MH370 Doppler
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
407-982-0446
I thought that Inmarsat terminals had AFC to the sat's down-link. Not to the degree of true phase-lock like DSN has but enough so that the sat's abillity to do doppler correction on the uplink is valid to help with BER, etc... Otherwise the doppler correction would be of no help and not be needed.
-Brian, WA1ZMS/4
iPhone
On Aug 18, 2014, at 12:53 PM, Joe Leikhim jleikhim@leikhim.com wrote:
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
407-982-0446
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On Mon, Aug 18, 2014 at 04:18:30PM -0400, Brian, WA1ZMS wrote:
I thought that Inmarsat terminals had AFC to the sat's down-link. Not
to the degree of true phase-lock like DSN has but enough so that the
sat's abillity to do doppler correction on the uplink is valid to help
with BER, etc... Otherwise the doppler correction would be of no help
and not be needed.
I beleive most Aero Classic terminals use a fairly good
OCXO. Somewhere I may have a limit spec on stability, but those
docs are not immediately handy.
Normally a demod in the terminal is kept tuned to one of the
continuous L band control channels which I believe may be Doppler
compensated in the ground uplink transmitter for the 6 Ghz C band uplink
Doppler and LO drift on the satellite so it is correctly on frequency as
radiated on the L band downlink. This could supply a frequency
reference to the terminal that could be used to AFC the terminal
frequency standard so it is close to right on. Doing this would
require terminal firmware to determine estimated Doppler at the L band
control channel downlink frequency from the satellite based on some
estimate of the planes position, satellite position and relative
velocities.
The QPSK DSP modems used at both ends would be easily able to
supply estimated frequency offset, both on the ground at ground earth
station and in the plane. It is presumably true that this measurement is
corrected on the ground end for the Doppler due to movement of the
satellite relative to the ground station on the C band downlink relaying
the L band uplinks from the plane so it reflects frequency error as seen
at the satellite on the L band uplink with the downlink and satellite LO
drift terms removed. I presume this is what INMARSAT is reporting, but
am not sure.
IIRC the plane is expected to adjust its burst uplink frequency
and timing to come out right at the satellite receive antenna... thus
compensating for the uplink Doppler at L band and the time delay too.
But I do remember that the ground supplies feedback on the control
channel as to how much the plane is off so it can adjust...
Guess it might be time to dig out the docs again.
--
Dave Emery N1PRE/AE, die@dieconsulting.com DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."
As I understand it, Doppler will give you the magnitude of the velocity
vector for an aircraft with respect to the satellite, but it won't give
you the actual direction of the aircraft.
Why does the stability of the oscillators matter if you can't determine
the direction? Is there another satellite involved? Can you learn
something if you assume a velocity for the aircraft? In which case the
error in the assumed velocity would swamp the oscillator error, no?
Bill Hawkins
-----Original Message-----
From: Joe Leikhim
Sent: Monday, August 18, 2014 11:53 AM
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.
You CAN determine the ground track if you assume the altitude above
sea level is constant and the aircraft's speed is also constant. But
you are correct that Doppler alone would not be enough.
The question I have to people here is: How does error in the dopler
translate to error in the ground track. In other words what is the
function that maps oscillator stability to distance on the ground.
On Tue, Aug 19, 2014 at 3:25 PM, Bill Hawkins bill@iaxs.net wrote:
As I understand it, Doppler will give you the magnitude of the velocity
vector for an aircraft with respect to the satellite, but it won't give
you the actual direction of the aircraft.
Why does the stability of the oscillators matter if you can't determine
the direction? Is there another satellite involved? Can you learn
something if you assume a velocity for the aircraft? In which case the
error in the assumed velocity would swamp the oscillator error, no?
Bill Hawkins
-----Original Message-----
From: Joe Leikhim
Sent: Monday, August 18, 2014 11:53 AM
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.
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--
Chris Albertson
Redondo Beach, California