Guide Position prediction

Guide: position prediction

What it is. Aircraft maps in OnAir (ADS-B, UAT, ACARS, VDL2 and HFDL) can smooth and extend positions with an estimator. It fills the gaps between position reports so that an ADS-B aircraft moves smoothly instead of jumping once a second, and it dead-reckons ACARS, VDL2 and HFDL aircraft (which report only every few minutes) along their track, with an uncertainty ellipse that grows with time.

The two switches

On the map's button line:

• Predict positions: "Fills the gaps between position reports: the aircraft moves smoothly and, when messages stop, is dead-reckoned along its track (and along the route when a message named the destination)." A predicted position is drawn as a ghost marker with a dashed ring.
• uncertainty: appears when prediction is on. Draws the 95 % uncertainty ellipse of every estimated position.

Both settings are remembered per screen.

Reading the map

• A solid marker is the last reported position. A ghost with a dashed ring is the estimate for now.
• The tooltip says for example "predicted from the last position 3 min ago, +-4 NM".
• An aircraft that has no position at all but talks to a ground station is shown as the reach of that station: "estimated from the ground station only, within about N NM". For VHF the reach is line of sight (about 240 NM for an aircraft at FL350); for HFDL it is taken as 1500 NM.
• When a message names a destination or an ETA (for example DEST, a flight plan line, ETA hhmm), the prediction knows where the aircraft is going and parks it there on arrival.

Step by step

1. Open a map-capable aircraft mode (ADS-B, ACARS, VDL2, HFDL) and start the receiver.
2. Tick Predict positions. For ADS-B, watch the aircraft glide between fixes.
3. Tick uncertainty to see how sure the estimate is. A small ellipse means a recent, accurate fix; a long one means a long gap.
4. For ACARS, VDL2 and HFDL, wait for an aircraft's second position. The marker starts as a dot and becomes a plane icon after the second position.

How good is it

Measured in simulated flights (12 flights between real city pairs), median error between fixes: ADS-B with a fix every second about 4 m (linear extrapolation of the last velocity: about 50 m); ADS-B every 5 s about 9 m; ACARS text reports 5 minutes apart about 300 m (linear: about 450 m); VDL2 2 minutes apart about 170 m. Holding patterns with sparse reports cannot be tracked: the ellipse is then wrong. The simulator's noise is the author's own model, so real traffic will differ (check the project documentation for updates).

What it does not do

It never invents a position for the public map: the shared positions are the ones the signals sent. It does not use another program's data. Route knowledge is limited to a table of about 300 major airports; unknown destinations simply give no route.

Under the hood

An interacting-multiple-model Kalman filter mixes three motion models (constant velocity, coordinated turn, manoeuvre), takes position, velocity and bank-angle measurements with noise set by the source (ADS-B accuracy codes, 60 m for ACARS text and ADS-C, and so on), and handles altitude separately. It runs on a local map frame around each aircraft, correct near the poles and the date line.

Troubleshooting

• Aircraft jump back after a message: the report was older than the estimate; this is correct behaviour.
• Ellipse huge: a long gap since the last fix.
• Nothing to predict: an aircraft with only a flight number has no position yet.

Related

ADS-B | ACARS message log | VDL2 | HFDL | Live map