For the ISS, do you compensate for Doppler shift on 2 meters?

44% (71 votes)
56% (92 votes)
Total votes: 163
N5VHO – Thu, 2006 – 09 – 28 09:01

My e-mail address at ARRL.NET is good

Scott, email me at AH6RH@ARRL.NET. There are a handful of low elevation descending and high elevation ascending passes that might do the trick. I'd love to work the San Fran area, +/-. Doppler won't be needed on the descending passes, but will probably be needed for the ascending passes. APRS would do the trick.

Ron H

Submitted by AH6RH on Thu, 2006-09-28 19:14.

Specs for pass


Assuming the ISS goes back to APRS mode, these are the specs for the pass.

10/28 07:58:24z-07:59:04z or about 40 seconds of passtime.
Elevation in Hawaii starts at 1 degree, azimuth 58 degrees
and drops to zero elevation, azimuth 66 degrees.

Doppler is -1,100 Hz to -1,503 Hz, which is not enough for a 5 Khz radio.

Range is approx 1,000 nm.

Elevation at your end starts at zero and rises slowly to .70 degrees.
Azimuth ranges from 251 degrees, down to 243 degrees.
Doppler starts at 797 Hz and drops to 321 Hz.

These specs are approximate, subject to new kep elements and actual field results.

Too bad the cross-band repeater wasn't in operation. The specs looks like it would work due to the low doppler shift.

Ron H

Submitted by AH6RH on Sat, 2006-09-30 05:39.

2 meter Doppler

I don't adjust for Doppler on 2 meters. Just like Ron AH6RH states,
most of the time it is not necessary, unless on a high Doppler pass and bad attitude (poor signal) from ISS.

I am STILL looking for someone in the Hawaiian Islands to exchange reports with via ISS. There might be half a dozen opertunities between HI and CA, each month and those passes are less than 2 degrees total on each side above the horizion.

73, Scott WA6LIE

Submitted by wa6lie on Thu, 2006-09-28 18:32.

When I do compensate for Doppler...

I find most of the passes are below 25 degrees. During such passes, I find little value in going through the calculations and effort in performing Doppler compensation.

About a fourth of the passes, the ISS climbs higher than 25 degrees. On such passes, the doppler shift gets more pronounced. As the elevation rises higher than about 60 degrees, the shift due to doppler increases. Then it's worth CONSIDERING doppler compensation.

When it's above 60 degress, the doppler shift is slightly more than +/- 3 khz. About the first third of the pass, you transmit 5 khz low, and receive 5 khz high. During the middle of the pass, one disregards doppler and transmits and receives on frequency. During the last third, one uses doppler again and transmits 5 khz high, and receives 5 khz low.

I find that during most passes, when no one else is active, the audio quality is such that one doesn't need doppler compensation. The FM capture with the ISS is pretty much good down to the horizon even if the signal is off freq by more than 3 khz. I find it more than adequate when working APRS. 5 watts into a vertical is adequate when the ISS is overhead, but one needs at least 10 watts as it approaches the horizon.

But, if you expect others to be operating towards the perimeter of the footprint of the ISS -- those stations who would be on frequency and closer to the ISS -- or if you expect to operate on voice or the packet mailbox, you would gain back some of the disadvantage if you used doppler compensation. Otherwise, your signal would appear to the ISS to be off frequency by 3 khz and the station below the ISS would be favored and captured by the radio onboard the ISS as being on frequency. Your distance could be something like 2,000 km, and the other person might be 350 km away from the ISS. So you would need something like 25 times more power than the other person to be received at the same signal strength by the ISS -- and you would still have to overcome the handicap of being off frequency by 3 khz.

So compensating for doppler makes sense if you want to continue working the ISS during the lower thirds of your pass when others are operating at the edge of the footprint. Combine that with a directional antenna like the Arrow beam antenna, and you'ld have a much better chance of making/maintaining contact at the lower portions of the pass.

Most of the times these days, I don't compensate for doppler. I have more problems due to tall buildings and mountains blocking the path to the ISS so I compensate for that and drive to the part of the island that consistently favors the ISS pass without obstructions. I use a quarter-wave vertical on the rear trunk of my car. If I want good signals down to the horizon, I'll put a quarter-wave mag mount vertical in the center of the roof of the car to better ensure consistent coverage in all directions. The trade-off for not having to worry about doppler is that the digipeating and cross-band repeater work is limited to Hawaii and as a result, few people monitor and work the ISS on a regular basis.

Ron Hashiro, AH6RH
Honolulu, HI BL11

Submitted by AH6RH on Thu, 2006-09-28 10:56.

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