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'[EE:] GPS vs LPS'
2003\11\11@163829 by llile

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OK, we all know that GPS is good at locating positions down to some
accuracy that is good enough to tell if you are on Mount Everest or not,
but generally not accurately enough for many tasks.  For instance, a
surveyor needs to know where he is within 2 cm, a robot lawnmower needs to
know where it is within 6 inches maybe.  An automated farming machine, say
a robot 4 bottom plow or a robot planter needs to know where it is
withinhalf a meter maybe.  Any of these would benefit from a Local
Positioning System, something that would cover just a few thousand square
feet with immensely better relative accuracy.

This might allow a robot lawnmower to employ efficient mowing algorithms,
instead of random (efficient in terms of processor power) blundering
about.

How would one go about cooking this up?  I have scratched my head about it
a little.

1.  Have a central GPS reciever at a stable position, say on a pole or one
the roof of a house.  Read it's location once.  Then forever after, read
it's location again, and rebroadcast the *error* from the most recent
reading versus the very first reading to your robot cotton picker on a
different frequency.  Your robot cotton-picker recieves GPS, also recieves
this error correcting signal, and with a little math gets a little more
preceise indication of where it happens to be.  Doesn't seem like this
scheme could get you down to an inch though

2. Duplicate GPS on a small scale.  Have 3 or more transmitters near the
corners of your property/farm field/Yard at stable positions.  Continually
broadcast signals from the three and your robot needs to figure out how to
decode them into GPS-like distance signals.  Unfortunately, the GPS
transmitters are synched in a very accurate way linked to the atomic
clock, something that would be hard to manage in a small, cheap, homemade
system.

3. Duplicate GPS with ultrasonic waves.  The way I figure it 40KHZ sound
has a wavelength on the order of .7 cm (this could be way off I did it
from memory)  and if one could use three or more ultrasonic transmitters
to determine distance from known points, one might be able to calculate a
local position within centimeter accuracy.  One scheme that comes to mind
is this:  Each transmitter sends a radio pulse, considered to be
instantaneous at these scales, and it's address by radio at the beginning
of an ultrasonic pulse.  The delay between the leading edge of the radio
pulse and the leading edge of the sonic pulse would translate directly
into distance.  The three transmitters could do this in sequence, thereby
avoiding "packet collision" problems.  Each one would have to be addressed
so the reciever would know which one it was looking for. Sionce the
transmitters poll, they can share the same transmit frequency and each
ahave a reciever so they know when to fire.   The recieved signal delay
would tell you how far in space you were from each unit, and if I do my
geometry correctly 3 transmitters would result in two possible positions
for your hapless robot, one of them being several feet in the air. However
the 2D coordinates (generally what matters anyway) would only have one
possible position.    Sounds like a lot of math for a poor PIC.  However,
might be accurate enough to keep your robot mower out of the flower bed!



any other ideas?  None of these are simple enough.


-- Lawrence Lile
Senior Project Engineer
Toastmaster, Inc.
Division of Salton, Inc.
573-446-5661 voice
573-446-5676 fax

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2003\11\11@165947 by Mike Hudson

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Do a web search for RTK (Real-time kinematic) GPS.

This is a carrier-phase-measurement technique that's used for
agriculture and surveying. It claims to be accurate down to a few
centimetres, but I haven't used a unit myself so I'm unable to verify
their possibly exaggerated claims.

By the way, you describe classical differential GPS in (1). It'll give
an error similar to that of WAAS, a couple metres.
--
Mike Hudson

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2003\11\11@172445 by John Tserkezis

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llile@SALTONUSA.COM wrote:

> 1.  Have a central GPS reciever at a stable position, say on a pole or one
> the roof of a house.  Read it's location once.  Then forever after, read
> it's location again, and rebroadcast the *error* from the most recent
> reading versus the very first reading to your robot cotton picker on a
> different frequency.

