Amateur radio operators have built many satellites since the first one was launched in 1961. Amateurs use these Orbiting Satellites Carrying Amateur Radio (OSCAR) to communicate with other amateurs around the world. The satellites often use the VHF and UHF bands because radio signals on those bands normally go right through the ionosphere. The satellites retransmit signals to provide greater communications range than would normally be possible on those bands. While it is possible to use HF signals for satellite operation (and some satellites do), more of the HF signal energy may be bent back to the Earth rather than going through to the satellite.
Since satellite communication uses line-of-sight propagation, two amateurs can communicate through a satellite as long as the satellite is in view of both stations.
Notice in the picture above that stations 1 and 2 can communicate through the satellite at this time, but neither station could communicate with station 3 at this time. Later, as a satellite orbits the Earth, stations 2 and 3 will be able to use the satellite, but not station 1.
It is helpful to have a directional antenna for some satellite operation so you can point the antenna in the direction of the satellite. This also requires that you be able to aim your antenna in different compass directions (azimuth) as well as change the elevation angle to keep the antenna pointed at the satellite.
When the satellite is almost directly overhead, and you can point your antenna at the satellite, relatively low power is required. It is even possible to operate through some satellites using simple vertical antennas and low power handheld radios. When a satellite is near the horizon, however, you may need more power even if you can point your gain antenna directly at the satellite. That is primarily because your signal must travel a longer distance through the relatively dense air close to Earth.
All current Amateur Radio satellites are in non-geostationary orbits. This simply means that the satellites are not in fixed positions in the sky from our perspective here on Earth. They are like tiny moons, rising and setting rapidly over your local horizon. While the satellites zip around the Earth at tremendous speeds, the Earth is turning beneath them. The result is that you can't rely on satellites to appear in the same places at the same times each day.
So how can you know when a satellite is about to make an appearance in your neighborhood? To answer that question you need to know the satellite's orbital elements.
As incomprehensible as it may seem, an orbital element set is merely a collection of numbers that describes the movement of an object in space. By feeding the numbers into a computer program, you can determine exactly where a satellite is (or will be) at any time. Don't worry about the definitions of mean anomaly, argument of perigee and so on. If you're curious, get a copy of the Satellite Experimenter's Handbook* and you'll learn all about those definitions-and more. For the moment, consider the words as labels for the numbers that appear beside them.
Amateur Satellite Frequency Guide
Compiled December 1996
1. The modes in ( ) are the new designations to be instituted with Phase 3D.
2. The AO-10 beacon is an unmodulated carrier. This satellite has suffered computer damage making it impossible to orient it for optimum service or solar illumination. It may mot be present for weeks at a time and may also disappear suddenly, apparently when it enters darkness. In order to preserve it as long as possible, do not transmit to it when you hear the beacon or the transponded signals FMing.
3. RS-10 and its backup RS-11, and RS-12 and its backup RS-13 are each mounted on common spaceframes, along with communication and navigation packages. RS-11 & 13 are currently turned off. All of these packages have the capability of being operated in modes A, K, & T, with modes K & T running simultaneously.
4. Transmitters on both AO-16 & LU-19 are currently using Raised Cosine Mode.
5. AO-16 users are encouraged to select 145.900, 145.920 and 145.940 for uploading and 145.960 for directory and/or file requests.
6. DOVE is designed to transmit digital voice messages but, due to hardware and software difficulties, it has not yet met this objective.
7. Letters in [ ] represent digital formats, which are as follows:
[a] = 1200 bps PSK AX-25
[b] = 1200 bps AFSK AX-25
[c] = 9600 bps PSK
[d] = digital voice
Finding the Elements
There are several sources of orbital elements:
W1AW RTTY and AMTOR bulletins***
Packet bulletin boards \par Telephone bulletin boards****
World Wide Web******
If you have an HF radio, RTTY capability, a packet TNC, a telephone modem or the necessary cash for a subscription, you'll always be able to get the latest orbital elements for the satellites you want to track. If all else fails, there is probably someone in your area who has access to the elements. Ask around at your next club meeting.
Computers are common in most Amateur Radio stations today. If you have a computer in your shack, you're in luck! There are many programs on the market that will take your orbital elements and produce detailed satellite schedules.
Among other things, the programs tell you when satellites will appear above your local horizon and how high they will rise in the sky (their elevation), When working satellites, the higher the elevation the better. Higher elevation means less distance between you and the satellite with less signal loss from atmospheric absorption.
*The Satellite Experimenter's Handbook is available from your local dealer or direct from ARAL HO.
**TheAMSATJournalis included with membership in AMSAT, P0 Box \par 27, Washington, DC 20044. ($30 per year US, $36 Mexico and Canada) \par OSCAR Satellite Report ($29 per year US, $32 Canada) and Satellite Operator ($33 per year US, $36 Canada) are available from R. Meyers Communications, P0 Box 17108, Fountain Hill, AZ 85269-7108.
***W1AW transmits satellite orbital elements on Tuesdays and Saturdays at 2330 UTC using 45.45-baud Baudot RTTY, 110-baud ASCII and Mode B (FEC) AMTOR. Frequencies are 3.625, 7.095, 14.095, 18.1025,21.095, 28.095 and 147.555 MHz.
****Satellite orbital elements may be downloaded from the following telephone bulletin boards:
ARRL BBS-tel 860-594-0306
Dallas Remote Imaging Group (DRlG)~teI 214-394-7438
*****AMSAT Information Nets:
Sunday at 1800 UTC on 21.280 and 14.282 MHz
Tuesday at 2100 Eastern Time on 3.840 MHz
Tuesday at 2100 Central Time on 3.840 MHz Tuesday at 2000 Pacific Time on 3.840 MHz