Ham Radio Repeater frequencies

May 25, 2022

Occasionally, the ISS amateur radio equipment is switched into repeater mode, with uplink at 437.800 MHz and downlink on 145.800 MHz. Most of the time the equipment is in APRS mode (you can hear distintive packet sounds when ISS is overhead). Several amateur radio operators have used it to make contacts over distances hundreds of miles.

ISS repeater works like a local mountaintop repeater (exception doppler effect) but much more crowded. Stations you will hear are from other areas of the country (do not expect ISS crews to join in, but they may if ISS workload permits), and you will hear it since it is extremely busy as it comes overhead. Frequency adjusting per doppler effect, proper microphone technique, and patience seem to be the best efforts when establishing contact.

Success Tips for Using the ISS Voice Repeater
Info from

  • Downlink/Uplink Frequencies

    ISS repeater downlink is 145.800 MHz (uplink is 437.800 MHz), which is the normal FM voice.

    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, Check your transceiver 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.

    Working through the ISS repeater:

  • Make sure your transceiver can transmit on the ISS uplink frequency.
  • Obtain accurate pass predictions.
  • Adjust your transmitter frequency to correct for Doppler.
  • Keep your modulation and deviation within the normal range for terrestrial repeater hardware.
  • Use full duplex whenever possible to monitor your signal quality.
  • Always be courteous. Please don’t monopolize the repeater.
  • Once you’ve made a contact give someone else a chance. Activity can be heavy!
  • Listen before transmitting to make sure you aren’t stepping on another station.
  • Try not to talk too loud or close to the microphone, the repeater is very sensitive to over-modulation.
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    Source: hamradio.arc.nasa.gov
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