New Leica GPS Earth Monitoring Technology

Thomas A. Stansell Jr.

Leica Geosystems Inc., GPS, Torrance

Presented at IUGG IAG Meeting, Wuhan, China

November, 1997

Leica Geosystems Inc., GPS, Torrance, USATelephone +1 310 791 5300, Fax +1 310 791 6108

Earth Monitoring Technology

Contents

Biography 5

Introduction 6

Corporate Capability 7

Product Configuration 9

Technology 13

Resulting Earth Monitoring Capabilities 18

References 19

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New Leica GPS Earth Monitoring Technologyby

Thomas A. Stansell, Jr.Leica GPS

BiographyTom Stansell is a Vice President of Leica GPS in Torrance, California, where he is involved in special projects,technology development, and strategic relationships. Tom received his BEE degree in 1957 and his MEE degreein 1964, both from the University of Virginia. At the Johns Hopkins University Applied Physics Laboratory heparticipated in development of the Transit Satellite Navigation System. At Magnavox, he lead the development ofmany Transit and GPS products and their underlying technology. He is the author of many technical papers andreceived the ION Weems Award in 1996 for continuing contributions to the art and science of navigation.

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IntroductionLeica intends to be a major provider of Continuously Operating GPS Reference Stations. This paper describeshow Leica has prepared to assume a leadership role in this category of products which is so important to earthmonitoring applications.A leadership role requires much more than a competent product, although this is vital. It requires that a companybe committed to the application and to its customers. It requires both product reliability and excellent productsupport. It requires a willingness to resolve problems, to learn from experience, and to improve constantly. Leicahas this commitment.Therefore, this paper is more than a simple description of products and technology. It explains how Leica hasbecome a world leader in precision GPS applications in order to fulfill our mission of being �the world�s firstchoice provider of professional GPS products for high precision navigation, measurement, and control�. Thedescription will focus on convergence toward this objective in three key areas: corporate capability, productconfiguration, and technology. The paper then describes two Leica solutions for earth monitoring which resultfrom this convergence.

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Corporate Capability

Historical BackgroundLeica Geosystems was formed by combining three major businesses - Wild, Kern and Magnavox. The parent unitwas Wild Heerbrugg, a Swiss company which was founded in 1921 and has became one of the world�s largest andmost successful providers of surveying instruments. In 1988 Wild acquired Kern, another famous Swiss surveyinginstruments company, which had been founded in 1819. This acquisition further strengthened the Wild leadershipin high precision optical surveying instruments. In 1990, due to other acquisitions, the corporate name waschanged to Leica, which is very well known because of the famous Leica Cameras.In the mid-1980�s Leica recognized that GPS soon would play a major role in the survey market. Therefore, in1985 a joint venture called the WM Satellite Survey Company was formed between Wild Heerbrugg and Magnavox.Magnavox had been involved in GPS user equipment longer than any other company and was a world leader inGPS products and technology. In 1986 the joint venture introduced the WM101 GPS Surveyor, which was thefirst truly compact and fully sealed GPS survey instrument. In 1988, the joint venture introduced the WM102dual frequency GPS Surveyor featuring a patented code aided squaring technology providing a 13 dB advantagein tracking the encrypted GPS L2 signal.When it was time to develop the next generation product, Magnavox, for internal reasons, was unwilling tocontinue in the joint venture. Showing its commitment to the market, to the technology, and to the developmentteam, Leica then paid Magnavox to develop and to manufacture the SR299 GPS receiver, a key part of the WildSystem 200, which was introduced in 1991. The System 200 was the most compact and portable GPS SurveyInstrument of its era; it was the first to have a Rapid Static survey mode, and it pioneered the use of a handheldGPS Controller. It is interesting to note that in 1992 the System 200 was used in the first GPS survey of MountEverest.All of Magnavox Electronic Systems Company was sold in late 1993, and almost immediately afterwards Leicawas able to purchase the Magnavox Commercial GPS Business Unit from its new owner. Thus, in February of1994 this new Leica Business Unit, now called Leica GPS, was established in Torrance, California.The first GPS survey product from an all-Leica team was the System 300. This product, which was introduced in1995, incorporated the SR399 GPS receiver, which was completed by the Leica team in Torrance. The receiverfeatured a narrow correlator for C/A code tracking and a newly patented code aided cross correlation processwhich provides full wavelength phase measurements while retaining the 13 dB L2 tracking advantage.

