Journal of Global Positioning Systems (2003)
Vol. 2, No. 2: 73-82
High Precision Indoor and Outdoor Positioning using LocataNet
Joel Barnes1, Chris Rizos1, Jinling Wang1, David Small2, Gavin Voigt2, Nunzio Gambale2
1School of Surveying and Spatial Information Systems, The University of New South Wales, Sydney, NSW 2052, Australia
2Locata Corporation Pty Ltd, Australia
Received: 16 December 2003 / Accepted: 29 December 2003
Abstract. Today, GPS is the most popular and widely
used three-dimensional positioning technology in the
world. However, in many everyday environments such
as indoors or in urban areas, GPS signals are not
available for positioning (due to the very weak signals).
Even with high sensitivity GPS receivers, positioning for
urban and indoor environments cannot be guaranteed in
all situations, and accuracies are typically of the order of
tens to hundreds of meters at best. Other emerging
technologies obtain positions from systems that are not
designed for positioning, such as mobile phones or
television. As a result, the accuracy, reliability and
simplicity of the position solution is typically very poor
in comparison to GPS with a clear view of the sky.
Locata is a new positioning technology, developed to
address the failure of current technologies for reliable
ubiquitous (outdoor and indoor) positioning. In this
paper key aspects of the new technology are discussed,
with particular emphasis on the positioning network
(LocataNet). An innovative characteristic of the
LocataNet is its ability to propagate (autonomously) into
difficult environments and over wide areas. Through an
experimental LocataNet installation, a key mechanism for
achieving this is tested, and real-time stand-alone
positioning (without a base station and additional data
link) with sub-centimeter precision is demonstrated.
Key words: Locata, Pseudolite, Indoor positioning,
Network propagation.
1 Introduction
Accurate spatial information is becoming increasingly
important in today’s society, and location aware
applications cover a broad range, from mobile phones to
machine control. Today, GPS is the most popular and
widely used three-dimensional positioning technology in
the world. However, the GPS signals received on the
Earth are extremely weak, and not reliably available in
many everyday environments such as indoors, or in urban
areas where buildings block the line-of-sight to GPS
satellites (Figure 1). This failure in the GPS technology,
and a huge market for location aware applications, has
led to a large number of new positioning technologies.
Currently, the basic approaches of these emerging
technologies have been:
• Use GPS signals, but build special high sensitivity
GPS receivers that try to detect and use very weak
attenuated signals.
• Use signals from systems not designed for positioning,
including mobile phone networks and television.
• Use a combination of 1 and 2.
Fig. 1 Positioning problem with GPS.
These solutions are attractive because they make use of
existing infrastructure. However, they often perform
poorly in comparison to that achievable with GPS in a
benign environment (with a clear view of the sky), in
terms of accuracy, reliability and simplicity. An
alternative approach to extending the capability of GPS
into urban environments and indoors is to increase the
number of GPS signals through ground-based

74 Journal of Global Positioning Systems
transmitters of GPS-like signals, called pseudolites (short
for pseudo-satellites). Although the pseudolite
technology has been around since the 1970s, for GPS
receiver equipment testing before the launch of GPS
satellites (Harrington & Dolloff, 1976), their use has not
been widely adopted. There are several reasons for this:
Hardware – Pseudolites (PLs) are available from a very
small number of companies, and their price tends to be
high (at least US$10,000). The majority of off-the-shelf
(OTS) GPS receivers do not have the capability to use
pseudolite signals. The few OTS GPS receivers that are
able to track pseudolites, can only record pseudolite data
for post-processing, and cannot use the signals in real-
time.
Near constellation – There are some challenging
operational and modelling issues due to the
comparatively small separation between PLs and user
receivers, including near-far signal strength, PL location
errors, tropospheric delays, multipath and non-linearity.
These issues have been discussed in a series of papers by
SNAP researchers (Barnes et al., 2002; Wang et al.,
2001, 2002; Dai et al., 2001).
Unsynchronised – Standard pseudolites are not
synchronised to GPS time or to one another. Therefore,
single-point positioning is not possible, and differential
operation is necessary, similar to DGPS or RTK.
