Locata: A New Positioning Technology for High Precision Indoor and Outdoor Positioning

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ABSTRACT

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Locata: A New Positioning Technology for High
Precision Indoor and Outdoor Positioning
Joel Barnes, Chris Rizos, Jinling Wang
School of Surveying & Spatial Information Systems, The University of New South Wales, Australia (UNSW)
David Small, Gavin Voigt, Nunzio Gambale
Locata Corporation Pty Ltd, Australia
BIOGRAPHY
Joel Barnes is one of the senior researchers within the
Satellite Navigation and Positioning (SNAP) group, at the
School of Surveing & SIS, the University of New South
Wales (UNSW), Sydney, Australia. He obtained a Doctor
of Philosophy in satellite geodesy from the University of
Newcastle upon Tyne, UK. Joel has assisted in the
development of a Locata (the mobile positioning device)
and testing of the “Locata technology”. Other current
research interests include pseudolites, GPS receiver
firmware customization and high precision kinematic GPS
positioning.
Chris Rizos is a graduate of the School of Surveying, The
University of New South Wales (UNSW), Sydney,
Australia; obtaining a Bachelor of Surveying in 1975, and
a Doctor of Philosophy in 1980 in satellite geodesy. Chris
has been researching the technology and high precision
applications of GPS since 1985, and is currently leader of
the Satellite Navigation and Positioning group at UNSW.
Chris is a Fellow of the Australian Institute of Navigation,
a Fellow of the International Association of Geodesy
(IAG), and is currently president of the IAG’s
Commission 4 “Positioning and Applications”.
Jinling Wang is a Lecturer in the School of Surveying &
Spatial Information Systems at the University of New
South Wales (UNSW). He is Editor-in-Chief of the
Journal of Global Positioning Systems and Chairman of
the international working Group on pseudolite
applications with the International Association of
Geodesy’s (IAG).
David Small is director of research and development at
Locata Corporation, and is also president. He founded
Locata corporation (formely QX corporation) with his
partner Nunzio Gambale in 1995. He has invented almost
all the technical aspects of the Locata enabling technology
Currently he is the holder of 11 patents granted in 8
countries, with many more in the process. David is
responsible for conceiving, conducting and leading the
R&D program for Locata Corporation.
Gavin Voigt is principal design engineer for Locata
Corporation. He is the principal engineer in the
development of the Locata technology and is involved at
all levels of the design process. He was directly
responsible for the design and implementation all aspects
of the hardware and low level software of the Locata
technology. He has a BEng in Electronics and
Communications from University of Canberra, Australia.
Nunzio Gambale is chief excutive officer of Locata
Corporation. . He founded Locata corporation (formely
QX corporation) with his partner David Small in 1995.
He has conceived the business, patent, trademark and
marketing concepts, and consumer devices embodied
within Locata technology.
ABSTRACT
The use of GPS for indoor positioning poses difficult
challenges due to very weak signal levels, and accuracies
are typically of the order of tens to hundreds of metres at
best. To overcome this severe limitation Locata
Corporation has invented a new positioning technology
called Locata, for precision positioning both indoors and
outside. Part of the “Locata technology” consists of a
time-synchronised pseudolite transceiver called a
LocataLite. A network of LocataLites forms a LocataNet,
which transmits GPS-like signals that allow single-point
positioning using carrier-phase measurements for a mobile
device (a Locata). The SNAP group at UNSW has
assisted in the development of a Locata and testing of the
new technology. In this paper the prototype “Locata
technology” is described, and the results of indoor
positioning performance test experiments are presented.
The experiments demonstrate proof-of-concept for the
“Locata technology” and show that carrier-phase point
positioning (without radio modem data-links) is possible
with sub-centimetre precision.
1 INTRODUCTION
A real-time positioning technology that can operate
indoors and outside anywhere in the world, with sub-cm
accuracy, and low cost is the ultimate goal for many
researchers. GPS can achieve cm-level kinematic
positioning accuracy, but with some major constraints.
First and foremost the use of GPS signals for indoor
positioning poses difficult challenges, due to the very

weak signal levels. Indoor positioning using high
sensitivity GPS receivers cannot be guaranteed in all
situations, and accuracies are typically of the order of tens
to hundreds of metres at best. Of course GPS is widely
used outdoors for real-time cm-level positioning in
numerous applications. In these situations real-time
kinematic GPS techniques (RTK) are used, where a base
station transmits data to a rover unit via a radio modem.
