Journal of Global Positioning Systems (2002)
Vol. 1, No. 1: 48-56
Pseudolite Applications in Positioning and Navigation:
Progress and Problems
J. Wang
School of Surveying and Spatial Information Systems, University of New South Wales, Sydney, NSW 2052, Australia
e-mail: jinling.wang@unsw.edu.au; Tel: +61(2)9385 4203; Fax: +61(2)9313 7493
Received: 4 May 2002 / Accepted: 18 July 2002
Abstract. Global navigation satellite systems have been
revolutionising surveying, geodesy, navigation and other
position/location sensitive disciplines. However, there are
two intrinsic shortcomings in such satellite-based
positioning systems: signal attenuation and dependence
on the geometric distribution of the satellites.
Consequently, the system performance can decrease
significantly under some harsh observing conditions. To
tackle this problem, some new concepts of positioning
with the use of pseudo-satellites have been developed and
tested. Pseudo-satellites, also called pseudolites, are
ground-based transmitters that can be easily installed
wherever they are needed. They therefore offer great
flexibility in positioning and navigation applications.
Although some initial experimental results are
encouraging, there are still some challenging issues that
need to be addressed. This paper reviews the historical
pseudolite hardware developments and recent progress in
pseudolite-based positioning, and discusses the current
technical issues.
Key words: GPS, Pseudolites, Indoor Positioning,
Navigation
1 Introduction
Global navigation satellite systems, such as GPS and
Glonass, have been playing an increasingly important
role in surveying, geodesy and navigation, in which
positioning is a major component. It is well known that
for such space-borne satellite positioning systems the
accuracy, availability and reliability of the positioning
results is very dependent on both the number and
geometric distribution of satellites being tracked.
However, under some harsh observing environments,
such as in urban canyons and deep open-cut mines, the
number and geometry of visible satellites may not be
sufficient to reliably carry out the positioning operations.
In the worst situations, such as underground or inside
buildings, the satellite signals are completely lost. Such
problems with space-borne satellite positioning systems
can be addressed by additional ranging signals
transmitted from ground-based "pseudo-satellites"
(pseudolites).
The concept of the pseudolite was proposed in the
1970’s, even before the launch of the GPS satellites. In
fact, pseudolites were originally designed to test the
initial GPS user equipment (Harrington & Dolloff, 1976).
During the past decade, new pseudolite concepts and
hardware have been developed for a variety of
positioning and navigation applications. Pseudolites can
be used as an augmentation tool for space-borne satellite
positioning systems. This augmentation can improve the
system performance because the availability and
geometry of positioning solutions are significantly
strengthened. Further more, a pseudolite-only positioning
system is possible, which can replace the space-borne
satellite constellation where the use of satellite signals is
not feasible, such as underground and indoors.
This paper presents an overview of the pseudolite
hardware developments and recent progress in pseudolite
positioning applications, and discusses the current
challenging issues, such as pseudolite and receiver
hardware development, pseudolite synchronization,
multipath effects and modelling errors.
2 Hardware Developments
During the early days of GPS development, the test
facility, the Inverted Range, was established. In this test
range at Yuma Proving Ground (USA) four ground
transmitters provided the simulated GPS satellite signals
Wang: Pseudolite Applications in Positioning and Navigation 49
for testing GPS receivers. These ground transmitters
(GTs) were so-called pseudolites (Harrington & Dolloff,
1976). These first pseudolites were designed to transmit
GPS L1 signals, although the navigation message for
these pseudolites was different from that for the GPS
satellites. In fact, the pseudolites just transmitted the
fixed coordinates for the pseudolite locations (while the
other portions of the navigation message for the satellites
are not applicable in the case of pseudolites).
The pseudolite concept developed during the GPS
development stage is being used again during the current
GPS modernization programme (ITT, 2002). A new
Inverted GPS Range (IGR) will be developed at
Holloman Air Force Base (USA) to support validation of
new military and civilian signals planned for the
modernised GPS constellations. This new GPS user test
facility will be open to both military users and
manufacturers of civilian receivers.
Similar to the GPS, a new global navigation satellite
system GALILEO under development by the European
Union will also use the pseudolite concept to test and
validate frequency allocation and user equipment. A
GALILEO pseudolite is currently being developed by the
Institute of Geodesy and Navigation (IfEN) at the
University FAF Munich (Hein, 2002).
