GPS for Pitot-Static calibration
Guest
Posts: n/a
GPS for Pitot-Static calibration
Like a lot of other people, I'm using GPS a lot nowadays for pitot-static calibration work (particularly for ASI errors).
Personally I favour an into-wind / downwind racetrack method to determine mean TAS, and then all the sums from there. Others I know use an equilateral triangle course method, which I can see is more accurate but personally I don't favour since I don't consider the extra 0.2 of a knot or so of accuracy usually necessary.
I'm currently reading Dr John Lowry's book, "Light aircraft performance" where he uses a sort of horseshoe pattern which seems workable but to my mind far too damned fiddly.
What method is anybody else using, does anybody have a particular slant on the matter that others of us might learn from?
G
Personally I favour an into-wind / downwind racetrack method to determine mean TAS, and then all the sums from there. Others I know use an equilateral triangle course method, which I can see is more accurate but personally I don't favour since I don't consider the extra 0.2 of a knot or so of accuracy usually necessary.
I'm currently reading Dr John Lowry's book, "Light aircraft performance" where he uses a sort of horseshoe pattern which seems workable but to my mind far too damned fiddly.
What method is anybody else using, does anybody have a particular slant on the matter that others of us might learn from?
G
Join Date: Jul 2004
Location: Bristol
Age: 61
Posts: 10
Likes: 0
Received 0 Likes
on
0 Posts
Genghis
I have been trying to calibrate the pitot-static on our RV4. I looked at the two methods that you describe but finally settled on a method described by Doug Gray in a paper 'Using GPS to accurately establish True Airspeed (TAS)' of 1998 that is widely available on the internet.
In this you fly any 3 headings and his supplied spreadsheet equations will calculate the TAS. It was quite easy and the results are in the area I expected from observed wind, estimate of speed. But to be sure as a calibration I need to understand the derivation of the equations and this has got me beaten. Are you familiar with the method ?
I have been trying to calibrate the pitot-static on our RV4. I looked at the two methods that you describe but finally settled on a method described by Doug Gray in a paper 'Using GPS to accurately establish True Airspeed (TAS)' of 1998 that is widely available on the internet.
In this you fly any 3 headings and his supplied spreadsheet equations will calculate the TAS. It was quite easy and the results are in the area I expected from observed wind, estimate of speed. But to be sure as a calibration I need to understand the derivation of the equations and this has got me beaten. Are you familiar with the method ?
Join Date: May 2004
Location: Arlington, TX
Posts: 11
Likes: 0
Received 0 Likes
on
0 Posts
GPS for Airspeed Calibration
I have used very easy method. I simply fly down the section lines and keep the aircraft in trim, using the reverse course method. I have compared this with the trailing bomb method and the agreement has always been within 1 knot.
For getting STC's in the U.S., it is good enough.
For getting STC's in the U.S., it is good enough.
PFA The three heading method has been around since the 1930s, GPS is just a relatively new slant on it. The original reference is:-
F L Thompson (Langley Memorial Laboratory, NACA), The measurement of air speed of airplanes, NACA Technical Note No. 616, October 1937 Which you can download from the NACA reports server if you hunt around a bit.
Also worth looking at the download section of NTPS' website at http://www.ntps.edu/Files/ which has an explanation and a reduction spreadsheet (Looks like Doug Gray's paper is very similar). The basic maths is just trig, which is easy enough to derive if you enjoy that sort of thing (personally I think it's a pain and best avoided). I can see why you like his approach however, since it uses GPS track (rather than compass heading, which will contain errors).
Don't forget that you also need the relationship between TAS and CAS, which is the square root of sigma, the relative air density. There's quite a nice free reduction sheet for this online at http://www.bmaa.org/techinfo.asp (form 043).
If you want a really thorough proof, there's a book called "Performance of Light Aircraft" by an American Boffin called John Lowry - ISBN 1-56347-330-5 which covers it very well. Having said that, he's a little overfond of maths for my taste - as evidenced by his book being the only I've ever read which includes a correction for the distance between the static port and the tyres, which personally I think is going a little far.
