Author: GPS World Staff

  • GeoEye at the ESRI International Users Conference

    GPS World magazine interviews at the ESRI show, talking with Deke Young, the GIS Sales Manager at GeoEye.

  • TomTom at the ESRI International Users Conference

    GPS World magazine interviews at the ESRI show, talking with Dan Adams of TomTom.

  • Carlson Software at the ESRI International Users Conference

    GPS World magazine interviews at the ESRI show, talking with Bruce Carlson of Carlson Software.

  • Hemisphere GPS at the ESRI International Users Conference

    GPS World magazine interviews at the ESRI International User Conference 2012, talking with Garry Hurkens of Hemisphere GPS.

  • Lockheed, Raytheon Complete First Launch Exercise for Next-Gen GPS Satellites

    Raytheon Company and Lockheed Martin have successfully completed the first launch readiness exercise for the U.S. Air Force’s next generation GPS III satellites. The exercise is a key milestone demonstrating the team remains on schedule to achieve launch availability in 2014, the companies said.

    The Lockheed Martin-built GPS III satellites and the Raytheon-developed next generation GPS operational control system, known as OCX, are critical elements of the U.S. Air Force’s effort to affordably replace aging GPS satellites while improving capability to meet the evolving demands of military, commercial and civilian users worldwide. This is the first space and ground enterprise successfully building the ground control and space vehicles by two independent prime contractors.

    The launch readiness exercise, completed over a three day period by mission operations personnel, validated the basic satellite command and control functions, tested the software and hardware interfaces and demonstrated basic on-console procedures required for space vehicle contacts during the launch and early orbit mission.  The event sets the stage for the first GPS III satellite’s mission readiness timeline, which includes five short-duration exercises and six, five-day mission rehearsals leading up tolaunch.

    “Completion of our first GPS III launch readiness exercise is a major milestone for the entire GPS enterprise and is a solid indictor that our space and ground segments are well synchronized,” said Col Bernie Gruber, the director of the U.S. Air Force’s Global Positioning Systems Directorate.

    To achieve first launch availability in the 2014 timeframe, the U.S. Air Force awarded Lockheed Martin and Raytheon contracts in January of this year to provide a Launch and Checkout Capability (LCC) for launch and early on-orbit testing of all GPS III satellites.  At the heart of the LCC is Raytheon’s Launch and Checkout System that will provide satellite command and control capability, an integral part of OCX’s  support of the first GPS III launch.

    “The completion of our first launch readiness exercise is an important milestone for the entire GPS enterprise,” said Keoki Jackson, vice president of Lockheed Martin’s Navigation Systems mission area. “This achievement is a testament to efficient planning and synchronization by the U.S. Air Force and demonstrates that we are on track to deliver critical GPS III capabilities to military, commercial and civilian users worldwide.”

    “This milestone represents the hard work and dedication of the entire GPS III and OCX government-industry team,” stated Ray Kolibaba, a vice president of Raytheon’s Intelligence and Information Systems business and GPS OCX program manager. “This is another demonstration of the rapid progress we’re making on OCX development, while maintaining GPS space-ground enterprise alignment. I’m confident that we’ll be prepared to support the first GPS III launch with an efficient, evolvable and secure ground control system built independently.”

    The GPS III team is led by the Global Positioning Systems Directorate at the U.S. Air Force Space and Missile Systems Center. Air Force Space Command, based at Schriever Air Force Base, Colo., manages and operates the GPS constellation for both civil and military users.

  • Galileo Satellite Navigation Agency Moved to Prague

    Galileo Satellite Navigation Agency Moved to Prague

    Credits: Astrium/Raoul Kieffer
    Credits: Astrium/Raoul Kieffer

    On September 6, the European GNSS Agency (GSA) inaugurated its new premises in Prague, Czech Republic, in the presence of Commission Vice-President Antonio Tajani, in charge of Industry and Enterprise, and Minister of Transport Pavel Dobeš. Previously headquartered provisionally in Brussels, the headquarters of the Galileo program moved its seat to Prague over this summer, as had been agreed by the EU heads of state and government on December 10, 2010.

    Galileo is expected to be partly operational by the end of 2014.

    Tajani said two satellites will be launched in October, and beginning in 2013 four more Galileo satellites will be launched every six months until the network of 30 is completed in 2020.

