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By Dick Jansson, WD4FAB VP Engineering, AMSAT
And Bill Tynan, W3XO President, AMSAT
QST May & June 1993
The size of the new Phase 3D satellite and its launch adapter compared to OSCAR 13 (l), and average human and a Microsat (r).
The number of hams who've tried to work through Amateur Radio satellites is still quite small-despite the fact that OSCARs have been around for about 30 years. Why is this? Don't amateur satellites offer consistent, predictable cover age? Yes, they do. In fact, depending on which satellite you select, the coverage is far greater than any repeater or network of linked repeaters.
Amateur satellites offer transcontinental coverage. In the case of the high-altitude birds (OSCARs 10 and 13), the coverage approaches worldwide. Amateur satellite uplinks are at VHF and UHF frequencies, so they're open to all classes of licensees except Novices.
Satellites are particularly attractive when you consider solar cycle declines. As HF conditions deteriorate, HF operators will squeeze themselves into the lower bands where they'll compete with thousands of others. Satellites offer an excellent alternative! By making an investment in satellite equipment, you can maintain regular communications with friends thousands of miles away-regardless of propagation conditions. Satellites afford extended and worldwide coverage with much less crowding than you'll find on 20, 40, 80 or 160 meters. And they don't require large antennas and 1 kW RF amplifiers!
So What's the Problem?
Why haven't more hams become ardent satellite fans? A substantial number, albeit a minority compared to the total amateur population, are. For these enthusiasts, satellite operating represents a significant portion of the time and effort they put into Amateur Radio.
The reason that more have not embraced satellite operation seems to lie in the perception that it's extraordinarily difficult, particularly with the high-altitude birds. For example, many hams balk when they discover that they can't use FM on most of today's amateur satellites. There is a reason for avoiding FM, however.
FM requires more bandwidth than SSB or CW. If the OSCAR 10 and 13 passband--each approximately 140 kHz wide--were dedicated to FM, there would be space for only nine channels (assuming 15-kHz spacing). With CW and SSB, the same satellites can handle more than 100 users at a time. Imagine how the 40-meter phone band would sound if it were carved into only nine FM channels!
The Components of a Mode-B amateur satellite station for OSCARs 10 and 13
Power is another factor. To provide those nine FM channels, a satellite would have to generate between 10 to 100 times the power required to support CW, SSB and similar modes. Not only does FM require a higher signal level for comfortable copy, its carrier is at full power all the time--whether or not any speech is present at a particular instant. To a large extent, it is the power-averaging/power-demand nature of CW and SSB signals that allows so many users to share a satellite's available power. (The probability of all users being key down or at the peak of their speech envelope at any instant of time is quite low.) If all these operators were using FM, their signals would be at peak power 100% of the time, forcing the satellite to generate considerably more power on its downlink. This kind of power generation is beyond the current capabilities of amateur satellites. Not only would the satellites have to be much larger, their cost would escalate as well.
The Case for VHF/UHF Multimode Rigs
Most newcomers and many old-timers seem to hold the mistaken opinion that FM is the only active mode on the bands above 30 MHz. When contemplating the purchase of VHF equipment, they don't even consider multimode (FM/SSB/CW) gear.
If given a choice between a single or multimode transceiver, why not choose multimode? Yes, cost is a factor, but this is also true of HF rigs. For example, few would consider buying a CW-only HF rig-even though it would be less expensive than a transceiver with SSB capability.
An FM-only VHF rig deprives the owner of much of what the VHF bands have to offer-just as a CW-only HF transceiver would limit access to a large amount of HF action. Sure, HF CW offers interesting and challenging hamming; just as FM provides enjoyable local QSOs and crystal-clear communications. But, just as many hams are not satisfied with limiting themselves to CW operation on the HF bands. neither should they settle for VHF equipment capable of only a small percentage of what the VHF and UHF bands have to offer.
In addition to satellite operation, you can Use Multimode equipment to explore terrestrial VHF and UHF. VHF/UHF enthusiasts enjoy contacts spanning hundreds and even thousands of miles without repeaters. Those who limit themselves to FM are missing most of the excitement available on the bands above 50 MHz.
