DIGITAL SATELLITES.
In 1984 the first ‘flying mailbox’ was put into orbit. UoSat-Oscar-11, built by the University of Surrey, England, carried the DCE (digital communications experiment) a proof of concept testbed for the future digital satellites. In August 1986 Japan launched Fuji-Oscar-12 and hams had their first opportunity to try digital satellite transponders.
In January 1990 four microsats were launched from a single Ariane rocket, as secondary payloads along with two new UoSats - Oscar-14 and UoSat-Oscar-15. The ‘amateur’ microsats were Amsat-Oscar-16, DOVE (digital orbiting voice encoder)-Oscar 17, WEBERSAT-Oscar-18 and LUSAT-Oscar-19.
AO-16, an Amsat-NA project and LO-19 (LUSAT), an Amsat-Argentina project provide 1200BPS packet operation using FM Uplinks and PSK Downlinks. PSK (phase shift keying) modulation is used because of it's ability to copy very weak signals. PC based broadcast protocol packet software is used to communicate with these satellites.
AO-19, sponsored by Weber State University, Utah, USA sends images in a binary format, which can be viewed using special software called Weberware.
DOVE-Oscar-17, sponsored by Amsat-Brazil (Bramsat) sends telemetry using the standard amateur AX25 packet protocol which can be copied on an ordinary packet TNC. The concept of DOVE is to collect short messages from schoolchildren, encode them then have DOVE broadcast them world-wide.
In the years between 1990 and 1998 several nations have been responsible for other digital satellites. UO-22 (University of Surrey), KO-23 (Korea, but built at the University of Surrey), KO-25 (Korea, built in Korea) AO-26 (ITAMSAT) Italy, UNAMSAT (Mexico) and SUNSAT (South Africa) with launched scheduled for 1998 to name just a few. FO-20 and FO-29 were Japan's replacements for FO-12. Others are planned. for future launch.
Some of these satellites use higher data rates, namely UO-22, KO-23 and KO-25 operating at 9600BPS (bits per second). Note that higher data rates require more bandwidth. Even higher data rates are planned for future satellites.
KO-23 is ‘famous’ for it's images taken with it's pair of CCD cameras, one with wide image lens the other one using a narrow image lens. One of the interesting experiments for amateurs as well as the usual ‘mailbox’ communications, is downloading the images, noting the date and time the picture was taken, checking the pass against a favourite tracking program to figure out exactly where the satellite was at the time - by looking for detail in the images.
AO-27 also has a digital transponder. AO-27 is described in it's own section.
MIR also uses digital communications as well as analogue.
At the time of writing (June 1998) the digital transponders on FO-20 and FO-29 are non-operational, though both are currently operating in analogue mode and providing excellent service.
The equipment needed to operate digital satellites is a bit more complex than an analogue only station. As well as the transceivers in use, 2m, 70cm 1.3ghz and antennas, you'll also require a TNC (terminal node controller) connected to your PC and a satellite modem. Depending on whether you wish to work the PSK or FM birds you'll need either (or both!) a PSK modem or 9600BPS modem. (Contact a ‘digital satellites’ Amsat Elmer for advice, 1st page in Oscar News).
|
|
Example of a packet station.
Note:
Packet satellites use either data rates of 1200BPS, 9600BPS or both.
FSK and AFSK are both used! You should seek expert help in selecting equipment for your Digital Satellite Station. Check the transponder tables elsewhere in the booklet for your satellite(s) of interest.
In addition you will need PC based software, either PB/PG (Packet Broadcast/Packet Groundstation) written specifically for the job at The University of Surrey, in DOS, or WISP, a Windows based program for digital satellite operating. Those not used to digital satellites will find it odd at first but will soon be drawn by the challenge.
PB/PG
The PB/PG package comes in two parts, Packet BROADCAST and Packet GROUNDSTATION.
Because there are several ‘users’ within the footprint at the same time who would normally try to download some of the same files, the satellite
‘broadcasts’ these files so that all or any of the users can copy them at the same time. PB takes care of this, captures all the files the satellite
broadcasts, labels them with a unique number and makes a directory. You can see what is being broadcast on your screen. Obviously the LEO satellite can only send a certain amount of data while it's passing near your QTH so you may not receive a complete file or picture. The bits that are missing are called ‘holes’. PB is a program that allows you to communicate with the satellite.
If for instance you see that you have only received part of a message addressed to you, you can request the satellite to fill the holes on the next pass.