Integrating CapabilitiesIt is always difficult to merge organizations, partly because every organization has its own culture and its ownway of doing business. It is more difficult when the organizations are separated by nine time zones. Leica�s firststep in this process was to create a separate organization in Torrance which concentrated on building the necessarylocal infrastructure while cooperating with other Leica business units. During this stage, the responsibility forGPS Survey products remained with a business unit in Switzerland. This process was completed in August of1996, at which time a new business unit called Leica GPS was formed. The new business unit is headquartered inTorrance, California; it is responsible for the development and manufacture of all Leica GPS products, and itincludes facilities in Heerbrugg, Switzerland as well as in Copenhagen, Denmark.Creating Leica GPS required many transitions: from a joint venture relationship with Magnavox, to a contractrelationship with Magnavox, to founding a new Leica business unit in California, to now having a consolidatedand focused GPS business which is fully integrated within Leica. Understanding these transitions is very importantto those interested in Leica GPS products, whether for earth monitoring or for any other application. The processillustrates Leica�s substantial commitment to GPS, its long term vision, and its tenacity to succeed. It means thereis now a consolidated and focused Leica business with the technology, the talent, and the charter to be �the world�sfirst choice provider of professional GPS products for high precision navigation, measurement, and control�. It

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means that this new business unit is fully imbued with the Leica heritage of producing excellent products, withexcellent reliability, and for backing these products with exceptional customer support.No other company in this field has the extensive and professional distribution and customer support system ofLeica. No other company has the depth of geodetic and survey experience of Leica. Leica�s commitment to itscustomers is legendary, and the new Leica GPS business unit is no exception.

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Product ConfigurationSurvey Product Application to Earth MonitoringGPS survey products must acquire precise carrier phase measurements in order toprovide sub-centimeter positioning, and Leica products are no exception. Therefore,from the very beginning, Leica GPS survey products have had the inherent abilityto support earth monitoring projects. However, there are several reasons why thiscapability was seldom used.As illustrated by Figure 1, Leica GPS survey receivers were optimally configuredfor the professional surveyor, particularly by emphasizing portability. Small andlight antennas were integrated within the GPS receiver housing. This actuallyprevented connection of an external antenna, which prevented use of the larger andheavier antennas favored by the earth monitoring community. Also, receiver controlwas commanded by and data logging was performed within a handheld controller,and the message data format between the receiver and the controller was heldproprietary. Although this was an ideal configuration for field surveying, it wasan obstacle for earth monitoring. More data storage and a convenient way totransfer data by modem was needed for earth monitoring, and the handheld controller and the proprietary dataformat were impediments. Furthermore, the relationship between Leica and Magnavox made it difficult to adaptto these other requirements.

After acquisition of the Magnavox commercial GPS business, theseimpediments began to disappear. A very important change was introductionin 1996 of the rugged and fully sealed SR9500 GPS receiver, shown inFigure 2. The primary objective was to enhance reliability. In addition,partly because of an improved thermal design, the SR9500 provides 24 parallelchannels for tracking 12 GPS satellites on both the L1 and L2 frequencies. Italso has a broader temperature range specification than prior Leica GPSsurvey products. Because the GPS antenna is not built inside the SR9500,several types of antennas can be used, including the Leica AT304 choke ringantenna, as shown in Figure 3, which fully meets rigorous UNAVCOrequirements.Equally important for earth monitoring, special Windows-based softwarecalled Multistation was developed. Multistation allows a personal computer(PC) to control the SR9500 receiver and log vast amounts of data in anon-proprietary format. Using the PC�s flexible ability to communicate viaa modem or other interface, a central control station can access each receiverin a network, control each one, and acquire the stored data.These recent changes have removed all of the previous barriers for use ofstandard Leica survey receivers in an earth monitoring network. As a result,Leica GPS receivers now are being used for earth monitoring, and the resultsbeing reported are excellent.

Figure 1 - System 200

Figure 2 - SR9500 Receiver

Figure 3 - UNAVCO definedLeica AT304 Antenna

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Machine Control Product Application to Earth MonitoringIn 1995 the U.S. Government selected Leica to head a team of companies, including Caterpillar Inc. and SpectraPhysics Laserplane, to develop a system capable of directly controlling the three-dimensional position of anearthmoving machine�s blade using GPS measurements, optionally aided by a laser plane. Leica provided theGPS portion of this contract, and a successful field test was performed at the Caterpillar proving grounds inJanuary, 1997.

