An introduction to GPS technology Accuracy

Acknowledgements Thank you to the University of New Hampshire Cooperative Extension Program for providing these materials!

GPS Accuracy – what are your needs? How accurate does your GPS data need to be? Is vertical accuracy important? What are the ways to improve accuracy? It is important to evaluate your accuracy needs before beginning a project or purchasing GPS equipment. While handheld GPS units provide sufficient accuracy and data quality for the majority of applications, it is possible that your goals might require a different approach.

GPS Accuracy – signal delay Atmosphere, clock errors, multipath error 6 secs 4 secs Idealized situation One major sources of error which will affect our ability to accurately locate ourselves using GPS is signal delay. While GPS satellites orbit high above the earth in a relative vacuum, the signal must travel through the earth’s atmosphere, and errors can be introduced. The lower the satellite is above the horizon, the greater the atmospheric travel path will be.

GPS Accuracy – signal delay Atmosphere, clock errors, multipath error 6 secs 4 secs 4 secs uncertainty real situation One major sources of error which will affect our ability to accurately locate ourselves using GPS is signal delay. While GPS satellites orbit high above the earth in a relative vacuum, the signal must travel through the earth’s atmosphere, and errors can be introduced. The lower the satellite is above the horizon, the greater the atmospheric travel path will be. 6 secs uncertainty

GPS Accuracy – satellite arrangement Another major sources of error which will affect our ability to accurately locate ourselves using GPS is the arrangement of the satellites during data collection. GPS satellites are not geostationary, but orbit the earth. This means that their position relative to one another changes with time. This changing satellite geometry will influence our ability to accurately determine location. When the visible satellites are clustered in one area above us, our positioning error relatively large. Conversely, when the satellites are well separated spatially, we find much higher accuracy. The highest degree of uncertainty will always be associated with z - height. To determine z to the same level of accuracy as x and y, we would need information from satellites positioned below us, and these signals are block by the earth. bad arrangement good arrangement

GPS Accuracy – satellite arrangement Geometric arrangement of satellites in space When the GPS satellites are clustered more closely together, the uncertainty in our position increases. In a simple example using only two satellites, the increased error introduced by closely clustered satellites is clearly visible in the increasing size of the pink polygon from the “good” to “bad” satellite constellations. 6 secs 4 secs uncertainty uncertainty good constellation

GPS Accuracy – satellite arrangement Geometric arrangement of satellites in space When the GPS satellites are clustered more closely together, the uncertainty in our position increases. In a simple example using only two satellites, the increased error introduced by closely clustered satellites is clearly visible in the increasing size of the pink polygon from the “good” to “bad” satellite constellations. bad constellation

GPS Accuracy – differential correction Base stations measure inaccuracies and send out correction signal 15m: typical handheld GPS accuracy without corrections 3m: typical handheld GPS accuracy with corrections WAAS: most common handheld correction The differential GPS system utilized by handheld receivers is known as WAAS (Wide Area Augmentation System). This system, when fully operational, will be used for aircraft navigation, in conjunction with a LAAS (Local Area Augmentation Systems) in the immediate vicinity of local airports. The WAAS system consists of 25 base stations located throughout the U.S. and a series of geostationary WAAS satellites above the equator. The land based stations collect data from the GPS satellites, relay this information to a ground based control station where corrections are relayed to WAAS satellites for re-broadcast. There are two additional satellite based differential GPS systems in operation. EGNOS (the European Geostationary Overlay System) is currently in development by the European Space Agency. MSAS (MTSAT Satellite based Augmentation System) is being developed to provide Japan with satellite correction information. It is important to realize that all satellite based augmentation systems are regional in nature. For GPS users in the northeast U.S. WAAS signal availability is now in a state of flux. The WAAS satellite (35) which had been serving this region was recently moved further west and the signal is now marginal in this region. Two additional WAAS satellites which will provide signals for users in this region (42N, 71W) are in testing mode. Garmin SV Number Current Status (July06) Approx. Elevation 35 operational 5° 48 testing 12° 51 testing 29° 47 operational below horizon http://www.ae.utexas.edu/~shep/

Questions?

This GPS presentation includes slides from Trimble Navigation and Tom Luther, USDA Forest Service.