Difference between revisions of "Beginners Guide to LEO Satellites"
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==== MOVING UP TO LINEARS ====
==== MOVING UP TO LINEARS ====
Revision as of 22:00, 18 November 2020
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 pre-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 ("beam") 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. There are a few small, hand-held dual-band beam antennas on the market, or they can be easily built (see the portable equipment section of this wiki). 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 fixed-station equipment section 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 that happens to be the uplink or the downlink -- and to recall that uplinks move up, and downlinks move down!
If one has a proper mathematical description of a satellite's orbit -- such as its height above the ground, its inclination, and a few other bits, such as its eccentricity (how close it is to an actual circle as opposed to more of an oval) -- and if one knows the exact time, some calculation will predict where the satellite will be at any given moment and when it will next be in range of a particular point on earth. These mathematical bits are called Keplerian Elements, named after the German astronomer and mathematician, Johann Kepler, who worked out all of this math way back in the early 17th century. If you are a budding Kepler, you are welcome to get your pencil and scratch pad and work it out for yourself.
Most amateur satellite operators these days prefer to let a computer do all of these calculations instead. It's not that a super-computer is required -- there are all sorts of smart phone apps and little utility programs for desktops and laptops that do just fine, and can make calculations for dozens of satellites all at once. Many of these programs are free. Some of the more complete ones, that can steer antennas to the proper azimuth and elevation, and tune the radio to correct for Doppler Effect all at the same time, are still not terribly expensive. The Store on the AMSAT-UK website will sell you a very nice program that is very widely used on PCs. There is a list of tracking programs and apps elsewhere on this wiki. Or just do an internet search for "satellite tracking" to find programs or apps for your device of choice.
All tracking programs share some common requirements. They all need to know the time accurately (most phones and computers update the time quite accurately from their networks). They need to know your location (the GPS on most phones today will provide that information). And they need fresh Keplerian Elements (usually updated once a week or so). The Keplerian Elements need updating because orbits change over time due to a variety of factors, such as the vanishingly thin atmosphere that still reaches up into the LEO altitudes, the unevenness of the earth's mass (one might say that the earth is "lumpy"), and even particles of solar wind cast out from our Sun. When new satellite operators have difficulty finding a satellite, the culprit is almost always an incorrect time zone, an error in the location, or old "Keps."
HEARING A SATELLITE
The easiest signals from space to hear are those from ARISS (Amateur Radio aboard the International Space Station). Many of the astronauts and cosmonauts aboard the International Space Station (ISS) are licensed amateurs, and there are two sets of amateur equipment aboard. One is on the U.S. side and another is on the Russian side. One of these radios is operational at almost all times, except when docking or undocking operations are underway, or during EVAs (spacewalks outside the station). If fortunate, one might actually hear one of the crew talking on the radio on 145.800 MHz. These are most often school contacts, when crew members answer questions from students (learns more at teh ARISS website). Rarely, crew members will make casual contacts with other amateurs. Most often, the ARISS gear is operated in a digital mode called APRS on 145.825 MHz (learn more about APRS here). These signals are relatively strong, and can often be heard, IF the ISS is overhead, with a normal amateur hand-held radio or FM scanner.
Hearing actual Amateur Radio operations on one of the FM satellites will require a bit more preparation and a bit of skill. Above, the hand-held beam antenna was mentioned. Such an antenna, whether home-built or commercially purchased, is necessary for two reasons. First, signals from the satellites are usually not strong enough to be heard with simple antennas such as those used for mobile or hand-held operation through earth-based repeaters. Second, the satellites are not just moving swiftly across the sky in their orbits, they are also tumbling in space. That means that the antenna polarity (sometimes called antenna polarization) is unpredictable and constantly changing. Earth-bound repeaters usually have vertical antennas (the stick up, perpendicular to the ground), and so we use vertical antennas on our cars and houses, and hold our hand-held radios with the antenna pointing up. But the antennas on a satellite might be vertical, or horizontal, or more often, something in between. So we not only have to point our beam in the right direction (azimuth), and point it at the sky at the proper angle (elevation), we also have to twist it from vertical to horizontal to somewhere in between until the satellite signal is the strongest. It sounds complicated, but with a bit of practice, and occasional glances at the tracking program to get updated azimuth and elevation directions, one catches on quickly.
