18
Global positioning systems
Martin Davis
Of the plethora of technologies developed during the Cold War, few would proceed to have
such a substantial and lasting effect on cartography as the Global Positioning System (GPS).
The United States Department of Defense’s (DoD) 24-satellite NAVSTAR GPS (NAVigation
System with Time and Ranging Global Positioning System), which facilitates the determination
of latitude, longitude and altitude at any point on or above Earth at any time of day, began as
a solely military venture, though the subsequent willingness of the US Government to make
NAVSTAR data available for civilian applications has profoundly impacted both the collection
and visualization of spatial data (Dorling and Fairbairn, 1997). The recent development of satellite navigation systems by the Russian Federation (GLONASS), the European Space Agency
(Galileo) and China (Beidou) indicates that the technology remains a desirable and necessary
part of numerous military and civil applications around the world. Indeed, the acronym ‘GPS’
commonly refers to any satellite positioning system, although the more generic term is properly
‘Global Navigation Satellite System’ or (GNSS). This chapter gives a brief outline of how cartographers have used this technology since its inception, the development and organization of
the system and the fundamental principles behind its operation. Similar systems, in operation or
development, are then outlined.
GPS and cartography
The availability of GPS has wholly transformed the processes of surveying and mapping in recent
years and the technology is now an indispensable tool for professional and amateur cartographers
alike. The ever-increasing availability, affordability and portability of GPS receivers has only
increased applications of GPS in cartography (Xiao and Zhang, 2002). Between 23 March 1990
and 2 May 2000, with the exception of an 11-month period during the first Gulf war, GPS signals were systematically degraded for civilian users; a policy known as selective availability (SA).
This resulted in a two-stream framework within the GPS – a Precise Positioning Service (PPS)
being made available for a selected group of authorized (mainly military) users and a degraded
Standard Positioning Service (SPS) for civilian users (Sleewaegen, 1999; Xiao and Zhang, 2002;
Seeber, 2003). As part of an effort to broaden the civil and commercial applications of a number of technologies developed by the US Government, President Bill Clinton announced the
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removal of SA, predicting that this action would ‘allow new GPS applications to emerge and
continue to enhance the lives of people around the world’ (Clinton, 2000). Mapping was one
such application; the near ten-fold increase in accuracy overnight made GPS considerably more
attractive to surveyors and cartographers by providing a new source of medium to large scale
data for use in Geographical Information Systems (GIS) (Xiao and Zhang, 2002). SPS and PPS
still exist as distinct entities, differentiated by the fact that SPS broadcasts on one frequency,
whereas PPS uses two (NCO, 2015).
Military and civilian navigation by land, sea and air undoubtedly constitute the largest application of GPS, and indeed is the system’s raison d’être. However, in addition to displaying a
real-time position on a base map, most commercial GPS receivers now facilitate the recording
of point, line and polygon data, which can be converted to most major vector formats. In 2002,
Ordnance Survey Director General Vanessa Lawrence acknowledged that GPS had become a
vital technology in the creation of Ordnance Survey products, stating that the UK’s national
mapping agency had ‘invested heavily in specialist GPS equipment that is fundamental to [producing] data of the highest precision’ (Lawrence, 2002).
Since the removal of SA, GPS has also changed the medium through which many consumers
of map products receive geographical data. In-car and smartphone-based satellite navigation has
largely replaced the market for printed road maps, with studies indicating that, for some user
groups, ‘sat-navs’ are now by far the preferred means of navigation (Speake, 2015). Research
conducted by Ochieng and Sauer (2002) accounts for this surge in popularity, concluding that,
since the removal of SA, GPS data in urban road navigation applications reach a 20-metre
accuracy level 99 per cent of the time, compared with only 44 per cent of the time prior to the
removal of SA. Recreational use of GPS (e.g. walking and cycling) has also grown since the turn
of the century, aided by the portability of modern GPS receivers.
