I am "pleased" to report that my £12k TAS605 has possibly paid for itself today...
Were you flying from an aerodrome with ATC? If so do you have to file an MOR?
A mid-air happened several months ago at one notable place on the south coast.
That is why I prefer to fly in CAS.
...which is actually (certainly in the US) where most of the mid-airs occur.
Are there any possible holes in the TCAS technology which could cause a spurious contact to appear, i.e. an indication where there is in fact nothing....? Or where that contact is appearing in an erroneous position and is in fact elsewhere?
I have a PCAS (the Zaon MRX) and it's a cheap solution for traffic avoidance. I've had several times that the PCAS saw my own transponder signal as a thread. That is really scary, as it gives an alert for traffic at the same altitude, range 0.1NM
Graham wrote:
Are there any possible holes in the TCAS technology which could cause a spurious contact to appear, i.e. an indication where there is in fact nothing....? Or where that contact is appearing in an erroneous position and is in fact elsewhere?
Peter, you will be interested in this and might want to do some checking of your own. I just installed a Garmin GDL88 UAT based ADS-B system in my airplane. It has a traffic display capability with a dual frequency receiver (both 1090ES and UAT). The version I installed on my all metal Bonanza has a single antenna on the bottom of the aircraft. Garmin offers a dual antenna version for more money. I wanted to evaluate how well my single antenna version worked before expending the extra money for the second antenna upgrade.
A friend who owns a plastic airplane (2010 Cirrus SR22TN) just had his aircraft transponder updated to support 1090ES ADS-B Out. He happens to have a Avidyne TAS605 installed as his traffic solution. He also has a portable Stratus that receives both ADS-B frequencies and displays on his iPad using the Foreflight app.
We mutually agreed to conduct some flight tests. The ADS-B worked perfectly for both of us, I always had him at the correct position on my GNS530W and he had me on his iPad. He flew at an altitude above me as I expect that this would present a worse case scenario and I approached him from behind and below. When we got close, I moved to his 7 O'clock and continued past him while climbing, but still below to allow my right wing to block line of sight from my antenna. All results were impressive and we returned to the airport and vectored ourselves to the ILS. I remained about 3 NM in trail and with the approach loaded on my GNS530W, I could easily see him joining the approach on the 530W although he was out of my direct sight range. I certainly could have given him vectors from my presentation.
Now to the point of this. On debrief, he said he always had me on his iPad, but lost me briefly using the TAS605. This doesn't seem totally abnormal as long as the loss of contact is brief. More troubling was that he also said that every once in a while, I showed up in a mirrored location at the correct distance, but the azimuth was on the wrong side. This apparently occurred several times. When it occurred, he said that the target would correct back to its proper azimuth in several steps following an arc as its presentation.
Since he just had his aircraft mode S transponder updated to support the ES capability, the installer noticed an anomaly on the mutual suppression buss shared with the TAS605. When the 605 was turned on, the mutual suppression buss would break into what looks like oscillation when it attempted to assert the suppression. The GTX33ES would output a normal square wave pulse when it was transmitting and visa versa with the TAS605, but only if the other device was powered off. When both are on the GTX33ES still outputs a square wave pulse, but the TAS605 appears to output an extended period of short high frequency (from memory I would estimate several hundred pulses and for a duration 20 times the GTX33ES transmission period). The pilot has a video of the suppression buss and I have asked him to provide me with a URL so it could be viewed.
Cirrus has been able to duplicate the problem on a new production aircraft with the TAS605 and is exploring it. They claim the issue doesn't show up with the other manufacturer TAS options.
NCYankee
By "suppression bus" you presumably mean the TAS6xx-GTX330 RF interconnection (page 50 of this PDF) which stops the TAS transmitting when the GTX is transmitting.
There is a similar arrangement with the DME but it is done with just a wire (page 47 of the PDF).
On page 47 is a cryptic note about conforming to ARINC standards, and Fig 13. Maybe this was not done correctly? The sort of oscillation you describe can be the result of some output stage being shorted to ground (or to a more powerful output of something else), so I would check that the suppression is correctly wired.