Based on this development, in 1997 Leica introduced the MC1000 product,shown in Figure 4. For machine control applications, this receiver provides10 independent position solutions per second (10 Hz solutions) with an accuracyof one centimeter and a latency (delay between the antenna�s true position at amoment in time and output of that GPS coordinate solution) of 30 milliseconds(0.03 second). These characteristics are needed to permit rapid blade controlby a hydraulic servomechanism, and we believe this amazing capability, whichwas first developed by Leica, continues to be unique. The rugged housing isfully sealed and is designed for extreme environments, such as a bulldozer.

The 10 Hz output rate and the 30 millisecond latency of the MC1000 are notrequired for earth monitoring. It is interesting to note, however, that they are useful in monitoring the motion oftall buildings under wind load in order to predict the damage potential of earthquakes.

The MC1000 application to earth monitoring is based on its powerful internal computer and its 60 megabyteinternal data storage capacity. The memory can store up to 200,000 data records, and the receiver is designed torecover automatically from a power interruption and continue logging data. Simply by connecting a modem orother communications device to one of the four serial data ports, a central control station can access each receiverin a network, control each one, download new software to it if desired, and acquire the stored data. One of manyexternal antenna options can be used, including the Leica AT304 choke ring antenna which fully meets rigorousUNAVCO requirements.

The MC1000 is now being used in earth monitoring applications, and the results being reported are excellent. Insome applications, an external tilt sensor and external meteorological sensors have been attached to the availabledata ports. The MC1000 provides a rugged and flexible solution for earth monitoring without the need for anexternal PC.

Network Control Product Application to Earth MonitoringIn addition to GPS receivers for earth monitoring, Leica is a leading supplier of networked GPS solutions. Thispaper will mention just two:

Øresund ProjectIn 1991 the Swedish and Danish governments decided to build a Fixed Link (motorway and a double-trackrailroad) across the Øresund between the Danish capital city Copenhagen and the Swedish city Malmö. The FixedLink will consist of a 3.5 km tunnel starting at Kastrup nearby Copenhagen international airport. It leads towardsan artificial island south of Saltholm, from which a 7.8 km bridge will continue to Malmö. The project aims toimprove the traffic connection and thereby strengthen the regional infrastructure significantly. This will aid theØresund region to better compete with Europe�s other major commercial centers.

When construction first began, Real Time GPS (RT-GPS) was being used by a number of contractors, includingStenkon, which uses the Leica Real Time System 300 for machine control of their excavators. In order tominimize the number of radio frequencies required for this project, the government decided that only a joint

Figure 4 - MC1000 GPS MachineControl Receiver

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network of reference stations would cover the growing demand for the use of RT-GPS and avoid foreseeableproblems. Therefore at the end of 1995 the government opened a tender.

From out of 20 companies initially applying for pre-qualification, Leica AG was finally awarded the contract inMarch 1996 for a Multi Purpose Reference System (MPRS) providing an integrated solution which combines thefunctions of a standard GPS reference station, control and monitor system (CMS), and bulletin board and datamanagement/archive system. The system design of hardware and software components are specifically designedto meet the following goals:

• Employ redundancy within the MPRS to guarantee 100% availability of the GPS data for logging andtransmission to the users

• Get full information on the status of the MPRS and the neighbor MPRSs by analysis of the received RTCMv.2.1 messages and a check of position and satellite status

• Check the availability of the integrity monitor on neighbor MPRSs• Archive and manage the Multistation and CMS log and data files• Provide remote access for the supervisor and �normal users� by telephone link• Provide remote control for the supervisor via telephone link• Inform the users and supervisor using a pager system in case of unrecoverable errors within the MPRS or the

neighbor MPRSs• Provide continuous operation in case hardware or software components fail, by switching to the back-up

system.

In order to manage this variety of tasks, an integratedcomputer network, connected to the GPS hardware andradio modems, was designed. Figure 5 gives an overviewof the MPRS�s main hardware and software componentsand interconnections. This Leica system is in daily use, isvital to the success of the Øresund project, and illustratesLeica�s ability to design, install, and support a sophisticatedGPS network system. This capability is available for earthmonitoring projects as well.

Beacon Reference SystemsLeica is one of the world�s leading providers of MarineRadiobeacon Systems which transmit Differential GPS(DGPS) correction signals along the coastlines of manycountries. Leica beacon systems are operational in Australia, Belgium, China, Denmark, Poland, and SaudiArabia. Furthermore, Australia, which now operates Leicasystems, has ordered four new and three upgrade systems toreplace those originally provided by another company, China,which operates five Leica systems, has ordered four more,and India has ordered 10 Leica systems.

