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'[OT]: While on the subject of GPS...'
2002\11\20@162814 by Barry Gershenfeld

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>Does the error of a standard GPS receiver affect it's ability to determine a
>vehicle's or vessel's speed accurately, or is it safe to assume that the
>error will be constant and the speed it reports will be accurate?

"Error of a ...receiver" implies that you're talking about transmission
path and receiver noise effects.   These receivers use the doppler
shift of the signal to determine speed and direction, so it's
not connected to position.   Even in the bad old days of Selective
Availability, speed and direction were more accurate than position.

Barry

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2002\11\20@181215 by John Ferrell

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Should altitude indications be accurate at this time?
I use a older system (Sony-Etak) on a w98 Compaq laptop in our motor home
and have never felt the altitude aspect was meaningful.

John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"

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2002\11\20@200917 by Barry Gershenfeld

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>Should altitude indications be accurate at this time?
>I use a older system (Sony-Etak) on a w98 Compaq laptop in our motor home
>and have never felt the altitude aspect was meaningful.

We live in a horizontal world.   1000 feet looks a lot worse
as an altitude error than it does on the ground.

Barry

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2002\11\20@211508 by John Ferrell

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That it does. I recall a math book detailing a 2-diminsional world called
"Flatland".

John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



{Original Message removed}

2002\11\21@011338 by Dave Tweed

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Barry Gershenfeld <spam_OUTbarry_gTakeThisOuTspamZMICRO.COM> wrote:
> > Does the error of a standard GPS receiver affect it's ability to determine
> > a vehicle's or vessel's speed accurately, or is it safe to assume that the
> > error will be constant and the speed it reports will be accurate?
>
> "Error of a ...receiver" implies that you're talking about transmission
> path and receiver noise effects.

Not necessarily. The errors could arise from receiver clock errors and
geometric dilution of precision (GDOP).

> These receivers use the doppler shift of the signal to determine speed
> and direction, so it's not connected to position.

No, GPS receivers don't use Doppler to determine receiver motion; the
velocity of the transmitters (satellites) is typically 2 orders of
magnitude greater than that of the receiver.

> Even in the bad old days of Selective Availability, speed and direction
> were more accurate than position.

Mainly because the "frequency spectrum" of the SA noise is well below the
frequencies associated with vehicular motion; if you're on foot, however,
the effects are quite noticable.

-- Dave Tweed

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2002\11\21@044013 by Chris Loiacono

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OK, so I was asked to put together a GPS speedometer. I thought it would be
fairly simple, but now I'm not so sure I can expect accuracy without a fair
amount of complexity.
Is it worth the trouble?
C

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2002\11\21@044843 by Alan B. Pearce

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>Should altitude indications be accurate at this time?
>I use a older system (Sony-Etak) on a w98 Compaq laptop in our
>motor home and have never felt the altitude aspect was meaningful.

My understanding is that the elevation given by a GPS is above a perfect
sphere which is centred on the centre of gravity of the earth. Because the
earth is not a true sphere (it is expanded at the equator due to centrifugal
forces of the earths rotation) the displayed height can be quite different
to actual height AMSL. It is possible to correct for this using tables.

There was quite a discussion on this list about this very thing about a
month or two back.

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2002\11\21@053936 by Russell McMahon

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> OK, so I was asked to put together a GPS speedometer. I thought it would
be
> fairly simple, but now I'm not so sure I can expect accuracy without a
fair
> amount of complexity.
> Is it worth the trouble?

Speed can be deduced from position variation with time BUT you have a speed
signal already available based on phase shift of received signals. It would
be trivial to use this for a speedometer function and you could add code to
make an informed decision about whether something "intetresting" was
happening.

Given that it's so easy to try, why not try it and see how it works out.


       RM

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2002\11\21@084628 by Dave Tweed

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"Alan B. Pearce" <A.B.Pearcespamspam_OUTRL.AC.UK> wrote:
> My understanding is that the elevation given by a GPS is above a perfect
> sphere which is centred on the centre of gravity of the earth. Because the
> earth is not a true sphere (it is expanded at the equator due to centrifugal
> forces of the earths rotation) the displayed height can be quite different
> to actual height AMSL. It is possible to correct for this using tables.
>
> There was quite a discussion on this list about this very thing about a
> month or two back.

