Samar J. Singh, Ex.C (UK), MPhil (HKU), PhD (Wales)
The Hong Kong Polytechnic University
Abstract
This paper undertakes a brief review of the Global Positioning System
(GPS) and considers the role of GPS derived outputs, in terms of their potential for
future deployment in the sea-going environment. The outcome of observations from High
Speed Craft are used to illustrate errors that may arise, when navigating in close
proximity to land, or, from damped observations. Representative results of manoeuvring
trials on a large container ship are used to depict the potential that GPS derived outputs
have for providing a hitherto unknown quality of real time feedback to the navigator. The
paper also explores the role of Conventional Differential GPS (CDGPS) and Wide Area
Differential GPS (WADGPS).
Keywords:
Global Positioning System, Errors,
Outputs, Differential, WADGPS, Accuracy
A Brief Review of GPS
The Global Positioning System has become increasingly relevant to the
demands of modern life. Its applications stretch from its use in the prevention of car
theft to the measurement of deformation on bridges. In the civilian context, it's growth
has been primarily driven by the requirements of precise positioning from the surveying
and hydrographic community, where it has created a revolutionary change in the way in
which position data is captured, processed, analysed, verified and presented.
The full configuration of 24 Satellites was achieved in 1993, and made
the Standard Positioning Service (SPS) a viable civilian utility. This provides 100 metre
accuracy at the 95% level. The Precise Position Service (PPS) is available to restricted
users who have the necessary control codes. The services are available twenty four hours a
day in all weathers and many receivers available today are capable of providing several
observations per second.
The Satellites transmit at two frequencies, L1, at 1575.42 MHz and, L2,
at 1227.6 MHz. These carriers are modulated with two coding systems and a navigation
message. The codes are referred to as the C/A Code and the P Code and provide the means
for positioning. However, at the present time, the C/A code is further modulated by an
imposed error in the satellite clock and in the navigation message, thereby lowering the
accuracy levels to the SPS specification. This phenomenon is called Selective Availability
(SA). The P Code is encrypted to transmit a Y Code which requires an encryption key to be
usable.
The civilian GPS world has reacted to the challenge of SA by various
means which use the well known principle of differential correction to improve the
accuracy available. This is made possible by the fact that many errors sources, though not
all, correlate well over large distances. Hence, the existence of a monitor station which
continuously compares it's observed measurements with the expected measurements arising
from its known position, can be used to transmit corrections to remote users. Corrected
observations at the remote position can have errors which are less than 5 meters but such
errors will Increase with distance from the monitor station almost linearly as a function
of range from the monitor station. Such monitor stations are available at the present time
in many ports, and transmit the corrections as a public service without charge. Full
coverage of the US coastal region is expected in the near future.