Beacon DGPS systems also illustrate the ability of Leica toprovide a complete systems solution. For example, Figure 6shows how a Leica Central Control station running LeicaControl Station software under Windows® NT software canmonitor and control multiple remote sites. In this case eachstation consists of two single or dual-frequency Leica GPSreference receivers, a single-frequency Leica GPS integritymonitor receiver, a computer operating the remote site, also

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using Base station software under Windows® NT, as well as the modulators and transmitters required to broadcastthe signals. One reference station creates the DGPS corrections for transmission, and the computer monitors allthree receivers to assure full system integrity. If a problem occurs, the software automatically switches to thebackup reference receiver and/or modulator. All of these functions also are monitored by the Central ControlStation.

Figure 7 shows one of the many displays used by the Central Control Station to monitor the remote sites. Softwaremodules and the systems expertise exist at Leica to implement a complete earth monitoring system. For example,

the Central Control Station can be programmed to polleach of the remote sites via a dial-up telephone line.Each connection automatically would download the datastored by the remote station. If an operational problemwere detected, a human operator would be summonedto evaluate the situation and take appropriate action.Complete remote control of each remote receiver isprovided, including diagnostics and the ability to transfercompletely new software when required. The data fromall remote sites routinely would be archived in a standardformat such as RINEX. An integrity computer wouldevaluate the data from each download by computingknown baselines, thus assuring full data integrity. Onceagain, if a problem was detected with one of the remotereceivers, the human operator would be alerted toevaluate the situation and take corrective action.

Figure 7 - Beacon System Integrity Monitor Screen

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Technology

Technology RequirementsEarth monitoring demands the highest level of accuracy, local data storage, automatic recovery from powerfailure, simplicity of operation, remote data access and control, the flexibility to meet varying mission requirements,and the ability to withstand the environment. Some of these characteristics were discussed above. This sectionconcentrates on technical aspects of Leica receivers which enhance accuracy and which aid robust performanceby rejecting interfering signals.

Because signal tracking performance is very good and essentially equivalent in all professional GPS receivers,there are only two key accuracy issues which are practically important. One is the antenna phase center accuracyand stability, and the other is the ability to mitigate multipath signals. These topics are considered first, followedby the important issue of interference rejection.

Antenna Phase Center Accuracy and StabilityFor an ideal GPS antenna, the L1 phase center and the L2 phase center should be superimposed and at a preciselyknown position relative to the physical housing. In addition, the phase center position should not be a function ofsignal reception angle. Another way to say the same thing is that a locus of constant phase would be a sphericalsurface, at least over the elevation angles of interest, centered on the defined phase center position.

Every antenna design is a compromise. For example, for portability in surveying applications it is often equallyimportant to achieve light weight and small size than to optimize the antenna phase center characteristics. Fortunately,over the relatively short baselines which are practical for GPS surveying, these errors are canceled if antennaswith nearly identical phase distortion characteristics are oriented in the same direction. This is because theelevation and azimuth angles to each satellite at every site are nearly identical. This makes it possible to achievesub-centimeter surveying accuracy with quite small and lower cost antennas.

If the span of an earth monitoring network is only a few hundred kilometers, the same technique of orientingsmaller and less expensive antennas, such as the Leica AT302, is entirely practical. However, if the network isextremely large or part of a worldwide network, for example, then the elevation and azimuth angles to eachsatellite are likely to be quite different between receiver sites. In this case it is vital to employ antennas havingphase centers with much better accuracy and stability. For these purposes, a de facto standard has been createdby UNAVCO, based on a design from the Jet Propulsion Laboratory (JPL). Although Leica continues to investigateother antenna designs which could achieve equal or better results at lower cost, we also offer the AT304 antennawhich fully meets the UNAVCO requirements.

Multipath Mitigation

OverviewIn most applications of high precision GPS, multipath is the most significant error source. In addition to the directsignal path from a GPS satellite to the receiving antenna, there are many indirect paths, thus the name multi-path.In a typical environment there may be hundreds of multipath reflectors. The GPS receiver processes the combinationof the direct signal with all of the indirect signals. Any difference between measurements which would be obtainedif there were no multipath signals and the actual measurements is the error due to multipath, which affects bothcode (pseudorange) and carrier phase measurements. Leica has been and continues to be a leader in multipathmitigation technology.