As with any aspect of Amateur Radio, it is best to listen, and listen, and listen some more, before trying to transmit and make a contact. By listening, one becomes familiar with tracking and twisting the antenna for best signal. One also becomes accustomed to the rapid pace of operation on the satellites. Contacts are brief, because many stations are attempting to share a scarce resource: Only one channel is available, and the channel is only available for a short time on each satellite pass. Generally, only two items of information are shared: the callsign and the grid locator. Exchanges almost always use the phonetic alphabet for greater clarity.
Unless you live on a desert island, FM satellites are almost always busy. Turn off the receiver squelch, because signals may be too weak to break it. Certainly, if the pass is over Europe, North America, Japan, or other industrialized areas, there will be users activating the satellite on every pass, even in the middle of the night. If you aren't hearing activity, check once again your time, your location, and your Keplerian elements, then check the downlink frequency, squelch, and volume controls. DO NOT make "blind" calls to a satellite you are not hearing. To do so will almost certainly cause interference to those using the satellite.
If you have been practicing tracking satellites and listening, you will have discovered that there are a great many things to keep track of. Pointing the antenna properly at the satellite, correctly frequencies for Doppler shift, and listening to what is being said, is quite a lot to juggle! To actually transmit and make a contact add more balls to the juggler's act. Now, one will have to find a way to hold a microphone, use a push-to-talk (PTT) switch, keep a log, and remember what to say when the transmitter is keyed. One will soon discover that humans do not possess enough hands and arms to do all of this at once.
Various methods to aid the process have been adopted, and those you select will be largely up to personal preference and what equipment you have or can afford at the moment. Most operators use some sort of audio recording device -- it might be a dedicated digital audio recorder, or a phone app. Many operators use a headset with boom microphone. Experiment, find out what works for you.
Beginners will be most successful when activity is lighter. Avoid holidays, weekends, or early evening passes. If you've been listening enough, you'll know how to time your call and what to say. Again, callsign and grid locator, there is no time for "CQ," or "hello the bird," or other such nonsense. When you are ready, be bold and speak up. If at first you don't succeed, listen some more, then try again. Nothing is easy at first. It's the challenge that makes it fun! And remember, you are talking to outer space -- it really IS rocket science. But you can do it.
MOVING UP TO LINEARS
After dealing for a time with the fast pace and limited exchanges on the FM satellites, one might begin to become curious about the linear satellites. The big difference here is that these satellites offer a band of frequencies, usually 20 to 30 kHz in width, in which multiple stations can carry on multiple contact simultaneously. Because all of the operators are no longer competing to use that single channel, stations can spread out, contacts can go on longer. Operators often exchange names, discuss their gear, talk about other interests, and actually get to know one another. It's a refreshingly different style of operation.
A satellite carrying a inverting linear transponder will re-transmit in its downlink passband a mirror image of whatever signals appear in its uplink passband. A Lower Sideband (LSB) voice signal near the low end of the uplink passband will appear as an Upper Sideband (USB) signal near the high end of the downlink passband. A CW signal just above the center of the uplink passband will appear as the same CW signal just below the center of the downlink passband, and so forth. One tunes around in the passband to find stations very much as one does on the shortwave bands. The transmitter frequency must also tune, but in the opposite direction.
Unfortunately, the design restraints on such a transponder make it very unwise to use FM signals. An FM signal in the passband will essentially suck up all of the transponder's power and bandwidth, making it very difficult for any other stations to use the satellite. Thus, one who transmits FM to a linear satellite will quickly become very unpopular.
Add to the complexity of the inverted tuning our old friend, the Doppler Effect. Because Doppler shift varies according to frequency, the shift will always be greater at UHF than at VHF. To keep a SSB or CW signal tuned in properly requires much more precise correction than the more forgiving FM mode. Thus, finding one's uplink signal on the passband downlink, and keeping it tuned in, requires either superb operator skill, or a computer to tune the radio for the operator. Most operators today let the computer do it.