View Full Version : Garmin 196 accuracy
19th Apr 2006, 16:32
this is my first post so please be kind!
I have recently started using a Garmin 196 and find that when I am flying via VORs the heading the VOR wants me on is slightly off from the line shown on the G196 (I put the VOR code in as a waypoint e.g. BPK).
I know that there is a margin of error from the VOR, but is there one for the GPS as well?
19th Apr 2006, 16:38
Don't use a G196 myself, but surely somewhere it reports its EPE (Estimated Position Error)? This will give an estimate of how accurate it thinks it is.
Compare that to the stated accuracy of a VOR and you will find it is almost certainly the VOR and/or your receiver that are inaccurate!
19th Apr 2006, 17:16
I've not encountered your particular concern on VOR tracking, but I have noticed that my 196 has some position mapping errors. Most noticeably: heading east off the south coast at about 600ft , with the shoreline definitely at least 100 yds off to my left, the 196 map placed me clearly inland. On that basis I certainly wouldn't rely on the 196 to skim me past controlled airspace! At ground level the error is less, but still noticeable: it shows my take-offs as being to one side of the runway, and thats just not possible ..... :D
19th Apr 2006, 17:31
The aeronautical database and the ground features database are handled differently, with the ground feature less accurately represented.
A VOR traditionally may be 2-3 degrees out, so a few miles at a distance, would be common.
A GPS with a good view of the sky would be accurate to 100ft or so. My 196 is usually +/- 50ft.
20th Apr 2006, 03:22
Assuming the Garmin is getting proper reception (always a question on anything with an integral aerial, operated in a metal cockpit) it should be accurate to within a few yards.
The land database on a GPS can be hundreds of yards out. Nobody really cares about it too much. Try using an aviation GPS on the road, and you find yourself half a mile off the M25 for example. The only way round that is to run the Ordnance Survey 1:25k or 1:50k chart on a GPS; then you can often tell if you are 5 yards off some footpath...
The airspace shapes are done differently though. They are defined in the relevant national AIPs, as a coordinate list. This coordinate list is lifted and stuffed whole into the GPS database, by Jeppesen (nearly all aviation GPS data is done by Jepp). This may sound like famous last words, but there really is very little room for subtle errors there.
Same with navaid and airport positions; they are lifted from the AIP data.
In reality I think Jepp will use various sources. There is the US DAFIF database, Eurocontrol run a big database of coordinates too which anybody can query. These are machine readable whereas the AIPs are usually delivered printed, which is useless.
Errors do happen but they tend to be major c0ckups, not subtle shifts in airspace.
I'd say a cheap handheld GPS will be more accurate than the ATC radar, too.
There is a different issue with alleged airspace infringements: the controller sees your current track, and if this shows you heading for his airspace, he will get nervous. He may well tell you that you are infringing now. But you could be miles out, just messing about with your track, making him nervous. I happen to know this first hand, from a certain large UK unit. People with autopilots that can track a GPS track or the heading bug can easily make controllers nervous just by switching between HDG and NAV modes; the resulting temporary heading changes (especially from HDG to NAV, if the plane is say half a mile off track) are going to look quite bizzare on a radar screen.
20th Apr 2006, 03:37
Wot Justinmg said. The ground database, like the marine database, may not have been resurveyed for a hundred years, furthermore, air charts don't include the reliability diagrams that would alert you to the potential inaccuracy like a marine chart does.
Your Garmin should be accurate to a few yards in position (but not altitude, thanks to WGS 84). I have experienced one satellite failure (thankfully at sea) where the GPS transported me instantly thousands of miles to South East Asia until it was gagged.
20th Apr 2006, 07:20
I've got a Garmin 196, and there have been a couple of times when I've been a bit suspicious of it. Once I had it on when flying between Shoreham and Biggin, although I wasn't using it to navigate, I was tracking VORs and looking out the window. When I checked my route on it later it showed me clipping the corner of Gatwicks airspace.
Now I know most of you are probably going to think that I did, and in some circumstances I might agree with you, but I fly around the area a lot and I'm paranoid about Gatwick, and know exactly where it is, so I'm 100% positive I was a good few miles away from where it showed me.
Having said that, I've zoomed in to an airport and it's shown me right on the centreline of the runway - and obviously that's where I always land :ok:
20th Apr 2006, 07:44
<< I was a good few miles away from where it showed me.>>
There are only two possibilities that come to mind - either your GPS is faulty or it was being screened somehow to prevent it receiving enough satellites. Modern GPS receivers - even the little cheapo ones - are exceptionally accurate. If they have EGNOS capability the accuracy is down to a few feet.
For Garmin users, the company has an extraordinary after-sales service. I have twice had faulty equipment exchanged for brand new, the last being a GPS12XL which was 12 years old. They delivered a replacement by courier within 3 days with no charge. Might be worth JL having a chat with Garmin...
IO540 wrote: "He may well tell you that you are infringing now. But you could be miles out, just messing about with your track, making him nervous. I happen to know this first hand, from a certain large UK unit. "
Well, I was a Heathrow radar controller for 31 years and spent thousands of hours talking to clockwork mice on 119.9 and I find that statement very, very difficult to believe. It's ludicrous to think an air traffic controller would make such a statement. If he was in contact with the pilot there would be no reason for him to get "nervous" - ATC people aren't like that! ATC radars are very accurate and if a pilot was "miles out" a controller would NOT tell him he was infringing..