Spotlight On: Fuji-2/OSCAR-20
By: John A. Magliacane, KD2BD
[This article originally appeared in The AMSAT Journal, Volume
15 No. 5, November/December 1992]
Name: Fuji-OCSAR 20 Object # 20480 Launch: 7 February, 1990, Tanegashima Space Center, Japan Period: 112.23 Minutes Orbit: Low Altitude, elliptical, non-sun-synchronous Altitude: 1745 km apogee (1,084 miles) 912 km perigee (567 miles) Height: 470 mm Width: 440 mm Depth: 440 mm Weight: 50 Kg Shape: Polyhedron of 26 faces Operating Modes: Mode J - Analog Mode J - Digital
Fuji-OSCAR-20 is the second in a series of Japanese Amateur communications satellites having both analog and digital Mode J transponders. The first spacecraft, Fuji-OSCAR-12 was launched on August 12, 1986, and was the first OSCAR satellite to utilize the AX.25 packet radio communications protocol. It was followed by the currently active Fuji-OSCAR-20 several months after FO-12 had to be removed from service due to a deteriorating power budget.
Fuji-OSCAR-20 was launched on February 7, 1990 at 01:33 UTC from the Tanegashima Space Center, National Space Development Agency of Japan (NASDA) on an H-1 two-stage rocket. Its orbit differs slightly from most current OSCAR satellites, being slightly elliptical with a high inclination. This assures that the satellite will remain in sunlight for the majority of its orbit all year long.
The physical structure of FO-20 is that of a 26 sided polyhedron, with a weight of approximately 50 kg, so it is much larger than the Microsat satellites. Although Fuji-OSCAR-20 uses Mode J AX.25 packet radio communications links as the Microsat satellites do, one big difference between FO-20 and the Microsats is that FO-20's packet radio mailbox can be access without the need for special Microsat terminal software. Any computer or terminal that can be used to access terrestrial packet radio bulletin board systems (BBSs) can be used to access the FO-20 mailbox.
The other big difference between FO-20 and the Microsat satellites is that in addition to the packet mailbox features of the satellite, FO-20 also supports a Mode J analog transponder for SSB and CW communications.
The Mode JD beacon serves as the single AX.25 Mode JD downlink to ground stations, and also carries telemetry in either a ASCII (most often used), or binary formats. The Mode JD beacon operates on a frequency of 435.910 MHz with 1 watt of output power, and uses BPSK modulation.
The CW telemetry carries 12 analog status items and 33 status items, while the BPSK packet telemetry beacon carries 29 analog items, plus 33 status items.
The Mode JA analog transponder system consists of an inverting heterodyne transponder with a bandwidth of 100 KHz, with an uplink in the 145 MHz band, and a downlink in the 435 Mhz band. An uplink power of about 100 watts EIRP is required for access to the transponder, but of course, the more sensitive your downlink receiver is, the less uplink power is required to achieve an adequate downlink signal-to-noise ratio.
The Mode JD digital transponder system functions as a mailbox using the AX.25 level 2 protocol. It receives Manchester encoded FSK on any one of four uplink frequencies, and transmits on a single downlink frequency of 435.910 MHz at 1200 bps using BPSK modulation. An uplink of about 100 watts EIRP is the minimum required for Mode JD transponder access. The satellite transmits with 1 watt of transmitter power.
Fuji-OSCAR-20 uses circular antenna polarization on all uplink receivers and all downlink transmitters and beacons. The 145 MHz uplink antenna is a ring turnstile antenna mounted below the bottom side panels of the spacecraft, and exhibits a maximum gain of about +0.5 dBi. The 435 MHz transmitting antenna is a turnstile array mounted on the top of the spacecraft, and exhibits a maximum gain of about +4 dBi.
Communications via Fuji-OSCAR-20's Mode JA analog transponder is similar to operating through any of the other analog OSCAR transponders. The only difference is in the way Doppler effect is compensated.
During normal communications through any non-geosynchronous satellite, Doppler shift is encountered between the uplink to the spacecraft, and the downlink to the ground station. Different ground station within the footprint of the satellite will experience varying degrees of Doppler shift. In order to minimize the effects of Doppler shift on frequency sensitive communication modes, two things are done. First, spacecraft designers use "inverting transponders", so as the spacecraft sees an uplink signal that is drifting lower in frequency, it is translated to a downlink that drifts higher in frequency within the transponder passband. Second, users of the transponder adjust their transmitter and receiver tuning frequencies during a contact to prevent excessive frequency drift throughout the transponder passband.
