Time-error would be rings for sure.
Doppler errors could also behave with these rings, but there is a much
more complex scenario of ground speed and angle of observation. As the
transponder passes under the satellite-earth line there is no
observeable doppler, just like a train passing by has no doppler just as
it passes by. Similarly as you fly in a ring pattern centered around the
sat-earth line there is no doppler, it takes that you travel
orthogonally to these rings for maximum doppler, it depends on the
ground speed and geometry.
Thus doppler gives by itself fuggy info. It's only when you combine
doppler info and timing info that you can start making some reasonable
observations about ground speed and to some degree rule out parts of a
later ring as it would have required too high air speed to be that
airplane. If sufficient distance in time, you can draw a expanding path
of likely true-track and get a rough idea of what area it could be. For
this to give reasonable result, initial vector would need to use other
observations for approximate startingpoint.
Does anyone has the set of timing and doppler measurements, and position
of the observing satellite?
Cheers,
Magnus
On 08/21/2014 02:32 AM, Chris Albertson wrote:
Yes, that is what they are doing. A given Doppler shift corresponds
to a certain "ring" on the Earth's surface. Each Hertz of Soppler
shift corresponds to a certain number of miles on the radius of the
ring.
At 1.6GHz one part per billion is 1.6Hz. 175Hz of shift gives
something like a 2,400 mile radius ring.
On Wed, Aug 20, 2014 at 8:47 AM, Joe Leikhim <[email protected]> wrote:
The L Band uplink was reported to be transmitting at around 1.6435/ghz.
/
Assuming, we actually knew what the tolerance of the OCXO (If it is an
OCXO) was under the environment of the mishap, and assuming it /was/ 10ppm
for example. The error would be (1,650 X1,000,000) * 10ppm or _16,500 hz_.
I think we can discount the error being that large, but could still
rationalize it being a significant portion of the reported BFO value./
/
Also the ground track is unknown, they are attempting to reconstruct the
ground track from the BFO (Burst Frequency Offset "Doppler") and from the
BTO (timing pings) the BTO supposedly offers range information, hence the
concentric rings corresponding to pings._
_
_Chris Alb__ertson wrote:_
"The total Doppler in this case is on the order of 100 Hz. The tiny
frequency shifts of an out of spec OCXO is just to small to measure.
The data says at UTC 18:30 the shift was in the mid range and was
about 175Hz. Assume the OCXO drifts 10 parts per million. That
is a lot for an OCXO. But maybe the effect is only about 50 feet on
the ground.
The OCXO error of even 1E-5 is just not very important as it does not
move the aircrafts ground track enough to matter."
--
Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida
[email protected]
407-982-0446
WWW.LEIKHIM.COM
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