What DME is your colleague using?
I have not seen the problem with traffic walking gradually from one side to the other, but certainly some traffic does momentarily disappear. This happens often with traffic more than a few miles away, and doesn't suprise me (marginal transponder installations, etc).
With respect to locating aircraft visually, many suggest leaving the landing light on to aid acquisition. I don't believe this works during the bright sunlight portion of the day as the eye uses the rods to pickup a target via the contrast the target makes with the bright sky. The rods are very sensitive to light and do a great job of detecting contrast. Turning the landing light on reduces the contrast and makes the aircraft more difficult to see against a bright background.
During WWII, before radar was generally available in aircraft, the US conducted experiments with a B24 by mounting landing lights along the leading edge and turned them on so that the B24 could get closer to a U boat before being spotted. It was an early stealth method that was abandoned with the advent of smaller radar units that could be outfitted in aircraft. The project was named Yahudi. You can read about it from this write up: http://jmrc.tripod.com/fa/stealth/stealth2.htm
As I said, most pilots are trained to turn landing lights on while in the pattern. The next time you are at an airport during the bright of the day, watch the aircraft on final approach. Answer the following questions: When do you first spot the aircraft? When can you determine if the landing light is on?
On the other hand, aircraft that are equipped with the pulse light system that flashes the landing lights on and off are very easy for the eye to detect. I have read where the change in contrast is what attracts the eye and brain to recognize the aircraft and that rod's are sensitive to as little as a single photon of light.
It became nice and technical there - over my head but clearly of interest to you chaps!
Provoked some thought, anyway. Just wondering if sometimes those 'invisible' contacts weren't actually there, or were somewhere else. Most of the time I'm sure they are though.
I've no TCAS experience, but I often get traffic called by a radar unit which I 'should' be able to see but cannot for the life of me. I guess our eyes really are awful.
Peter,
He has a TAS610. The mutual suppression buss is P1-7, see PDF page 28. It is not compatible with all transponders or DME units, but is used to suppress simultaneous transmission for any system that transmits in the 900 to 1200 MHz area, such as DME, transponder, ADSB, or TAS/TCAS unit. Here are links provide by the avionics shop.
With only the 33ES turned on (good square wave): https://dl.dropboxusercontent.com/u/300799/IMG_2371.MOV
With only the TAS turned on: https://dl.dropboxusercontent.com/u/300799/IMG_2372.MOV
With both on: https://dl.dropboxusercontent.com/u/300799/IMG_2370.MOV
I have a PCAS (the Zaon MRX) and it's a cheap solution for traffic avoidance. I've had several times that the PCAS saw my own transponder signal as a thread. That is really scary, as it gives an alert for traffic at the same altitude, range 0.1NM
Yes, that is a limitation of those passive devices. They have a hard time distinguishing Mode C replies of a Mode S transponder. They have to decide whether they are both from the same transponder or from two. Just be aware of it and don't panic if it suddenly shows up at your position and altitude. For a real target, it is highly unlikely to only show when it is exactly at your position.
In general I am very happy with my PowerFLARM. It shows almost everything I can spot with my eyes (and those targets I see but the device doesn't could be without transponder, I sometimes ask ATC whether they got a primary or secondary target). In addition, PowerFLARM helps with the gliders which is very much necessary because I happen to live in what is possibly the most active gliding region in the world. Fair cumulus clouds and gliders is like dogs*** and flies 
NCYankee - those signals look like two outputs are being shorted together.
One needs to get the circuit diagrams (schematics) of the equipment in question.
Yes, that is a limitation of those passive devices. They have a hard time distinguishing Mode C replies of a Mode S transponder.
I think the problem is that they don't have a connection to the aircraft's pressure altitude source (so they try to receive the aircraft's transponder transmission from which to extract the data) and also they don't have a connection to the aircraft transponder so they can tell when that is transmitting (as opposed to some transponder nearby).
The fixed installations solve the above by wiring up to the Gray code pressure altitude wires (or its ARINC429 digital form) and by connecting to the transponder's output cable.