 Your first suggestion describes DGPS, where the known location is plugged
into  the transmitting reference end, and the receiver takes a sample of the
GPS determined location, plus the transmitted error information, giving a more
accurate result. (typcially 1 metre or so)

 The comms link and the mobile reciever might be easy, but the reference might
be a little more difficult.  I think the Motorola Oncore series support DGPS
data out.

 However, when you look at the cost (and before you ask, there is no such
thing as a "poor man's DGPS", it's been asked before) it almost certainly won't
be worthwhile for a mere lawnmower.

 I think WAAS (typcial 3 metre accuracy) with boundary sensors for obstacles
and edges, all of which involve relatively cheap and easily available
technology, is the way to go.

 If you're talking about a typical sized yard, (not a large field), then I
would not even use GPS, but look at something like grass height sensors, and
get the mower to go around in a spiral route.  You doing the edges manually.

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2003\11\11@173932 by Tim ODriscoll

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On Tue, 11 Nov 2003 EraseMEllilespam_OUTspamTakeThisOuTSALTONUSA.COM wrote:
> any other ideas?  None of these are simple enough.

How about some kind of RF triangulation?

Is it viable to stick three transmitters (at different frequencies) around
the area you want to navigate, then have your bot measure the RF strength
of each frequency?

Could you then work out where you were in relation to the transmitters?

Damn, I knew I shouldn't have read my email before I went to bed.. *Never*
gonna sleep now!


Tim

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2003\11\11@175629 by Marcel van Lieshout

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I'm am studying this subject for a while now and have decided to try
ultrasound. I'm still working things out, so no results or code yet.

I have a slightly different setup. I have one central transmitter which
sends a synchronization signal once in a while (let's say once per second).
Every beacon and the moving object(s) have a receiver. As there is only one
transmitter, only one channel is needed which is only used by one
transmitter.
Every receiver receives the sync-signal. The time between sync-signals is
(fixed) divided into several timeslots. Every beacon has an address.
At timeslot zero, beacon zero will send it's US. The moving object(s) know
that it's beacon0 that's sending. In the next slot (1), beacon 1 sends it's
US. All moving objects know it's beacon 1 that is sending. etc, etc. If you
have four beacons and give each beacon a timeslot of 250 msec, a syncsignal
can be sended by the central transmitter every second.
The timeslots should be relatively large to allow the beacon to send it's
ultrasound (let's say 5 periods of 40kHz) and allow for the sound to travel
and "die".

HTH

Marcel

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2003\11\11@181743 by Stef Mientki

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"Sounds" Interesting ;-)
It is a project that's also on my todo list.
What range and accuracy are you gaining at ?
(Mine is 50..100 cm range with 0.5 .. 1 cm resolution).
I think the main challenge is to get a good transmit pulse (in a simple
way).
The ideas I've gathered of generating the US pulse till so far are:
generating a spark or high voltage discharge on the transmitter.
Stef Mientki

Marcel van Lieshout wrote:

{Quote hidden}

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2003\11\11@182329 by Marcel van Lieshout

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My ultimate goal is a range of at least 5 meters with a repeatable accuracy
of 1 millimeter. Yes, I know one should have goals that can be met ;-)

BTW it's my first PIC project (exept for the blinking led thing), even
worse: it's my first embedded project.

Marcel

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2003\11\11@224139 by Hopkins

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There is a system called dead reckoning in witch you start your robot of at
a know location and us a gyro to sense every movement along with a distance
recorder - like a Speedo.



The Gyro will give you an output that can be translated into degree's.



I have seen cheap gyros sold for robot systems.



I do not know how accurate they are but could be a starting point.



This is a simple system and with an accurate gyro & distance recorder you
will have accurate results.



This is how plains, ships & submarines can passively keep track of where
they are, is no radio transmissions required that the enemy may hear or
block.



:-) Roy

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2003\11\11@225008 by Liam O'Hagan

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Analog.com (http://www.analog.com) have MEMS gygroscopes available in an 8
pin LCC package, unfortunately no samples are available yet...

