Follow this link to skip to the main content NASA Home Page JPL Home Page Caltech Home Page
NASA - Jet Propulstion Laboratory GDGPS vertical separator + View the NASA Portal
JPL Home JPL - Earth JPL - Solar System JPL - Stars and Galaxies JPL - Technology
The Automatic Precise Point Positioning Service of the Global Differential GPS System
Home
Unique Features
About GDGPS How to use APPS
Under The Hood Instant Positioning Registered Users Register
APPS with Pressure Data

  This is a unique feature designed to enhance the accuracy of the position estimates, especially in Kinematic mode. It is most powerful in improving positioning accuracy for airplanes because it helps decorrelate the estimates of the vertical position and the wet zenith delay.
 
  APPS offers the user an option to upload pressure measurements, which are converted internally to hydrostatic delay value s and used in the modeling of the tropospheric delay. If the user opts not to upload pressure measurements, APPS will mod el the hydrostatic delay based on the altitude of the receiver, using the model: 1.013 * 2.27 * exp(-.000116 * h) meters, where h is the station height above the ellipsoid in meters. Any errors in this model are usually fully absorbed by the estimated wet zenith delay and total delay gradients that are estimated as random walk processes every measurement epoch.
 
  For airplane/helicopter positioning, which is much more sensitive to errors in the modeled hydrostatic delay, APPS automa tically extracts pressure data from weather fields obtained in near-real-time from the National Center for Environmental Prediction (NCEP). Currently these fields cover the continental U.S., Alaska, and Hawaii. These objective analysis press ure fields are interpolated to the computed flight path. The derived pressure is typically accurate to better than 10 mba r (roughly equivalent to 2 cm of zenith delay).
 
  User-provided pressure measurements must have the following format:
 
  UTC_Time(yyyy:mm:dd:hh:mm:ss.ssss) Pressure(mbar)
 
  No spaces are allowed within a field. Arbitrary number of spaces can separate the two fields. No headers or comments are allowed. Partial time format is allowed (as long as it contains no spaces (for example, 2007:7:19:17:0 is ok). For exampl e:
 
 
2007:07:19:17:32:8.0  931.59
2007:07:19:17:32:10.000     931.59
2007:07:19:17:32:15.0000  931.590
2007:7:19:17:32:20.0000  931.6
2007:7:19:17:32:25.0000  931.59
2007:07:19:17:32:30.0000  931.59
2007:07:19:17:32:35.0000  931.60
2007:07:19:17:32:40.0000  931.60
2007:07:19:17:32:45.0000  931.60
2007:07:19:17:32:50.0000  931.60
 
  Important Note: If pressure measurements are provided by the user, only the RINEX records within the span of the pressure data will be processed.
link to First Gov NASA logo