Differential operation requires another GPS receiver at a
known point to provide data to the user GPS receiver. A
wireless communication link must therefore be used
between the base station and user receiver, for example
radio modem or GSM. This requirement significantly
adds to the cost and complexity of a system for
widespread use. Until now, attempts to synchronise
pseudolites have resulted in position solutions that are up
to six times worse in comparison to an unsynchronised
approach using double-differencing (Yun and Kee, 2002).
Despite the problems associated with pseudolites, the
technology has been applied to niche applications such as
the precision approach and landing of aircraft (Soon et
al., 2003, Hein et al., 1997, Bartone, 1996, Barltrop et al.,
1996, Cobb et al., 1995, Galijan et al., 1993). Integrated
GPS and pseudolite systems developed for this purpose
are expensive (US$100,000s) and custom built for a
particular installation.
Locata is a new positioning technology, developed to
address the failure of current technologies for reliable
ubiquitous (outdoor and indoor) positioning. In the
following sections important aspects of the new
technology are discussed, with particular emphasis on the
positioning network (LocataNet). One innovative
characteristic of the LocataNet is its ability to propagate
(autonomously) into difficult environments or over wide
areas. Through an experimental LocataNet installation, a
key mechanism for achieving this is tested, and real-time
stand-alone positioning (without a base station and
additional data link) with sub-centimeter precision is
demonstrated.
Fig. 2 The Locata technology positioning concept.

Barnes et al.: High Precision Indoor and Outdoor Positioning using LocataNet 75
2 The Locata Technology
The Locata technology was designed with four key
objectives (Locata, 2003):
1. Available in all environments.
2. High reliability.
3. High accuracy.
4. Cost effective.
In Locata these objectives are achieved through a
network of ground-based transmitters that cover a chosen
area with strong signals, suitable for accurate positioning
in all environments. A Locata receiver can track both
GPS and Locata signals, thereby providing a seamless
transition between environments where a user can utilise
Locata signals, GPS signals, or both. Figure 2 illustrates
the positioning concept behind the Locata technology.
Locata is designed to enhance and improve GPS,
extending its positioning capability into difficult urban
environments and indoors. Therefore, Locata can
seamlessly work with GPS or entirely independently of it,
and is not designed to replace GPS entirely.
2.1 Core Components
At the heart of the Locata technology are two core
components (see Barnes et al., 2003a for further details):
1. LocataLite – A transceiver which generates a GPS-like
signal. The prototype device shown in Figure 3 transmits
a GPS L1 signal and C/A code pseudorange, and
incorporates the same receiver hardware as the Locata.
2. Locata – A stand-alone low cost GPS-like receiver that
can track both GPS and LocataLite signals. The
prototype hardware, shown in Figure 4, is based on an
existing GPS chipset. When four or more Locata Lite
signals are tracked the Locata receiver is capable of 3-
dimensional positioning with sub-centimeter precision.
Fig. 3 Prototype LocataLite (transceiver) hardware.
Fig. 4 Prototype Locata (receiver) hardware.
2.2 The Positioning Network - LocataNet
When four or more LocataLites are deployed they
cooperate to form a positioning Network called a
LocataNet. This positioning network is time-
synchronous, which means a stand-alone Locata receiver
can compute its position without any additional
information or correctional data. The time
synchronisation procedure is called Time-Loc, and is a
key innovation of the Locata technology (see section
2.3).
When building a Locata Net there are two basic
considerations for the position of the LocataLites. First,
the LocataLites must be able to receive the signal from at
least one other LocataLite. The other basic consideration
is that the geometry of the network (dilution of precision,
DOP) is suitable for the positioning precision
requirements.
The establishment of a LocataNet is designed to be a
simple autonomous process, which is best explained
through the following steps:
A LocataLite self-surveys using the GPS constellation
and begins transmission of its own unique ranging signal
(Figure 5).
A second LocataLite, placed within range of the first
LocataLite self-surveys from the GPS constellation and
the first LocataLite. It time-synchronises to the first
LocataLite signal, and then begins transmission of its
own unique ranging signal (Figure 6).
