1. How the GPS works: principles
· Satellite and receiver clock errors
3.2 Differential Code-Phase
GPS (DGPS)
4.2 GPS co-ordinates systems:
WGS84 and ETRS89
As Global
Positioning System (GPS) may be used either as a positioning tool or as a
survey tool, archaeologists are introducing this technique to many of their
survey tasks on site. This guide outlines the basic principles of GPS and its
use in archaeological work.
This
introduction to GPS is focussed on applications in archaeology. It is not
intended as a ‘do and don’t do’ manual.
The use
of GPS may appear at first complicated, but the principle is quite simple.
GPS stands
for Global Positioning System -a shorted term for NAVSTAR GPS (NAVigation Satellite Timing and Ranging) -a system for
locating ourselves on earth. It is a satellite-based system created and
controlled by the US Department of Defense, initially
for military purposes but extended later for civilian usage. It consists of a
constellation of 24 satellites (4 satellites in 6 orbital planes) orbiting at
an approximate altitude of 20200 km every 12 hours.
Each satellite broadcasts two
carrier waves in L-Band (used for radio) that travel to earth at the speed of
light. The L1 channel produces a Carrier Phase signal at 575.42 MHz as well as
a C/A and P Code. The L2 channel produces a Carrier Phase signal of 1227.6 MHz,
but only P Code.
These
codes are binary data modulated on the carrier signal. The C/A or
Coarse/Acquisition Code (also known as the civilian code), is modulated and
repeated every millisecond; the P-Code, or Precise Code, is modulated is
repeated every seven days.
The GPS
system works with a receiver (essentially a radio receiver) that acquires
signal from satellites in order to locate its position geographically. The GPS
receiver simply calculates the distance to the satellite by measuring the
travel time of the signals transmitted from the satellite and multiplying it by
the velocity.
Distance =
velocity (speed of light) x Time
The GPS
receiver computes its position and time by making simultaneous measurements to
the satellites. A signal from three satellites will sort out a 2-dimensional
position or horizontal position. In order to get a 3 dimensional position
(latitude, longitude and height) at least four satellites are needed within
signal range.
Additional
explanations of how more complex GPS function will be described further on. For
more information on satellite signals consult:
http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap3/311.htm#prn%20code
There has
been a misconception over the past years about the accuracy of GPS. It is true
that for many years the US Department of Defense
maintained intentional degradation of accuracy called Select Availability
(S/A), a system for randomly degrading the accuracy of the signals being
transmitted to civilian GPS receivers. However, the S/A was removed in May
2000.
Therefore,
the accuracy of GPS should be a discussion based on the type of system (device)
and its ability to eliminate error sources and not on the availability of
reliable satellite signals.
Error sources are variable; here are some of the more commonly occurring:
·
Ionospheric delays. The ionosphere is the upper layer of the atmosphere ranging in
altitude from 50 to 500 km. It consists largely of ionized particles which
cause a disturbing effect on the GPS signals. Since the density of the
ionosphere is affected by the sun there is less ionospheric
influence during night time. The ionosphere has also a cyclical period of 11
years which reaches a maximum and a minimum of the magnitude of its effect. For
the current cycle, it reached its maximum in 1998 and its minimum in 2004.
In addition, low elevation satellite
signals (anywhere between the horizon and up to 15 degrees above it) will be
affected by a longer ionospheric delay as the
distance the signal has to travel further and generally “noisier”. In the more
sophisticated GPS receivers an “elevation mask” can be set so that satellites
below the mask are not used in computing position.
·
Satellite and receiver clock errors. Each satellite is equipped with a very accurate clock which is
continuously monitored by ground stations (US Department of Defense).
Despite this, errors of precision can be up to one metre.
Each receiver also has a clock but less accurate than the satellite’s
clock (its cost – around $50000- and weight –20kg- would not be suitable for a
land GPS).
·
Multipath error. This is where more than
one signal is received due to a reflection on other objects nearby (tall
buildings or lakes) causing erroneous measurements.
· Satellite geometry. This means the relative position of the satellites at a specific moment. When satellites are located at wide angles relative to each other, the possible error margin is small. On the contrary, when satellites are grouped together or located in a line the geometry will be poor. The effect of the geometry of the satellites on the position error is called Geometric Dilution of Precision (GDOP). GDOP comprises the components shown below, which can be individually computed but are not independent of each other:
PDOP - Position Dilution of Precision (3-D)
HDOP -
Horizontal Dilution of Precision (Latitude, Longitude)
VDOP - Vertical Dilution
of Precision (Height)
TDOP - Time
Dilution of Precision (Time)
3. Types of GPS
Broadly
speaking, there are three types of GPS, depending on the level of acquired accuracy.
