Anyone ever tried decoding tcas?

[ogoun]

Hi,

Anyone ever tried decoding the TCAS (traffic collision and avoidance system) signals on 1030mhz/1090mhz? These sigs are wideband, and a bit tricky to decode, but are more common than ACARS sigs, as TCAS is part of the SSR (secondary surveillance radar) system, used for aircraft info and other stuff...

L8r,

Pete

 

[K2KOH]

What software would you use for that? This sounds interesting...and I have an R7100 that will receive those frequencies...

 

[edN]

Hi,

Anyone ever tried decoding the TCAS (traffic collision and avoidance system) signals on 1030mhz/1090mhz? These sigs are wideband, and a bit tricky to decode, but are more common than ACARS sigs, as TCAS is part of the SSR (secondary surveillance radar) system, used for aircraft info and other stuff...

L8r,

Pete

Pete,

I'm fairly familiar with TCAS operation, but I've never personally heard of anyone decoding TCAS.

 

[kg4icg]

Question is how do you decode the radar signal with transponder info in it?

It's part of the aircraft avoidance radar which i don't think there is a way to decode it. Unless you have a lot of time and money to to buy the aircraft avoidance radar a transponder and a a few other peicers of aviation equipment to wonder how it works.

R Collins

 

[ogoun]

The spec is readily available, and describes in painful detail exactly how it all works.

What is probably most interesting (?) to us acars listeners is the fact that the system does not require a dialog with a transponder to provide info.. About every second or so, a "squit" is sent out, containing basic aircraft info, similar(?) to acars. Of course, you can also receive traffic between one transponder and ATC, or another plane, assuming you have two receivers (one for the uplink and downlink freqs).

The format is quite simple, and builds on mode a and mode c messages, which are just AM pulses (technology dates to WW2, and early IFF systems).

Mode S is the one TCAS uses. It phase modulates the modea/c pulses to encode more info, 64 bit packets if I recall.

I think the tcas system is also called ADS-B, if anyone wants to do a search..

This decoding task should be fairly easy, I would think.

Any thoughts on this?


l8r,

Pete

 

[ogoun]

sorry for this (now deleted) double post..

Pete

 

[Videokid]

The idea is very cool. But, unlike VHF signals wouldn't 1GHz signals require a clear view of the sky and the only aircraft one would be able to receive would be those in line of sight, directly overhead? Would an omni directional antenna suffice in this case?

 

[jetmech96]

Just clarify TCAS stands for Traffic alert and Collision Avoidance System.

The question of why you would want to decode it for anything other than the fun of it is what I would ask. Compared to ACARS the information is relatively useless to a ground based user. You can only decode the squawk code and aircraft altitude. If you can decode the Mode S stuff you will only get the aircrafts N-number.

If you were interested in listening to SSR/TCAS, you really only need one receiver. You only want to listen to the downlink freq. of 1090 MHZ, because that will provide you with any information that you would want to decode. The uplink freq. of 1030 MHZ will only have the interrogation messages.

By the way Ogoun, I think you are mixing SSR/TCAS and ADS-B together. ADS-B uses just the Mode S transponder. TCAS is a separate system that is used to interrogate other aircraft like other SSR systems. The mode S transponder replies to SSR/TCAS interrogations and squitters the ADS-B message on 1090 MHZ. Current TCAS systems will have to modified to receive and decode ADS-B transmissions

Also, ADS-B is just in the testing stages. There really no one using it yet. So you will only be getting SSR/TCAS message for the time being. It is going to be quite a while before it becomes used. But interestingly enough UPS is one of the biggest promoters and developers of the technology. Yes, the United Parcel Service.

 

[ogoun]

Thanks jetmech for the informative reply

If you live near an airport, I was under the impression that mode S tcas signals are used for lots of stuff, not just the squitter info.. I think it can even be used as a very slow and simple 2 way data comms (i.e. test based screen tx/rx), although I would imagine that this would be quite rare.

I read something about ADS-B a while ago.. This looks pretty neat, but as you said, a ways off yet.

As to why do it, the usual reason: because its there/because I can (well, not yet), because no one else has.

I understand your reason for asking why though. I read a very interesting/disturbing technical discussion about tcas squits a short while ago, and without going into detail, it gave me pause. Sometimes I wish life were as simple/safe as I remember it being when I was younger...


Cheers,

Pete

 

[jetmech96]

If you are refering to the "disturbing" discussion about free flight and possible overload of the 1090 MHZ freq. Your right it is disturbing. ADS-B is a great idea but no one seems to be addressing the downsides. With all the different transmission of the 1090 Mhz, there is going to be a lot of transmissions stepping on each other.

Also, I mispoke about TCAS having to be upgraded to recieve ADS-B. They have been require to be updated to the Change 7 software. I read that it is for the TCAS to "better Receive" ADS-B. If that means it can decode the transmissions I don't know.

By the way, if there was software out there to decode ADS-B, I would probably use it myself. But for the time being ACARS seems to do a better job.

 

[Arcticpeak]

TCAS does not have anything to do with ADS-B.
But it is correct that ADS-B can be used on Mode S extended squitter, but ADS-B can also be sent via one of the VHF Data Links.

The difference between TCAS Change 6.04A and Change 7.0 is the way the Voice Announcement are announced.
Change 6.04A has both corretive announcements and preventive announcement, to reduce pilots workload and to prevent misunderstandings there are only corrective announcements in Change 7.0.

 

[jetmech96]

Look Arcticpeak, I am not trying to get into a pissing match with you. This is twice now, we have gone back and forth. I understand the systems, you understand the systems. I am just trying to provide people that don't understand anything about avionics some background info.

The biggest thing I want to point out is that this stuff, specifically ADS-B, has not formally finallized and is at least a few years off from implementation. Otherwise, all these discussions are just academic.

In regards to TCAS change 7, when I was doing STC for TCAS I and II, I was under the impression that it had to do with updated TAU's due to RVSM. I known nothing about the changes to RA's. It has been a couple of years since I did that stuff, I have not been keeping up with this stuff with the diligence that I use to.

 

[Arcticpeak]

First, I want to point out that I didn't want to step on the toes of jetmech96. I'm sorry that he feel it that way. I just wanted to point out that TCAS and ADS-B is not related to each other, other than that TCAS and ADS-B share the same frequency when Mode S extended squitter is used for ADS-B.

I will now withdraw from this discussion because I don't want to be blamed for trying to wind up a "pissing contest" when I'm only trying to tell the facts. English is also not my native language, so I'm may choose the wrong words to state my view sometimes.

For anyone who is interrested in these topics and may have an interrest in trying to decode this data, there is a lot of information on the EuroControl website, not everything is applicable to US, but most of it is the same. (Note: TCAS is called ACAS in Europe.)
http://www.eurocontrol.int/acas/
http://www.eurocontrol.int/mode_s/
http://www.eurocontrol.int/cascade/public/standard_page/ads.html