    Credits: Astrium/Raoul Kieffer
    Galileo In-Orbit Validation satellites Flight Model 3 and 4 being worked on at the Guiana Space Centre on 27 August 2012. Multi-layer insulation is being applied to FM3. (Credits: Astrium/Raoul Kieffer)

    GSA ensures security of satellites and prepares ground for new GNSS products. The agency is responsible for a number of implementation tasks for the European Satellite Navigation programmes Galileo and EGNOS (European Geostationary Navigation Overlay Service), which are managed by the European Commission. Its two main tasks are:

    • Security (security accreditation of satellites, launchers, and sites, and the operation of the Galileo Security Monitoring Centre), and
    • Market Development for the European satellite navigation systems (for example, see MEMO/12/601, New products and services possible using Internet access to satellite navigation data).

    Additionally, the GSA has been assigned other tasks by the commission by delegation, for instance promoting GNSS applications and services, supporting the development of a Public Regulated Service (PRS) and preparing the exploitation of the GNSS systems.

    Security of Galileo Programme. The GSA’s security accreditation activities are of key importance for the satellite launches. After a successful first launch of two satellites on October 21, 2011, the “In-Orbit Validation” phase will be accomplished with a second launch of two satellites on October 10, 2012. From 2013 on, the deployment of the satellite infrastructure will continue faster, with several launches per year until the full constellation of 30 satellites (which includes six in-orbit spares) is reached before the end of the decade.

    Future role of the GSA. A commission proposal for revising the GNSS Regulation, which is now before Parliament and Council, foresees that operational responsibility for the GNSS Programmes will be gradually transferred from the European Commission to the GSA over the next multi-annual financial framework (2014-2020). This process will start with EGNOS in 2014, and already a number of preparatory tasks have been allocated to the GSA, including the procurement for the future operations of EGNOS.

    To carry out these new functions, the GSA’s staff is expected to increase over the coming years from about 60 today to more than 180 by the end of next financial framework in 2020.

    The Budget. The GSA has an annual budget of about €12,750 million (2012). In addition, it manages the budget for activities that are entrusted to it under delegation from the European Commission. These amount to €34.4 million for exploitation activities.

    According to the commission’s calculations, a total budget of € 7000 million is necessary to complete the deployment phase of the Galileo programmes and finance the exploitation phase of the GNSS programmes over the 2014-2020 period. The commission’s proposal for a new GNSS Regulation foresees that the GSA will manage the budget necessary to operate EGNOS and Galileo and ensure service provision. This budget will be assigned under a delegation agreement signed with the commission, a mechanism foreseen under the European Union’s Financial Regulation. Under this arrangement, the commission would remain responsible for the overall political supervision of the GNSS Programmes. However, the GSA would ensure the exploitation of the GNSS systems with the appropriate level of autonomy and authority.

    The Structure of the GSA. The GSA today is composed of a security department, a market development department, and an organizational entity charged with preparing the GSA’s future responsibilities in the management of the GNSS Programmes. In addition to a number of horizontal departments that ensure the agency’s functioning, the Galileo Security Monitoring Centre is an organizational component of the GSA.

  • Upcoming Navigation Satellite Launches Scheduled

    News courtesy of CANSPACE listserv.

     

    Launch dates this fall for GNSS satellites are as follows, according to various sources:

    Compass M2 and M5: September 18, 18:12 UTC (speculative); Compass G6: No earlier than October 1.

    GSAT-10 (includes a GAGAN SBAS transponder): September 21.

    GPS IIF-3: October 4, 2012. Launch window: 12:10-12:29 UTC.

    Galileo IOV FM3 and FM4: October 10, 18:31 UTC.

    Luch-5B: Originally scheduled for October 15, launch has slipped to no earlier than November 1 due to an issue with the “Briz-M” upper stage, which caused the loss of the Telkom-3 and Ekspress-MD2 communication satellites during their launch on August 6.

    GLONASS-K1 (block K2s): November 14.

  • The System: Next GPS IIF in October

    Next GPS IIF in October

    The next GPS satellite, Block IIF-3 (SVN65), scheduled to be launched on October 4, will be positioned in orbital slot 1, which is in plane A. This slot is currently occupied by a Block IIA satellite, SVN39, operating as PRN09. SVN39 is one of the oldest operating satellites in the GPS fleet, dating from June 1993. SVN65 will the the third of a projected 12 IIF satellites to attain orbit.

    Galileo IOV Tandem in October, Too

    The previously announced September 28 launch date for the second set of Galileo IOV satellites has reportedly been pushed back to October 10.