The use of pseudolites in positioning and navigation was
first discussed by Beser & Parkinson (1982) and Klein &
Parkinson (1984). In the mid 1980s, the Radio Technical
Commission for Maritime Services (RTCM) defined a
pseudolite which can receive GPS satellite signals,
compute pseudorange and range-rate corrections, and
transmit the correction information at 50 bits per-second
on an L-band frequency. In addition, the transmitted
signal should be GPS like and the signal is designed to
prevent interference to GPS and other equipment. The
RTCM committee SC-104 ('Recommended Standards for
Differential Navstar GPS Service') designated the Type 8
Message for the pseudolite almanac, containing the
location, code and health information of pseudolites
(Kalafus et al., 1986). However, at that time, the
development of a prototype pseudolite of the type defined
by the RTCM was costly, with the indicative prices
ranging from US$100K-200K (Parkinson & Fitzgibbon,
1986).
In the early 1990s, researchers at Stanford University
developed a low cost GPS L1 C/A code pseudolite for
use in a CAT III automatic landing system (Cohen et al.,
1993). During the past decade, commercial pseudolite
hardware products have become available on the market.
In the mid 1990s, the first commercial pseudolite product
was manufactured by the IntegriNautics company
(www.integriNautics.com). In 2001, another
manufacturer, Navicom, launched a new pseudolite
product called NGS1T (http://www.navicom.co.kr).
Another pseudolite product for indoor tracking and
navigation services is under development in Finland
(Söderholm et al., 2001). These pseudolites transmit
GPS-like ranging signals. For this reason, they are called
GPS pseudolites (Elrod & Van Dierendonck, 1996). They
can be programmed or preset to broadcast any of the gold
codes of GPS (i.e., PRN codes from 1 to 37) on GPS L1
at the frequency of 1575.42Mhz. Some types of GPS
signal simulators, such as the Stanford Telecom Model
7201 wideband Signal generator (Holden & Morley,
1997) and GSS simulators (Weiser, 1998), can be
configured to transmit a GPS-like C/A code signal on L1.
Thus, these GPS signal generators/simulators can
essentially be used as a pseudolite.
In principle, pseudolites can transmit their ranging signals
on different frequencies, just as the GLONASS satellites
do. Australia’s CSIRO Telecommunications and
Industrial Physics is currently developing a high precision
location system (PLS) which uses the ISM band
frequencies (http://www.tip.csiro.au/ICT/PrecisionLocator
/index.htm). Zimmerman et al. (2000) proposed a
pseudolite design that uses up to five frequencies (two in
the 900MHz ISM band, two in the 2.4GHz ISM band, and
the GPS L1 frequency). An advantage of such multi-
frequency pseudolite systems is that the integer carrier
phase ambiguities can be resolved instantaneously, due to
redundant measurements and the extra wide-lane
observables that can be constructed from these
frequencies.
New pseudolite hardware designs have been proposed
during the past few years. The latest hardware designs are
closely connected to new applications. Some examples
are:
In order to use the pseudolite signals in single point
positioning, it is necessary to synchronise the
pseudolite ranging signals to the GPS signals. This
kind of pseudolite is called a Synchrolite (Cobb, 1997).
To implement a Mars pseudolite array navigation
system as proposed in LeMaster & Rock (1999), the
pseudolites have been designed to be capable of both
receiving and transmitting ranging signals at GPS
L1/L2 or other frequencies. This type of pseudolite can
‘exchange’ signals, self-determining the geometry for a
pseudolite array. These pseudolites are referred to as
Transceivers. Stone et. al. (1999) have reviewed
transceiver applications.
A pseudolite can be installed on stratospheric
platforms, as shown in Figure 1, to broadcast both
ranging signals and differential corrections for GPS,
GLONASS and GALILEO systems (Dovis et al.,
2000). Such a pseudolite design is called Stratolite.
