Radar Resolution

[Nipper2]

I overheard a radio exchange yesterday between a controller and a pilot where the question was asked, "Are you navigating by GPS?". The pilot replied, "Yes" and was then advised that he had transgressed the Heathrow Zone.

I am not for one minute advocating using GPS to skirt around 200 meters outside CAS and nor do I wish to get into a debate about GPS versus steam navigation.

Through my work in the marine industry I have a pretty good idea about the accuracy (and potential errors) inherent in GPS. I do not however have any idea about the accuracy and potential errors in the radar system.

Please can a knowledgeable ATC person enlighten me.

[Scooby Don't]

I'm making a few assumptions here....but I'm pretty sure radar processors are accurate to within a pixel as displayed on the screen for range. However, they can only display the slant range. At its most extreme, an aircraft at FL360 could be over the radar head but displayed at a range of 6nm laterally from the radar head. This supposes that the radar is "visible" to the radar within the overhead. In reality its not really an issue, as aircraft which are close enough to each other to need close attention will be within the same portion of the radar's envelope.

For accuracy within azimuth, assuming the radar is correctly set up, the issue becomes one of PRF - pulse repetition frequency. For a radar head which rotates at 15rpm (5400 degrees a minute) and sends out 3600 pulses every minute, the distance in azimuth between pulses is 1.5 degrees. On the few remaining radar displays in the western world which can display raw, unprocessed primary returns, it's actually quite obvious. The blip shows up as an arc, and the longer the range from the radar head, the broader the arc. I've seen blips with a scale width of 2nm or more! All the radar can tell you is that the target is somewhere within that arc of 1.5 degrees. A processed radar doesn't have any greater accuracy - it just displays its best guess.

Whether that means the radar can conclusively show when an aircraft is busting airspace depends on several factors, including how the airspace boundary is alligned with the radar head. In the case of an arc around the radar head for example, the aircraft would show up slightly BEYOND the arc when actually directly overhead its boundary, due to slant range. How much beyond it would show would then differ with altitude.

So, the answer to teh question is.....it depends!

[LH2]

Thinking laterally, maybe the issue is not one of measurement accuracy. It could be the route the aircraft was flying (well, at least attempting to) that made the controller think "this guy is following the arrow". You haven't said if the pilot suggested the controller might have been mistaken, or if he was skimming the zone; maybe he just flew straight into it (literally following the arrow) without paying any attention to the airspace warnings on his GPS. It could have been not even an aeronautical GPS, perhaps a Tom-Tom or something. Less likely, it could also have had an outdated and erroneous database.

Just food for thought. Btw, there was a discussion not long ago (with interesting answers) about radar precision. Hopefully someone with better luck will find it and post a link.

[Nipper2]

Thanks Scooby. Your reply is most helpful, but also generates more questions. I fully understand your point about any two things likely to bump into one another having roughly the same errors but...

Your suggestion that the slant range will work to 'help' the supposed airspace buster is a good one, always providing that the radar head is 'inside' the controlled airspace. This might not be the case with respect to data from the head at Pease Pottage and the London TMA

At a typical resolution (area and approach control for example) how big in scale terms is a pixel? How many pixels wide is the TMA boundary on the display?

For processed radar images does the system use data from just one head or a composite of data from more than one? Presumably (depending on geometry) a multi-data system would be more accurate.

So far as I am concerned, all good reasons to stay outside airspace boundaries by a good margin.

[Scooby Don't]

LapSap - accepted re: beam width, though that and the PRF will be designed to give 360 degree coverage. And now you have me wanting to get someone to switch off Mode C at altitude, just to see if the blip does a sideways jump!

[HEATHROW DIRECTOR]

My brain is too dead for me to enter serious technical discussions! However, most of my life spent using radar within the London Control Zone and many years using handheld GPS for car navigation and outdoor hobby work, plus more recent acquisition of SBS, all give me reason to trust radar above GPS any day. Our in-car nav is based on the popular Memory Map programme, versions of which are used by some pilots. It is usually extremely accurate but just occasionally it does strange things. Travelling along a very straight section of the M3 recently it tracked us about a quarter of a mile to one side for several miles and then regained the road. It's a road we have driven many times and it is usually accurate. We have experienced this before but have no explanation for why it happens.

