Post by thread:[EE:] GPS vs LPS

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

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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

2003\11\11@165947 by Mike Hudson

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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body

2003\11\11@172445 by John Tserkezis

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. -- -o) /\\ Message void if penguin violated _\_V Don't mess with the penguin Linux Registered User # 302622 <http://counter.li.org> Fido: 3:712/610 BBS/FAX: +61-2-9716-8310 Internet: jtKILLspamtechniciansyndrome.org -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam.....mitvma.mit.edu with SET PICList DIGEST in the body

2003\11\11@173932 by Tim ODriscoll

On Tue, 11 Nov 2003 EraseMEllilespam_OUTTakeThisOuTSALTONUSA.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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

2003\11\11@175629 by Marcel van Lieshout

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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email @spam@listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

2003\11\11@181743 by Stef Mientki

"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}>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 > >-- >http://www.piclist.com#nomail Going offline? Don't AutoReply us! >email KILLspamlistservKILLspammitvma.mit.edu with SET PICList DIGEST in the body > > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

2003\11\11@182329 by Marcel van Lieshout

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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spamBeGonelistservspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body

2003\11\11@224139 by Hopkins

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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email TakeThisOuTlistservEraseMEspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

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

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

WLAN positioning? http://www.wi-fiplanet.com/tutorials/article.php/1487271 -----Original Message----- From: RemoveMEllileTakeThisOuTSALTONUSA.COM Sent: Nov 11, 2003 4:35 PM To: PICLISTEraseME.....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> -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservmitvma.mit.edu with SET PICList DIGEST in the body

2003\11\12@030723 by Wouter van Ooijen

> 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 -- ------------------------------------------- Van Ooijen Technische Informatica: http://www.voti.nl consultancy, development, PICmicro products -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestEraseMEEraseMEmitvma.mit.edu

2003\11\12@095549 by Chuck Busch

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_OUTKILLspamGLI.COM.AU> To: <RemoveMEPICLISTTakeThisOuTspamMITVMA.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

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 <EraseMErdhopkinsspamspamBeGoneIHUG.CO.NZ> Sent by: pic microcontroller discussion list <RemoveMEPICLISTKILLspamMITVMA.MIT.EDU> 11/11/2003 09:40 PM Please respond to pic microcontroller discussion list To: PICLISTSTOPspamspam_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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spamBeGonelistservSTOPspamEraseMEmitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestspamBeGonemitvma.mit.edu

2003\11\12@110747 by Olin Lathrop

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 -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestEraseMEmitvma.mit.edu

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

{Quote hidden}> From: @spam@llile@spam@spam_OUTSALTONUSA.COM[SMTP:spamBeGonellileKILLspamSALTONUSA.COM] > Sent: Tuesday, November 11, 2003 4:35 PM > To: .....PICLISTspam_OUTMITVMA.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. > 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. ... > 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. ... 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 -- http://www.piclist.com hint: To leave the PICList TakeThisOuTpiclist-unsubscribe-request.....TakeThisOuTmitvma.mit.edu

2003\11\12@170053 by Tom

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: TakeThisOuTllileKILLspamspamSALTONUSA.COM[SMTP:.....llileRemoveMESALTONUSA.COM] >> Sent: Tuesday, November 11, 2003 4:35 PM >> To: RemoveMEPICLISTspamBeGoneMITVMA.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. -- http://www.piclist.com hint: To leave the PICList spamBeGonepiclist-unsubscribe-request@spam@spam_OUTmitvma.mit.edu

2003\11\12@170054 by Stef Mientki

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 -- http://www.piclist.com hint: To leave the PICList TakeThisOuTpiclist-unsubscribe-requestspammitvma.mit.edu

2003\11\12@172340 by Brooke Clarke

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 {Quote hidden}>Date: Tue, 11 Nov 2003 15:35:10 -0600 >From: llileEraseMESALTONUSA.COM >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. > >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 > > > > > -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestEraseMEspam_OUTmitvma.mit.edu

2003\11\12@180109 by Marcel van Lieshout

Stef, ----- Original Message ----- From: "Stef Mientki" <@spam@s.mientkiRemoveMEEraseMEMAILBOX.KUN.NL> To: <EraseMEPICLIST@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 > > -- > http://www.piclist.com hint: To leave the PICList > @spam@piclist-unsubscribe-requestspam_OUT.....mitvma.mit.edu > -- http://www.piclist.com hint: To leave the PICList spamBeGonepiclist-unsubscribe-requestEraseMEmitvma.mit.edu

2003\11\12@182347 by Stef Mientki

hi Marcel, Marcel van Lieshout wrote: {Quote hidden}>Stef, > >----- Original Message ----- >From: "Stef Mientki" <s.mientkispamBeGoneMAILBOX.KUN.NL> >To: <RemoveMEPICLIST@spam@spamBeGoneMITVMA.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. > 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}> 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. > Aha, that's a new point (or I missed it), very good. {Quote hidden}>>>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. > > 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 -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-request@spam@EraseMEmitvma.mit.edu

2003\11\12@182556 by llile

Ingenious! -- Lawrence Lile Tom <.....kristRemoveMETHEGRID.NET> Sent by: pic microcontroller discussion list <.....PICLISTSTOPspam@spam@MITVMA.MIT.EDU> 11/12/2003 04:00 PM Please respond to pic microcontroller discussion list To: PICLISTEraseME@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: RemoveMEllilespamBeGoneSALTONUSA.COM[SMTP:spamBeGonellileKILLspam@spam@SALTONUSA.COM] >> Sent: Tuesday, November 11, 2003 4:35 PM >> To: PICLISTspam_OUT@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. -- http://www.piclist.com hint: To leave the PICList spamBeGonepiclist-unsubscribe-request@spam@mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestEraseMEKILLspammitvma.mit.edu

2003\11\12@182803 by Marcel van Lieshout

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 -- http://www.piclist.com hint: To leave the PICList spamBeGonepiclist-unsubscribe-requestspam_OUTRemoveMEmitvma.mit.edu

2003\11\12@184219 by Olin Lathrop

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. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestRemoveMEmitvma.mit.edu

2003\11\12@185809 by Stef Mientki

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 -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-request@spam@mitvma.mit.edu

2003\11\12@193609 by Jack Smith

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

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. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestRemoveMESTOPspammitvma.mit.edu

2003\11\13@040107 by Marcel van Lieshout

> > > I'ld love to see your results ! > All in due time, my friend. All in due time... -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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