    Meanwhile, after more than four years of service as a Galileo testbed satellite, GIOVE-B was retired on July 23. Its navigation transmitters were switched off, according to an announcement from the European Space Agency, and the satellite’s height was raised in a series of steps to a graveyard orbit where there will be no danger of it interfering with the operational Galileo satellites or other spacecraft.

    The SES-5 geostationary communications satellite (also known as Sirius 5 and Astra 4B), launched in July, arrived at its orbital slot of 5 degrees east longitude late that month. The current position is actually about 5.2 degrees. The satellite carries L1 and L5 transponders for the European Geostationary Navigation Overlay Service (EGNOS) satellite-based augmentation system. The GPS Directorate has assigned C/A PRN code 136 and L5 PRN code 136 for use by the satellite.

    GAGAN in September

    India’s GSAT-10 telecommunications satellite — one of two passengers for Arianespace’s upcoming Ariane 5 mission on September 21 — has completed pre-flight preparations at the Spaceport in French Guiana. Aboard GSAT-10 is the GAGAN (GPS and GEO augmented navigation) payload, which will support the Indian government’s implementation of a satellite-based regional capability to assist aircraft navigation over Indian airspace and in adjoining areas. GSAT-10 is expected to be positioned at 83 degrees east longitude and use PRN code 128. It will join the first GAGAN-equipped satellite, GSAT-8, launched in May 2011, and now at 55 degrees east longitude and transmitting test signals on the L1 frequency using C/A PRN code 127. Although GSAT-8 reportedly carries a dual-frequency transponder, no L5 signals from this satellite have yet been detected by International GNSS Service tracking stations.

    GLONASS SBAS in September as Well

    Luch-5B, the second of three geostationary satellites to reactivate Roscosmos’s Luch Multifunctional Space Relay System, is scheduled for launch no earlier than November 1, 2012, to be positioned at 16 degrees west longitude. The system’s multi-functional satellites carry transponders for the System for Differential Correction and Monitoring (SDCM), Russia’s satellite-based augmentation system. The transponders will broadcast GNSS corrections on the standard GPS L1 frequency using C/A PRN codes assigned by DoD’s GPS Directorate.

    Luch-5A, launched in December 2011, has been placed in an orbital slot at 95 degrees east longitude. It began transmitting corrections on July 12, using PRN code 140.

    SVN49 Back on the Air, Unhealthy

    The GPS Block IIR-M satellite, SVN49, briefly resumed transmissions as PRN24 on August 9. The signals were marked unhealthy and the satellite was not included in broadcast almanacs. SVN49 was launched in March 2009, but remains out of service until an L1/L2 satellite multipath issue is resolved. Although not in the almanacs, a number of stations of the International GNSS Service tracked SVN49. See http://gge.unb.ca/test/IGS_stns_tracking_G24_223.pdf. SVN49 stopped transmitting signals as PRN24 on August 22. SVN49 previously operated between March 28, 2009, and May 6, 2011, as PRN01, and between February 2 and March 14, 2012, as PRN24.

    Beidou Begins Testing Network

    China will build a Beidou testing and certification network over the next three years to sharpen the system’s global competitiveness, according to a statement from China’s Certification and Accreditation Administration. By 2015, a national testing center will be set up in Beijing, while seven local sub-centers will be established across the nation, it said. The centers will test the safety and accuracy of products designed for use with Beidou and qualify them for civilian use. China plans to launch 30 satellites to complete the system by 2020.

    The launch of next two Beidou-2/Compass medium-Earth-orbit satellites, M2 and M5, did not occur in August as was previously speculated. A knowledgable source states: “All three active Chinese tracking ships have retreated to their home base Jiangyin, north of Shanghai. (Two ships are required for tracking down-range for a typical Chinese beyond-low-Earth-orbit launch.) The launch was put off for the remaining part of August and at least the first couple of weeks in September. The most recently speculated launch date is September 18.”

     

  • First Results: Precise Positioning with Galileo Prototype Satellites

    First Results: Precise Positioning with Galileo Prototype Satellites

    By Richard B. Langley, Simon Banville, and Peter Steigenberger.

    For a brief period, and for a few hours on certain days, signals from the first four orbiting Galileo satellites could be received by state-of-the-art multi-frequency, multi-constellation GNSS receivers. Although not intended for actual positioning tests, the satellites did provide a first opportunity to assess the prototype Galileo signals in the positioning domain. The results obtained bode well for the future operational Galileo constellation.

    The launch and successful operation of the two Galileo In-Orbit Validation Element (GIOVE) satellites — GIOVE-A and GIOVE-B — followed by the two Galileo In-Orbit Validation (IOV) satellites — ProtoFlight Model (PFM) and Flight Model 2 (FM2) — were important steps in the development of Europe’s Galileo satellite navigation system.

