ISS Amateur Radio Status: May 28, 2001

Adjacent Channel Interference:

By Miles Mann WF1F,

MAREX-MG (Manned Amateur Radio Experiment, North American Division)

One of the minor issues with Interference (QRM) on the ISS packet uplink on 145.990, is the FM repeater input on 146.010.

There is a Repeater pair on 146.010 input with output on 146.610 (in the USA) Most repeater channels in the USA have approximately 100 repeaters on each channel. (maybe some one at the ARRL can come up with the current channel count for this frequency)

On Earth, we do not have a problem with 100 FM repeaters sharing the same channel. This is because of the term Terrain Shielding. If you place two repeaters using the same channel 200 miles apart, there is a 99.99% chance they will not cause each other interference (assuming 2-meters and no ducting). The curvature of the earth, trees, hills, etc. help block the signals. Terrain shielding helps us load up a channel and reuse a channel over and over again.

ISS and Terrain Shielding:

ISS is at an altitude of approximately 220 miles (400~km). This altitude gives ISS a LINE-OF-SITE radio range of 1500 miles (2000~km). When ISS is over South Dakota, its radio range covers the USA coast to coast. It is even possible for a station in Boston Mass to Digi to a station in California. Once ISS is above your local horizon there is no terrain shielding. Any station operating on the ISS uplink channel improperly can inadvertently cause serious interference (QRM).

Adjacent Channel Interference:

We all know, that you can not run 2 FM repeaters in the same town with a 10k channel separation. Even 15k channel separation does not work (recommended terrestrial spacing for FM repeaters using a 15k channel is 75 miles between repeaters).

A FM voice channel (5k deviation) needs 20k of channel spacing on EARTH to prevent interference from bleeding over to the next channel (adjacent channel interference)

In space, it is a whole other story. In space, you must also add to your channel spacing the Doppler frequency correction. In this case, it is 3.6k, so lets round to 5k that brings us to 25k. Now lets add another 5k to help prevent the FM capture effects from hearing the repeater inputs. That puts us at 30k channels.

After 12 years of experiment with Mir, we came to the conclusion that 25k channel spacing was minimum to avoid qrm from terrestrial stations or other satellites and 30k channels spacing was ideal. For the Mir space station, we compromised on 145.985 FM simplex. This put Mir 25K away from the 100 repeater inputs on 146.010.

In my opinion, a small percentage of the QRM being received by ISS on 145.990 is caused by adjacent channel interference caused by the repeater inputs on 146.010 and other users close to the 145.990 channel. There is not much we can do about the QRM at this time. If we do install a filter on ISS to notch out the qrm from, we can look into adding an extra filter for blocking signals above 146.010.

Interference Sources:

Here is a partial list of interference’s sources you will need to be aware of, and contend with while operating LOE Satellites.

  1. Excessive Traffic loads to the ISS Packet System
  2. If you are a low power station, try accessing ISS packet while everyone is sleeping or at work.

  3. UO-14 Uplink traffic on 145.975
  4. Monitor both satellites and try to avoid overlapping orbits.

  5. Repeater inputs on 146.010 adjacent channel interference
  6. ISS VHF Activity on (desense)
  7. Stations not following band plan suggestions
  8. Tight Receiver squelch setting
  9. RF Intermode caused by RF energy from urban areas

The best ways to overcome these interference problems is:


good luck,

knowledge is greater than QRO



New MAREX Web pages:

Check out our future ISS Projects and a large list if Mir related links and tips on how to use the Chat room on ISS and voice frequencies.

Copyright 2001 Miles Mann, All Rights Reserved. This document may be freely

distributed via the following means - Email (including listservers), Usenet,

and World-Wide-Web. It may not be reproduced for profit including, but not

limited to, CD ROMs, books, and/or other commercial outlets without prior

written consent from the author.

Until we meet again



You are visitor # since MAY 2001

Last updated May 2001
©(2001) MAREX-MG