Barnes Etal2003a

5/26/2018 Barnes Etal2003a

1/14

PAPER 49 (peer-reviewed) 1

Presented at SatNav 2003The 6

thInternational Symposium on

Satellite Navigation Technology IncludingMobile Positioning & Location Services

Melbourne, Australia

2225 July 2003

Locata: the positioning technology of the future?

J. Barn es, C. Rizos, J. WangSatellite Navigation and Positioning (SNAP) Group

School of Surveying and Spatial Information SystemsUniversity of New South Wales, Sydney, NSW 2052, Australia

Tel: +61 2 9385 4174 Fax: +61 2 9313 7493 Email:[email protected]

D. Small, G. Voig t, N. Gamb aleLocata Corporation Pty Ltd

401 Clunies Ross Street, Acton, ACT 2601, AustraliaTel: +61 2 6229 1777 Fax: +61 2 6229 1778 Email: [email protected]

ABSTRACT

Locata Corporation has invented a new positioning technology called

Locata, which is designed to overcome severe limitations in otherpositioning systems currently available. Part of the Locata technologyconsists of a time-synchronised pseudolite transceiver called aLocataLite.A network of LocataLites forms a LocataNet, which transmits GPS-likesignals that allow single-point positioning using carrier-phase measurementsfor a mobile device (aLocata). The SNAP group at UNSW has assisted inthe development of a Locata and testing of the new technology. In thispaper, the prototype Locata technology is described, and the results of aperformance test experiment are presented. The test experimentdemonstrates proof-of-concept for the Locata technology and shows thatcarrier point positioning (without radio modem data-links) is possible withsub-centimetre precision.

KEYWORDS: High-precision, Kinematic-positioning, Time-synchronisednetwork,LocataLite, Pseudolite.

1. INTRODUCTION

GPS is used for precise centimetre-level positioning in a variety of outdoor applications,where a relative carrier-phase differential technique (using a base station) is almostexclusively used. Differential operation, most commonly using the double-differencedobservable, is necessary to reduce orbit errors, spatially correlated errors due to the

atmosphere, and to eliminate both receiver and satellite clock biases. For real-timepositioning (i.e., RTK), the roving GPS receiver must receive base station data via a radio


2/14


modem (or other wireless link) from a base station. This, combined with the fact that RTKGPS only works well with a relatively unobstructed and geometrically good satelliteconstellation, and the operating range of a rover receiver is typically limited to less than 10kmdue to ionospheric effects, is a significant limitation of the RTK-GPS technology.

In situations where GPS satellite geometry is poor or the signal availability is limited, ground-based transmitters of GPS-like signals (called pseudolites) can be used to augment GPS.They therefore have the potential to be used for both outdoor and indoor positioning. Withenough pseudolites it is theoretically possible to replace GPS entirely, though in practice thishas been difficult to achieve. Typically pseudolites use cheap crystal oscillators and operateindependently (in the so-called unsynchronised mode). In this case, double differencingmust be used to eliminate the pseudolite and receiver clock biases.

The SNAP group has been conducting pseudolite research for the past three years andexperimenting with them in the unsynchronised mode for a variety of applications (Barnes etal. 2002a, Barnes et al. 2002b, Wang 2002, Wang et al. 2001, Dai et al. 2001). Therefore,

real-time centimetre-level positioning with unsynchronised pseudolites can only be achievedwith a base station that provides data to a rover unit via a radio modem (as with standardRTK-GPS). If pseudolites can be synchronised, stand-alone positioning can be achievedwithout base station data (and without the radio modem data link). Until now attempts tosynchronise pseudolites have resulted in position solutions that are up to six times worse incomparison to an unsynchronised approach using double-differencing (Yun and Kee, 2002).

Locata Corporation has invented a new positioning technology (Locata), that consists of anetwork (LocataNet) of time-synchronised pseudolite transceivers (LocataLites). In thefollowing sections, the Locata technology is described, and real-time stand-alone (without

base station data) high precision (sub-cm) point positioning is demonstrated.

