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Transponder Modes and Modems

Transponder Modes

To be written.

Beacon/Telemetry Modes

Beacons are autonomous transmitters on the satellite (they do not repeat ground-station transmissions). Generally, they send either pre-programmed messages ("HI" in the early OSCARs, synthesized voice greetings on the UoSATs and DOVE, and so on), or telemetry . Telemetry is typically engineering data (temperatures, voltages, status values) encoded as text. To decode this information we need to consider the modulation format (hence the type of modem required), the code (to convert from the binary modem output to text), and the telemetry conversion format (to convert from text - decimal, hexadecimal, or alphanumeric - to engineering units ).

Modulation Schemes

CW (Continuous-Wave). Also known as On-Off-Keying (OOK), the binary form of AM (amplitude modulation). Normally uses International Morse Code. A signal at a single radio-frequency (the carrier) is turned on and off. Typical reception involves conversion (heterodyning) to an arbitrary audio frequency such as 800 Hz for demodulation and decoding by the human ear.
FSK (Frequency-Shift Keying). The binary form of FM (Frequency Modulation). Typically used with standard asynchronous teleprinter codes such as ASCII or Baudot. Two different radio-frequencies are selected alternately to represent mark (on) and space (off) binary values. Typical reception involves heterodyning to a standard pair of audio frequencies, and demodulation by a phase-locked-loop, filter/rectifier/comparator, or DSP techniques such as cross-correlation, followed by decoding of the binary information.
AFSK (Audio Frequency-Shift Keying). A variation of FSK where two audio frequencies are alternately used to frequency-modulate an rf carrier. Sometimes used with asynchronous teleprinter codes (ASCII: UO-9, UO-11), more often with synchronous coding (HDLC, AX.25, etc. as in the various microsats). Reception requires a standard FM receiver, the audio output of which is applied to the same sort of modem and decoder as used for FSK. The commonly-used standard is called "Bell 202".
PSK (Phase-Shift Keying). A binary (or m-ary) form of PM (Phase Modulation). The phase of a radio-frequency carrier is shifted depending on the information bits. Almost always used with synchronous codes. Reception involves heterodyning to audio frequencies, and detection by a synchronous modem or DSP, followed by decoding.
RTTY (Radio TeleTYpe). A generic term for digital communication of text. Most often used in the Amateur Radio community to refer to Baudot-coded FSK or AFSK transmissions.

Telemetry Decoding

Once the telemetry information is received, it must be decoded - converted to engineering units such as temperature, voltage, etc. A wide variety of formats has been used. Here are some references.
"Amsat Oscar 10 RTTY Format", anonymous, Amsat Satellite Journal #1, Jan/Feb 1985, p. 13.
"The Telemetry Formats of JAS-1b/Fuji-OSCAR 20", JR1NVU, The AMSAT Journal, v. 13, no. 4, Sep 1990, p. 20.
"Decoding RS-12/13 Telemetry", Keith Bergland, WB5ZDP, The AMSAT Journal, v. 15, no. 1, January/February 1992, p. 13.
"The Evolution of Amateur Satellite Telemetry", Gould Smith, WA4SXM, The AMSAT Journal, v. 15, no. 4, July/August 1992, p. 11.

Telemetry/Beacon Index

AFSK - 1200 baud ASCII code - UO-11
AFSK - 1200 baud AX.25 code - DO-17
400 baud PSK - AO-10/AO-13
1200 baud PSK - AO-16, LO-19, FO-12, FO-20
9600 baud - UO-14


"The Phase III IFDEM Telemetry Beacon Decoder", John DuBois, W1HDX, Orbit #5, Jan/Feb 1981, p. 10.
"Modifications to the G3RUH 400 Baud Telemetry Demodulator", KA9LNV, Ed Krome, The AMSAT Journal, volume 13, number 3, July 1990, page 19. See ham radio, April 1985, page 50.
"A PSK Demodulator for the JAS-1 Satellite", Fujio Yamashita, JS1UKR, QEX, August 1986, p. 3.
"A Packet Radio PSK Modem for JAS-1/FO-12", James Miller, G3RUH, ham radio, February 1987, p. 8.
"A 1200 Bit/s Manchester/PSK Encoder Circuit for TAPR TNC Units", Barry McLarnon, VE3JF, QEX, November 1987, p. 10.
"The TAPR TNC Modem", Lyle Johnson, WA7GXD, QEX, September 1987, p. 3.
"Construction of the TAPR Phase Shift Keying Modem", WD0E, Jim White, The AMSAT Journal, volume 13, number 1, March 1990, page 15.
"Using the G3RUH 1200 PSK Modem with the AEA PK-232", WB6GFJ, Ross Forbes, The AMSAT Journal, volume 13, number 2, May 1990, page 7.
"Low-budget UoSAT-OSCAR-14 9600 baud Reception", GM4IHJ, John Branegan, The AMSAT Journal, volume 13, number 4, September 1990, page 8.
"Telemetry Reception with the PK-232", W6SHP, Howard Sodja, The AMSAT Journal, volume 13, number 4, September 1990, page 4.
"Low Budget UoSAT-OSCAR 14 9600 Baud Reception", John Branegan, GM4IHJ, The AMSAT Journal, v. 13, no. 4, September 1990, p. 8
"Microsat Demodulator", John C. Reed, W6IOJ, QEX, September 1990, p. 3. Corrigendum, QEX, November 1990, p. 18.
"Microsat Biphase Modulator Interfaces with any VHF TNC", John C. Reed, W6IOJ, QEX, January 1991, p. 10.
"FO-20 PSK Modem Improvement for Microsat Use", G3RUH, J. Miller, The AMSAT Journal, volume 14, number 3, May 1991, page 12.
"PK-232 Telemetry Reception Modification", Piero Moroni, I5TDJ, QEX, September 1991, p. 3.
"The KD2BD PacSat Modem", John Magliacane, KD2BD, QEX, August 1994, p. 20. Reprinted in The AMSAT Journal, volume 18, number 4, July/August 1995, p. 19.

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