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 values and used in the modeling of the tropospheric delay. If the user opts not to upload pressure measurements, APPS will model 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 automatically 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 pressure fields are interpolated to the computed flight path. The derived pressure is typically accurate to better than 10 mbar (roughly equivalent to 2 cm of zenith delay).
Format of User-Provided Pressure Data
User-provided pressure measurements must have the following format:
Data 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 example:
Example Data
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