Can your GPS give you a solid, fixed solution,

I would certainly be interested in the results of such a test.

One of those stations should be sitting at the edge of a wall of trees. Half the sky open, the other half covered with trees. Thats where I get the most error and still show a "fixed" solution with RTK.

James

What good would it do to know that brand A works in the deep woods 35 percent of the time, and brand B works 55 percent of the time, etc.... The problem with the RTk in the woods is that it might work great, sometimes, but you can never be certain which side of the fence you are on at any given moment.

Agreed, only real confidence builder is a redundant observation, however if one brand always gave correct results 100% of the time in said test, that might be something to consider.

SHG

Pseudo,

What difference does it make?

Well, those of us who know how, can move the gps receiver over 5'-10', for a DIFFERENT multipath environment, compass and tape between the points, and now we have an independent check of the shot.

Now we never return to the point, and we are good.

N

Actually the Spectra Precision / Asthech ProMark 800, ProMark 220 and ProMark 120 will also fix GLONASS only, US only or a crippled (very low SV count) US/GLO mix only.

It has been advertised for some time and I have not paid much attention to it, but the other day I noticed that in Carlson SurvCE V 3, you can actually turn off US tracking and get solutions that are GLONASS only (see [ this ] for pictures.)

Now that you can turn off US SV tracking, it is easy to test fixing GLONASS only and I have done so in the local TURN (The Utah Reference Network) VRS network. It is a bit trickier to test with say 3 US SV's and 3 GLO SV's as the count is always high enough to fix with either alone.

Just now I am wondering if I could simulate a very low combined count by raising the elevation mask to a very high value?

I don't doubt that Javad instruments do a great job in this regard. Last week I read the results of a GAL only fix from Javad.

But, I can't name a reason that I would want to turn off either constellation. I am getting spoiled running with crazy high SV counts, 24-7-365.

Not sure I would ever want to go back to an RTK instrument tracking 5 SV's. Although I did pull a tape last week, so the time may come 🙂

M

So, RTK in the deep woods with one redundant check against an other another RTK point 5 feet away and under basically the same conditions?

What you described will probably work ... Sometimes ... I just wouldn't bet my license on "probably" and " sometimes".

In the woods I would not trust a fix unless I re-initialized a least two times for a check. The stuff under canopy is RISKY if its for a boundary point.

John Harmon

I would be very interested in test results such as these and I would use it to make my GPS purchase decisions. It seems like GPS sales and advertising wants its customers to view the decision of what brand to buy similar to an automobile purchase. Ambiguous specs are spun around the advertisement but the real target point is brand, loyalty, pier image and overall coolness. They never tell you that the meat of the product is probably produced by someone else and the same board or antenna manufacturer may contribute or produce several competing models. On the other side of the coin, truth in advertising may not produce the greatest sales results unless everyone participated, which we know would be impossible.

Depends on HOW you do it. Loose lock, re initilize. And, watch that you actually change the Multipath environment. I have done this for as far as 40 feet away, in order to get an adequate change in environment. Those of us that have experience with compass and tape surveying, know how to use them.

If in doubt still, get a 3rd shot. Now you actually have a triangle, and you have a real check on it all.

N

Hm. Reckon we could take up a collection and buy some cheap tax auction land somewhere, with trees of course. Erect those pillars and keep the whole place staffed with volunteers year round.

And BBQ. Perpetual BBQ. Supplied by the GPS vendors.

Try to pick a place with good hunting and fishing and I'll come help with the tests.

I guess the real questions then become, 1)Which system will give your the best fix given a certain coverage. 2)Which system will give the accurate fix under continually greater cover.

My problme has never been bad fixes, as with checks I have always ben comfortable with the results and check shots that I have gotten. My biggest problem is getting that fix to begin with.

I will save you plenty of time and effort...GPS does not work under canopy. Period. You can run all the tests you want, it still won't work. The truth in advertising is that there isn't any. There are plenty of people who will sell you the snake oil you are seeking though.

My argument is that IF it fixes faster, then you will get MORE shots, in the same time, and the data now has been checked. So what it lies occasionally? But, if you have to wait for an hr for that check shot, then you get alot less work done.
N

Most of the time, depending on the density of the canopy, the geometry of the constellation and the mood of the ghost in the machine. However, it has been my personal experience that even with GNSS, tracking beacoup sv, every now and then I will get a 'solid fix' under canopy only to find out later that it flat out lied and I'm off by several feet.

The problem is I can't and won't trust RTK under canopy, so when it comes to tieing crtical control, I don't chance it. Doesn't take that long to set a couple points out in the open and break out the gun and make the tie conventionally, even if it's just as a check. Ol' Murphy is hunting my *ss and getting lazy is leaving the door wide open for him with a welcome mat.

exactly, you cant rely on it and you cant check it with a tape, because software is funny, it likes to give the same bad answer under the same or nearly the same conditions, so your check shots may (or may not) be just as precisely bad. Now, I realize someone will come along and say they have gotten good results by doing this or that and holding their mouth just right.Doesnt matter what the box says, surveyors need to understand the proper procedures. GPS under canopy is never the right solution. Run your GPS outside the canopy and conventional under. I dont trust salesman as they have told me the same bunk for years.

Williwaw is 100% correct and that comes from 10 years of RTK experience.

> Most of the time, depending on the density of the canopy, the geometry of the constellation and the mood of the ghost in the machine. However, it has been my personal experience that even with GNSS, tracking beacoup sv, every now and then I will get a 'solid fix' under canopy only to find out later that it flat out lied and I'm off by several feet.
>
> The problem is I can't and won't trust RTK under canopy, so when it comes to tieing crtical control, I don't chance it. Doesn't take that long to set a couple points out in the open and break out the gun and make the tie conventionally, even if it's just as a check. Ol' Murphy is hunting my *ss and getting lazy is leaving the door wide open for him with a welcome mat.

Yep.

> I propose that we build a gauntlet. 10 or so permanently mounted in concrete pegs, that are about 6' tall, and are in the woods. Good woods. Deep woods, and we can place all the current GPS receivers on the top, at the same time, and hook them to laptops, and STORE all the data from them. Set the Stations all in a straight line, about 10' apart, so that we can truly compare the data. Let them sit, for 24 hrs straight, then swap the receivers, so that in 10 days, they have all sat on a peg for 24 hrs. Now, take this pile of data, and REVIEW it, and find out how many bad inits it got. How many fixed solutions it got. Find out how accurate it was. Find out "If you were doing RTK for "this 5 mins, and saved these 5 shots, you can see, you would have ____ error".
>
> Now, after the 10 days are up, you can take any NEW gps receiver, and one of those original 10, run them for 1 day, swap pegs, and run for another day, and compare the new products, against some sort of standard.
>
> Would you be interested in the results of such a test?
>
> Nate

We would all like to see this, but...who will pay for it?

The magazines won't do it, for obvious reasons...

Time Is The Problem

You need at least 4 satellites with a common time component to start getting a good position. GPS time and GLONASS time are not interchageable. So at a minimum 4 and 4 are required. However the number of satellites does not guarantee anything. I have seen very poor precision with 8 GPS satellites because they are in a narrow range and not well populating the sky.

When considering the Javad guru every time he speaks he claims to have improved everything. But it seems along the way the great majority who have this, that or the other manufacturer is very unwilling to give up what they already have. I surmise he has never really done it better, only differently to avoid his own patents.

Paul in PA