To allow three-dimensional positioning from a LocataNet
alone, two additional LocataLites are deployed (Figures 7
and 8). The process described in step 2 is repeated for
each additional LocataLite that is deployed, using all the
available signals from the LocataLites and GPS.
Figure 9 illustrates a basic LocataNet with the four time-
synchronised LocataLites. Each LocataLite knows its
precise position, and the four transmitted ranging signals
are all time-synchronised. The established LocataNet can
operate independently of GPS.

76 Journal of Global Positioning Systems
Once the LocataNet is established, additional LocataLites
can be added, even indoors (Figure 10). A fifth
LocataLite placed inside a building can self-survey using
only the LocataNet and then begin transmission of its
own unique ranging signal.
Figure 11 shows an example of a Locata receiver outside
using both GPS and LocataLite signals for positioning.
Figure 12 shows a Locata receiver inside a building using
only Locata Lite signals for positioning, since GPS
signals are blocked.
Fig. 5 Establishing a LocataNet, step 1.
Fig. 6 Establishing a LocataNet, step 2.
Fig. 7 Establishing a LocataNet, step 3.
Fig. 8 Establishing a LocataNet, step 4.
Fig. 9 Establishing a LocataNet, step 5.
Fig. 10 Propagating the LocataNet indoors.
Fig. 11 Positioning outside using LocataNet and GPS.
Fig. 12 . Positioning indoors using LocataNet alone.
The Locata Net concept is powerful and has some
important characteristics including:
Autonomous installation – LocataLites can autonomously
survey and negotiate themselves into a positioning
network. The self-survey can be from GPS or from an
existing LocataNet. This capability makes LocataNet
easily expandable to increase signal coverage where

Barnes et al.: High Precision Indoor and Outdoor Positioning using LocataNet 77
necessary, with LocataLites autonomously joining or
departing.
Ad hoc capability – As well as permanent installation of
LocataNets, the autonomous installation and built in
networking capability allows for ad hoc networks to be
created when required. This is very useful for emergency
response type applications or civil engineering projects.
Signal penetration – In comparison to GPS signals,
LocataLite signals are orders of magnitude stronger.
Because of this, signals from a LocataNet can provide
significant building penetration. In situations where
“deeper” coverage is required, additional LocataLites can
be added to the LocataNet inside the building.
Seamless positioning with GPS – LocataNet uses the
same WGS-84 coordinate system as GPS, allowing a
Locata receiver to seamlessly transition from outdoors to
indoors, maintaining consistent coordinates.
Expansion and coupling – In addition to autonomously
adding LocataLites to expand a network, the unique
Locata technology provides the capability for LocataNets
to join together to form a single continuous network from
two or more individual LocataNets .
Scalability - LocataNets can be large or small, ranging in
size from a room to a large city. Networks could have as
few as four LocataLites to thousands of devices.
Time-synchronised – LocataNet positioning signals are
time-synchronised, which allows single-point positioning
in the same manner as GPS. However, unlike GPS the
level of synchronisation between LocataLites allows
single-point positioning with sub-centimeter precision.
The time synchronisation is achieved through an
autonomous process known as Time-Loc.
2.3 Time-Loc
The time synchronisation accuracy requirements for
LocataLites is very high if sub-centimeter positioning
precision is desired for a Locata receiver, since a one
nanosecond error in time equates to an error of
approximately thirty centimeters (due to the speed of
light). LocataLites achieve high levels of synchronisation
without atomic clocks, external cables, or a master
reference receiver. Time-Loc provides an autonomously
synchronised network. The Time-Loc procedure is best
described in the following steps for two LocataLites A
and B (Figure 13):
1. LocataLite A transmits a unique signal (code and
carrier).
2. The receiver section of LocataLite B acquires, tracks
and measures the signal generated by LocataLite A.
Step 1. LocataLite A transmits a unique signal.
Step 2. LocataLite B receives signal from A.
Step 3. LocataLite B transmits a unique signal.
Step 4. LocataLite B computes difference between transmitted and
received signals.
Step 5. LocataLite B makes difference (in step 4) zero.
Step 6. LocataLite B corects for A & B seperation.
Fig. 13 Time-Loc procedure.

