hevansrv7a

hevansrv7a

Well Known Member
This is a subject that arose in my earlier posting but after I thought about it some more I decided it deserved a new thread. Moderators welcome to over ride me.

I'm not disputing that the 3 leg and 4 leg methods of speed measurement (using ground speed via GPS) are best. But how good is good enough?

If an airplane flies directly into the wind so that the magnetic heading and the GPS track are 5 degrees apart and assuming 180 sm/h or 180 kts, then the difference in speed caused by the angle alone will be 0.69. For three degrees it will be 0.25. This is just basic trig and it doesn't matter how much xwind there is because that's being corrected by the alignment or near alignment of the heading and the track. In other words, if there is more wind you would have to be closer to perfect.

So if you fly upwind and downwind and your magnetic heading and your track are 3 degrees or less apart upwind and then you fly 180 degrees different downwind then you are into an area of precision that would be hard to beat with any method. Of course, this requires a very precise magnetic compass. In the case of the GRT we have that. I would not think a whiskey compass good enough for this.

The GRT calibration routine probably makes the correction for the angle, too, but my point is that it's a minor error in any case.

If you average the upwind and downwind you have the TAS. Example: 210 downwind and 150 upwind is a TAS of 180 and a wind of 30, plus or minus .25 for 3 degrees or .69 for 5 degrees.

Opposing views welcome.
 
On all the runs I made if my final result is within 1 mph of actual I would be very happy.

I have given up on the 3-4 leg GPS runs and now just use IAS to see if any improvement is made. It's close enough for me. I get there when I get there.
 
It is a difficult task

If you are satisfied with your results that is what matters. When you are struggling for a fraction of a knot and want to measure the difference as accurately as possible I think it is better to use a test method that eliminates the variables as much as practical. Testing early in the morning when the winds are dead calm I think is best, otherwise the winds seem to be changing in direction and velocity. I simply could not get the consistent results that I was looking for until I went to the three track NTPS method at 6,000 ft density altitude. Before that my demonstrated error range was 4 kts. with the same configuration. Now it seems to me, it is in the tenths of knots but I have not demonstrated that. Most of my modifications actually slowed the plane down and it was important to me to back them out when the results were negative. With some the change is so small a test method allowing a large error will never see the benefit or the degradation of performance. The two track method is championed by Paul Lipps as well but there is too much talent required to get it right for my liking. I climb to find 6,000 ft density altitude per the www.us-airrace.org handicap procedure then call Razorback Approach to fly a triangular speed test at that altitude. Once approved and a unique squawk code is assigned I turn to 000 close the vents, wide open throttle, max RPM (2720-2730), lean mixture to around 1300 F EGT on cylinder #4 (hottest one), trim for straight and level flight, engage the autopilot for 000 track and engage altitude hole. I wait until the speed stabilizes on the GPS while tweaking the mixture some to see if I get a peak. Then I record speeds at 20 second intervals until I have 5 in sequence that do not vary by more than 1 kt. Once the first leg is done I disengage the autopilot turn to 120 and record the speeds in that direction. Finally, I repeat the speed measurement task with a 240 track. Back on the ground I average the speeds for each leg and plug the result into the NTPS spreadsheet and it gives me a speed which seems very reliable. The entire test flight typically lasts .7 hrs from starting the engine to shutting it down. That is the way I do it and it works for me.

Bob Axsom
 
Which way does it blow?

hevansrv7a said:
But how good is good enough?
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How do you now where the wind is coming from? That seems to be the big hang up. Lets say the wind is ZERO; than one run would be good enough, right. But what contribution is the wind making is always our problem. Even if you did two 2-leg runs, each leg 90 degrees from each other and averaged those, you will end up with more error, depending on winds, which I think is a real problem knowing, unless on the deck with a wind sock right there.