Currently the majority of the pseudolites transmit GPS-
like signals at the L1 frequency (1575.42MHz) and
possibly on L2 (1227.6MHz). With this configuration,
standard GPS receivers can be used to track pseudolite
signals with the modification of the firmware. Currently,
50 Journal of Global Positioning Systems
it has been identified that NovAtel Millennium and
Canadian Marconi Corp. Allstar GPS receivers can be
used to track pseudolite signals. In addition, some GPS
receiver development kits, which include receiver
firmware source code, can be modified for pseudolite
applications. For example, the Mitel (now Zarlink) GPS
Architect 12 Channel Development Kit has been used for
this purpose (e.g., LeMaster & Rock, 1999; Stone &
Powell, 1998; Wawrzyniak et al., 2001)
Fig. 1 Differential satellite positioning with a stratolite
(http://www.helinet.polito.it)
Although pseudolites transmit ranging signals similar to
GPS satellites, the pseudolite signals can be much
stronger than GPS signals. Therefore there is a potential
interference with the satellite signals due to the pseudolite
transmitter(s) being very close to the receiving antenna
compared to the GPS satellites. However, if the
transmitters are too far from the receiver antenna, the
pseudolite signals will be too weak to be tracked. This is
referred to as the 'near-far' problem, which is caused by
the higher dynamic range of the signal strength a user
receiver will experience when the receiver is in motion
within the proximity of pseudolite signal transmitters
(e.g., Cobb, 1997).
Klein & Parkinson (1984) have proposed three potential
solutions for the near-far problem: (1) to pulse the
pseudolite signals at fixed cycle rates; (2) to transmit the
signals at a frequency offset from GPS L1, but within the
same frequency band as GPS; (3) to use different codes
that have a longer sequence than the existing GPS codes.
Galijan & Lucha (1993) proposed a GLONASS
pseudolite concept, which is similar to solution (2). The
major advantage is that the GLONASS pseudolites will
have a larger near/far ratio, approximately 20 times that
of C/A code GPS pseudolites. However, there are
potential problems with the receiver designs, one of
which is inter-channel biases varying with the antenna
temperature. In addition, this may also lead to
complicated modelling and ambiguity resolution
procedures (Wang, 2000; Wang et al., 2001a). Because
solutions (2) and (3) require modifications of the GPS
receivers, solution (1) is the preferred choice for general
applications.
Recently, Madhani et al. (2001) proposed a successive
interference cancellation approach to mitigate the near-far
problem. This approach is based on a signal processing
technique which does not require receiver hardware
modification. A theoretical analysis has shown that a
combination of code and phase can deal with the near-
fare problem (Progri & Michalson, 2001). It is also
reported that the special pseudolite transmitter antennas
with appropriate radiation patterns can address the near-
far problem (Söderholm et al., 2001).
3 Pseudolite Positioning and Navigation Applications
As ground-based radio signal transmitters, pseudolites
have been used to augment the GPS constellation, to form
an independent system for positioning and navigation
applications, and to integrate with other sensors. During
the past decade, with a variety of pseudolite hardware
designs, the investigations into pseudolites have
intensified across a wide range of applications.
3.1 GPS Augmentation with Pseudolites
The applications of pseudolites in augmenting GPS
satellite constellation had been exploited even at the GPS
development phase. In fact, the first pseudolites deployed
at the Inverted Range were also used to test the
differential GPS (DGPS) concept (Beser & Parkinson,
1982). The role of pseudolites in differential GPS
applications was discussed in, for example, Kalafus et al.
(1986), Parkinson & Fitzgibbon (1986), Stansel (1986),
and Van Dierendonck et al. (1989). In DGPS
applications, a pseudolite can be used to provide not only
an additional ranging signal, but also a differential data
link. Holden & Morly (1997) have reported the
development and test of a pseudolite-augmented DGPS
system GuideNetTM which is built around the Stanford
Telcom Model 7201 Wideband Signal Generator
(pseudolite).
Fig. 2 A navigation and positioning service using stratolites (Tsujii et
al., 2001)
A concept for a navigation and positioning service using
pseudolites on Japan’s airship-based stratospheric
platforms (SPF) has been proposed (Tsujii et al., 2001).
As shown in Figure 2, the airships will be deployed at an
Wang: Pseudolite Applications in Positioning and Navigation 51
altitude of about 20km, and thus the separation between
pseudolites (stratolites) and users varies from 20 to 70km.
The near-far problem will not be as serious a problem as
in the cases of ground-based pseudolite applications. In
this concept, pseudolite signals are considered as extra
satellite signals in navigation and positioning solutions, at
both code and carrier phase levels.
