Thoughts on GPS and altitude measurement
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Posts: 46,084
The typical GPS measures about +-30 feet in the XY direction on
average and double that for the Z direction. I noticed several GPS
manufacturers including Garmin are using altimeters instead of
relying on the reading from the GPS unit. After a bit of research I
found Motorola's MPXA6115AC6U pressure sensor which is used in OEM
altimeters. I've been playing with it and the darn thing is pretty
accurate about +- 15 feet. The millivolt reading is proportional to
kpa which can then be correlated to altitude. I have yet to check the
effects of a high pressure front of weather moving in. Anyway just
some thoughts for those who might be interested in altitude
measurements.
average and double that for the Z direction. I noticed several GPS
manufacturers including Garmin are using altimeters instead of
relying on the reading from the GPS unit. After a bit of research I
found Motorola's MPXA6115AC6U pressure sensor which is used in OEM
altimeters. I've been playing with it and the darn thing is pretty
accurate about +- 15 feet. The millivolt reading is proportional to
kpa which can then be correlated to altitude. I have yet to check the
effects of a high pressure front of weather moving in. Anyway just
some thoughts for those who might be interested in altitude
measurements.
Comments
What if you were to tie that reading to the local airport barometric
reading? You could then compensate for that cold front. Over time
and leaving the sensor in a single location of know altitude you
should be able to work out a formula. I have no clue HOW accurate it
could be but the airports update hourly and the data is readily
available in XML format for integrating into a calculation of some
kind. Now if you are out in the boonies, just get the reading off
the radio and then enter it into via two push buttons. Barometric
pressure doesn't vary a whole lot and you can easily use one button
for up and the other for down until you have the correct amount on
the LCD screen.
Weather is facinating. I highly recommend taking a short aviation
weather course at a local community college as I did. It was a
blast! Free tutorial on weather by Univ of Illinois is at
http://ww2010.atmos.uiuc.edu/(Gh)/home.rxml
The multimedia CD is only $7.00 USD
Finally, for you weather buffs that want to make your own weather
station using a Basic Stamp, browse a textbook that I got once where
the author goes through the process of designing and building a home
weather station and the challenges. He used a different processor
and the C language but round about page 254 he starts in on the
concept, design, program and test phases of the weather monitor.
I don't want to offend our hosts so look up the book at amazon as
ISBN 1-4018-1206-6
http://www.amazon.com/exec/obidos/tg/detail/-/1401812066
--- In basicstamps@yahoogroups.com, "Eric Berg" <khufumen@y...> wrote:
> The typical GPS measures about +-30 feet in the XY direction on
> average and double that for the Z direction. I noticed several GPS
> manufacturers including Garmin are using altimeters instead of
> relying on the reading from the GPS unit. After a bit of research I
> found Motorola's MPXA6115AC6U pressure sensor which is used in OEM
> altimeters. I've been playing with it and the darn thing is pretty
> accurate about +- 15 feet. The millivolt reading is proportional to
> kpa which can then be correlated to altitude. I have yet to check
the
> effects of a high pressure front of weather moving in. Anyway just
> some thoughts for those who might be interested in altitude
> measurements.
I have a Bario altimeter that can measure within about +/- 4 feet or so. I'm
not sure what sensor it uses, and I don't want to take it apart, as it is a
nice and expensive unit. Moving weather fronts, especially at higher
altitudes, can make a big differnce. I was on top of Mount Shasta (14,126
feet) with a big front moving in, and the altimeter suddenly gained about
150 feet. A barometric altimeter must be calibrated before a climb and
interpreted. Mine has a some cool features to help compensate for pressure
changes. However, it still beats the pants off of a GPS unit. I never bring
my GPS climbing.
Jonathan
www.madlabs.info
Original Message
From: "johncouture" <mcu@j...>
To: <basicstamps@yahoogroups.com>
Sent: Saturday, June 05, 2004 2:06 AM
Subject: [noparse][[/noparse]basicstamps] Re: Thoughts on GPS and altitude measurement
> Eric,
>
> What if you were to tie that reading to the local airport barometric
> reading? You could then compensate for that cold front. Over time
> and leaving the sensor in a single location of know altitude you
> should be able to work out a formula. I have no clue HOW accurate it
> could be but the airports update hourly and the data is readily
> available in XML format for integrating into a calculation of some
> kind. Now if you are out in the boonies, just get the reading off
> the radio and then enter it into via two push buttons. Barometric
> pressure doesn't vary a whole lot and you can easily use one button
> for up and the other for down until you have the correct amount on
> the LCD screen.
