In our quest to create the simplest and cheapest autopilot available, we're debating whether we need a barometric pressure sensor or can do reasonable altitude control with GPS alone. The old answer to this was the GPS alone wouldn't work--not only were errors of as much as 10s of meters, but glitching could leave you without any altitude data at all for many seconds at a time. But the newer SiRF III chipsets are much better at altitude calculation, and the even newer yet uBlox5 GPS module is almost as good as a pressure sensor at altitude (it's too expensive for our purposes, but a glimpse of the way things are going). So let's look at that question agan: do we need a pressure sensor?
The way to tell is to test a pressure sensor head-to-head against GPS under varying condition. Here's a paper that did just that, using the older (and worse) SiRF II chipset. A sample of the data is the graph above, which as you can see shows errors of as much as 60m in some cases. Needless to say, that would be unusable. (It turns out that many of those errors were due to GPS lag, and shifting the GPS line forward by 13 seconds improves the fit considerably. But if you're trying to fly on that data, you're stuck in real time, so those sort of corrections don't help.)
Jack Crossfire has been testing more modern GPSs, including both the SiRF III-based EM406 that we use and the uBlox5, and his data is more encouraging. A sample post on EM406 data is here, and a chart from that post follows:
Unfortunately this isn't calibrated so there are no units on the scale, and we can't calculate the absolute error. (Jack, any chance you still have that data and can oblige?). But given that the data was taken in a heli and he's probably flying at around 30m, my sense is that we're looking at errors of less than 6m and no crazy glitches.
If so, that's within the usable range for us. We're typically looking at holding altitude around 50-100m, so single-digit variation is acceptable. It's not perfect, but doing without a pressure sensor would save us about $25 on the autopilot (keeping it below the magic $100 figure), and simplify calibration and instrument-compartment design (no need to pressure-isolate it).
I've got a pressure sensor coming and will do a proper head-to-head with the EM406 and the surface-mount EM312 (which will be built in to the autopilot on the production version) under various conditions before making the final decision, but for now we're betting on GPS being good enough (and getting better) to do the trick. Anybody disagree?
I have used both GPS and barometric altitiudes for topographic surveys for water resources studies in remote parts of Africa. The barometric altimeters tended to provide results which were repeatable and these were corrected to allow for changes of atmospheric pressure during the day by having a second altimeter at the base station (back at the hotel!) But these corrections were minor if I recall and usually of the order of less than 2m.
Ian, I'd be interested to know more about the PIR system but to log heights rather than as an altitude correction. I have been looking into this but haven't found suitable rangefinder that is light, cheap and accurate. My plan was to log barometric altitude and simultaneously log height using a range finder. This could be used for feasibility level topographic surveys in remote areas that might be accurate to +/- 1.5m.
I've done lots of tests with the SiRF III, and the altitude is accurate enough, provided it's tracking enough satellites. In my testing I just hooked up the GPS to the PC, and collected a load of data, and then post-processed the altitude.
In my experiance in the airforce with various camera pods and mapping cameras attached to a pod on board an mirage jet in the 70,s, we first recorded the height before taking the frame.
If your intention is to map then use more distance between earth and craft avoiding the obstructions problem and adjust your focus on your camera using trig to counter alt diffrence.
IE take alt reading first ,store and then take frame noting frame to alt.
Reduce or enlarge sections to be laced together using the data stored.
My experiance with barometer alt systems is the low arructe figures.
As said by Jack Crossfire pressure affects these devices.
What height above ground are you operating at?
I will show you how to modify a microwave pir to do the job as long as it is not to high(-+ 200 metres)
i will be playing with the eb-85a and alt, this summer i have a beromerter to compare with. in the near future gps will replace most sensors ie.. speed , alt. , and imu. and fyi cloud caps AP uses GPS to calibrate the barometer every so often .
Well you really can't get anything except delay & rate of drift out of a GPS vs barometer test. For sensor testing, our best results came from video. Onboard video can show exactly how bad the IMU is. Ground video can show exact altitude.
You raise a good point that there's no need for 1m accuracy--we're just trying to maintain altitude within a band. If that band is +-5-7m that's fine, but if it's +- 20m that's not okay (for reasons from possible obstructions and difficulty in stitching imaging from different altitudes) . Worse of all would be data dropouts. I'm just trying to confirm that SiRF III can reliably get us <10m errors and no dropouts.
Why the need for accurate alt.
If it is to accurate then your uav and system is going to chase the height.
In real flight as a pilot the craft is going to rise and fall due to turbelance and other factours but on average will remain constant.
You will have trouble trying be accurate within at lest 15%.
i feel gps is more than accurate in the time span of things.
The only other method is a microvave alt meter
You can make one from the many security Pir dectors in the market
more on that at a late date.
As far as how fast the barometer drifts from GPS over time, there's no record of that. People usually blend their barometer height with GPS to keep it from drifting.
Comments
Ian, I'd be interested to know more about the PIR system but to log heights rather than as an altitude correction. I have been looking into this but haven't found suitable rangefinder that is light, cheap and accurate. My plan was to log barometric altitude and simultaneously log height using a range finder. This could be used for feasibility level topographic surveys in remote areas that might be accurate to +/- 1.5m.
Mike
If your intention is to map then use more distance between earth and craft avoiding the obstructions problem and adjust your focus on your camera using trig to counter alt diffrence.
IE take alt reading first ,store and then take frame noting frame to alt.
Reduce or enlarge sections to be laced together using the data stored.
My experiance with barometer alt systems is the low arructe figures.
As said by Jack Crossfire pressure affects these devices.
What height above ground are you operating at?
I will show you how to modify a microwave pir to do the job as long as it is not to high(-+ 200 metres)
You raise a good point that there's no need for 1m accuracy--we're just trying to maintain altitude within a band. If that band is +-5-7m that's fine, but if it's +- 20m that's not okay (for reasons from possible obstructions and difficulty in stitching imaging from different altitudes) . Worse of all would be data dropouts. I'm just trying to confirm that SiRF III can reliably get us <10m errors and no dropouts.
If it is to accurate then your uav and system is going to chase the height.
In real flight as a pilot the craft is going to rise and fall due to turbelance and other factours but on average will remain constant.
You will have trouble trying be accurate within at lest 15%.
i feel gps is more than accurate in the time span of things.
The only other method is a microvave alt meter
You can make one from the many security Pir dectors in the market
more on that at a late date.
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