Your understanding is wrong. GPS elevation is given above a theoretical
surface called the "geoid", which is the shape the Earth would have without
tides. It starts with the basic oblate spheroid caused by the Earth's
rotation and incorporates certain distortions that are caused by the
nonuniform distribution of mass in the crust (e.g., oceans and mountain
ranges). The technical definition of this surface is "the equipotential
gravitational surface" and it corresponds to mean sea level, even on land.

That's not to say that GPS elevations are as accurate as horizontal
positions. In most consumer receivers, the "error budget" is skewed to
favor the horizontal dimensions over the vertical, because the former are
more important to most users. This is also governed somewhat by the basic
geometry of the most common usable satellite configurations.

-- Dave Tweed

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2002\11\21@085459 by Dave Tweed

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Russell McMahon <KILLspamapptechKILLspamspamPARADISE.NET.NZ> wrote:
> > OK, so I was asked to put together a GPS speedometer. I thought it would
> > be fairly simple, but now I'm not so sure I can expect accuracy without a
> > fair amount of complexity. Is it worth the trouble?
>
> Speed can be deduced from position variation with time BUT you have a speed
> signal already available based on phase shift of received signals. It would
> be trivial to use this for a speedometer function and you could add code to
> make an informed decision about whether something "intetresting" was
> happening.
>
> Given that it's so easy to try, why not try it and see how it works out.

Trivial? Easy to try? What kind of receivers do you have access to that
provide phase information?

Even if you could get it, you'd need to compensate for the ~5800 mi/hr
velocity of the satellites themselves before you could determine anything
about the receiver velocity.

-- Dave Tweed

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2002\11\21@090125 by Alan B. Pearce

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> OK, so I was asked to put together a GPS speedometer. I thought it would

IIRC one of the NMEA0183 messages gives speed, possibly in knots, but I
thought it was MPH.

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2002\11\21@090718 by rad0

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----- Original Message -----
From: "Dave Tweed" <TakeThisOuTpicEraseMEspamspam_OUTDTWEED.COM>
To: <RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU>
Sent: Thursday, November 21, 2002 7:53 AM
Subject: Re: [OT]: While on the subject of GPS...


> Russell McMahon <apptechEraseMEspam.....PARADISE.NET.NZ> wrote:
> > > OK, so I was asked to put together a GPS speedometer. I thought it
would
> > > be fairly simple, but now I'm not so sure I can expect accuracy
without a
> > > fair amount of complexity. Is it worth the trouble?
> >
> > Speed can be deduced from position variation with time BUT you have a
speed
> > signal already available based on phase shift of received signals. It
would
> > be trivial to use this for a speedometer function and you could add code
to
> > make an informed decision about whether something "intetresting" was
> > happening.
> >
> > Given that it's so easy to try, why not try it and see how it works out.
>
> Trivial? Easy to try? What kind of receivers do you have access to that
> provide phase information?

pardon me, but doesn't the gps give velocity in knots?  Is this completely
useless?

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2002\11\21@090728 by ar=E3es?=

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Hi,


> > OK, so I was asked to put together a GPS speedometer. I thought it would
>
> IIRC one of the NMEA0183 messages gives speed, possibly in knots, but I
> thought it was MPH.


   It is surely in knots in the $GPRMC message. I am not sure why but it
does seem to get crazy less often than the position information.

Best regards,
Alexandre Guimaraes

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2002\11\21@102823 by Mike Harrison

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On Thu, 21 Nov 2002 12:06:26 -0200, you wrote:

>Hi,
>
>
>> > OK, so I was asked to put together a GPS speedometer. I thought it would
>>
>> IIRC one of the NMEA0183 messages gives speed, possibly in knots, but I
>> thought it was MPH.
>
>
>    It is surely in knots in the $GPRMC message. I am not sure why but it
>does seem to get crazy less often than the position information.
>
>Best regards,
>Alexandre Guimaraes
Probably because GPS was originally aimed as sea and air navigation, I
believe both traditionally use knots.  
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2002\11\21@111450 by Dave Tweed