Post-SA GPS has also enabled a significant democratization of mapping, notably including the growth of community mapping and online-based open source map platforms, such
as OpenStreetMap (OSM), established in 2006. Perkins (2007) highlighted that, for the first
time, such initiatives had enabled the long-established paradigm of ‘top-down’ mapping to be
challenged. No longer was cartography exclusively reserved for professional cartographers, as it
broadly had been for centuries, but anyone with a GPS device and computer could now collect
spatial data and incorporate it into a map, individually or collaboratively, anywhere on Earth.
This democratization of mapping has led to a new wave of map types emerging, separate from
traditional mapping. Maps, often together with GPS, are being increasingly used in innovative applications as diverse as contemporary artistic projects (Schulz, 2001), travel patterns and
spatial mobility (Duncan and Regan, 2015), animal tracking and habitat mapping (Hulbert
and French, 2001), offshore survey (NCO, 2015), and infrastructure maintenance (Kumar and
Moore, 2002).
NAVSTAR GPS: principles of operation, structure and organization
The development of the NAVSTAR GPS programme, more simply referred to as GPS, began
in 1973, and the system has been fully operational in its current form since 1995. By calculating ‘pseudoranges’ between a ground receiver and at least four satellites, the exact locations of
which are known at all times, the position of the ground receiver can be determined geometrically almost instantaneously. This is achieved by timing how long signals from the satellite take
to reach the ground receiver (the difference between the times of transmission and reception).
This process is accurate to within approximately 10–15 m in ideal conditions (Seeber, 2003).
GPS consists of three segments, each with vital functions. The space and control segments are
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largely operated and maintained by the United States Air Force (USAF), whereas the user segment now encompasses a broad range of users, no longer confined to the USA.
Space segment
The space segment of the GPS currently consists of 31 satellites, known as the ‘baseline constellation’ (NCO, 2015). These orbit the Earth at an inclination of 55° along one of six equally
spaced orbital planes (A–F) (Seeber, 2003). Of the 31 satellites, at least 24 are operational 95
per cent of the time, with the ‘spare’ satellites kept available for use during maintenance, or
in the event of a fault (NCO, 2015). In addition, decommissioned NAVSTAR satellites are
kept in orbit in case a need to reactivate them emerges (NCO, 2015). GPS satellites orbit
the Earth at an altitude of 20,200 km and have a repeat phase of 12 hours (Seeber, 2003;
NCO, 2015). The number and configuration of the satellites and their orbits has been determined to ensure that at least four satellites are ‘visible’ at all times from all points on Earth,
ensuring continuous global coverage. Given the importance of accurate timing to the calculation of pseudoranges, perhaps the most important, and costly, elements of GPS satellites are
their highly precise, synchronized clocks. GPS satellites also contain several spare clocks for use
in the event of the main clock malfunctioning.
Control segment
The control segment of the GPS consists of a series of ground facilities which control and
track the satellites, while monitoring the data being transmitted from them. In order to perform these functions across the entire constellation of satellites, the control segment comprises
12 command and control antennae and 15 monitoring stations distributed across the globe
(Figure 18.1); coordinated by a Master Control Station (MCS) at Schriever USAF Base, near
Colorado Springs, USA (which also functions as a further monitoring station) (NCO, 2015).
Monitoring stations are responsible for observing the transmissions and orbits of satellites
passing overhead. Pseudoranges between monitoring stations and satellites, together with
meteorological data, are transmitted to the MCS in real time (Seeber, 2003). Originally, a
total of six monitoring stations were used, all operated by the USAF (three in the USA –
Hawaii, Cape Canaveral and Schriever MCS – in addition to Ascension Island, Diego Garcia
and Kwajalein). In 2008, as part of the Legacy Accuracy Improvement Initiative (L-AII),
Figure 18.1 The control segment (adapted from NCO, 2015)
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ten further monitoring stations, operated by the National Geospatial-Intelligence Agency
(NGA), were brought into use (in Alaska and Washington, DC, Argentina, Australia, Bahrain,
Ecuador, New Zealand, South Africa, South Korea and the UK). Vandenberg USAF Base in
California is equipped to function as an alternate MCS if necessary (NCO, 2015).
