Moose,
Here is my take on GPS and Trimble:
The accuracy of GPS varies greatly because of so many different factors involved. You may have been lucky to get so close to elevation from the sign. Back in the day, I think it was a Tuesday, the control segment continually deployed selective availability (S/A) that dithered the timing of the signal to artificially increase the error on the position estimates. The idea of S/A was to selectively allow the best available accuracy to only those with the proper key given out by our government. This effect created up to 100 meters of error, with a standard deviation of about 35 meters. If one were patient enough, after a day of averaging out this error, one could get down to something less than 5 meters of error. This was due to the particular polynomial nature of the errors. A lot of time and money was spent trying to remove this error.
The most direct method to remove S/A was to use a stationary GPS receiver to determine the amount of S/A that was applied to each satellite, and then remove this error by applying these corrections to the measurements in the other receiver that was going about its business. One could send these corrections over a radio link and quite effectively disable S/A. Because the nature of the corrections is the difference between an observed measurement and a mathematically predicted estimate, these were labeled as differential corrections, and gave rise to the term differential GPS, and DGPS for short.
The average user of GPS would not have access to DGPS correction and would have to be satisfied by 35 meters of accuracy. So fisherman could find their favorite spots, would-be sailors could get a bearing to their marina, and hikers could be happy to find the ranger station or their car again. Happy campers all.
Trimble started out with a Loran receiver marketed to mariners and created GPS/Loran hybrid receivers while the satellite constellation was still being deployed. When there were enough satellites to guarantee a position fix throughout the day, Trimble moved on to GPS-only receivers.
As the technology improved, companies started moving into surveying and the differential technique became a critical component in marketing. The technology of DGPS advanced so far that one could survey across 1000 Km with an accuracy of 1 meter, and with another technique called ‘kinematics’ one can obtain an accuracy of 1 cm over a 10 Km range, roughly.
As more and more companies got into the act, and more and more equipment allowed users to more cheaply remove S/A, I think the government finally decided to drop S/A for good. Well almost. They can still enable S/A while a satellite is over, say China for example, and disable it again as that satellite passes over the US. The US Department of Transportation also got into the act and created a program call WASS – for Wide Area Augmentation System. This was to improve the quality of the corrections over a much wider area, specifically to cover the continental US and Canada. Interesting to me at least, is that we have one branch of our government that wanted to degrade the quality of GPS for most, while another branch wanted to improve the accuracy of GPS for everyone. Some people blame the US administration at the time for giving our enemies access to the full quality of GPS, but the act of disabling S/A was an acknowledgment that the intent of S/A had long been rendered irrelevant by DGPS advances.
When I first started looking into GPS, and Trimble in particular, I believed the marketing that GPS would become another utility like electricity or water services or phones. In other words, GPS would become ubiquitous in our lives and we would subconsciously reach for our GPS boxes when we needed to orient ourselves or needed directions. I waited a very long time, but I eventually moved on. Everything took much longer than I would have thought. I guess now that I was thinking that GPS would have a similar uptake as the VCR technology, or the CD. The story has slowly unfolded as I imagined, but at a pace unsuited to my investment mode at the time. <g> I wish I had a 10- or 15-year commitment mode for investments.
Back in the late 90’s, I got wind of E-911 and the possibility of GPS technology being used to locate cell phones. I actually thought that Trimble might make a foray into E-911; I was wrong again. SnapTrack was bought by Qualcomm and that ended that story as far as I was concerned.
Best to you. |