>
> Weather is facinating. I highly recommend taking a short aviation
> weather course at a local community college as I did. It was a
> blast! Free tutorial on weather by Univ of Illinois is at
> http://ww2010.atmos.uiuc.edu/(Gh)/home.rxml
>
> The multimedia CD is only $7.00 USD
>
> Finally, for you weather buffs that want to make your own weather
> station using a Basic Stamp, browse a textbook that I got once where
> the author goes through the process of designing and building a home
> weather station and the challenges. He used a different processor
> and the C language but round about page 254 he starts in on the
> concept, design, program and test phases of the weather monitor.
>
> I don't want to offend our hosts so look up the book at amazon as
> ISBN 1-4018-1206-6
> http://www.amazon.com/exec/obidos/tg/detail/-/1401812066
>
>
>
>
>
>
>
> --- In basicstamps@yahoogroups.com, "Eric Berg" <khufumen@y...> wrote:
> > The typical GPS measures about +-30 feet in the XY direction on
> > average and double that for the Z direction. I noticed several GPS
> > manufacturers including Garmin are using altimeters instead of
> > relying on the reading from the GPS unit. After a bit of research I
> > found Motorola's MPXA6115AC6U pressure sensor which is used in OEM
> > altimeters. I've been playing with it and the darn thing is pretty
> > accurate about +- 15 feet. The millivolt reading is proportional to
> > kpa which can then be correlated to altitude. I have yet to check
> the
> > effects of a high pressure front of weather moving in. Anyway just
> > some thoughts for those who might be interested in altitude
> > measurements.
>
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
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Body of the message will be ignored.
>
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>
>
>
>
>
>
Warning.
If you are going to use gps to measure altitude then plan to use 2
units compared together.
One unit at a benchmark or other accurate altitude point. Then adjust
your x altitude unit by the difference between the benchmark unit and
the actual benchmark value.
Department of defense forces the greatest error in the gps to civilian
use, in the altitude reading.
Ie. The warden of a local lake read his gps alt and drove around the
lake and in less than one hour on returning, read his gps again. The
reading was over 1000 feet different from what it was when he left the
exact same parking space at his office.
Unless you are realy going hi-tech and using d g p s then play it safe.
73
spence
Original Message
From: "BOYD S.(SPENCE) MINER" <k4kep@b...>
To: <basicstamps@yahoogroups.com>
Sent: Saturday, June 05, 2004 2:12 PM
Subject: [noparse][[/noparse]basicstamps] Re: Thoughts on GPS and altitude measurement
> EXCUSE THE CAPS.
>
> WARNING.
>
> IF YOU ARE GOING TO USE GPS TO MEASURE ALTITUDE THEN PLAN TO USE 2
> UNITS COMPARED TOGETHER.
>
> ONE UNIT AT A BENCHMARK OR OTHER ACCURATE ALTITUDE POINT. THEN ADJUST
> YOUR X ALTITUDE UNIT BY THE DIFFERENCE BETWEEN THE BENCHMARK UNIT AND
> THE ACTUAL BENCHMARK VALUE.
>
> DEPARTMENT OF DEFENSE FORCES THE GREATEST ERROR IN THE GPS TO CIVILIAN
> USE, IN THE ALTITUDE READING.
>
> IE. THE WARDEN OF A LOCAL LAKE READ HIS GPS ALT AND DROVE AROUND THE
> LAKE AND IN LESS THAN ONE HOUR ON RETURNING, READ HIS GPS AGAIN. THE
> READING WAS OVER 1000 FEET DIFFERENT FROM WHAT IT WAS WHEN HE LEFT THE
> EXACT SAME PARKING SPACE AT HIS OFFICE.
>
> UNLESS YOU ARE REALY GOING HI-TECH AND USING D G P S THEN PLAY IT SAFE.
>
> 73
> SPENCE
>
>
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
>
> Yahoo! Groups Links
>
>
>
>
>
Original Message
>From: SB [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=NC5sdw5P-pFCvL726_Z725iF_ap5w7ez3am98lQrd-Sk_izLSPOmT6eM7MVhOllCQgCQ7Eoz8aG672Ik]steve.brady@r...[/url
>Does WAS or WASS?? affect altitude at all?
Yep, but it's still not terribly accurate. I wouldn't count on anything
better than 30-50 meters, even with 3 meter WAAS accuracy.
BTW, it is *WAAS*, an acronym for Wide Area Augmentation System.
