[time-nuts] Fwd: Re: Fwd: MH370 Doppler

Joe Leikhim jleikhim at leikhim.com
Mon Aug 18 20:57:51 UTC 2014


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 <jleikhim at leikhim.com <mailto:jleikhim at leikhim.com>>
>> *Date:* August 18, 2014 at 12:53:24 PM EDT
>> *To:* time-nuts at febo.com <mailto:time-nuts at febo.com>
>> *Subject:* *[time-nuts] MH370 Doppler*
>> *Reply-To:* Discussion of precise time and frequency measurement 
>> <time-nuts at febo.com <mailto:time-nuts at febo.com>>
>>
>> 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
>>
>> JLeikhim at Leikhim.com <mailto:JLeikhim at Leikhim.com>
>>
>> 407-982-0446
>>
>> WWW.LEIKHIM.COM <http://WWW.LEIKHIM.COM>
>>
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-- 
Joe Leikhim


Leikhim and Associates

Communications Consultants

Oviedo, Florida

JLeikhim at Leikhim.com

407-982-0446

WWW.LEIKHIM.COM


-- 
Joe Leikhim


Leikhim and Associates

Communications Consultants

Oviedo, Florida

JLeikhim at Leikhim.com

407-982-0446

WWW.LEIKHIM.COM






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