GPS Ephemeris Data
Online Prediction and Planning websites
(For location of Bellingham use Latitude = N 48.75° - Longitude = W 122.5°)
- Trimble's Planning Online page
- Navcom's Satellite Predictor Tool
- Here's a snapshot of current locations of GPS satellites (& WAAS = #133, 135 & 138)
(Green = Precision Approach, available for WAAS, Yellow = low precision, Blue = out of range for WAAS, Red = unhealthy, Do Not Use)
GPS satellites transmit information about their location (current and predicted), timing and "health" via what is known as ephemeris data. This data is used by the GPS receivers to estimate location relative to the satellites and thus position on earth. The Ephemeris Data can also be used to predict future satellite conditions (for a given place and time) providing a tool for planning when (or when not) to schedule GPS data collection.
Until recently ephemeris data had to be downloaded and used with special desktop software (i.e., Trimble Planning or Quick Plan). But now the ephemeris data can be accessed online (see links above). Ephemeris data is considered good for up to 30 days (maximum). If you are attempting to predict GPS conditions for more than 30 days into the future it is recommended that you repeat your predictions a week or two before collection to verify the results (satellite conditions are constantly changing).