Equipment is the Key
The key to successful satellite operation is buying or building the right VHF/UHF station equipment. First, your station must be capable of SSB and CW operation. (See Your VHF Companion, or check Chapter 23 of the ARRL Handbook, for examples of currently available commercial equipment suitable for satellite work.) Second, you must have sufficient power for reliable uplinking. Although many have demonstrated satellite access with only a few watts or less, reliable and comfortable contacts generally require from 50-100 watts. Because many commercial VHF rigs put out only 10 watts, some sort of amplifier is in order.
As in any type of radio communication, the antenna is critical. To communicate with the high-altitude amateur satellites, such as OSCARs 10 and 13, you'll need a directional antenna system that can be steered in azimuth and elevation. For the most popular mode of operation, Mode B, the uplink to the satellite is on 70 cm just above 435 MHz. The downlink is on 2 meters between 145.8 and 146 MHz. This means that you'll need two antennas-one for each band. To avoid the effect of satellite attitude and polarization rotation caused by the ionosphere, circularly polarized antennas are the norm.
Usually, both antennas are mounted at opposite ends of a horizontal pipe that passes through a rotator to provide the elevation control. The whole assembly mounts on another rotator that provides the azimuth control.
A typical antenna installation for working OSCAR 10 or 13. The antenna must be able to rotate horizontally and vertically.
The antenna assembly can be aimed by determining the correct azimuth and elevation of the satellite, and then using the rotator controls to point the antennas in the right direction. Or, the rotators can be controlled automatically by a computer running satellite-tracking software. Computer control is not a necessity, however, especially with the high-altitude birds, such as OSCARs 10 and 13. When these satellites are close to apogee (the high point of their orbits), they move across the sky so slowly that antennas need only be realigned about once per hour.
Why Do We Need Phase 3D?
Let's assume that you've purchased the right equipment and installed it properly. Now you're able to enjoy worldwide communications via OSCARs 10 and 13 several hours each day. If you're having so much fun, why do you need another satellite?
Since the early '80s, radio amateurs the world over have had access to satellites of the Phase 3 series-first OSCAR 10 and, more recently, OSCAR 13. Both satellites have served us well for more than a decade. Unfortunately, these spacecraft won't last forever.
Van Allen Belt radiation has severely degraded OSCAR-10's onboard computer memory, rendering its various subsystems uncontrollable. Despite this handicap, the satellite remains a viable communications tool. It can't be controlled as effectively, however, and it's not clear how much longer it will continue to operate.
The difficulty with OSCAR-13 is more complex. It remains healthy, but it too is confronted with a serious problem. Hitherto, little-known gravitational interactions between the spacecraft, the earth, the moon and the sun are causing OSCAR-13 to slowly de-orbit. Professionals using sophisticated computers have confirmed this unfortunate situation and predict that OSCAR 13 will reenter the atmosphere sometime in 1996.
The good news is that OSCAR 13's demise coincides with the scheduled launch of the most ambitious amateur satellite yet. Its prelaunch designation is Phase 3D, but it represents far more than a mere replacement for OSCARs 10 or 13. Phase 3D is specifically designed to open an entirely new world of satellite operation for all amateurs!
The Potential of Phase 3D
If you were turned off by the description on the kind of station needed to be effective on OSCARs 10 and 13, you're not alone. Satellite operation, while it can be very gratifying, has not yet proven popular for everyone. Phase 3D is designed to remedy that situation and make satellite operation much more accessible to amateurs throughout the world.
It probably won't be possible to directly access Phase 3D with a hand-held rig (like the one you use on your local repeater). Direct access should be possible, however, from portable or mobile stations running 25 to 50 watts. All they'll need is a whip or a crossed-dipole antenna on the roof of the vehicle. Imagine cruising down the road while talking to someone a half a world away--and knowing you'll be able to do it again at a specified time tomorrow, next week or a month from now. Likewise, apartment dwellers with medium-power rigs and small antennas on their balconies will be able to talk allover the world almost any time they wish!