Yes you can average two legs, 180 degree apart if flown directly in and down wind (or close). The math is a simple average of the two separate run ground speeds. You will enter some error, but how much? If you want a quick a dirty yes its OK. It would be interesting to find the winds aloft (read below) and than do both 2, 3 and 4 leg methods and compare results. If you know the winds, you really know them exactly, with some math or trig you can do one run. It's the winds that are our pain.

The 3 or 4 leg methods you alluded to are to resolve not knowing the wind and not relying on IAS indicator at all. That is their charm. The 3 leg constant track method has more complicated math (easy with the spread sheet floating around), but it better or best method in my opinion. It is better to fly less legs since it takes out the variability in the additional legs, and it also takes the wind out of the equation automatically.

A stool needs at min three legs. The rule of three in nature and statistics is nice. Four makes a nice chair but all legs have to be even or it wobbles. The 3 leg method is just one more leg than 2. It is also constant track which is easier to fly, I think. Also if constant heading and wind direction is not constant than your track will vary. To more you control or keep constant the better. Last it does not care what three tracks you choose, its up to you. You can imagine all this was impossible without GPS. (note: depending where the GPS birds are in the sky GPS is not 100% accurate.)

To do the two leg method you have to know winds fairly well. You could calculate winds but it will be another step. If you have track, mag heading, TAS (AS corrected for temp) and GPS ground speed you can figure the aloft winds with a simple E6B. Than you can choose your headings for the 2-leg, 180-degree apart runs method, directly head/tail wind. Youu already alluded to the down side, indications can be off: heading, IAS indicator error and temp/altitude error. The other method is down the runway on the deck both ways. You know length, winds and track. The down side is it's short. The freeway was a much longer run, thus giving better results.

My two way method, involved flying over a strip of straight freeway, which is about 5 miles from an un-towered field with AWOS. The "run" or freeway section is of known length. I can see the mile markers and ground ref points from the air, which I know from driving this stretch of freeway. That is how the HP planes catch speeders, they time it from those markers. Flying down on the deck (500 feet agl), knowing heading, track (both GPS and freeway) and winds from the nearby field, I can do some calibration with a stop watch. It's more of a sanity check and IAS indicator check at higher speeds, from the test at altitude.

The reason for the "advanced" 3 leg method using constant track not heading is consistency and eliminating wind unknown. We really fly tracks now a days with GPS any way, right. We fly point to point direct, which is track. We guess or assume the GPS track is pretty accurate and easier to fly, with the heading as a backup or double check.

The real bugaboo in all methods is you assume the atmosphere is homogeneous at least in layers and winds don't change horizontally. Not true.

I mention double check or sanity check several times, overlapping data or redundant information you can correlate. Even if doing the down low stop watch timing method, where a GPS is not needed, I still use it and record it. When doing 3 legs aloft, constant track GPS method, I'm still measuring and recording IAS, temp, density altitude and BARO. The trick is repeatability. Test over and over at different times. If they are repeatable, your results are within about 1 mph every time, than your confidence goes up. It either means your numbers are close as you can get it. or your making the same mistake over and over again. :D

In the old days before GPS, all you could do is use your IAS and do speed check with that. In that case one run was good. To calibrate the IAS you could fly besides other planes or do low time/dist runs. The fly-side-by method comparing to other planes is still a good sanity check. Once you IAS is calibrated and you have a correction chart, its about as good as it gets and is fairly accurate if you know the Baro and temp accurately.
 
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gmcjetpilot said:
How do you now where the wind is coming from? That seems to be the big hang up. Lets say the wind is ZERO; than one run would be good enough, right. But what contribution is the wind making is always our problem...
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If there is little or no difference between the magnetic heading and the track then you have no XW therefore you are flying either upwind or downwind. That was the main point along with a mathematical measurement of possible error.

If you turn slowly at a steady altitude you will easily see where the two come together and you already had some idea before you took off what the wind would be so that's a good starting point. To repeat an obvious point, you would want a good, smooth magnetometer with digital readout like the GRT, not a whiskey compass.

I agree that other methods are "more" exact. My point is to ask/suggest how good is good enough. If I can get to 0.25 knots I'm happy. I can do that with 3 degrees from perfect. That's what the trig says.