A general "rule of thumb is to vary the transmitter or receiver operating on the highest frequency band to minimize frequency drift observed by the downlink receiver. In the case of Mode J, the transmitter frequency should remain fixed during a contact, while each ground station adjusts their downlink receiver in compensation for Doppler shift. This is done because Doppler shift is proportional to RF operating frequency, and since the Mode J downlink is on the higher of the two bands used by the transponder, adjusting the downlink receiver will have the greatest effect in compensating for Doppler effect.
Communications via Fuji-OSCAR-20's Mode JD digital store-and-forward communications transponder requires the use of a packet radio terminal node controller (TNC), a 1200 bps "Pacsat Modem" along with a 2-Meter FM uplink transmitter, and a 70-cm SSB receiver or combination of HF receiver and downconverter. Some means of routing automatic frequency control (AFC) signals from the modem to the downlink receiver will be required so the modem can tune the downlink receiver in compensation for Doppler shift. The uplink transmitter does not require Doppler compensation, however.
Since a single downlink is used by all users of FO-20's mailbox, the ground station TNC parameter MAXFRAME should be set to 7 or higher to allow FO-20 time to respond to all the packets it receives from ground stations.
Connection to the FO-20 mailbox is made by issuing the TNC connect command to FO-20's callsign, "8J1JBS":
cmd:CONNECT 8J1JBSUpon successful connection to FO-20, the ground station TNC will announce the connection with the message:
*** CONNECTED TO 8J1JBSafter which FO-20 will provide a greeting and mailbox prompt:
FO-20/JAS1b Mailbox ver. 2.00 commands [B/F/H/M/R/U/W] Use H command for Help JASFO-20 mailbox commands differ somewhat from what most packet users are used to from terrestrial packet bulletin board systems. FO-20 mailbox commands are as follows:
B : List file headers addressed to ALL F : List latest 15 file headers F * : List latest 50 file headers F d : List file headers posted on day d H : Show help message (list of available commands) K d : Kill a file number n M : List file headers addressed to current user R n : Read a file numbered n U : List callsigns of those currently connected W : Write a fileFiles are ended by entering a period on a line by itself, after which the mailbox responds:
END JASand is ready for the next ground station command.
FO-20 does not have a "bye" command to log off the mailbox. Users must manually issue the DISCONNECT command to their TNC to log off the mailbox and disconnect from FO-20.
Fuji-OSCAR-20 spends most of its time in Mode JD. The analog transponder is usually activated on Wednesdays, UTC. Operating schedules are published frequently in SpaceNews and are also made available through the AMSAT News Service (ANS) bulletins.
Fuji-OSCAR-20 can be a great start on the road to digital satellite communications. Its Mode JD mailbox requires no special ground station terminal software, and most packet users already the needed 2-Meter FM transmitter required for the Mode JD uplink. The Mode JA transponder is sensitive, and a great way to talk to all the people active on Mode JD!
TABLE 1 - Fuji-OSCAR-20 operating frequencies and modes.
FUJI-OSCAR-20 Catalog Number: 20480
Transponder: Mode JA - Linear, Inverting
Uplink Passband: 145.900 MHz to 145.800 MHz
Downlink Passband: 435.800 MHz to 435.900 MHz
Beacon Frequency: 435.795 MHz CW or BPSK
Transponder: Mode JD - Digital, 1200 bps, AX.25 Protocol
Uplinks: Uplinks are Manchester encoded FSK using a maximum frequency shift of 3.5 KHz
145.850 MHz
145.870 MHz
145.890 MHz
145.910 MHz
Downlink: 435.910 MHz 1200 bps, AX.25, NRZI, BPSK
TABLE 2 - Sample beacon downlink data.
8JlJBSBEACON UI C: JASlb RA 92/11/18 16:15:00 000 656 654 652 705 831 841 826 000 566 613 000 457 497 493 480 482 495 650 000 671 699 679 720 999 641 873 290 8FD 000 010 101 000 001 111 100 001 100 000 000 8JlJBSBEACON UI C: JASlb MO 92/11/18 16:15:00 Mailbox is at your service from 92/11/05 00:45:00 The JD Transmitter is available in all orbits during JD mode. FO-20's BPSK beacon transmits telemetry and status information as an unnumbered information UI packet frames once ever minute.
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