2. LOCATA CORPORATIONS LOCATA TECHNOLOGY

Locata Corporations Locata is a positioning technology that is designed to overcome thelimitations (outlined in section 1) of GPS and other positioning systems currently available. Ithas 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 toallow 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

Locatatechnology, which is described in the following sections.

2.1 LocataLite

The LocataLitecan be described as an intelligent pseudolite transceiver. The transmitterprototype hardware used is such that the intelligence of the unit is in its software. This is anextremely flexible approach and allows major design changes without requiring completelynew hardware. The receiver part of the prototype is based on an existing GPS receiverchipset, which is described in section 2.3. The receiver chipset and the transmitter share thesame 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 atthe GPS L1 frequency. The signal is generated digitally (unlike most existing pseudolites,


3/14


which use analogue techniques) and can be operated in a pulsing mode with different dutycycles, power output, and any PRN codes can be generated. Pulsing is commonly used with

pseudolite signals (instead of a continuous transmission, like GPS), to reduce interference andincrease the working range (the near-far problem). The duty cycle refers to the percentage oftime the pseudolite is transmitting when pulsing. Commercially available GPS patch

antennas are used for the receiver and transmitter, in addition to a custom built waveantenna for one of the LocataLite transmitters. The prototype LocataLite and antennas areshown in Figure 1.

Figure 1. PrototypeLocataLitehardware and antennas.

2.2 TimeLoc

In order for a mobile receiver to carry out carrier-phase point positioning without the need forbase station data, the LocataLite devices must be time-synchronised. The level ofsynchronisation required is extremely high, considering a one nanosecond error in timeequates 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 theLocata technology and is know as TimeLoc. The TimeLoc procedure to synchronise one

LocataLite(B) to anotherLocataLite(A) can be broken down into the following steps:

1.LocataLiteA transmits a C/A code and carrier signal on a particular PRN code.

2. The receiver section of LocataLiteB acquires, tracks and measures the signal (C/A codeand carrier-phase measurements) generated byLocataLiteA.

3.LocataLiteB generates its own C/A code and carrier signal on a different PRN code to A.

4.LocataLiteB calculates the difference between the code and carrier of the received signaland its own locally generated signal. Ignoring propagation errors, the differences between thetwo signals are due to the difference in the clocks between the two devices, and the geometricseparation between them.

5.LocataLiteB adjusts its local oscillator using Direct Digital Synthesis (DDS) technology to

bring the code and carrier differences between itself and LocataLiteA to zero. The code andcarrier differences betweenLocataLiteA and B are continually monitored so that they remainzero. In other words, the local oscillator of B follows precisely that of A.


4/14


6. The final stage is to correct for the geometrical offset between LocataLiteA and B, usingthe known coordinates of theLocataLites, and after this TimeLocis 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 usingTimeLoc.

2.3 A Locata

To speed up the development of a prototype system it was decided to use existing GPShardware for the receiver section in the LocataLite and the Locata (the mobile positioningdevice). The SNAP Group at UNSW has assisted in the development of theLocatathroughMitels (now Zarlink) GPS Architect development system (Zarlink, 1999). The developmentsystem 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 thesystem includes GPS firmware C source code that can be modified, compiled and uploaded tothe GPS receiver. However, the GPS Architect hardware is designed as an indoor laboratorydevelopment tool and not suited to outdoor use.

Instead of designing and building GPS receiver hardware (using the GP2000 chipset), suitablefor outdoor use, a different approach was taken. This was to modify a Canadian MarconiCorp (CMC) Allstar GPS receiver, which uses the Mitel GP2000 chipset, so that it wouldoperate in exactly the same way as the GPS Architect hardware. The original GPS Architectfirmware source code has been extensively modified and improved, by the LocataCorporation and the SNAP group. The modifications have been in signal acquisition, thetracking loops and the navigation algorithm. The prototype Locatahardware and antenna (acommercially available patch antenna) are shown in Figure 2.

