division between near and far.  Table 10 1 above also shows how the reference

station receiver noise and multipath errors are included in the differential

corrections and become part of the user s error budget (root sum squared with the

user receiver noise and multipath errors).

10.5  SYSTEM BLOCK DIAGRAM

Figures 10 2 and 10 3 show an example reference station and DGPS user

equipment.  Two receivers are shown at the Reference Station to increase the

station reliability and to provide station integrity.  Nominally each receiver will track

all satellites in view in order to assure that differential corrections are determined

for all satellites.  With the full GPS constellation as many as eight to ten satellites

may be in view.  If eight satellites are visible, the reference station would have to

broadcast data for 8 satellites.  If SPS equipment is used, the broadcast can be

unencrypted.  If PPS equipment is used, the transmission of SA corrected errors

requires the use of an encrypted data link.

From a military standpoint, DGPS no longer remains a passive system.  DGPS

transmitters have a limited range application (up to about 800 km).  There will also

be increased system cost for communications and processing equipment.

GPS

ANTENNA

GPS REFERENCE

PSEUDORANGE

STATION

PROCESSOR

CORRECTIONS

DIFFERENTIAL GPS

DATA LINK TX

GPS INTEGRITY

MONITOR

DIFFERENTIAL GPS

DATA LINK RX

Figure 10 2.  Typical Reference Station

10 9