Tracking Satellites: The Nitty Gritty Details

If you want to listen to satellites, you have to be able to track them as they pass over the sky. When I first started tracking amateur satellites, computing the satellite’s location in the sky was a part of the challenge. Nowadays, that’s trivial. What’s left over are all the extremely important real-world details.  Let’s take a look at a typical ham satellite tracking setup and see how it all ties together.

Rotators for Steering

The popularity of robotics, 3D printing, and CNC machines has resulted in a deluge of affordable electric motors and drivers. It’s hard to imagine that an electric motor for rotating an antenna would be anything special, but in fact, antenna rotators are non-trivial engineering designs. Most of the challenges are mechanical, not electrical — the antennas that they drive can be huge, have significant wind loading and rotational inertial, and just downright weigh a lot. A rotator design has to consider bearings, weather exposure, all kinds of loads, not just rotational. And usually a brake is required to keep the antenna pointed in windy conditions.

There’s been a 70-some year history of these mechanisms from back in the 1950s when Cornell Dubilier Electronics, the company you know as a capcacitor manufacturer, began making these rotators for television antennas in the 1950s. I was a little surprised to see that the rotator systems you can buy today are not very different from the ones we used in the 1980s, other than improved electronic controls.

Typical Azimuth Rotator

These rotator systems tend to be quite beefy, as HF antennas can be large.  Fortunately in the case of amateur satellite communications only small Yagi antennas are needed. This simplifies the design if only because the equivalent surface area and weight of the antennas are much less. Commercial manufacturers have developed two-axis rotator combinations, such as this Yaesu model below, which is more compact and easier to setup. But some people might argue that this takes the fun out of the installation.

Example of Compact El over Az Rotator

For those people, the smaller size and less stringent requirements means that homebrew rotators are well within reach and suitable to this environment. Furthermore, some satellite tracking stations these days are portable and can be installed on a camera tripod in an hour’s time, the weatherproofing requirement all but goes away. Heavy winds that might damage the bearings aren’t much of concern if the whole tripod topples over before the damage can be done.

One final advantage for doing it yourself is that pressing a normal rotator into service on the elevation axis might be tricky. Conventional rotators are designed to operate vertically. Turning one sideways might not work. Here’s where spinning your own design might be easier than adapting an existing rotator. Browse the Internet for satellite tracking designs, or just design your own. Use a tripod, don’t worry about the weather and wind, and enjoy the satisfaction of seeing your firmware moving those antenna across the sky.

One Loop Can Be Ignored

Rotating the antennas to a commanded location represents one control loop. But there is another control loop that has been traditionally ignored: is your RF beam truly pointed at the satellite? Getting feedback to close this loop is a much more difficult problem, and fortunately an unnecessary one in ham satellite communications. But for the curious, there are a couple of ways this has been solved in the past, both requiring a steady signal from the satellite to use as a beacon.

One technique is to continually “wobble” your antennas in a circle around the expected pointing angle. Let the diameter of the wobble circle be the 3 dB beamwidth of your antenna. Now dedicate a separate receiver to listen to the beacon signal (don’t forget it has to track doppler, too), and observe the signal strength versus the wobble, you can get the tracking error. If you are perfectly on-track, the signal strength will not vary with the wobble. But if you are off-track, then the signal will be vary, and the pointing error can be caclucated from this variation. This adds quite some complexity to the design, mechanically and electrically. Given the large beamwidths of antennas used for ham satellite operations, that wobble would be a crazy sight to behold. This kind of tracking is much better suited to smaller beamwidth antennas that are lighter in mass, and therefore easier to wobble.

Or better yet, wobble virtually. Without going in to the gory details, you can calculate the off-axis angles by using multiple antennas in a phased array receiver. But if you want to apply this technique, you would need at least four antennas instead of one.

Why would the calculated position be wrong? Truth is, it’s rarely wrong. Satellite orbits are well established and their parameters are updated frequently. In fact, they are often integral components of calibration procedures themselves. If your station clock is accurate and its location is accurately known, the only real errors are going to be with the antenna system: error in the feedback, error in the orientation of the antenna mast, and mis-alignment of the RF beam vs. the mechanical axis of the antenna. Fortunately, the broad beamwidths of the typical ham satellite antennas mean that success can be had without needing to take extraordinary measures.

Antennas: Crossed Yagis and Polarization

You often see Yagi antennas for satellite communications arranged in an “X” pattern, called “crossed Yagis”. The reason is to accomodate different signal polarizations, either during a pass as conditions change, or for satellite to satellite variations. The best case is when the transmitted signal has the exact same polarization as the receiving antenna when it arrives. The worse case is if the two polarizations are orthogonal — say, you have a horizontally polarized antenna receiving signals from a vertically polarized one. In theory, you won’t hear anything — a complete mismatch. This is the same concept as polarized light filters we’re all familiar with.

I used the term arriving polarization, because the signal can be modified as it passes through the atmosphere. The satellite antenna itself might be moving, too, as it passes overhead. There are several tricks to deal with this. One is to use a single antenna positioned at a 45 degree angle. Then we will always receive H and V polarized signals, but both will come with a 3 dB loss. Another method is to physically rotate the Yagi polarization as needed to match the incoming signal. This can be done manually or by motors: conceptually yet another “axis” of rotation to consider.