GPS augmentation signals can be transmitted from
airborne or space-borne platforms, such as airplanes and
spacecraft. Airborne pseudolites (APL) have been tested
for military applications (Tuohino et al., 2000). Just like
GPS, such APL research development, driven by the
military purposes, may also benefit the civil pseudolite
applications. With high orbits, GPS has been widely used
in the navigation and attitude determination for low earth
orbit (LEO) spacecraft, such as the International Space
Station (ISS). However, for some spacecraft approaching
the ISS for docking and/or other operations in the vicinity
of the ISS, there may not be sufficient GPS signals for a
navigation solution, as the ISS is a huge structure, which
can block GPS signals. An investigation is underway as
to the feasibility of installing pseudolites on the ISS
structure, transmitting ranging signals to the approaching
space vehicles (Wawrzyniak et al., 2001). In other
developments, pseudolites have been considered for
onboard orbit and attitude control of geo-stationary
satellites (Altmayer et al., 1998) and Spacecraft
Formation Flying (Corazzini & How, 1999).
During the last decade the most notable pseudolite
application has been in aviation for precision approach
and landing (e.g., Brown, 1992; Cohen et al., 1993; Hein
et al., 1997; Bartone & Kiran, 2001). In these
applications, pseudolite measurements can provide an
extra check on the integrity of navigation solutions
(Pervan et al., 1994). In addition, rapid change of the
geometry between the pseudolites and the users can speed
up the carrier phase integer ambiguity resolution, which
is a key prerequisite for precise navigation operations. A
motion-based ambiguity resolution approach has been
developed and tested by Cohen et al. (1993).
It has been established that the pseudolite ranging signals
can contribute to the satellite positioning systems by
enhancing the geometric strength, improving the
availability, integrity, and reliability, and increasing the
accuracy of the positioning solutions, especially in the
height component (e.g., Morley & Lachappelle, 1998;
Stone & Powell, 1998; Dai et al., 2001a; Wang et al.,
2001b).
It has been demonstrated that the pseudolite carrier phase
measurements are of high precision, even at a very low
elevation angle (e.g., Wang et al., 2001b). Precise
pseudolite carrier phase measurements are under study
for a wide range of applications, such as machine control
at mining sites (Stone et. al., 1999), and deformation
monitoring applications (Dai et al., 2001b; Barnes et al.,
2002; Meng et al., 2002). With the introduction of
pseudolite measurements, the success rate of ambiguity
resolution and the reliability of positioning can be
improved (Verhagen, 2001; 2002).
3.2 Pseudolite-only Positioning
In principle pseudolites can replace the satellite
constellation for positioning and navigation wherever
satellite signals are unavailable, such as indoors,
underground carparks, long tunnels, and even on other
planets. Actually, the very first pseudolite application was
pseudolite-only positioning (Harrington & Dolloff,
1976). A pseudolite-only positioning and navigation
concept has been proposed and tested for indoor
positioning (e.g., Zimmerman, 1996; Kee et al., 2000).
The basic principle behind such an indoor positioning
concept is still the ‘double-differencing’ procedure as
used in precise GPS relative positioning.
As with satellite-based positioning systems, the reliability
of a pseudolite-based positioning system is very
dependent on the strength of receiver-pseudolite
geometry in the system. A simulation has been carried
out to analyse the geometry strength for an indoor
application scenario, in which five pseudolite transmitter
antennas were installed on the ceiling (10 metres above
the floor). Figure 3 shows the RDOP values for a rover
moving around on the floor of the room (Wang et al.,
2001c). In this scenario, the RDOP varies from 1.2 to 3.8,
suggesting a reasonably good positioning geometry.
Fig. 3 Indoor positioning scenario: 5 pseudolites 10m above the floor
Simulation is a useful tool for pseudolite-based
positioning design studies. A software simulation tool has
been developed to predict achievable accuracies from an
array of six pseudolites within a tunnel (Calijan, 1996).
Such a simulation study has shown that deployment of
six pseudolites with a good geometry can potentially
provide 1-5 cm horizontal positioning accuracies within a
tunnel of 150m in length. These positioning accuracies
are sufficient for vehicle tracking and control in future
Automated Highway Systems (AHS).