- Hand-held GPS or
Navigational (> c. 10m)
- Differential Code-Phase GPS
(DGPS) (< 1m)
- Carrier-Phase GPS (< cm)
3.1. Hand-held GPS
The
Navigational or hand-held GPS consists of a single receiver, as easy to use as a
mobile phone and around the same cost. It is the simpler technique of GPS but
also the least accurate. The position calculated from the satellites’ signal is
frequently distorted by sources of error, which can degrade its accuracy by
several metres (about 15 to 100 m).
3.2
Differential Code-Phase GPS (DGPS)
This
differential measurement technique eliminates most of source errors, achieving
results of sub-metre accuracy. It is obviously a more complex system than
hand-held GPS - which is reflected in its substantially higher cost.
It consists of a base station and
a rover receiver connected by a radio link. The base station or reference
receiver when located at a known point can estimate what the ranges to the
satellites should be and work out the differences between the computed and
calculated range values. These differences are known as corrections. The base
station transmits these real time differential corrections to the rover
receiver (through the radio) so they can be used to correct its measurements.
The DGPS corrections are transmitted in a standard format specified by the
Radio Technical Commission Marine (RTCM).
One of
the powerful radio transmitters is the Radio Beacon. Set up around the coastline
of many countries, these transmitters are located at old Radio Beacon stations,
and have ranges of 100-150 miles. The DGPS signals are radiated on frequencies
in the old MF (medium frequency) Beacon band, around 300 kHz. (For a detailed
table with Radio Beacons available in the
http://www.nlb.org.uk/dgps/dgpschart.htm
The users of these transmitters were mainly marine
craft navigators, but in some countries such as the
Another radio transmitter is the OmniSTAR Inc, working in a similar way to the beacons. It
consists of a network of GPS base receivers around the world, which broadcast
corrections to user receivers. Access to these corrections is available by subscription. For more information
consult:
There are also new satellite-based
differential systems, free of charge, such as WAAS, EGNOS and MSAS. The Wide
Area Augmentation System (WAAS) is designed to provide a higher confidence
level in autonomous GPS positioning for use in aviation. Unlike radio and
satellite differential, WAAS corrects the atmospheric and orbital data so that
autonomous calculations can better determine true position. But as the system
is designed for aircraft, there are still some limitations for non aviation
users.
The European Geostationary Navigation Overlay Service
(EGNOS) is
The Japanese Multi-function
Transport Satellite Augmentation System (MSAS), sponsored by the Japanese Civil
Aviation Bureau, is designed to provide a satellite-system in some of the Far
Eastern areas.
3.3 Carrier-Phase GPS
This differential system achieves accuracy ranging from centimetre
to millimetre, depending on the measuring technique. The Carrier-Phase GPS uses
a minimum of two receivers simultaneously.
After an autonomous position
is calculated using differential code methods, clock errors can be annulled by
observing two satellites from two receivers by a method known as double differencing.
Once the better
approximation of the position is known, a statistical calculation of phase
intersections from multiple satellites can be used to resolve ambiguous
results.
GPS Measuring Techniques
There are
several measuring techniques that can be applied when surveying with Carrier-Phase
GPS. A brief description of the more commonly used is given below.
§
Static
Used for high accuracy
(about 5mm + 1ppm), measuring long distances. It requires data to be collected
for several hours on two receivers simultaneously to achieve the best results.
The time of data collection is relative to the length of the baseline between
the receivers.
§
Rapid Static
A form of static GPS
which requires minutes instead of hours for satellite observation due to
special ambiguity resolution techniques which use extra information. Accuracy can reach the centimetre on
baselines less than 20km.
§
Real Time Kinematic
This technique uses a
radio to link so that the reference station broadcasts the data obtained from
the satellites to the rover instantaneously. As data
is transferred by radio, it limits baseline lengths and accuracy will be in the
range of 1-5cm. Nevertheless, it is becoming the most popular technique as
results are fast and co-ordinates are displayed in real time.
Most of GPS
measurements techniques mention above collect data for post-processing, with
the exception of Real Time Kinematic. Data collected
by both receivers can be processed to obtain a better accuracy and/or to
eliminate the noise caused by the real-time operation.
It is essential to mention some
elements of geodesy as the study of the Earth’s shape and its representation,
for a better understanding of GPS survey and its relation to local mapping. The
Earth is represented by various co-ordinate systems made to fit specific areas
of its surface. Each mapping system is based on a local ellipsoid, designed to
match the geoid. The ellipsoid is a mathematical
surface that approximates the shape of the earth and the geoid
is a theoretical surface which most closely matches mean sea level, both created to
ease the representation of the Earth.