But the more common approach is to use two Yagis mounted on a common boom. You can just switch between H and V to get the best signal, using a RF relay. Or, you can obtain circular polarization by adding a phase delay line and a combiner, and a relay will let you switch between clockwise or counterclockwise rotation. The math here is really crazy, but the bottom line is that any linear polarization will couple to a circularly polarized antenna with a 3 dB loss, no matter its angle.

Auxilliary Equipment

Because of cable loss at VHF and UHF frequencies, you typically need to locate additional equipment, such as a power amplifier, receiver pre-amps, RF relays and power supplies, at the antenna and not in the shack. These things need power, control, and status feedback, and will inevitably add complexity to the satellite station controller. Except for maybe getting mains power safely to the roof, these issues are much simpler to solve today than back in the 1980s.

Making Everything Play Together, 35 Years On

Controlling traditional rotators hasn’t really changed much. You basically mimic a human pressing the buttons by wiring a relay in parallel with the switches. If you have a modern controller, it might be even easier. The Yaesu rotator mentioned above has an RS-232 interface to control both azimuth and elevation. It even has an internal table of positions vs time, which you can pre-load with a satellite pass and let the smart rotator drive the antennas. But this takes away all the fun of building a tracker yourself.

I see no reason to make an open-loop system anymore, at least from a cost perspective. There are many approachable ways to close the pointing loop today. One technique would be to use a MEMS chip, such as TDK’s MotionTracking or ST Microelectronics iNEMO family of chips that Ted Yapo wrote about last year. Another idea would be to use a monitoring camera and computer vision algorithms to calculate the pointing angle (although you might need to put some strategically placed LEDs on your antennas for night operations). Or you could do it the usual way, incorporating a position sensor in each axis. Usually the feedback signal would be sent by wires, which have to jump across a rotating joint. While such feedback could be sent wirelessly, adding a few more wires isn’t really a problem — you already need cables to power and control the rotators, and of course RF coaxial cables to connect to the antennas.

Kepler’s laws of orbital mechanics haven’t changed. Whether you write your own algorithms, borrow some from an open source repository, or purchase a commercial software package, there are plenty of software choices to match your budget or skill level. But the options for user interface have changed drastically, and for the better.

While talking with some C64 folks online about my tracker restoration project, I realized just how dumb my old tracker program UI was. But today, using various libraries and data sets, your program could easily draw satellite data visually in real time. I was even able to draw a satellite orbit in OpenSCAD in short order.

One huge advancement is the ease of getting satellite tracking data from NASA. No more paper tables in the mail or modem links to BBS systems — you can get satellite parameters with a few clicks of the mouse and an internet connection.

If you are interested in Ham radio satellites and antenna tracking, there’s never been a better time to get involved. The price of entry, monetarily- and technologically-speaking, has never been better.

 

16 thoughts on “Tracking Satellites: The Nitty Gritty Details

  1. “Heavy winds that might damage the bearings aren’t much of concern if the whole tripod topples over before the damage can be done.”

    I used a steel pole buried into concrete pretty deep. Have to be a serious wind to move that.*

    *Considering I live in tornado alley, pretty possible.

  2. One good example of how to do it is a maritime stabilized antenna such as an Azimuth, Intellian or Seatel that uses 3 axis motors (azimuth, elevation and polarization) to track satellites on boats and ships. As the boat rocks and heaves, the dishes maintain within 1/2 degree pointing accuracy. They use counterweighted elevation gimbals and radomes which eliminate effects from the wind.
    They constantly move very slightly as described around the peak beam which they call “boxing” the beam. Another example is O3b that uses general dynamics dishes which are simply motorized with big motors and no radome. Considering you know location lat and lon, which way is north, orbital position and elevation angles you probably don’t need boxing, just math and decent angle sensors.

  3. I use Orbitron software for tracking, personally. It has DDE support, so it can calculate Doppler shift and automagically tell your SDR client what the correct frequency should be at any given moment, as well as directing your (hopefully) computer-interfaced rotor controller software the correct direction to point based on the predicted ephemeris elements.

    The software hasn’t been supported in a while(like most good ham software, sadly), but the elements are all that really matters, and they are automatically updated from NASA and NORAD(TLE) – and you can make your own custom element catalogs fairly easily.

    And it’s Linux-friendly for “you people” :P

  4. Seems like building a computer controlled tracking antenna presents quite an engineering challenge! If it is solely for the engineering challenge, I suppose that is a great reason. Or if you live somewhere where the weather is crappy all the time. But getting outdoors is, for me, half the fun.

    I’ve had near 100% success with my Kenwood 5W HT and an Arrow handheld antenna. It is super easy to rotate the antenna for best polarization, and point it in the right direction too. Basically a subconscious version of the wobble method described in the article. I considered mounting it on a tripod, but even that was much more of a hassle than simply pointing it at the sky. On high overhead passes I can easily get “in” with a whip antenna. Plus it rolls up and can be stuffed in a backpack and was easy (pre-COVID) to bring along to a park or something for the day.