A novel approach has been suggested for the use of
pseudolites in Mars explorations (LeMaster & Rock,
1999). Figure 4 shows a Mars pseudolite array, designed
to provide centimetre-level location and attitude
52 Journal of Global Positioning Systems
information to robotic rovers. This high accuracy
navigation capability is also a key technical requirement
for future astronaut/robot exploration teams to Mars.
Fig. 4 Mars Pseudolite Array (LeMaster & Rock, 1999)
Just like other pseudolite-based positioning systems, the
locations of the pseudolites need to be precisely
determined. However, this will be a difficult task when
placing the pseudolites on another planet like Mars. To
address such a challenge, a new pseudolite positioning
system, a Self-Calibrating Pseudolite Array (SCPA), has
been proposed by the Aerospace Robotics Laboratory at
Stanford University (LeMaster, 2002). As mentioned
earlier, the pseudolites used in such a system are actually
transceivers. The transceivers can transmit and receive
ranging signals to determine the relative locations of all
the pseudolites in the array. As shown in Figure 5
(LeMaster, 2002), a transceiver consists of a transmitter
(PL) and a receiver (Rec), which can essentially receive
the signals from all the transmitters including the one
inside the transceiver itself. This hardware design enables
the cancellation of both transmitter and receiver clock
errors without using a separate reference station, which is
normally required for typical differential satellite
positioning.
Fig. 5 Double differencing with transceivers (LeMaster, 2002)
Pseudolite-only postioning can also be based on the so-
called inverted positioning concept (Raquet et al., 1995).
In such a positioning scenario a reference pseudolite, a
user/mobile pseudolite and four or more receivers are
needed. Similar to GPS relative positioning, the double-
differenced measurements between pseudolites and
receivers can be formed to remove most of the systematic
errors, such as transmitter and receiver clock errors. The
receiver and reference pseudolite locations should be
precisely pre-determined. With the known coordinates for
the receivers and the reference pseudolite, the coordinates
for the user pseudolite can be determined. Extensive
experiments have been conducted to evaluate the
performance under various operating environments (e.g.,
O’Keefe et al., 1999; Tsujii et al., 2001; Dai et al., 2002;
Barnes et al., 2002).
As with satellite-based positioning systems, the reliability
of an inverted pseudolite positioning system is very
dependent on the strength of geometry of the receivers
and pseudolites used in the system. For an inverted
pseudolite positioning system, a poor geometry will be
one in which all the receivers and pseudolites lie
approximately in the same plane (Pachter & Mckay,
2000). Such poor geometry will amplify the errors in the
positioning solutions. Also, ‘unfavorable’ geometry may
occur, for example, when the transmitter antenna stays
directly over the centre of a planar four-receiver square.
In this situation the design matrix of the measurement
equations will become singular, and thus there is no
unique positioning solution. Such situations can be
identified through a full simulation for all possible
trajectories and excluded from positioning operations.
3.3 Integration of GPS, Pseudolites and INS
Given the flexibility that pseudolites can offer,
pseudolites can be combined with other sensors such as
INS. In contrast to satellite/pseudolite-based positioning
systems, INS is self-contained and autonomous. Thus,
INS systems are independent of any external signals.
However, one of main drawbacks of INS, when operated
as a stand-alone system, is the time-dependent growth of
systematic errors. GPS measurements are typically used
to calibrate INS systematic errors. GPS signals might be
obstructed for extended time periods under difficult
operational conditions, during which the performance of
integrated GPS/INS systems may degrade rapidly. This
issue can be addressed by the inclusion of pseudolite
signals. An integrated GPS/INS/pseudolite or
INS/pseudolite system would be able to improve system
performance under a wide variety of poor operational
environments. Such an integration concept has been
proposed and tested (Wang et al., 2001c, Grejner-
Brzezinska et al., 2002), demonstrating the feasibility and
potential for a GPS/INS/PL system.
Extensive simulation studies have demonstrated that an
integrated PL/INS system can provide a stable and high
precision navigation solution for indoor applications (Lee
et al., 2002). The reliability and accuracy of an integrated
PL/INS system is dependent on the geometric distribution
of the pseudolites deployed within the system. Figure 6
shows the DOP values for two different pseudolite
geometric scenarios, indicating a significant difference in
the geometric strength. As shown in Figure 7, the better
pseudolite geometry will lead to better performance
Wang: Pseudolite Applications in Positioning and Navigation 53
Pseudolite hardware
within an integrated PL/INS system. It is noted that the
vertical component can be even more accurate than the
horizontal components. This is due to the use of
pseudolite measurements from low elevation angles.