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rad0 <RemoveMErad0TakeThisOuTspamspamATTBI.COM> wrote:
> Dave Tweed <EraseMEpicspamspamspamBeGoneDTWEED.COM> wrote:
> > Russell McMahon <RemoveMEapptechKILLspamspamPARADISE.NET.NZ> wrote:
> > > Speed can be deduced from position variation with time BUT you have a
> > > speed signal already available based on phase shift of received
> > > signals. It would be trivial to use this for a speedometer function
> > > and you could add code to make an informed decision about whether
> > > something "intetresting" was happening.
> > >
> > > Given that it's so easy to try, why not try it and see how it works out.
> >
> > Trivial? Easy to try? What kind of receivers do you have access to that
> > provide phase information?
>
> pardon me, but doesn't the gps give velocity in knots?  Is this completely
> useless?

Of course it does, but it's computed from successive positions. You seemed
to be implying that you could do better "based on phase shift of received
signals." That's the aspect I was questioning.

The very high accuracy "kinematic" receivers used for precision surveying
do in fact measure carrier phase, but these units cost thousands of dollars
and are not the type of thing you'd use for a "GPS speedometer".

-- Dave Tweed

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2002\11\21@123802 by Peter L. Peres

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On Thu, 21 Nov 2002, Dave Tweed wrote:

*>The very high accuracy "kinematic" receivers used for precision surveying
*>do in fact measure carrier phase, but these units cost thousands of dollars
*>and are not the type of thing you'd use for a "GPS speedometer".

And they take a while to do that. 15 minutes or more for a fix and much
longer for a precise fix. They must stay in the same place while doing
this. Maybe things have improved but I doubt it.

Peter

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2002\11\21@174915 by John Ferrell

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The Sony/Etak unit seems to be very consistant with the speed indication. It
is always a little higher than my vehicals speedometer. As far as I can tell
with highway mile markers, it is accurate.


John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



>
>

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2002\11\21@202423 by Jim

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  "Probably because GPS was originally aimed as
   sea and air navigation, I believe both traditionally
   use knots."


This is MORE a function of NMEA output - I have
a JRC OEM GPS receiver that natively puts out
Meter per second in the JRC proprietary JRC
format ...

RF Jim


{Original Message removed}

2002\11\22@031138 by jason.sherlock

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Let's say I do have access to phase shift information.  Am I only able to
derive speed (directionless magnitude) from these measurements or can I
actually calculate a velocity vector?

Thanks,
Jas.

{Original Message removed}

2002\11\22@090729 by Dave Tweed

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Jason Sherlock <EraseMEjason.sherlockspamEraseMEBIRDSTEP.COM>
> Let's say I do have access to phase shift information.  Am I only able to
> derive speed (directionless magnitude) from these measurements or can I
> actually calculate a velocity vector?

You could compute a velocity vector. The Doppler/phase information gives
you a set of velocity components along lines between the receiver and each
satellite. Since these are never orthogonal, you need to solve a system of
equations to find the overall velocity of the receiver relative to the
whole system (the center of the Earth).

-- Dave Tweed

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2002\11\22@122247 by Martin Darwin

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How good (accurate) is GPS during hard braking, cornering and acceleration
(of a normal road car -- not a race car)? I had thought if you could get a
GPS uint with a 10Hz update rate it would be pretty good for calculating
lap times, lines through corners, braking points, etc.. of a car on a race
track.

MD

On Thu, 21 Nov 2002, John Ferrell wrote:

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2002\11\22@123113 by Dr Martin Hill

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It's not brilliant for it.  We have a 20Hz output GPS unit, but that doesn't
always mean that the data is really calculated 20 times per second.  We
produce a data logger which combines GPS data with data from accelerometers
and enables you to calculate lap times, braking points and so on, it works
better with a 1Hz GPS unit and 100Hz accelerometer data than just with a
20Hz GPS unit (much cheaper to produce as well).  Search for DL90 Data
Logger on Google for more info.

We see offsets of upto 2kph under hard acceleration and braking from the raw
GPS data compared to combined data or data from an optical speed sensor.

Regards,

Martin

{Original Message removed}

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