4.1 OSGB36
In
All
For a
detailed explanation to
4.2 GPS co-ordinates systems: WGS84 and ETRS89
Data
received from GPS is related to a global co-ordinate system known as WGS84 or
World Geodetic System 1984. GPS position will be expressed in latitude, longitude
and ellipsoid height.
However, the WGS84 co-ordinate
system will become unacceptable when using fixed points for land surveying.
This is caused by the constant motion of continents with respect to the WGS84
co-ordinate system: in Great Britain, it moves on a rate of 25mm per year away
from the WGS84, meaning that in reality there are no fixed points.
For this reason, the European
Terrestrial Reference System 1989 (ETRS89) is used as the standard precise GPS
co-ordinate system throughout
If co-ordinates are required in a
local mapping system, a transformation from GPS WGS84 or ETRS89 is needed.
For
Furthermore,
Additionally, there are 900 passive
stations but with the disadvantage of having to be occupied e by the user’s own
GPS receiver.
ETRS89 co-ordinates and full
information from both active and passive stations are supplied by Ordnance
Survey through their website ready to use for post-processing.
GPS
proves to be an excellent tool for surveying. In archaeological fieldwork,
Global Positioning Systems may well be used for mapping find-spots, earthworks
and other archaeological features without the need of conventional techniques
(i.e. triangulation, off-set grids). The
correct choice of GPS for a job depends upon the acceptable level of accuracy.
Many
archaeologists are happy with the level of accuracy obtained by hand-held GPS
(navigation grade) for archaeological field-work. Whilst this is an
increasingly indispensable (and inexpensive) item of equipment for field
reconnaissance and walkover surveys- especially in the more remote regions
(e.g. uplands) of the UK- it is clearly inappropriate for positioning
evaluation trenches or conducting field-walk surveys, where tighter controls
are required over each transect to be walked.
The level
of accuracy may be hugely variable from one point to the next within a space of
minutes. Experience in central and southern
Where
accuracy within a metre is acceptable, such as trench stake-outs, field-walking
transects or recording sites at scales up to 1:2500, a Differential GPS is
suitable. For more detailed recording, however, such as topographical surveys,
excavation plans or grid layouts, a Carrier-Phase GPS is preferable. This will
assure millimetric accuracy.
These are
not the only applications of GPS in archaeological survey, but perhaps the more
commonly used.
5.1 GPS versus
Total Station
Over the
last decade, the Total Station Theodolite (TST) has
become increasingly the preferred tool of archaeologists for setting out
trenches, surveying sites or undertaking topographical surveys. Frequently in
archaeological work, the TST becomes the less attractive option when compared
to GPS:
·
Where sites are remote or hard
detail is poor, positioning may be unreliable.
·
Two people are required unless a
robotic system is used.
·
Line of sight must be maintained
between the instrument and prism.
On the
contrary, the use of Global Positioning Systems in archaeological field-work
has distinct advantages:
·
There is no dependency on
permanent landscape features.
·
A single operator may carry out
the survey.
·
There is no dependency on a
maintained line of sight between the base receiver and rover.
Additionally,
both setting up and surveying time is considerably reduced. For example, two
operatives using TST to set out 50 standard length archaeological trenches, to
provide conventional ground coverage in rural conditions, may take two days to
complete. The same coverage may be achieved by a single operative using GPS in
half the time.
There
are, however, some limitations with GPS that should be taken into account. As GPS receivers listen to signals from
satellites they must have a clear view of the sky at all times. In proximity to
tall buildings or in dense forest satellite signal may be poor.
5.2
Considering a GPS
It’s
worth mentioning that accuracy comes with a price tag. The more expensive the
GPS, the more accurate it will be.
Buying a
hand-held GPS does not seem to be much of a problem, as nowadays prices are
very reasonable and are widely available in most outdoor leisure stores. But if
an accurate survey is needed then the use of a higher precision GPS should be
considered.
As the
purchase cost of a precision GPS may be outside the budget range of many
companies, the alternative is to sub-contract a specialist. Most GPS surveying
contractors are engineering-based, although there are a few companies that
specialize in archaeological requirements.
There is
also the possibility of hiring GPS equipment, but rates are still relatively
high and its use by skilled operators is strongly recommended.
Whether
subcontracting another company, buying your own equipment or hiring it, the use
of GPS is an option to consider on an archaeological field survey.
For any
additional information on GPS visit:
http://www.garmin.com/aboutGPS/manual.html
http://www.gmat.unsw.edu.au/snap/gps/gps_survey/principles_gps.htm
For case
studies applications of GPS on archaeological field work visit:
For a
detailed glossary, visit:
http://www.garmin.com/aboutGPS/glossary.html