multi path

jd, it is my understanding that multipath is a 'reflected' signal. foliage is likely causing cycle slips (interruptions of individual signals), so the ambiguities are having to be constantly resolved. either way you slice it, neither condition is good.

what is your work around? my guess is to navigate/stakeout as near to the point as possible, then change to compass and tape stakeout from there.

or am I visualizing this wrong?

There are some work arounds. Say for example you layout your fire hydrant. Its under a tree so your not sure the shot is good. If its a single tree go out in the open and if it says that point in stakeout is 80 feet away, check it with a cloth tape. Then go in a different direction and check the distance again. Depending on what you are doing, this may be good enough.

GPS works by measuring the time and hence the distance from satellites. If the signal has more than one path to the antenna due to reflections, then the receiver will see an ambiguous signal arriving at multiple times. Foliage also will attenuate the signal, making it easier for a reflected signal to swamp out the right one.

The receiver may take the stronger signal, but which one is stronger may change as you or the satellites move. It may take some average of the multiple signals. And if the signals are near the same strength they can cancel themselves out in certain positions.

I would stake 3 point and pull tapes. Until you are comfortable, pull tapes between the points and to the point in question. Get the GPS in the wide open.

I will leave the technical explanation to others, but I can give you some practical tips.

1. There is a difference between a real fix and a momentary fix.
-reject any fix with HRMS OR VRMS greater than 0.30'
-if it bounces in and out of fix, or you have to "walk it in", that solution is suspect
-shoot suspect points a second time with an hour or two separation. Bonus for shooting all points twice.
2. Base placement makes a difference. Multi-path on the base is an even bigger deal.
3. Updated firmware can help.
4. Some DC's and receiver combinations allow for settings that tell the software that you are in a high multi-path environment.
5. Checks with hand tape from a "good" shot might help you sleep the next night.

Thanks, all

hitting it again this morning. Still working thru that learning curve. Office guy tells me yesterday that I shoulda brought the total station with me....considering the fog and other weather issues, I would have been sitting in camp...we'll see what today holds.

again, thanks!!
-JD-

Thanks, all

Set three points in the open and distance-distance to it, as said earlier.

I like the distance-distance intersect thingy myself when staking out design points under canopy or in a building shadow with RTK and don't have a TS handy. Typically I'll set a lath out in the open and write the distance to the point I'm staking, do the same with another lath at something between 45-90 degrees. Whip out the logger's reels or rag tapes and two tape the point in off the laths. When in doubt, check it with a third distance shot.

As another check measure between the "two" or "three" trilateration points. If they measure what the coords. say, you have it locked up. Sleep well.

P.S., I try to keep my trilateration points less that 50 feet from the unknown point.

my topcon equipment has a setting for "multipath reduction and CO-OP tracking"

with these features on i have never received a false fix with topcon equipment unless the fix was over 0.03m or it will not hold a steady fix for more than 10+seconds then weird things can happen.

i have tested it over and over again marking points close to buildings, canopy etc with the total station and always to my suprise well within the tolerance required. gives a guy good confidence in his equipment. demoed a new spectra sp80 and i cant say the same false fixes where i normally would have zero issue with the topcon.

>unless the fix was over 0.03m or it will not hold a steady fix for more than 10+seconds then weird things can happen.

Yep.

For the record, Topcon no longer recommends using the Co-op tracking option in your settings.

That function as I recall was intended to synthesize the data you should have been getting from a satellite if the signal from a particular satellite became blocked for a brief time because of obstruction.

The thought being, if you are in an open position receiving several satellites' signals and then needed to move to a point nearby where one or more would be blocked, this function would sort of fabricate the data you should have been getting. This was an attempt to wrangle a usable position out of your equipment when the available constellation was considerable less robust than it is today. A Javad idea, this. Apparently there was/is significant likelihood that your allegedly fixed solution obtained with this option enabled may well also be a fabrication.

Not recommended to use. Probably never was more than an interesting idea.

Perhaps someone more knowledgeable than I will chime in but that is my understanding.

It's looking like the engineer that did the layout for the fire hydrants somehow shifted the basemap East about 30'. I did have trouble in a few spots, but was able to get what I needed with about 16' of rod! Other issue is that there was another survey crew on a road job that had a much more "robust" radio on their base, switched frequencies, but that made no difference. Drafter is saying ALL my points, power poles, manholes etc were falling in the same offset as the fire hydrant points. Picked up some property corners around town, so that should correct the problem. Field fit the dang hydrants anyhoots, doubt that anyone would want one in their living room!! Thanks for all the advice.

-JD-