Figure 2. PrototypeLocatahardware.

2.4 Navigation Algorithm in a Locata

The Locata uses carrier point positioning (CPP) to determine its three-dimensional positionfrom at least fourLocataLites. As the name suggests, CPP uses the carrier-phase as its basicmeasurement and it is therefore useful to consider the carrier-phase observations in the case of


5/14


GPS. The basic GPS L1 carrier-phase observation equation between receiverAand satellitejin metres can be written as:

1

j j j j

A A trop A ion A

L

cc T c T N

f! " # $ $ # % & ' ' ( ( ( ' (1)

where 1Lf is the frequency of the L1 carrier-phase observable; c is the speed of light in a

vacuum; jA" is the geometrical range from station A to satellite j; AT$ is the receiver clock

error for stationA; jT$ is the satellite clock error for satellite j; jAN is the integer ambiguity

(the unknown number of carrier cycles between the receiverAand satellitejat lock-on); ion#

is the atmospheric correction due to the ionosphere; trop# is the atmospheric correction due to

the troposphere; % represents the remaining errors, which may include orbital errors, residual

atmospheric effects, multipath error and receiver noise etc.

For kinematic GPS, the above equation contains parameters that are not known with a highenough accuracy to enable a single GPS receiver to perform CPP and determine the receivers

position and clock error at the centimetre-level. Instead, another GPS receiver (base station)is used and double differencing is commonly performed to eliminate both receiver andsatellite clock errors, and reduce the effects of orbit errors (baseline length dependent), andthe spatially correlated errors due to the troposphere and ionosphere.

If real-time kinematic positioning using carrier-phase is desired, the base station data must beavailable at the rover receiver, usually via a radio modem. The carrier-phase integerambiguities must be determined before centimetre-level carrier-phase positioning can be

realised. There are numerous ambiguity resolution approaches used, but they can basically bebroken down into geometry and geometry-free approaches (Leick, 1995). However, reliablerapid (less than a minute) On-The-Fly (OTF) ambiguity resolution is only possible when L2carrier-phase data in addition to L1 is used, and at least five satellites with good geometry arevisible. The cost of a commercial RTK system with dual frequency GPS receivers istherefore relatively expensive, and typically costs US$ 30,000.

In comparison to GPS, the basic LocataNet carrier-phase observation equation betweenreceiverAandLocataLitejin metres can be written as:

1

jj j j

A A trop A A

L

c

c T Nf! " # $ % & ' ' ( ' (2)

where the terms are the same as for GPS, except they refer to LocataLitesinstead of satellites.In equation 2 there is no clock error due to the LocataLitessince they are time-synchronisedto each other (see Section 2.2), and because the devices are ground based there is noionospheric correction term. The tropospheric correction will depend on the separation

between the Locata and the LocataLite, the elevation angle to the LocataLite, and theatmospheric conditions (pressure, temperature, humidity and pressure) along the line-of-sightsignal path.

The term that poses the most difficulty in the above equation is the unknown number ofcarrier wavelengths between theLocataand theLocataLitewhen TimeLocis achieved. In the


6/14


prototype system, the ambiguity term and the initial receiver clock error are determinedthrough a static initialisation at a known point. Assuming that the tropospheric effects aremodelled 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

jj j

A A A

L

cB c T Nf

$ %& ( ' (3)

j j j

A A AB ! "& ( (4)

The basic observation equation (2) therefore becomes:

j j j

A A A AB dT! " $ % & ' ' ' (5)

and2 2 2( ) ( ) ( )j j j jA A A AX X Y Y Z Z" & ( ' ( ' ( (6)

where AdT$ is the change in the receiver clock error from the static initialisation epoch, and

this together with the Locata coordinates , ,A A AX Y Z give four unknowns; which can be

solved with a minimum of four LocataLite carrier-phase measurements and least squaresestimation. 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. Afterthe carrier-phase bias is determined through static initialisation the Locatais free to navigatekinematically. The positioning algorithm is embedded in the GPS firmware of theLocatatoallow for real-time positioning. It should also be stressed that each positioning epoch isindependent and no smoothing or filtering is carried out in the prototype system.