- Robert
I used to run an Etrex and I could get down to about 9ft (3m) accuracy --
well, that's what the display told me!
I'm always surprised when ppl say their GPSs' are so far off!
Original Message
From: "Robert Ussery" <uavscience@f...>
To: <basicstamps@yahoogroups.com>
Sent: Saturday, June 05, 2004 10:29 PM
Subject: RE: [noparse][[/noparse]basicstamps] Re: Thoughts on GPS and altitude measurement
>
>
> >
Original Message
> >From: SB [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=TVZJ-SsgYqbplHA7bJKEKj1RJphDd-MLJihboFpwxEF2dPfKmqvbpdQrtk6e6XPSByy-wq4TI4fnRnYAB0GJ]steve.brady@r...[/url
>
> >Does WAS or WASS?? affect altitude at all?
>
> Yep, but it's still not terribly accurate. I wouldn't count on anything
> better than 30-50 meters, even with 3 meter WAAS accuracy.
>
> BTW, it is *WAAS*, an acronym for Wide Area Augmentation System.
>
> - Robert
>
>
>
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
>
> Yahoo! Groups Links
>
>
>
>
>
>Haha...figures...it's the one abbreviation combination I didn't try!
>
>I used to run an Etrex and I could get down to about 9ft (3m) accuracy --
>well, that's what the display told me!
>I'm always surprised when ppl say their GPSs' are so far off!
>
>
>
I have an Etrex with WAAS, and regularly get 14-15 ft accuracy (~3m).
Unfortunately, this is only lateral accuracy. The Z axis is never as
accurate as the X and Y axes. I've seen my GPS jump up and down 30 feet
in indicated altitude, and it is rarely more accurate than 100-150 feet
in altitude between uses.
- Robert
> DEPARTMENT OF DEFENSE FORCES THE GREATEST ERROR IN THE GPS TO
CIVILIAN USE, IN THE ALTITUDE READING.
Not so, Selective Availability (the DOD's wandering error set into
the GPS signal) has been turned off since 2000. Any errors you see
now in GPS are just it's natural inaccuracy.
>IF YOU ARE GOING TO USE GPS TO MEASURE ALTITUDE THEN PLAN TO USE 2
UNITS COMPARED TOGETHER. ONE UNIT AT A BENCHMARK OR OTHER ACCURATE
ALTITUDE POINT.
Sounds like a home version of DGPS. DGPS enabled GPS's are available
widely in marine applications, where the coast guard has a GPS
mounted at fixed sites, and transmits the current GPS signal error
to nearby ship's and aircraft. Probably not available in the rocky
mountains for you climbing types.
Who do these metric guys think they are eh! haha
I think, since the GPS really only needs a few sats to get good accuracy
with 2D readings (lateral), that you need twice as many for alt. And I
wonder where the sats have to be for the best alt reading....overhead or
near the horizon?? I'm thinking that the ones on the horizon are more
likely to view the GPS as a lateral movement and it might indicate
properly...unsure.
Then again the clutter affects of being on the horizon might negate any
benefits....
Original Message
From: "Robert Ussery" <uavscience@f...>
To: <basicstamps@yahoogroups.com>
Sent: Sunday, June 06, 2004 9:30 PM
Subject: Re: [noparse][[/noparse]basicstamps] Re: Thoughts on GPS and altitude measurement
> SB wrote:
>
> >Haha...figures...it's the one abbreviation combination I didn't try!
> >
> >I used to run an Etrex and I could get down to about 9ft (3m)
ccuracy --
> >well, that's what the display told me!
> >I'm always surprised when ppl say their GPSs' are so far off!
> >
> >
> >
> I have an Etrex with WAAS, and regularly get 14-15 ft accuracy (~3m).
> Unfortunately, this is only lateral accuracy. The Z axis is never as
> accurate as the X and Y axes. I've seen my GPS jump up and down 30 feet
> in indicated altitude, and it is rarely more accurate than 100-150 feet
> in altitude between uses.
>
> - Robert
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
>
> Yahoo! Groups Links
>
>
>
>
>
Original Message
From: SB [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=jaIeciRt8Z6aDo1MKg8dZLzkK33YhtbYSaS_qtlF1o5O_y82uLzt__eFrs8YE6pV5uHomFCCqh4WmUTj]steve.brady@r...[/url
Sent: Monday, June 07, 2004 4:24 PM
To: basicstamps@yahoogroups.com
Subject: Re: [noparse][[/noparse]basicstamps] Re: Thoughts on GPS and altitude measurement
psst...3meters = 10feet! [noparse];)[/noparse]
Who do these metric guys think they are eh! haha
I think, since the GPS really only needs a few sats to get good accuracy
with 2D readings (lateral), that you need twice as many for alt. And I
wonder where the sats have to be for the best alt reading....overhead or
near the horizon?? I'm thinking that the ones on the horizon are more
likely to view the GPS as a lateral movement and it might indicate
properly...unsure.