User segment
Although applications of GPS have been dealt with in more detail elsewhere, it should not be
overlooked that the user segment is the largest of the three components of the GPS, both in terms
of the number of people involved and the variety of applications of GPS data, especially since the
removal of SA. As GPS receivers are incorporated into an ever-increasing variety of commercial devices (e.g. smartphones, smart watches (see Figure 18.2) and Unmanned Aerial Vehicles
(UAVs)), the growth of the user segment is likely to continue. The falling price of receivers
accounts for much of this growth (Lechner and Baumann, 2000). Early receivers, produced during the mid-1980s, cost in excess of US$100,000. However, prices fell swiftly as mass-production
of receivers began; the first receiver costing under US$1,000 appeared on the market in 1992
and the $100 milestone was reached in 1997 (Kumar and Moore, 2002). Entry-level commercial
receivers, including in-car navigation units, are now available at less than US$50.
Figure 18.2 A 2015 Apple watch, which is GPS-enabled when connected to a compatible
smartphone (Macworld, May 2015)
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Major sources of error
Inherent in several elements of GPS data collection is scope for varying degrees and types of error
to be introduced. As Seeber (2003) highlights, the GPS is reliant on modelling – most notably
the modelling of a reference system for the Earth (WGS 84) and modelling the behaviour of
transmissions between space-borne satellites and receivers on the ground. Any inaccuracies or
uncompensated fluctuations in these physical frameworks will lead to the inaccuracy of such
models, which will in turn inevitably lead to the introduction of errors in GPS data.
The first of these error sources is the Earth’s atmosphere. The degradation and delay of signals
as they pass through the ionosphere and troposphere can interfere with the resulting pseudoranges which need to be highly accurate in order to provide accurate position data ( Jin, 2004).
The second is Dilution of Precision (DOP) (see Figure 18.3); a phenomenon through which the
precision of data is reduced due to the geometric arrangement of visible satellites from a receiver
at a given time. DOP is reduced when there is a greater angle between visible satellites in the
sky (Dussault et al., 2001). Different indexes exist to measure the effect of DOP on different
aspects of accuracy; Horizontal Dilution of Precision (HDOP), Vertical Dilution of Precision
(VDOP), Position Dilution of Precision (PDOP) (a combination of HDOP and VDOP) and
Time Dilution of Precision (TDOP) (Seeber, 2003). Geometric Dilution of Precision (GDOP)
incorporates both satellite location and time.
Given the importance of accurate timing to the measurement of pseudoranges, clock
errors are another potential source of error. Lack of synchronization between the satellite clocks and receivers, even by the smallest margins, can lead to substantial errors in
positioning. According to Seeber (2003), an error of 1µs in a satellite clock will result in a
pseudorange error of 300 m. It for this reason that the close observation of ‘GPS time’ in the
control segment is vital.
Although a lack of visible satellites is perhaps the most fundamental cause of poor accuracy
of GPS data, the reflection or delay of signals by objects in close proximity to the receiver
(e.g. trees, buildings, terrain) can disrupt pseudorange measurement, and therefore overall
data accuracy, regardless of the number of satellites visible.
Improving accuracy
Differential GPS (DGPS) refers to a means of improving the accuracy of data by correcting or
compensating for it based on measurable discrepancies between the true location of a receiver
and the location of the same receiver as stated by the GPS. This is achieved by measuring the
error in the GPS position of a receiver at a reference station, the true position of which is precisely and accurately known. The positions of other nearby GPS receivers can then be corrected
based on this known error, often providing accuracy to within 1 m (Kumar and Moore, 2002).
Figure 18.3
Example graphical representations of good and bad Position Dilution of Precision
(PDOP) (adapted from Seeber, 2003)
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The increase in accuracy provided by DGPS makes it a more viable means of positioning in
some applications where precision is vital (Lechner and Baumann, 2000).
