Combining/Fusing GPS, Pressure, Height data

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I have a RC model airplane, currently carrying  a RC receiver, 900MHz radio
modem, temperature sensor, pitot and static pressure sensors, batteries,
GPS, and a mini video camera. I'm also planning to add an accurate
altitude/hight sensor to measure airplane's height above the ground during
the final landing approach.(I could not find an accurate range sensor yet).

When there are different sensors which provide the same type of data with
different accuracy/uncertainities, how we can combine/fuse these data to
improve the accuracy ie reduce the uncertainity of the measurement. For
example, both the static pressure sensor and GPS produce altitude data. If I
can add another more accurate sensor to be used on final leg of he landing
approach I will have one more sensor producing the altitude/height data. So
3 sensor generating same type of data (height/altitude).

I can code a simple "if then else" logic into my flight controller to switch
from one sensor to another depending on the airplane's altitute. But I'm
sure there must be better methods to utilize, fuse the altitude/height data
from the static pressure sensor, GPS and planned new height sensor instead
of having "if then else" kind of swithcing code.

Do you know how should I fuse the altitude/height data from the static
pressure sesor, GPS and new height sensor so that I can obtain the best and
most realistic height value for my RC model airplane?

Thank you

Falco



Re: Combining/Fusing GPS, Pressure, Height data


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Your "if then else" logic is probably the best way to go, actually.  In
order to advantageously fuse sensor data like this you either need
sensors whose errors are very close, or you need sensors whose error
have markedly different behaviors over time.

As an example of the latter you can get phenomenal improvements in
system performance by using a GPS, three gyroscopes and three
accelerometers, because the GPS tends to have very good accuracy over a
long period and the gyro/accelerometer package has good accuracy over
short periods.  Merging these disparate measurements into one sensible
whole takes a great deal of work by someone with a pretty high-level
knowledge of the subject, and adequate gyro/accelerometer packages are
pretty expensive.

--

Tim Wescott
Wescott Design Services
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Re: Combining/Fusing GPS, Pressure, Height data



| Do you know how should I fuse the altitude/height data from the static
| pressure sesor, GPS and new height sensor so that I can obtain the best and
| most realistic height value for my RC model airplane?

Well, GPSs are a good deal less accurate giving the altitude than they
are giving the XY coordinates of something, so an error of 50 feet or
so should not be unexpected, and that's probably not acceptable at all
for landing, though in practice the accuracy may be low but the
precision (over a short period of time) should be higher, so it might
not be quite so bad.

In case the terms are not familiar, accuracy refers to many
measurements all averaging out to the correct value -- the closer the
average is, the more accurate it is.  Precision refers to many
measurements all coming close to each other, nevermind how far they
are from the correct value.  You can be very precise and very
inaccurate and vice versa.

The pressure sensor should be a lot more accurate (both precise and
accurate) than the GPS altitude.  Errors of only around a foot aren't
uncommon at all.

(Of course, it's accurate because you calibrate it when you turn it on
-- this reading corresponds to zero feet above ground.  But it's very
precise as well, with the precision decreasing over time as the
pressure and temperature change.)

And your new height sensor is likely to be the most accurate and
precise of all three, and shouldn't vary depending on the weather.

So, the solution is pretty obvious ...

Don't use the GPS altitude at all, unless you have to.  (But go ahead
and use the XY coordinates provided by GPS, because you're not likely
to find anything better, at least not without spending lots of weight.)

Use the pressure sensor altitude when you're too high to use your
height sensor, and ...

Use your height sensor for landing and any other time when you're
close to the ground and high precision matters.

(Your height sensor may become inaccurate if you're banking -- it will
depend on exactly how it works.)

So, it looks like if/then/else.

--
Doug McLaren, snipped-for-privacy@frenzy.com
You're getting old when you get the same sensation from a rocking
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Re: Combining/Fusing GPS, Pressure, Height data


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since He's already got a radio link to the aircraft differential GPS may be
an option, that'll give a significant improvement in positioning,
prolly still need the gyros to track attitude etc.

Bye.
   Jasen

Re: Combining/Fusing GPS, Pressure, Height data


  > since He's already got a radio link to the aircraft differential GPS may be
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    Now that would be useful if you could do it with lightweight gear,
because, with differential GPS, the error approaches zero as the
mobile unit approaches the pseudolite.  Which, of course, is why
the psuedolites for airports are near the runway end, the one place
where an aircraft really needs very precise vertical position.
Autoland for model aircraft would be a very nice feature.