2.5 Advantages of the LocataNet

There are several major advantages to the LocataNet approach in comparison to othercurrently available positioning technologies (including GPS), which include:

1. No data links The base station concept is meaningless in theLocataNetapproach and noradio modem is required at theLocata. Additionally there are no radio modems or hard-wires connecting any of theLocataLitedevices.

2. Reduced latency In a differential-based navigation system, the highest positioningaccuracies are achieved when the rover uses time-matched base station data (with nointerpolation). Therefore, the rover unit must wait to receive base station data before itcan compute a position. TheLocatacomputes a carrier point position (CPP) using time-synchronised signals from the LocataLites and does not have to wait for any additionaldata in order to compute a position.

3. Intelligent signal transmissions Standard pseudolites typically use pulsing to preventjamming and reduce the near-far problem. However, when operating pseudolites in thismanner it is still possible that multiple devices may be transmitting at exactly the sametime and could cause interference problems. In theLocataNet, signal transmissions are

precisely controlled to ensure that LocataLites do not transmit at the same time,minimising interference between signals from differentLocataLites.


7/14


4. Theoretically greater precision In differential GPS the double-differenced observable isformed from four carrier-phase measurements. Assuming all measurements have equal

precision and are uncorrelated, the precision of the double-differenced measurement istwo times worse than a single carrier-phase measurement (the basic measurement used bytheLocata).

5. Time solution In differential GPS the double-differencing procedure eliminates theclock biases and hence time information is not determined. For certain applications,

precise time is important and the LocataNet approach allows time to be estimated alongwith position (as is the case of standard GPS single point positioning).

3. LOCATANETTEST NETWORK

To prove the concept of LocataNetand to test the accuracy of the TimeLocmethodology, anoutdoor test network has been established in Canberra, not far from Locata Corporations

offices. The network consists of fiveLocataLitedevices, with a configuration as illustrated inFigure 3. 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. The

North and South poles are approximately thirty metres from the Master while East and Westare approximately one hundred meters from the Master. The LocataLites transmit andreceive antennas are mounted on three-metre poles, which are concreted into the ground andstabilised using guy ropes. The positions of the poles in the test network were established atthe cm-level, using GPS data collected (with NoVatel Millenium receivers over one hour at aone second rate) between the Master pole and other poles in the network.

Figure 4 shows a view from the EastLocataLitepole (on a hill) looking down over the entiretest network. An additional one metre pole (rover) is positioned near the Master pole, and can

be used to initialise the Locata before navigation or to perform static tests. The geometricconfiguration of the LocataLites is such that the dilution of precision (DOP) values at therover pole, in East North and Up are 0.97, 0.70 and 4.25 respectively. The poor DOP in theUp component is due to the fact that the greatest elevation angle from the rover pole any

LocataLite is 24.5 degrees (Master). The elevation angles and distance of the LocataLitesfrom the rover pole are given in Table 1. Because of the poor DOP in the Up component, thefollowing tests will concentrate on horizontal components only.

LocataLite PRNused

Transmit/ReceiveAntennas

Elevation anglefrom rover pole

(Degrees)

Distance fromrover pole (m)

SNRmean/stdev

(unit)

Singledifference

stdev (mm)

Master 32 wave/NA 24.5 5.5 20.1/0.019 Reference

North 12 Patch/Patch 3.4 36.9 20.1/0.051 9.6

East 14 Patch/Patch 9.1 103.8 21.2/0.101 8.1

South 21 Patch/Patch 5.2 29.6 19.8/0.041 6.6

West 29 Patch/Patch -4.0 108.1 18.8/0.080 7.0

Table 1.LocataLitetrial details: elevation angle and distance from rover pole, SNR and singledifference statistics.