The double-differenced carrier-phase observable is
commonly utilised, to reduce spatially correlated errors
due to the atmosphere and orbit errors, and to eliminate
both receiver and satellite clock biases. The GPS
hardware is of the dual-frequency variety and therefore
quite expensive (typically US$30,000 for a RTK system
utilising two receivers), and only works well with a
relatively unobstructed and geometrically favourable GPS
constellation.
Ground-based transmitters of GPS-like signals (called
“pseudolites”) can be used to augment GPS where the
satellite geometry is poor or the signal availability is
limited. They therefore have the potential to be used for
both outdoor and indoor positioning. With enough
pseudolites it is theoretically possible to replace GPS
entirely, though in practice this has been difficult to
achieve. Typically pseudolites use cheap crystal
oscillators and operate independently (in the so-called
“unsynchronised mode”). In this case, the GPS data is
double-differenced to eliminate the pseudolite and
receiver clock biases.
The SNAP group has conducted pseudolite research for
the past three years, and experimented with them in the
unsynchronised mode for a variety of applications (see
Barnes et al., 2002a; Barnes et al., 2002b; Wang, 2002,
Wang et al., 2001; Dai et al., 2001). Real-time
centimetre-level positioning with unsynchronised
pseudolites can only be achieved with a base station that
provides data to a rover unit via a radio modem (as with
standard RTK-GPS). If pseudolites can be synchronised,
stand-alone positioning can be achieved without base
station data (and without the radio modem data link).
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).
Locata Corporation has invented a new positioning
technology (Locata), that consists of a network
(LocataNet) of time-synchronised pseudolite transceivers
(LocataLites). In Barnes et al. (2003), at an outdoor
LocataNet test network, real-time stand-alone positioning
(without a base station) at centimetre-level precision was
demonstrated for a kinematic rover (a Locata). If a
LocataNet is established indoors, and there are direct line-
of-sight signals from the LocataLites to a Locata then cm-
levels of precision can be expected. In real-world indoor
positioning applications, such as the tracking of people or
assets in an entire office building, with many rooms, it is
uneconomical to install LocataLite devices in every room
to achieve a direct-line of sight signal between
LocataLites and a Locata.
This paper concentrates on the use of LocataLite signals
that arrive at a Locata via a non line-of-sight path,
specifically by penetrating an office building. In the
following sections, the “Locata technology” is described,
and real-time stand-alone (without base station data)
indoor positioning with up to sub-cm precision is
demonstrated.
2. LOCATA CORPORATION’S “LOCATA
TECHNOLOGY”
Locata Corporation’s Locata is a positioning technology
that is designed to overcome the limitations (outlined in
section 1) of GPS and other indoor positioning systems
currently available. It has invented a time-synchronised
pseudolite transceiver called a LocataLite. A network of
LocataLites forms a LocataNet, which transmits GPS-like
signals that have the potential to allow point positioning
with sub-cm precision (using carrier-phase) for a mobile
unit (a Locata). A prototype system has been built to
demonstrate the proof-of-concept of the “Locata
technology”, and is described in the following sections.
2.1 L
OCATA
L
ITE
The LocataLite can be described as an “intelligent
pseudolite transceiver”. The transmitter prototype
hardware used is such that the intelligence of the unit is in
its software. This is an extremely flexible approach, and
allows major design changes without requiring completely
new hardware. The receiver part of the prototype is based
on an existing GPS receiver chipset, which is described in
section 2.3. The receiver chipset and the transmitter share
the same clock, which is a cheap temperature-
compensated crystal oscillator (TCXO). The transmitter
part of the prototype generates C/A code pseudorange and
carrier-phase signals at the GPS L1 frequency. The signal
is generated digitally (unlike most existing pseudolites,
which use analogue techniques) and can be operated in a
pulsing mode with different duty cycles, power output,
and any PRN code can be generated. Pulsing is
commonly used with pseudolite signals (instead of a
continuous transmission, like GPS), to reduce interference
and increase the working range (the near-far problem).
The duty cycle refers to the percentage of time the
pseudolite is transmitting when pulsing. Commercially
available GPS patch antennas are used for the receiver and
transmitter, in addition to a custom built ¼ wave antenna
for one of the LocataLite transmitters. The prototype
LocataLite and antennas are shown in Figure 1.

Figure 1. Prototype LocataLite hardware and antennas.