    For sat tracking I use a free iPhone app that even has augmented reality.

    The only problem I’ve had with this rig is getting stepped on hard by people sitting in their shacks, probably with computer controlled antennas (sorry couldn’t resist), and certainly putting out greater than 5 watt. I’m certain you don’t need anything more than that because even on low, say 10 degree passes, in the wee hours of the morning when the sats are almost unused I’ve had no problem hitting and even activating sleeping com sats with the 5W rig mentioned above.

    TL:DR if you are going to work the birds, please dial back your transmission strength to “minimum necessary” per ham regulations

    cheers
    Craig KG5YVN

    1. I agree 100% with Craig KG5YVN that far too many amateur operators use WAY too much power which is not needed when working FM or SSB birds. 5W on UHF driving at minimum a 7 element beam is more than adequate and 5W on VHF with 3 elements does just fine too, rotator or not. Sure, some people feel the need to spend more and make fancy stations, but your still reaching the same satellite a few hundred miles away. Please do us all a favor in dialing the power back!. I miss how a few satellites used to have better watch dog systems that would punish people by trying to use more power than what was needed.

  5. “One technique is to continually “wobble” your antennas in a circle around the expected pointing angle.”

    Actually the most common practice is to move the antenna off axis slightly in elevation and azimuth while interpolating the satellite’s measured beacon signal level and moving to a new position where the beacon level is calculated to be a maximum. This technique is termed “step-tracking” and is used with slowly moving satellites, such as tracking the diurnal motion of geosynchronous satellites.

    “…you can calculate the off-axis angles by using multiple antennas in a phased array receiver.”

    The proper term for this is “Monopulse Tracking”. See this site for more on this:

    http://earthstationnotes.blogspot.com/

    You might also want to mention the problem of “azimuth correction for elevation in elevation over azimuth mounts” (the most common type of mount). As the elevation angle increases you have to move more and more in azimuth to obtain a given offset from the antenna’s beam center (boresight). At some point the amount of azimuth movement required is too much for the antenna mount to keep up with and the satellite can no longer be tracked. The limiting case is when the satellite is directly overhead (90 degrees elevation), in that case any amount of movement in azimuth will never move the antenna off beam center. This paper [PDF] explains this problem in detail:

    Azimuth Correction for Elevation-over-Azimuth Positioners

    https://de.eutelsat.com/files/contributed/support/pdf/Azimuth_Correction.pdf

    One case where this azimuth correction problem is critical is in earth stations located on or near the earth’s equator tracking overhead geosynchronous satellites. In this case elevation over azimuth mounts have so much azimuth “wind-up”, even tracking the diurnal motion of a geosynchronous satellite can wear out the azimuth motor drive over time. This problem is even worse when the earth station is mobile. The best solution is to replace the elevation over azimuth mount with an azimuth over elevation mount. However, azimuth over elevation mounts are more complex and costly compared with elevation over azimuth mounts.

    Just forget about the tracking!

    For VHF/UHF LEO satellites skip the tracking and use two “eggbeater” omnidirectional satellite antennas, one for VHF and one for VHF. Eggbeaters don’t have the gain and directivity of Yagi antennas, but in most cases they are good enough, especially if you locate the receiver pre-amplifier and transmit power amplifier at the antenna feed point. Here is a picture of a commercial dual VHF/UHF eggbeater system:

    https://static.dxengineering.com/global/images/prod/xlarge/msq-satpack1_xl.jpg

    Here is the homepage for that eggbeater – M2 Antennas Satellite Antenna Packages SATPACK1 $572.99:

    https://www.dxengineering.com/parts/msq-satpack1

    Yeah $572.99, that’s not a typo. Fortunately eggbeater antennas are pretty easy to make. I’m sure there are plans online somewhere. And remember to simulate your design before you start bending metal. This antenna simulation tool is free:

    https://www.qsl.net/4nec2/

    1. Great resources and comments. I wasn’t too sure how deep to go, hence only the passing mention of monopulse. I haven’t encountered stepped tracking before, but at these wavelengths it would be much easier to implement than conical scanning. I did mention a three-axis rotator arrangement in the first article, thanks for mentioning the simpler Az over El arrangement which also can help solve the zenith crossing problem. If the link budget allows, you are right, forget tracking and use those egg-beater style antennas. The small external antennas we used to use for GPS receivers come to mind, and I vaguely recall reading that some portable commercial satellite terminals had fixed antennas as well.

  6. I’ve got no rotor yet and use a bigwheel antenna system,
    but I’m using a tracker program, too. :D

    The old STS Orbit Plus for DOS uses cool vector graphics and runs fine on several systems here:

    My Macs with VirtualBox/SoftWindows, my vintage 286/386 systems external math coprocessor, a blu-ray player (android) with a PC emulator, my smartphone running DOSBox..

  7. It’s a blast to talk through an OSCAR satellite to someone over in Eastern Europe. The OSCAR was furthest out at its apogee, so not difficult to maintain tracking once contact was made.

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