I just saw the headline Malaysia Airlines First to Track Fleet With Satellites. But the article mentions that “[m]ost international flights are already transmitting their position with technology known as ADS-B and the signals can be tracked from the ground or space.”
So what is this new system? Is it an alternative to ADS-B, or an upgrade to it? And how does ACARS fit into all this? I thought that was an existing network that airlines used to send data.
That’s all kinds of tangled up. Let me try to clarify / simplify.
ACARS is a 1970s era low speed (2400 baud actually) short range data link system. It carries whatever data the airplane and airline want to send to each other. It is not used for anything related to ATC control of aircraft because it’s inherently not very reliable nor real time. It’s also overland and near-shore only. Although it can be rerouted over satellite based comm if the aircraft is so equipped. ACARS is a standard about transmission mechsanism, not about content.
ADS-B is a type of data link where the aircraft broadcasts it’s position, speed, etc. every few seconds. The standard defines both content of the messages and the technology of the network. It’s also a short range system.
The original intent was to transmit the data to nearby aircraft and to ATC ground stations so ATC could save money and not have to buy radar. There was no particular intent to share that info directly with the airlines. The original intent also was that this would work over land where there would be receivers on the ground. It would also work over the edges of oceans where receivers could be placed at the coast and on islands. It was not expected to work in the open ocean other than as direct nearby airplane-to-airplane collision awareness.
The aircraft position that might be sent out over ACARS or over ADS-B is created by the systems on the aircraft. Some of which include GPS receivers as source data. Which leads to journalists describing this as “GPS satellites tracking the airplane” when what they really mean is “the airplane is receiving GPS satellite signals, computing a position from them, then somehow, completely unrelated to GPS satellites, transmitting that position (and other data) to the outside world.”
For the last several years, and well before MH370, there has been a hodgepodge of different ground-based and communications satellite-based systems where an airline could arrange for the position data onboard the airplane to be transmitted via those systems back to airline HQ. Where a tracking app could in principle be created to keep watch over the fleet. The issue is the total amount of data for the total industry fleet would grossly overtax the satellite datalinks in place. So it was/is common for such legacy systems to only position report every 15 or 30 minutes.
What there was not, and still is not, is a single unified technology standard that works everywhere worldwide, has the bandwidth for everybody to use it, and that everybody has signed up to begin using as fast as they can re-equip their aircraft. Right now each airline has a mix of various airplanes with various legacy systems that each work in some places some times. AFAIK there is no airline with 100% real-time tracking coverage planetwide. If there is, they’re doing it by dint of stapling together data from a bunch of different local and aircraft systems.
Meanwhile the various avionics and satellite comm companies are rushing products to market hoping to become the *de facto *standard just by winning the land rush before the standards setting bodies slowly congeal on a solution.
In your cited article, Maylasian is crowing that they’ve signed a contract with one of those satellite comms companies that hopes to have a system in orbit by 2018. But they don’t have it up there yet. Maylasian is crowing that they’re the first customer of that company that hopes to have a product next year. Whether they will or they won’t remains to be seen.