    The GIOVE test-bed satellites were orbited to secure the use of the frequencies allocated by the International Telecommunication Union for the Galileo system; to verify the most critical technologies of the operational Galileo system, such as the on-board atomic clocks and the navigation signal generator; to characterize the novel features of the Galileo signal design, including the verification of user receivers and their resistance to interference and multipath; and to characterize the radiation environment of the medium Earth orbits planned for the operational Galileo constellation. The IOV satellites, of which there will be four with two more to be launched this fall, are prototype operational satellites designed to validate the Galileo concept in both space and on Earth. Once all four IOV satellites are in orbit, it should be feasible to carry out positioning exercises using just Galileo satellite signals. It was not intended for the GIOVE plus two initial IOV satellites to be used for positioning demonstrations. However, it turns out that (before GIOVE-A and GIOVE-B were recently decommissioned) for a few hours on certain days, signals from all four satellites could be received simultaneously by state-of-the-art multi-frequency, multi-constellation GNSS receivers.

    Dual-frequency measurements from the GIOVE satellites and triple-frequency measurements from the IOV satellites have been archived by a number of continuously operating receivers including those in the COoperative Network for GIOVE Observation (CONGO) and those contributing to the International GNSS Service’s Multi-GNSS Experiment (M-GEX) observing campaign. Before joining the M-GEX campaign as the receiver at station UNB3 at the University of New Brunswick (UNB) in Fredericton, Canada, a Trimble Navigation NetR9 receiver fed by a Zephyr Geodetic II antenna was continuously tested at UNB for a couple of months and its 30-second-interval measurements were locally archived. These measurements included (in the terminology used by the Receiver Independent Exchange (RINEX) version 3 format): C1X, L1X, and S1X (pseudorange, carrier-phase, and carrier-to-noise-density-ratio measurements for combined data-plus-pilot tracking of the Open Service signal on the E1/L1 carrier frequency (1575.42 MHz)); C5X, L5X, and S5X (the corresponding in-phase and quadrature (I+Q) measurements on the E5a carrier frequency (1176.45 MHz)); C7X, L7X, and S7X (the corresponding I+Q measurements on the E5b carrier frequency (1207.140 MHz)); and C8X, L8X, and S8X (the corresponding I+Q measurements on the effective E5a+E5b carrier frequency (1191.795MHz)).

    The first two of four Galileo IOV satellites were launched on October 21, 2011. Credit: ESA.

    Although the IOV satellites are in synchronized orbits in the same plane with mean orbital periods of 1.70475 orbits per day, those orbits are not coordinated with those of the GIOVE-A and GIOVE-B satellites, which had mean orbital periods of 1.69434 and 1.70960 orbits per day, respectively. (The orbit of GIOVE-B was recently raised, following decommissioning.) This means that all four satellites will not generally be in view at a ground station at the same time. However, at a given location on certain days, four-satellite visibility did occur for periods up to a few hours. We identified several such days but were hampered in our efforts to obtain positioning solutions due to the testing programs of the satellites.

    Our first constraint concerned GIOVE-A. The European Space Agency carried out tests with this satellite for more than six years and decided to decommission the satellite for its purposes on June 30, 2012, and switched off the navigation signals. This narrowed our window of possible four-satellite-visibility days. Secondly, the clocks on the IOV satellites were allowed to drift so that their offsets with respect to GPS System Time could be very large with offset values of tens to hundreds of milliseconds. Some GNSS receivers cannot make usable measurements when presented with such large clock offsets. This behavior further limited our windows of opportunity for four-satellite Galileo positioning. Nevertheless, we found that on May 17, 2012, the receiver at UNB successfully tracked the four satellites with a period of common visibility of two and a half hours. See Figure 1 for the time series of the occurrences of actual measurements made by the receiver. Common visibility extended from 03:04:30 to 05:34:30 GPS Time with the receiver tracking the satellites without any elevation-angle cutoff imposed.

    In the remainder of this article, we describe the procedures used to obtain precise positions from the measurements, including the technique used to determine precise orbit and clock data for the Galileo satellites, and the results we obtained.

    Source: Richard B. Langley, Simon Banville, and Peter Steigenberger
    Figure 1. Visibility of Galileo satellites from UNB on May 17, 2012.