                John Nagle

Re: Combining/Fusing GPS, Pressure, Height data


        The barometric altimeter is actually accurate to within about
20 feet, but getting a reliable static air pressure on a moving
airplane is the hard part. In the fullscale world, the designer spends
some time messing with different locations on the airframe, and with
different port syles, and even then there's some error. Varying
barometric pressure and temperature are bigger problems but can be
calculated out.
        My son has a Magellan handheld GPS that is accurate to within a
couple of feet in altitude. We've tried it t known altitude benchmarks,
and if he holds it in his hand, gets an altitude, then moves it up a
foot, the thing will detect that change. The accuracy and repeatability
is astounding and is better than any of the five GPS nav units we have
in our real airplanes.  The problem with the handheld is their update
rate of only once or twice per second.
        The ultrasonic tape measures wouldn't work so well in a model,
I think. The roughness of the surface would absorb or scatter too much
of the signal.
        
        Dan


Re: Combining/Fusing GPS, Pressure, Height data


If you need accurate altitude for an autoland system, why not use what we
used at NASA for landing on the moon and for the Shuttle autoland, and that
is a landing radar.  Moving from radar you can use a system that uses the
same mechanism used in some auto focus cameras which is an infrared sensor.
Many of the electronic hobbyist and robotics books and mags have circuits
for electronic distance measurements.  If you use pressure, GPS, and
infrared distance (which is affected by airframe attitude) you will need an
algorithm to mix these values together to solve for altitude.  Maybe giving
more weight to the infrared data as you approach the ground.  It is an
interesting project.

Also you can mount the altitude sensor from below the model on a two axis
gimbal that always points the sensor straight down.  It is driven by two
servos being fed signals from an FMA Co-Pilot system.  I saw a write up on
this in a recent AMA Model Aviation mag article, except it was used to keep
an aerial camera pointed in a certain orientation.  Neat idea.


<Falco> wrote in message
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Re: Combining/Fusing GPS, Pressure, Height data


You can find cheap low range (30-40 ft) electronic tape measures at most
hardware or building supply stores. I believe they operate using ultrasonic
sound pulses. At least mine does. They are very directional and accurate to
about 1 inch.
To my mind the way to do this would be to calibrate a pressure altimeter to
0 ft against the U/S one on the ground prior to take off. The pressure
altimeter shouldn't change very drastically over the span of one flight so
should be used for any reference above 40 or 50 ft. . You could also create
some kind of averaging algorithm to monitor the GPS altitude and use that to
check for drastic pressure changes. For example flying through a weather
front. You don't indicate what sort of distance or time you plan to fly so
this may not be required. If you do use a GPS, make sure it supports WAAS.
This will increase the accuracy of both Lat/Long and Alt. a lot.
HTH - Sounds like an interesting project

<Falco> wrote in message
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Re: Combining/Fusing GPS, Pressure, Height data


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Barometric pressure will never tell you how far above the ground you
are, only how far above sea level you are, based on standard conditions
of course.   That's assuming you have adjusted it properly for local
pressure variations.  Even then you would never want to rely on it for
anything finer grain than say 50-75 ft.  You need to overlay that data
on top of a terrain map to tell how far you are above the ground, and
for it to be useful for navigation.

If you are in an aircraft and flying a precision GPS approach, you will
use the readout from your barometric altimeter until you descend down to
roughly 200' AGL (varies by approach and runway).  At that point you are
expected to see the runway environment (lights, runway marking, etc.)
and use your eyeballs.  If you do not see anything, you go around and
try again or go elsewhere.

I would focus on the sensor that gives you the most reliable indicator
of your current situation.  I would follow this approach:

1. Using DGPS and runway elevation data, determine a 3 degree glide
slope intercept waypoint for final leg of approach.
2. Descend to pattern altitude using Barometric altimeter and DGPS data.
3. Enter pattern using DGPS data
4. Fly landing pattern using DGPS data.  Follow pattern until you reach
the glide slope intercept way point.
5. Begin descent (setup landing configuration)
6. Start looking at radar or laser height data.
7. Use DGPS altitude data until sensor starts picking up the ground.
8. Use DGPS data for runway centerline alignment and laser/radar sensor
for height indication.  Control airspeed, power, etc as needed to stay
on glide slope and proceed to landing.

Don't use the altimeter below 75 ft. or the DGPS altitude data below 6
meters.  You will be in the margin of error.  This makes your decision
pretty simple.

Shawn

Re: Combining/Fusing GPS, Pressure, Height data


Other posters have replied with valid techniques for the problem you are
trying to solve, but your original question about "data fusion" made me
think of the Kalman Filter algorithm which is used in high-end INS systems
to achieve similar types of results to what you are looking for (integrate
different sensor with different error models).  It is complex math and may
be overkill for your application, but if you're interested, do a Google
search for "Kalman Filter".

Regards, Tristan.


<Falco> wrote in message
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