Doing the upwind-downwind test is very quick and easily repeatable and can thus be done with a minimum of concern for changing winds. If you want to do other tests that's OK with me. Heck, someday I might do one and use up almost an hour when five minutes was within half a knot or a quarter of a knot. I'm not that good a "stick" that I can fly precisely for three or four legs better than that.
 
hevansrv7a said:
So if you fly upwind and downwind and your magnetic heading and your track are 3 degrees or less apart upwind and then you fly 180 degrees different downwind then you are into an area of precision that would be hard to beat with any method. Of course, this requires a very precise magnetic compass.
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It is very hard to know exactly how accurate our magnetic compasses are. Are the compass swings we do on the ground at low power completely representative of what we would see at higher power?

And, to further complicate things, the magnetic track output by the GPS is based on true track, corrected with its internal table of magnetic variation vs position. How accurate is that internal table of magnetic variation? Does it account for any local magnetic anomalies? Probably not.
 
Yes and no

Kevin Horton said:
It is very hard to know exactly how accurate our magnetic compasses are. Are the compass swings we do on the ground at low power completely representative of what we would see at higher power?

And, to further complicate things, the magnetic track output by the GPS is based on true track, corrected with its internal table of magnetic variation vs position. How accurate is that internal table of magnetic variation? Does it account for any local magnetic anomalies? Probably not.
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The GRT does a self-correct on the ground with a swing during a special phase. It writes a full 360 degree curve to get all the readings correct.

No, it has no idea about local anomolies, but at least some of them are on the sectional, like in Essex Co., Ontario, for example.

Good points. Don't they apply equally to the 3-leg and 4-leg methods?
 
hevansrv7a said:
Good points. Don't they apply equally to the 3-leg and 4-leg methods?
Click to expand...
It depends on which methods you use. There are methods that rely on flying runs at specific headings - those methods are affected by any errors in magnetic headings. But, I like the methods based on the work done by RV builder Doug Gray. Those methods don't rely at all on the magnetic compass. They only rely on the accuracy of the GPS track and ground speed. The NTPS PEC spreadsheet is a good example of such a method. I like the four leg variation of this method, as the redundant data provides a check on the quality of the test point.

Ideally, we would use true track, so there was no effect of any potential variations in the magnetic field model that the GPS uses to convert true track to magnetic track. But, as near as I can tell, our GPSs don't have any knowledge of local magnetic anomalies, so the magnetic variation model will produce variations that vary smoothly with location. Assuming all legs of the 3 or 4 leg test are done not too far apart, the magnetic variation that the GPS uses won't vary much at all between legs, and the accuracy using magnetic track should be essentially equivalent to that using true track.
 
If you're serious about accurate testing you really need to
do multiple runs and find a big mass of stable air. The more you do the more data scatter you will get. Omit the best and the worst and average the rest. In a perfect world the best method is to find an airplane with the same speed as yours and do side by side testing. If you're like Bob Axom and you enjoy racing multiple turn courses you will go faster by learning how to turn your airplane (turn radius, g loading, turn straight and level?, climb in dive out? dive in climb out? unload before the rollout? lots of stuff to play with. The reality is that when flying a multiship everyone will slow down for the slowest airplane anyway.
Have fun
Tom RV3
 
As I wrote before, with a 200 mph TAS airplane, a 20 mph wind would have to be 90 deg to the ground track to give an error of 1 mph. If you can fly ground tracks on your GPS, not headings, 180 deg apart, and within 30 deg of the wind, you will have less than 1/2 mph error. Use forecast winds and temps in your testing, since many OATs have considerable error which translates into errors in going from IAS to TAS. Convert the winds from true to magnetic and fly ground tracks with and against the wind then average the obtained ground speeds to get your TAS. Your error from this will be mostly your ability to keep altitude constant if you don't have an altitude hold. I've done lots of testing this way, and I always get consistent results.
 