    Generating the Orbits and Clocks

    GIOVE and IOV satellite orbit and clock parameters are determined at Technische Universität München with a modified version of the Bernese GPS Software in a two-step procedure based on GPS and Galileo observations of 23 CONGO stations. After a common preprocessing step (detection of outliers and cycle slips), GPS and Galileo observations are treated separately. Station coordinates, tropospheric delay parameters and receiver clocks are obtained from GPS observations only. GPS satellite orbits and clocks as well as Earth rotation parameters from the Center for Orbit Determination in Europe (CODE) are kept fixed in this step. In the second step, the ionosphere-free linear combination of E1 and E5a observations is used to estimate the Galileo-related parameters, namely the satellite orbits and clocks. The station coordinates and the troposphere and receiver clock parameters are fixed to the GPS-derived results of the first step. To account for systematic differences between the GPS and Galileo code signals as well as biases between the different receivers, differential code biases (DCBs) are estimated for all stations but one. Separate biases are set up for the GIOVE and IOV satellites. To strengthen the stability of the orbital arc, five daily solutions are combined into a multi-day solution and consistent Galileo clock parameters are recomputed. Only the middle day of the5-day solution is used for the positioning discussed in this article. Based on internal consistency tests and satellite laser ranging residuals, the accuracy of these orbits is assumed to be on the one-to-two-decimeter level.

    The Positioning Technique

    A preliminary assessment of the quality of Galileo-only positioning could be achieved using the four satellites simultaneously in view at UNB. The second author’s GNSS positioning software was used to process the UNB data. Applying a 7.5-degree elevation cutoff angle to remove low-elevation-angle measurements resulted in an observation session of 1 hour and 48 minutes. The east or longitude dilution of precision (DOP) component starts out at 0.829 at the beginning of this session, gradually dropping to 0.688, and then rising to 1.285 at the end of the session; while the north or latitude DOP component starts out at 2.683 at the beginning of the session, rising to 4.233 at the end (see Figure 2).

    Source: Richard B. Langley, Simon Banville, and Peter Steigenberger
    Figure 2. North (N), east (E), vertical (V), and geometrical (G) dilution of precision (DOP) values.

    Even though the receiver was tracking signals on E1, E5a, E5b, and E5a+b, carrier-phase and code observations on E1 and E5a were selected to match the satellite-clock datum. Ionosphere-free combinations were formed to eliminate first-order ionospheric effects, while the tropospheric delay was modeled using local measurements of temperature, pressure, and relative humidity provided by UNB’s meteorological station. No residual delay was estimated. Phase center offsets (PCO) and variations (PCV) for the Trimble Zephyr Geodetic II antenna were obtained through anechoic chamber calibrations (see Further Reading). The same satellite PCO as the ones used in the generation of the satellite orbits and clocks were applied, and no satellite PCV were considered. Other error sources required for precise positioning were also modeled such as solid Earth tides, ocean tide loading, and phase wind-up.

    Since separate biases were set up in the estimation of the GIOVE and IOV satellite clock estimation, the same approach should be used on the user side. Unfortunately, solving for this additional parameter in the navigation filter is not possible when tracking only four satellites. To overcome this limitation, a GIOVE/IOV offset was estimated using 24 hours of combined GPS-plus-Galileo observations in static mode (one position solution for the whole observation period), and was introduced as an additional correction in the Galileo-only solution. The estimated coordinates from this combined solution were also used as a reference in computing the errors in latitude, longitude, and height presented next.

    Results and Discussion

    Three solutions were computed to demonstrate the quality of Galileo-only navigation. In the first scenario (see Figure 3), ionosphere-free code observations solely contributed to the epoch-by-epoch estimation of receiver position and clock offset. The estimated coordinates are largely contaminated by code noise, which is amplified by a factor of approximately three when forming the ionosphere-free combination. In the absence of redundancy, any errors in the observations (such as noise) propagate directly into the estimated quantities and, in this case, affected particularly the latitude component. An analysis of the noise and multipath characteristics of each signal revealed the presence of time-varying effects in the C5X observations. Further investigations are required to properly identify the cause of those effects. As a result, the root-mean-square (RMS) error of the latitude, longitude and height components were 3.084, 0.658 and 1.617 meters, respectively (see Table 1).

    Source: Richard B. Langley, Simon Banville, and Peter Steigenberger
    Figure 3. Code-based solution. Differences in latitude, longitude, and height with respect to reference coordinates.
    Source: Richard B. Langley, Simon Banville, and Peter Steigenberger
    Table 1. Summary of the RMS errors for the three solutions computed.