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Comparing Wide and Narrow C/A Code Trackers

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There are three primary multipath mitigation techniques in current use by GPS manufacturers: (1) antennacharacteristics, (2) filtering by carrier aided code smoothing, and (3) use of narrow correlators.

Antenna characteristics which tend to minimize reception of multipath signals are a high front-to-back gain ratioand excellent circular polarization. These characteristics are inherent in all Leica antennas, but especially in theAT304, defined by UNAVCO.

Filtering code measurements with carrier aided smoothing is a widely used technique invented by Leica (Magnavox)but, unfortunately, not patented. [1] The basic idea is to use the very precise measure of pseudorange changeprovided by the integral of carrier phase measurements to remove the effects on the code tracking function ofsatellite motion, user motion, and oscillator drift. As a result, the code loop and/or subsequent code filtering canemploy very long time constants to filter out much of the multipath noise. The bandwidth of this filtering processmay be variable to rapidly remove start-up code offsets. Otherwise, its smoothing limit is determined by the needto track ionospheric refraction, which affects code and carrier measurements equally but in opposite directions.

Leica (Magnavox) also invented an important extension of carrier aided code smoothing which permits a singlefrequency carrier aided code smoothing time constant to increase continually as long as signal lock is maintained.In this case the idea was patented (U.S. Patent 5,471,217).

The Narrow Correlator is a signal processing technique whichreduces the code measurement error caused by multipathsignals, as compared with the measurement error when usinga full-width or wide correlator. Figure 8 shows this effect byplotting the envelope of maximum code tracking error causedby a single multipath signal with half the voltage amplitude ofthe direct signal as a function of the multipath signal delayrelative to the direct signal. Both axes are scaled in C/A codechips (293 meters/chip). The outer envelope shows the erroras a function of delay when tracking a GPS C/A code with awide correlator. The maximum error is 25% of a C/A codechip, or 73.3 meters. The error is present for any multipathcode delay from zero to 1.5 chips, or 440 meters. In contrast,the maximum error with a correlator having 10% the width ofthe wide correlator is 10% as large, and the error ends formultipath delays greater than 1.05 chips, or 308 meters.

Figure 9 is a practical example illustrating the differencebetween tracking a GPS satellite with a wide correlator andwith a 10% narrow correlator. The plots show the raw,unfiltered output of two code tracking loops, each tracking thesame satellite. The two outputs are intentionally offset fromeach other, and the common long term trends are due to theionosphere. The important observation is that the noisefluctuations, which are due primarily to multipath, are manytimes less with a narrow than with a wide correlator. Thisdifference results directly in improved code measurementaccuracy.

It is evident that the narrow correlator is a very effective multipath mitigation technique. Leica is licensed to

Effect of Half Voltage Amplitude Multipath Signal - C/A Code Scale

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Figure 9 - Unfiltered Code Noise with Wide andNarrow Correlators tracking the same satellite

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practice the most advanced, patented, variable width, narrow correlator technology, and current Leica productswhich track the C/A code all employ 10% narrow correlators.

P Code Multipath MitigationIn addition to the wide and narrow correlator error envelopes,Figure 8 also shows the error envelope of a P code correlator.Because the GPS P code clock rate is 10 times higher than theC/A code clock rate, even a full width P code correlator producesan error envelope which is one tenth as large as the C/A codefull width correlator envelope. Also, the maximum error is thesame as the 10% C/A code narrow correlator, but the errorfunction reaches zero for multipath delays greater than 0.15 ofa C/A code chip, or 44 meters. Thus, the P code correlator isan even better way to mitigate multipath than the narrow C/Acode correlator.

The immediate response to this statement is likely to be thatGPS satellites don�t transmit the P code; they transmit an

encrypted P code called the Y code. Fortunately, there are ways to track the P code, which is embedded in the Ycode, as described by Leica�s U.S. patents numbered 4,972,431 and 5,535,278. As a result, these patentedtechniques allow Leica receivers to track the P code using a P-squared correlator with single frequency as well aswith dual frequency receivers. The result is illustrated by Figure 10, which is the same as Figure 9, except that theraw P code loop output also is shown. It is evident that P code tracking has the least multipath noise.

We believe that applications requiring the best possible code accuracy should employ these P code trackingtechniques. Every Leica dual frequency receiver is capable of independently tracking P code on either or both L1and L2 for optimum code measurement performance.