Use the GPS to calibrate the compass

Kevin Horton said:
It is very hard to know exactly how accurate our magnetic compasses are. .........
Click to expand...

When I installed my second D10 to use as an HSI, I calibrated its compass with my GPS. Since the 496 has a 200ms update rate, this is very easy on the ground. Roll a few feet in one direction and you know what the mag heading of the airplane is. This is way more easy than using a compass rose!

I also used it for the in flight portion of the cal in a virtual no-wind situation.

Anyway, for this discussion, make a compass correction table using the GPS on the ground. In the air, you'll know how to correct the compass to be sure you are pointed direct into or away form the wind.

The errors are different when banked into a turn, so you must be going straight for this to work.
 
mansboat said:
When I installed my second D10 to use as an HSI, I calibrated its compass with my GPS. Since the 496 has a 200ms update rate, this is very easy on the ground. Roll a few feet in one direction and you know what the mag heading of the airplane is. This is way more easy than using a compass rose!
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This probably works OK, as long as there isn't anything in the area that affects the local magnetic field.

There are quite a few airports that have all kinds of magnetized stuff underneath taxiways and runways. In my day job, I am privy to numerous reports of problems with compasses on a certain Canadian-built regional jet not agreeing with runway heading at numerous airports. It turns out that the wing is low enough to put the flux valve too close to all the junk, rebar, etc that is underneath the aircraft. This AAIB report provides examples of several airports with local magnetic anomalies.

In theory, whoever sets up a compass rose should be confirming that the local magnetic field is such that it agrees with the lines painted on the rose (see the Australian compass rose guidelines for example). So, if you want to use the GPS to do your compass swing, it still might make sense to do it at the compass rose.
 
Update

I still believe the 2-way method is good enough and Ellipse seems to agree.

That said, I checked my GRT's TAS against the NTPS method using 120 degrees between the three legs and got a much different result. The NTPS number was 6 or 7 kts lower. I then re-calibrated the GRT and it brought the disagreement down to near nothing. I think this means that the method is good but somehow I did not get it right the first time when applying it to the instrument. A possible reason might be that I only had one IAS entry in the GRT's table. Now there are two. I don't know, but perhaps that allows a calculation of a slope that could not be done with only one. The error correction was only 1 kt different but it made a much bigger difference in the TAS window immediately.

I also had a thought about magnetic issues as raised by Kevin. A double check would be to slowly vary the heading back and forth to find the highest and lowest speeds by the GPS. If the headings are right they will already be at the highest and lowest groundspeeds.
 
About GRT TAS Correction Function

I confirmed today with GRT that the function may not work right unless there are a minimum of three entries and it works best if they are at high, medium and low speeds.

For me this means: be more careful in using these fancy new instruments. But it also means that the 2-way speed test is still valid within reasonable limits.
 
I have done a rough 2 way as you describe and "finding the wind" is VERY easy. this is a little different as I don't have the GRT to compare heading and track but the principle is the same. I just let the plane turn in a very shallow bank and watch the ground speed vary. Using the DG, if I'm turning NE to SE and hit my peak at 80 degrees, and that peak holds until 110 degress then I know the wind is coming from 270 degress. Fly the speed run heading 90 degress, turn and fly at 270 degress. Average them. Done.

Finding the wind is FAR more accurate than using the forcast wind and takes literally less than a minute if you have GPS ground speed available.

If the 3 and 4 leg runs are designed to eliminate wind errors then I fail to see how they can make much of a difference in the world of GPS and a good 2 way with KNOWN wind direction.

Magnetic variation doesn't come into play in this and compass error is irrelevant. Unless your DG precesses 90 degress in 10 minutes or less!!:rolleyes:

So IMHO your 2 way with GRT ought to do as well as 3/4 leg runs especially for "close enough". I'll have to go try comparing them now, whats 10 gallons for trivias sake.:eek:
 
To each his own but two observations:

1. I think you guys are missing the point of the *method*. The very elegance of it is that you need ANY three GPS track and G/S readings to give you a solution. The accuracy comes for free. No other instrument or its accuracy enters the equation.

