94 POSITION-FIXING
so it can use them to correct its measurements. The
corrections are only relevant to those satellites used in
the position calculation (figure 11.12).
When this method was first developed, operators had
to set up their own reference stations for their particular
projects. Today many authorities have set up chains of dif-
ferential stations around much of the coast and in many
harbours. Corrections can also be received from satellites
in a large proportion of the world. The International
Association of Lighthouse Authorities (IALA) has set up
a system that is free to users and extends from the north-
ern North Sea into the Mediterranean. Corrections are
received by radio link and the signal is decoded and sent
to the GPS receiver. With the introduction of differential
range corrections, positional accuracy with SA turned off
is now of the order of <1 m to 4 m (3 –13 ft) at a 95 per
cent confidence level, depending on the time of day, loca-
tion and the quality of the receiving unit.
At the time of writing, enhanced systems are being devel-
oped that are due to come online imminently. Termed
augmentation systems, they will provide differential
range corrections, more satellite ranges as well as system
status information. This information will be broadcast
from satellites and will be available in specific regions.
The European system is known as EGNOS (European
Geostationary Navigation Overlay Service), with the
compatible system in the USA known as WAAS (Wide Area
Augmentation System) and MSAS (Multifunctional
Satellite Augmentation System) in Japan. These systems
will be a significant step forward, as they will provide
dynamic accuracies of 3 to 6 m (10 –20 ft) from com-
patible standard hand-held GPS units. This will have
serious implications for marine archaeology, as they will
significantly bring down the cost of remote-sensing sur-
veys and coastal-evaluation surveys, particularly in areas
that do not benefit from coverage of national differential
networks.
Real-time kinematic (RTK)
RTK GPS is similar to differential GPS in that a receiver
is set up on a known point or reference station and cor-
rections are transmitted to the remote unit. The nature
of the corrections, however, is a little more complex as they
are based on the phase, a property of the radio signal, not
the derived ranges as in a differential system. The cor-
rections are broadcast to the mobile receiver. The advan-
tage of this system is that centimetric accuracy in three
dimensions can be achieved in real time. The disadvant-
age of this system, at present, is the cost of the equipment.
The system is also limited to a maximum range of
approximately 40 km (64 miles) due to the changes in the
properties of the atmosphere and the curvature of the earth.Equipment
With recent developments in electronics, the cost (and,
indeed, size) of GPS receivers has drastically reduced,
making GPS one of the most cost-effective means of
position-fixing. It is possible to purchase a hand-held unit
with a differential receiver that is able to pick up the IALA
correction service, or an EGNOS compatible unit, for less
than the price of a medium-quality survey sextant. Most
leisure craft and all commercial craft carry a GPS unit.
Receiver quality and, therefore, price is dependent on the
number of receiving channels, the quality of the internal
clock and the sophistication of the algorithms used to cal-
culate position.
It is important to stress at this stage that GPS under-
takes all position calculations on the WGS84 datum.
Most receivers have the ability to convert from WGS84 into
other local datums. So, once again, it is vital to know which
datum the receiver is set to and which datum is being
worked in. Some receivers will also give an indication
of the quality of the calculated position. This can be
expressed as an accuracy figure or a DOP value (DOP being
the ‘dilution of precision’) and it can be prefixed by H
for horizontal or G for geometric. Basically, the higher
the number, the worse the accuracy of the position.Limitations of GPS
The fundamental principle of GPS is that all calculations
are made on the WGS84 spheroid or datum and this in
itself can cause problems to the unwary. If an alternat-
ive datum is selected in a receiver, a transformation is
undertaken between WGS84 and the alternative datum.
Few makes and models of GPS units have consistent
transformation parameters (the method of converting
from one datum to another between local datums and
WGS84). Therefore, different coordinates can be derived
for the same point, giving rise to low repeatability.Figure 11.12 Differential GPS system with satellites, shore-
based reference stations and in-boat mobile receiver. (Based
on original artwork by Ben Ferrari)