- Stu WØSTU
Amateur Satellite Contacts
Amateur satellites function as repeaters in orbit. By transmitting to an amateur satellite in low earth orbit you can make contacts hundreds of miles distant. It is not unusual for the instantaneous coverage diameter of a satellite’s footprint to be 1500 miles. Many amateur satellites operate as digital repeaters, and some may contain broadband linear transponders that provide multiple simultaneous channels and modes of operation. However, the simplest method of getting started with amateur satellite contacts is with birds having FM phone (voice) capability on VHF and UHF bands available to the Technician Class operator.
And you don’t need a super powerful transmitter to work a satellite – a 5 watt HT and a hand-held directional antenna is usually sufficient. Let’s take a look at how you can easily get started making satellite contacts!
AMSAT: How do radio amateur satellites get into orbit? The Radio Amateur Satellite Corporation (AKA AMSAT) is a worldwide non-profit organization organized for scientific purposes carried on, in part, by means of developing and providing satellites and related equipment useful for amateur radio communications. [Derived from Articles of Incorporation, Radio Amateur Satellite Corporation.] This organization raises funds and conducts activities for the creation and launch of amateur radio satellites. You can learn more about AMSAT at amsat.org.
Big Picture: Before jumping into some details, let’s make sure you have the big picture understanding of basic amateur satellite operations. Here is a simplified step-by-step list of what you might do to work a satellite and make phone contacts with other hams:
Research upcoming amateur satellite passes that will be within range of your earthly location and select one or more that you wish to try to work.
Establish a split channel in your HT specific to the satellite frequencies required, including any CTCSS tones (or other squelch methods).
Before the satellite pass occurs, set up your equipment (HT and directional antenna) and envision the path across the sky through which the satellite will travel, making particular note of altitude and azimuth milestones over the minutes of the pass. (i.e. The satellite will be near the top of that tree at 08:43:00, and it will move just west of the garage roof by 08:45:00.)
During the time of the satellite pass, point your directional antenna(s) in the direction you expect the satellite to be. This means moving the pointing direction of the antenna across the sky as accurate to the predicted path as possible.
Listen on the downlink frequency for other operators calling and answer their call, or make your own call (usually just call sign and Maidenhead grid square location).
Continue to track the satellite path with your directional antenna over the time of the pass making / answering calls and exchanging call sign and position information.
With that overview of how the operation will transpire, let’s now consider some important details that will help you to better understand and conduct radio amateur satellite operations.
Orbits: Knowing the characteristics of satellite orbits is important for making radio contacts because you will usually want to track the satellite using a directional antenna. That is, as the satellite moves across the sky you will point the main lobe of your antenna in its general direction. More about antennas and tracking in a moment after we consider some features of satellite orbits.
The vast majority of amateur satellites are placed into low earth orbit (LEO). This means that the satellite is no more than a few hundred miles above the surface of the earth. It is common for the birds to fly between 200 and 300 miles high. For comparison, the International Space Station (ISS) has a mean orbital altitude of about 240 miles.
Low earth orbits may be equatorial, polar, or inclined. Equatorial orbits are those that circle the earth over the equator, and these are rarely used with amateur satellites. Polar orbits have the satellite pass over the north and south geographic poles of the earth. An inclined orbit is somewhere in between equatorial and polar, with some angle of inclination measured upward from the equator.
It is very common for amateur satellites to be placed in a high inclination orbit, one that is nearly polar. Polar or high-inclination orbits have a particular advantage in that they will pass over essentially every position on the earth. Since the typical LEO requires about 90 minutes per orbit, and since the earth spins underneath the orbiting bird, a high inclination or polar satellite can cover practically every position on the earth within the paths of several orbits. Of course, the other relevant operational implication of this type of orbit is that the satellite will move rapidly across the sky, limiting the amount of time available for radio contacts during any given pass over your region.
Equipment & Modes: Despite an amateur satellite being a couple hundred miles away, you have a clear line of sight to a passing bird. An ordinary handheld FM transceiver (HT) with an antenna that provides a bit of directional gain is usually all you need to work a satellite and contact other amateur operators. A handheld Yagi attached to a 5 watt HT radio is the typical starting setup for most hams seeking satellite contact.
