A downlink option is also possible from the users to the reference station or other

central collection point.  In this case the differential solutions are all calculated at a

central location.  This is often the case for test range applications where precise

vehicle tracking is desired but the information is not used aboard the vehicle.  The

downlinked data can be  position data plus the satellites tracked, or pseudorange and

deltarange measurements, or it can be the raw GPS signals translated to an

intermediate frequency.  The translator method can often be the least expensive with

respect to user equipment, and therefore is often used in munitions testing where the

user equipment may be expendable.

10.3.4  Local Area and Wide Area Systems

The accuracy of a DGPS solution developed using a single reference station will

degrade with distance from the reference station site.  This is due to the increasing

difference between the reference and user receiver ephemeris, ionospheric, and

tropospheric errors.  The errors are likely to remain highly correlated within a distance

of 250 km, but such systems are often limited by the data link to an effective range of

around 170 km.  Such systems are usually called local area DGPS (LADGPS) systems.

DGPS systems that compensate for accuracy degradations over large areas are

referred to as wide area DGPS (WADGPS) systems.  They usually employ a network of

reference receivers that are coordinated to provide DGPS data that is valid over a wide

coverage area.  Such systems typically are designed to broadcast the DGPS data via

satellite, although a network of ground transmission sites is also feasible.  A user

receiver typically must employ special algorithms to derive the ionospheric and

tropospheric corrections that are appropriate for its location from the observations

taken at the various reference sites.  The U.S., Canada, Europe, Japan, and Australia

are planning to deploy WADGPS systems transmitting from geostationary satellites for

use by commercial aviation.  The satellites will also provide GPS like ranging signals. 

Other nations may participate by providing clock corrections only from single sites or

small networks, requiring the user to derive ionospheric corrections from an

ionospheric model or dual frequency measurements.  Similar systems limited to military

use have also been discussed.

Some commercial DGPS services broadcast the data from multiple reference stations

via satellite.  However, several such systems remain a group of LADGPS rather than

WADGPS systems.  This is because the reference stations are not integrated into a

network, therefore the user accuracy degrades with distance from the individual

reference sites.  

10.4  SOLUTION ERROR SOURCES

The major sources of range error for nondifferential GPS are:

10 6