[ao16aprs] Re: PACSAT

[Mike Gilchrist <kf4fdj@amsat.org>]

Tate,

At 10:29 PM 02/03/2000 +0000, you wrote:
>How critical is it to adjust for Doppler shift on the uplink and/or
>downlink of the 9600 baud pacsats.  Could one set the transmitter and
>receiver on the nominal frequency and still copy transmit and receive small
>files and APRS position reports?


It all depends on pass band widths.  If the 2 m receiver is sufficiently 
wide to allow an off frequency signal into the pass band, then you don't 
need to correct on the uplink.  Fortunately, the pacsat receivers are 
sufficiently wide.

2 m Doppler shift is around 3 kHz, while 70 cm Doppler can be around 10 kHz.

So, you must correct for YOUR receive on the downlink.  Since Doppler can 
be 10 kHz high at the beginning of the pass, and 10 kHz low at LOS, that is 
a 20 kHz spread.  Since a properly deviating 9k6 signal will occupy 20 kHz 
of bandwidth, your receiver must pass more than 30 kHz of signal to capture 
the coherence. What is the bandwidth selectivity of your rig on 70 cm?

In practice, it is a good idea to correct both for uplink and downlink on 
the pacsats.  You could program non-standard splits in your rig, and just 
tune frequency pairs as the pass progresses.  On 2 meters, 5 khz low at the 
beginning of the pass, right on frequency at the middle, and 5 kHz high 
towards LOS will suffice, if your rig will only tune in 5 kHz steps.

Here is a suggested table of frequencies to program into your dual band rig 
for UO-22.

Downlnk -  Uplink
_________________
435.130 - 145.895
435.125 - 145.900
435.120 - 145.900  Mid Pass
435.115 - 145.900
435.110 - 145.905

I hope this helps a little.

73, Mike


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