Amateur satellites most commonly use a split mode for transmitting and receiving. For instance, the current AO-85 “Fox-1A” satellite is received on earth on the 2-meter band (downlink frequency), but earthbound operators transmit to the satellite on the 70-centimeter band (uplink frequency). A common dual-band HT for 2-meters and 70-centimeters may be programmed with a split channel to operate this way, automatically shifting to the proper transmit band and frequency when the push-to-talk button is activated. This is identical to setting up a normal repeater channel, only the pairing of frequencies is between two different bands rather than within a single band. Check your HT user’s manual for instructions on establishing a split channel, or “odd split.”
Of course, a couple of single-band HTs can be used instead, one for receiving and one for transmitting. This method provides the advantage of full duplex operations, and you will be able to check that the satellite has received and retransmitted your signal since you will hear your own transmission on the receiving radio. In this scenario, and especially if each radio has a directional antenna connected for its band of operation, the tracking mechanics are a bit more involved than using one dual-band transceiver and directional antenna. The paired antennas will usually need to be mounted together pointing in the same direction, and possibly mounted on a pivoting base or tripod to allow ease of satellite tracking across the sky. But, a willing friend or family member can also lend a hand, mirroring your transmitting antenna pointing direction over the time of the satellite pass.
A fine point of nomenclature: With amateur satellites the “satellite mode” does not refer to the method of modulation such as FM, AM, CW, etc. Rather, the mode of the satellite describes the uplink/downlink arrangement. As an example, the AO-85 FM voice function is Mode U/V. This means the uplink frequency (always the first letter) is a UHF frequency in the 70-centimeter band and the downlink frequency (always the second letter) is a VHF frequency in the 2-meter band. Other satellite modes are feasible as listed in the table.
Directional antennas for providing gain may be homebrewed or commercially obtained. My favorite satellite antennas are manufactured by Arrow Antenna of Cheyenne, Wyoming. Their Alaskan Arrow satellite antenna is a dual-band (2m / 70cm) Yagi offering excellent gain on each band in a very manageable, hand-holdable, lightweight package. It may also be mounted on a standard photography tripod for ease of pivoting and tracking in two dimensions. A similar, lower gain and lower cost model of earlier design is the Arrow II Satellite.
Home-brewing a satellite antenna is a terrific beginner’s project, and several designs are found at online locations. A couple of note are the VE2ZAZ model, the AMSAT “Cheap and Easy Yagi” pairing, and the W6NBC tapemeasure Yagi dual-band. Check out these fine designs and create your own satellite antenna. You’ll find that homebrewing an antenna that you can use to reach into orbit a very rewarding experience, and one that will impress your friends!
Tracking: If a LEO amateur satellite passes directly overhead of your location on earth it will require roughly 7 to 8 minutes to travel from horizon to horizon. If the pass is not overhead, but rather off to one side of your position, the travel time from horizon to horizon will be reduced. In either case you will need to be knowledgeable of the track and timing of the satellite across the sky in advance of the attempted contact.
The easiest resources with which to obtain amateur satellite pass information are found online and in smart phone apps. I’ll note a couple of my favorites, but be aware that several additional resources are freely available and found with a quick online or app search.
AMSAT provides an online pass prediction utility. Simply select a satellite from the pulldown list, enter your Maidenhead grid square locator, and click Predict. You will get a list of upcoming passes over your location including azimuth and elevation figures, duration, and date and time information. AMSAT also maintains a page listing currently active satellites.
A more visually intuitive pass prediction utility is provided by Heavens-Above.com. After providing your location information and selecting a bird to track, sky maps and ground track maps are generated that are very easy to interpret. Heavens-Above also provides smart phone apps with similar mapping functions and tabular lists of pass information.
Heavens-Above.com ground track map example with AO-85. (Courtesy Heavens-Above.com)
Whether you are hand-holding your directional antenna, slewing it on a tripod, or tracking with a motorized gimbal, it is crucial to maintain the pointing vector of the main lobe toward the moving satellite. It sounds easy enough, but it takes a little practice sometimes to fully coordinate manual tracking, careful listening, proper transmission verbalization, and recording of contacts. Getting the assistance of another person to help record contacts or to perform accurate tracking with the antenna is a good idea, especially for the beginner.