2.2 T
IME
L
OC
In order for a mobile receiver (a Locata) to carry out
carrier-phase point positioning (CPP) without the need for
base station data, the LocataLite devices must be time-
synchronised. The level of synchronisation required is
extremely high, considering a one nanosecond error in
time equates to an error of approximately thirty
centimetres (due to the speed of light). The time-
synchronisation procedure of one or more LocataLite
devices is a key innovation of the “Locata technology”
and is know as TimeLoc. The TimeLoc procedure to
synchronise one LocataLite (B) to another LocataLite (A)
can be broken down into the following steps:
1. LocataLite A transmits a C/A code and carrier signal on
a particular PRN code.
2. The receiver section of LocataLite B acquires, tracks
and measures the signal (C/A code and carrier-phase
measurements) generated by LocataLite A.
3. LocataLite B generates its own C/A code and carrier
signal on a different PRN code to A.
4. LocataLite B calculates the difference between the code
and carrier of 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 code and
carrier differences between itself and LocataLite A to
zero. The code and carrier differences between
LocataLite A and B 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 TimeLoc is achieved.
Importantly, the above procedure does not require
expensive atomic clocks, and there is in theory no limit to
the number of LocataLites that can be synchronised
together using TimeLoc.
2.3 A L
OCATA
To speed up the development of a prototype system it was
decided to use existing GPS hardware for the receiver
section in the LocataLite and the Locata (the mobile
positioning device). The SNAP Group at UNSW has
assisted in the development of the Locata, through Mitel’s
(now Zarlink) GPS Architect development system
(Zarlink, 1999). The development system uses the Mitel
GP2000 chipset comprised of the GP2015 RF front end
and GP2021 12-channel correlator, together with the
P60ARM-B microprocessor (Ibid). Importantly the
system includes GPS firmware C source code that can be
modified, compiled and uploaded to the GPS receiver.
However, the GPS Architect hardware is designed as an
indoor laboratory development tool and not suited to
outdoor use.
Instead of designing and building GPS receiver hardware
(using the GP2000 chipset) suitable for outdoor use, a
different approach was taken. This was to modify a
Canadian Marconi Corp (CMC) Allstar GPS receiver,
which uses the Mitel GP2000 chipset, so that it would
operate in exactly the same way as the GPS Architect
hardware. The original GPS Architect firmware source
code has been extensively modified and improved, by the
Locata Corporation and the SNAP group. The
modifications have been in signal acquisition, the tracking
loops and the navigation algorithm. The prototype Locata
hardware and antenna (a commercially available patch
antenna) are shown in Figure 2.
Figure 2. Prototype Locata hardware.
2.4 NAVIGATION ALGORITHM IN A L
OCATA
The Locata uses carrier-phase point positioning (CPP) to
determine its three-dimensional position from at least four
LocataLites. As the name suggests, CPP uses the carrier-
phase as its basic measurement and it is therefore useful to
consider the carrier-phase observations in the case of GPS.
The basic GPS L1 carrier-phase observation equation
between receiver A and satellite j in metres can be written
as:

1
jj j j
A A trop A ion A
L
c
cT cT N
f
ϕρ
τδδτ ε
=+ + − −− + (1)
where
1L
f
is the frequency of the L1 carrier-phase
observable; c is the speed of light in a vacuum;
j
A
ρ
is the
geometrical range from station A to satellite j;
A
T
δ
is the
receiver clock error for station A;
j
T
δ
is the satellite clock
error for satellite j;
j
A
N is the integer ambiguity (the
unknown number of carrier cycles between the receiver A
and satellite j at lock-on);
ion
τ
is the atmospheric
correction due to the ionosphere;
trop
τ
is the atmospheric
correction due to the troposphere;
ε
represents the
remaining errors, which may include orbital errors,
residual atmospheric effects, multipath error and receiver
noise, etc.
For kinematic GPS, equation (1) contains parameters that
are not known with a high enough accuracy to enable a
single GPS receiver to perform CPP, and determine the
receiver’s position and clock error at the cm-level.
Instead, another GPS receiver (a base station) is used and
the data is double-differenced to eliminate both receiver
and satellite clock errors, and to reduce the effects of orbit
errors (baseline length dependent), and the spatially
correlated errors due to the troposphere and ionosphere.