78 Journal of Global Positioning Systems
3. LocataLite B generates its own unique signal (code
and carrier).
4. LocataLite B calculates the difference between the
received signal and its own locally generated signal.
Ignoring propagation errors, the differences between the
two signals are due to the difference in the clocks
between the two devices, and the geometric separation
between them.
5. LocataLite B adjusts its local oscillator using Direct
Digital Synthesis (DDS) technology to bring the
differences between its signal and LocataLite A to zero.
The signal differences are continually monitored so that
they remain zero. In other words, the local oscillator of B
follows precisely that of A.
6. The final stage is to correct for the geometrical offset
between LocataLite A and B, using the known
coordinates of the LocataLites, and after this Time-Loc is
achieved.
In theory, there is no limit to the number of LocataLites
that can be synchronised together using the Time-Loc
procedure described previously. Importantly, the Time-
Loc procedure allows a LocataNet to propagate into
difficult environments or over wide areas. For example,
if a third LocataLite C can only receive the signals from
B (and not A) then it can use these signals for time-
synchronisation instead. Moreover, the only requirement
for establishing a Loca taNet using Time-Loc is that
LocataLites must receive signals from one other
LocataLite. This does not have to be the same ‘central’
or ‘master’ LocataLite, since this is not possible in
difficult environments or when propagating the
LocataNet over wide areas. The proof-of-concept of
LocataNet propagation, and its influence on signal
quality, is investigated in section 3.
2.4 Locata Receiver Positioning Accuracy
The positioning accuracy of a Locata receiver using only
LocataLite signals has been assessed through two
experimental LocataNet installations, comprised of five
LocataLites. One of the installations is designed for
positioning ouside while the other is for indoors. At the
outdoor test network (approximately 200x60 meters) with
direct line-of-sight signals between the LocataLites and
the Locata receiver, static and kinematic point
positioning with sub-centimeter and centimeter level
precision can be achieved respectively (Barnes et al.,
2003a). For the indoor test network (approximately
65x35 meters), the LocataLites are located outside a two-
story office building, and the signals penetrate the
building (brick walls and a metal roof). Using this
LocataNet, indoor static and kinematic real-time
positioning can be achieved with sub-centimeter and sub-
meter precision respectively (Barnes et al., 2003b). For
indoor kinematic positioning with line-of-sight signals,
centimeter level positioning is possible. A Locata
receiver using a LocataNet has several major advantages
in comparison to other currently available positioning
technologies (including GPS) which include:
Reduced latency – In a differential-based navigation
system, the highest positioning accuracies are achieved
when a user waits for time-matched base station data
(with no interpolation). There is therefore latency
associated with base station data transmitted on the
communication link. The Locata receiver does not have
to wait for any additional data in order to compute a
position and therefore has less latency.
Theoretically greater precision – In differential GPS the
double-differenced observable is formed from four
carrier-phase measurements. Assuming all
measurements have equal precision and are uncorrelated,
the precision of the double-differenced measurement is
two times worse than a single carrier-phase measurement
(the basic measurement used by the Locata receiver).
Time solution – In differential GPS the double-
differencing procedure eliminates the clock biases and
hence time information is lost. For certain applications
precise time is important, and the LocataNet approach
allows time to be estimated along with position (as is the
case of standard GPS single-point positioning).
No data-links – Centimeter level positioning precision
can only be achieved with GPS in a differential operating
mode. A base station is used along with a wireless link
(radio modem or GSM) to communicate data to the user
receiver. The base station concept is meaningless in the
LocataNet approach, and no radio modem is required at
the Locata. Additionally there are no radio modems or
hard-wires connecting any of the LocataLite devices.
3. LocataNet Propagation and Signal Quality
As discussed in section 2.3 the Time-Loc methodology
allows a LocataNet to autonomously propagate into
difficult environments and over wide areas, such as an
entire city. To demonstrate the proof-of-concept of
LocataNet propagation, and its influence on signal
quality, an investigation was conducted at an
experimental outdoor LocataNet (Figure 14), near
Canberra in October 2003. The approach for this
investigation was to establish the LocataNet using the
Time-Loc procedure in two different ways:
1. Master Time-Loc: The LocataNet was established
through all Locata Lites time synchronising to a central
‘master’ LocataLite (32), as illustrated in Figure 15.
