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GPS instability


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Thread starter #1
Just out of curiosity, I tested this GPS Module with Ceramic Antenna for DIY Handheld Positioning System for $7.86
http://www.dx.com/p/open-smart-gps-...ino-apm2-5-flight-control-480918#.WiDKElWnH4Y because without moving, the Latitude & Longitude numbers go up and down independently. It seems like it is wandering around but stays within a 25 feet diameter circle for the most part. I guess that is pretty good trying to triangulate my location from 3 or more moving objects in space.
That is not good enough for a GPS controlled lawn mower.
I took a few sets of data with a Stationary GPS sampling once per second then plotted the 3 least significant digits in Excel.

Plot 3.jpg

Plot 2.jpg

Plot 1.jpg


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Informative. Don't land-based positioning systems, for e.g. farming and surveying, use 'Differential GPS', involving a fixed local beacon as well as the satellites, to get extra precision?


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Thread starter #3


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and a common problem with GPS,
alot of the apps use algorithms to compensate .... i only know cos if i go off route with my phone it takes about 5 seconds to realize it and re coordinate

until the point where you go off route far enough the gps will assume its just drift and keep you moving on the original course at your calculated speed

I hope you get this working it is definitely a problem that you need a dog fence to run one of these mowers ....

OMG ima genius!!! ... .I just had a thought.... how hard would it be to build our own local triangulation system ... so like we set up 3 beacons for reference in the corners of the yard ... then the bot would have better resolution ... also there are less clouds throwing things off down here

... i am thinking more now ... a wifi module has rssi for wifi signal strength ... i wonder if we could use that for local pos (we dont need gps coorinates really just local boundary coordinates) with 3 routers in the area(dont even need to be connected) we could maybe do a pinpoint with that?
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dr pepper

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LoranC is (was) accurate to about 50 meters.
It has been put back into service here in the Uk, but is unuseable for navigation, just for frequency.
Rssi on wifi isnt a great deal of use for navigation directly, if you walk round a room it will go all over the place, not necessarily in relation to the distance to the router.
A dog collar type boundary system would be effective, I think they use Rf around 100kc.
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thats what they use now is the dog collar system .... it seems crule to me
more importantly the cost to install a dog fence is more expensive than a state of the art mesh network
Signal strength alone is no good for distance or position over a small area. Phasing effects and reflections can cause the signal to vary dramatically within a couple of feet at VHF and above.
That's why such as WiFi gear has multiple antennas, so not all combinations of path distances are generally in nulls at the same instant.

Anyone that's worked with radio comms is familiar with mobile flutter and even aircraft flutter (the reflection from an aircraft overhead causing dropouts).

A high precision system needs either distance by signal timing from each beacon, or be able to measure bearing to beacons.

It does not help that precise distance by radio timing is inherently difficult as you need nanosecond accuracy just to get to about a one foot error margin; fractional nanoseconds to get better.

For a short range system around a house for such as a mower, an ultrasonic beacon system could possibly work?
That makes distance by timing easy for a simple MCU, it's about one millisecond per foot for sound.


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but what about in a mostly open yard, there will be less reflection there, a quick calculation for std wifi range is about 150ft and signal drops out at about -70 & - 80, so thats about 1db per half foot(ok so i round off non linear ), I have noticed when scanning inside homes for wifi that a stationary item will fluctuate about 2db....

plus not sure of the scope for others but i am calculating for a urban yard where most yards are less than 1/4 acre ... and there is a wifi broadcast in every direction (in this situation you wouldn't need the "satellites" since the robot would load the map based on the SSID's near by) ... but even for the larger rural home without wifi you could set up something like this around the perimeter:

take one of these:
and put it in one of these:

.... and maybe add a RF trigger for the bot to wake it from sleep

and there we go for only 30$ we have our satellite station
.... put about 10 or 20 around the yard (every 50ft) (still less than fence) ... and now we have multiple points to navigate from even if one or 2 satellites drop out(neighbours move...ect) ... since we dont even need network access to measure the SSID and RSSI

would that be able to compensate for Phasing effects... or maybe 2 radios on the bot itself?

dr pepper

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Give it a try, though you might be dissapointed.
Imagine 1000s of rubber balls being dropped from a great height into said garden, that is what microwave Rf would do, bounce all over the place.
Waves hitting the ant from different directions at the same time add & cancel each other confusing the issue.
Wifi only has a length of a few cm, to build a reasonably simple radio locating device a wavelength longer than the accuracy you want would be in order.
You can get into detecting unreflected wavefronts on rf signals, that gets real complex real quick though, or if you could somehow get your wifi router to 'ping', that too would probably be a tricky session with the router setup.

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