If real-time kinematic positioning using carrier-phase is
desired, the base station data must be available at the rover
receiver, typically via a radio modem. The carrier-phase
integer ambiguities must be determined before cm-level
carrier-phase positioning can be realised. There are
numerous ambiguity resolution approaches used, but they
can basically be broken down into geometry and
geometry-free approaches (Leick, 1995). However,
reliable rapid (less than a minute) On-The-Fly (OTF)
ambiguity resolution is only possible when L2 carrier-
phase data, in addition to L1 data, is used, and at least five
satellites with good geometry are visible. The cost of a
commercial RTK system with dual-frequency GPS
receivers is therefore relatively expensive, and typically
costs US$30,000.
In comparison to GPS the basic LocataNet carrier-phase
observation equation between receiver A and LocataLite j
(in metric units) can be written as:
1
j
jj j
AAtrop A A
L
c
cT N
f
ϕρ
τδ ε
=+ + − + (2)
where the terms are the same as for GPS, except they refer
to LocataLites instead of satellites. In equation (2) there is
no clock error due to the LocataLites since they are time-
synchronised to each other (see Section 2.2), and because
the devices are ground-based there is no ionospheric
correction term. The tropospheric correction will depend
on the separation between the Locata and the LocataLite,
the elevation angle to the LocataLite, and the atmospheric
conditions (temperature, humidity and pressure) along the
line-of-sight signal path.
The term that poses the most difficulty in the above
equation is the unknown number of carrier wavelengths
between the Locata and the LocataLite when TimeLoc is
achieved. In the prototype system the ambiguity term and
the initial receiver clock error are determined through a
static initialisation at a known point. Assuming that the
tropospheric effects are modelled or negligible due to
relatively short distances between the Locata and
LocataLite, the initial bias (clock error and ambiguity) in
metres can be written as:
1
j
jj
AA A
L
c
BcT N
f
δε
=− + (3)
jjj
AAA
B
ϕρ
=−
(4)
The basic observation equation (2) therefore becomes:
jjj
AAA A
BdT
ϕρ
δε
=++ + (5)
and
22 2
()()()
jjjj
AA A A
XX YY ZZ
ρ
=−+−+−
(6)
where
A
dT
δ
is the change in the receiver clock error from
the static initialisation epoch, and this together with the
Locata coordinates , ,
AA A
XYZ give four unknowns;
which can be solved with a minimum of four LocataLite
carrier-phase measurements and least squares estimation.
The least squares estimation procedure is similar to that
for standard GPS single point positioning (SPP), except
that the very precise carrier-phase measurement is used.
After the carrier-phase bias is determined through static
initialisation the Locata is free to navigate kinematically.
The positioning algorithm is embedded in the GPS
firmware of the Locata to allow for real-time positioning.
It should also be stressed that each positioning epoch is
independent and no smoothing or filtering is carried out in
the prototype system.
2.5 ADVANTAGES OF THE
L
OCATA
N
ET
There are several major advantages to the LocataNet
approach in comparison to other currently available
positioning technologies (including GPS), which include:
No data links – 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.
Reduced latency – In a differential-based navigation
system, the highest positioning accuracies are achieved
when the rover uses time-matched base station data (with
no interpolation). Therefore, the rover unit must wait to

receive base station data before it can compute a position.
The Locata computes a carrier point position (CPP) using
time-synchronised signals from the LocataLites and does
not have to wait for any additional data in order to
compute a position.
Intelligent signal transmissions – Standard pseudolites
typically use pulsing to prevent jamming and reduce the
near-far problem. However, when operating pseudolites
in this manner it is still possible that multiple devices may
be transmitting at exactly the same time and could cause
interference problems. In the LocataNet, signal
transmissions are precisely controlled to ensure that
LocataLites do not transmit at the same time, minimising
interference between signals from different LocataLites.
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).
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).
3. L
OCATA
N
ET
TEST NETWORK FOR INDOOR
POSITIONING
To demonstrate the concept of LocataNet for indoor
positioning, and to test the accuracy of the TimeLoc
methodology, a test network has been established at the
Locata Corporation’s offices. The offices are located in a
two-storey building with double brick external and
internal walls, and with a flat corrugated metal roof
(Figure 3).