2. Cascaded Time-Loc: To simulate the propagation of a
LocataNet, time synchronisation was established in four

Barnes et al.: High Precision Indoor and Outdoor Positioning using LocataNet 79
steps: 14 to 32, 29 to 14, 12 to 29, and 21 to 12, as
illustrated in Figure 16.
In both Time-Loc configurations pairs of LocataLites
were time-synchronised in less than ten minutes. It is
important to note that the Locata receiver uses the five
LocataLite signals from the two LocataNet
configurations in exactly the same manner for
positioning, as illustrated in Figure 15.
Fig. 14 Outdoor experimental LocataNet comprised of five
LocataLites.
Fig. 15 Master Time-Loc procedure using ‘master’ LocataLite 32.
For each LocataNet configuration, the Locata receiver
antenna was mounted on a static pole (with known
coordinates), at the centre of the LocataNet. The
geometric configuration of the LocataLites is such that
the dilution of precision (DOP) values at the Locata
receiver antenna, in East North and Up are 0.97, 0.70 and
4.25 respectively. The poor DOP in the Up component is
due to the fact that the greatest elevation angle from the
Locata receiver pole to any LocataLite is 24.5 degrees
(32).
Before a Locata receiver (in the prototype system) can
compute its position using the LocataNet signals, it must
first determine carrier-phase biases using the known
coordinates of the initial receiver position and the
LocataNet (see Barnes et al., 2003a). For each LocataNet
configuration these were determined, and then for
approximately twenty-three minutes the Locata receiver
independently computed real-time position and time
solutions once a second. The real-time positions together
with the raw measurement data were logged using a
laptop computer via a serial interface.
Fig. 16 Cascaded Time-Loc procedure, simulating network propagation.
Fig. 17 Locata receiver positioning using LocataNet signals from either
Master or Cascaded Time-Loc.
3.1 LocataLite Signal Precision
A good way to assess the quality of the signals from the
LocataLites and how well the LocataNet is time-
synchronised is to compute single-difference
measurements between the LocataLites. This eliminates
the Locata receiver clock error, and shows any time
synchronisation errors, and also multipath error. Using
the logged measurement data for both Time-Loc
procedures, single-difference measurements were
computed between LocataLite 32 and all the other
LocataLites. The ambiguities of the single-differences
were resolved using the known coordinates of the
LocataLites and the Locata receiver pole.
Figures 18 and 19 show the four single-differences
between 32 and the other LocataLites for the Master and
Cascaded Time-Loc LocataNets respectively. Most
importantly, visually all the single-difference time series
on average fit a horizontal line, and do not have any long-

80 Journal of Global Positioning Systems
term drifts during the twenty-three minute test. The
single-difference standard deviations (Table 1) for the
Master Time-Loc LocataNet range from 3.9 to 10.4 mm.
Interestingly, the standard deviations of LocataLites 12
and 21 are at least 40% smaller than 29 and 14. These
LocataLites are approximately 30 m from 32, as opposed
to approximately 100 m for 29 and 14. However, in a
previous experiment using the same LocataNet (Barnes et
al., 2003b), the standard deviation of 12 was slightly
greater than 14, 29 and 21, all with similar values
(difference of less than 20%). Therefore, the size of the
values do not appear to be correlated with the distance
over which Time-Loc is conducted, and requires further
investigation.
