[amsat-bb] ISS Packet Frequency?

Stewart Todd Morgan morgan_st at bellsouth.net
Tue Apr 25 14:10:24 UTC 2017

I used an Alinco DJ-X11T receiver in SDR mode to record an 80 degree ISS 
pass at 13:33:53 UTC this morning.  The packet signals did appear on the 
expected frequency given Doppler shift (centered on 145.825 MHz).

However, there does appear to be an issue with the packet 
transmissions.  Scott's Dropbox image of the pass over several minutes 
compressed the packet transmissions to the point that the issue is not 
visible in his image, but it is quite apparent in my waterfall which 
only shows about 20 seconds at a time in KGSDR. Namely, the centerlines 
of all packet transmissions describe a "J" shape, even at AOS and LOS 
when Doppler should not be noticeable at all.  In other words, there is 
a rapid increase in transmitted frequency at the start of a packet 
transmission, and this frequency change rate diminishes toward the end 
of the transmission.  This phenomenon could very well cause some 
receiving systems to fail to decode packets.

I tested my system against local terrestrial packet stations just prior 
to the pass, and the packets show vertical lines in the waterfall, so 
this is not some systemic issue in my receiving system.


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