The network comprises of five LocataLite devices located
on and around the outside of the two storey office
building, as illustrated in Figures 3 & 4. Four of the
devices are orientated approximately North, East, South
and West, while the fifth device (Master) is located
approximately at the centre of the other four, with a direct
line-of-sight to each of them. The LocataLite’s transmit
and receive antennas are mounted on poles bolted to the
office building. The positions of the poles in the test
network were established to cm-level accuracy, using GPS
data collected (with NovAtel Millennium receivers over
one hour, at a one second rate) between the Master pole
and other poles in the network. On the first floor of the
building, the position of an indoor test location (rover in
Figure 3) was determined using traditional surveying
methods. This point can be used to initialise the Locata
before navigation, or to perform static accuracy tests. The
dilution of precision (DOP) values at the rover point in
East, North and Up are 0.71, 0.73, 1.4. The elevation
angles and distance of the LocataLites from the rover pole
are given in Table 1. The master LocataLite has the
largest elevation angle (65.1) from the rover, while the
elevation angles of the others range from –2.7 to 7.7
degrees.
Table 1. LocataLite trial details: elevation angle and
distance from the indoor rover point.
From rover pole
LocataLite PRN
used
Transmit/
receive
antennas
elev. angle
(degrees)
distance
(m)
Master 32 ¼ wave/NA 65.1 3.7
North 12 Patch/Patch -2.7 37.2
East 14 Patch/Patch 7.7 14.5
South 21 Patch/Patch 4.3 30.4
West 29 Patch/Patch 8.2 18.3
Figure 3. LocataNet test network for indoor positioning.
−40 −30 −20 −10 0 10 20 30 40
−30
−20
−10
0
10
20
30
40
East (m)
North (m)
rover
master
north
east
south
west
Figure 4. ‘Map’ showing position of LocataLites and
indoor rover test point.
3.1 INDOOR POSITIONING PERFORMANCE OF
THE “L
OCATA
TECHNOLOGY”
On 19 December 2002, a test was conducted at the
LocataNet test network (described in section 3) to assess
indoor positioning accuracy. After turning the
LocataLites on, the North, South, East and West devices
time-synchronised to the signal transmitted by the Master
using TimeLoc. Time-synchronisation of the LocataLites
was typically achieved in less than 10 minutes, and
remained time-synchronised for several hours, which
indicates the very good reliability and stability of the
TimeLoc procedure. The LocataLites used GPS satellite
PRN codes 12 (North), 14 (East), 21 (South), 29 (West)
and 32 (Master), as listed in Table 1. All the LocataLites
used patch antennas for the transmitter and receiver, with
the exception of the Master pseudolite, whose transmit

antenna was a ¼ wave vertical. Table 1 summarises the
configuration of the LocataLites.
3.1.1 INDOOR STATIC ACCURACY TEST
A static positioning test was first performed at a known
location (‘rover’ in Figure 4), to assess the indoor
positioning accuracy and the LocataLite TimeLoc
technique. The rover point only has a direct line-of-sight
(through glass) to the North LocataLite, and the signals
from the other devices must pass through the structure of
the building. In particular signals from the West and
South LocataLites must penetrate several double-brick
walls and a metal roof.
As described in section 2.4, in order for a Locata to carry
out CPP, the carrier-phase biases must first be determined.
With the Locata antenna mounted on the known
coordinates of the rover point (as illustrated in Figure 5)
the carrier-phase biases were determined. Then for
approximately 42 minutes the Locata independently
computed real-time position and time solutions once a
second, giving 2500 epochs of data. The real-time
positions together with the raw measurement data were
logged using a laptop computer via a serial interface.
One interesting measurement logged during the test was
the signal-to-noise ratio (SNR) values of the five
LocataLite units, recorded by the Locata, and these are
plotted Figure 6. Also, the mean and standard deviation
of the SNR time series are given in Table 2. If the
LocataLites and the Locata are stationary, and the
measurement environment remains constant, it is expected
that the SNR values should be random with a constant
mean, unlike GPS SNR values which typically increase as
the satellite elevation angle increases. Overall, the signal
strength from all the LocataLites was good, with mean
values ranging from 18.7 to 21.42 dB. These mean values
are largely a function of what materials (brick walls, metal
roof etc) the signals must penetrate and the elevation angle
of the LocataLite (the antenna gain pattern). The signals
from the South (21) and West (29) LocataLites must
penetrate the most material and therefore have the
smallest mean values. In terms of the variation of the
SNR values, the Master (32) LocataLite has the least
variation, with the smallest standard deviation of 0.024
dB, while the greatest variations are for the North (12) and
South (21) LocataLites. The larger variations in SNRs for
these LocataLites can be explained due to people walking
around the offices during the experiment, whereas the
signal from the Master is almost directly above the Locata
antenna and the signal path environment (metal roof) does
not change. It is important also to note that during this
period the Locata tracked the LocataLite signals without
difficulty.