But direct access is only part of the story. Because of the higher signal levels available through Phase 3D, many repeaters can be expected to use it for interconnecting with other repeaters hundreds, and perhaps thousands, of miles away. Hams working through such interconnected repeaters will be able to use their hand-helds to communicate with amateurs on the opposite side of the country. This gateway technique has already been tested on OSCAR 10 and 13, and it works. Phase 3D will be in an even higher orbit than the current Phase 3 satellites, providing more hours of communication. All of these features should persuade many repeater owners to employ it regularly for long distance linking.
Nor is improved performance the only advantage that Phase 3D will bring. As stated, Mode B (70 cm up and 2 meters down) is presently the most popular mode on OSCAR13 and the only one now available on OSCAR-10. OSCAR-13 also supports Mode J (the reverse of Mode B), Mode L (23 cm up and 70 cm down) and Mode S (23 cm up and 13 cm down). Crowded conditions on 2 meters, from new satellites and from growing numbers of terrestrial users, is making the reception of satellite signals on the band increasingly difficult. To alleviate this problem, Phase 3D will incorporate a flexible approach to the selection of uplinks and downlinks. It will be capable of operation well into the microwave bands right up to 10 GHz. Because of its current popularity, Phase 3D will support Mode-B operation as well.
In addition to substantially reducing ground-station requirements, Phase 3D is designed to assist the continued march of Amateur Radio toward higher frequencies. This is important if amateurs are to retain the use of these bands, which in the next century may turn out to be some of the most valuable assignments we have.
As commercial and government communicators have discovered, satellites make the upper reaches of the spectrum useful for communications between widely scattered points on the Earth. In addition, the time may not be far off when we'll be using the GHz bands to talk to hams on space stations, the moon and planets. Phase 3D will give us an incentive to make more use of these valuable frequencies.
Flexible Transponders
Amateur Radio satellites over the past 20 years have used communications transponders. A transponder receives signals on one band of frequencies and transmits amplified replicas on another band of frequencies-sort of like a crossband repeater. A repeater generally consists of a complete receiver and transmitter wired together so that the receiver's audio output modulates the transmitter, which is keyed on and off by received signals.
Transponders convert the received signals to an intermediate frequency (IF), which is amplified and then converted to another frequency for retransmission. Amateur satellite transponders have transmit/receive bandwidths of 20 to 800 kHz. For example, the Mode-B transponders on OSCARs 10 and 13 are approximately 140 kHz wide. By using transponders, many QSOs can take place through a satellite simultaneously, rather than just one at a time.
Instead of dedicated transponders, which limit flexibility, Phase 3D employs the equipment architecture shown in Fig 1. The satellite's communications package will consist of a series of receiver front ends and mixer/power amplifiers linked through an IF-bus controller unit. Fig 2 illustrates the currently planned uplink/downlink combinations this technique makes possible on Phase 3D. Each dot is a controllable communications link.
The output of any receiver can be connected to any of the mixer/power amplifiers all under computer control. This means that uplinks and downlinks can be configured on any bands for which hardware exists on the satellite. This is important because no one can be sure what bands will be most viable for uplinks and downlinks in, say 2005--the year Phase 3D will be nine years old and, we hope, still going strong.
Putting the "Spin" on Phase 3D
It was noted that Phase 3D will permit greater use by less-capable stations than do OSCARs 10 and 13. Also, the satellite's orbital parameters have been designed to increase coverage and make the satellite more intuitively easy for people to understand and use. Four features are being incorporated into the satellite's design to provide this enhanced service.
First, the transponders will feature significantly higher output-power levels. Table 1 shows how Phase 3D stacks up against the older OSCARs in this respect. Second, the antennas on Phase 3D will have higher gain than their cousins on AO- 10 and AO-13. Third, the antennas on Phase 3D will always point toward Earth.