> {Original Message removed}

2003\11\11@232707 by Charles Craft

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WLAN positioning?

http://www.wi-fiplanet.com/tutorials/article.php/1487271

-----Original Message-----
From: RemoveMEllilespamTakeThisOuTSALTONUSA.COM
Sent: Nov 11, 2003 4:35 PM
To: PICLISTEraseMEspam.....MITVMA.MIT.EDU
Subject: Re: [EE:] GPS vs LPS

OK, we all know that GPS is good at locating positions down to some
accuracy that is good enough to tell if you are on Mount Everest or not,
but generally not accurately enough for many tasks.  For instance, a
surveyor needs to know where he is within 2 cm, a robot lawnmower needs to
know where it is within 6 inches maybe.  An automated farming machine, say
a robot 4 bottom plow or a robot planter needs to know where it is
withinhalf a meter maybe.  Any of these would benefit from a Local
Positioning System, something that would cover just a few thousand square
feet with immensely better relative accuracy.

<snip>

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2003\11\12@030723 by Wouter van Ooijen

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> 1.  Have a central GPS reciever at a stable position, say on
> a pole or one
> the roof of a house.  Read it's location once.  Then forever
> after, read
> it's location again, and rebroadcast the *error* from the most recent
> reading versus the very first reading to your robot cotton picker on a
> different frequency.

In the Netherlands there are some radio stations that broadcast their
current GPS position.

Wouter van Ooijen

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2003\11\12@095549 by Chuck Busch

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Can you provide a link, spec sheet or any other information on this?  I went
up on the website and came up empty

Thanks in advance,

Chuck

----- Original Message -----
From: "Liam O'Hagan" <RemoveMEliamspam_OUTspamKILLspamGLI.COM.AU>
To: <RemoveMEPICLISTTakeThisOuTspamspamMITVMA.MIT.EDU>
Sent: Tuesday, November 11, 2003 7:50 PM
Subject: Re: [EE:] GPS vs LPS


> Analog.com (http://www.analog.com) have MEMS gygroscopes available in an 8
> pin LCC package, unfortunately no samples are available yet...
>
> > {Original Message removed}

2003\11\12@104424 by llile

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Dead reckoning is useless outdoors.  The wheels grind and slide and hump
over gopher mounds or rocks.  Even worse for big machinery, farm machinery
wheels slip and slide in loose durt.  Might work OK on carpet though.

My goal in this is a yard-sized project.  I have a mowbot in the works
that is designed to use random mowing algorithm (blundering about) and
bump sensors inside a fenced yard.  That works OK for the back yard, but
the front yard will require an expensive fence or a different idea such as
local positioning.  So my range is on the order of 150'.

Ultrasonics might not work so good in a farm-sized operation - they might
not have the range for a farm field. Having raised a little wheat and
soybeans, I think about this problem some.



-- Lawrence Lile





Hopkins <EraseMErdhopkinsspamspamspamBeGoneIHUG.CO.NZ>
Sent by: pic microcontroller discussion list <RemoveMEPICLISTKILLspamspamMITVMA.MIT.EDU>
11/11/2003 09:40 PM
Please respond to pic microcontroller discussion list


       To:     PICLISTSTOPspamspamspam_OUTMITVMA.MIT.EDU
       cc:
       Subject:        Re: [EE:] GPS vs LPS


There is a system called dead reckoning in witch you start your robot of
at
a know location and us a gyro to sense every movement along with a
distance
recorder - like a Speedo.



The Gyro will give you an output that can be translated into degree's.



I have seen cheap gyros sold for robot systems.



I do not know how accurate they are but could be a starting point.



This is a simple system and with an accurate gyro & distance recorder you
will have accurate results.



This is how plains, ships & submarines can passively keep track of where
they are, is no radio transmissions required that the enemy may hear or
block.



:-) Roy

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2003\11\12@110747 by Olin Lathrop

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Hopkins wrote:
> There is a system called dead reckoning in witch you start your robot
> of at a know location and us a gyro to sense every movement along with
> a distance recorder

Actually that's called "inertial navigation".

> The Gyro will give you an output that can be translated into degree's.
> I have seen cheap gyros sold for robot systems.
> I do not know how accurate they are but could be a starting point.
>
> This is a simple system and with an accurate gyro & distance recorder
> you will have accurate results.

