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Chapter 23
Space
Communications
An Amateur Satellite Primer
Most amateurs are familiar with
repeater stations that retransmit signals to
provide wider coverage. Repeaters
achieve this by listening for signals on one
frequency and immediately retransmitting
whatever they hear on another frequency.
Thanks to repeaters, small, low-power
radios can communicate over thousands
of square kilometers. Unfortunately,
many amateurs are
not
familiar with the
best repeaters that have ever existed.
These are the amateur satellites that hams
have been using for 40 years. (See the
sidebar “Tired of the Same Old QSOs?”)
This is essentially the function of an
amateur satellite as well. Of course, while
a repeater antenna may be up to a few hun-
dred meters above the surrounding terrain,
the satellite is hundreds or thousands of
kilometers
above the surface of the Earth.
The area of the Earth that the satellite’s
signals can reach is therefore much larger
than the coverage area of even the best
Earth-bound repeaters. It is this charac-
teristic of satellites that makes them at-
tractive for communication. Most amateur
satellites act as analog repeaters, retrans-
mitting CW and voice signals exactly as
they are received, as packet store-and-for-
ward systems that receive whole messages
from ground stations for later relay, or as
specialized Earth-looking camera systems
that can provide some spectacular views.
See
Fig 23.2
, an image of a town in the
western US.
Amateur satellites have a long history
of performing worldwide communica-
tions services for amateurs. See the
sidebar “Amateur Satellite History.”
LINEAR TRANSPONDERS AND
THE PROBLEM OF POWER
Most analog satellites are equipped with
linear transponders
. These are devices
that retransmit signals within a band of
frequencies, usually 50 to 250 kHz wide,
known as the
passband
. Since the linear
transponder retransmits the entire band, a
Tired of the Same Old QSOs? Break out of Orbit and Set your Course for the
“Final Frontier”
Satellite-active hams comprise a relatively small
segment of our hobby, primarily because of an unfortu-
nate fiction that has been circulating for many years—
the myth that operating through amateur satellites is
overly difficult and expensive.
Like any other facet of Amateur Radio, satellite
hamming is as expensive as you allow it to become. If
you want to equip your home with a satellite communi-
cation station that would make a NASA engineer blush,
it will be expensive. If you want to simply communicate
with a few low-Earth-orbiting birds using less-than-state-
of-the-art gear, a satellite station is no more expensive
than a typical HF or VHF setup. In many cases you can
communicate with satellites using your present station
equipment—no additional purchases are necessary.
Does satellite hamming impose a steep learning
curve? Not really. You have to do a bit of work and
invest some brain power to be successful, but the same
can be said of DXing, contesting, traffic handling, digital
operating or any other specialized endeavor. You are,
after all, communicating with a spacecraft!
The rewards for your efforts are substantial, making
satellite operating one of the most exciting pursuits in
Amateur Radio. There is nothing like the thrill of hearing
someone responding to your call from a thousand miles
away and knowing that he heard you through a satellite.
(The same goes for the spooky, spellbinding effect of
hearing your own voice echoing through a spacecraft as
it streaks through the blackness of space.) Satellite
hamming will pump the life back into your radio experi-
ence and give you new goals to conquer.
Space Communications
23.1
(A)
Fig 23.2—UO-36 captured this image of
a well-known city in the western US.
Need a hint? Think “Caesar’s Palace.”
uplink passband, your signal will appear
at the
top
of the downlink passband. In
addition, if you transmit in lower side-
band (LSB), your downlink signal will be
in upper sideband (USB). See
Fig 23.4
.
Satellite passbands are usually operated
according to the courtesy band plan, as
shown. Transceivers designed for satel-
lite use usually include features that cope
with this confusing flip-flop.
Over the years, the number of amateur
bands available on satellites has in-
creased. To help in easily identifying
these bands, a system of “Modes” has
been created. In the early years reference
to these Modes was by a single letter
(Mode A, Mode B, etc), but with the
launch of AO-40 the opportunities greatly
increased and it was necessary to show
both the uplink and downlink bands. See
Table 23.1,
Satellite Operating Modes.