I know little about modern avionics but understand that aircraft ADS-B transmissions (which provide lat and long info on a/c position) are derived from some form of GPS equipment. Watching these transmissions on SBS is very interesting and it is not unusual to observe position errors of 2nm or more. A few days ago I saw one of at least 5nm.

All of this leads me to have very considerably more confidence in ground radar than current GPS derived information and I am sure that if radar observed a pilot infringing an airspace boundary then he really did. Having dealt with many, many such incidents whilst at Heathrow I would just add that some pilots would argue black was white when accused of such action!

[055166k]

Nipper 2

I would challenge the authority of the controller to ask the question in your post; and then to make the comment that you report. Controllers are not policemen.
There are several areas of interest here, for example a processed radar picture suffers from "coasting" if the return is weak or intermittent, and so for a small slow moving contact at the extremity of primary cover and/or in an area of patchy secondary cover, it may be that the aircraft in reality was not exactly where the displayed symbol said it was.
An additional defence might be that the controller would have to prove the accuracy of his/her display video map for the control zone against the laid down geographic parameters and how that accuracy and precision were correlated for that duty period against the geographic representation of the control zone on a pilot's navigation chart.
A more interesting question from your scenario....if the controller comment was as you suggest....if the controller was providing a radar service to the aircraft then how was the alleged transgression allowed to occur; additionally, how and using what method was the aircraft identified, and with what degree of surety could the claimed breech be attributed to that aircraft? [Label swapping does occur]
Your radar accuracy question can wait for a rainy day, the answers are wide and various depending on particular equipment and environment....we need more facts and ATC unit identity, and aircraft type is very significant.

[HEATHROW DIRECTOR]

<<I would challenge the authority of the controller to ask the question in your post; and then to make the comment that you report. Controllers are not policemen.>>

Similar situations have occurred many, many times and I see no reason why a controller should not ask such a question. I've done it myself on countless occasions with light aircraft flying down the west side of the London Control, cutting corners. I have asked which navaid was being used, told them they have entered controlled airspace and given advice on getting back out. The pilot might be communicating with ATC but not under radar control. Even when under radar control the ATCO cannot be watching every aeroplane every second, nor is he required to. It is in everyones interest to let a pilot know that his navigation is a bit awry. When I worked at Heathrow it was standard procedure to track zone infringers who were not communicating with ATC to see if they could be identified. If the aircraft was seen to join the circuit of a nearby airfield, ATC at that airfield would be asked to tell the pilot to squawk. If the squawk coincided with the unidentified target, identity was confirmed. Such evidence has been used successfully in court cases.

Back in the past, if 939 action was anticipated the pilot would be asked to telephone ATC and he would be cautioned that "anything you say", etc, etc., similar to police action.

I expect it's all different nowadays..

[HeliCraig]

Out of interest, a question for both current ATCO's and HD.

I recently flew this section of airspace in a R44 (EGBE - EGKA), with GPS; but had put way points in which kept me well clear (4nm W of Bagshot mast @2,000ft).

I was receiving a FIS from Farnborough (as part of the new LARS service) squawking as directed, and confirmed identified. If I had started to transgress towards / slightly into the zone as described above what would have been the most likely outcome?

I presume, that if Farnborough had noticed before infringement they would have advised me to change heading. Had I have infringed I presume LHR ATC would have seen the squawk and asked Farnborough to get me out of the way?

Now, a colleague suggested I could have planned a route through the zone via BUR and Ascot under a SVFR clearance. Having only flown in the zone once before - what would the likelihood of that clearance being given have been? Are the ATCOs at Heathrow Special kind to singles wishing to do that??

C.

