GPS systems, accurate to <1m

This is the unit that was installed in the Ford Versatile I was working on towing the Air Seeder , also the Versatile speedo works from a radar under the tractor pointing at the ground , measuring ground speed

Doug the systems now can split last years seed rows I believe at 10" so just go the 5" centering to pick up the residual fertility of last years crop. Pretty fantastic at that level. Not sure if the stuff you were using will go that close. The Farmscan system I had here sounds ancient taken from the front driveshaft.

JohnR
The system I was using was not quite like the newest one in the above photo but almost, about the only change was the colours on the buttons to the eye, No his system was not that accurate or had auto steer, one had to keep an eye on the overlap but at the same time I was given a chance to gain some new experience , wouldn't mind going back again for next year, trouble is with my plans and what I'm up to at present it will be much to far for 9 days of work unles I add an adventure like CSR on the end of it

took the words out of my mouth

My small farm was quoted by 2 survetors, $18k and $21k
I did it with my quest as I found out most of rural fences are "about" in the right place so a couple of metres here and there, no worries. According to my measurements, the neighbour to the north has about 1m of my land and I have about 2m of another neighbours, and has been like that around the area since it was first fenced.
Mike

Hi Big Mike,

How did you work out the accuracy of the fencing to the nearest metre or two with a consumer grade porduct?

The accuracy of the Quest is as follows:

Position: <15 meters (49 feet), 95% typical
DGPS Position: 3-5 meters (10-16 feet), 95% typical

Your readings obtained may be precise, however that doesn't necessarily mean they are accurate.

Andrew

Hi Andrew,
Waited until accuracy showed on the gps as between 2 - 5 metres and fence posts were within that range so I was happy. Also found a site peg on the corner of one of the boundaries and and using a bearing off a survey map, worked out that stretch of the fence. As I said, not dead accurate but it works for me.
Mike

Thanks Big Mike,

As long as it works for you and you are aware of the accuracy of the readings, then all's well. :-)

Andrew

Yeah Andrew, Would love to be able to say, that point is exactly my boundary but have better things to spend $18k on. And besides, I dont think the locals would have appreciated a city slicker moving in and telling them their fence is out by a coupla metres. I had a look at the systems the surveyors use and they are pretty special, plus they would have had 1 week walking boundaries, cutting rainforest back etc to mark lines so am not saying costs are unjustified, just not practical for me.
Cheers
Mike

Mike I think I would rather get a surveyer to look at the local survey pegs than start a court case with my neighbours on a GPS reading. Even public statements could be embarassing.

Sorry John,
Dont understand what you are trying to say here. All I was saying is that I have roughly worked out where the boundary is and have no want, or issue, to bother finding out exact boundaries. I get along really well with my neighbours and I didn't even suggest court cases and the like. I am not sure how you made that assumption, by its way off line.
Cheers
Mike

As I said Mike, best not make issues because you could be well off with a GPS. Just making sure you didn't even think of going to the extreme.

Hey John,
Not worth going to the extreme on stuff like that, lifes too short.
I like the look of your truck. Am thinking in the next 12 months of getting a 4.2L with a slide on canopy, or waiting to see what nissan do to compete with the toyota v8 td. Have enjoyed reading your posts re warrawena, hope all is ok now. Will have to get to a gathering, sounds great fun.
Regards
Mike

Thanks for the comment. If you are building a vehicle mate, watch the weights. The mass of a tray can be a penalty, I just have a frame to carry th ebox now as the tray was a penalty of another 250-300kgs.

Nissan need to get their act into gear to provide the market signals to a loyal market. If they don't act in the next few months they may well have missed the boat.

The Warraweena episode isn't over yet though. I may have to write up extended warranties one day. Managers have yet to restore my faith {;-(

did you read the post

Spot on Robin. One friend who has lived the last few years near the sea has found his computer and GPS wandered out over night hundreds of metres from his home. Still connected to the power point too ;-))))) Just left the track in Ozexplorer where it had apparently gone fishing.

Clarification of the second point.........

Am talking about the relatively low cost retail units here, not the professional diffGPS units.

The GPS systems for recreational use are just not powerful enough to constantly lock onto the 12 satellites all the time - that's why when you use one it fluctuates between accuracy ranges - normally if you are lucky you average between 3 - 5 satellites.

3-5 satellites? I must be one of the lucky few to average more than that.... ;-)

Andrew

It also should be noted that if the spread of the satellites is not even, the accuracy diminishes. You may have 3 satellites low on the horizon and only 1 above or several satellites clustered together then readings will be less accurate than if you have a good spread of 4 or more satelites evenly dispersed with good signal strength indicated (in simple terms domestic GPS receivers are just maths processors that use time signal and position data transmitted from the visible satellites and triangulate your position from that data).

We find variation of still up to +/- 90m on routinely hit waypoints on a day to day basis. The other issue that tricks people at my work is when data is collected in one datum, batteries changed or unit serviced and the datums are reset to factory default and people just assume it is still set to the same datum - some wicked miss plots with these results.

