thanks brembo.
can you get backscatter off aluminium in a boat?
i learned a bit about gps back when they had selective availability. i understood speed to be calculated using the doplar effect, and so even when they only gave us 40 metre position accuracy 95 percent of the time speed was always spot on.
so a surveyer, with a 100k dollar unit, can position to a millimetre ? that is just so cool.
i spose first they use gps to work out the location of the satellites relative to known terrestial locations then work out your location relative to the satellites.
cheers
I dunno about the doppler effect being used in GPS for speed calcs. My strong area was always in time-stamp vector driven calculations. Doppler is used but well beyond my knowledge base.
An AL boat will send signal all over the place. Put the antenna facing upwards on something that blocks any possibility of signal being relected upwards to it (like on top of a cooler or something). Most units have a screen with two circles on it, outer ring is the horizon and the inner ring is a 45 degree zenith. If there are a lot of sats bunched in the 45 ring, expect weird data at times. With a handheld or general navigation GPS backscatter will simply slow it's lock down and give slightly off numbers, on the order of a few feet. With precision measurements that surveyors have to attach a signature and liability too, a wonky satellite signal is a no-no. Often times I'd have to go into the data files and remove a satellite that was hovering near the horizon due to spotty reception.
The old SAS stuff really didn't effect surveyors all that much. The really tight locations were all post-processing. Back in the day of 486 processors a 1/2 day data collection could tie up a high-end station for hours translating and rotating the data. In 2010 it took longer to get the data out of the units via firewire than it took the computer to r and t. All we cared about was relative location and we would set "control" to base our traverse and "sideshots" (angular observations from a control point). A "control station" would be placed on a control point and allowed to collect data all the time a rover was in use. The rover is in communication all the time with the control station and the time of any data point collection is synched. So even if a Titan were to reach down and rip the entire site and toss it two miles it would not matter a lick to a surveyor. The relative accuracy would be intact.
A few surveys we did had to be tied down to known and "real" coordinates. We used Geodetic Survey markers and translated to that system once back in the office (or truck if engineers needed onsite data).
I only ever staked out one house with GPS, it was an architect that was so incredibly anal about solar orientation that I ended up having to research the suns traverse. I could have doe it with a traditional total station but this guy wanted GPS because "it is better". I personally don't argue with lasers but whatever the lease on the RTK stuff cost him a bundle.
If any of this stuff interests you have a look at Cyrax systems. Scanning technology with 2mm accuracy. It's amazing stuff. I demoed one for a basement retrofit(structural) and was blown away with the amount of data these things parse. A good engineer can do anything with a proper Cyrax scan. The newest of the new total stations have this ability built-in, millimeter GPS built-in AND calibrated digital cameras that will overlay the photos as a "skin". Star Treks "tricorder" technology is quickly approaching.