karn (11/11/82)
PACSAT -- A New AMSAT Satellite Project
Den Connors, KD2S and Tom Clark, W3IWI
A new type of amateur satellite was proposed by Tom Clark,W3IWI, at the
packet radio working group meetings held 8-11 October, 1982 at the AMSAT
laboratory in conjunction with AMSAT's annual general meeting. The working
group meeting was also noteworthy in that it provided a focus for the
various packet radio groups to coordinate their activities and resulted in a
new unifed protcol which is described by Hank Magnuski, KA6M, in a companion
document. The new satellite project has been tentatively dubbed PACSAT -- a
final name is yet to be chosen, pending resolution of some potential copy-
right conflicts. This satellite would build upon AMSAT's experience in low-
cost spacecraft development, the current upsurge of interest in digital and
computer techniques, and the technology being developed for amateur packet
radio applications. The basic idea is to implement an orbiting digital
packet radio repeater with store-and-forward capabilites. This satellite
would allow amateur "electronic mail" service with a few hour delivery time
to anywhere in the world -- a virtual "flying mailbox" for amateur radio and
computer enthusiasts.
The "strawman" system proposed by Tom and discussed during the three
days of meetings, would have one or more high-speed packet radio channels,
and possibly other ASCII or RTTY input/output channels. An onboard computer
system would control a large amount of memory storage -- perhaps as much as
one megabyte for messages. Access to the message system would be somewhat
like using a more traditional computer bulletin board system (CBBS) using
landline dial-up capability. Using a Phase-2 type of low-earth orbit
("LEO"), the satellite would be available several times a day for up to 15
minutes worth of message reading and writing. Although the LEO satellites
are limited in their coverage, the store-and-forward capability could extend
an AMSAT-OSCAR-8 type satellite to provide global coverage.
One of the major problems with such a concept is 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 accptable bit error rates and practical contraints 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 tele-
command 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 (simult-
aneous transmission and reception) involving two bands (probably 435 Mhz
uplink and 145 MHz downlink). The design assumed that 0.5-1.0 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
communications would be by an active, onboard computer. Other functions to
be performed by this computer would include the access mechanism to manage
the "virtual disk" storage, handling of the protocols to allow multiple
users to get their messages up and down during a single pass, and possibly
inter-spersing of "QST" bulletins in the downlink data stream during moments
of inactivity.
On Saturday, the AMSAT members attending the technical forum were
allowed to hear packet radio in action on AMSAT's W3ZM (146.235/.835)
repeater. W3IWI brought a microprocessor-based Terminal Node Contoller (TNC)
developed by the Vancouver Area Digital Group (VADG), a "202A-type" AFSK
modem, and a standard 2m FM transceiver. Some 50 miles away in Sterling
Park, VA, Dave Borden (K8MMO) had his Z-80 CP/M computer hooked to a similar
TNC and modem. Amidst normal talk-in activities, the two packet radio
terminals were sending brief data packets through the repeater so that those
attending the meeting saw a game of "ADVENTURE" being played in the squelch
tails. This impressive demonstration was the first exposure for many AMSAT
members of the capabilities of digital techniques in amateur radio.
As the weekend proceeded, a number of informal discussions on packet
radio continued. Among the experts there grew a general agreement on the
data link protocol, with the AMSAT AMICON ADDCP-HDLC definition emerging as
being the best for the "LEO" PACSAT too. Arguments for and against Tom's
proposal for multiple uplinks (supporting a single downlink) centered on the
estimated relative traffic loading on satellite uplink and downlink. These
discussions included the observations:
-- CBBS experience shows that users "browse" through stored information
much more than generating new information. If the PACSAT is to be op-
erated as a flying CBBS, then the design should be reconsidered.
-- The "QST" bulletin transmissions will be a very important PACSAT
function for the users.
On Sunday, a proposal was made by Den Connors and Lyle Johnson which
included a number of the above concepts. Different mechanisms for access
using full-duplex uplink/downlink pairs were presented. The AMSAT AMICON
(AMRAD AX.25) HDLC logical link protocol was discussed in as a PACSAT
standard and a number of network-related issues, including message classifi-
cations and buffer allocation/deallocation mechanisms were raised. Lyle
presented another possible system block diagram, showing different
input/output channels, including command, multiple CPU's and different
memories for program store, file directories, buffers and message mass
storage. The tasks of the CPU were further detailed, and the different
algorithms needed were identified.
During this discussion, Tom Clark outlined the possibility of a truly
international implementation stategy. Tom described one scenario with the
system design and user eduction/interfacing being the prime U.S. respons-
ibility, on-board hardware constructed in South Africa, satellite int-
egration performed in the United Kingdom and actual launch handled by the
commercial group in Texas which has recently flown a successful test mission
(see A.S.R. #41, September 13, 1982). A part of the discussion involved the
possibility that part of the US role might also include a new role for AMSAT
in being responsible for the distribution of the hardware unique to packet
radio unless commercial interest is seen soon; this could serve as a method
for generating much needed revenues to support all of AMSAT's activities.
To help get this activity moving, Den Connors agreed to act as interim
PACSAT coordinator.
The general PACSAT concept was presented at the AMSAT general meeting,
and the AMSAT membership welcomed the idea, with the only caveats being
expressed as "Can we afford it? Are we stretching ourselves too thin?" Tom
indicated that these were AMSAT management concerns too but that the key
individuals were very enthusiastic about the concept. He stated that without
a dedicated "hardcore" of technical volunteers, AMSAT loses its vitality.
Since no further opposition was raised, Tom indicated that further develop-
ment of the PACSAT concept can be expected.
Since the meetings at Goddard, Den has accepted the position of Project
Manager for the US part of the PACSAT project, and several developmental
efforts are in the formative stages. ZS1FE has confirmed the South African
interest in pursuing the project, and G3YJO has begun "selling" the rest of
the payload (which may well be called UoSAT-2). Key developments in the next
few months will include ground station design, technology investigations to
identify large flyable memories and modem (both MSK and PSK) design,
planning for maximum utilization of the Phase-3B designs, conceptual designs
of the on-board communications and processing hardware and software and
coordination of the roles between the various groups that can contribute to
this project.
It is hoped that this satellite will not only give the packet radio
experimenters a much-awaited international linking capability, but also
provide a new, challenging and unique outlet to the many thousands of new
hams who are quite savvy in computer techniques and utilization. The AMSAT
team solicits your indications of interest and assistance. This, as with all
AMSAT projects, is a volunteer effort -- can we serve as a focus for your
creative ideas?