2.

Ionospheric and Tropospheric Delays

.  For users near the reference

station, the respective signal paths to the satellites are close enough

together that the compensation is almost complete.  As the user to

reference station separation is increased, the different ionospheric and

tropospheric paths to the satellites can be far enough apart that the

ionospheric and tropospheric delays are no longer common errors.  Thus,

as the distance between the Reference Station and user receiver

increases the effectiveness of the atmospheric delay corrections decrease.

3.

Ephemeris Error

.  This error is effectively compensated unless it has

quite a large out of range component (for example, 1000 metres or more

due to an error in a satellite  navigation message).  Even then, the error

will be small if the distance between the reference receiver and user

receiver is small.

4.

Satellite Clock Error

.  Except in a satellite failure situation, this error is

more slowly changing than the SA dither error.  For all practical purposes,

this error is completely compensated, as long as both reference and user

receivers employ the same satellite clock correction data.

The correlation of the errors experienced at the Reference Station and at the user

location is dependent on the distance between them, but they are normally highly

correlated for a user within 350 km of the Reference Station.  Table 10 1 shows the

error budget for a PPS DGPS system, i.e., there is no added SA error.  The error

budget assumes that the common range error sources due to the Space and

Control Segment are eliminated in a PPS DGPS system.

Table 10 1.  PPS DGPS Error Budget

Differential Mode

Segment

Normal

Near

Far

Source

Error

Mode (m)

(m)

(m)

Space

Clock and Nav Subsystem Stability

6.5

0.0

0.0

Predictability of SV Perturbations

2.0

0.0

0.0

Other

1.0

1.0

1.0

Control

Ephemeris Prediction Model

8.2

0.0

0.0

Implementation

Other

1.8

1.8

1.8

User (P(Y) Code)

Iono Delay Compensation

4.5

0

4.5

Tropo Delay Compensation

3.9

0

3.9

Receiver Noise and Resolution

2.9

4.1

4.1

Multipath

2.4

3.4

3.4

Other

1.0

1.0

1.0

UERE (95%)

13.0

5.8

8.3

Atmospheric errors can be eliminated if the user is close to the reference station,

but if they are more than 250 km apart, the user will obtain better results using

correction models for ionospheric and tropospheric delay.  250 km is a reasonable

10 8