Ground Based Transponders

[Admin]

Pete Davies reported an interesting 'anomoly' that he was seeing a number of ground based Mode-S signals, which he suspected of being vehicles at Manx Airport.
After inquiring he got the following reply

As part of the new mode S radar system we have four ground based reference transponders which usethe 24-bit identifiers 43BF78, 43BF79, 43BF7A and 43BF7B so that would explain why you are getting these constantly. There are two at the airport, on in Santon and one on Snaefell. They should be using the mode A transponder test codes 7777 or 7776 so perhaps your app can filter these?

Ok, so this is interesting, it would be easy for me to filter out these squawk codes, but I wanted to open this up for discussion to get some other points of view, wikipedia reports ...

7710-7776    
    External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]

7777    
    Non-discrete code used by fixed test transponders (RABMs) to check correctness of radar stations (BITE) (Belgium, Germany, Netherlands, USA)
    DOD interceptor aircraft on active air defense missions and operating without ATC clearance in accordance with FAA Order 7610.4 (USA)[15][23]

interested on views, if I filter 7776 & 7777, then this will make them totally invisible.

Edit:
I also found this, so sounds plausible ?

7776 - 7777 SSR Monitors (See note 5)
Note 5: Ground based transponder equipment

Thx
Lee

[Admin]

A follow up to my own post, these transponders don't appear to be squawking !
This was the capture below.

[AlanB]

From the UK AIP ENR 1.6 on SSR Code Allocation

Codes 7776 - 7777 SSR Monitors (Refer to ENR 1.6, paragraph 2.2.4)


ENR 1.6 Para 2.2.4

2.2.4 SSR Site Monitors/Far Field Monitors
2.2.4.1 SSR site monitors, sometimes referred to as Far Field Monitors (FFMs) for Mode S radars, shall use Mode A codes *7776 and *7777, along with Mode C pressure-altitude data set to either a high value (eg over 60000 ft) or an unrealistically low value (eg 2000 ft below ground).
2.2.4.2 Mode S FFMs will also require an ICAO 24-bit address to uniquely identify them. These addresses shall be allocated as part of the National IFF/SSR Committee approval process. For further details contact: nisc@caa.co.uk

[exfirepro]

Hi Lee,

I thought that transponder code looked familiar, so have just been checking back my screen grabs from my Mode S tests.

On 14th January, while testing on the ground from my house, 5 miles due south of Edinburgh Airport, I recorded a traffic entry at 15.43

Hex 43BF7E   Reg  43BF7E   Squawk  7777   Dist  -  Alt  61265ft

Now I know what the h-ck it was!!

Peter

[Admin]

Hi Peter,

Well more interestingly, you are seeing the squawk code, whereas in Pete's screengrab there is no squawk code, and no altitude 
so I could not filter this - even if I wanted to.

I also read that the altitude is set to something unrealistic such as 60,000ft or -2000ft, which concurs with your observation of 61265ft

Thx
Lee

[peteD]

I've never seen a squawk code associated with those hex codes, despite being told that they are test codes.
I have asked ATC where else in the uk this system has been installed...I recall Cardiff & Bristol and 3/4 others but will confirm.

Regards
Pete

[AlanB]

I've never seen a squawk code associated with those hex codes, despite being told that they are test codes.
I have asked ATC where else in the uk this system has been installed...I recall Cardiff & Bristol and 3/4 others but will confirm.

Regards
Pete

I think you will find that all SSR Radar Sites have an associated Ground Based Transponder which is set to a low power and usually on a Yagi, I speak as an ex NATS Radar Engineer.

As there are a number of these throughout the UK for en-route and Airport Services then you may come across them on PiWare. The ones associated with a rotating antenna usually have a directional antenna therefore unlikely you will often see them unless between it and the associated radar head.

With the introduction of Multi-lateration SSR interrogation  to replace the the rotating antenna systems there are a number of fixed Receiver Antennae in a Matrix with an associated set of fixed base Tx sites that transmit the 1030 interrogation pulses and the received 1090 signals are then processed to produce target position form the response - they are Mode A/C and S compliant and currently not enabled for ADSB. Aircraft Positions are calculated from the received signal in space to each antenna using timing, direction and a complex algorithm in a fast processing system.

Isle of Mann was one of the earlier systems together with a number of others at Airports and North Sea. They are also used around the world and in some countries ADS-B enabled where the matrix is substantially large and covering areas that conventional rotating antenna systems are difficult. An ADS-B receiver in a mountain top could be solar powered and only require a fibre link of even a low powered microwave link to get the information back to the user.

