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 Previous: Short-Term Expedition 9 Crew Congratulates SpaceShipOne Success |  Next: NTIA's BPL Position a Moving Target; Agency Edges Closer to FCC's Stance |
By Emily Clarke, W0EEC
October 6, 2004
It's easy to make QSOs through the Amateur Radio on the International Space Station (ARISS) repeater aboard the ISS.
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To the delight of many hams, the Amateur Radio on the International Space Station (ARISS) Phase 2 gear in recent weeks has been turned on periodically as a voice repeater [the system was in repeater mode as of October 7--Ed.]. Unfortunately for every amateur making a successful contact through the ISS repeater, many others go away disappointed when they can't seem to transmit through the repeater. This can be very frustrating because unlike other amateur satellites--where hearing the bird often is the major challenge--the ISS is very easy terrestrial copy. It generally puts out 5 to 10 W as opposed to the typical 0.5 to 1 W OSCAR downlink power. Nonetheless, some of those attempting to work through the ISS repeater experience difficulty or only have limited success, even when the ISS is directly overhead.
Unlike any other FM satellites that operate in Mode V/U (Mode J) the ISS is operating in Mode U/V (Mode B). While this seems to be of little consequence to most owners of dualband handheld transceivers, mobile rigs and base stations, some important factors can spell the difference between success and failure. Let's consider these.
Downlink/Uplink Frequencies
The ISS repeater downlink is 145.800 MHz, which is the normal FM voice and the RS0ISS packet system downlink worldwide. (The non-repeater FM voice uplink frequencies are 144.49 MHz in Regions 2 and 3 and 145.20 MHz in Region 1. The packet uplink frequency is 145.99 MHz worldwide.)
The ISS repeater uplink is 437.800 MHz, which is in the high end of the satellite subband. Not all dualband FM transceivers will transmit on that frequency, however. For example, a stock Icom IC-W32A will not transmit there. So, before you attempt to transmit through the ISS repeater, check your transceiver's owner's manual to make sure you will be able to transmit on the uplink frequency. No CTCSS tone is needed to access the ISS repeater.
Doppler Shift
Dealing with Doppler is a fact of life for all satellite work. Most hams who have tried to work Mode V/U satellites are used to transmitting on a single frequency and listening on a frequency 5 to 10 KHz above the "published" downlink frequency, then tuning lower in frequency as the satellite approaches. This is because on a 2-meter uplink frequency Doppler shift is less of a factor--on the order of perhaps 3 kHz. At 70 cm, however, Doppler shift is substantial and can easily exceed the ability of the receiver to capture the downlink signal.
 This diagram illustrates how Doppler shift will require you to change your transmit frequency when working through the ISS Mode U/V repeater. Transmit frequency must change according to the position of the ISS relative to the operator's location on Earth. [Emily Clarke, W0EEC] |
To see this for yourself, have a friend transmit on a 440 MHz simplex frequency while listening first 10 kHz away, then 5 kHz away. See what happens to the signal.
The published uplink frequency of 437.800 MHz is the frequency the ISS receiver--a Kenwood TS-D700E in this case--is tuned to. Since there's only a brief time when the receiver will actually hear a signal transmitted on that frequency, adjusting your transmit frequency to match the receiver's tuning is critical. This is because Doppler shift causes the transmitted signal to shift up as the satellite passes from horizon to horizon overhead.
So, to be "on frequency" with the ISS repeater, you have to begin transmitting at a lower frequency--starting at around 437.790 MHz when the ISS first comes over the horizon, tuning up to 437.795 MHz about a third of the way into the pass (approximately 3 minutes on a good overhead pass). When the ISS reaches its time of closest approach--TCA--tune your transmitter to 437.800 MHz. At this point, Doppler should be essentially neutral (0 Hz). Then, as the satellite moves toward the opposite horizon, continue tuning up to 437.805 MHz and finally--when the ISS is about to go over the horizon again--to about 437.810 MHz.
Actual transmit frequencies will vary depending on your position on Earth with respect to the ISS, so using a computerized tracking program that can calculate the exact frequency is a helpful tool.
This technique also
will vary depending on the tuning steps available on your transceiver. If your 70 cm transmitter can only tune in 5 kHz tuning steps, you
can't tune precisely. If you're within 2.5 kHz, however, your signal will get
captured and repeated. If you're equipped for computer-controlled Doppler
tuning, make sure you are tuning both
uplink and downlink frequencies, not just the downlink. Many operators owning
radios designed for satellite work are used to locking their transmit frequency
and only tuning the receiver. In the case of the ISS, however, one should tune
both transmit and receive.
 Expedition 9 Commander Gennady Padalka, RN3DT, uses the ARISS Phase 2 equipment, located in the crew's quarters. The crew periodically sets up the Phase 2 gear as a Mode U/V repeater. [NASA Photo] |
Modulation and Deviation
The ISS receiver seems to be more sensitive to excessive deviation and overmodulation. These factors are generally not a problem on OSCAR satellites. To avoid problems in this area, be sure to turn off any speech compression or processing (a good idea for most FM and satellite work in general) and consider lowering your microphone gain (or speaking a bit farther away from your microphone). If you are using full duplex and monitoring your own downlink signal, you will notice your audio become cleaner and more natural sounding.
Pass Predictions
The ISS is moved from time to time to boost its orbit or to avoid space debris. This means its crucial to have the latest Keplerian elements. Serious users of the ISS repeater likely will update their Keps daily. I recently updated the AMSAT Pass Prediction Calculator to update Keplerian elements automatically twice per day, thus increasing the accuracy of its predictions.
Schedule
The ISS voice repeater is not on all the time. When not in repeater mode, the Kenwood D700 serves as the RS0ISS packet system (145.990 MHz up/145.80 MHz down). The ISS crew generally switches to repeater mode before an ARISS school group contact and back to packet after the school contact. (This could change when the Expedition 10 crew takes over the ISS in late October, and the Kenwood Phase 2 equipment becomes the primary transceiver for school group contacts.) Given this uncertainty, it's important to monitor the ARISS Web site for more information. You can get additional information by subscribing to the SAREX mailing list. The ISS Fan Club Web site also lists the current ARISS equipment operating status.
ISS Expedition 9 crew member Mike Fincke, KE5AIT, has been active making casual Amateur Radio QSOs and occasionally gets on to talk with stations while the system is in repeater mode. Expedition 9 Commander Gennady Padalka, RN3DT, also has been active during the current crew's tour of duty.
In Conclusion
Success working through the ISS repeater is easy. Remember to:
Emily Clarke, W0EEC, is an ARRL and AMSAT member. In
addition to designing the new AMSAT Web site,
she serves as AMSAT's San Francisco Bay Area Coordinator. Clarke also is the
vice president for membership and on the board of directors of the Project OSCAR Amateur Radio Club.
| Previous: Short-Term Expedition 9 Crew Congratulates SpaceShipOne Success | Next: NTIA's BPL Position a Moving Target; Agency Edges Closer to FCC's Stance |