Different files within the program do different things but the program is
fully automated so once it is set up correctly you can just sit back and watch the data flash across your screen.
The scope of this booklet doesn't allow for a ‘course’ in setting up PB/PG or WISP but Amsat offer an ‘Introduction to Digital Satellites’ comprising a very informative tutorial booklet and disc containing the program.
The WISP program has extensive help files and full instructions for setting up the fully automated software.
DIGITAL SATELLITE TRANSPONDERS
|
SATELLITE |
UPLINKS |
DOWNLINKS |
CURRENT
ACTIVITY/ NOTES |
|
UoSAT2 |
NONE |
145.825 435.025 2401.500 |
1200 BAUDS
ASCII (BELL 202) |
|
U-O-14 |
---------- |
---------- |
9600 BPS
COMMERCIAL SERVICE |
|
A-O-16 |
145.900 145.920 145.940 145.960 |
437.051 437.026 2401.143 |
1200 BPS FSK/PSK ‘PB’ PB
CALL=PACSAT-11 PG CALL=
PACSAT-12 |
|
D-O-17 |
NONE |
145.825 2401.220 |
1200 BPS FSK AX25 DATA |
|
W-O-18 |
NONE |
437.075 437.100 |
1200 BPS PSK BINARY DATA
& PICTURES |
|
L-O-19 |
145.840 145.860 145.880 145.900 |
437.150 437.125 |
1200 BPS
FSK/PSK ‘PB’ PB CALL =
LUSAT-11 PG CALL =
LUSAT-12 |
|
F-O-20 |
145.850 145.890 145.910 |
435.910 |
AX25 BBS
1200BPS FSK/PSK |
|
U-O-22 |
145.900 145.975 |
435.120 |
9600BPS FSK
‘PB’ PB CALL =
UOSAT5-11 PG CALL =
UOSAT5-12 |
|
K-O-23 |
145.850 145.900 |
435.175 |
9600BPS FSK
‘PB’. PB CALL =
HL01-11 PG CALL =
HL01-12 |
|
K-O-25 |
145.870 145.980 |
436.500 435.175 |
9600 BPS FSK
‘PB’ PB CALL =
HL02-11 PG CALL =
HL02-12 |
|
I-O-26 |
145.875 145.900 145.925 145.950 |
435.867 435.822 |
1200 BPS
FSK/PSK ‘PB’ PB CALL =
ITMSAT-11 PG CALL =
ITMSAT-12 |
|
P-O-28 |
145.925 145.975 |
435.250 435.275 |
9600 BPS FSK
‘PB’ |
|
MIR |
145.550 |
145.550 |
1200 BPS AX25
BBS |
|
SAREX |
145.490 |
145.550 |
1200 BPS AX25
ROBOT |
|
TMSAT |
NONE |
436.926 |
9600bauds FSK 38400bauds FSK high res image
data (see text) |
NOTES.
PRIMARY downlink frequencies are listed first.
All uplink frequencies are available simultaneously.
Operating procedure on the digital satellites using the dedicated PACSAT software is pretty much the same as a terrestrial bulletin board system - apart from the ‘filling of holes’ procedure, where you need to request ‘hole fills’ from the satellite by specifying the unique file number of the ‘hole’ you need filled. (Your directory will specify ‘files’, ‘part files’ and ‘holes’, each with a unique identifying number. When a message is successfully downloaded completely the program will sound an alarm and inform you that it is complete so that you always know whether you have a complete message or only part of one.
The ‘store and forward’ system of operation of the PACSATS (packet satellites) is very effective. You can upload a message from England to Japan on a particular pass, the operator in Japan receives the message as the satellite passes over his QTH 45 minutes later. If it's a short message, he uploads his reply and you receive it the next time the satellite passes over your QTH.
In other words, a lot faster than waiting for the message to pass through several BBS's in the terrestrial packet system. It is quite obvious that the normal practice is to compose messages and SAVE them between passes so that when the satellite is in range the control program can automatically link to the satellite, send the message(s) and disconnect, thereby saving unnecessary ‘connected’ time. The digital transponder uses 4 frequencies for the Uplink, allowing several users, but only a single Downlink. The reason should be fairly obvious. Most people wanting to transmit their messages at the same time as other users, if there was only 1 Uplink frequency the system wouldn't be very effective. The satellite ‘broadcasts’ to everybody, at the same time on the same frequency.
Files are automatically compressed by the software used, to save transmission time. There is a program built into the software to decompress them into readable text on successful receipt of the file with a utility that allows post processing while waiting for the next pass.