This is downright silly for an affordable lawnmower system.

Note that position is the double integral of accelleration.  Any
accelleration measurment error will therefore result in a position error
proportional to the square of the elapsed time.  Not good.

As an exercise, think about navigating a lawn mower to a flower bed just
10 meters away.  To simplify things, just consider movement along a
straight line, meaning the problem is only about a single accelleration
sensor.  Make resonable assumptions about the lawn mower's speed and
accelleration/deceleration capability.  A) Report the accuracy required of
the accellerometer to achieve 500mm position accuracy at the flower bed.
B) Report on the cost and availability of such an accellerometer.

For extra credit: For $100 or less in sensors, how many seconds will the
lawnmower run before its inertial navigation system becomes useless?


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2003\11\12@163946 by John N. Power

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{Quote hidden}

It isn't really feasible to compete with the GPS system, at least not at its level
of complexity. This also eliminates radio methods, which might have to be licensed
and which have synchronization problems. You have the advantage that this is
for your own use (not for commercial sale), and will only be used over a small area.

Triangulation would be the way to go. First, consider this ideal example. Set up three
differently colored poles around the edge of the region of interest. On top of the
vehicle, put a rotating sensor which spins continuously in the horizontal plane. Each
time one of the poles comes into the sensor's center of vision, the sensor puts out a
pulse, along with a signal which identifies the color of the particular pole. The timing
of these three pulses could be analyzed to give the vehicle's position.

Now eliminate color as the determiner of pole identity. Put a flashing IR lamp on each
pole, with flash frequency different for each pole. This would also allow operation at
night, and would make it easier to filter out background light (look only for an AC
signal, within one of three frequency bands). Each lamp would have to have a
omnidirectional light pattern, which would rule out LEDs (or would it?). Use an
array of LEDs shining from within on an external diffuser?

The general idea is to use a simple mechanical scanner to perform the sampling of
the reference points. How to couple the signal between the fixed and rotating parts?
Don't. Use a rotating mirror to reflect the field of view down onto a stationary photo-
detector. The biggest problem will be discriminating the individual fiducial points.
A good solution, if it were possible, would be to place the reference points such that
there would be no ambiguity in identifying them. For example, if #1 were at 0 degrees,
#2 were at 45 degrees, and #3 were at 200 degrees, and if the "shorter" spacing
between #1 and #2 always held up, there would be no doubt which was #1. In the
continuous pulse train from the detector, #1 would always be the first in the pair
closest together. It would take a considerable amount of analysis to find 3 positions
which would always offer unambiguous identity and also allow unambiguous
position detection, but once found, the system would be easy to build.

John Power

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2003\11\12@170053 by Tom

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Lawrence,

Find a student from your local university who desires to learn about PIC's.
In exchange for tutoring that person, have that person mow your lawn for
you.

While you lay in your hammock listening to the sound of the grass being
cut, you can devote your energies to solving the problem of how to have
your lawn trimmed autonomously.  Wait! You just did!

See how that works?

At 04:37 PM 11/12/03 -0500, you wrote:
>> From:         TakeThisOuTllileKILLspamspamspamSALTONUSA.COM[SMTP:.....llilespamRemoveMESALTONUSA.COM]
>> Sent:         Tuesday, November 11, 2003 4:35 PM
>> To:   RemoveMEPICLISTspamspamBeGoneMITVMA.MIT.EDU
>> Subject:      Re: [EE:] GPS vs LPS
>
>> This might allow a robot lawnmower to employ efficient mowing algorithms,
>> instead of random (efficient in terms of processor power) blundering
>> about.

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2003\11\12@170054 by Stef Mientki

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Marcel van Lieshout wrote:

>My ultimate goal is a range of at least 5 meters with a repeatable accuracy
>of 1 millimeter. Yes, I know one should have goals that can be met ;-)
>
I like that attitude, ...
... but ever calculated what the wavelength of a 40 kHz sound wave is ?
340 m/s  , 40 kHz  --->  ...... ;-(
And then I don't even talk about the change of sound speed due to
temperature and humidity !