Figure 5. Indoor static test at ‘rover’ point: Locata &
antenna, and laptop for data logging.
0 500 1000 1500 2000 2500
18
18.5
19
19.5
20
20.5
21
21.5
22
SNR 32
Epoch (s) stdev 0.024 mean 20.20
SNR (dB)
0 500 1000 1500 2000 2500
18
18.5
19
19.5
20
20.5
21
21.5
22
SNR 12
Epoch (s) stdev 0.218 mean 20.75
SNR (dB)
0 500 1000 1500 2000 2500
18
18.5
19
19.5
20
20.5
21
21.5
22
SNR 14
Epoch (s) stdev 0.129 mean 21.42
SNR (dB)
0 500 1000 1500 2000 2500
18
18.5
19
19.5
20
20.5
21
21.5
22
SNR 21
Epoch (s) stdev 0.186 mean 19.55
SNR (dB)
0 500 1000 1500 2000 2500
18
18.5
19
19.5
20
20.5
21
21.5
22
SNR 29
Epoch (s) stdev 0.117 mean 18.65
SNR (dB)
Figure 6. Signal-to-noise ratio (SNR) values of the five
LocataLites: master, north, east, south and west (top to
bottom).

Table 2. LocataLite SNR and single-difference statistics.
SNR (dB)
LocataLite
mean stdev
Single difference
stdev (mm)
Master (32) 20.2 0.024 Reference
North (12) 20.8 0.218 8.7
East (14) 21.4 0.129 8.2
South (21) 19.6 0.186 7.8
West (29) 18.7 0.117 5.3
A useful way to assess how well the LocataLite units are
time-synchronised and the quality of the carrier-phase
measurement data is to compute single-difference
measurements between the LocataLites. This will
eliminate the Locata clock error, and show any errors due
to the LocataLite clocks and also multipath. Using the
logged measurement data, single-difference observables
were computed between the Master and all other
LocataLites. The ambiguities of the single-differences
were resolved using the known coordinates of the
LocataLites and the rover point.
Figure 7 shows the four single-difference time series
between the Master and the other LocataLites. Most
importantly, visually all the single-difference time series
on average fit a horizontal line and do not appear to have
any long-term drifts. The overall standard deviations of
the single-difference time series are all less than 9mm (see
Table 2), and in terms of how well the LocataLite clocks
achieve TimeLoc, and this equates to approximately 30
pico-seconds. Interestingly, the single-difference time
series for the South (29) LocataLite has the lowest
standard deviation (5.3mm), even though the line-of-sight
signals for this LocataLite pass through a several internal
walls and a corridor that is commonly used by people
walking between offices. The standard deviations for the
other LocataLite single-differences are very similar.
Visually, all the time series do not appear entirely random
and the cause of the fluctuations requires further
investigation. One likely factor is the changing multipath
conditions as people walk around the office building. The
single-difference time series for North (12) and East (14)
visually appear the most random, and multipath conditions
along the line-of-sight from the rover point to these is
least likely to change. The effect of building propagation
on signal propagation is one area that requires further
investigation.
To assess the accuracy of the real-time indoor positioning
results, the known (sub-cm) coordinate of the rover pole
was used to compute the positioning error for each epoch.
Figure 8 shows the East and North errors for the real-time
positions of the Locata. The mean error of the both time
series is less than 2.1mm, with the standard deviations and
root-mean-square values less than 4mm. Clearly sub-
centimetre indoor positioning precision has been achieved
with 99% of the East and North errors less than ±1cm.
Importantly there are no long-term drifts in the position
time series. The time series are not entirely random, as
expected, and the fluctuations present correlate with those
in the single-difference time series (Figure 7). The above
results demonstrate that in a real-world office environment
sub-centimetre indoor positioning precision can be
achieved with the “Locata technology” even with non
line-of-sight signals.
