Altitude sensing - Ublox versus SCP1000

I got a SCP1000 absolute pressure sensor and have been looking at the relative performance versus ublox.

Here is the data from my first flight test - ublox is blue, scp1000 is pink. The data was taken while manually flying a SkyFun "pusher jet" in somewhat gusty wind conditions.

There are several things that stand out to me. First is that the scp data is a lot "noisier". Does this actually represent the movement of the aircraft? Perhaps. I cannot really say after this one test.

Second is that the ublox appears to lag badly during fast descents. Seems to keep up going up, but not so much coming down. I suspect this has something to do with the ublox filtering algorithm.

The third thing I see, about which I am the most curious, is that something produced some kind of offset to the pressure at the beginning and end of the flight. If you look closely at the beginning of the graph, you can see that the pressure altitude starts at the same value as the gps altitude (by definition in the software). However, while still on the ground, something causes the pressure altitude to jump down about 22 meters. It bounces back and forth for one period before takeoff. Then at the end of the flight, you can see that the landing altitude is about 22 meters below actual, but then something happens on the ground that causes it to bounce back up around the correct value. This is a real problem. My only theory so far is that putting the canopy on may be trapping some small amount of pressure. However, if this were the case I don't know how the sensor would have the relative level of accuracy it does during the flight. The effect does not appear to be related to airspeed, as the changes noted at the beginning and end were while not in motion.

I also noted that except for the bottom of the "second valley", the effect I noted above, if it is coming and going, could produce the "noise" seen in the pressure altitude. You can see that the peaks of the "noise" correspond fairly well with the gps altitude for much of the flight.

I welcome all thought and comments.....

Views: 1326

Comment by Yves Gohy on April 9, 2010 at 1:57pm
Ah!, here is what to compare. What is the duration of the registration? Can you also posted the raw data. Good job!
Comment by Jack Crossfire on April 9, 2010 at 2:01pm
GPS altitude is a house of pain. When we did this, it sometimes worked & sometimes didn't. It depends on if a satellite is directly overhead, weather conditions, radio interference.

Comment by James F. on April 9, 2010 at 2:01pm
What does your pressure setup look like? Is the tube out in free stream? What direction and location on the plane is the pressure tube located?

Often if the tube isn't located in clean flow, you can get noise from the turbulent air.

Comment by Doug Weibel on April 9, 2010 at 2:18pm
WOW!! The strange "offset" issue is solved!

The SCP1000 hates sunlight! I played with putting the canopy on and off to check for a pressurization issue, but could not produce anything interesting. Then I remembered this -

and the note about direct sunlight. So I figured I'd test it out. My sensor is mounted facing up under a clear plastic canopy. With the canopy off and waving my hand back and forth shading and un-shading the sensor I could make a square wave with a 25 meter swing.

So I guess I need to devise a sunshade. Or spring for the 95 cents plus shipping gasket from Sparkfun.

Yves - the time scale was 3.6 minutes start to finish.

James - the sensor is open to the interior of the cockpit. I looked for airspeed effects but saw none.

Jack - IKWYM - Indoors I see my ublox wander all over the place. Outdoors here in Colorado it seems to do very well. However, I may be flying below the roofline at Sparkfun (if I get really brave) so I was very interested to check out the relative performance.

I will devise a sunshade and get new data.
Comment by Jack Crossfire on April 9, 2010 at 2:21pm
More recent flights had a bit more tuned algorithm than years ago. Main problem is if you want dead accurate altitude, the SCP1000 needs temperature compensation or GPS blending.

Comment by Doug Weibel on April 9, 2010 at 2:29pm
Jack - Can you elaborate on temperature compensation. Do you mean to account for the lapse rate in the altitude calculation, or do you mean to compensate the scp1000 pressure reading based on a bias curve like you would with a gyro? Supposedly the device does the latter itself with calibration data stored in onboard non-vol memory.
Comment by Jack Crossfire on April 9, 2010 at 3:12pm
Obviously the error only happened outside the normal RC regime. It did a good job up to 130m & then it hit a sudden transition into cold air that you'd never encounter in normal use. If you're staying in normal altitudes & not trying to measure relativity with it, it's a good product.
Comment by Michael Zaffuto on April 9, 2010 at 4:31pm
I'm glad you two are discussing this as I respect your opinions on these topics. I'm curious about the relative performance difference between the BMP085 and the SCP1000. I've looked at both datasheets so I have an idea where they differ, but I wanted to be able to put their performance in perspective relative to their cost. As this is a DIY site, cost of components can be a barrier etc. At one major supplier of components, in onesies, the BMP085 is priced at $8.79 ($4.25/3000) on the other hand the SCP1000 is priced at $29.73($24.50/100).
That is over 3 times the the SCP1000 really 3 times better (accuracy wise if both a optimally compensated and with proper curve fitting etc)?

Comment by Jani Hirvinen on April 9, 2010 at 6:28pm
GPS altitude measurement is accurate only on some locations and if US military wants, they can "crash" your plane pressing one button at their GPS control center. I live in Asia area and even when I have 10 or more satellites locked and 3D Fix. My altitude for fixed antenna can jump -+ 70 meters in just matter of minutes. so at least on this area GPS altitude measurement is big NONO.

I've been making those measurement with ublox 5 too. Heres few pics taken from u-console while antenna was stationary on my balcony on 10th floor. One measurement was made on 10 May and another just few days ago. So you should not trust too much on your GPS altitude data.

Comment by Doug Weibel on April 9, 2010 at 6:57pm
@Michael - I had not looked at the BMP085 before, but it looks pretty good. You will note that VTI does not give any accuracy specifications, only resolution. After my testing today I can tell you that the accuracy and resolution differ by at least an order of magnitude, perhaps 2. So I wouldn't get concerned with the "low" accuracy spec on the Bosch - it may actually be better than the VTI. The Bosch is faster too.


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