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Satellite
Gateways.
The terrestrial
packet network has been with us now for over 15 years and it now seems
to be in a decline. There are several reasons for this decline in popularity.
At its peak, packet radio was new, no other mode existed like it, and
it was a great time for experimenting to see just what could be done.
Once established, with links and nodes, passing traffic became more of
a service. With the explosion of the Internet and mobile telephone networks,
a lot of the magic of amateur radio has gone. However, there are still
some of us "old-timers" who still try to maintain the status
quo, and to try to make the amateur network evolve further. Unfortunately
some of the original founding amateurs who started the data revolution
have also deserted the sinking ship!
The Amateur Satellite
organizations have put an immense amount of work into designing, building
and launching numerous satellites over the years. This helps to expand
the horizons of a lot of amateurs who are unable to erect large antenna
systems in their back yard. There are also several Low Earth Orbiters
that have been tirelessly working and orbiting the Earth for a number
of years. These are the satellites that are of interest to the Data enthusiast,
and in particular UO-22. This satellite is the one used for the transfer
of data from the terrestrial network up to UO-22 and back down again when
it appears on the other side of the globe. It is a very satisfying feeling
to know that amateurs here can pass mail in this way on earth, but even
this has declined in popularity, largely due to a migration to the Internet
and Email. I hope that by describing the system, we can encourage more
amateurs to use amateur radio for amateur traffic and help support a system
that can evolve to even greater things. If we don't do this, there are
greedy commercial eyes looking at our VHF/UHF bands right now and rubbing
their hands. If those frequencies are "sold off" we will never
get them back again. This is a classic case of "use it or lose it".
Anybody can pick up a telephone.
System
Hardware
Radio
As the data link
operates full duplex, it is necessary to have either two transceivers,
or one that can operate both bands simultaneously. The modern ones, such
as the FT-847, can do this, so this simplifies the radio requirement.
I run the Icom 271/471 combination and it works just as well.
Antennas
The uplink to the
Pacsat is on 2 meters and the downlink is on 70 cms. Obviously signals
are not enormous, so some gain is needed. The minimum requirement for
the uplink frequency is something like a 10xy, preferably arranged and
phased to give Right Hand Circular Polarisation (RHCP). For 70 cms a multi
element beam can be used, or a 16 turn helix. James Miller, G3RUH, produces
a kit for a 16-turn helix, and it works very well, is easy to assemble
and has a very low SWR when tuned correctly.
These antennas are mounted on a 9ft boom and, in my case, the whole assembly
is on top of a 25ft home made tower. The assembly has to be rotated in
azimuth and elevation and for this I use a Yaesu G-5600B rotator combination.
I use Landwehr pre-amps, also mounted on the top of the tower and the
feeders, which should be as short as possible, are a piece of LDF-250
for 70 cms and UR-67 for 2 meters. My feeder length is about 35 ft, as
the tower is just outside the shack. There are many variations on this
theme of course, but this is about the average installation.
TNC/Modem
The TNC is the
old version 1 Tiny-2 running in KISS mode together with a 9k6baud modem,
which in my case is the G3RUH modem. This I mounted in a similar box to
the TNC and put this on top of the Tiny-2. Connections are straightforward
and easy to do. However, these days, there are 9k6baud units all ready
to go and these can be used with no construction time needed.
Computer
This can be anything
from a 386 upwards, preferably not less than a 486 however! It is actually
much easier to run two computers, networked together. On one the Satgate
runs and this talks to the other, running the BBS. For example, in my
case I have a 486 100mHz for the Satgate and a Pentium 150 for the BBS.
Control
Running all this
equipment every day for a large percentage of the day, it is essential
that the satellite is tracked on the nose, and also the radio(s) are moved
in frequency to compensate for Doppler shift. On a Low Earth Orbiter (LEO)
such as UO-22, Doppler can move the frequency some 15kHz.
In order to automate the system and save Sysop time and worry, there has
to be a method of controlling these parameters. The TrakBox is one such
way, and was designed by JA6FTL of Jamsat. It is a stand-alone unit, supplied
as a PCB, and has to be built and mounted in a box. With a weekly feed
of Keplerian elements, it tracks up to 16 satellites and controls the
radio gear, compensating for Doppler shift.
Another method is the PCB from Dave Lamont, ZL2AMD. This is a PCB that
mounts inside the PC, does not use a COM port and is conveniently out
of the way. It does a similar job to the TrakBox and takes up less room,
plus it saves an IRQ.
Software
Most amateur computers
now use Windows 9x and this is the platform that most Satgates use. It's
possible to run both the BBS and WISP software on one machine, but it
is easier to use two PC's, networked together. The main BBS program will
be F6FBB 7.00g. This runs on one PC, taking the messages from the terrestrial
network and forwarding them to a file in a directory in the root of the
BBS PC. These come from VHF/UHF and in my case also HF as I run an HF
Gateway too. At a pre-determined time the Satgate PC collects these files
and processes them prior to a pass of UO-22. The Satgate PC runs a program
called WISP, written by Chris Jackson, G7UPN/ZL2TPO. Chris works at the
University of Surrey. This continuously tracks many satellites and priority
can be given to certain satellites. In the case of the Gateway, priority
will be given to UO-22 over all other satellites. If you are tracking
KO-25 for example and UO-22 came within range, the gear would leave KO-25
and go and track UO-22.
Prior to the pass,
the mail is processed to conform to the protocol used and a similar post-pass
processing takes place and the mail is then passed to the BBS computer.
All this processing software is written by Andrew Sellers, G8TZJ.
WISP automatically updates the Keps when downloaded from UO-22. These
are normally uploaded to the satellite on a regular basis so are usually
the latest set. However, it is also possible to subscribe to receive them
weekly on the Internet. Obviously it would be a manual job then to update.
UO-22 also updates the computer clock; the satellite itself is updated
from a very accurate source. When tracking satellites, the Keps and the
time are extremely important.
For updating the TrakBox with the latest Keps and also for station details,
such as Latitude and Longitude, callsign etc., Procomm is used.
Set-up
Wiring of the hardware
is fairly straightforward. Checking out the SWR on the antennas is very
important, as is the alignment of the antenna system. I use the Pole Star
to align the antennas. The only disadvantage is that it has to be done
at night, so I usually do it in the summer!! Once the software has been
loaded and set up correctly, and this can take quite some time, tracking
of one satellite can be attempted. I cannot stress enough the need to
have the computer clock as accurate as possible, the latest Keplerian
elements, and the antennas aligned properly. It will take a few attempts
to see any visible data from the satellite, but once that happens, it
is very satisfying indeed.
Problems will occur, there is no perfect system, so don't be discouraged
if the first few attempts end up as a big zero! There are quite a few
Elmers around to ask if you have any queries and you will have, believe
me! Don't be afraid to ask, we all had to. The satisfaction you will achieve
at sending a message this way, whether it be direct or via a Satgate,
is much greater than that obtained from using the telephone, so even if
you don't intend to set up a satellite station yourself, please try to
make use of the Satellite Gateway network.
Reqsat
Try sending mail
via SATGATE. GB7LDI is the only UK Satgate and handles traffic to most
parts of the world. If you have contacts abroad who are active on packet,
then use amateur radio instead of the Internet. Unless the traffic is
of sufficient quantity, the Satgate network itself is in jeopardy. I cannot
justify running the gear 24/7 if there is nothing to send. I might just
as well close down and use Email myself. However, I refuse to be a defeatist,
and will keep struggling to maintain the station.
If you are unsure
how to use the Satgate, then follow the instructions and you cannot go
wrong.
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