0 500 1000 1500 2000 2500
−0.05
−0.04
−0.03
−0.02
−0.01
0
0.01
0.02
0.03
0.04
0.05
L1 single difference 12−32
Epoch (s) stdev 8.7mm
Metres
0 500 1000 1500 2000 2500
−0.05
−0.04
−0.03
−0.02
−0.01
0
0.01
0.02
0.03
0.04
0.05
L1 single difference 14−32
Epoch (s) stdev 8.2mm
Metres
0 500 1000 1500 2000 2500
−0.05
−0.04
−0.03
−0.02
−0.01
0
0.01
0.02
0.03
0.04
0.05
L1 single difference 21−32
Epoch (s) stdev 7.8mm
Metres
0 500 1000 1500 2000 2500
−0.05
−0.04
−0.03
−0.02
−0.01
0
0.01
0.02
0.03
0.04
0.05
L1 single difference 29−32
Epoch (s) stdev 5.3mm
Metres
Figure 7. Single-differences of the LocataLites using
master as reference (north, east, south and west, top to
bottom).
0 500 1000 1500 2000 2500
−0.05
−0.04
−0.03
−0.02
−0.01
0
0.01
0.02
0.03
0.04
0.05
East
Epoch (s) stdev 3.1 mm mean 1.5 mm rms 3.4 mm
Error (metres)
0 500 1000 1500 2000 2500
−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 3.1 mm mean −2.1 mm rms 3.7 mm
Error (metres)
Figure 8. The Locata East and North static positioning
error.

3.1.2 INDOOR KINEMATIC ACCURACY TEST
It is difficult to asses the kinematic indoor positioning
performance of a Locata without a ‘truth’ positioning
system with greater positioning accuracy. However, in an
indoor office environment the path of a moving Locata is
restricted by the internal wall structure of the building.
Therefore, the approach in this experiment was first to
determine the carrier-phase biases at the rover point, and
then to move around the building, finally returning to the
initial rover point. This allows the real-time trajectory of
the Locata to be compared with a floor plan of the
building. Additionally, returning to the ‘rover’ point at
the end of the kinematic session allows comparison to a
‘true’ position.
In the kinematic tests both real-time positions and raw
measurement data were recorded using a laptop via a
serial interface, as illustrated in Figure 9. The positioning
results for a typical kinematic test are shown in Figure 10,
where the horizontal real-time position results are overlaid
on a floor plan map of the offices. The path of the Locata
was from rover point, down (South East) and up (North
West) the main corridor, and lastly back to the rover point.
It is important to note that while the Locata antenna was
in the corridor there was no direct line-of-sight to any of
the LocataLites positioned outside the building. The main
corridor is approximately 1.5 metres wide and clearly all
positions lie within this, demonstrating typically sub-
metre precision in this difficult environment. The final
position of the Locata compares to the known coordinate
of the rover point to less than 20cm. This offset is due to
undetected cycle slips experienced during the test. The
level of accuracy achieved by the “Locata technology” is
extremely good considering the multipath error and
varying delays induced from LocataLite signals
penetrating brick walls and a metal roof. Additionally,
this level of accuracy is more than adequate for tracking
people and assets in an office environment. However, if
cm-level kinematic precision is desired indoors (in, for
example, machine control applications) then this can be
achieved by ensuring the LocataLites are positioned to
provide a direct line-of-sight signal to the Locata (Barnes
et al., 2003).
Figure 9. Locata indoor positioning test.
Figure 10. Kinematic indoor positioning results.
4. CONCLUDING REMARKS
In this paper the fundamentals of the Locata technology
have been described and the prototype LocataLite
hardware discussed. At a test network located outside a
two-storey office building, LocataLite signals penetrated
the building (brick walls and metal a roof) and allowed
real-time static positioning inside with sub-cm precision
for a Locata (mobile unit). This level of precision is
extraordinary considering the fact that some LocataLite
signals had to penetrate several solid brick walls and a
metal roof. Moreover, these results were achieved using a
carrier-phase point positioning technique (without the
need for base station data), and this clearly demonstrates
the proof-of-concept of a time-synchronised network for
positioning.
Also, using the same test network, a Locata was tracked in
real-time as it moved around the office building. Through
a comparison of the path of the Locata with the internal
structure of the building, the estimated positioning
precision was at the sub-metre level. These results are
remarkable considering the changing signal penetration
path through the building as the Locata moved, and the
difficult multipath environment. This level of precision is
at least ten to one hundred times better than can currently
be achieved using high sensitivity GPS receivers indoors.
The Locata technology has the potential to deliver sub-
centimetre positioning precision, both indoors and outside,
and at low cost. The Locata Corporation and SNAP have
set their sights to achieve this with continued research and
development.
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