AMSAT-OSCAR 13
(Phase 3C)

Spacecraft Summary

Detailed Description

AMSAT-OSCAR-13 was the most powerful, and probably the finest Amateur Radio communications satellite in operation at the present time. AMSAT-OSCAR-13 was modeled after AMSAT-OSCAR-10, which was launched in 1983. However, unlike OSCAR-10, OSCAR-13 was in a near-Molniya orbit, which provideed the spacecraft with outstanding DX potential over the world's most populated regions. Roundtable contacts between Asia, North America, and Europe were commonplace through AO-13, using less transmitter power than was commonly used for HF communications.

AMSAT-OSCAR-13 was the third in a series of "Phase 3-type" high-altitude, elliptical orbit amateur communications satellites. The first Phase 3 spacecraft, known as "Phase 3A before launch, was lost in the Atlantic when its launch vehicle malfunctioned after launch. "Phase 3B" became AMSAT-OSCAR-10 after launch. OSCAR-10's launch vehicle bumped the spacecraft after deployment, causing damage to one of OSCAR-10's antennas. Later, a problem with OSCAR-10's kick motor failed to transfer the spacecraft to it intended 57 degree inclination orbit, placing it in a 26 degree inclination orbit instead. After several years of operation, OSCAR-10 experienced a "stroke", when radiation induced damage to the spacecraft's Integrated Housekeeping Unit caused the satellite to go "brain dead", leaving only its Mode B transponder in operation. The non-optimal kickmotor burn left AO-10 in an orbit which caused it to spend much more time in the Van Allen Radiation Belts than it was originally designed for, hence the reason for the IHU failure.

AMSAT-OSCAR-13 was launched on July 15, 1988 from Kourou, French Guiana, South America, by the European Space Agency on an Ariane-4 rocket. OSCAR-13 was initially placed in a transfer orbit having an apogee of 36,077 km, a perigee of 223 km, and an inclination of 10 degrees. Through a series of kick motor firings, OSCAR-13 controllers were able to carefully boost the spacecraft into its final 36,265 km x 2545 km x 57 degree inclination orbit. OSCAR-13's sub-satellite point at apogee was slowly moving northward, and reached a maximum latitude equal to it orbital inclination of 57 degrees in November 1991. At that time, the SSP at apogee began to slowly move back south.

Careful analysis of OSCAR-13's orbit revealed that resonant perbutations existed that were leading the satellite into a "negative perigee" altitude by December 1996. The perigee was expected to be down to only 150 km by August 1996. This will drastically increase atmospheric drag on the satellite, which was predicted to eventually lead to the decay of the spacecraft by late 1996.

OSCAR-13 spacecraft re-orientation occurs several times a year when the solar panels on the satellite no longer point directly toward the sun. Instead of using propellants to change the attitude of the spacecraft, a series of "magnetorquer" coils were energized by pulses of current controlled by OSCAR-13's on-board computer. The magnetic field produced by these coils interacted with the earth's magnetic field and produceed a force capable of changing the attitude of OSCAR-13 or modifying the spin rate of the satellite. The magnetorquing procedures were always done near perigee.

AMSAT-OSCAR-13 carrieed four beacon transmitters and four linear transponders. Transponder scheduling was based on sun angles, power budget, and mean anomaly. Mean anomaly (in this case), was a modulo 256 orbital "clock" that indicateed where the spacecraft was located in its orbital plane. A mean anomaly value of 0 indicateed the spacecraft was located at perigee, beginning a new orbit. A mean anomaly of 128 occured half way through the orbit when the spacecraft was at apogee, its farthest point from the earth's surface.

General spacecraft operations were controlled through an Integrated Housekeeping Unit designed around an RCA 1802 central processing unit supported by 32 kilobytes of RAM. This processor ran software written in Interpreter for Process Structures (IPS), a multitasking programming language developed by Dr. Karl Meinzer, DJ4ZC. IPS was similar to Forth and was used to control other 1802-based satellites, such as OSCAR-10 and OSCAR-11 until its "Diary" operations were written in Forth.

OSCAR-13 also contained a digital communications transponder called "RUDAK-1". This was a store-and-forward mailbox designed around a 65SCO2 CPU. However, attempts to get the RUDAK experiment operating correctly failed. The beacons carried spacecraft telemetry data and general spacecraft operating schedules and news using CW, RTTY and ASCII formats. Radioteletype (RTTY) was sent at 60 WPM using 170-Hz shift. ASCII bulletins were sent at 400 bits per second (bps) using binary phase shift keying (BPSK) modulation.

The Mode B transponder was by far the most popular transponder in use on OSCAR-13. 2-Meter downlink signals could be copied using a simple monopole antenna, GaAsFET pre-amplifier, and a 2-Meter SSB/CW receiver or HF receiver with suitable downconverter. Much better performance could be obtained with a circularly polarized antenna with at least 13dBc gain (bigger was better), along with a mastmounted GaAsFET preamplifier. Uplink antennas with at least 15dBc gain enabled solid Mode B communications with 50-watts or less transmitter power.

Reference Documents:

• James Miller, "A PSK Telemetry Demodulator for OSCAR 10," Ham Radio, Apr 1985, pp 50-51, 53-55, 57-62; also appeared in Electronics and Wireless World, Part 1, Oct 1984, pp 37-41, 59-60, Part 2, Nov 1984, pp 37-38. Now superceded (1992) by a much improved MK II P3 Data Demodulator.
• "Phase 3C System Specifications," Part 1 and Part 2, Amateur Satellite Report, no. 177, 8 Jun 1988, pp 3-4.
• Vern Riportella, "Introducing Phase 3C: A New, More Versatile OSCAR," QST, Jun 1988, pp 22-30; reprinted in The ARRL Satellite Anthology
• Peter Gulzow, DB2OS, AMSAT OSCAR-13 Telemetry Block Format OSCAR News, no. 73, Oct 1988, pp 8-14.
• Amsat Satellite Report, No. 177, 178, 179, Jun 8, Jul 5, Jul 18, 1988.
• Karl Meinzer, "The Orbital Decay of AMSAT-OSCAR-13?," The AMSAT Journal, Vol. 13, No. 3, Jul 1990, p. 1.
• John A. Magliacane, "Spotlight On: AMSAT-OSCAR-13," The AMSAT Journal, Vol. 15, No. 2, Mar/Apr 1992, p. 17.
• James Miller, "Measure AO-13 Squint Directly," The AMSAT Journal, Vol. 16, No. 1, Jan/Feb 1993, p. 20.
• James Miller, Managing Oscar-13, (206kB) The Amsat Journal, Vol. 17, No. 1, Jan/Feb 1994.
• James Miller, The Re-Entry of Oscar-13, (95kB) The Amsat Journal, Vol. 18, No. 3, May/Jun 1995.
• Martin Davidoff, Satellite Experimenter's Handbook, 2nd edition, The American Radio Relay League, Newington, CT., 1990.
• Keith Berglund, Beginner's Guide to AO-13, AMSAT