How NDGPS Works
NDGPS improves the radio navigation accuracy of the GPS solution by providing a differential correction based on
the known location of a GPS reference station antenna and the calculated position of the GPS receiver. Each
NDGPS broadcast facility has two reference stations (figure 1). Each reference station has a precisely
surveyed GPS antenna. This survey typically has an accuracy of a few centimeters, although newer sites are being
used to measure plate tectonic movement and are surveyed more accurately and employ more stable reference station antenna structures. When a site is brought on-line, the GPS
receiver calculates the psuedorange from the antenna to each of the satellites in view. This is compared to where the
receiver’s antenna is known to be by the survey. The difference between these two values is the correction factor
or differential correction. This differential correction is then broadcast for users in the coverage area of the
NDGPS facility. Rather than a single value for the entire satellite constellation, a psuedorange correction for each
visible satellite is generated and broadcast.
This ensures that users can use the optimum distribution of satellites for their location. Figure 2
illustrates the area for which NDGPS will provide coverage by the end of 2001.
The technique for generating the differential corrections used by NDGPS is described as a code phase correction since it primarily corrects for errors
in the observed code ranges supplied by the satellites. Where do these errors come from? There are many sources. Table 1 indicates, using observed data from
the satellites, where the errors are generated and how much error each source can contribute.
NDGPS is also designed to improve the availability of GPS by correcting for clock errors that would normally make
the satellite unusable, and to provide an integrity function. As described earlier, the timing of the
signal from the satellite to the user is used to determine the users range from
the satellite. If one of the cesium clocks on a satellite begins to drift, the health message the satellite
broadcasts as part of its signal will usually be set to unhealthy once the
error goes beyond a predetermined drift point or drift rate. The NDGPS service provides a correction for
this error that is applied within the user’s GPS receiver, allowing the satellite to still be used for a navigation solution.
Integrity refers to the ability of the service to notify users that a condition exists such that they should not rely on certain satellites for a navigation solution. Currently, the GPS satellites must pass over a satellite uplink
station to have the satellite set its broadcast as unhealthy. There are scenarios where this may not occur
for several hours. The NDGPS service provides this integrity function within 5 seconds of detecting a satellite
anomaly.
NDGPS Fact Sheet
