## Math, Doppler And The Missing Jetliner

I just finished reading some technical data from Inmarsat and Ministry of Transport Malaysia concerning the analysis of satellite data transmissions from MH370. It’s this data that’s shifted the searchers from MH370’s planned route to a tract in the Southern Indian Ocean well west of Australia–nearly the opposite direction expected!

It is a brilliantly concocted method to get usable information from what should have been meaningless housekeeping transmissions.

Radio signals travel at the speed of light. If we know how long those signals take to go satellite-to-plane (or vice versa) we can start doing calculations and find the distance between the two.

Inmarsat was then able to calculate the range of the aircraft from the satellite, and the time it took the signal to be sent and received, to generate two arcs of possible positions – a northern and a southern corridor.

As you probably know the northern track was thrown out. But why? That’s where the plane should have been flying. It was the most logical direction.

Enter Doppler!

Because the satellite and plane were both moving, their radio waves were subject to Doppler shift. This is an expected part of satellite work and equipment to compensate for it is built into the system.

The Inmarsat technique analysed the difference between the frequency that the ground station expected to receive and the one actually measured, known as the Burst Frequency Offset.

Because the satellite wasn’t at the midpoint of the two project tracks, the expected northbound offset or shift was different than the southbound shift. What was actually seen only matched the southern track.

Depending on the plane’s speed the same Doppler shift could indicate slightly different positions. Unfortunately, that’s an unknown. It’s a good guess to estimate 400-450 knots. That’s why the area now being searched isn’t a single point, but a larger area.

Obviously, the plane hasn’t been found, there’s still no real explanation for what went wrong. However, this clever use of math helps bring those looking one step closer.

I know this is somewhat complex. I’m not 100% sure my explanation will be clear to everyone. Questions are welcome.

## What’s Left Unsaid With MH370

Today’s news concerning Malaysian Airlines Flight 370 is sad, but expected. The plane’s course and probable last location have been narrowed further by Inmarsat and the British equivalent of the NTSB using “groundbreaking maths.” This means the passengers are dead and the plane might someday be found in the vast expanse of the Southern Indian Ocean.

Like I said, sad but expected.

However, there is a new tantalizing thread being dangled today. The Guardian quotes Chris McLaughlin, an Inmarsat senior vice president.

Ships have to log in every six hours; with aircraft travelling at 500 knots they would have to log in every 15 minutes. That could be done tomorrow but the mandate is not there globally.”

The operative word is, “tomorrow.” McLaughlin seems to be verifying a piece of this puzzle I suspected right away.

Let’s say you buy a new car. Today, most come with a satellite radio. If you don’t pay for a subscription the satellite radio won’t work.

I think it’s similar with this Malaysian Airlines 777. It came equipped with terrestrial (ground based) and satellite ACARS. Malaysian Airlines didn’t buy the satellite ACARS package.

Once an hour the plane would tell Inmarsat, “Hey, I’ve got data.” Inmarsat would answer back, “You’re not a subscriber,” and the conversation would end. It was logged and noted, but assumed to be worthless.

This is why in the first days of this tragedy when satellite reception of MH 370’s signal was mentioned Malaysian officials seemed surprised. They had no idea this unused, unpurchased functionality was even in the plane.

If I understand correctly, what McLaughlin’s saying is, why isn’t this data link required to be active as a matter of course? Even if only GPS coordinates were sent it would mean a lot.

There are a few ‘big rig’ pilots who read my blog. I hope you’ll take a moment to correct me where I’m wrong.

Meanwhile, will this data spigot be turned on tomorrow for planes still flying? I hope so. I fear not.