    As a second step, both code and carrier-phase observations were combined into a single adjustment (see Figure 4), yielding what is often referred to as precise point positioning (PPP). To accommodate the initial carrier-phase ambiguities, additional parameters were estimated in the filter. While adding carrier phases clearly reduces the noise in the solution, the estimated coordinates do not converge to cm-level accuracies, as typically expected in PPP. Despite weak geometry and range errors, the main reason for poor convergence is again the presence of biases in code observations. Without redundancy, carrier-phase observations only act as a filter for code observations, without reducing the contribution of biases. The RMS errors are 0.422 meters in latitude, 0.150 meters in longitude, and 0.389 meters in height.

    Source: Richard B. Langley, Simon Banville, and Peter Steigenberger
    Figure 4. Combined code and carrier-phase solution. Differences in latitude, longitude, and height with respect to reference coordinates.

    To get an independent assessment of carrier-phase observations, a phase-only solution was computed (see Figure 5). For this test, a different methodology was adopted in which we simulated starting the positioning at a known precise location. At the first epoch, the receiver coordinates were constrained to the estimated values from the 24-hour GPS-plus-Galileo positioning solution, and the receiver clock offset was fixed to an arbitrary value (in this case zero). This initial epoch thus allowed estimation of the carrier-phase ambiguities, which remained constant for the rest of the session. For subsequent epochs, the receiver position and clock offset were estimated on an epoch-by-epoch basis. Even though the errors in the initial ambiguity estimates propagated into the following epochs, the estimated coordinates remained at the centimeter level throughout the nearly two-hour common observing period.

    Source: Richard B. Langley, Simon Banville, and Peter Steigenberger
    Figure 5. Phase-only solution, starting at a known location. Differences in latitude, longitude, and height with respect to reference coordinates.

    Conclusion

    We have obtained what we believe to be the first positioning results using observations from the four Galileo satellites launched to date. The results are very respectable given that the observing geometry was far from ideal and there was no redundancy for epoch-by-epoch solutions. Furthermore, the satellites were not operating at a performance level to be expected for the fully operational future constellation. Both GIOVE satellites have been retired and we must now wait for the second set of IOV satellites to be orbited before we can continue our investigations in Galileo-only positioning with live signals.

    Acknowledgments

    We thank the operators and station managers of the CONGO network for supplying the data used to model the orbits and clocks of the Galileo satellites.


    Richard B. Langley is a professor in the Department of Geodesy and Geomatics Engineering at the University of New Brunswick (UNB) in Fredericton, Canada.

    Simon Banville is a Ph.D. candidate in the Department of Geodesy and Geomatics Engineering at UNB. He is also working for Natural Resources Canada on real-time precise point positioning.

    Peter Steigenberger is a staff member in the Institut für Astronomische und Physikalische Geodäsie of the Technische Universität München in Munich, Germany.

    FURTHER READING

    “Anechoic Chamber Calibrations of Phase Center Variations for New and Existing GNSS Signals and Potential Impacts in IGS Processing” by M. Becker, P. Zeimetz, and E. Schönemann, presented at the IGS Workshop, Newcastle upon Tyne, England, June 28–July 2, 2010. Available online: http://www.igs.org/event/newcastle2010/ (scroll to “0205” and click on “PDF.”)

    A Guide to Using International GNSS Service (IGS) Products” by J. Kouba, an IGS resource.

    “Precise Orbit Determination of GIOVE-B Based on the CONGO Network” by P. Steigenberger, U. Hugentobler, O. Montenbruck, and A. Hauschild in Journal of Geodesy, Vol. 85, No. 6, 2011, pp. 357–365, doi: 10.1007/s00190-011-0443-5.

     

  • Expert Advice: GNSS in the Global Economy

    By Irving Leveson.

    The $100 billion GNSS industry is already stressed. How deeply and how long the pressures persist depends to a great extent on the performance of the world economy. In a time of extraordinary uncertainty and change, the industry faces great challenges over the next 2–3 years and beyond: potential delays in availability of satellites and ground support, adaptation to multiple constellations, shifts associated with the proliferation of portable electronics, and fluctuating demands from governments, businesses, and consumers. Vulnerabilities are already increased with the weakening of the slow U.S. recovery, with recession in Europe, and slowdowns in many other nations. Potential shocks could cause economic conditions to deteriorate further.

    Outcomes for the GNSS industry will depend very much on developments in the U.S. and global economy and associated government decisions. Effects on the industry will be far-reaching. Of course, non-economic factors will weigh in as well, but are beyond the present scope.