8/14


100 80 60 40 20 0 20 40 60 80 100

80

60

40

20

0

20

40

60

80

East (m)

North(m)

rover master

north

eastsouth

west

Figure 3. Map showing position ofLocatLitesand rover pole.

Figure 4.OutdoorLocataNettest network.

3.1 Performance Analysis of the LocataTechnology

On 18 December 2002, a test was conducted at the outdoor test network to assess thepositioning accuracy of the Locatatechnology. After turning theLocataLiteson, the North,South, East and West devices time-synchronised to the signal transmitted by the Master.

LocataNet TimeLoc was achieved in less than ten minutes and the devices remained so forseveral hours that day, which indicates very good reliability and stability of the TimeLoc

procedure. TheLocataLitesused GPS satellite PRN codes 12 (North), 14 (East), 21 (South),29 (West) and 32 (Master), as listed in Table 1. It should be noted that the researchexperiment was conducted in a remote location away from potential users of GPS, and thefinal system will not use GPS PRN codes. All theLocataLites used patch antennas for thetransmitter and receiver, with the exception of the Master pseudolite, whose transmit antenna

was a wave vertical. Table 1 summarises the configuration of theLocataLites.


9/14


3.1.1 Static accu racy t est

To test how well the LocataLite units achieve TimeLoc, a static positioning test was firstperformed. 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 knowncoordinates of the rover pole (as illustrated in Figure 5) the carrier biases were determined.Then for approximately 35 minutes the Locata independently computed real-time positionand time solutions once a second. The real-time positions together with the raw measurementdata were logged using a laptop computer via a serial interface.

Figure 5. Static test: TheLocataand masterLocataLiteantennas.

The signal-to-noise ratio (SNR) values of the five LocataLiteunits, recorded by the Locata,during the static test are given in Figure 6. Also, the mean and standard deviation of the SNRtime series are given in Table 1. Because theLocataLiteunits and theLocataare stationary itis expected that the SNR values should be random with a constant mean, unlike GPS SNRvalues which typically increase as the satellite elevation angle increases. Overall, the signalstrength from all theLocataLitesare good and similar, with mean values ranging from 18.8 to21.2 dB. The variation of the SNRs is relatively small, with the absolute variation less than0.5 dB for allLocataLites. This indicates that the signal power from theLocataLitesis stableand that theLocatais able to track the signal without difficulty.

It is interesting to note that there does appear to be some correlation between standarddeviation of SNR values and the distance between theLocataLitesand theLocata(Figure 6).The SNR values from the Master, only 5.5 metres from theLocatahave the smallest standard


10/14


deviation (least variation) of 0.02 dB. The SNRs of the East (PRN 14) and West (PRN 29)LocataLites have the largest variation with a standard deviation of approximately 0.1 dB.These devices are the greatest distance from theLocata(approximately 100 m).

0 200 400 600 800 1000 1200 1400 1600 1800 200018

18.5

19

19.5

20

20.5

21

21.5

22SNR 32

Epoch (s) stdev 0.019 mean 20.06

SNR

0 200 400 600 800 1000 1200 1400 1600 1800 200018

18.5

19

19.5

20

20.5

21

21.5

22SNR 12


SNR

0 200 400 600 800 1000 1200 1400 1600 1800 200018

18.5

19

19.5

20

20.5

21

21.5

22SNR 14


SNR

0 200 400 600 800 1000 1200 1400 1600 1800 200018

18.5

19

19.5

20

20.5

21

21.5

22SNR 21


SN

R

0 200 400 600 800 1000 1200 1400 1600 1800 200018

18.5

19

19.5

20

20.5

21

21.5

22SNR 29


SN

R

Figure 6. Signal-to-noise ratio (SNR) values of the fiveLocataLites.

A good way to assess how well the LocataLite units are time-synchronised is to computesingle-difference measurements between the LocataLites. This will eliminate the Locataclock error, and show any errors between the LocataLite clocks. Using the logged

measurement data, single difference observations were computed between the Master and allother LocataLites. The ambiguities of the single differences were resolved using the knowncoordinates of theLocataLitesand the rover pole.