When you think you might want to take a data set - you can use whatever heading you currently have, stabilize - record the data + altitude and OAT for good measure, turn right or left to another convenient heading somewhere around 80 to 130 degrees , stabilize and record T & G/S, another turn in the same direction approx same amount but any heading will do - record the last data point.

Probably done in the time you have sniffed around for the wind direction.

2. In calibrating the GRT ASI using the GPS method you have closed the loop on the instrument. You have positively determined the instrument+pitot+other minor errors and dialled in the corrections and can now use the TAS readout as an accurate measure.

Careful initial calibration will define the error tolerance - or expressed another way the confidence you can apply to that TAS figure.

Doug Gray
 
Doug said:
To each his own but two observations:

1. I think you guys are missing the point of the *method*. The very elegance of it is that you need ANY three GPS track and G/S readings to give you a solution. The accuracy comes for free. No other instrument or its accuracy enters the equation.

When you think you might want to take a data set - you can use whatever heading you currently have, stabilize - record the data + altitude and OAT for good measure, turn right or left to another convenient heading somewhere around 80 to 130 degrees , stabilize and record T & G/S, another turn in the same direction approx same amount but any heading will do - record the last data point.

Probably done in the time you have sniffed around for the wind direction.

2. In calibrating the GRT ASI using the GPS method you have closed the loop on the instrument. You have positively determined the instrument+pitot+other minor errors and dialled in the corrections and can now use the TAS readout as an accurate measure.

Careful initial calibration will define the error tolerance - or expressed another way the confidence you can apply to that TAS figure.

Doug Gray
Click to expand...
Doug, I completely agree with you. If the GRT would use 3-way, I'd do it. The beauty of the GRT calibration is that you only have to do it once and then you can get highly accurate readings of changes with much less time and fuel used.
 
Well, I disagree with David. Doug's method is much easier than the 4-way 90 degree method and the spreadsheet produces very accurate results. I used it to fix a 3-4 knot error with my Safair static ports.
 
To make things even easier, I made a web site calculator version of Doug Gray's method of computing TAS from GPS. Hopefully he doesn't mind. I actually improved on the spreadsheet slightly by avoiding division by small numbers.

Calculator is at: www.dualrudder.com/tas.php
Code is at: www.dualrudder.com/tas.phps in case somebody wants to check my work.

Dave
 
All this disagreement :) Did anyone notice that the "Doug" as mentioned is the same "Doug" the published the original 3 *and* 4 leg method? So, I would suggest, you don't need to disagree with him as you all are espousing using his *methods* :)...

Suffice it to say, if you want to use the 3 leg, do it, if you like the 4, then go for it as well.

In the beginning, I actually followed Kevin's lead and use and most likely will continue to use the 4 leg version, however, I may have to try this 3 leg version just to see how it compares... I've got lots of 4 leg data

Just thot I'd point the above out...
 
Davepar said:
To make things even easier, I made a web site calculator version of Doug Gray's method of computing TAS from GPS. Hopefully he doesn't mind. I actually improved on the spreadsheet slightly by avoiding division by small numbers.
Dave
Click to expand...

Dave,
This is a great little tool, thanks for putting it together and also sharing the script. I for one will use it.

Only comment - the 'units' part may confuse.

I have not tested it myself - I'll leave that for others. I have an instrument panel to wire and just received some shielded cable in the mail, so that's what I'll be doing.

Regarding the 4 track variation on the *method* I have not entered the debate. Kevin's spreadsheet employs this additional leg to validate the remaining data points and I believe his motivation is from his actual flight test experience. Often when the rubber hits the road different priorities are in play. I am not a test pilot and deeply respect Kevin who is.

Furthermore the data I have seen using Kevin's variation has been nothing short of remarkable in terms of the precision and repeatability.

