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sci.geo.satellite-nav (Global Satellite Navigation) (sci.geo.satellite-nav) Discussion of global navigation satellite systems (GNSS). Topics include the technical aspects of GNSS operation, user experiences in the use of GNSS, information regarding GNSS products and discussion of GNSS policy (such as GPS selective availability).

MH 370



 
 
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  #11  
Old March 19th 14, 11:31 PM posted to sci.geo.satellite-nav
Alan Browne
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Posts: 1,339
Default MH 370

On 2014.03.19, 20:19 , Happy Trails wrote:
On Tue, 18 Mar 2014 20:09:08 -0400, Alan Browne
wrote:


the INMARSAT satellite reports the power level of
received messages and from that the 'arc' was developed ... a line of
"equal" power received from the surface by that satellite. (I guess they
ignore a/c altitude... or maybe not)

The errors in power level, as well as the pointing accuracy of the space
based antenna, it's actual lobe shape, attenuation at different points
in the chain (transmitter power, cables, antenna) add up to those arcs
being roughly +/- 156 km laterally from the nice clean arc in the Malay
presentation, by his estimation.

His caveat there is that the antennas may be twice as bad in platform
pointing stability ... so over +/- 300 km from the arc....


Thanks for the Inmarsat info - I only know gps, so I was not aware
there could be any position inference whatsoever from Inmarsat.

The scenario posed in the link from Michna seems very plausible, and I
would not trust to any degree of accuracy at all any line of position
from Inmarsat. There are too many factors that could reduce the power
level of a signal received at the Inmarsat satellite - the position
assumes full power available at the transmitter and a properly aimed
antenna on the transmitter, both of which are dubious in these
circumstances.

I have said all along they will find the plane/crash site on an
autopilot-maintained line west of Malaysia in the Indian Ocean about
6-7 hrs flying time to the west. That is what I originally meant by
"mirror-imaged" arcs - not exactly, and not centered on the Inmarsat
satellite, but way west of where they are looking and curved the other
way from the news articles.

You read it here first.



I wouldn't make any prediction at all anymore. The fact that the
Aussies have focused on a mere 600,000 square km area (near Amsterdam
Island) is interesting. Perhaps that's where a Bayesian search is pointing.

--
Those who have reduced our privacy, whether they are state
or commercial actors, prefer that we do not reduce theirs.
- Jaron Lanier, Scientific American, 2013.11.

  #12  
Old March 20th 14, 07:06 PM posted to sci.geo.satellite-nav
Happy Trails
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Posts: 341
Default MH 370

On Thu, 20 Mar 2014 10:18:08 +0000, Jeff wrote:

. . . all you need to know is the time difference in
the transmitted request and the received reply . . .


And without the timestamp indicating exactly when the transmission
took place, how do you propose that be calculated?

In actual fact, the degree of timing synchronization required to do
any position determination based on a time difference is far more
precise than you will ever get from hardware and software that is not
designed specifically to provide this, like gps.

Just for starters, a difference in REPORTING of only 1 second will
give you a range estimation error of approx. 300,000 meters, not to
mention any calculation delays at both ends!

  #13  
Old March 20th 14, 07:56 PM posted to sci.geo.satellite-nav
Terje Mathisen[_3_]
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Default MH 370

Happy Trails wrote:
On Thu, 20 Mar 2014 10:18:08 +0000, Jeff wrote:

. . . all you need to know is the time difference in
the transmitted request and the received reply . . .


And without the timestamp indicating exactly when the transmission
took place, how do you propose that be calculated?

In actual fact, the degree of timing synchronization required to do
any position determination based on a time difference is far more
precise than you will ever get from hardware and software that is not
designed specifically to provide this, like gps.

Just for starters, a difference in REPORTING of only 1 second will
give you a range estimation error of approx. 300,000 meters, not to
mention any calculation delays at both ends!


It is _slightly_ worse than that:

With c at 300 000 km/s, you get that 300 km range error in a single ms.

The only way they can get accurate ranging info from Inmarsat would be
if there is some kind of echo message which the sat can send out and
which the receiver will echo immediately: If so, then the time delay can
give a pretty good range estimate.

Otherwise, if you can track the response time for all non-streaming
request/reply pairs in the protocol, then you can use the lowest of
these as the range estimator.

Using just the received power would be really error-prone unless you
have such data also for the period before the flight transponder
stopped, i.e. from the period where we know the exact location.

