TCAS logic
Thread Starter
Joined: Feb 2011
Posts: 2
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From: England
TCAS logic
Hi everybody,
I am working on mid-air traffic collision avoidance system like TCAS, and I have some doubts. It might seem obvious for most of you, I am sorry for that.
So in threat detection, TCAS logic is using time-to-go to the CPA. This is equal to the slante range divided by the closure rate. But I can't find what exactly is the slant range, is it the range to go to the CPA or the range between the two aircrafts? Concerning the closure rate, does it take into account both aircraft velocities, is it only the own aircraft velocity?
Another question, does anybody has heard about "horizontal or vertical miss distance" and explain me what it means?
Thank you very much for your answers,
Best regards,
Célia
I am working on mid-air traffic collision avoidance system like TCAS, and I have some doubts. It might seem obvious for most of you, I am sorry for that.
So in threat detection, TCAS logic is using time-to-go to the CPA. This is equal to the slante range divided by the closure rate. But I can't find what exactly is the slant range, is it the range to go to the CPA or the range between the two aircrafts? Concerning the closure rate, does it take into account both aircraft velocities, is it only the own aircraft velocity?
Another question, does anybody has heard about "horizontal or vertical miss distance" and explain me what it means?
Thank you very much for your answers,
Best regards,
Célia
Joined: Dec 2010
Posts: 192
Likes: 0
From: MC80 Home One type Star Cruiser
Not 100% sure, but I guess the time-to-go to the CPA (called TAU) = Slant range between the 2 AC / Closure rate
Example:
2 AC with 1Nm separation.
Closure rate = 1Nm/min
TAU = 1 min
If you take the distance to the CPA with an aircraft speed of 6Nm/min, this would be 6Nm to go before reaching the CPA.
If you divide this range by the closure rate you get 6Nm/(1Nm/min) = 6 min
Which is incorrect. You would need to divide range to CPA by speed of AC, but because closure rate is used, this calculation is not being made by the TCAS.
It takes both AC into account, but it's a relative speed between those 2 AC.
E.g. if both AC are flying parallel, with only a few degrees difference in heading and are slowly converging, it doesn't matter which speed they are flying. The TCAS will calculate both AC trajectories and their closure rate.
A320 FCOM:
http://www.smartcockpit.com/site/pdf...Navigation.pdf
TCAS: p188-p224 (P195 explains briefly the principle)
Example:
2 AC with 1Nm separation.
Closure rate = 1Nm/min
TAU = 1 min
If you take the distance to the CPA with an aircraft speed of 6Nm/min, this would be 6Nm to go before reaching the CPA.
If you divide this range by the closure rate you get 6Nm/(1Nm/min) = 6 min
Which is incorrect. You would need to divide range to CPA by speed of AC, but because closure rate is used, this calculation is not being made by the TCAS.
Concerning the closure rate, does it take into account both aircraft velocities, is it only the own aircraft velocity?
E.g. if both AC are flying parallel, with only a few degrees difference in heading and are slowly converging, it doesn't matter which speed they are flying. The TCAS will calculate both AC trajectories and their closure rate.
A320 FCOM:
http://www.smartcockpit.com/site/pdf...Navigation.pdf
TCAS: p188-p224 (P195 explains briefly the principle)
Thread Starter
Joined: Feb 2011
Posts: 2
Likes: 0
From: England
Thank you Bus Driver Man! Now it makes more sense.
The thing that I have been having difficulty with is contradictory information regarding TCAS logic on the Net. On the one hand I have read that TCAS logic doesn't use the bearing (because of errors due to antennas) and it only uses range and closure rate to compute the time-to-go to CPA, and in doing so it compares this value to TAU threshold (Is that right??). But it simulates the case in which the two A/C would be on crossing trajectory and their CPA would be confonded with that crossing point! But obviously it doesn't always happen and even more so in 3D. For me CPA means that there can be horizontal and vertical miss distances at the CPA, so to find the Time-to-go to the CPA I would have expected to find an expression to derive... I must have missed something..
The other thing I have read, which is also on the document you gave me (thanks for that btw) is that TCAS logic uses bearing, range, closing rate, altitude and its own position to simulate both aircraft trajectories and find out where the CPA is, check the horizontal and vertical miss distance and then derive the Time-to-go to the CPA to compare it to TAU threshold... and Eureka! I personnaly would prefere this method which seems far more logical to me.
What do you think about this? Because I don't know which method TCAS logic uses. I am thinking of using the second one for my program.
Thank you for your answer,
Célia
The thing that I have been having difficulty with is contradictory information regarding TCAS logic on the Net. On the one hand I have read that TCAS logic doesn't use the bearing (because of errors due to antennas) and it only uses range and closure rate to compute the time-to-go to CPA, and in doing so it compares this value to TAU threshold (Is that right??). But it simulates the case in which the two A/C would be on crossing trajectory and their CPA would be confonded with that crossing point! But obviously it doesn't always happen and even more so in 3D. For me CPA means that there can be horizontal and vertical miss distances at the CPA, so to find the Time-to-go to the CPA I would have expected to find an expression to derive... I must have missed something..
The other thing I have read, which is also on the document you gave me (thanks for that btw) is that TCAS logic uses bearing, range, closing rate, altitude and its own position to simulate both aircraft trajectories and find out where the CPA is, check the horizontal and vertical miss distance and then derive the Time-to-go to the CPA to compare it to TAU threshold... and Eureka! I personnaly would prefere this method which seems far more logical to me.
What do you think about this? Because I don't know which method TCAS logic uses. I am thinking of using the second one for my program.
Thank you for your answer,
Célia
Joined: Dec 2010
Posts: 192
Likes: 0
From: MC80 Home One type Star Cruiser
The thing that I have been having difficulty with is contradictory information regarding TCAS logic on the Net. On the one hand I have read that TCAS logic doesn't use the bearing (because of errors due to antennas) and it only uses range and closure rate to compute the time-to-go to CPA, and in doing so it compares this value to TAU threshold (Is that right??).
Although, I have been told that the indication we have in the cockpit of the bearing of an other aircraft is not 100% accurate, while the range is being calculated very accurately. (Because of the time between interrogation and reply)
If I have to create a failsafe system, I would only take the accurate measurements, which are range, closure rate and altitude (measured by the other aircraft) and only use the bearing for situational awareness.
I'm afraid I can't help you any further.