Linear transponders can repeat any type
of signal, but those used by amateur sat-
ellites are primarily designed for SSB and
CW. The reason for the SSB and CW pref-
erence has a lot to do with the hassle of
generating power in space. Amateur sat-
ellites are powered by batteries, which are
recharged by solar cells. “Space rated”
(B)
Fig 23.1—At A, in the foreground, Phase 3E (P3E) spacecraft is illustrated within
the SBS (specific bearing structure) in the final assembly room at the CSG
Arianespace launch facility in Kourou, French Guiana. P3D can be seen in the
background. Part B is an actual top-view photo of the original P3D satellite (now
AO-40) sitting inside the launch stage of its Ariane 5 launcher. Photo/illustration A
courtesy of Wilfried Gladisch, Philipps-Universitaet Marburg.
number of signals may be retransmitted
simultaneously. For example, if three SSB
signals (each separated by as little as 5
kHz) were transmitted to the satellite, the
satellite would retransmit all three sig-
nals—still separated by 5 kHz each (see
Fig 23.3
). Just like a terrestrial repeater,
the retransmissions take place on frequen-
cies that are different from the ones
on which the signals were originally
received.
Some linear transponders invert the
uplink signals. In other words, if you trans-
mit to the satellite at the
bottom
of the
23.2
Chapter 23
Fig 23.4—The OSCAR satellite band
plan allows for CW-only, mixed CW/
SSB, and SSB-only operation.
Courteous operators observe this
voluntary band plan at all times.
Fig 23.3—A linear transponder acts much like a repeater, except that it relays an
entire group of signals, not just one signal at a time. In this example AO-40 is
receiving three signals on its 23-cm uplink passband and retransmitting them on
its 13-cm downlink passband.
kilometers. From this vantage point the
satellites can “see” almost half of our
planet. Their speed in orbit matches the
rotational speed of the Earth itself, so the
satellites appear to be “parked” at fixed
positions in the sky. They are available to
send and receive signals 24 hours a day
over an enormous area.
Of course, amateur satellites
could
be
placed in geostationary orbits. The prob-
lem isn’t one of physics; it’s money and
politics. Placing a satellite in geostation-
ary orbit and keeping it on station costs a
great deal of money—more than any one
amateur satellite organization can afford.
An amateur satellite group could ask simi-
lar groups in other areas of the world to
contribute money to a geostationary satel-
lite project, but why should they? Would
you contribute large sums of money to a
satellite that may never “see” your part of
the world? Unless you are blessed with
phenomenal generosity, it would seem
unlikely!
Instead, all amateur satellites are either
low-Earth orbiters (LEO), or they travel in
very high, elongated orbits. See
Fig 23.5
.
Either way, they are not in fixed positions
in the sky. Their positions relative to your
station change constantly as the satellites
zip around the Earth. This means that you
need to predict when satellites will appear
in your area, and what paths they’ll take as
they move across your local sky.
You’ll be pleased to know that there is
software available that handles this pre-
diction task very nicely. A bare-bones pro-
gram will provide a schedule for the
satellite you choose. A very simple sched-
ule might look something like
Fig 23.6
,
showing the antenna pointing angles for
each minute of a pass for AO-16.
The time is usually expressed in UTC.
AO-16 will appear above your horizon
Table 23.1
Satellite Operating Modes
Frequency Band
New Designation Old Designation
(Transmit, First Letter;
Receive, Second Letter)
Mode U/V
Letter
Designation
HF, 21-30 MHz
H
Mode B
VHF, 144-148 MHz
V
Mode V/U
Mode J
UHF, 435-438 MHz
U
Mode U/S
Mode S
1.26-1.27 GHz
L
Mode L/U
Mode L
2.40-2.45 GHz
S
Mode V/H
Mode A
5.6 GHz
C
Mode H/S
Mode L/S
Mode L/X
Mode C/X
10.4 GHz
X
24 GHz
K
solar arrays and batteries are very expen-
sive. They are also heavy and tend to take
up a substantial amount of space. Thanks
to meager funding, hams don’t have the
luxury of launching satellites with large
power systems such as those used by com-
mercial birds. We have to do the best we
can within a much more limited “power
budget.”