OSCARs 10 and 13 were designed to be spin-stabilized in inertial space. Thus, for one part of the orbit, the gain antennas might be oriented toward Earth, but for the rest of the orbit they'd be pointed out into space. To provide operation during this time, both satellites include low-gain antennas. The high-gain antennas are used near apogee, the high part of the orbit, and the low-gain antennas near perigee. But there are lots of times when neither antenna is optimum.1
Keeping Phase 3D continuously oriented toward the Earth is quite a trick. Just meeting this single objective adds considerable complication to the spacecraft's design. First, the satellite must "know" its orientation with respect to space and then calculate its orientation with respect to Earth--depending on where it is in its orbit. To determine spatial orientation, several schemes have been suggested. One approach is the use of sun, star and Earth sensors. Another proposal involves the use of signals from global positioning satellites (GPS). Phase 3D could make use of the phase difference between GPS signals arriving at different points on the spacecraft.
Determining the orientation of the satellite is only part of the problem. Once the orientation is known, it's necessary to correct the continual misorientation caused by the satellite traversing its orbit, and smaller drifts that build up over time. The way big TV satellites do it is to use stored-gas thrusters to maintain the proper attitude and orbit. The TVsats are in geostationary orbits, so they must maintain their orbits precisely. When their gas is used up, their life is over. Gas depletion is the principal cause of geostationary satellite failure.
In addition, big TV satellites spin to keep one side from becoming too hot and the other too cold. Their antennas are mounted on motor-driven platforms that turn at the same speed as the spacecraft's spin, but in the opposite direction. In this way, their antennas are always aimed toward Earth.
To assure a long lifespan for Phase 3D, it was decided that a means other than maneuvering thrusters had to be used to provide continuous orienting of the satellite. The method adopted is too complex for this discussion, but it essentially employs a set of spinning discs called reaction wheels. The momentum energy associated with the spin of these wheels acts to reorient the satellite, trading wheel-spin energy for satellite-motion torque, not unlike a gyroscope. This is one of the major disciplines being dealt with in the design of Phase 3D, and its successful pursuit is one of the most important tasks before the Phase 3D design team. It's essential if Phase 3D is to meet its main objective of bringing satellite operation to many more hams on the ground.
One problem associated with spinning spacecraft, such as big TVsats, is the mechanisms used to despin their antenna platforms. These mechanisms are exposed to space, and this presents reliability problems. Moving parts must have lubrication, and lubricants tend to evaporate in the vacuum of space. These considerations led to the decision that Phase 3D would be three-axis stabilized with as few moving parts as possible. The reaction wheels will be magnetically suspended in sealed containers. This circumvents the lubricant difficulties, but presents a new set of engineering challenges.
Three-axis stabilization carries another consideration. As we've discussed, the TVsats spin to maintain an even temperature. In the case of Phase 3D, this problem will be handled by careful thermal design-including the use of heat pipes to carry the heat from the sunlit side to the cold side, where it will be radiated into space.
The Power Challenge
A design feature intended to make Phase 3D accessible to smaller ground stations is the use of higher-power transmitters. This creates a new problem: Adequate power generation.
Satellites don't get their primary power by plugging into a wall outlet! The only economical means of generating power on amateur satellites is the use of solar panels.2 Phase 3D will employ deployable, or unfolding, solar panels to augment those mounted on two of the satellite's faces. This will be the first use of such panel mechanisms on an amateur satellite. This requires reliable mechanisms to provide the deployment, latching and hinging.
Generating the power needed to support the transmitters aboard Phase 3D will require large solar panels. The current design calls for 4.3 square meters (44 square feet) of solar array, using cells of 15% efficiency. These are considerably better than the cells one can usually find at hamfest flea markets, but not as good as the best available. One of the major aspects of satellite design involves the tradeoff between solar efficiency, cost, and the size and complexity of the structure needed to support them. Solar cells represent one of the highest-cost items that go into building a spacecraft. The Phase 3D arrays will produce about 730 watts of power at the beginning-of-life (BOL), with a 40% reduction after 10 years in orbit. Like almost anything else, solar arrays deteriorate with age.
To produce relatively high RF power and live within the tight power budget imposed by solar cells, high-efficiency power amplifiers are a must. Attaining high efficiency, particularly at microwave frequencies, is a formidable task. Fortunately, the amateur community has already addressed this problem with High Efficiency Linear Amplification by Parametric Synthesis (HELAPS). This concept was the subject of a doctoral dissertation by Dr Karl Meinzer, DJ4ZC, president of AMSAT-DL and one of the principals on the Phase 3D design team. Karl's concept has been proven on the Mode B and J transponders used for OSCARs 7, 8, 10 and 13.