Dave I occasionally have trouble justifying the method with some of the more old-fashioned regulators over here. You wouldn't have a graph or two of GPS PEC .v. static bomb shoing your accuracy values that I can steal from you? Oh yes, and welcome to the Flight Test Forum, glad to have somebody of your experience around.
G
F L Thompson (Langley Memorial Laboratory, NACA), The measurement of air speed of airplanes, NACA Technical Note No. 616, October 1937 Which you can download from the NACA reports server if you hunt around a bit.
Also worth looking at the download section of NTPS' website at http://www.ntps.edu/Files/ which has an explanation and a reduction spreadsheet (Looks like Doug Gray's paper is very similar). The basic maths is just trig, which is easy enough to derive if you enjoy that sort of thing (personally I think it's a pain and best avoided). I can see why you like his approach however, since it uses GPS track (rather than compass heading, which will contain errors).
Don't forget that you also need the relationship between TAS and CAS, which is the square root of sigma, the relative air density. There's quite a nice free reduction sheet for this online at http://www.bmaa.org/techinfo.asp (form 043).
If you want a really thorough proof, there's a book called "Performance of Light Aircraft" by an American Boffin called John Lowry - ISBN 1-56347-330-5 which covers it very well. Having said that, he's a little overfond of maths for my taste - as evidenced by his book being the only I've ever read which includes a correction for the distance between the static port and the tyres, which personally I think is going a little far.
Dave I occasionally have trouble justifying the method with some of the more old-fashioned regulators over here. You wouldn't have a graph or two of GPS PEC .v. static bomb shoing your accuracy values that I can steal from you? Oh yes, and welcome to the Flight Test Forum, glad to have somebody of your experience around.
G
Join Date: Jul 2004
Location: Bristol
Age: 61
Posts: 10
Likes: 0
Received 0 Likes
on
0 Posts
Genghis
Having re-read your original post I see you mentioned THREE methods, the Gray one is the general case of the equilateral triangle.
Shawn
The NTPS method written by your colleague Lewis is an enhancement of the Gray one and the Gray paper is available on the NTPS website. Could somebody there point me towards a derivation of the spreadsheet equations ? I assume you are the Shawn Coyle of 'The Art & Science of Flying Helicopters' so thank you for a very good book.
Dave
It looks like the into wind/downwind reverse course method is the most favoured in practice.
Thanks all, Alan
Genghis, we posted at the same time, yes it is that trigonometry that is causing me a pain.
Having re-read your original post I see you mentioned THREE methods, the Gray one is the general case of the equilateral triangle.
Shawn
The NTPS method written by your colleague Lewis is an enhancement of the Gray one and the Gray paper is available on the NTPS website. Could somebody there point me towards a derivation of the spreadsheet equations ? I assume you are the Shawn Coyle of 'The Art & Science of Flying Helicopters' so thank you for a very good book.
Dave
It looks like the into wind/downwind reverse course method is the most favoured in practice.
Thanks all, Alan
Genghis, we posted at the same time, yes it is that trigonometry that is causing me a pain.
The only difficulty, in my experience, in the into-wind and downwind method is to accurately determine wind direction. My approach to this is to trim to a slow cruise then point the aircraft roughly into wind, then fly a series of headings at constant IAS & Alt, until I've determined the heading that gives me the lowest GPS groundspeed. Then since I'm only interested in that heading and the reciprocal, I fly on the GPS - ignoring the magnetic compass completely.
If you want the derivation of Gray's maths, and are no more in the mood to work it out yourself than the rest of us, his paper lists his Email address as [email protected] ; presumably he wouldn't put that on a public domain paper if he objected to being asked questions about it - so if I were you, I'd do just that.
G
If you want the derivation of Gray's maths, and are no more in the mood to work it out yourself than the rest of us, his paper lists his Email address as [email protected] ; presumably he wouldn't put that on a public domain paper if he objected to being asked questions about it - so if I were you, I'd do just that.