>BTW it's my first PIC project (exept for the blinking led thing), even
>worse: it's my first embedded project.
>
And of course you are going to program it in assembler ?
And program the PIC with a selfmade programmer ?


Stef

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2003\11\12@172340 by Brooke Clarke

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Hi Lawrence:

Your #1 is not Differential GPS, but what's commonly called "Poor Man's
DGPS".  To get DGPS what's done is to use a base receiver at a known
location and it determines the range and range rate errors to each
satellite.  This information is transmitted by the Low Frequency beacons
and by the WAAS satellites.  The rover GPS receiver applies the
corrections for range error to each of those satellites it's tracking
and also makes a correction because of the time delay in the correction
based on the range rate.  This gets you into the area of 1 meter.

There's a neat simplification of DGPS patented by CSI.  Now, with
Selective Availability turned off, the errors to each satellite is
stable with time and so you don't need to constantly be measuring it.
Instead you can take a single receiver to a known location and let the
receiver determine range and range rate corrections for all the sats
that are visible (a 12 channel receivers is very desirable for this) and
then the receiver sends these corrections to it's self and as the single
receiver moves about it is a true DGPS receiver with an accuracy that
slowly degrades with time and once all the satellites have set will be
back to a plain GPS receiver so this method is only good for a
relatively short period of time.

Your #2 is called a pseudolite system.  It was used when the GPS system
spec was published and before there were any satellites in orbit to test
receivers.  It's also used on missile ranges and other places to improve
the geometry by adding pseudo satellites on towers, hilltops, etc.  Note
that the Dilution Of Position gets lower as the geometry of the sats
gets better and/or denser.  the DOP is bad if all the satellites you are
tracking are in the same general direction.  Ideally, in the northern
hemisphere,  there would be a satellite to the West, South, East and
directly overhead.  There will never be one at the North horizon because
of the orbit inclination.

Your #3 is using what's called carrier phase is applied to the GPS
system since you are looking at the phase of a cycle of the received
signal.  If you envelope detected the ultrasonic signal and just looked
at the rising edge then it word be analogous to what's called code phase
GPS reception.  The difference in the GPS detection methods is huge.
Code phase is used for civilian receivers and gets you in the 10 meter
area, and with DGPS into the 1 meter area.  Surveyors developed the
carrier phase method and they get down into the mili meter area.  The
Real Time Kinematic system achieve mm accuracy while the receiver is moving.

Note that the GPS system depends on absolute time.  Think of a boat out
in S.F. Bay that's all fogged in.  If a fog horn #1 starts exactly on
the hour and the boat hears the horn exactly 10 seconds past the hour
then the boat is on a circle with a radius of about 10,000 feet from the
for horn.  If fog horn #2 starts exactly 1 minute past the hour and the
boat hears that horn at 1:05 past  the hour then the boat is about 5,000
feet from horn #2.  But if the captains watch is off then there is an
error in the boats position.  One way around the clock error is to add
another fog horn.  After noting the times from all 3 fog horns it's
possible to calculate the error in the boat's clock and so a timex watch
can be used instead of an Rolex.  For a 3 dimensional fix 4 sources are
needed.

GPS in fact works in a very similar way.  The prior Transit system
required the Nuke subs to have atomic clocks, expensive even for the
military.

Note that in the fog horn analogy no information was gained by the phase
of the sound.  Now suppose that the clock error has been calculated and
the boat has a precise 1 pulse per second sampling signal to trigger an
A/D connected to a microphone tuned to pickup the foghorn frequency.
Now you will have a phase angle for each fog horn, say 37 degrees for
horn #1, 273 degrees for horn #2 and 50 degrees for horn #3.  But you
don't know the number of full wavelength to each ot the horns.  In GPS
this is called the integer ambiguity.  There are a large number of
possible positions where the solution to the integer ambiguity is
valid.  One way around this with the GPS system is to wait a moment to
let the satellites move a long way and then take another data point.
After some number of minutes with a new fix once every second there is
only one solution to where the fixed point is on the ground and this
position is known very precisely.