    At mid-summer 2012, the economic environment is too fluid to rely on a single forecast. To explore the issues, I compare four scenarios in a discussion that considers what the scenarios depend on, their likelihood, and their consequences for the GNSS industry.

    At the time of this writing, the consensus is that the U.S. and Europe will muddle through and that economic growth will be somewhat higher in 2013 than in 2012. This is evident in the forecasts of the Organization for European Cooperation and Development (OECD) and the International Monetary Fund (IMF). For example, the IMF in its July report expects world gross domestic product (GDP) growth to slow to 3.5 percent in 2012 from 3.9 percent in 2011, but then to rise to 3.9 percent in 2013. It expects GDP in advanced economies to be up to 1.9 percent in 2013 from 1.4 percent in 2012. This is in spite of a greater decline in public consumption.

    I consider global recession scenarios to be more probable than such a rebound. Moreover, there is a risk of a severe world recession led by developments in both Europe and the United States. In all four scenarious outlined here, serious long-run problems remain unresolved.

    Scenario Traits and Probabilities

    Table 1 lists the four scenarios and suggested probabilities, contributing factors, and global manifestations. A fuller description of each scenario comes in the next section.

    In the two Global Recession scenarios, the problems in the United States and/or Europe lead to worldwide recession, which is defined by a sharp slowdown in global growth. The Europe- and U.S.-led global recession scenarios are associated with greater government budget cuts and tax increases than in the other scenarios, and with greater political uncertainty, gridlock, and substantial contagion effects. Governments are less able to act, and some policies may be ineffective or counterproductive. Output declines in the U.S., Europe, and Japan, and slowdowns in growth occur in developing countries.

    The recession scenarios are negative for consumer spending, business investment, hiring, and risk-taking. Information technology spending is cut back. While cost pressures abate, companies have little ability to influence pricing; profitability declines.

    In the Muddling Through scenario, crises go to the brink, and little is done to immediately solve fundamental problems, but policies temporarily prevent severe economic and financial disruption. The Rebound scenario is facilitated by the most extensive delays in spending cuts and tax increases, together with increased confidence from agreement on long-term solutions. Serious but incomplete efforts are made to reduce impediments to growth and adjustment.

    As of mid-summer, I rate the probability of a Europe-led global recession and a U.S.-led global recession each at 25–30 percent, with global recession most likely fully underway some time in 2013 in each case. The probability of a more severe global recession led by both Europe and the United States I put at 30–40 percent. The probability for muddling through is 25–30 percent, and for the rebound scenario it is 15–20 percent.

    Developments and impacts are not quantified here, nor are longer-term prospects considered. More extreme possibilities are not addressed; these include wars, a major breakup of the euro in the next two or three years, a large energy price shock, massive immediate U.S. budget cuts beyond the sequester, extensive increases in U.S. regulation after the election, or a Chinese economic collapse.

    I now turn to elaboration and discussion of each scenario.

    Europe-Led Global Recession

    Efforts by European institutions and the IMF to prevent debt defaults by southern European countries by extending credit only delay financial crises into 2013 or early 2014. With a major problem of insolvency (liabilities greater than assets) and not simply liquidity, a Europe without a fiscal union, common banking rules or even deposit insurance is unable to implement new structures in time to forestall severe adjustment. Increased bank capital requirements on January 1, 2013 also restrain lending. Financial market contagion spreads with rising interest rates on debt of already stressed countries, accelerated bank runs, and capital flight.

    These problems spill over to the United States and the rest of the world through declining securities values, losses of financial institutions that are then less able to lend, and declining trade. U.S. business is adversely affected by a strong currency as investors seek relative safety in the dollar. This slows U.S. exports and eventually expands exports from Europe and other countries into the United States. It also leads to lower overseas earnings for U.S. companies as a result of less favorable currency translation.

    Efforts to reduce debt in Europe create ongoing financial pressures on many countries, including Brazil, India, and China, and other countries whose economies are already slowing. While the greatest problems are in southern Europe, many other impacted countries including the United States take years to return to pre-crisis levels of growth.

    Source: GPS
    Table 1. Global economic scenarios.
    Source: GPS
    Figure 1. General government gross financial liabilities as a percent of gross domestic product (GDP), with OECD projections to 2013.

    U.S.-Led Global Recession

    The U.S. economy is thrown into recession by a combination of tax increases and budget cuts (the sequester) that together constitute the January 1, 2013 so-called fiscal cliff. Tax and spending changes are modified, but the remaining tax increases from the end of the Bush tax cuts, together with those in the Affordable Care Act, weaken incentives to save, invest, and take risks. Additional pressures come from increased bank capital requirements and other financial regulations that restrain lending.

