Thread: GPS accuracy

I think there is a misconception here...

GPS recievers can return a fix from 3 satelitites, but 'guess' the elevation.

To give an accurate fix they require 4.

Ideally the satelites need to be evenly scattered in the sky, the closer the satelites are together, the less accurate the positioning.

More satelites does not mean a more accurate positioning, the GPS unit only uses 4. More satelites increases the likelihood of a more accurate position as it gives the receiver a wider choice of satelites!

GPS units that receive a correction from a ground station broadcasting from a known location are great, except the further you are from the ground station, the less reliable the correction. After a couple of hundred kilometres they are no more accurate than a normal commercial unit.

You either have to spend up big, or put up with accuracy of about +- 10 metres. I reckon I can see what I'm looking for from 10 metres away, so I'm very happy with commercial GPS!

Originally Posted by theresanothersteve

I think there is a misconception here...

GPS recievers can return a fix from 3 satelitites, but 'guess' the elevation.

In guessing the elevation, they are also, to a certain extent, guessing the lat/long.

To give an accurate fix they require 4.

Isn't that what I said?

Ideally the satelites need to be evenly scattered in the sky, the closer the satelites are together, the less accurate the positioning.

Isn't that what I said?

More satelites does not mean a more accurate positioning, the GPS unit only uses 4. More satelites increases the likelihood of a more accurate position as it gives the receiver a wider choice of satelites!

So, the GPS receiver is using more satellites in order to decide which ones to use at each update.

GPS units that receive a correction from a ground station broadcasting from a known location are great, except the further you are from the ground station, the less reliable the correction. After a couple of hundred kilometres they are no more accurate than a normal commercial unit.

I didn't mention DGPS! It isn't available with his gps receiver.

You either have to spend up big, or put up with accuracy of about +- 10 metres. I reckon I can see what I'm looking for from 10 metres away, so I'm very happy with commercial GPS!

That is the quoted figure, but with increasing Nos. of satellites, it is somewhat less in practice.

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I'm sorry you have taken exception to my post and consider me an instant expert.

I have a backgroung in IT training, including the delivery of training courses in GPS to emergency services personnel.

I at no stage refuted what you said, I am trying to point out that GPS only uses 4 satelites, irrespective of howm many the unit can detect. If you read the thread there is a ambiguity about the more satelites the GPS can 'see', the more accurate it is. The inference is that the reciever is using all the satelites.

I like to clear up misconceptions so that everyone is well informed. Information should be shared.

If I have caused you offence, I apologise.

I have been reading a few of the magazines from the Kondonin Group (farming association). In there, I read of a direct drill seader that was accurately seeding between the previous year's stubble. This was achieving accuracy of around 2~3 cm, although this was with auto steer and using a transponder in the field. Accuracy this great is not needed for spraying. It would be much cheaper to use the foam dollop method described before.

Aaron.

Originally Posted by Aaron IIA

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This was achieving accuracy of around 2~3 cm, although this was with auto steer and using a transponder in the field.
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Aaron.

Using a transmitter which is transmitting it's know position combined with the data received from the GPS satellite system gives extremely accurate positioning.
This sort of system is readily available for anyone to buy and use.


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