G
Join Date: Jul 2004
Location: Bristol
Age: 61
Posts: 10
Likes: 0
Received 0 Likes
on
0 Posts
I was in the mood but despite my efforts could not do it. My copy of the paper had a different, and now not valid, email address so I will try the one you have given. Thanks.
As someone who once spent a lot of time analysing sources of GPS error, it's worth noting that the instantaneous speed indications on a commercial type GPS may well be in error. 2D RMS position errors are now pretty low, thanks to the switching off of Selective Availability (an odd term for deliberate time code jitter I've always thought....).
If seeking ultimate accuracy I'd be inclined to use time and distance between two GPS fixed waypoints rather than rely on the instantaneous indicated ground speed. Speed filtering is notably better on more modern GPS receivers, but is still inherently limited by instantaneous, and essentially random, position variations that can be around the 20m - 30m in each second region. The speed errors are related to ground speed, in that the filtering is intelligent enough to increase the speed averaging time constant at lower speeds. This serves to cause the display to read near zero velocity when stationary, which may fool the unwary into thinking the thing is accurate!
The same goes for height accuracy BTW. Because of the shape of the spherical geometry presented by the best selected Space Vehicle (SV) set for 2D accuracy (which is what commercial sets seek to optimise when selecting available SVs) the vertical errors will always be higher than the horizontal ones.
If anyone has a spare moment, try drawing a scale sketch of the pseudo range tracks from a typical 4 or 5 SV set to a receiver. This will clearly show the vertical geometry problem caused by the relatively low orbital altitude of the constellation. A low orbital altitude is obviously good from the horizontal fix accuracy perspective but is poor for vertical position determination.
Surveyors use phase measurement systems to add position accuracy, but as far as I am aware this technology only works for stationary receivers, as it takes several seconds to average the relative phase from each SV. Similarly Differential GPS (DGPS) corrects the gross errors via a second mean local error transmitting data link, but again this will be the mean position error in that region at the time, not the instantaneous error seen by the receiver, so it doesn't fix the speed error problem effectively.
If seeking ultimate accuracy I'd be inclined to use time and distance between two GPS fixed waypoints rather than rely on the instantaneous indicated ground speed. Speed filtering is notably better on more modern GPS receivers, but is still inherently limited by instantaneous, and essentially random, position variations that can be around the 20m - 30m in each second region. The speed errors are related to ground speed, in that the filtering is intelligent enough to increase the speed averaging time constant at lower speeds. This serves to cause the display to read near zero velocity when stationary, which may fool the unwary into thinking the thing is accurate!
The same goes for height accuracy BTW. Because of the shape of the spherical geometry presented by the best selected Space Vehicle (SV) set for 2D accuracy (which is what commercial sets seek to optimise when selecting available SVs) the vertical errors will always be higher than the horizontal ones.
If anyone has a spare moment, try drawing a scale sketch of the pseudo range tracks from a typical 4 or 5 SV set to a receiver. This will clearly show the vertical geometry problem caused by the relatively low orbital altitude of the constellation. A low orbital altitude is obviously good from the horizontal fix accuracy perspective but is poor for vertical position determination.
Surveyors use phase measurement systems to add position accuracy, but as far as I am aware this technology only works for stationary receivers, as it takes several seconds to average the relative phase from each SV. Similarly Differential GPS (DGPS) corrects the gross errors via a second mean local error transmitting data link, but again this will be the mean position error in that region at the time, not the instantaneous error seen by the receiver, so it doesn't fix the speed error problem effectively.
Join Date: Apr 2001
Location: Germany
Posts: 19
Likes: 0
Received 0 Likes
on
0 Posts
GPS speeds
VP959
AFAIK all but the cheapest GPS receivers (which, I assume, we are not using for flight tests...) use doppler information on the GPS signals (change of frequency due to movements of transmitter or receiver) to compute velocity, or speed, whatever. This doppler shift data is very precise, and usually much more reliable than position data.
ref: I think I read that in Hofmann/Wellenhoff's GPS book, easily to be found on the internet.
