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Otterman speaks...

Cycling, macintosh, natural history and life in Singapore - Archives

List of Categories : travel * museum * cycling * Singapore Naturalist * science * kakis * mangroves * movies * mac and the internet * meow * NUS * life in Singapore * lit * world *

Tue 08 Aug 2006

Understanding the limits of GPS

Category : Singapore Naturalist

Field researchers need to provide reference to named places and not just rely on a GPS fix which could well be meaningless. This complexity is compounded by the fact that few read a GPS mannual thoroughly to understand its limits.

A reference to a named place and cross-checking your specific location with a paper map is critical. In reference to the differential GPS that the new Singapore Land Authority's (SLA) network system will use, Goh Pong Chai explains why 'a good GPS fix is almost impossible in Shenton Way':

"Debunking that all seeing myth." By Goh Pong Chai (NTU's School of Civil and Environmental Engineering). Today, 08 Aug 2006.

Some excerpts:

"Misguided reports and fine-print caveats create confusion as to how accurate GPS really is - from tens of metres to millimetres. Is millimetre-accuracy a fact or a myth?

GPS simply measures the distance from the receiver to a handful of GPS satellites. The principle is easy to understand: If you cannot see at least four satellites, GPS does not work. It will not work in buildings and under dense foliage. ...

Accuracy also depends on the technique used. Low-cost receivers use only GPS codes, which are easy to measure but less accurate. This is like measuring with a tape that has only 100-metre markings. One can increase accuracy by using what is called carrier-phase GPS - which requires special, more complicated receivers - the equivalent of using another tape with 1m markings.

The next most important factor in accuracy is how biases are mitigated. While most biases may be mathematically modelled and mitigated, multipath - the corruption of GPS signals due to reflection off nearby structures - is a real nuisance, and difficult to control, in environments with high-rise buildings.


In short, it is like trying to measure something when the measuring tape meanders (caused by signals passing through the lowermost part of the earth's atmosphere), bends (caused by multipath) and stretches (caused by clock errors and noise) - the result is inaccurate.


... you can have the best equipment, but you cannot control the operating environment and how people use it. Theoretical high-precision limits can be achieved only under ideal conditions - real life is more unpredictable.

Read the article.

Posted at 12:13AM UTC by N. Sivasothi | permalink | , .