So an RTK survey system can give you mm accuracy on a moving GPS
receiver, but at considerable cost.

There are a number of what you call Local Positioning Systems.  Usually
they are radio based.  One is the aircraft Distance Measuring Equipment
(DME) and there are a number of patented variants on this theme. An
early 911 location system for cell phones used the difference of the
time of arrival from one cell phone to multiple cell towers to locate
the phone in the triangular cell.  A good place for a lot of ideas is
the USPTO search site. But to get started you need to find some class
numbers that are for what you are looking for, so go here first:
http://www.uspto.gov/go/classification/uspcindex/indexa.htm


Note that of all the things that can be measured such as distance,
volume ,mass, etc. the one that is heads and shoulders the most precise
is measuring time intervals, so you want a system that measures a time
interval rather than some other quantity.

Have Fun,

Brooke Clarke, N6GCE
http://www.RPC68.com

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2003\11\12@180109 by Marcel van Lieshout

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Stef,

----- Original Message -----
From: "Stef Mientki" <@spam@s.mientkiRemoveMEspamEraseMEMAILBOX.KUN.NL>
To: <EraseMEPICLISTspam@spam@MITVMA.MIT.EDU>
Sent: Wednesday, November 12, 2003 11:00 PM
Subject: Re: [EE:] GPS vs LPS


> Marcel van Lieshout wrote:
>
> >My ultimate goal is a range of at least 5 meters with a repeatable
accuracy
> >of 1 millimeter. Yes, I know one should have goals that can be met ;-)
> >
> I like that attitude, ...
> ... but ever calculated what the wavelength of a 40 kHz sound wave is ?
> 340 m/s  , 40 kHz  --->  ...... ;-(
> And then I don't even talk about the change of sound speed due to
> temperature and humidity !

Yes, I know that it would be better to choose a higher frequency. I know
that I'm trying to achieve a better accuracy than the wavelength makes
possible. I think it may be possible. It's a tradeoff between price, area
coverage, and resolution. The higher the frequency, the better resolution
but the ultrasound travels shorter distances and the transducers are more
costly. For the moment I keep this higher frequency in the back of my head.

As for the fluctuation in the speed of sound: The beacons are at fixed
positions. Measuring the TOF (time of flight) of the sound between two
beacons at a well known distance let's me calculate the current sound speed.
This measurement is done frequently (eg. once a minute). The result is
(through rf) broadcasted to all interested objects.

>
> >BTW it's my first PIC project (exept for the blinking led thing), even
> >worse: it's my first embedded project.
> >
> And of course you are going to program it in assembler ?
> And program the PIC with a selfmade programmer ?

I will write everything in C. Required optimizations might be rewritten in
assembler. BTW I'm a experienced C-programmer. The use of assembly is not
unknown to me (no PIC's though). My programmer is bought, has a usb
connection and is, next to being a programmer, also an ICD.

>

Never said it would be easy...

>
> Stef
>
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2003\11\12@182347 by Stef Mientki

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face
hi Marcel,

Marcel van Lieshout wrote:

{Quote hidden}

Yes, it's possible, when you're signal to noise ratio is low enough you
achieve 0.05 wavelength (I did it once ;-)
So that's enough for you.

{Quote hidden}

Aha, that's a new point (or I missed it), very good.

{Quote hidden}

Oh that's 'sounds' different, no doubts anymore, you'll probably succeed !
Maybe I can even hear the sound, in the neighboorhood of Nijmegen  ;-)

>Never said it would be easy...
>
>
I'ld love to see your results !

Stef


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2003\11\12@182556 by llile

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Ingenious!