Figure 7 shows the four single differences between the Master and the other LocataLites.Most importantly, visually all the single difference time series on average fit a horizontal lineand do not have any long-term drifts during the thirty-five minute test. The overall standarddeviations of the single difference time series are all less than 1 cm (see Table 1), and in termsof how well the LocataLite clocks achieve TimeLoc this equates to better than 33 pico-seconds. However, visually the time series do not appear entirely random and the cause of thefluctuations requires further investigation. Also interesting to note is that the standard

deviations of the time series do not appear to be correlated with the standard deviations of theSNRs, and is worthy of further investigation.


11/14


0 200 400 600 800 1000 1200 1400 1600 1800 2000

0.05

0.04

0.03

0.02

0.01

0

0.01

0.02

0.03

0.04

0.05

L1 single difference 1232

Epoch (s) stdev 9.6mm

Me

tres

0 200 400 600 800 1000 1200 1400 1600 1800 2000

0.05

0.04

0.03

0.02

0.01

0

0.01

0.02

0.03

0.04

0.05



Me

tres

0 200 400 600 800 1000 1200 1400 1600 1800 2000

0.05

0.04

0.03

0.02

0.01

0

0.01

0.02

0.03

0.04

0.05



Metres

0 200 400 600 800 1000 1200 1400 1600 1800 2000

0.05

0.04

0.03

0.02

0.01

0

0.01

0.02

0.03

0.04

0.05



Metres

Figure 7. Single differences of theLocataLitesusing master as reference.

To assess the accuracy of the real-time positioning results, the known (sub-cm) coordinate ofthe rover pole was used to compute the positioning error for each epoch. Figure 8 shows theEast and North errors for the real-time positions of the Locata. The mean error of the bothtime series is less than 2 mm, with the standard deviations and root-mean-square values lessthan 6 mm. Clearly sub-centimetre positioning precision has been achieved with 93% of the

East and North errors less than )1 cm. As expected from the analysis of the single differenceresiduals, there are no long-term drifts in the time series (Figure 7). Also as expected,visually the time series do not appear entirely random due to the patterns in the singledifference residuals as noted previously. The above results demonstrate that CPP with sub-centimetre precision is clearly achieved with the Locatatechnology.

0 200 400 600 800 1000 1200 1400 1600 1800 2000

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 4.5 mm mean 1.8 mm rms 4.8 mm

Error(metres)

0 200 400 600 800 1000 1200 1400 1600 1800 2000

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 5.5 mm mean 0.2 mm rms 5.5 mm

Error(metres)

Figure 8. TheLocataEast and North static positioning error.

3.1.2 Kin emat ic accu racy test

Assessing the kinematic performance of the Locata is difficult due to the lack of a truthpositioning system with greater positioning accuracy. Therefore, the approach taken was tomake theLocata antenna repeat a circular path, and analyse deviations from a circle. Thetest equipment was constructed from an old record turntable and is shown in Figure 9. A


12/14


box mounted on the turntable houses the Locata, battery and laptop computer for datalogging. The antenna was mounted on a bar attached to the top of the box, giving a seventy-centimetre circle radius.

Figure 9. Circle repeating kinematic test equipment.

During the test, lasting sixteen minutes, the Locata antenna rotated at an approximate linearvelocity of 2.4 m/s, and an angular velocity of 3.4 rad/s. Both real-time positions and rawmeasurement data were recorded using a laptop via a serial interface. Before the antennastarted to rotate, the carrier biases were determined through initialisation at the rover polewith known coordinates.

To assess how close theLocatacoordinates lie on a circle, a least squares procedure was usedto estimate the radius and centre of the circle using the Easting and Northing data. Thecomputed radius differs to that measured (using a steel tape measure) by less than 3 mm, andFigure 10 shows the horizontal position of the Locata. Visually, the plot depicts a circle andsuggests that North-South precision is slightly better than East-West, and is partly due to thefact that the dilution of precision North-South is approximately 28% better than East-West.