Advanced Code Multipath MitigationLeica has applied for patents on an advanced codemultipath mitigation technique, and the first of these patentsis expected to issue shortly. To illustrate this technique,Figure 11 shows, as dashed lines, the same C/A code widecorrelator and 10% narrow correlator error envelopes asFigure 8. The error envelope of the new Leica MultipathMitigation (MM) Correlator also is shown, as a solid line.It is evident that this C/A code MM Correlator behavesvery much like a P code correlator, i.e., the error returnsto zero for all delays greater than 0.15 C/A code chip.Note that the MM Correlator can be narrowed further,with obvious benefits. Without question, the MMCorrelator is superior to an equivalent narrow correlatorand represents a significant advance in the state of the art.Importantly, this advanced technique also can be used tofurther improve P code tracking.

Comparing Wide & Narrow C/A Code Trackers With a P-Squared Tracker

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Effect of Half Voltage Amplitude Multipath SignalC/A Code Example

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Figure 11 - Error Envelopes of the C/A Code Wide andNarrow Correlators compared with the Leica MultipathMitigation Correlator

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Carrier Phase Multipath MitigationLeica also has applied for a Phase Multipath Mitigation patent. This technique provides the same benefits forphase measurements as the Multipath Mitigation technique provides for code measurements. In other words,only multipath signals with very short delays will affect carrier phase measurement accuracy. Multipath signalswith longer delays will have no effect on the carrier phase measurements.

Carrier phase multipath mitigation is more important for precise applications than code multipath mitigation forthree reasons. (1) Before Leica first announced this capability [2], it was generally considered impossible tomitigate the effects of multipath signals on carrier phase measurements. (2) P code tracking already providesexcellent code multipath mitigation in Leica receivers, so the new code multipath mitigation technique has lessrelative significance. (3) Even when tracking P code, L1 carrier phase measurements are obtained from the C/Acode signal rather than the P/Y code signal. Therefore, phase multipath mitigation provides a very substantialbenefit, equivalent to changing from code tracking with a wide correlator to code tracking with P code. We expectthis breakthrough to substantially improve performance of GPS receivers used for survey, earth monitoring, andmachine control.

Interference RejectionEven a receiver which provides very accurate measurements is of no value if local interfering signals prevent theGPS signals from being received. Therefore, Leica continues to improve the interference rejection capability of itsreceivers. [4] The most recent interference rejection technology significantly improves the tracking performanceof receivers at both the L1 and L2 GPS frequencies, even under high levels of interference.

To show the improvement, two generations of Leica receivers were compared. The older of the two technologies,developed several years ago, uses one bit (two level) signal sampling. Ceramic bandpass filters are used in theantenna preamplifier and at the L-band input stages of the receiver, and inductor-capacitor (LC) filters are used inthe Intermediate Frequency (IF) stages. This receiver is referred to as the Prior Technology (PT) receiver. Thenewest receiver design employs multi-level signal sampling and Surface Acoustic Wave (SAW) filtering. Thisreceiver is referred to as the New Technology (NT) receiver.

The multi-level sampling technique used by the NT receiver has been described by Amoroso [3]. It employs anadaptive A/D converter to maintain a specified statistical frequency at which the sampling thresholds are exceeded.This sampling technique is effective in improving in-band rejection of narrowband interfering signals.

SAW filters are employed because their out-of-band signal rejection profile approaches the ideal. An ideal filterpasses only the desired signals while completely rejecting interference and noise at frequencies above and belowthe bandpass. SAW filters also are temperature-stable, with repeatable performance from unit to unit, requiringno tuning or alignment.

Tests were conducted to compare the trackingperformance of the PT and the NT receivers inthe presence of both in-band and out-of-bandinterference on both the L1 and the L2 channels.Figure 12 shows the jammer to signal (J/S) ratioat which the jamming signal caused loss of lockas a function of the interference frequency. ThePT receiver performance curve is marked withcircles and the NT performance curve bycrosses. As can be seen, interfering frequencieswere injected over the range from 1,100 MHzto above 1,700 MHz, which covers the out-of-

J/S AMPLITUDE AT LOSS-OF-LOCK VS. INTERFERENCE FREQUENCY

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band, the near-band, and the in-band regions for the L1 frequency, centered at 1575.42 MHz, and for the L2frequency, centered at 1227.6 MHz. It should be noted that at the maximum J/S value of 95 dB, no out of bandloss of lock was observed for either receiver.