-- Lawrence Lile





Tom <.....kristRemoveMEspamTHEGRID.NET>
Sent by: pic microcontroller discussion list <.....PICLISTSTOPspamspam@spam@MITVMA.MIT.EDU>
11/12/2003 04:00 PM
Please respond to pic microcontroller discussion list


       To:     PICLISTEraseMEspam@spam@MITVMA.MIT.EDU
       cc:
       Subject:        Re: [EE:] GPS vs LPS


Lawrence,

Find a student from your local university who desires to learn about
PIC's.
In exchange for tutoring that person, have that person mow your lawn for
you.

While you lay in your hammock listening to the sound of the grass being
cut, you can devote your energies to solving the problem of how to have
your lawn trimmed autonomously.  Wait! You just did!

See how that works?

At 04:37 PM 11/12/03 -0500, you wrote:
>> From:         RemoveMEllilespamspamBeGoneSALTONUSA.COM[SMTP:spamBeGonellileKILLspamspam@spam@SALTONUSA.COM]
>> Sent:         Tuesday, November 11, 2003 4:35 PM
>> To:   PICLISTspam_OUTspam@spam@MITVMA.MIT.EDU
>> Subject:      Re: [EE:] GPS vs LPS
>
>> This might allow a robot lawnmower to employ efficient mowing
algorithms,
>> instead of random (efficient in terms of processor power) blundering
>> about.

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2003\11\12@182803 by Marcel van Lieshout

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Stef,

Well, Nijmegen - Heerde (between Zwolle and Apeldoorn) is only slightly more
than 5 meters... ;-)

You have got my attention with the 0.05 wavelength story you told. Do you
have any more info about that?

Marcel

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2003\11\12@184219 by Olin Lathrop

face picon face
Brooke Clarke wrote:
> Your #1 is not Differential GPS, but what's commonly called "Poor Man's
> DGPS". ...

Nice writeup on various GPS and other positioning schemes, thanks.

One thing I haven't heard anyone mention is a scheme I thought of 20 years
ago before there was GPS.  I do some trail maintenence and other outdoor
things, and I wanted to somehow record my position as I walked so that the
path could be plotted on a map after I got home.

I was thinking of using existing AM radio stations as beacons at fixed
locations.  This scheme requires a base unit which can support any number
of mobile units.

Plant the base unit at a fixed known location.  Calibrate by taking mobile
unit readings for a minute or so at two known directions and distances
from the base unit (50 meters west, 50 meters south).  Then you can walk
around wherever you want to.  When you get back home, download the data
from the base and the mobile unit, and the computer can reconstruct your
track.

The way it works is that each unit locks onto the carrier of at least
three, preferably 5-10 AM radio stations.  At regular intevals, like every
100mS, each unit stores the current count of carrier cycles for each
station.  The counter only needs to be big enough to not wrap within the
sample period.

The sequence of counts therefore saves the phase relationship between the
carriers over time.  Since only whole carrier cycles are counted, each
individual measurement is +- 1/2 carrier cycle.  However, with 10
measurements per second and considering the walking speed of a human, the
error can be greatly reduced by filtering nearby samples.  For greater
accuracy, sample more often or move more slowly.

The purpose of the base station is to track the phases of all carriers at
a known fixed location over time.  Frequency shifts of the transmitters
are therefore recorded and can be cancelled out later.  The original
calibration step in effect allows for triangulating the location of each
transmitter.

This scheme was interesting to think about, but didn't seem useful anymore
once GPS became available.  I never did get around to building real
hardware.  Oh well.


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(978) 742-9014, http://www.embedinc.com

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2003\11\12@185809 by Stef Mientki

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hi Marcel,

Marcel van Lieshout wrote:

>Stef,
>
>Well, Nijmegen - Heerde (between Zwolle and Apeldoorn) is only slightly more
>than 5 meters... ;-)
>
Maybe at a lower frequency ;-)

>
>You have got my attention with the 0.05 wavelength story you told. Do you
>have any more info about that?
>
Well it's some years ago, and distance was only 5 cm (we wanted to
measure very low air speed, fast).
We had a sawtooth running, synchronized with the transmitter, and
sampled the analog value of the sawtooth at a number (because you'll see
that the period of the subsequent detected waves isn't constant) of zero
crossings (another point you'll see is that you'll have far more than 5
received waves, even if you transmit just 1 half), then do some
averaging math, et voila.
One other trick, I don't remember if was essential, we did it
bi-directional.
Maybe it'll give you some ideas.