From my perspective as a statistician and engineer I have trouble with how to deal with the data set with a single outlier. Do you use the remaining 3 points, do you discard all 4 points, do you average... In reality I expect to see scatter in data like this and usually we cannot decide on a single point if an outlier is 'normal' scatter or not.

If I look at the 4th point as a quality control sample - it seems a small price to pay for the confidence it returns.

Great to see people using the *method*.
Doug Gray
 
Doug said:
Regarding the 4 track variation on the *method* I have not entered the debate. Kevin's spreadsheet employs this additional leg to validate the remaining data points and I believe his motivation is from his actual flight test experience. Often when the rubber hits the road different priorities are in play. I am not a test pilot and deeply respect Kevin who is.
Click to expand...
Actually, I did not develop that spreadsheet, nor did I create the four leg variation on your method. All that work was done by the National Test Pilot School. I happened to be done there for two weeks taking a refresher course, and I wasn't happy with the methods they used to determine TAS from GPS data. I told them about your method - they looked into it, and decided it was the best method yet for determining TAS based on GPS data. They then came up with the clever four leg twist, and put all their work in a spreadsheet.

The four leg variation method works like this:

  1. You gather data on four legs, with the legs roughly 90 degrees apart, although the exact track difference between the legs doesn't matter.
  2. You only need data from three legs to calculate a TAS, so you do four calculations, dropping data from one of the four legs out of the calculation each time.
  3. If the air conditions were good, and the test was accuratly flown, the data was correctly recorded, the four TASs you calculated should be essentially identical. But, if the data is bad (e.g. you copied a number down wrong, or the wind changed, etc), the four TASs will not be the same. In this case you simply throw all the data from all four legs away, and repeat the test on another day. The NTPS spreadsheet calculates the average and standard deviation of the four calculated TASs. If the standard deviation is small, that means you have good data.
  4. If you use the three leg method, it works perfectly well if you have good data, and the wind didn't change. But there is no way to tell if there is a problem, other than the fact that this calculation doesn't match the results from other flights.
 
Kevin
That is a very nice explanation of why the four leg test is preferable to the three leg system. I collected data using the three leg system for quite a while and I often wondered at what appeared to be inconsistencies. This meant that you never really knew if it was the test or the change that I had made to the airplane. Multiple runs are required for accurate data but at least using this four leg system I would have more confidence in a particular flight. It would only add a few minutes to each flight to collect the extra run so the additional cost would be negligible for the piece of mind that what you had collected was accurate.
The big mistake that I made was that I did not allow for differences in power between runs on different days. I always set myself up at 8000 p. altitude and wide open throttle at 2500 rpm. The problem is that one day the temperature could be 10C the next day or month -10C. Manifold pressure also varied for any given day. This makes a huge difference in the actual power used and as such the differences made much of the data I had collected over a couple of years of not much value for comparison purposes. The next time I start to collect data I will fly my tests at 6000 p. altitude so that I can make changes to the manifold pressure to take differences in temperature into account as it relates to power. Paul Lipps has spoken often about the errors in OAT probes and I am going to try some different locations for the probe to see if I can get better readings than I do currently.
 
A little more data

Today I tested WOT speed at about 75% and got 175.3 as compared to last time's 175.4, both using NTPS spreadsheet, 3 autopilot legs 120 GPS degrees apart. While I was at it and still using the autopilot I added a calibration point to the GRT and it says 175 (it used the two-way method). The results are actually closer than they have a right to be, since altitude, wind, mixture and temperature were not identical. This test was at a slightly higher actual altitude and 1-2% less power.

Conclusion 1: If you want to be really certain, use NTPS or some other multi-leg method you believe in.

Conclusion 2: If you want to know, while you are up there, how fast you are going in the air, use your GRT which was previously calibrated per maker's instructions. It is good to know the GRT agrees with the multi-leg method.

Conclusion 3: If, while you are flying, you suspect your instrument, use your GPS, find the fastest and slowest speeds at a given altitude and average them.

