Now I get it...
The satellite "pings" the plane, which responds with "I am here." By measuring the round trip time between the transmit and receive signals, the satellite can determine the aircraft's distance, and thus the angle to it.
Article here: http://theaviationist.com/2014/03/16/satcom-acars-explained/
Tom WB8WOR
On Mon, Mar 17, 2014 at 1:28 PM, Chandler Heath convergx@gmail.com wrote:
I read that the "handshakes" were still occurring when they lost contact with the aircraft. In an article posted here from the Inmarsat web site Inmarsat mentioned that they and their partner Sita are working with officials to use handshakes to triangulate the position.
On the topic of Steering, the satellites have the capability to "steer" spot beams to address capacity needs, but to "steer" for one subscriber would make things worse for the others.
Here is a link on the Sita service OnAir. I don't know how accurate it is, but here it is for anyone that may want to learn more.
http://en.m.wikipedia.org/wiki/OnAir_(telecommunications)
Sent from my iPhone
On Mar 17, 2014, at 11:11 AM, Tom Busch tom@bloomington.com wrote:
Looks like I'm wrong. The "Steering" info wasn't from CNN. It's from
this
Huffington Post article:
http://www.huffingtonpost.com/2014/03/16/malaysia-airlines-takeover_n_497288... .
I still would like to know how the satellite knows.
It also says that these "pings" occur every hour, so I suppose it could have kept flying for an hour after the last contact. Tom WB8WOR
On Mon, Mar 17, 2014 at 11:28 AM, Tom Busch tom@bloomington.com
wrote:
The news has been reporting that they are using the angle of the last known ping from the ACARS system to the satellite. This is where the 40-degree arc around the satellite comes from.
What I don't understand is how INMARSAT knows what that angle is. CNN says that the satellite steers its antenna to the location where it
expects
the next ping, but that doesn't make sense.
I have been looking for the algorithm, but I can't find it. Signal strength? Some sort of electronic steering? Trade secret? I don't
know.
Tom WB8WOR
On Sun, Mar 16, 2014 at 10:57 PM, R.T.Liddy k8bl@ameritech.net
wrote:
Tony,
They would use the time differential between receipt to measure the distance versus the location of the satellites. The more satellites, then the more accurate the triangulation.
73, Bob K8BL
From: Anthony Japha tjjapha@earthlink.net To: amsat-bb amsat-bb@amsat.org Sent: Sunday, March 16, 2014 4:49 PM Subject: [amsat-bb] Malaysian airliner puzzle
Thanks Rick, How do those sats determine distance to the source? Tony, N2UN
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