Figure 12 shows that within the in-band regions of 1,565-1,585 MHz and 1,217-1,238 MHz, an averageimprovement of about 7 dB is noted for the NT receiver. This improvement is primarily due to the use of multi-level signal sampling.

In the near-band regions of 1,180-1,218 MHz, 1,239-1,605 MHz, 1,540-1,564 MHz, and 1,586-1,605 MHz, theNT receiver shows even more improvement, up to about 35 dB. This additional improvement is due primarily tothe tighter bandpass filtering obtained with the SAW filters. Because governments attempt to protect the GPSin-band signals, interference is more likely to fall within the near-band and out-of-band regions. One particularexample of how this additional improvement can be quite important are the Digipeater transmitters which arewidely used in Germany and which previously caused significant interference problems.

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Resulting Earth Monitoring CapabilitiesThis paper shows how Leica has positioned itself to be a leading provider of Continuously Operating GPSReference Stations (CORS) for earth monitoring applications. To achieve this goal has required the convergenceof three key factors: (1) the corporate capability, (2) the product configuration, and (3) advanced GPS technology.The convergence of corporate capability was achieved by purchasing the Magnavox commercial GPS business in1994 and, in 1996, fully consolidating its resources into a single GPS business unit headquartered in California.This new and fully integrated Leica business unit is now even better able to provide the quality of product andservice epitomized by the name Leica.

The second convergence has been in product configuration and functionality. Although Leica GPS survey equipmentalways had the ability to make very precise measurements, its configuration was specialized for the survey market.Recent changes have permitted use of an external antenna, improved reliability, and increased the number oftracking channels. In addition, a UNAVCO approved antenna is now available as an option, and specialized datalogging and remote control functions have been added. Leica GPS receivers are now ideally configured for earthmonitoring.

The third convergence is a continuing process, which is improvement of the GPS receiver technology. This paperdescribed two important technical improvements: multipath mitigation for both phase and code measurementsand improved rejection of interfering signals. Because of Leica�s commitment to high precision GPS applications,such technical improvements will continue indefinitely.As a result of this convergence, Leica now offers two optional earth monitoring configurations. One uses theSR9500 GPS receiver coupled to a PC running Multistation software. In this configuration the PC controls thereceiver, provides data storage, and permits remote control and data retrieval by a control center over a variety ofcommunication channels.

The second configuration employs the MC1000 receiver which includes a powerful built-in computer, an internal60 megabyte flash memory card, and four data ports. By connecting the MC1000 to a modem or othercommunications device, the control center is able to remotely control the receiver and acquire its recorded data.Finally, Leica has developed several systems which control a network of GPS receivers. These systems useadvanced computer software based, for example, on Windows NT. The systems permit a Central Control Stationto collect data from each remote site and to perform diagnostics. Alarms are given if a problem is detected. As aresult, data integrity is assured, operational costs are low, and the system is easy to understand and to use.

Perhaps more important than corporate capability, product configuration, superior technology, or systems expertiseis the commitment and reliability of the supplier. No other company providing CORS equipment has a moreextensive or a more dedicated customer support organization than Leica. In earth monitoring as in survey, Leica�scustomers can rest assured of high quality products with continuing support.

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References[1] Hatch, R. R., �The Synergism of GPS Code and Carrier Measurements�, Proceedings of the Third InternationalGeodetic Symposium on Satellite Doppler Positioning, DMA, NOS, New Mexico State University,Las Cruces, NM, February 8-12, 1982, Vol. 2, pp.1213-1232.

[2] Hatch, R., Keegan, R., Stansell, T., �Leica�s Code and Phase Multipath Mitigation Techniques�, Proceedingsof the ION National Technical Meeting, Santa Monica, California, January, 1996.

[3] Amoroso, F., �Adaptive A/D Converter to Suppress CW Interference in DSPN Spread SpectrumCommunications�, IEEE Transactions on Communications, October 1983.

[4] Maenpa, J., Balodis, M., Walter, G., Sandholzer, J., �New Interference Rejection Technology from Leica�,Proceedings of ION GPS-97, Kansas City, Missouri, September 1997.

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LLeica Geosystems Inc.,

GPS,23868 Hawthorne Blvd

Torrance, CA 90505USA

www.leica.com

© Leica Geosystems Inc., Torrance, USA, 1997

1007 Letter - 11/97