Beginners Guide to LEO Satellites
This page will cover topics such as satellite hardware, orbits, frequencies, doppler, operating techniques, satellite tracking.
As with any specialized or technical endeavor, the language of amateur satellites is filled with terms, abbreviations, shorthand's, and acronyms that become second nature to those who use them daily, but can be obscure to newcomers -- or even to old hands who begin to explore new aspects of satellite construction or operation. When confused by an unfamiliar batch of "alphabet soup," consult the Acronyms list
A detailed, but somewhat dated beginners guide and other resources may be found at https://www.amsat.org/introduction-to-working-amateur-satellites/
Most current Amateur Radio satellites (or OSCARs, for Orbiting Satellite Carrying Amateur Radio) are in Low Earth Orbit (LEO). That is, they are in orbits that are mostly circular at altitudes somewhere between 400 kilometers (250 miles) to 1300 km (800 mi) above the surface of the earth. The higher the satellite's orbit, the larger its footprint -- that is, the larger the circle of stations on the ground that will have the satellite in view, above their horizon, at any given time.
In addition to the altitude, another significant attribute of a satellite's orbit is its inclination, that is, the angle at which it crosses the equator as it circles the earth. Some satellites are in near polar orbit -- they have an inclination approaching 90 degrees so that they pass very near to both the North Pole and the South Pole on each orbit. Other satellites have a much lower inclination, so that they never pass near the poles, but track north and south across middle latitudes. Both types of orbits are useful to amateurs for communication. Higher inclination satellites will tend to pass over a given location on earth from north to south or from south to north, and may tend to pass that location at roughly the same time of day. Lower inclination satellites will generally track across a given location from west to east (due to the rotation of the earth) at times that vary greatly from day to day.
In nearly all cases, any particular LEO satellite will pass over a given earth location several times daily -- usually two or three orbits clustered 1.5 to 2 hours apart -- and 12 hours or so later, another two or three orbits clustered similarly. It is usual to find that a satellite will have several passes in the morning and several more in the evening, or several passes at mid-day followed by several more in the middle of the night. Each orbital pass will vary in duration, depending on how near the orbital track is to being directly over the observer's location. But most passes, during which the satellite is above the horizon and usable by a particular station, will be on the order of 5 to 20 minutes each orbit.
There are three basic types of Amateur Radio satellites: Those that use various digital modes for communication between earth stations or to transmit telemetry data from onboard experiments, those that use FM phone for voice communication between earth stations, and those carrying linear transponders in order to allow communication between earth stations using CW or SSB phone across a narrow band of frequencies. Many digital stations can be heard with very simple receivers, if one knows when to listen and on what frequency to tune.
The easiest satellites to communicate through, from a technical standpoint, are the FM satellites. They are essentially single-channel repeaters in space. So while they are relatively easy to hear and to access, they are also very busy, with many stations attempting contacts on the single channel during the short time the satellite is overhead. This can make them somewhat frustrating to work from an operational standpoint, especially on a weekend or holiday. Nevertheless, the FM satellites are a good place to begin.
These FM satellites are cross-band repeaters -- that is, they either listen on the 2 meter amateur band (145 MHz) and transmit on the 70cm amateur band (435-436 MHz), or they do the reverse, listening at 70cm and transmitting on 2m. There have been FM satellites using other amateur radio frequency bands, but none are presently in operation. Currently operational FM satellites include:
- AO-91 (RadFxSat / Fox-1B)
- SO-50 (SaudiSat-1C)
- PO-101 (Diwata-2 -- available by a varying schedule)
- AO-27 (Currently on for four minutes on ascending and descending passes over mid-latitudes of the Northern Hemisphere)
- IO-86 (LAPAN-A2) In equatorial orbit, activated by schedule
- Several additional FM satellites are currently in orbit undergoing per-commissioning, or are soon to be launched.
While it is possible to make contacts through these satellites with a single, dual-band (2m/70cm) hand-held FM radio, it is highly desirable to have "full-duplex" capability -- that is, to be able to hear the satellite at the same time that one is transmitting to it. A few radios on the market have this capability, but they are generally more costly. Many beginners fulfill the requirement using two, less expensive radios -- one for transmitting and the other for receiving.
Also, some sort of small, directional antenna is necessary, along with some means to aim it at the satellite as it passes overhead. For beginners, the aiming of the antenna is often done simply by hand. More advanced stations make use of electric motors to rotate the antennas both horizontally (azimuth) and vertically (elevation), often under computer control. More on that in the equipment sections of this wiki.
Once the basic equipment is in hand, the next step is to tune to the proper frequencies. In satellite operation, being on the right frequency is complicated by a phenomenon called "Doppler Shift" or "Doppler Effect." It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842, and is the change in frequency of a wave caused the motion of the wave source in relation to an observer. In other words, a satellite in space is moving so fast that it distorts the frequencies at which it transmits and receives. The direction and magnitude of this shift in frequencies varies depending on whether the satellite is approaching or receding, its distance from the observer, and the original frequency of the wave being distorted. Thus, the Doppler Shift will be different for every user of the satellite at any given moment.
FM, due to its inherent characteristics, is rather forgiving about being precisely on frequency. But Doppler Shift becomes more pronounced as frequency increases. So for FM satellites, we can usually get by ignoring the Doppler Effect in the 2 meter band, but we often need to make some corrections in the 70cm band. For example, FM satellite AO-91 transmits down toward earth (the downlink) on 145.960 MHz. In most instances, one may tune their receiver to that frequency and hear the satellite just fine. However, AO-91 receives signals coming up from the ground (the uplink) at 435.250 MHz. A station transmitting on that frequency will probably only be successful in being heard through the satellite when it is near its closest approach to that station -- when the Doppler Shift is near minimum. At the beginning of the satellite's pass over that station, as the satellite is approaching, the operator will have much greater success by tuning to a lower frequency -- perhaps about 435.240 MHz. Once the Doppler Effect has made its impact, that transmitted signal will appear to the satellite's receiver to be very nearly on frequency (435.250). As the satellite moves a bit closer, but is still approaching, the operator will be more successful raising the transmitted frequency a bit -- perhaps to 435.245 MHz -- because the Doppler Effect will have become somewhat less pronounced as the satellite draws closer. Similarly, as the satellite begins to recede from the station, the operator will keep tuning upward -- perhaps to 435.255. And as the satellite nears the horizon, still going away, the operator might tune up still more to 435.260 MHz.
Conversely, FM satellite SO-50 transmits its downlink at about 436.795 MHz while listening for uplink signals at 145.850 MHz. In this case, the operator may tune the transmitter to 145.850 MHz and be quite successful being heard through the satellite throughout its pass. But in order to receive SO-50 well, the operator will have to begin the pass (at AOS, or Acquisition of Signal) by tuning the receiver at bit higher -- perhaps to 436.805 or so. The operator will then tune the receiver downward in frequency, passing the satellite's actual transmitted frequency of 436.795 near the Time of Closest Approach (TCA), and continuing down perhaps as low as 436.785 at Loss of Signal (LOS), when the satellite disappears below the horizon once again.
This may all seem very confusing, but it is easy to remember that one only needs to adjust the 70cm frequency -- no matter whether than happens to be the uplink or the downlink -- and to recall that uplinks move up, and downlinks move down!