In some parts of the world, commercial DGPS reference stations have been established, giving users the opportunity to obtain correction data. Examples of such stations can be found in
Germany (Satellite Positioning Service of the German Topographical Survey Administration
[SAPOS]), The Netherlands (Eurofix, with coverage across Europe) and the UK (General
Lighthouse Authority) (Lechner and Baumann, 2000).
A Satellite-Based Augmentation System (SBAS) is a means of improving GPS accuracy
within a large region by incorporating DGPS data from multiple reference stations with clock
error and ionospheric corrections. The corrected data is then transmitted to receivers across
the region via a geostationary satellite. The SBAS for the USA is known as the Wide Area
Augmentation System (WAAS) and was primarily developed to improve GPS accuracy in the
aviation industry (Lechner and Baumann, 2000). Similar systems are in operation elsewhere in
the world, notably in Europe (European Geostationary Navigation Overlay Service – EGNOS)
and Japan (Multi-Functional Satellite Augmentation System – MSAS).
The development of GPS
The technology which would become vital to the modern GPS perhaps has its origins in 1948
when the US Army Signal Corps successfully transmitted radar waves to the moon and detected
their deflection from the Earth’s surface – indicating for the first time that microwave transmission in space was possible (Kumar and Moore, 2002). The first artificial satellite, Sputnik 1, was
launched on 4 October 1957 by the USSR, prompting interest in space technology in the US
and the start of the ‘space race’. Scientists studying the satellite at Johns Hopkins University,
Baltimore, Maryland, observed a variance in the frequency of the ‘beeps’ transmitted by Sputnik
I as it orbited overhead. They reasoned that if the orbit of the satellite was known, as well as the
exact degree to which the signal frequency varied during each pass, the position of the receiver
on Earth could be determined (Ramsey, 1984).
It was not until 31 January 1958 that the first US satellite, Explorer 1, was launched. Explorer
1 collected pioneering data regarding conditions outside of the Earth’s atmosphere, although it
was not used for terrestrial positioning. The first system used for this purpose was TRANSIT,
launched in 1964. Used to aid the navigation of US submarines, this primitive system had several major limitations; given that it only consisted of a single satellite, position data could only
be obtained every 35–40 minutes. Furthermore, the submarine needed to be stationary in order
to obtain this information as there was no way of determining how long a transmission between
the satellite and submarine had taken and it was therefore not possible to accurately compensate
for any movement. This issue was rectified in 1967 with the launch of the TIMATION I satellite. TIMATION I had an atomic clock on board, making it possible to determine the length
of time signals had taken to reach the receiver on Earth. Nevertheless, due to the continuing
dependence on one satellite, positional accuracy remained low (Kumar and Moore, 2002).
The development of the TRANSIT system continued throughout the 1970s, with multiple satellites becoming operational before the end of the programme in the late 1980s. Use of TRANSIT
data was not restricted to military users. Of the 10,000 receivers active in 1984, approximately
90 per cent were operated by civilian users, notably in the shipping industry (Ramsey, 1984;
Gooding, 1992). However, TRANSIT was never a GPS as its constellation was too small to
achieve constant, global coverage (Ramsey, 1984). From its inception, the NAVSTAR GPS programme was intended to be the first system to achieve this, while also providing altitude and
velocity information, making it suitable for airborne navigation (Ramsey, 1984; Ford, 1985).
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Early development of the NAVSTAR GPS programme began in 1973, separate from previous projects (Kumar and Moore, 2002). The first four satellites were launched in 1978 and
the ensuing years saw significant expansion of the NAVSTAR constellation at a cost of over
US$2 billion (Ramsey, 1984). The system became available to private users in 1995 (Lechner
and Baumann, 2000).
Alternative and future GNSS
GLONASS (Russian Federation)
After NAVSTAR GPS, the largest GNSS in operation is GLONASS (GLObalnaya
NAvigatsionnaya Sputnikovaya Sistema), operated by the military of the Russian Federation.
