I have an airspace aware, and the other day it yelled at me that I had reached all of 3011 feet, the limit for that area being 3000 feet... I had been trying to fly between an ATZ and a TMA and got distracted for a moment. Whoops.
I fixed it within a couple of seconds and don't feel any overwhelming wish to 'fess up to the CAA. However, I find myself wondering how accurately they monitor airspace. Clearly there must be (varying) limitations on the accuracy of their radar systems - especially secondary surveillance where a portion of the accuracy will depend on the aircraft transponder. Googling around shows that Mode C has to be accurate to within 125 feet, which could of course work either 'for' or 'against' you in terms of keeping clear of airspace, on the radar operator's screens.
Now, it seems to me that if the transponder and correctly set aircraft altimeter differ by 125 feet, the controller could see you flying significantly inside airspace, whilst your instruments (including the GPS) indicated that you were just outside it - e.g. squeezing between a danger area to 2400 feet and airspace at 2500 to save a long detour. GPS also gives the option to maneuver far closer to airspace than I would dare if I were navigating visually.
Do ATC leave buffers around the margins of CAS to allow for such inaccuracies, and if so does anybody know how big they are? Not that I plan to invade airspace, of course, but both to know whether I should be phoning people after relatively minor infringements such as the above, and to judge the risk of having one's wrists slapped without having actually infringed airspace. If the latter is a risk, is it a good idea to keep GPS logs that may be more accurate than radar returns in order to defend oneself in the event of any action taken?
I think this exercise is pointless.
What do you do when the base of a CTA falls below MSA?
It's wise not to scud run.
Instead you are far safer negotiating a clearance through the TMA or ATZ rather than being squeezed between the two.
I blame poor airspace design as well.
I really wouldn't worry about it.
Firstly, the GPS altitude doesn't (usually) define the airspace: vertical extents can be defined as one of
Barometric Altitude
pressure altitude
Height above ground, measured barometrically
None of these will ever exactly equal GPS altitude - I can think of 2 reasons and I'm sure there are more: temperature skews the barometric readings and the GPS altitude is derived from a different datum to the barometric data. (NOTE - does anyone know if danger areas are defined barometrically or using GPS/Trig altitude?)
Additionally, primary radar can't detect altitude (well, some military systems can, but I doubt the CAA have access to that data) and as you say, there is a huge margin for error built into the Mode C altitude reporting technology.
My understanding is that while ATC don't maintain full separation from traffic outside their zones (that traffic is assumed to be VFR), there is a minimum height (500ft rings a bell) that traffic will be kept away from any vertical extents. Additionally ATC will keep traffic away from anything outside the zone that looks like its about to infringe (as soon as there is an infringement they have to maintain 5nm and 3000ft separation, which can be quite difficult!).
GPS altitude is great for making an educated guess to warn about infringement of airspace (although in extreme high or low pressure they can be quite far off warning about altitudes defined as flight levels). Its also great for cross-checking your altimeter during an instrument approach. It's not really useful for eking out that last 10 feet of uncontrolled airspace.
One thing that does help when squeezing through narrow gaps is having your Mode C reported altitude derived digitally from the same sensor that displays your altitude (ie. using a ADAHRS to feed a screen and the transponder, rather than an altitude encoding steam altimeter or a blind encoder). Also, set your QNH to that of the airspace above. This way the controllers are guaranteed to see the same altitude that you are seeing.
EDIT: what fuzzy said is much better advice!
the GPS altitude doesn't (usually) define the airspace
I would say that assuming the GPS altitude is the true height above MSL (which itself is another debate, but generally it should be within 10-20ft) then GPS altitude does define the CAS base.
If a piece of CAS has a base (on the map) of say 2500ft then that means simply 2500ft AMSL; how you actually maintain or measure that during flight is immaterial IMHO.
In reality we are supposed to use barometric altitude but strictly speaking that would mean e.g. if flying under the 1500ft Class D base of Gatwick you are supposed to dial their ATIS and set their QNH. Almost nobody is going to be doing that so in reality there are margins built in.
there is a huge margin for error built into the Mode C altitude reporting technology.
I don't know the figure elsewhere in Europe but in the UK you are allowed up to 200ft off on your Mode C return, I seem to recall (G-reg; the FAA has its own more detailed requirements for the 2-yearly altimeter check). In reality many are further off than that - as is obvious when you switch on a TCAS system on the ground and look at what other planes who are still on the ground are returning 
Regarding lateral radar accuracy... I suspect it is a lot worse than one might think. Probably something like 1nm at 30nm from the radar installation. Some feedback I've had from ATC during flight does suggest figures like that. For traffic separation this doesn't matter, but ATC may well see you in CAS when you are just outside it.
GPS altitude is a great aid for cross-checking one's QNH!
As regards GPS altitude accuracy, handhelds are often very suspect.
The SIRF-2 chip which was used in nearly all handheld GPSs up to a few years ago had a bug which produced an error of about 200ft in the southern UK. I have had a load of gadgets which showed that. In fact I think my brand new Nokia 808 phone has a similar bug, and that is a much newer design.
"Proper" aviation handhelds like say a Garmin 496 are spot on; usually within 20ft and better still with EGNOS.
All panel mount IFR units are supposed to have a means of correcting from the geoid shape to the actual AMSL. This is normally done with a lookup table but there is a very complicated polynomial out there as well. My old KLN94 is accordingly within about 20ft as well. I am sure the upmarket aviation handhelds do the same thing, otherwise they would be way out.
