250 / 352
246
The second example of experience refers to an investigation of GPS application to
measurement of suspension bridge movement using pseudolites (PS), Fig. 7. This infor-
mation can be used to warn of dangerous conditions and estimate the long-term deterio-
ration of the structure. Additionally, this information is of value to bridge designers and
for traffic management. It is known that when using GPS, the accuracy, availability,
reliability and integrity of the position solutions is very dependent on the number and
geometric distribution of the available satellites. One option to improving the satellite
geometry is to use ground-based transmitters of GPS-like signals, “pseudolites” (PLs).
PLs can be properly located to provide additional ranging information, and therefore
improve the positioning precision.
In situations with limited or no visibility of the GPS
satellites, ground transmitters that
emulate
the signal structure of the GPS satellites (
PLs
) can be
used as additional or replacement
signal
sources.
The measurements to test the suitability of a
PL-augmented GPS system for bridge monitoring, were conducted at the Wilford
suspension footbridge over the river Trent in Nottingham, UK on 16 October 2002 /9/.
Figure 7: Pseudolite (PL) locations in relation to base and rover GPS receivers /9/
Pseudolites are ground-based pseudo-satellite transmitters which initialise carrier-
phase DGPS navigation systems (CDGPS) in seconds to perform RTK measurements
with error of about 1 cm. Also are used synchrolites (synchronized pseudolites) which
derive their timing from individual GPS satellites; they can replace the CDGPS reference
station and data link and simultaneously initialize CDGPS navigation.
The equipment used for the measurements consisted of two types of GPS receivers
used in the trial, and connected to the same Leica AT502 antenna via a splitter /9/. The
low-cost (few hundred $US) single-frequency Canadian Marconi Corp. (CMC) Allstar
receivers were used to track the PL and GPS signals, while the Leica SR530 receivers
tracked only the GPS satellite signals. The Allstar receiver allows the user to request
particular PRN codes (12, 16 or 32) to be tracked, and this is a basic requirement for a
PL-tracking GPS receiver. The Leica receivers were used to provide an independent
check on the quality of the GPS data from the CMC receivers and their dual-frequency
capability would enable ambiguities to be resolved cinematically On-The-Fly (OTF). The
three pseudolites were located on the river bank footpaths, Fig. 7.
By data processing of GPS-PL positioning it was found out that in the vertical
component time series the standard deviation was 7.2 mm (sub-cm) and smaller than the