TCAS alt readout
 

Anyone aware of tables/formulas/algorithm, etc for figuring out the delay/lag between altitude observed on a TCAS from another aircraft to its actual altitude in a climb or descent?

Simply put, I see an aircraft on my TCAS at 10,000 but climbing/descending, is there a formula published for determining the difference in altitude between what it's at and what I'm seeing on my TCAS?

Before it's pointed out to me that has no practical application for in flight use, I recognize that. The question came up while killing time on a long flight.

Internally, TCAS calculates the future position and if there is a conflict.

Yup, but not what I'm looking for.

Did not find it yet
 

but tried here:

http://www.faa.gov/documentLibrary/m...%2020-151A.pdf

maybe you will be more thorough.

FD.

If there was such a predictable formula then don't you think it would be utilised in the TCAS itself to give the most accurate information?

Yes: Reported altitude + Climb rate X ( Transmitting Transponder system latency + Radio propagation delay + Receiving TCAS system latency)

That formula will give you the altitude of the other aircraft at any moment. The difficulty is determining the value of those variables.

Thanks, makes sense.

If you don't know the answer, it's quite acceptable not to post anything.

Transmitting Transponder system latency < 100 milliseconds (mainly the refresh rate of the altitude from the air data computer. The internal interrogation / reply processing of the Transponder will be small in comparison)

Radio propagation delay <1millisecond

Receiving TCAS system latency [the display works on a 1s refresh so that is the maximum latency]

So potentially the height data is maximum 1101milliseconds old.
At 5000ft per minute that's around 92ft as a maximum. But as an average probably much less.

This is simply for the display of course. The internal tracking algorithms of the TCAS unit are not constrained to the 1s refresh rate.

Well, I would wonder about that latency, given the latency in the GPS position.

Depending on the system, they handle they track and handle the threats differently, hence, there is a proprietary factor as well.

The climb rate is unknown to the flight crew other than that it is more than 500 fpm, so it is not possible to come up with a meaningful formula. You can use assumptions but as you say the result is probably much less than 100' which means that most of the time the difference between displayed altitude and actual altitude is less than the display precision.

TCAS does not use GPS at all.

Altitude is barometric.
Range and range rate are calculated from interrogation pings.
Direction is highly approximate and for pilot situational awareness only.

TCAS is a specific implementation of ACAS. TCAS algorithms are standardised with the intention that there should be no variation between manufacturers.

Where I come from, we call the formula PFM.

Well that's very nice. But for those of us who don't know where you come from or what PFM stands for, we are no better informed.

I suppose Pretty, Fluffy, Meaningful it is not. :E

From the friendly folks at NASA...

TCAS-II resolution Advisory Detection Algorithm

DHU,
Sorry, I was thinking that it relied on Mode S for more than ID...