POSITION-FIXING 93
Loran C, were initially used in conjunction with GPS. They
have now been almost entirely phased out and will there-
fore not be covered in this chapter.
The Global Positioning System (GPS) has revolution-
ized position-fixing at sea and on land. GPS is a satellite
positioning system developed, owned and operated by the
American military but available to commercial and
leisure users worldwide. At the time of writing it is pos-
sible to purchase a hand-held GPS receiver that will give
a position, anywhere in the world, to within 15 m. With
the correct equipment, discussed later in this chapter, this
can be enhanced, through differential corrections, to 1 m
in 70 per cent of the world. On a local level, one person
can set up and use a system that will give centimetric
accuracy, in three dimensions. The ease of use and avail-
ability of equipment has led to the adoption of GPS as
the standard form of navigation and survey position-
fixing system both on land and at sea.
In the 1980s, GPS was developed on the back of an
earlier system called Transit. Initially, GPS used a network
of 18 satellites around the earth. Four of these satellites
would be visible at any one time, from any location, and
hence provide an instantaneous position fix. At the time
of printing, there are 24 satellites in orbit plus a number
of spare ones circling the earth in case of breakdowns.
Since the introduction of GPS, other interest groups have,
or indeed are, developing their own satellite navigation
systems, for the most part based on the same operating
principles as GPS. Other commercial users and other
military powers have felt that the control of the system
by the US military could result in the service being
withdrawn without prior notice. Not surprisingly, the
Russians developed their own system called Glonass,
which is operational and it is possible to buy dual
Glonass and GPS receivers. There is a European system
in development, known as Galileo, which should be fully
operational by 2010. Galileo is being developed by com-
mercial organizations for commercial and leisure users.
The derivation of position by GPS is relatively complex
and beyond the scope of this book. It is based on the basic
survey principles of trilateration discussed in chapter 14
(Underwater Survey). The positions of the satellites in space
are known and ranges are calculated from the satellites to
the receiver on earth. Each satellite transmits a signal con-
taining information about the satellite position and the
time of the signal transmission. The satellites are constantly
updated with their positions from a series of ground sta-
tions that continually track and position the network.
Within the receiver, there is a clock that measures the arrival
time of the signal. From the known travel time of the sig-
nal, and assuming a known speed of travel, a range can
be calculated. A position can be calculated using four satel-
lite ranges instantaneously. The system broadcasts on
two frequencies. This enables corrections to be made for
atmospheric errors in the signal. The lower frequency con-
tains the P code, which is the protected code for use by
the US military and maybe her allies, but not released
to civilian users. The higher frequency contains the C/A
(coarse/acquisition) code, which is the code used in com-
mercial and leisure receivers for position calculations.
The C/A code is less accurate than the P code because
of the pattern of coding used in the transmission signal.
It was further degraded to reduce accuracy. This cor-
ruption of the signal was termed ‘selective availability’
(SA). At present, SA has been switched off with the result
that a standard GPS receiver will give a position with an
accuracy of between 2 and 20 m (6.5 – 65 ft), depending
on the time of observations and the location. Prior to the
relaxation of SA, an accuracy of between 25 and 50 m
(81–162 ft) was obtainable but the system accuracy was
quoted as being ±100 m (325 ft) 95 per cent of the time.Enhanced accuracy methods
Several techniques have been devised to enhance the
accuracy of the GPS C/A code. These techniques were
devised when SA was implemented and have had a rad-
ical effect on the accuracy and reliability of the derived
position. With the suppression of SA, they are still rele-
vant today as they increase the accuracy still further.Differential range-corrections /
differential GPS
Differential range-corrections were initially used by the
offshore survey industry and aeronautical community.
Systems and networks that broadcast differential range-
corrections have subsequently been developed for both
commercial and leisure marine users on a national and
international scale. The improved accuracy provided by
differential range-corrections, otherwise known as differ-
ential GPS (DGPS), has had a profound effect on the
importance of GPS as a surveying resource.
The principle of operation is that a receiving unit is set
up on a known point and positions are derived. This
receiver is known as the reference station. The difference
between the derived satellite position and the known
position of the reference station is calculated. The cor-
rections between the observed ranges from the satellites
and the computed ranges are calculated to derive the
known position at the reference station. The stationary
receiver is the key because it ties all the satellite mea-
surements into an accurately surveyed reference point. This
reference station receives the same GPS signals as the mobile
receiver but, instead of working like a normal GPS
receiver, it works in reverse by using its known position
to calculate errors in the GPS signal. The receiver then
transmits these errors to the mobile receiver in real time