[055166k]

Interesting reply from the respected HD. I can't think of anything worse that would dissuade a pilot from communicating with an ATC unit. We're supposed to help our customers and not put the fear of God into them. I also question on what authority you can order another unit to issue a squawk instruction to a pilot at a possible crucial time in his/her flight, particularly in a circuit pattern when workload could be high and that instruction may compromise the safe conduct of the aircraft....landing checks for one example....traffic lookout for another......possibly a student.....BIG liability to take on!! To say nothing of the distraction to your primary task.
We can't have home-made rules without proper Authority and guidance; I say again that Controllers are not and should never be viewed as policemen. Fill the form in and let the system look after it; I'm not bothered if one or two get away.......education and awareness programmes are the friendly way to go.

[HEATHROW DIRECTOR]

055166k. I'm not sure of your background but all you disagree with was employed many, many, many times at Heathrow and, I know for sure, other airfields too. Nobody was trying to put the fear of God into our "customers" - urrgghh, I HATE that term but realise that it is beloved of modern managers. VFR light aircraft were not "customers"; we gave them a damned good service for free. However, we were required to report any zone infringements and always tracked the aircraft in an attempt to identify it. This was usually done by the Approach Supervisor; maybe the SVFR Director if he wasn't busy. Phone calls to other units were dealt with similarly.

<<I also question on what authority you can order another unit to issue a squawk instruction to a pilot at a possible crucial time in his/her flight, particularly in a circuit pattern when workload could be high and that instruction may compromise the safe conduct of the aircraft....landing checks for one example....traffic lookout for another......possibly a student.....BIG liability to take on!! To say nothing of the distraction to your primary task.>>

The responsibility for passing the instruction rested with the controller at the other airfield and he could have refused. It caused NO distraction to anyone's primary task. Everything worked very smoothly.

About 50% of pilots who infringed and then rang us would apologise and admit their error. Unless their actions had been reckless they were told to please be careful and no action was taken. If a serious infringement occurred requiring us to delay Heathrow traffic, or if the pilot was adamant that he couldn't possibly be wrong, then we tended to throw books around.

[Nipper2]

Thanks for the enlightening replies.

The controller in question was providing a FIS so no element of control was involved. The station concerned does have radar and routinely issues discreet squawks to aircraft in receipt of an FIS.

Interestingly several of the scenarios discussed here also played out on Sunday afternoon.....

Only a few minutes earlier the controller had issued an instruction to an aircraft receiving a flight information service "G-XXXX, you are identified at YYYY. Your current heading will take you into the Heathrow Zone. Turn south immediately to remain clear". Suffice to say the pilot responded and from the sound of his voice was very relieved. In the same situation, I would certainly hope that any controller I was working with would pass similar information. They certainly have done in the past and saved my bacon.... (VOR set in error to 190 not 170 - an easier mistake to make than you might imagine).

Not much later, while at my destination airfield, the desk received a call from Swanwick asking them if they were able to to identify an aircraft in a certain position close to the field. Unable so to do, the A/G radio operator was then requested to broadcast asking the unidentified aircraft to squawk NNNN. There was no response. My understanding was that the target aircraft had busted the Southampton Zone in a big way.

Finally, on the way back to my base, another service provider requested a police aircraft to try and identify another zone buster.

As a pilot of a very small aircraft my personal view is that any kind of "them and us" situation is bad. I do not regard myself as a customer of ATC services but as a cooperative party. I hope that the information I give them is as useful in helping us all keep to our own bit of the sky and not bumping into one another as the information they give me. Despite the fact that it will cost damn near 20% of the value of my aircraft I have a mode S unit on order for this very reason.

Keep up the good work guys.

[ISaidRightTurns]

Most likely culprit for airspace violations in the US:

GPS in statue miles, rest of the World in nautical.
Don't know how common that would be elsewhere.

Also, someone mentioned an a/c at FL360 depicting 6 miles from the site. In the US, our sites overlap enough that a mosaic (composite) picture is formed and displayed on the scope. At altitude, over a long range radar site, you would not be picked up by that site, instead, your transponder would be received by the other sites that overlap. Position is then accurately interpolated by the machine.

Do other countries not use mosaic radar for their area control?

[anotherthing]

Nipper2 wrote

Finally, on the way back to my base, another service provider requested a police aircraft to try and identify another zone buster.