GLONASS operates by the same principles as NAVSTAR GPS, though it uses a smaller
constellation of 27 satellites, organized in 3 orbital planes at a wider inclination of 64.8° and
slightly lower altitude of 19,100 km (Lechner and Baumann, 2000; Federal Space Agency,
2016a). As with GPS, GLONASS operates two levels of service for different users; Channel
of Standard Accuracy (CSA) and Channel of High Accuracy (CHA). CSA data has been freely
available to civilian users since 1996 and offers 60 m horizontal accuracy and 75 m vertical
accuracy 99.7 per cent of the time (Lechner and Baumann, 2000). GLONASS uses a Russian
geodetic framework (PZ-90) and operates on Moscow time (GMT+3). Greater coverage,
and potentially accuracy, can be achieved by using a receiver which combines both GPS and
GLONASS data (Melgard et al., 2009; Pan et al., 2014).
The USSR launched its first prototype navigation satellite, Cosmos 192, on 23 November
1967. As with its early American counterparts, the positional accuracy of data from Cosmos
192 was low; after a software upgrade in 1969, average horizontal error remained as high as
100 m if measured over a five-day period (NASA, 2014). The Soviet Union embarked on the
development of its first multi-satellite navigation system in 1979. Cicada consisted of four lowlevel satellites which were used to gather positional data for maritime applications. The first test
flights for the GLONASS programme took place in October 1982, although the newly formed
Russian Federation did not begin operational testing of the system until 1993 (Federal Space
Agency, 2016b). An operational constellation of 24 satellites was in place by 1995 (Lechner and
Baumann, 2000). The command centre, or Information Analytical Centre, of GLONASS is in
Korolyov, near Moscow.
Galileo (European Space Agency)
The European Space Agency’s (ESA) desire to build a European, civilian GNSS has its roots
in collaborative discussions which took place between the ESA and the European Union
throughout the 1990s. The first product of these discussions was the aforementioned EGNOS
augmentation system, launched in 2009, which improves GPS accuracy across Europe. The
second was Galileo, a new GNSS, independent of existing systems.
Still in its infancy, relative to GPS and GLONASS, ESA launched its first test satellites,
GIOVE-A and GIOVE-B, in 2005 and 2008 respectively. This was followed by the launch
of the first operational Galileo satellite on 21 October 2011 (ESA, 2015). Twelve operational
Galileo satellites have been launched to date, with 30 proposed in total. The constellation will
be spread across three orbital planes with an inclination of 56°. The programme is partly funded
by the European Union and is operated from two ground control centres in Fucino, Italy and
Oberpfaffenhofen, Germany.
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Beidou (China)
Between 30 October 2000 and 24 May 2003, China launched three navigation satellites as part
of its Beidou programme, a regional initiative intended to facilitate civilian positioning applications in Chinese territory (Forden, 2004). Almost immediately afterwards, development began
on the Beidou-2 programme, intended to be the first GNSS independently operated by China.
By the end of 2012, 16 Beidou-2 satellites had been launched, with a fully operational, global
system expected by 2020 (Li et al., 2014).
Regional satellite positioning systems
The Indian Space Research Organization (ISRO) is currently developing a satellite navigation
system, the Indian Regional Navigation Satellite System (IRNSS) (ISRO, 2016). The first
IRNSS satellite was launched in 2013 and five of the seven proposed satellites are now in orbit.
The Japan Aerospace Exploration Agency (JAXA) is also in the early stages of developing
a regional satellite navigation system, designed to complement NAVSTAR GPS, rather than
replace it. The first and currently only satellite of the Quasi-Zenith Satellite System (QZSS) was
launched on 11 September 2010. The development of three further satellites has been approved
by the Japanese Government, with JAXA planning to have a seven-satellite constellation in
operation sometime after 2023 (QZSS, 2016).
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