Viavi 87339, Operation Manual

Page: 137 / 198

Principals of Operation
6 -20
PDOP (Positional Dilution OF Precision) is a measure of overall uncertainty in a GPS
position solution with TDOP not included in the estimated uncertainty. The best PDOP
(lowest value) would occur with one satellite directly overhead and three others evenly
spaced about the horizon. PDOP = SQRT(HDOP^2 + VDOP^2).
GDOP (Geometric Dilution Of Precision) is a measure of the overall uncertainty in a GPS
position solution. GDOP = SQRT(TDOP^2 + HDOP^2 + VDOP^2) or in another form
GDOP = SQRT(PDOP^2 + TDOP^2). GDOP value should be less than 5.
The Position Accuracy = Dilution Of Precision (DOP) times Measurement Precision. So, if
the Measurement Precision = 1m and the DOP = 5, then the best position accuracy will be
5m.
6.1.7 GNSS Augmentation
Augmentation of GNSS, is a method of improving the navigation system accuracy,
reliability and availability through the integration of external information into the
calculation process. SBAS (Satellite Based Augmentation System) and RAIM (Receiver
Autonomous Integrity Monitoring System) are GNSS augmentation systems.
Satellite Based Augmentation System (SBAS)
SBAS is a system that supports wide-area or regional augmentation through use of
additional satellite broadcast messages that contain correctional data obtained from
multiple ground stations at surveyed locations.
The effects of the ionosphere generally change slowly and can be averaged over time.
The effects for any particular geographical area can be easily calculated by comparing
the GPS-measured position to a known surveyed location. This correction is also valid for
other receivers in the same general location.
The data is transmitted via satellites in the SBAS system and is transmitted on the GPS
L1 frequency using a special pseudo-random number, allocated for SBAS use. This
allows the civilian L1 C/A code receivers that support SBAS, to use the correctional data.