So what does this have to do with SSB
or any other mode? Think
duty cycle
—the
amount of time a transmitter operates at
full output. With SSB and CW the duty
cycle is quite low. A linear satellite tran-
sponder can retransmit many SSB and CW
signals while still operating within the
power generating limitations of an ama-
teur satellite. It hardly breaks a sweat.
Now consider FM. An FM transmitter
operates at a 100% duty cycle, which
means it is generating its full output with
every transmission. Imagine how much
power a linear transponder would need to
retransmit, say, a dozen FM signals—all
demanding 100% output!
Having said all that, there
are
a few,
very popular FM repeater satellites. How-
ever, these are very low-power satellites
(typically less than 1 W output) and they
do not use linear transponders. They re-
transmit only one signal at a time.
FINDING A SATELLITE
Before you can communicate through a
satellite, you have to know what satellites
are available and when they are available.
(See sidebar “Current Amateur Satel-
lites.”) This isn’t quite as straightforward
as it seems.
Amateur satellites do not travel in geo-
stationary orbits like many commercial
and military spacecraft. Satellites in geo-
stationary orbits cruise above the Earth’s
equator at an altitude of about 35,000
Space Communications
23.3
Amateur Satellite History
The Amateur Radio satellite program began with
the design, construction and launch of OSCAR I in
1961 under the auspices of the Project OSCAR
Association in California. The acronym “OSCAR,”
which has been attached to almost all Amateur Radio
satellite designations on a worldwide basis, stands
for
O
rbiting
S
atellite
C
arrying
A
mateur
R
adio. Project
OSCAR was instrumental in organizing the construc-
tion of the next three Amateur Radio satellites—
OSCARs II, III and IV. The Radio Amateur’s Satellite
Handbook, published by ARRL has details of the
early days of the amateur space program.
In 1969, the Radio Amateur Satellite Corporation
(AMSAT) was formed in Washington, DC. AMSAT has
participated in the vast majority of amateur satellite
projects, both in the United States and internationally,
beginning with the launch of OSCAR 5. Now, many
countries have their own AMSAT organizations, such
as AMSAT-UK in England, AMSAT-DL in Germany,
BRAMSAT in Brazil and AMSAT-LU in Argentina. All
of these organizations operate independently but may
cooperate on large satellite projects and other items
of interest to the worldwide Amateur Radio satellite
community. Because of the many AMSAT organiza-
tions now in existence, the US AMSAT organization is
frequently designated AMSAT-NA.
Beginning with OSCAR 6, amateurs started to
enjoy the use of satellites with lifetimes measured in
years as opposed to weeks or months. The opera-
tional lives of OSCARs 6, 7, 8 and 9, for example,
ranged between four and eight years. All of these
satellites were low Earth orbiting (LEO) with altitudes
approximately 800-1200 km. LEO Amateur Radio
satellites have also been launched by other groups
not associated with any AMSAT organization such as
the Radio Sputniks 1-8 and the ISKRA 2 and 3
satellites launched by the former Soviet Union.
The short-lifetime LEO satellites (OSCARs I
through IV and 5) are sometimes designated the
Phase I satellites, while the long-lifetime LEO satel-
lites are sometimes called the Phase II satellites.
There are other conventions in satellite naming that
are useful to know. First, it is common practice to
have one designation for a satellite before launch and
another after it is successfully launched. Thus,
OSCAR 40 (discussed later) was known as Phase 3D
before launch. Next, the AMSAT designator may be
added to the name, for example, AMSAT-OSCAR 40,
or just AO-40 for short. Finally, some other designator
may replace the AMSAT designator such as the case
with Japanese-built Fuji-OSCAR 29 (FO-29).