The HELAPS precepts are a bit complex for discussion here, but Jan King, W3GEY, gave a clear description of the principle in AMSAT's ORBIT magazine.3 These techniques are the mainstay on the high-power amplifiers aboard Phase 3D. Designing HELAPS amplifiers is tricky business, understood by many microwave designers--amateur or commercial. Design, construction, troubleshooting and final checkout of these amplifiers is another major task confronting the Phase 3D design team.
Improved Antenna Designs
Another of Phase 3D's design features intended to accommodate smaller ground stations is the use of antennas with higher gain than employed on previous amateur satellites. The subject of antenna design has been a major thrust in AMSAT programs over the past five years.
As mentioned, Phase 3D will incorporate receivers and transmitters on many bands. Various antennas will be required for these bands. Maximizing gain while minimizing the size of each antenna is another challenge facing the Phase 3D design team. Installing long-boom, high-gain Yagis on satellites is a mechanical-engineering nightmare! Take a 16-dBic 70-cm antenna, for example: The antenna should be circularly polarized to prevent satellite orientation from affecting the signal received on the ground.4The obvious way to produce a circularly polarized wave is with crossed Yagis or a helix. Either type of antenna would have to be about 15 feet long to achieve the desired gain. The AMSAT Phase 4 (geostationary) program examined the mechanization of these kinds of VHF, UHF and microwave antennas onto a satellite platform. In the street vernacular, the problems made the situation a "real bear!"
Antenna gain doesn't have to come from boom length, however. The geometry of concern is the real or effective capture area (in wavelengths) of an antenna. The longer booms of Yagi antennas create larger effective capture areas. In the case of Phase 3D we have a spacecraft top-plate area greater than 3.68 square meters (approximately 40 square feet). Antenna designers can easily compute a real or virtual area (the latter, in the case of a Yagi) for an antenna and tell you exactly how much gain should be achieved from that area.
With practical Yagi or other long-boom antennas out of the question, the Phase 3D design team sought alternative answers and found them in patch and short backfire antennas. A patch is a square or circular conductor whose dimensions are approximately 1/2 wavelength and spaced from its reflector by a small dimension (see Fig 3). This kind of physical geometry offers the added benefit of rugged construction, to withstand the rigors of the launch environment. The short-backfire antenna is a two-wavelength-diameter dish with straight sides and a small disk mounted in the centerline (Fig 4). The short backfire behaves almost like a resonant cavity, offering substantial gain in a small space.
Fig 4--An example of a typical short backfire antenna.
You Can Help
The system-design considerations we've just discussed, and extensive prelaunch testing, are the tasks before the Phase 3D design team at this moment. Small wonder Phase 3D is a five-year effort, involving many dedicated, highly competent people from more than a dozen countries.
The result will be a powerful satellite that virtually all hams will be able to use. Phase 3D is an effort in which we can all take pride, but the project has a long way to go. With OSCAR 13 scheduled to reenter the Earth's atmosphere within three years, time is running out. The Phase 3D team needs your help to fund this ambitious project. Please send your contribution to the Phase 3D Fund, c/o AMSAT-NA, PO Box 27, Washington, DC 20044. AMSAT-NA is a non-profit organization under Section 501(c)(3) of the Internal Revenue Code, so US taxpayers may be able to claim a deduction for their contribution.
With the launch of Phase 3D only a few years away, we can expect not only a long-life replacement for OSCARs 10 and 13, but a new era for Amateur Radio communications.
Notes:
1Because OSCAR 10 can't be commanded to any particular orientation, its low-gain antenna is used at all times.
2Some government-produced satellites in the US and the (former) Soviet Union have employed nuclear power. The use of such power sources isn't open to most other satellite builders.
3King, Jan, "The Third Generation," ORBIT, November/December 1980, pp 12-18.
4Observers on the ground can't predetermine the rotational orientation of a satellite, and thus can't place the polarization plane of an antenna at an optimum orientation. If the satellite and the ground station are circularly polarized, polarization rotation isn't a problem.