AFAIK all but the cheapest GPS receivers (which, I assume, we are not using for flight tests...) use doppler information on the GPS signals (change of frequency due to movements of transmitter or receiver) to compute velocity, or speed, whatever. This doppler shift data is very precise, and usually much more reliable than position data.
ref: I think I read that in Hofmann/Wellenhoff's GPS book, easily to be found on the internet.
SF,
True for the expensive specialist market GPS kit, but not for handhelds or commercial aviation GPS sets, which still just use time and distance to compute velocity.
I know that G (and a fair few other light aircraft FT people) use relatively cheap commercial GPS sets that don't use doppler velocity correction, hence my post pointing out the potential errors.
Having just looked at the specs for a few relatively top end commercial GPS receivers, I noticed that they haven't yet chosen to use doppler measurement in the main. The main growth area for these specialist sets seems to be in the race car instrumentation market of all things.
True for the expensive specialist market GPS kit, but not for handhelds or commercial aviation GPS sets, which still just use time and distance to compute velocity.
I know that G (and a fair few other light aircraft FT people) use relatively cheap commercial GPS sets that don't use doppler velocity correction, hence my post pointing out the potential errors.
Having just looked at the specs for a few relatively top end commercial GPS receivers, I noticed that they haven't yet chosen to use doppler measurement in the main. The main growth area for these specialist sets seems to be in the race car instrumentation market of all things.
Join Date: May 2004
Location: Arlington, TX
Posts: 11
Likes: 0
Received 0 Likes
on
0 Posts
Genghis,
Sorry it takes me so long to get back. I haven't found the data you asked about, but I'll continue to look. The last time the FAA asked me to justifiy it I did not have a calibrated trailing bomb and I used a ground course over a surveyed runway and the data was within .5 knots. It was good enough for them at the time. Of course dealing with the FAA is very personality dependent and next time I may have problems.
Dave
Sorry it takes me so long to get back. I haven't found the data you asked about, but I'll continue to look. The last time the FAA asked me to justifiy it I did not have a calibrated trailing bomb and I used a ground course over a surveyed runway and the data was within .5 knots. It was good enough for them at the time. Of course dealing with the FAA is very personality dependent and next time I may have problems.
Dave
Join Date: Oct 2003
Location: Canberra Australia
Posts: 1,300
Likes: 0
Received 0 Likes
on
0 Posts
Genghis the Engineer
GPS use to measure sideslip angle ?
Do you have a good method?
Guess you would need a stable air mass and distant ground features on which to align.
GPS use to measure sideslip angle ?
Do you have a good method?
Guess you would need a stable air mass and distant ground features on which to align.
Theoretically possible I suppose, but it would assume that you knew with near-absolute certainty the wind vector and the compass deviations in the configuration that you were flying at. That and you could only use it if prepared to hold the beta angle long enough, in steady heading, for the GPS to stabilise. Can't see that working in real life - too many sources of error.
There is a tried and tested method of estimating Beta, by establishing steady heading sideslip, noting the compass heading, then swinging the aircraft back onto balanced flight and noting the new heading - the difference being roughly the sideslip angle. We tend to quote to the nearest 2° or so (giving about a 5° error bound), but realistically I'd not trust it to more than that. Good enough for testing the low speed aeroplanes I usually deal with that have can sideslip 30-35° degrees at full rudder deflection, but far too crude for anything fast that above Va probably won't slip more than 10° except as a very rough and ready guesstimate. Beyond that, my opinion would be that you need to bite the bullet and fit a yaw vane.
G
There is a tried and tested method of estimating Beta, by establishing steady heading sideslip, noting the compass heading, then swinging the aircraft back onto balanced flight and noting the new heading - the difference being roughly the sideslip angle. We tend to quote to the nearest 2° or so (giving about a 5° error bound), but realistically I'd not trust it to more than that. Good enough for testing the low speed aeroplanes I usually deal with that have can sideslip 30-35° degrees at full rudder deflection, but far too crude for anything fast that above Va probably won't slip more than 10° except as a very rough and ready guesstimate. Beyond that, my opinion would be that you need to bite the bullet and fit a yaw vane.
G
