Navman 11 User Manual

Page 79

reserved for use by the DoD and certain authorised

users. SPS is less accurate and intended for

general public use. This is the level of accuracy

used by the Jupiter family of GPS receivers.

The SPS signal can be intentionally degraded to

a certain extent by a process known as Selective

Availability (SA). SA is used to limit access to the

full accuracy of SPS in the interest of D.S. national

security.

Differential GPS (DGPS) Description

The following general description of DGPS is from

the document RTCM Recommended Standards

For Differential NAVSTAR GPS Service. Refer to

that document for more speciﬁc details of DGPS

operations (see Appendix B).

Differential operation of the GPS offers the

possibility of accuracies from 1 m to 10 m for

dynamic, navigation applications. DGPS operation

requires a reference receiver to be placed at a

known, surveyed-in point. By comparing the known

location with that predicted by the GPS, corrections

are determined. These corrections are then

broadcast to nearby users who use them to improve

their position solutions.

Sources of bias error

The differential technique works if the main errors

are bias errors due to causes outside the receiver.

This is the case for GPS. The major sources of

error are the following:

SA errors: artiﬁcial errors introduced at the

satellites for security reasons. Pseudorange

errors of this type are about 30 m, 1-sigma.

PPS users have the capability to eliminate them

entirely.
Ionospheric delays: signal propagation group

delay, which is typically 20 to 30 m during the

day and 3 to 6 m at night.
Tropospheric delays: signal propagation

delays caused by the lower atmosphere. While

the delays are as much as 30 m at low satellite

elevation angles, they are quite consistent and

modellable. Variations in the index of refraction

can cause differences (between reference

station and user) in signal delays from 1 to 3 m

for low-lying satellites.
Ephemeris error: differences between

the actual satellite location and the location

predicted by the satellite orbital data. Normally

these are quite small, less than 3 m, but they

could be more than 30 m under SA.
Satellite clock errors: differences between

the satellite clock time and that predicted by

the satellite data. The oscillator that times the

satellite signal is free-running; the GPS ground

control station monitors it and establishes

corrections that are sent up to the satellite

to set the data message. The user reads the

data and adjusts the signal timing accordingly.

Satellite clock errors are completely

compensated by differential operation as

long as both reference and user receivers

are employing the same satellite data.

Ephemeris errors, unless they are quite large

(30 m or more) are similarly compensated by

differential operation. SA errors that affect the

timing of the signals are also compensated

by differential operation, except that the

corrections lose their validity after a period of

time.

For users near the reference station, the respective

signal paths to the satellites are sufﬁciently close

that compensation is almost complete. As the

separation increases between user and reference

station, the different ionospheric and tropospheric

paths to the satellites may be sufﬁciently far

apart that the atmospheric heterogeneities cause

the delays to vary. The extent of the difference

constitutes an error in the DGPS measurement,

called ‘spatial decorrelation’. This type of error

will be greater at larger ‘user-to-reference station’

separations.

Required DGPS Equipment

The equipment consists of a GPS receiver with

antenna, a data processor, a data link receiver with

antenna, and interfacing equipment as illustrated

in Figure C-5. The data processor applies the

corrections received from the reference station to

the pseudoranges measured by the sensor.

Application of DGPS Corrections

For each satellite employed by the user’s receiver,

the correction obtained from the reference

station (message type 1 or 9) is added to the

pseudorange measurement. The correction itself is

derived from the range and range-rate, adjusted to

account for the time elapsed between the time of

reception of the correction and the time of the user

pseudo-range measurement, as follows:

PRC(t) = PRC(t

) + RRC x [t-t

]

where:
PRC(t) is the correction to be applied
PRC(t

) is the range correction from the

message RRC is the range-rate correction

from the message
(t

) is the time reference of the correction

(t) is the time associated with the pseudo-

range measurement.

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