Overall, these simulation results show that an integrated
PL/INS system can achieve centimetre-level accuracy
even in indoor operating environments (ibid, 2002).
All commercial pseudolite products are currently using
GPS L1 frequency. Operating such pseudolites requires
care so that pseudolite signals do not jam or interfere with
the operation of nearby GPS receivers. Although pulsing
pseudolite signals can reduce the potential interference
with GPS signals, there could be some other frequency
choices suitable for pseudolite applications. To select
optimal pseudolite frequencies, careful considerations
should be given to such factors as hardware
implementation, frequency allocation/licensing,
ambiguity resolution, multipath mitigation, as well as
integration with GPS/GLONASS/GALILEO or even
mobile phone signals for mobile location applications.
Mobile phone signal transmitters can also be considered
as ‘pseudo-satellites’ in some sense (Zhao, 2000).
Although there are several solutions proposed to the
‘near-far’ problem, new solutions are still emerging that
may offer more flexibility and improved performance in
pseudolite positioning.
5.045.041 5.0425.043 5.044 5.045 5.046 5.0475.048 5.049
x 105
0
5
10
15
RDOP
RHDOP
RVDOP
5.045.041 5.0425.043 5.044 5.045 5.046 5.0475.048 5.049
x 105
0
1
2
3
4
5
GPS second
Value
RDOP
RHDOP
RVDOP
Mean RDOP : 4.7
Mean RDOP : 2.1
Fig. 6 DOP values for two different pseudolite scenarios
Pseudolite receivers
5.045.041 5.042 5.043 5.0445.045 5.046 5.047 5.048 5.049
x 105
-1.2
-0.9
-0.6
-0.3
0
0.3
0.6
PL/INS Integation
Position Difference(m)
North Coordinates
East Coordinates
Vertical Coordinates
5.045.041 5.042 5.043 5.0445.045 5.046 5.047 5.048 5.049
x 105
-1.2
-0.9
-0.6
-0.3
0
0.3
0.6
GPS second
Position Difference(m)
North Coordinates
East Coordinates
Vertical Coordinates
Standard Dev.
N : 20.1mm
E : 20.9mm
V : 30.7mm
Standard Dev.
N : 18.8mm
E : 13.0mm
V : 10.8mm
Experiments with current pseudolite and receiver
hardware have revealed some characteristics of
pseudolite signals, which need to be considered in the
receiver tracking loops (Ford et al., 1996; Biberger et al.,
2001). To develop a robust pseudolite tracking receiver,
more investigations are required to gain insights into the
pseudolite signal propagation and reception under a
variety of operating conditions, for example, high
dynamics and severe multipath. Based on the ultra-tight
GPS/INS/PL or INS/PL integration concept, a new
receiver design with an inertial aid may improve the
signal tracking and enhance the reliability of the
positioning solutions. Software receiver architecture
appears to be a good platform for such developments.
Fig. 7 Differences between the reference and computed coordinates for
the PL/INS simulation (Lee et al., 2002) Multipath
Although these simulations demonstrate the potential of
GPS/INS/PL or PL/INS integration, there have been
some concerns about the systematic biases in pseudolite
measurements (e.g., Wang et al., 2001b;c; Dai et al.,
2001).
In pseudolite applications, particularly for indoor
positioning, multipath is a major concern. In static
positioning, pseudolite multipath biases appear to be
constant. In kinematic mode, however, the possible biases
are most likely randomised and thus are difficult to deal
with. This issue may be addressed by using appropriate
transmitting and receiving antennas, robust tracking
techniques, as well as sensor integration. Helical
transmitting antennas are commonly used to mitigate
multipath, whilst various well-designed GPS receiver
antennas can also contribute to the reduction of multipath.
