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CX3ACS > SAREX 21.06.97 20:57l 151 Lines 4919 Bytes #-10422 (0) @ AMSAT
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To : SAREX@AMSAT
=========
SpaceNews
=========
MONDAY JUNE 16, 1997
SpaceNews originates at KD2BD in Wall Township, New Jersey, USA. It
is published every week and is made available for non-commercial use.
* P3D ENGINEERING SOFTWARE NEWS *
=================================
Regular users of FO-20 may have been puzzled by a strange buzzing sound
on
its mode JA transponder over the last couple of weeks. This was due to
command stations testing the new P3D Range Determination software and
hardware.
P3D's orbit will change significantly after motor firings, and NORAD
will
lose track of us. So would we! The P3 ranging system enables our world
wide network of Command Stations to measure distance to the satellite
from
different locations and at different times and so compute the new
orbit's
Keplerian elements. This information is used by us, the AMSAT
community,
and of course given back to NORAD so that they can re-acquire our
satellite
by radar. This methodology was used to remarkable effect in picking up
OSCAR-13 after both its motor firings.
Back in 1988, the P3C engineering software was still based on the
Atari-800XL
computer. This was a legacy from the very successful designs
originating as
far back as 1979 and the ill-fated P3A satellite (1981).
However, the prospect of working on P3D yet again using a dual audio
cassettes
for storage and 8-bit 2 MHz processors has never been very attractive,
and a
re-engineering of many of the tools has been undertaken by James Miller
G3RUH
over the last couple of years.
The P3 Range Determination package marks the successful completion of a
substantial development programme that includes the following principal
packages:
- Tracking
- Telemetry Display
- Command Uploader
- IPS-X1802
- Ranging
- OrbitFit
Tracking of course needs no introduction.
Telemetry Display is essentially as already available for AO-13, but
with P3D
specific changes. This will be publicly released for several platforms
when
P3D is finally "nailed down".
Command Upload software is used for commanding the spacecraft both
during
lab-testing and in space. These first three packages were used for
AO-13.
P3D, like its predecessors, has a flight computer based on a radiation
hardened CDP-1802 microprocessor. It runs an operating system called
IPS.
But generation of the flight operating system and on-board control
software
is done on a ground-based host computer. The IPS-X1802 package is, as
its
name suggests, a cross compiler.
Source files for P3D are written using the IPS language. They are
compiled
by the IPS-X1802 development system (itself written in IPS) which
outputs a
target binary in 1802 machine code. In the days of P3A/B/C, this
compilation
used to take half an hour on the Atari-800XL. Imagine how tedious a
simple
edit used to be. Flight software compilation, from source files to
uploadable binary, now takes half a second.
The IPS-X1802 cross-compiler is a major part of the P3D development
programme. Without it, the flight software which controls the
spacecraft,
and is much different than that of P3A/B/C, would be impossible to
produce
efficiently in the short timescales left to us.
The Ranging software, mentioned earlier, measures range to the satellite
with a basic accuracy of about 150m (1 microsecond). Comparison of
ranges
measured via FO-20, with ranges displayed by regular tracking programs,
shows
agreement within 5 km, often better. During tests, signal strengths
have
been kept to the absolute minimum needed for "lock", and are weaker than
AO-13's general beacon used to be. The uplink power to FO-20 is
typically
under 1 watt to a KLM14C antenna, rather less than a typical SSB user's.
The ranging software can track down to a level where the signal is
virtually
inaudible. Sample GIF (5 kbytes) taken during an FO-20 pass is at:
http://www.jrmiller.demon.co.uk/IPS/range.gif
The final program OrbitFit takes range measurements from the network of
command stations, and computes an orbit that best fits the data. It was
written 10 years ago by Stefan Eckart DL2MDL, and has been adapted for
our
current needs by G3RUH. Tests processing FO-20 ranging data have been
completely successful.
Software is written in BASIC and ARM assembler for the Acorn Risc
Computer.
Current machines use the DEC SA-110 processor at typically 200 MHz/700
mW/$50,
representing the highest MIPs/mW and MIPs/$$ (taken together) in the
business
and currently the embedded systems processor of choice. It makes a cool
personal computer too.
Acorn Risc Computer:
http://www.acorn.co.uk/acorn/products/strongarm/
DEC SA-110 information:
http://www.europe.digital.com/info/semiconductor/sa110.htm
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