[NOAA] APT transmission's SyncA markers controlled?

From: Andreas Hornig via Seesat-l <seesat-l_at_satobs.org>
Date: Sat, 26 Feb 2022 23:16:12 +0100

I was checking NOAA18's APT transmission. I recorded it with some of my
stations to test my orbit determination.

Does anyone else ever checked or has further knowledge about how the NOAA
sats control their clocks for the APT transmissions? [0]

They seem very stable and the spikes in my graph are most likely just due
to my script finding the SyncA[1] markers. For every spike there is a
counter spike directly next compensating it. So the periods measured as
diffs between the marker timestamps are plausible for me.

I just expected more drift and a small slope due to the clock.
I used Space-Track.org TLEs on the same day of recording. I used that to
correct the propagation delay from my stations towards the known position
in orbit based on the TLE. In this way I was able to determine the
transmission time of each SyncA that I was able to receive and are graphed

My OrbitDeterminator finds this orbit/TLE
1 00000U OrbDet   22044.42720540  .00000000  00000-0  00000+0 0    08
2 00000  98.9781 114.8904 0013875 325.1502 152.9600 14.12701478    01

This is  for one passage and almost all of the passage from ascending 18°
until the descending 10° elevation.

From Space-Tracks.org the closest TLE is this...
1 28654U 05018A   22044.47479952  .00000147  00000-0  10377-3 0  9991
2 28654  98.9643 114.9421 0013527 324.7524  35.2752 14.12687670862566

I know this is RF and not optical observations, nevertheless I am quite
happy about my result from above matching so nicely.

[0] https://twitter.com/andreashornig/status/1497691242200772619
[1] https://www.sigidwiki.com/wiki/Automatic_Picture_Transmission_(APT)

Would be nice to understand more about what NOAA is doing on APT mode :).

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Received on Sat Feb 26 2022 - 22:16:12 UTC

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