I wouldn't use a GPS to go within 10ft of the base of CAS, not because you will be in CAS but because you may well spook ATC who will be wondering what kind of stunt you will be doing next - expecially if your Mode C return is also showing you 200ft higher than you are
Maybe some lawyer can come in and report on whether a GPS track log is any defence against a radar track, in a CAS bust case? IMHO, technically, the prosecution would not have a leg to stand on because even the cheapest and nastiest GPS - provided it is actually functioning - is way more accurate laterally than ATC radar.
I would say that assuming the GPS altitude is the true height above MSL (which itself is another debate, but generally it should be within 10-20ft) then GPS altitude does define the CAS base.
I stand corrected - I had thought that each bit of airspace had an associated QNH station, but nothing of the sort seems to be mentioned in the AIP.
In reality we are supposed to use barometric altitude but strictly speaking that would mean e.g. if flying under the 1500ft Class D base of Gatwick you are supposed to dial their ATIS and set their QNH. Almost nobody is going to be doing that so in reality there are margins built in.
Why not? It takes 30 seconds and if you're going to be flying 100 feet below the base it seems like good manners to check you won't be setting off unnecessary alarms for the controllers.
Maybe some lawyer can come in and report on whether a GPS track log is any defence against a radar track, in a CAS bust case? IMHO, technically, the prosecution would not have a leg to stand on because even the cheapest and nastiest GPS - provided it is actually functioning - is way more accurate laterally than ATC radar.
You are probably right here, but I wonder if we should all just stay that extra mile away from CAS if in doubt/not talking to them? One of the biggest issues that CAT has with light GA is the rate of infringements. Even a non-infringement that sets off an ATC alarm has to be treated as one, at huge cost. A lot of goodwill could be gained if we stopped the mindset of "playing the CAS game to its limits".
Everyone carrying an airspace alerting GPS would be nice too, it is pretty hard to accidentally bust airspace if Skydemon/Aware/whatever is screaming at you. IMHO though the Garmin units don't do this well enough, at least for Europe. Peter, how does your panel setup alert on airspace?
I guess the need to dial the Gatwick ATIS depends on whose ATIS (or QNH) you already have. If you got the one from Shoreham or Biggin Hill, then you will be close enough.
But if you came off an IFR flight from Botswana that's different
OTOH as you are cleared to descent in CAS (either remaining in CAS or cleared to descend below CAS) ATC will always pass you the QNH. You will never get cleared to descend from a FL to an altitude without QNH being passed.
The problem might be if flying a long distance below CAS, uncontrolled, say after an IFR flight. Then you have to do "something" to make sure the QNH is reasonably current.
I don't know the rules but recall reading that ATC keeps traffic at least 500ft above CAS base.
Peter, how does your panel setup alert on airspace?
I don't have any warnings as such. The G496 does some but it does so many I tend to ignore them.
Normally, in VFR messing around I run the "real printed" VFR chart as a GPS moving map and that gives a pretty good picture. The only times I busted CAS were: I was talking to a passenger and forgot to descend; once we flew below CAS and as we crossed from one piece whose base was 4500ft to another whose base was FL045; one I forgot that the LTMA base changed from FL055 to 5500ft and I busted that by about 300ft. On the last occassion it was a week after the new map came out and I guess I wasn't the only one... I was asked to call up Gatwick Director who told me of the change, and was OK about it.
Panel mounts don't do airspace warnings (that I know of) and unless the handheld GPS is linked to your intercom, you won't hear any warnings anyway. My G496 is wired to the intercom but that carries only the terrain warnings; the airspace warnings are (in my case; it may be a config item) purely visual plus a beep.
I think GPS based airspace warnings have a very good place but unless wired into an intercom are unlikely to be heard.
Panel mounts don't do airspace warnings (that I know of)
The Garmins do, but they're worse than useless, lulling you into a false sense of security. I've turned mine off.
I think GPS based airspace warnings have a very good place but unless wired into an intercom are unlikely to be heard.
With the iPad volume right up, I find the alerts are quite clearly audible (with and without noise cancelling headsets).
If a piece of CAS has a base (on the map) of say 2500ft then that means simply 2500ft AMSL; how you actually maintain or measure that during flight is immaterial IMHO.
It is not immaterial as the means of determining 2500 feet is by reference to the QNH at an aerodrome in or under the CAS. As the pressure altitude may be considerably different from the real altitude you could infringe if you use the wrong datum.
Thanks...
I do have noise-cancelling headphones and don't have a problem hearing my Airspace Aware warnings. I think this may be in part because they are at a higher frequency than the engine noise that the headphones are mainly designed to suppress. Stall warnings can be a bit dicey though - I can hear them but not as prominently as I would like. It probably depends partly on the headphones.
I take Fuzzy's point re. squeezing in between airspace, but I don't quite get the link to scud-running or flying below the MSA, which at 3010 feet I certainly was not doing. FWIW I had planned to fly through the ATZ but didn't manage to sign off with London Information in time to contact them, so chose to fly over it.
I hadn't considered that even when remaining within Class G airspace I could still be causing problems for ATC - other perhaps than crossing approach paths. I'll endeavour to leave slightly larger margins in the future. That said, in the busier bits of airspace it can be quite a challenge. I suppose it's a special case, but I once had a radio failure over Guildford - trying to get to Blackbushe in a timely manner was not an easy thing to do. Presumably it'll get easier as I get more practice, but I've often taken the lazy route of trying to route around airspace rather than negotiating entry.
altitude busting means deviating more than 300 feet from your assigned altitude ... in IFR flying ... that's what I remember from 40 years duty time - am I wrong?