Keep in mind that tracking is a constant activity that must be correlated with time. That is, you must point the antenna over the predicted path through the sky in time with the prediction path time, and that alone requires significant concentration. However, be aware that with most directional antennas of moderate gain the forward gain RF lobe is going to be of sufficiently wide angle that extremely accurate pointing is not required. A typical 45-degree to 60-degree directional cone to -3 dB gain from peak gain direction provides quite a bit of slop for your pointing vector.
One last comment about tracking: Satellite passes that are of high elevation for your position – those that are closer to overhead than out at the horizon – are the passes for which you are more likely to be able to work a satellite with a low powered transceiver like an HT. Satellite passes near the horizon will be difficult or impossible to work due to the relatively low powered signal at those distances as compared to signals from operators for which the pass is more overhead. In selecting passes for contact attempts, try to find those that are of an elevation of 45-degrees or higher for the best chance of working the bird with an HT.
Doppler Corrections: Satellites are moving fast, and much like the tone of that speeding locomotive wailing its horn as it speeds past you, a satellite’s RF emissions will experience Doppler shift. As a satellite approaches your position its emitted signals will be shifted slightly higher in frequency, and as it departs its signals will be shifted slightly lower in frequency. Conversely, your transmissions to the satellite will be received as slightly higher during approach and slightly lower during departing motion. Particularly for satellite passes directly overhead, the Doppler shift can be significant enough to warrant tuning corrections during the pass. Just one more challenge to overcome in your tracking, listening, talking, logging exercise!
Doppler effect tends to manifest as a percentage of frequency, so it is common in U/V or V/U mode for the UHF 70-centimeter frequency to require some adjustment during the pass. The VHF 2-meter frequency can usually remain static with only minor frequency effects detected. The easiest method of accomplishing the Doppler correction with an HT is to program several satellite split channels in adjacent memory locations and snap through the sequence of channels during the satellite pass. From satellite’s approach to departure you should adjust the UHF frequency as necessary to help compensate for the Doppler effects during the pass.
In V/U mode where the downlink frequency is the 70-cm frequency, this will mean adjusting your receive frequency from a higher value received on approach to a lower value received as the satellite passes by and departs. For mode U/V, the uplink UHF frequency must be adjusted from a lower value to a higher value during the pass, as your transmitted frequency will be received by the satellite as a slightly higher value during approach and as a slightly lower value during departure.
For the AO-91 Fox-1B currently operating (late 2021 into 2022), AMSAT has published recommendations for Doppler corrections (here) and other operating recommendations. For Doppler corrections, five channels with uplink frequency increase in 5 kHz steps from 435.240 to 435.260 MHz are recommended. AMSAT also suggests a full duplex listening arrangement so that you can hear when frequency changes are warranted – listen with another HT, and perhaps use headphones to avoid the audio feedback in your uplink transmission.
On-Air Procedures: The on-air procedures and etiquette for amateur satellite contacts is only slightly different from normal repeater ops. Because the opportunity for contacts is brief – a few minutes at best – and because the area of coverage of a satellite is vast and may involve numerous operators attempting contacts, keep contact exchanges very brief. Usually just an exchange of call signs and locations is made in any single contact. Typically, the Maidenhead grid square locator is used to identify geographic position, so be sure that you know your grid square before you attempt satellite contacts. [Here is a great utility for finding and examining grid squares.]
Additionally, because numerous contacts are likely to be attempted simultaneously from across the satellite’s footprint area, interference and noise may make hearing difficult. Speak distinctly, clearly, and slowly, and utilize standard ITU phonetics to ensure an accurate and complete exchange.
Making satellite contacts is very rewarding, and quite easy to do. And don’t forget that the ISS is itself a satellite with an amateur radio station onboard. On occasion astronauts aboard ISS make contacts with ham operators, and the digital repeater onboard ISS is available on any pass over your area. However, most phone contacts with ISS are pre-arranged Amateur Radio on ISS (ARISS) events, and unplanned contacts are much rarer.
Gather your gear! Do your research! Select a pass! Make it happen! You can make amateur satellite contacts regularly with simple gear, a little planning, and a bit of practice and tenacity. Go for it! Good luck, and 73!