The single-difference standard deviations (Table 1) for
the Cascaded Time-Loc LocataNet range from 8.9 to
16.1 mm. In both Time-Loc procedures LocataLite 14
time synchronises to 32, and therefore the standard
deviations are expected to be almost the same. However,
the standard deviation of LocataLite 14 single-difference
time series is 14% smaller (1.5 mm), and visually appears
more random in the Cascaded Time-Loc LocataNet. The
reason for this requires further investigation. As
expected, for the other LocataLites the single-difference
standard deviations of the Cascaded Time-Loc are greater
than the Master Time-Loc LocataNet. It is difficult to
quantify exactly the increase in measurement noise due to
the Cascaded Time-Loc procedure, given that the single
difference standard deviation of LocataLite 14 is not the
same for both procedures. However, comparing the
LocataLite standard deviations for each step in the
Cascaded Time-Loc shows the values increase by 1.8mm
(29-14), 1.1 mm (29-12) and 4.3 mm (21-12). The size
of the increases requires further investigation, however
they do not appear to be correlated with the distance over
which Time-Loc is carried out. For example, the greatest
standard deviation increase (4.3 mm) is over the shortest
distance (60m) in the Cascaded Time-Loc procedure
(Figure 15). A possible cause for the variation in the
standard deviations is multipath error.
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Fig. 18 Master Time-Loc: LocataLite single-differences using
32 as reference (14, 29, 12, 21, top to bottom).
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Fig. 19 Cascaded Time-Loc: LocataLite single-differences
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Barnes et al.: High Precision Indoor and Outdoor Positioning using LocataNet 81
Single difference stdev
(mm)
LocataLite
Master
Time-Loc
Cascaded
Time-Loc
East (14) 10.4 8.9
West (29) 8.1 10.7
North (12) 3.9 11.8
South (21) 4.7 16.1
Tab. 1 LocataLite standard deviations.
3.2 Locata positioning accuracy
To assess the accuracy of the real-time positioning
results, the known (sub-centimeter) coordinate of the
Locata receiver pole was used to compute the positioning
error for each epoch in the two different Time-Loc
LocataNets. Because of the poor DOP in the Up
component, the following discussion will concentrate on
horizontal components only. Figures 20 and 21 show the
East and North errors for Master and Cascaded Time-Loc
LocataNets respectively. For all the time series the mean
error is 2mm or less, with standard deviations all less
than 8 mm. Clearly sub-centimeter positioning precision
has been achieved for both LocataNets. The standard
deviations of East and North positioning results in the
Cascaded Time-Loc LocataNet are approximately 30%
greater than the Master Time-Loc LocataNet. This
increase is expected since the LocataLite single-
difference measurement time series have greater standard
deviations (see section 3.1). However, importantly there
are no biases or long term trends introduced as a result of
the Cascaded Time-Loc procedure. As expected, the
North position standard deviation for both LocataNets is
smaller than the East, because the network geometry is
slightly better in the North (smaller DOP value, see
section 3).
4 Concluding Remarks
In this paper important aspects of the Locata technology
have been described, with particular focus on the
positioning network (LocataNet). Through an outdoor
experimental LocataNet installation, proof-of-concept for
a propagating positioning network has been
demonstrated. In two different LocataNet configurations,
one propagating (Cascaded Time-Loc) and the other non-
propagating (Master Time-Loc), stand-alone positioning
with sub-centimeter level precision was achieved. This
level of precision is very pleasing for a prototype system,
and is as good as (if not better than) GPS RTK using a
base station, radio modem and double differencing.
Moreover, the propagating mechanism of LocataNet is a
significant innovation, because it allows the positioning
network to extend into difficult environments and expand
over wide areas. The Locata technology has the potential
to deliver high-precision, ubiquitous (outside and inside)
positioning for an enormous range of location aware
applications, and research and development continues to
realise this potential.
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Gambale (2003a) Locata: A new positioning technology
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Epoch (s) stdev 5.4 mm mean 1.4 mm rms 5.6 mm
Error (metres)
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Epoch (s) stdev 4.1 mm mean −2.0 mm rms 4.6 mm
Error (metres)
Fig. 20 Master Time-Loc: Locata receiver East and North static
positioning error.
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0
0.01
0.02
0.03
0.04
0.05
East
Epoch (s) stdev 7.7 mm mean 1.0 mm rms 7.8 mm
Error (metres)
0200400 60080010001200140
0
−0.05
−0.04
−0.03
−0.02
−0.01
0
0.01
0.02
0.03
0.04
0.05
North
Epoch (s) stdev 5.9 mm mean −1.5 mm rms 6.1 mm
Error (metres)
Fig. 21 Cascaded Time-Loc: Locata East and North static
positioning error.

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