# Could TCAS be used as a backup source for position, attitude or air data?

Given the number of incidents related to freezing pitot tubes, is there any current/planned implementation to infer aircraft position/speed/attitude/bearing based on TCAS data as a last resort data point? If not, is this at least doable?

Even if TCAS gives only relative data, one may correlate the data at the last moment before pitot data are lost to assist in dead reckoning.

Edit: I am not sure if my question has been misunderstood, or that I am misinterpreting the answers. The question is more like inferring current aircraft's data from transponders of surrounding aircrafts. Let's suppose at time $t_1$ I have planes A, B, and C within my TCAS range. I will possess information regarding their bearing, distance, speed, and altitude. At time $t_2$, even without my own altimeter, I can at least infer my own vertical speed using trigonometry. A, B, C are like ocean currents, a background moving together with me. If I know enough information about them, depending on what TCAS's underlying data stream provides, I can probably infer my own position, altitude, speed, and heading. From my limited understanding, TCAS bearing, distance, and speed are relative to me. However if I have not mistaken, despite what is shown on the screen, altitude is absolute measurement from the other planes' altimeter. One side fixed on a similar triangle, you'll get the rest.

• Indeed it is amazing, the possibilities in FMS software development. I would think ADS-B could provide more data to the FMS than the limited scope of TCAS. Unless there's at least three other aircraft within scope, there'd be no data. Oceanic service would be nill for either. Here's FlightAware's coverage of ground-based ADS-B receivers: flightaware.com/adsb/coverage – STWilson Jun 15 '17 at 14:20
• GPS might be an option, combined with jetstream and wind plotting. – Koyovis Jun 15 '17 at 15:24
• And EGPWS contains a map of all of earth and knows the aircraft position and terrain altitude. – Koyovis Jun 15 '17 at 15:49
• GPS inferred data won't give you airspeed – Steve Kuo Jun 15 '17 at 15:53
• If we agree that is the capability of the sensor then there is no additional information available which would assist in DR or navigation. Integration of OTHER sensor data (GPS, ADS-B etc might yield usable data, but the bottom line is that TCAS alone provides no useful data for dead reckoning. While there might be an instant where you suggest that an inference could be made, most failures are not detected instantly, and by the time they are detected other aircraft have moved. – mongo Jun 21 '17 at 13:15

Can TCAS assist in dead reckoning?

First, understand that TCAS runs off secondary (transponder) radar data. Essentially your aircraft with TCAS is like ATC radar, interrogating other transponders, getting their pressure altitude and getting their bearing from your aircraft. That data combined is processed to create threat advisories.

If displayed on a moving map, the moving map is typically fed positional data for your aircraft from GPS, and the TCAS alert data is overlayed onto the map.

So to address your question, TCAS only provides data about transponders relative to your aircraft. TCAS does not determine where you are, or where you are going. (Depending upon your display it might appear that way, but that is not part of the TCAS system.)

So the answer to your question is no, TCAS does not effectively create a secondary navigation capability.

Addendum #1 TCAS can only sense altitude through the host aircraft's static port, so if it is iced over, TCAS has invalid altitude information.

TCAS does not sense other air data, such as pitot pressure information (airspeed).

TCAS does not know the host aircraft's position. Any positional information in TCAS is derived from pressure altitude information the host and all interrogated transponders. If the host static port is obscured, the information on threats will be invalid, because TCAS displays relative to the host pressure altitude.

In other words, there is no absolute positional information, and all TCAS information is relative to the current host aircraft, and derived from the secondary beacons of interrogated aircraft transponders.

Afair:

The ATC/SSR antena can barely discern azimuth on the responses, and effectivelly no elevation data.The only accurate info from TCAS is distance to intruder (and closure rate). This means you have some ideea of the direction of the intruder (not accurate enough mind you ,which is why RAs have only vertical guidance), but no real means of determining their relative altitude to you.

So basically you know you have another aircraft somewhere to your left, at FL110... how could you infer your altitude from that?

GPS altitude is another issue, and ,in fact, backup altitude from GPS does appear on some (A320s at least) PFDs when all ADR data is lost .

Later edit: if you bring ADS-B into the question, with a/c broadcasting their lat/long and flight level, you know the distance from your a/c to these points and can draw a sphere arround each of these points, at their intersection you can triangulate your position (including altittude) assuming you have enough traffic arround (min 3). Mathematically ,yes, posssible, but once you factor in the delays in ASD-B, the rounding errors in transmitting FLs, etc the accuracy would be horrible probablly within 100s and 1000s of feet