In order to provide wider coverage areas for longer
time periods, the high-altitude Phase 3 series was
initiated. Phase 3 satellites often provide 8-12 hours
of communications for a large part of the Northern
Hemisphere. After losing the first satellite of the
Phase 3 series to a launch vehicle failure in 1980,
AO-10 was successfully launched and became
operational in 1983. AO-13, the follow-up to the AO-
10 mission, was launched in 1988 and re-entered the
atmosphere in 1996. AO-10 provides some wide-area
communications capability at certain times of the year
despite the failure of its onboard computer memory.
The successor to AO-13, AO-40 was launched on
November 16, 2000 from Kourou, French Guiana.
With the availability of the long access time and
wide coverage of satellites like AO-10 and AO-40, it
may seem that the lower altitude orbits and shorter
access times of the Phase II series would be obso-
lete. This certainly might be true were it not for the
incorporation of digital store-and-forward technology
into many current satellites operating in low Earth
orbit. Satellites providing store-and-forward commu-
nication services using packet radio techniques are
generically called PACSATs. Files stored in a
PACSAT message system can be anything from plain
ASCII text to digitized pictures and voice.
The first satellite with a digital store-and-forward
feature was UoSAT-OSCAR 11. UO-11’s Digital
Communications Experiment (DCE) was not open to
the general Amateur Radio community although it
was utilized by designated “gateway” stations. The
first satellite with store-and-forward capability open to
all amateurs was the Japanese Fuji-OSCAR 12
satellite, launched in 1986. FO-12 was succeeded by
FO-20, launched in 1990, and FO-29, launched in
1996. In addition to providing digital store-and-
forward service. FO-20 and FO-29 also have analog
linear transponders for CW and SSB communication.
By far the most popular store-and-forward satellites
are the PACSATs utilizing the PACSAT Broadcast
Protocol. These PACSATs fall into two general
categories — the Microsats, based on technology
developed by AMSAT-NA, and the UOSATs, based
on technology developed by the University of Surrey
in the UK. While both types are physically small
spacecraft, the Microsats represent a truly innovative
design in terms of size and capability. A typical
Microsat is a cube measuring 23 cm (9 in) on a side
and weighing about 10 kg (22 lb). The satellite will
contain an onboard computer, enough RAM for the
message storage, two to three transmitters, a
multichannel receiver, telemetry system, batteries
and the battery charging/power conditioning system.
Amateur Radio satellites have evolved to provide
three primary types of communication services —
analog transponders for real-time CW and SSB
communication, digital store-and-forward for non real-
time communication, and direct “bent-pipe” single-
channel FM repeaters. Which of these types interest
you the most will probably depend on your current
Amateur Radio operating habits. If you enjoy real-time
DX QSOs on the HF bands, you may be most inter-
ested in the high-altitude wide-coverage satellites such
as OSCAR 40. On the other hand, if you are a com-
puter and terrestrial packet radio enthusiast you may be
more interested in the digital store-and-forward satel-
lites like AO-16, UO-22, KO-25 and UO-36. The FM
satellites (AO-27, UO-14 and now AO-51) have proven
to be wildly popular — although difficult to use because
of QRM — because of the simplicity of their use.
Whatever your preference, this section should provide
the information to help you make a successful entry into
the specialty of amateur satellite communications.
23.4
Chapter 23
Fig 23.6—Tabular output from an orbit prediction program showing time and
position information for AO-16.
Current Operational Amateur Satellites
OSCAR 7, AO-7, was launched November 15, 1974 by a Delta 2310 from
Vandenberg, CA. AO-7’s operating status is semi-operational in sunlight only.
After being declared dead in mid 1981 due to battery failure, AO-7 has miracu-
lously sprung back to life. It will only be on when in sunlight and off in eclipse.
AO-7 will reset each orbit and may not turn on each time.
OSCAR 11, UO-11, a scientific/educational low-orbit satellite, was built at the
University of Surrey in England and launched on March 1, 1984. This UoSat
spacecraft has also demonstrated the feasibility of store-and-forward packet
digital communications and is operational with telemetry downlinks only.