Then again the clutter affects of being on the horizon might negate any
benefits....
Original Message
From: "Robert Ussery" <uavscience@f...>
To: <basicstamps@yahoogroups.com>
Sent: Sunday, June 06, 2004 9:30 PM
Subject: Re: [noparse][[/noparse]basicstamps] Re: Thoughts on GPS and altitude measurement
> SB wrote:
>
> >Haha...figures...it's the one abbreviation combination I didn't try!
> >
> >I used to run an Etrex and I could get down to about 9ft (3m)
ccuracy --
> >well, that's what the display told me!
> >I'm always surprised when ppl say their GPSs' are so far off!
> >
> >
> >
> I have an Etrex with WAAS, and regularly get 14-15 ft accuracy (~3m).
> Unfortunately, this is only lateral accuracy. The Z axis is never as
> accurate as the X and Y axes. I've seen my GPS jump up and down 30 feet
> in indicated altitude, and it is rarely more accurate than 100-150 feet
> in altitude between uses.
>
> - Robert
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
>
> Yahoo! Groups Links
>
>
>
>
>
To UNSUBSCRIBE, just send mail to:
basicstamps-unsubscribe@yahoogroups.com
from the same email address that you subscribed. Text in the Subject and
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Yahoo! Groups Links
Original Message
From: <dmcguire@j...>
To: <basicstamps@yahoogroups.com>
Sent: Monday, June 07, 2004 4:31 PM
Subject: RE: [noparse][[/noparse]basicstamps] Re: Thoughts on GPS and altitude measurement
> psst...3meters = 9 feet 9 inches!
>
>
Original Message
> From: SB [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=HWfAcFqsfd12YjUrHJdTCnTF4qwpTQB2UqWfkCPLscuoPON6Ehj3Zcis8Wb6xwO7vGQC7y5RA_gdcS0sRTJYV1g]steve.brady@r...[/url
> Sent: Monday, June 07, 2004 4:24 PM
> To: basicstamps@yahoogroups.com
> Subject: Re: [noparse][[/noparse]basicstamps] Re: Thoughts on GPS and altitude measurement
>
>
> psst...3meters = 10feet! [noparse];)[/noparse]
>
> Who do these metric guys think they are eh! haha
>
> I think, since the GPS really only needs a few sats to get good accuracy
> with 2D readings (lateral), that you need twice as many for alt. And I
> wonder where the sats have to be for the best alt reading....overhead or
> near the horizon?? I'm thinking that the ones on the horizon are more
> likely to view the GPS as a lateral movement and it might indicate
> properly...unsure.
> Then again the clutter affects of being on the horizon might negate any
> benefits....
>
>
>
Original Message
> From: "Robert Ussery" <uavscience@f...>
> To: <basicstamps@yahoogroups.com>
> Sent: Sunday, June 06, 2004 9:30 PM
> Subject: Re: [noparse][[/noparse]basicstamps] Re: Thoughts on GPS and altitude measurement
>
>
> > SB wrote:
> >
> > >Haha...figures...it's the one abbreviation combination I didn't try!
> > >
> > >I used to run an Etrex and I could get down to about 9ft (3m)
> ccuracy --
> > >well, that's what the display told me!
> > >I'm always surprised when ppl say their GPSs' are so far off!
> > >
> > >
> > >
> > I have an Etrex with WAAS, and regularly get 14-15 ft accuracy (~3m).
> > Unfortunately, this is only lateral accuracy. The Z axis is never as
> > accurate as the X and Y axes. I've seen my GPS jump up and down 30 feet
> > in indicated altitude, and it is rarely more accurate than 100-150 feet
> > in altitude between uses.
> >
> > - Robert
> >
> >
> > To UNSUBSCRIBE, just send mail to:
> > basicstamps-unsubscribe@yahoogroups.com
> > from the same email address that you subscribed. Text in the Subject
and
> Body of the message will be ignored.
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
> >
>
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
> Body of the message will be ignored.
>
> Yahoo! Groups Links
>
>
>
>
>
>
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
>
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>
>
>
>
>