must come down" (i.e. the total message traffic up and down are about equal), led to the multiple uplink, single downlink proposal. Tom's proposal was that the uplink and downlink were full duplex (simultaneous trans mission and reception) involving two bands (probably 435 Mhz uplink and 145 MHz downlink). The design assumed that 0.51.0 Megabyte of storage was available onboard which could be treated as a "virtual disk" for planning. All messages would be "bit-regenerated" (i.e. no "direct" channel exists between the users except through the satellite), and control of the

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