PING
http://www.themalaysianinsider.com/sideviews/article/understanding-satellite-pings-tim-farrar
Key point 4: The “satellite pings” are due to the Inmarsat network checking that the terminal on board the aircraft is still connected to the Inmarsat satellite system and the terminal responding in the affirmative. So now the question is how accurately does the Inmarsat network know where the plane is located? To go back to my cellphone analogy, when the network is checking my phone is still connected, it looks in the last cell it was registered. If I move to a different cell, then my phone should check in with the network to request a new assignment. But AT&T doesn’t need to know my precise position within the cell, it just needs to know where to route an incoming call. Similarly with Inmarsat, there isn’t a need to know exactly where in a cell the plane is located, just that its there and not somewhere (or nowhere) else. Key point 5: The “satellite pings” indicate the plane is in a cell, but do not intrinsically give specific position information. How big is a “cell” on the Inmarsat network and why the confusion? First of all, we need to recognise that there are different Inmarsat network architectures for different generations of aeronautical terminals. Think of it like 2G, 3G and 4G phones. If I have a first generation iPhone then I can only use 2G (GSM+EDGE), an iPhone 3G can use 3G, and an iPhone 5 can use LTE. AT&T supports all of these phones, but in slightly different ways. Inmarsat introduced a new SwiftBroadband aeronautical service in 2010, using its latest generation Inmarsat 4 satellites (like AT&T’s LTE network). That has much smaller spot beams (“cells”) than the older Inmarsat 3 satellites. And the Inmarsat 3 satellites (like AT&T’s 3G network) in turn have regional spot beams as well as a “global” beam (covering an entire hemisphere) to support the oldest aeronautical terminals. As an aside, part of the SwiftBroadband communications protocol (essentially identical to BGAN) conveys (GPS-based) position information to the satellite when establishing a connection, so that the satellite can assign the terminal to the right spot beam. But it isn’t clear that GPS data is required as part of the “pings” which maintain registration on the network. That was one additional source of confusion about whether the specific position was being reported. In any case, it appears that MH370 had a Swift64 terminal onboard, not one of the latest SwiftBroadband terminals (that’s hardly surprising since SwiftBroadband is not yet fully approved for aeronautical safety services and is mostly used for passenger connectivity services at the moment, which don’t seem to have been available onboard). This is the equivalent of the iPhone 3G (not the oldest terminals, but not the newest either). In the Indian Ocean, the Swift64 service operates on the Inmarsat 3F1 satellite located at 64E (equivalent to AT&T’s 3G network not its latest LTE network), and can use both the regional and global beams, but it appears that Inmarsat’s network only uses the global beam for the “pings” to maintain network registration. Otherwise it would have been possible to rule out a location in the Southern Ocean.
Key point 6: The “satellite pings” were exchanged with the Inmarsat 3F1 satellite at 64E longitude through the global beam. So how can anyone find the position within this enormous global beam? There are two potential ways to measure the location: 1. Look at the time delay for transmission of the signal to the satellite. This would give you a range from the sub-satellite point if measured accurately enough, which would be a circle on the Earth’s surface.
2. Measure the power level of the signal as received at the satellite. The antennas on the satellite and the plane amplify the signal more at some elevation angles than others. If you know the transmission power accurately enough, and know how much power was received, you can estimate the angle it came from. This again would produce a similar range from the sub-satellite point, expressed as a circle on the Earth’s surface. We can see that the search locations are based on exactly these curves at a given distance from the sub-satellite point. However, it is unlikely that the measurements are more accurate than within say 100 miles.
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Post time 27-3-2014 12:03 AM
... nanti pesan pada akak 