Cheers

A

Tone,

Some good info certainly but a lot of rubbish too. As one reply said Selective Availability was turned off long ago as there was little point as the bad guys either had an alternative system or had beaten it. The Yanks are not the only ones with a satellite navigation system.
As for consumer equipment not being "powerful enough" to lock onto 12 satellites this is utter carp !
Most GPS units are 12 channel devices and IF they can receive 12 satellite signals they will certainly use them all to determine a position. Problem is it is not always possible to 'see' 12 satellites.
There are many facets to position accuracy but the key ones are satellite geometry and multipath signals. Satellites low on the horizon or directly overhead do not make for good positional accuracy, and like all radio systems any kind of interference with the transmission path can cause problems. The system measures the time signals arrive at the receiver. If anything changes the time the signal takes to get to the receiver then the accuracy of the location becomes incorrect. Averaging can minimise this an does improve accuracy. Sub metre accuracy can be achieved by post processing the location data and correcting it for the range of factors which controll position accuracy. This feature is not found in most recreational use GPS devices and you need to get into the low end survey devices such as the Trimble Geoexplorer and the Leica units.
there is a good booklet available from Information Victoria on GPS sytems & equipment and I'd commend it to forum readers.
Ken

Spot on Ken,
Amazing how many people have heard something in the pub and yet still make out their posts are factual. Unfortunately it makes you lose a lot of faith in some of the other posts.

"As for consumer equipment not being "powerful enough" to lock onto 12 satellites this is utter carp !"

Ken,

I am sorry if my information is 'utter carp'. Whether a GPS 'sees' or 'locks onto' a satellite is the samething - if it can't see it then or can't lock onto it.

"Amazing how many people have heard something in the pub and yet still make out their posts are factual"

Kiwi Kia,

My reply was based on my knowledge and experience that I gained from formal training and using GPS whilst in the Army as well as many years of 4WD and map reading etc - not in the pub.

FYI - all GPS system on the recreational market sold since about 2000 are 12 channel receivers before that they were 3 channel receivers.

In my post I tried to explain to Tonster the basics of GPS systems without being to technical - when using any navigational system there are a myriad of factors that will determine the accuracy of a system just as it is with radio signals etc.

Karl

Karl,

"FYI - all GPS system on the recreational market sold since about 2000 are 12 channel receivers before that they were 3 channel receivers."

Kiwi Kia analogy is correct in regards to pub information. Whether it's a pub, or your Army instructor, some of the technical information you are stating is slightly incorrect. My first GPS in 1996, a Garmin GPS 45XL, was a single channel version. They did however use multiplexing of 8 satellites which incidently didn't achieve a quick TTFF compared to mutli-channels versions. I'm sure there were other popular models on the market with single channel processors.

Andrew

gbc,
The accuracy of the system you are discussing is in the range 2 to 4 metres at 95% confidence.
The AMSA corrections currently running today show variations of 2.5, 3.2, 3.3, 3.1, 2.8 metres for the last 5 corrections recorded this morning from Corny.
Not sure you will get much better than this from Crib Point tower as the distance to Riverina from there is at the limit of the system over land.

You are correct about the system being a lot more accurate than it guarantees to be, but not sure it is down to less than 1 metre which is what Tonester was requesting advice on.
This has been a very good discussion point raised by Tonester.
Cheers

BennyGU,

The dithering (error) on the gps system was turned off a long time ago. The US military uses their own gps system totaly seperate to the civilian system these days. Also, commercial and private aviation uses gps extensively for navigation directly coupled to the aircrafts autopilot system. The US Government would not turn off or significantly 'adjust' the present civilian gps system.

Ben,

Things don't always go the way of the military - they make mistakes too. In the mid-nineties I was doing a gravity survey when the US turned Selective Availability off. It surely was no co-incidence that on the same day relations between the US and Cuba were at a minimum. It was all over the news.

For a couple of hours I was getting sub-meter accuaracy on the GPS. I would walk 1 metre north and the AMG northings would go up by one metre - no joke!

I also believe that SA kicks in again when the GPS is moving at a rate fast enough to be attached to a cruise missile.

Cheers

Alan

Equinox

Good to hear someone else in the gravity survey game. I have been doing it up here in QLD for the last 15 years. Heaps on in the industry at the moment. Love my old trusty LaCoste G Meter. We have seen SA turned off a few times in the 90's, finding Isogals/PSM's to the metre

Cheers

A

The information I was briefed on in regards to the US military system was supposedly current as on Jan 07 by Garmin Aust. If it has changed since then it is news to me. I know for a fact that I was unable to use my GPS system in the very early nineties because the accuracy had deteriorated to such a point that visual fixing was more accurate than the GPS network.

BennyGU,
The selective availability has been turned off since the second gulf war and has remained off ever since.