BTW - I also spent a few hours checking my OAT on the ground and it appears to be within 1 degree F of perfect - my facilities cannot do better than that as one of my check-references is the airport ASOS which reports in Deg C.
 
Is there a correction for air density?

I'm struggling a bit with the IAS calibration concept. I understand this is a good comparitive analysis, but does this actually check the IAS accuracy? If this is performed at 6000' pressure altitude, how is the pressure compensated for so the IAS is the same as it would be at ground level, where the GPS ground speed is being measured?

It seems that the IAS would be different at different altitudes since there is no compensation for the air density. Can someone explain this to me? If I use the procedure (3 or 4 leg) and the NTPS calculator, it appears to me that I would be calculating the IAS at ground level, but would need to compensate for recording IAS at the actual pressure altitude (6000' in this example).
 
Help with air density, etc.

trib said:
I'm struggling a bit with the IAS calibration concept. I understand this is a good comparitive analysis, but does this actually check the IAS accuracy? If this is performed at 6000' pressure altitude, how is the pressure compensated for so the IAS is the same as it would be at ground level, where the GPS ground speed is being measured?

It seems that the IAS would be different at different altitudes since there is no compensation for the air density. Can someone explain this to me? If I use the procedure (3 or 4 leg) and the NTPS calculator, it appears to me that I would be calculating the IAS at ground level, but would need to compensate for recording IAS at the actual pressure altitude (6000' in this example).
Click to expand...

This is easier to visualize if you look at the two scales on the E6B circular "computer". The same CAS will give you a higher TAS as you go up in density altitude. However, the difference between IAS and CAS is unique to the airplane and will vary at different speeds (and angles of attack), too.

Go to this link and select any of the spreadsheets. In the spreadsheet you will find a page (tabs at bottom) you can use for calibrating your IAS to CAS. The title of the page is "IAS-CAS". The page will take care of the air computations. You just have to supply the raw data as precisely as you can. Feel free to PM or email me if you have any difficulty using it.
 
trib said:
I'm struggling a bit with the IAS calibration concept. I understand this is a good comparitive analysis, but does this actually check the IAS accuracy? If this is performed at 6000' pressure altitude, how is the pressure compensated for so the IAS is the same as it would be at ground level, where the GPS ground speed is being measured?

It seems that the IAS would be different at different altitudes since there is no compensation for the air density. Can someone explain this to me? If I use the procedure (3 or 4 leg) and the NTPS calculator, it appears to me that I would be calculating the IAS at ground level, but would need to compensate for recording IAS at the actual pressure altitude (6000' in this example).
Click to expand...
Big picture description:
  1. Fly multiple legs at same IAS and altitude. Record GPS data, IAS, altitude and OAT.
  2. Use GPS data to calcualate TAS.
  3. Use TAS, OAT and altitude to calculate CAS.
  4. In a perfect world, IAS = CAS. The difference between the recorded IAS and the calculated CAS is the error in the airspeed indication system.
  5. If you do some ground testing to determine the ASI instrument error, you can know how much of the airspeed indication system error is due to instrument error, and how much is due to other static source position error.
There are many small details I didn't mention. The whole story was published as a two part series in Kitplanes a while ago. A somewhat old, shorter version is on my web site here
 
Thanks Kevin and H., that clears it up. I've already done the ground checks with a manometer and was within 3 mph on both the dynon and analog ASI I have installed.
 
TAS runs

I totally agree! Even if you have a 90 degree crosswind of 20 mph and you fly two ground tracks 180 degrees apart at 200 mph and average your groundspeeds, your error will be only 1 mph. The nice thing about the two-run method is that you have the results in hand and you can repeat the runs for consistency. I challenge anyone to get results within 1 mph unless they use an autopilot and altitude hold and have a very stable atmosphere and are flying over smooth ground with no up or down drafts. I've seen the results people have posted using the three and four leg methods and there is usually plenty of skew in the data, plus they didn't get the results until they got back on the ground and got to their computer unless they had a companion with a laptop!
 
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