Terje

--
- Terje.Mathisen at tmsw.no
"almost all programming can be viewed as an exercise in caching"
  #14  
Old March 20th 14, 08:03 PM posted to sci.geo.satellite-nav
Alan Browne
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Posts: 1,339
Default MH 370

On 2014.03.20, 06:18 , Jeff wrote:

To do so means you have to have an accurate timestamp for transmission
from the a/c and for reception at the satellite AND the satellite has to
have the same time base (GPS) in order to compute the difference (or
perhaps on the ground end but that means a lot more error).


Not quite, you do not need a time stamp as such, (mobile phones do not
have one). In that case all you need to know is the time difference in
the transmitted request and the received reply, and due to the accurate
time slotting of the system you can work out the range (with and
accuracy determined by the accuracy of the system clocks).


1. In the absence of a time stamp you would need two-way communication
that has 0 delay for the returned message at the a/c terminal (or a
fixed and known delay for a returned message).

2. It's not clear to me that the INMARSAT data uploads are with a
protocol or may be passively uploaded.

Again, however, I defer to the expert I was "talking" to via e-mail (he
is the colleague of a former work colleague of mine).

--
... it may be that "in the cloud" really isn't the best term
for the services these companies offer. What they really
want is to have us "on the leash."
-David Pogue, Scientific American, 2014.02
  #15  
Old March 20th 14, 11:37 PM posted to sci.geo.satellite-nav
Happy Trails
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Posts: 341
Default MH 370

On Thu, 20 Mar 2014 21:56:54 +0100, Terje Mathisen
wrote:

It is _slightly_ worse than that:

With c at 300 000 km/s, you get that 300 km range error in a single ms.


Oh yeah - dropped a few zeros there, didn't I, hahaha!
(old age slows down time, maybe, hahahahaha)

Back in the days when I wrote software for survey navigation and
sonar/magnetometer data recording, we used to get a variable from the
gps receiver every epoch which was the delay time to use for that
epoch, which was usually around 120 or so time units past the actual
tick (can't remember if it was millisecs or microsecs, and I'm not
going to go look it up).

In addition to this we had a time delay to estimate for the sonar
pings and our own software, and adjusted the positions recorded
accordingly based on current vessel speed. Even at that we were
usually skewed by a meter or 2 every recording.

I don't imagine that any programmer or analyst would be that
meticulous for something as simple as a time stamp on an aircraft data
message used recording purposes of accumulated engine and other data
that represented a much longer time span anyway. It would be very easy
and nobody would care if it were a few seconds out - it's not like you
were atually primarily using it for position calculations anyway.

  #16  
Old March 21st 14, 07:24 PM posted to sci.geo.satellite-nav
Alan Browne
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Posts: 1,339
Default MH 370

On 2014.03.21, 08:30 , Jeff wrote:

Not quite, you do not need a time stamp as such, (mobile phones do not
have one). In that case all you need to know is the time difference in
the transmitted request and the received reply, and due to the accurate
time slotting of the system you can work out the range (with and
accuracy determined by the accuracy of the system clocks).


1. In the absence of a time stamp you would need two-way communication
that has 0 delay for the returned message at the a/c terminal (or a
fixed and known delay for a returned message).



Exactly,that is how range is calculated on the mobile phone system; the
receiver replies in a specified fixed (for it) time slot, and the delay
between the allocated start of the time slot and the actual reception of
reply is directly proportional to the distance to the mobile unit. The
errors in the system are sub-microsecond due to the accuracy of the
synchronization of the system timings.


Another aviation system that has used time for measurement since the
60's is DME. In this the a/c sends out a brief, modulated pulse (unique
to the a/c), the DME ground transceiver delays for a fixed period (50
microseconds) and then spits it back. The a/c receiver subtracts the
fixed delay, divides by 2 which gives distance. Accuracy of about 1/15
NM (125 metres) - before slant range. More than sufficient for airways
nav, LOC/ILS+DME approaches, and beginning in the 70's RNAV systems that
would use several DME stations to compute position. (Short range -
about 200NM for an aircraft at high altitude).

2. It's not clear to me that the INMARSAT data uploads are with a
protocol or may be passively uploaded.

Again, however, I defer to the expert I was "talking" to via e-mail (he
is the colleague of a former work colleague of mine).


Indeed without more details of the Inmarsat system it is impossible to
say, however the error using power as the indicator is prone to
absolutely horrendous errors depending on many factors including actual
transmit power, atmospheric conditions the orientation of the antenna
etc etc.


Yes, I pointed those out in my first reply on the matter.

From what I was told it's quite consistent and even. That said, the
chain of errors mentioned adds up to +/- 300 km. ie: the errors aren't
horrendous - but they are there. Like GPS, the particular INMARSAT
bands are chosen to be least affected by atmospheric conditions - at
least wrt power. (They are very close to GPS frequencies).

