John, It's pretty clear from tracking the doppler effect during multiple passes that these frequencies are not correct. I believe I read somewhere that AO-27 was being quoted at 436.797, which is pretty much what my measurements showed. Of course, my measurements are going to be off by however much my old FT-726R is wrong. But I have reason to believe that's less than 1kHz.

Here's how I'm coming up with my figures: In the gpredict program, there is an area where the doppler shift is calculated. As posted recently, I hacked this to show the actual frequency that should be on my dial. I added the following line just after the doppler was calculated. I also added an if statement to select a specific satellite, but that is irrelevant to this discussion.

doppler=doppler*4.367950/100.0+7950.0;

In the program, the doppler is calculated in hz for a 100mHz signal. My idea was to simply rescale that to 436.7950 by multiplying by 4.367950. I need to move the decimal point over two more so that I can display the 100kHz and down frequency readout, so I divided by 100.

The problem was that the results were wrong by different amounts for the three satellites. The only way to get the right figure was to start moving the satellite's implied frequency. e.g.:

doppler=doppler*4.367920/100.0+7920.0;

The result from this calculation tracks SO-50 pretty well, so unless my idea is fundamentally wrong, I believe that SO-50 is transmitting near 436.7920.

Bob

--- John Kopala jkopala@gmail.com wrote:

The frequencies given for each of the easy satellites are:

AO-27 436.795 SO-50 436.795 AO-51 435.300

AMSAT gives the details for the individual satellites and their uplink and downlink frequencies. Go to the following site and then simply click on the satellite you are interested in. http://www.amsat.org/amsat-new/satellites/status.php

With doppler shifts as the satellites make a pass the actual receive frequency could vary by as much as about 10 KHz plus or minus the nominal frequency. The only time you would ever receive the satellite exactly on its downlink frequency would be when the satellite heading is exactly 90 degrees from your bearing to the satellite. At that point the satellite is tangential to a circle whose center is your location and is neither moving toward or away from your location.

I am using an IC-910H. For manual tuning at the beginning of a pass, I simply tune about 5 to 9 KHz higher (depending on the elevation of the pass) than the nominal frequency. With the AFC turned on, the radio will automtically lock into the signal when it becomes strong enough and track the frequency as the doppler shift changes the receiving frequency during the pass. At loss of signal, the receive frequency will be below the nominal frequency by about the same amount it was above at the start.

John Kopala N7JK

...

Date: Sun, 9 Sep 2007 19:44:15 -0700 (PDT) From: Bob Stewart bob@evoria.net Subject: [amsat-bb] FM sat frequencies? To: Amsat-BB amsat-bb@amsat.org Message-ID: 869490.76831.qm@web909.biz.mail.mud.yahoo.com Content-Type: text/plain; charset=iso-8859-1

Does anyone know the actual downlink frequencies of the easy sats? From messing around with my hack for gpredict, I get the values below, but it would be nice to know the correct values as a check against what I'm doing.

AO-27 436.7972 SO-50 436.7920 AO-51 435.3008

Bob - AE6RV

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