Figure 10 also shows the least squares residuals of the circle estimation using the positiondata. The overall standard deviation of the residuals is 16 mm, while 82% of values are less

than )20 mm from the best-fit circle. There are occasional residual values as large as 60 mm

in the time series, which could be due to multipath from people walking around the test areaduring the test or parked vehicles. Also, since the circle repeating test equipment is not

precisely calibrated, levelled or ridged, then this will impact on the accuracy of thepositioning results. Despite the errors induced by possible multipath and the test equipment,centimetre-level kinematic positioning can clearly be achieved using the Locatatechnology.


13/14


1 0.5 0 0.5 11

0.8

0.6

0.4

0.2

0

0.2

0.4

0.6

0.8

1Circle centre 0.1890 0.6659 radius 0.6971

Easting (m)

Northing(m)

100 200 300 400 500 600 700 800 9000.1

0.08

0.06

0.04

0.02

0

0.02

0.04

0.06

0.08

0.1Best fit circle residuals Std 16.0 mm Mean 0.0 mm

Min 58.2 mm Epoch (s) Max 66.8 mm

Residual(m)

Figure 10. Horizontal position plot (left) and least square residuals (right) of circle repeatingkinematic test.

4. CONCLUDING REMARKS

IsLocata the positioning technology of the future? In this paper it has been shown that theprototype Locata technology clearly demonstrates the proof-of-concept of a time-synchronised network for positioning. At an outdoor test network of LocataLites(LocataNet), approximately 200x60 metres, a mobile unit (aLocata) performed static carrier-

phase point positioning with sub-centimetre precision. Also, in a kinematic test centimetrelevel precision was obtained despite errors induced by possible multipath and the testequipment used. This level of precision is remarkable for a prototype system, and is as good

as (if not better than) GPS RTK using a base station, radio modem and double differencing.

A real-time positioning technology that can operate indoors and outside anywhere in theworld, with sub-cm accuracy, and low cost is the ultimate goal for many researchers. LocataCorp, with the assistance of SNAP (in software development and testing), is continuing todevelop the Locatatechnology to achieve this goal.

REFERENCES

Barnes J, Rizos C, Wang J, Nunan T, Reid C (2002a) The development of a GPS/Pseudolite

positioning system for vehicle tracking at BHP Steel, Port Kembla Steelworks,Proceedings of the 15

th

International Technical Meeting of the Satellite Division of The Institute of Navigation ION GPS2002,Portland, Oregon, 1779-1789

Barnes J, Wang J, Rizos C, Tsujii T (2002b) The performance of a pseudolite-based positioningsystem for deformation monitoring, Proceedings of 2nd Symp. on Geodesy for Geotechnical &Structural Applications, Berlin, Germany, 326-327

Dai L, Rizos C, Wang J (2001) The role of pseudosatellite signals in precise GPS-based positioning,Journal of Geospatial Engineering, 3(1), 33-44

Leick A (1995) GPS satellite surveying (second edition), John Wiley & Sons, Inc., 560pp


14/14


Yun D, Kee C (2002) Centimeter accuracy stand-alone indoor navigation system by synchronizedpseudolite constellation, Proceedings of the 15th International Technical Meeting of the SatelliteDivision of The Institute of Navigation ION GPS 2002,Portland, Oregon, 213-225

Wang J (2002) Applications of pseudolites in geodetic positioning: Progress and problems.Journal ofGlobal Positioning Systems,1(1), 48-56

Wang J, Tsujii T, Rizos C, Dai L, Moore M (2001) GPS and pseudo-satellites integration for precisepositioning. Geomatics Research Australasia, 74, 103-117

Zarlink (1999) GP2000 GPS receiver hardware design application note. Zarlink semiconductor, 54pp

Documents

Barnes Etal2003a