Spotlight On: AMSAT-OSCAR-13

By: John A. Magliacane, KD2BD

[This article originally appeared in The AMSAT Journal, Volume 15 No. 2, March/April 1992]


Name:     AMSAT OSCAR 13 aka AO-13 
Object #  19216
Launch:   15 June 1988
Period:   686.65 Minutes (11 hrs, 26 min)
Orbit:    Elliptical
Altitude: 8 hrs 30,000 Km, perigee: 720 Km, apogee: 38,000 Km
Height:   1.35 m (53 in)
Width:    2.0 m (78.75 in)
Weight:   140 Kg (at launch), 90 Kg (after engine firings, 198 lbs)
Operating Modes: Mode B, Mode J, Mode L, Mode S
Beacons:  145.812 MHz; 400 bps PSK, 50 baud RTTY, 10 wpm CW
          435.651 MHz; 400 bps PSK, 50 baud RTTY
          2400.325 MHz

AMSAT-OSCAR-13 is 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 is in a near-Molniya orbit, which provides the spacecraft with an outstanding DX potential over the world's most populated regions. Roundtable contacts between Asia, North America, and Europe are commonplace through AO-13, using less transmitter power than is commonly used for HF communications.

AMSAT-OSCAR-13 is 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 present 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. Since that time, the SSP at apogee has begun to slowly move back south.

Careful analysis of OSCAR-13's orbit reveals that resonant perbutations exist which are leading the satellite into a "negative perigee" altitude by December 1996. The perigee is expected to be down to only 150 km by August 1996. This will drastically increase atmospheric drag on the satellite, which is 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 are energized by pulses of current controlled by OSCAR-13's on-board computer. The magnetic field produced by these coils interacts with the earth's magnetic field and produces a force capable of changing the attitude of OSCAR-13 or modifying the spin rate of the satellite. The magnetorquing procedures are always done near perigee.

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

General spacecraft operations are controlled through an Integrated Housekeeping Unit designed around an RCA 1802 central processing unit supported by 32 kilobytes of RAM. This processor runs software written in Interpreter for Process Structures (IPS), a multitasking programming language developed by Dr. Karl Meinzer, DJ4ZC. IPS is similar to Forth and has been 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 contains a digital communications transponder called "RUDAK-1". This is a store-and-forward mailbox designed around a 65SCO2 CPU. However, attempts to get the RUDAK experiment operating correctly have failed. The beacons carry spacecraft telemetry data and general spacecraft operating schedules and news using CW, RTTY and ASCII formats. Radioteletype (RTTY) is sent at 60 WPM using 170-Hz shift. ASCII bulletins are sent at 400 bits per second (bps) using binary phase shift keying (BPSK) modulation.

The Mode B transponder is by far the most popular transponder in use on OSCAR-13. 2-Meter downlink signals can 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 can be obtained with a circularly polarized antenna with at least 13dBc gain (bigger is better), along with a mastmounted GaAsFET preamplifier. Uplink antennas with at least 15dBc gain will enable solid Mode B communications with 50-watts or less transmitter power.

OSCAR-13 operations are much more than the usual voice, CW, RTTY and packet radio contacts found on the HF bands. Users can also participate in a number of nets carried on the various transponders. These nets include AMSAT information nets, and Slow-Scan Television nets, just to name a few. There are also "Techno Sport" activities, such as ZRO Memorial receiver sensitivity tests designed to promote the technical skills of OSCAR enthusiasts. Every Monday, UTC, is a QRP day on OSCAR-13.

Getting started on OSCAR-13 is easy. Start by getting a 2-Meter receiving system in operation so the Mode B transponder downlink can be received. After listening to OSCAR-13 contacts for a while, you'll get a feel for what antennas and transceivers are the most popular for OSCAR-13 operation. You might even come across F9FT talking about high-performance antenna design! If you get "hooked", then you can add 70-cm uplink capability to your station and you'll be ready to get in on all the Mode B action there is to be had on AMSAT-OSCAR-13.

AMSAT-OSCAR-13: NASA Catalog Number 19216

AO-13's Analog Transponders:

Mode B Uplink:       435.423 MHz - 435.573 MHz LSB
Mode B Downlink:     145.975 MHz - 145.825 MHz USB

Mode L Uplink:       1269.641 MHz - 1269.351 MHz LSB
Mode L Downlink:     435.715 MHz - 436.005 MHz USB

Mode J Uplink:       144.423 MHz - 144.473 MHz LSB
Mode J Downlink:     435.990 MHz - 435.940 MHz USB

Mode S Uplink:       435.602 MHz - 435.638 MHz LSB
Mode S Downlink:     2400.711 MHz - 2400.747 MHz USB

Beacons: 145.812 MHz (general)
         145.985 MHz (engineering)
         435.652 MHz
         2400.664 MHz

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