Stef

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2003\11\12@193609 by Jack Smith

picon face
Clever idea. Most AM radio stations should have decent short-term phase
stability. From my experience in the industry, it's quite common to see the
frequency to stay within 0.5 Hz of the assigned frequency for days or even
weeks at a time and when the frequency moved, it was a very slow drift, like
0.1 Hz/day.

Some of the technical problems are interesting. You have to strip off the
sidebands to measure the carrier, so perhaps a limiter as found in an FM
receiver might be used. Or, PLL to the carrier with a long time constant and
measure the PLL.

Of course, you would have a sky wave problem to worry about as well, where
the signal arrives via both ground wave and sky wave and the phase
relationship between the two gets mixed up. That's more of a night problem,
of course, as the sky wave in daytime is usually pretty well down compared
with the ground wave. Say two hours after sunrise and an hour or so before
sunset for minimum sky wave problems.

If you were going to use this technique for long distance measurements, I
can see quite a few other difficulties, but for distances of a few hundred
yards, those wouldn't be a problem. (To take one, you are measuring from the
phase center of the antenna. In a directional array, that changes with
azimuth. But if you are a few miles from the station and only move a short
distance, you can take the phase center as a constant location.)

Another interesting ranging approach is used in analog cellular. The base
station generates a 6KHZ tone (called a SAT or supervisory audio tone) that
is repeated back to the base station by the mobile. The base station
measures the phase difference between the outgoing and returned 6 KHz SAT
and calculates the distance based on the phase difference. It's a rough
distance measurement of course. (The mobile has to meet a certain spec on
delay, which is factored into the measurement.) If you can measure to +/- 1
degree, you get a total (out and back) measurement error of 140 meters.

Jack


{Original Message removed}

2003\11\12@213021 by Olin Lathrop

face picon face
Jack Smith wrote:
> Clever idea. Most AM radio stations should have decent short-term phase
> stability. From my experience in the industry, it's quite common to see
> the frequency to stay within 0.5 Hz of the assigned frequency for days or
> even weeks at a time and when the frequency moved, it was a very slow
> drift, like 0.1 Hz/day.

But even small short term phase noise can be a problem.  That's why the base
station records the same data as the mobile units.  Any frequency or slow
phase drift is factored out.

> Some of the technical problems are interesting. You have to strip off the
> sidebands to measure the carrier, so perhaps a limiter as found in an FM
> receiver might be used. Or, PLL to the carrier with a long time constant
> and measure the PLL.

You really don't need anything very complicated.  The effect of the side
bands is just slow (compared to the carrier frequency) change in the carrier
amplitude.  The amplitude isn't important anyway, as long as the signal is
strong enough in the first place.  All you are looking for are the carrier
zero crossings.

> If you were going to use this technique for long distance measurements, I
> can see quite a few other difficulties, but for distances of a few hundred
> yards, those wouldn't be a problem.

I think it would work reasonably well for a few miles from the base station
before location got a bit distorted.  Of course if you can pass a known
location once or twice with the mobile unit, then the correctable range can
be greatly improved.  For example, mapping a 10 mile loop into a wilderness
area may have some error at the far distances, but a 10 mile trail that ends
at a known location should be quite accurate because you've got known anchor
points at each end.

> (To take one, you are measuring from
> the phase center of the antenna. In a directional array, that changes with
> azimuth. But if you are a few miles from the station and only move a short
> distance, you can take the phase center as a constant location.)

Most AM antennas are just big sticks in the sky.  A list of known
uni-directional stations would help.  There aren't that many of them that a
complete list in some PROM would be a problem.  Even so, as you said, the
phase center of a directional antenna isn't going to change much compared to
the typical distance you are from the antenna.


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2003\11\13@040107 by Marcel van Lieshout

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> >
> I'ld love to see your results !
>
All in due time, my friend. All in due time...

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