Given the complexity of the multipath issue associated
with spread spectrum ranging, there will be continued
research on multipath mitigation techniques. In another
development, new radio-location systems based on the
Ultra Wide Band (UWB) technology are being proposed
(http://www.uwb.org). It is expected that the new UWB
location systems may more efficiently mitigate multipath
4 Challenging Issues in Pseudolite Applications
From the positioning point of view, a pseudolite is just
like a satellite on the ground. However, different
locations of spaceborne satellites and ground-based radio
transmitters will have a variety of implications for
positioning performance. While pseudolites can offer
great flexibility in terms of geometry and signal
availability, the small separation between pseudolites and
users may cause, among others, a ‘near-far’ problem in
signal tracking, multipath, and tropospheric delay errors
in modelling. Therefore, there are challenging issues in
pseudolite applications, which need to be addressed.
54 Journal of Global Positioning Systems
in indoor positioning, as UWB radio signals are
transmitted as very short discrete pulses.
Pseudolite synchronisation
Unlike GPS satellites, pseudolites are usually equipped
with low cost clocks, which are not accurate enough to
synchronize the sampling time between the reference and
user receivers in a differential positioning mode. Further
more, in a single positioning mode, synchronization of all
the pseudolites used within a positioning system is
critical, which has been one of technical challenges in
pseudolite-only applications. Various techniques have
been proposed to deal with these problems (e.g., Cobb,
1997; Kee et al., 2000; Söderholm et al., 2001). It is
highly desirable to develop synchronizing strategies
suitable for various operating environments. If the
synchronization errors are reduced to the noise level of
the carrier phases, single-differenced integer carrier phase
ambiguities can be resolved and thus, centimetre-level
positioning accuracy can be achieved using just one
receiver.
Modelling errors
In pseudolite positioning, major error sources, such as
pseudolite location errors, multipath and troposheric
delays, need to be treated carefully. In a differential
positioning mode, pseudolite location errors can be
doubled in the differenced measurements with an
unfavourable geometry (e.g., Hein et al., 1997; Dai et al.,
2002). Therefore, pseudolite locations should be precisely
determined.
Multipath might be ‘amplified’ in the single-or double-
differenced measurements. Although multipath should be
carefully dealt with by using appropriate
hardware/firmware, optimal pseudolite-receiver geometry
(Michalson & Progri, 2000) and modelling procedures
can also contribute to the reduction of multipath.
It has been estimated that tropospheric delays in the
pseudolite measurements may reach up to 320ppm with
standard meteorological parameters, and even 600ppm
under extreme weather conditions (Dai et al., 2001a). To
reliably model the tropospheric delays, precise
measurements of atmospheric pressure, temperature and
humidity should be available. Barltrop et al. (1996) have
proposed an adaptive tropospheric delay estimation
method, in which troposheric delays are considered as
additional unknown parameters in the positioning
solutions. More investigations are required to evaluate the
accuracy and reliability of such estimated delays.
In the real world, precisely modelling range
measurements from radio signals is a challenge. Under
difficult operating conditions, for example, when signals
propagate within buildings and penetrate through walls
(Rappaport & Sandhu, 1994; Pahlavan et al., 1998;
Peterson et al., 2000), there are potentially many error
sources. The statistics of the errors relating to signal
propagation need to be investigated.
5 Concluding Remarks
Global navigation satellite systems are expected to play
an increasingly important role in the worldwide
geospatial information infrastructure. During the past
decade GPS has been the driving force behind numerous
position/location sensitive applications, such as car
navigation, mobile phone location, and many others.
Perhaps the most important contribution that GPS has
made is to raise the awareness of location information.
Ultimately we will expect to have precise and reliable
location information for any object in real-time anywhere
and at any time. Location technologies will be an
indispensable part of many emerging areas of business,
such as Location-Based Services (LBS) and future
personal navigation devices, “smart highway” systems,
and so on. However, current satellite-based positioning
systems cannot meet all the requirements for location
information, which include accuracy, reliability, integrity,
coverage and availability. Therefore, new location
technologies are desperately needed to enhance, even
replace under certain circumstances, the satellite
positioning systems. As a by-product of GPS system
development, the pseudolite concept and technology has
demonstrated great potential in meeting such a need.
Acknowledgements
The author would like to thank Prof. Chris Rizos for his
valuable comments on this paper. Thanks are extended to other
members of the Pseudolite Working Group
(http://www.gmat.unsw.edu.au/pseudolite) within the
International Association of Geodesy for many discussions and
the accumulated references they have provided on pseudolite
research.
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