For the last net of the year I decided to change it up and discuss something outside the realm of emergency communication. Tonight we’ll talk about the ARISS program (Amateur Radio on the International Space Station).
Under ARISS, a variety of amateur radio communication activities have been conducted at specified times as per declared schedules. Amateur radio operators around the world have made numerous FM radio voice contacts with astronauts onboard the ISS as well as received SSTV picture transmissions from the ISS.
However, the above activities in the past required adherence to ISS activity schedules. Radio amateurs could make radio contacts with the astronauts but only when they were free to operate amateur band radios…
A new version of the ISS Cross Band Repeater was activated on September 2, 2020 at 01:02 UTC. The ISS Cross Band Repeater allows two geographically separated operators on the ground to establish contact with one another. The ISS is only the facilitator just like any other communication satellite. One doesn’t need to wait for the astronaut to be available for contact. Moreover, with this repeater, it is no longer an earth station to space station contact but between two earth stations.
Let us see what this repeater system onboard the ISS has to offer…
The first important factor is that the ISS is large and therefore has a huge amount of resources in terms of physical space, electric power, etc. in comparison to smaller dedicated LEO satellites for amateur radio service. As a consequence, better equipment with higher transmitter power becomes feasible.
The repeater equipment used on ISS is a customized Kenwood TM-D710GA VHF/UHF FM radio transceiver. Unlike regular amateur radio satellites that typically have downlink TX power of around 1W or even less in many cases, the ISS repeater downlink RF power output is typically set at 5W, although the equipment has the capability to churn out 25W if needed.
The antennas used for communication on the amateur radio bands consist of VHF/UHF monopole antennas. There are three such antennas on the ISS and are used for various purposes. However, all said and done, they are all linearly polarized antennas with nearly omnidirectional radiation patterns. The effective antenna gains are nominal and close to 0 dB or perhaps a little more.
The uplink receiver front-end of the ISS Cross Band Repeater system is very sensitive and has low noise characteristics. Therefore, the ISS repeater can respond to relatively weak signals that might be produced from low power amateur radio rigs like the Hand-held (HT) radios outputting merely a couple of watts into a regular whip antenna.
Though it works with an HT whip antenna on most of the medium and high elevation passes, it would certainly be nice to have a set of 2m/70cm outdoor omnidirectional antennas like either the Turnstile, the QHA, Lindenblad, or the Eggbeater installed in the clear on a rooftop. This way a horizon-to-horizon strong and robust contact on every pass is guaranteed.
Another important factor that makes contacts via the ISS Cross Band Repeater possible with very moderate equipment is the fact that the orbital altitude of the ISS is quite low. It is approximately 420 Km above the earth’s surface. As a result of this, the link distance between the earth station and the satellite during its pass becomes less. Compared to a higher altitude satellite, the Line-of-sight (LOS) distance to the ISS is relatively smaller. On high elevation angle passes, the ISS is often visible with the naked eyes as it fleets across the sky.
However, there is a downside to the lower orbital altitude of the ISS too… The foot-print radius circle of communication coverage distance on the earth is less because of the lower orbital altitude. In other words, the distance between two earth stations that might simultaneously fall within the coverage area of the passing satellite gets reduced in comparison to what one might expect from another satellite at a higher altitude.
Important tips on working through the ISS cross band repeater:
First, the ISS features a V/U repeater. This means that the uplink frequency is VHF, at 145.990 MHz with a PL tone of 67.0. The downlink frequency is UHF and is 437.800 MHz, with no PL tone.
Orbiting satellites including the ISS produces Doppler downlink frequency shifts of ±9 KHz due to the continuously varying relative distance between the satellite and the observer on earth. The maximum 2m uplink Doppler shift for the ISS is within ±3 KHz, usually allowing the uplink frequency to remain constant at 145.990 MHz, with a PL tone of 67.0.
During the ISS pass, to account for the Doppler shift, it will be necessary to adjust the RX frequency incrementally while maintaining the VHF TX frequency of the split. It can be done manually, but it could be a rather distracting process.
One suggestion to overcome this on-the-fly adjustment is to program several versions of the split frequency into your memory locations using 5 kHz spacing for the downlink side of the split, and assign them friendly names.
Start with a receive frequency of 437.810 and transmit frequency of 145.990 with a PL of 67.0, then reduce the receive frequency of each additional saved split channel by 5 kHz before storing as a new memory. With 5 memory channels saved to the memory of your radio you can effectively cover the entire ±9 KHz receive range of the Doppler shift.
It is also a good idea to listen to your radio unsquelched. Even though unsquelched can be annoying, it does give you the best chance for success in working the ISS crossband repeater.
Along with the modest set of equipment and the information gathered in this article, we are almost set to go. However, we do not yet know as to when and where in the sky will the ISS appear. Hence, we need a reliable method of predicting and following the ISS orbital passes.
There are several internet options as well as mobile applications that will do the trick. ISS Detector is the application promoted on the website of the article. I also looked at a couple of internet sites with tracking displays such as isstracker.com and n2yo.com.
One of our net members, KE5HHW also suggested GO ISS WATCH as another mobile application option for tracking the ISS location.
I am interested to try this over the coming weeks to see how well I can hear this repeater on passes when it is in range. If you have experience communicating through the ISS or amateur satellites please feel free to share with the net.