Aircraft position reporting

An operations room is only as good as its picture of where the aircraft are. That picture is never the aircraft itself: it’s the last thing something told you. This module covers the four things that tell you, what each is good for, and what to do when they go quiet.

What you see on a fleet display is the last known position: where the aircraft was when it was last seen or last reported. Always know how old that is, and where it came from.

The four sources at a glance

Radar

Air traffic control’s radar positions, passed on to operations by a service provider. Positions refresh every few seconds as the radar sweeps, over land and near the coast only. An aircraft that flies out of cover simply stops moving on your display, at the last place the radar saw it.

• Radar gives a position and nothing else: no fuel, no estimate of landing.
• It is not a datalink. You can’t send the aircraft a message over it, or ask it for a report.
• It sees aircraft in the air. On the ground, go by the movement messages.

VHF ACARS

ACARS is the aircraft’s own datalink: short messages between its avionics and the ground, sent without the crew having to do anything. Over VHF radio it’s quick, cheap and dependable, but VHF travels in a straight line, so the aircraft has to be within sight of a ground station: about 200 nautical miles at cruising level, and much less lower down. On the ground an aircraft can only use the station at its own airport, if there is one.

Use it freely. It is the link to expect over land in busy regions.

SATCOM ACARS

The same messages by satellite, for when the aircraft is out of reach of a ground station. It works over the oceans and over thinly served land, on aircraft that carry the equipment: many short-haul types don’t. The satellites sit over the equator, so near the poles they are below the horizon and there is no service.

Each message is charged for, so SATCOM costs more than any other link. Position reports are set less often on it for that reason.

HF datalink

The same messages by high frequency radio, which reaches anywhere by way of the upper atmosphere. Every aircraft has it, and falls back to it when it has neither VHF nor SATCOM: across an ocean on a type without SATCOM, or over the poles on any type.

HF datalink: reports may be delayed or lost; do not rely on it alone to keep track of an aircraft. A message takes minutes rather than seconds, and some never arrive. The same goes for anything you send the aircraft. A gap in the reports from an aircraft on HF is more often the link than the aircraft, but you can’t tell which from the ground.

Which link an aircraft is on

You don’t choose. The avionics pick the best link they have, in this order: VHF, then SATCOM, then HF. The link changes during a flight without anyone being told: a transatlantic flight leaves on VHF, goes to SATCOM or HF a few hundred miles out, and comes back to VHF on the far side. Expect the pattern of its reports to change with it.

What the aircraft sends

Movement reports. The aircraft reports the four events of every flight itself, known as OOOI: Out of the gate, Off the ground, On the ground, and In at the gate. They arrive as movement messages (MVA), in the same form as a station’s MVT. With Off the aircraft gives its own estimate of landing.

Position reports. In the air it reports its position at a set interval, which depends on the link. Each report gives where it was, when, its flight level, the fuel on board, and its estimate of landing. The layout is published by the Air-Ground Message Standards Committee (AGMSC), the body that agrees the formats airlines and datalink providers exchange:

POS
UE101/05.GUEAA
5132N00027W 0712 F350
EA1010 JFK FOB12400

The aircraft’s estimate of landing is the crew’s and the avionics’ best figure at that moment. It allows for how the flight has gone, not for a wait at the destination. Early in a flight it can be a few minutes out; close to landing it is good to about a minute. Add the taxi in to know when the aircraft will be on stand.

Requested reports. You can ask an aircraft for a position report at any time (RQP). The request goes up on whichever link the aircraft is using, so on HF it may take minutes to get there, or not get there at all, and the same for the answer.

Good practice

• Track every aircraft at least every 15 minutes over oceanic and remote areas. If the routine reports come less often than that, ask for one.
• Before a flight leaves radar cover, know which link it will fall back to. A type without SATCOM over an ocean is on HF, and needs watching more closely.
• Read the age of a position before you act on it. An aircraft at cruise covers about eight nautical miles a minute: a report a quarter of an hour old is a hundred and twenty miles behind the aircraft.
• Treat the radar picture and the datalink picture as two witnesses. Where you have both, they should agree.

When a report is overdue

A position report is overdue when nothing has given the aircraft’s position for longer than its reporting interval and a few minutes’ grace. Most overdue reports clear by themselves: the next one arrives. Don’t wait for that.

1. Request a report from the aircraft.
2. Check what else you have: the last report’s position and time, the link it was on, and whether the aircraft should by now be back in radar or VHF cover.
3. If there is still no contact, follow your company’s procedures and alert air traffic services (ATS). Give them the flight, the last known position and its time.

Record what you did and when. An aircraft on HF that has missed one report is usually found by the next. One that has missed a report on VHF or SATCOM, or that should have come back into cover and hasn’t, is not routine.