OSCAR 16, AO-16, also known as PACSAT, was launched in January 1990.
A digital store-and-forward packet radio file server, it has an experimental S-
band beacon at 2401.143 MHz. AO-16 is only semi-operational with the 1200-
baud digipeater for APRS service.
OSCAR 22, UO-22, another of the UoSat series for both amateur and
commercial services, was launched in July 1991. UO-22 now operates in
amateur store-and-forward service as well as a 110°-wide CCD camera viewing
the Earth.
OSCAR 26, IO-26, was launched on September 26, 1993 is semi-operational
and now serves as a 1200-baud digipeater for APRS service.
RS 15, launched in December 1994, is a Mode V/H spacecraft; its uplink is
on the 2m band, and its downlink is on 10m.
OSCAR 29, FO-29, launched from Japan in 1996, in a low earth orbit. It
operates with a mode V/U analog transponder. It has a packet BBS with
9600-baud capability, but that service is of limited use.
OSCAR 40, AO-40, the fourth Phase 3 satellite, was launched on November
16, 2000, aboard an Arianespace AR507 rocket, and was placed in an elliptical
orbit. AO-40 is in normal service with mode U/S, L/S and L/K transponders
operating on set schedules. The S band telemetry beacon is available.
OSCAR 41, SO-41, also known as SAUDISAT-1A, was launched September
26, 2000 aboard a converted Soviet ballistic missile from Baikonur
Cosmodrome. When operational, SO-41 will operate as a 9600-baud digital
store-and-forward system. It has been authorized for service as an analog FM
repeater.
OSCAR 44, NO-44, also known as PCSAT was launched on September 30,
2001 from Kodiak, Alaska. PCSAT is a 1200-baud APRS digipeater designed
for use by stations using hand-held or mobile transceivers. The operational
status of PCSAT is uncertain and subject to change due to power availability.
OSCAR 46, MO-46, known as TIUNGSAT-1, was launched in September
26,2000 with commercial and Amateur payloads. The Amateur payload is
operational and provides FM and FSK communications.
OSCAR 50, SO-50 also known as SAUDISAT-1C, was launched December
20, 2002 aboard a converted Soviet ballistic missile from Baikonur
Cosmodrome. SO-50 carries several experiments, including a mode U/V FM
amateur repeater. The repeater is available to amateurs as power permits,
using a 67.0 Hz uplink tone for on-demand activation.
Fig 23.5—AO-40 travels in a high,
elliptical orbit.
beginning at 0516 UTC on January 30,
1994. The bird will “rise” at an azimuth of
164°, or approximately south-southeast of
your station. The elevation refers to the
satellite’s position above your horizon in
degrees—the higher the better. A zero-
degree elevation is right on the horizon;
90° is directly overhead.
By looking at this schedule you can see
that the satellite will appear in your south-
southeastern sky at 0516 UTC and will rise
quickly to an elevation of 70° by 0524.
The satellite’s path will curve further to
the east and then directly to the north as it
rises. Notice how the azimuth shifts from
164° at 0516 UTC to 0° at 0524. This is
nearly a direct overhead pass of AO-16
and it sets in the north-northwest at 348°.
The more sophisticated the software, the
more information it usually provides in the
schedule table. The software may also dis-
play the satellite’s position graphically as
a moving object superimposed on a map of
the world. Some of the displays used by
satellite prediction software are visually
stunning! This view,
Fig 23.7,
is provided
by Nova for Windows, from Northern
Lights Software Associates. It shows a
view of the Earth from space and the vis-
ibility circles, or “footprints” of both a
LEO satellite, AO-27, and a much larger
circle for the Phase 3 satellite, AO-40.
Satellite prediction software is widely
available on the Web. Some of the simpler
programs are freeware. The AMSAT-NA
Web site has the largest collection of satel-
lite software for just about any computer
Space Communications
23.5
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