In this case the fixed transponder is using a dipole as a number of receivers need to receive the signal in order for its position to be calculated and the system verify it continues to meet the monitoring of positional accuracy requirements.

Alan

[exfirepro]

I think you will find that all SSR Radar Sites have an associated Ground Based Transponder which is set to a low power and usually on a Yagi, I speak as an ex NATS Radar Engineer.

As there are a number of these throughout the UK for en-route and Airport Services then you may come across them on PiWare. The ones associated with a rotating antenna usually have a directional antenna therefore unlikely you will often see them unless between it and the associated radar head.

With the introduction of Multi-lateration SSR interrogation  to replace the the rotating antenna systems there are a number of fixed Receiver Antennae in a Matrix with an associated set of fixed base Tx sites that transmit the 1030 interrogation pulses and the received 1090 signals are then processed to produce target position form the response - they are Mode A/C and S compliant and currently not enabled for ADSB. Aircraft Positions are calculated from the received signal in space to each antenna using timing, direction and a complex algorithm in a fast processing system.

Isle of Mann was one of the earlier systems together with a number of others at Airports and North Sea. They are also used around the world and in some countries ADS-B enabled where the matrix is substantially large and covering areas that conventional rotating antenna systems are difficult. An ADS-B receiver in a mountain top could be solar powered and only require a fibre link of even a low powered microwave link to get the information back to the user.

In this case the fixed transponder is using a dipole as a number of receivers need to receive the signal in order for its position to be calculated and the system verify it continues to meet the monitoring of positional accuracy requirements.

Alan

Hi Alan,

Very useful information. Just a bit more upmarket than a Raspberry Pi  !!

Peter//

Lee,

I remember noticing these entries in the traffic log at the time and did wonder what the heck was flying at over 60,000 ft, but in the absence of any rational explanation presumed it had to be something military (or perhaps the space station), hence why it stuck in my mind and why I took the screen grabs.

I don't know where the 'beacon' is located, but the signal strength at my house varied between '10' with reported altitude 61265 when I first noticed it at 14.43 and '7' with reported altitude 61279 at 15.11, then it disappeared off my traffic screen. Compared to Pete D's 'beacons' which all have a much higher signal strength, mine is presumably either far lower power or much further off. I will try to see what I can find out locally.

Regards

Peter

[SteveHutt]

Don't forget that ADS-B is also designed for use by airside ground vehicles to aid collision aviodance while taxying, or to help ensure the runway is clear just prior to take-off. Maybe there is a flag in the ADS-B messages that indicates these are ground vehicles. I have not looked.

I also recall some discussion about NATS putting an ADS-B Out device on a crane as a trial. Don't know whether that actually happened.

Steve

[AlanB]

Don't forget that ADS-B is also designed for use by airside ground vehicles to aid collision aviodance while taxying, or to help ensure the runway is clear just prior to take-off. Maybe there is a flag in the ADS-B messages that indicates these are ground vehicles. I have not looked.

I also recall some discussion about NATS putting an ADS-B Out device on a crane as a trial. Don't know whether that actually happened.

Steve

Steve,
You are correct but currently the vehicle transponders are Mode S with no ES enabled at this time. The only ADSB network in UK is the development one used to provide the evidence for levels of accuracy and integrity analysis required to eventually allow operational use. No idea of timescales.

The Runway incursion monitoring is not just SSR based as not all incursions conveniently carry a transponder.

An ADSB out device on a London Crane was tried, and may still be, as part of the work after the incident in London between a Helicopeter and Crane. No idea what codes they were using but it was in the NOTAMS.

The SSR code allocation table in ENR 1.6 is the best way of deciding which codes are used for what reason and then Determining if they are of interest to an ADSB in or Mode S  detection device or not.

[exfirepro]

Steve/ Alan,

If they were ADSB contacts, they would show a distance in the traffic log. They don't, so must effectively be Mode S.

Peter

[SteveHutt]

Hi Alan/Peter,

Sorry, I was not intimating that these currently identified ground targets were ADS-B. I was just pointing out that this scenario will be repeated at some point in the future once ground vehicles with ADS-B out start to become commonplace.

So, PAW and PAW users will need to decide how they should be handled. When that day comes, I am not sure that simply trying to filter them out will always be the best option. This will need a bit more careful consideration. There is no reason why a GA aircraft shouldn't benefit from a warning of a vehicle on the runway, just the same as is the intention for CAT systems.

Regards,

Steve

[Paul_Sengupta]

I think the idea was to silence the audio warnings rather than filter them out entirely.