As a pilot of a very small aircraft my personal view is that any kind of "them and us" situation is bad.

It's not quite as clear cut as a 'them and us attitude' - several years ago, NATS identified zone infringers in the TMA environment as the biggest risk to safety. It was believed (and in some corners still is), that if there was to be a mid air collision within controlled airspace, it would be caused by a CAS or zone infringer.

NATS and other bodies have spent lots of money designing cutting edge new tools for controllers, and producing educational DVDS, workshops etc for light aircraft flyers. This has had a big impact on the number of infringers, but they still happen far too often.

Often these infringers, although avoided safely, cause multiple go arounds and thousands of pounds worth of fuel is burned in the process.

These people must be identified - not necessarily to punish, but to educate them and provide education for others.

The open reporting system employed in the UK is renowned globally for its positive impact on safety; if an infringer does not know he or she has infringed, then cause and effect is not learned!!

By your very tone Snapper2, you sound like a responsible pilot - unfortunately there are a few out there that think that once they have a licence, they are gods.

It's no different to a new or irresponsible driver who causes cars to swerve violently to avoid them because of their actions... I bet you would not hesitate to agree that these drivers need re-educated or punished. The aviation world is no different!

HeliCraig

If under a FIS, even if wearing a FIS squawk, you are not necessarily being tracked. However if Farnborough LARS saw you straying, they would gently advise you. If Heathrow saw you enter the zone (which they would), they would call Farnborough - thats the beauty and benefit of the Farnborough LARS setup - it can save pilots and controllers grief

[grob103]

Going close to the edges

I have to admit to being one of those GA pilots who goes right up to the edge of controlled airspace, because I'm trying to go from A to B to C as quickly as possible, without an engine.

A good example would be yesterday, which was probably the best gliding day I've ever experienced. I've checked my trace, and I'm clear of airspace, as least within the bounds of GPS accuracy and pressure altimetry, but I did all of the following:

* climbing to 100' below airspace at 5500' and FL65 (multiple times)
* skirting round the edge of some class D (within 1km for several track km, twice)
* over the top of several (M)ATZs and into a MATZ stub
* through (non-bye-lawed) danger areas (3 times)

All of this done without benefit of radio (although I would have liked one so I could call for a class D transit, and let Boscombe know what I was up to).

So, us glider pilots are a bit anti-social by power standards. I've always wondered how much we actually p*ss off ATC, and whether this is a factor in the CAA Mode S debate (not that I want to open that up here).

[HEATHROW DIRECTOR]

I started to respond to grob but then I thought... "this is either a wind up, or this bloke is definitely not the full box of chocolates".

[chevvron]

Unfortunately Bren, what he did was legal (a bit silly maybe going through a danger area). Pity the poor guys controlling the MATZ and class D airspace having to take avoiding action though; pity the people in any public transport aircraft who had to be rerouted to avoid him too.

[anotherthing]

Although Chevvron is correct about the legality of Grobs supposed actions this has definitely got to be a wind up!!

However, any muppet that goes through a danger area, and there are some out there (I am fully aware civvys do not need to heed them if not bye-lawed), is an idiot

[HEATHROW DIRECTOR]

Chevvron - yep, I know T. And I know from bitter experience, like you, that there really are people out there doing such things!

[DFC]

I've checked my trace, and I'm clear of airspace, as least within the bounds of GPS accuracy and pressure altimetry, but I did all of the following:

* climbing to 100' below airspace at 5500' and FL65 (multiple times)
* skirting round the edge of some class D (within 1km for several track km, twice)
* over the top of several (M)ATZs and into a MATZ stub
* through (non-bye-lawed) danger areas (3 times)

You could have climbed another 100ft higher and still been in class G

You could have gone closer to the Class D and still been in class G

Over or through a MATZ makes no difference (especially VFR) - class G

Of course being a glider you know that everyone has to get out of your way and not the other way round.

However,

Relying purely on GPS to keep you out of airspace / not knowing for sure if you were outside the airspace until you later checked the trace and of course recklessly endangering the aircraft by flying through a danger area...........well if that isn't a wind-up it should be.

Regards,

DFC