Based on where the latest debris has been located (still not proven at
all to be from MH 370) it's in the vicinity of the southern arc.

--

Privacy has become an essential personal chore that most
people are not trained to perform.
- Jaron Lanier, Scientific American, 2013.11.

  #17  
Old March 21st 14, 07:38 PM posted to sci.geo.satellite-nav
Happy Trails
external usenet poster
 
Posts: 341
Default MH 370

On Fri, 21 Mar 2014 17:24:38 +0000, Jeff wrote:
Yes, they do,


Hey Jeffy, have you considered the difference is system receiver
topology between the phone systems and Inmarsat?

  #18  
Old March 21st 14, 09:26 PM posted to sci.geo.satellite-nav
Alan Browne
external usenet poster
 
Posts: 1,339
Default MH 370

On 2014.03.21, 08:30 , Jeff wrote:

Not quite, you do not need a time stamp as such, (mobile phones do not
have one). In that case all you need to know is the time difference in
the transmitted request and the received reply, and due to the accurate
time slotting of the system you can work out the range (with and
accuracy determined by the accuracy of the system clocks).


1. In the absence of a time stamp you would need two-way communication
that has 0 delay for the returned message at the a/c terminal (or a
fixed and known delay for a returned message).



Exactly,that is how range is calculated on the mobile phone system; the
receiver replies in a specified fixed (for it) time slot, and the delay
between the allocated start of the time slot and the actual reception of
reply is directly proportional to the distance to the mobile unit. The
errors in the system are sub-microsecond due to the accuracy of the
synchronization of the system timings.


2. It's not clear to me that the INMARSAT data uploads are with a
protocol or may be passively uploaded.

Again, however, I defer to the expert I was "talking" to via e-mail (he
is the colleague of a former work colleague of mine).


Indeed without more details of the Inmarsat system it is impossible to
say, however the error using power as the indicator is prone to
absolutely horrendous errors depending on many factors including actual
transmit power, atmospheric conditions the orientation of the antenna
etc etc.


Another possibility I'm checking is whether the antenna system on the
a/c sends elevation angle data to the satellite in its comm overhead.

Where HGA Inmarsat antennas on aircraft point in both azimuth and
elevation, IIRC the MGA (or intermediate gain) antennas are only
controlled in elevation (and possibly transmit on one side (left or
right) at a time. If, in the data overhead, it uploads the elevation
angle, then the arcs we've seen can be derived from those. That said,
the angles are somewhat coarse - but a combination of power received
plus those angles could hone it down a bit.

Even so, the error would not be small - probably still on the order of
the errors that were mentioned earlier. +/- 100's of km.

--
Those who have reduced our privacy, whether they are state
or commercial actors, prefer that we do not reduce theirs.
- Jaron Lanier, Scientific American, 2013.11.

  #19  
Old March 21st 14, 11:53 PM posted to sci.geo.satellite-nav
David Chamberlain[_2_]
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Posts: 1
Default MH 370

On 3/19/2014 2:16 AM, Hans-Georg Michna wrote:
On Tue, 18 Mar 2014 17:07:31 -0400, Happy Trails wrote:

Does anyone still read msgs here?


Yes. Some more information:

Infographic by Bloomberg:
http://www.bloomberg.com/infographic...ince-1948.html

A Startlingly Simple Theory About the Missing Malaysia Airlines
Jet - wired.com
http://www.wired.com/autopia/2014/03...ectrical-fire/

Sounds very convincing to me.

Hans-Georg


Sorry. My first attempt at this post went to the sender, not the
newsgroup. Stupid news client...

http://www.slate.com/blogs/future_te..._langkawi.html

--
David Chamberlain
P27lc9bj
  #20  
Old March 22nd 14, 12:18 PM posted to sci.geo.satellite-nav
Alan Browne
external usenet poster
 
Posts: 1,339
Default MH 370

On 2014.03.21, 20:53 , David Chamberlain wrote:

Sorry. My first attempt at this post went to the sender, not the
newsgroup. Stupid news client...

http://www.slate.com/blogs/future_te..._langkawi.html


A fire.
A decompression.
A hijacking (botched or other).

Are all plausible (and not the sole possibilities). Until the FDR
is found speculation of any kind is silly. Articles attacking other
articles are silliest.

I suspect the CVR will be useless as they only record the last 30
minutes of cockpit sound. So the events at the critical time will have
been written over many times. (The FDR, OTOH runs 16+ hours).

--
Privacy has become an essential personal chore that most
people are not trained to perform.
- Jaron Lanier, Scientific American, 2013.11.
 




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