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Winter 2003 Issue |
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SATELLITES
Types of Satellites By Tom Webb,* WA9AFM
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Artist’s depiction (artist unknown) of AO-7 in orbit. (Courtesy AMSAT) |
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Low Earth Orbit (LEO) As mentioned in previous columns, a “low earth orbit” satellite, better known as LEO, is in a low-altitude orbit. These orbits can be as low as 100 miles. However, most of the amateur satellites are well above that: 500 miles for UO-14 and AO-27, 800-900 miles for FO-20 & 29 and AO-7. The main advantage of the LEO orbit is the minimal equipment needed for a ground station—i.e., low power, small antennas, no preamps, and so on. It’s possible to work a LEO satellite with a 5-watt HT and high-gain whip antenna! For the satellite builder, the LEO satellite downlink transmitter can be relatively low power, the up/downlink antennas can be simple, and the airframe small in size. Launches for the smaller LEO satellite are easier to find, as they don’t consume as much “throw weight” as their larger cousins. There is a downside with LEO satellites. Because of their low orbit, access time is measured in minutes. For example, even an overhead pass of UO-14 would last approximately 20 minutes. To say the least, the Doppler shift is dramatic. In addition, you might only get two, possibly three, passes a day where access is possible. However, considering that we have six LEO birds (UO-14, AO-27, and SO-41 on FM; FO-20, FO-29, and RS-15 on SSB), you have ample opportunity for satellite activity with a rather unsophisticated station. Digital satellites include AO-16, UO-22, MO-46, and GO-32. LO-19 and SO-33 are also digital satellites, but as of late December 2002 they were listed as not operational or very intermittently operational. For the latest information, check the AMSAT website: <http:// www.amsat.org/amsat/news/wsr.html>. There are even two APRS birds in orbit—IO-26 and NO-44. There are also several birds waiting for operational testing and commissioning for amateur service. For operational frequencies and modes of these satellites, go to AMSAT Weekly Satellite Report, <http://www. amsat.org/amsat/news/ans.html>. AO-7. One satellite, AO-7, has just become available . . . well, at least on a random basis (see figure 1). Launched November 15, 1974, AO-7 was designed for a three-year operational mission, but actually it soldiered on for 61/2 years before total failure because of a suspected short in a nickel-cadmium battery. The satellite community was rocked in June 2002 when AO-7 was heard again!
Telemetry suggests the shorted battery had
opened and allowed AO-7 to operate on its solar panels. When AO-7 enters
sunlight, the bird powers up. However, as there is no power to maintain
housekeeping computers and memory, the downlink frequency is selected at
random—either ”A” Mode (10 meters), “B” Mode (2 meters), or “C” Mode (a
low-power 2-meter downlink). Seventy-cm (435.1 MHz) or 13-cm (2304.1 MHz)
beacons may also be heard. |
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