Because of the "bent pipe" transponder there is a doppler shift on the
uplink from the terminal in the aircraft and doppler on the downlink
from the satellite. Each has to be calculated differently because of the
relative motion of the satellite (to ground station and aircraft) and
the gross difference in UL and DL frequency bands.
The local oscillator in the satellite is subject to drift due to solar
temperature changes as the satellite drifts in and out of eclipse. This
is apparently being considered.
However, the Doppler parameter called "burst frequency offset" is a
value reported by the satellite terminal to the ground station.
My point is that, all of the discussion I have read ignores the
possibility that the master oscillator aboard the aircraft is also
subjsect to drift due to thermal effects and those effects could be
quite significant if a fire on board. . Remember this "Doppler shift" is
not being tracked (observed) continuously, rather as a periodic data
point reported from the aircraft terminal on hourly intervals. Not like
averaging the tone from a train whistle to get a baseline.
On 8/18/2014 4:13 PM, Brian, WA1ZMS wrote:
Joe-
My understanding is that the Inmarsat link has doppler correction as
one of the parameters that the bird calculates from each "ping" heard.
So it matters not much if the LO on the sat drifts. So long as you
have the previous data you can plot a trend line. The trend is the sat
LO drift and the delta from that trend is the dopler.
At least that's how I read the raw Inmarsat data. I may be wrong.
Need to think about it more.
-Brian, WA1ZMS/4
iPhone
Begin forwarded message:
*From:* Joe Leikhim <[email protected] <mailto:[email protected]>>
*Date:* August 18, 2014 at 12:53:24 PM EDT
*To:* [email protected] <mailto:[email protected]>
*Subject:* *[time-nuts] MH370 Doppler*
*Reply-To:* Discussion of precise time and frequency measurement
<[email protected] <mailto:[email protected]>>
Is anyone paying attention to all the chatter about the lost aircraft
MH370, Inmarsat's supposed flight tracks based on 6 or 7 pings (1 per
hour), the Doppler shift (BFO) and transaction timing (BTO) etc??
Basically from my perspective they are putting too much stock into
the Doppler which relies in part upon the stability of the satellite
terminal in the 777 aircraft. My question is how stable an oscillator
(reported OCXO - not confirmed) would be under the extremes of
either or both a cabin fire or decompression event. There is a
website (Duncan Steel Blog) where some math brains are trying to sort
out the raw data provided by Inmarsat. They have made assumptions
about the stability of the local oscillator in the satellite, but I
think the aircraft satellite terminal's master oscillator is a
variable they have pushed aside.
--
Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida
[email protected] <mailto:[email protected]>
407-982-0446
WWW.LEIKHIM.COM <http://WWW.LEIKHIM.COM>
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--
Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida
[email protected]
407-982-0446
WWW.LEIKHIM.COM
--
Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida
[email protected]
407-982-0446
WWW.LEIKHIM.COM
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