    The recession in Europe and slowdowns in other countries further weaken the U.S. economy. High debt and unfunded obligations limit the ability to stimulate the economy with additional spending and limit the effectiveness of additional stimulation. Congressional gridlock prevents strong action, and the Federal Reserve has little additional room to stimulate the economy. The U.S. recession exacerbates the recession in Europe and weakens the global economy. U.S. and European recovery is very slow.

    Muddling Through

    The U.S. manages to “kick the can down the road” with enough policy changes to avoid the worst crises, but is unable to stimulate much growth. Tax increases and budget cuts are largely delayed in response to high and rising levels of unemployment but hold back recovery when they return. Economic and policy uncertainty and high levels of financial and business regulation continue to restrain growth and employment. However, underlying technological change is strong and enables continuation of modest growth, along with very low interest rates. Recovery in construction is limited.

    Europe also is able to delay the worst crises, such as would occur if there were insufficient resources to prevent major bank failures or one or more countries abandoning the euro. However, it must work through a recession that is severe in some countries and dampening growth in others. The United States, France, Japan, India, and China institute additional economic stimulus.

    Rebound

    In this scenario the United States temporarily avoids a recession by delaying most tax increases and budget cuts and delaying or modifying some of the most intrusive regulations. A new round of stimulus measures that includes major tax restructuring and infrastructure spending is instituted. A bipartisan plan for long-term fiscal discipline increases confidence. Businesses and consumers take advantage of technological opportunities, low interest rates, and moderated energy prices. Construction begins to recover with renewed housing demand and increased government spending on infrastructure. U.S. banks, with strong balance sheets and modest amounts of loans to Europe, are not heavily affected by the European financial crises and recession. Strong equity prices, bolstered by demand from foreigners seeking a safe haven, boost confidence and add purchasing power. Businesses are willing to take more risks.

    Improved U.S. growth somewhat tempers problems in Europe and elsewhere. Europe manages to implement policies to get through its challenges without a deep crisis or creating severe contagion effects. Counterproductive labor rules in Europe are modified, and tax avoidance is reduced. Austerity is modified and more emphasis is place on growth. The slowdown in the world economy abates, facilitated by the temporary resolution of problems and increased public and private investment in several countries.

    Implications for GNSS

    The most severe consequences for the GNSS industry come in the case of combined U.S.-led and Europe-led recessions, a prospect with a 30–40 percent probability. The reduced contribution of the GNSS industry will in turn impact economies, for which GNSS benefits are great. The effects of deep recession can be seen in the behavior of GPS equipment revenues in North America, which grew 7.9 percent in 2008 and declined by 3.6 percent in 2009, after earlier increases of 17.3 percent in 2006 and 14.5 percent in 2007. Table 2 summarizes the broad implications of the current possibilities for the industry.

    Source: GPS
    Table 2. Implications of global economic scenarios for GNSS.

    Overall Influences

    Even if the budget cuts from the U.S. sequestration are delayed or reduced, the Department of Defense faces severe pressures from the remaining 2013 budget and in out years that are likely to cause launches of GPS satellites to be stretched out. Efforts by House and Senate Appropriations Committees to dramatically reduce the civilian portion of GPS funding in the Federal Aviation Administration FY2013 budget, threatening the timing of civil signals and the ground support system, are a sign of things to come. Delays and modifications are greatest in the recession scenarios. In global recession, plans for GPS III crosslink and spot beam capabilities are dropped.

    The Air Force has requested funding to develop dual-launch capability for GPS III in its 2013 budget. Budget pressures could lead to a more final decision to proceed with dual-launch within the next two or three years if it can be shown to reduce costs. That could make up for the delays later on, but not before several years of falling behind schedules. Budget-induced delays in other programs could alleviate a shortage of launch capacity in the United States, offsetting some of the impacts of shortages on GPS. However, a slowdown in ordering launch vehicles could negate the lessening of delays. Budget pressures also could result in a reduction in the number of satellites in the GPS constellation below 30, as satellites age and replacement slows. Only 24 GPS satellites are guaranteed. Only in the rebound scenario could launches be on track for the next couple of years.

    Budget stringency also affects research and development and production for capabilities that are planned for later years. Military GPS user equipment purchases are stretched out by funding constraints to various degrees depending on budget levels. Military developments could change any aspects of the outlook.

    Budget pressures from the European recession could cause Galileo satellite launches to be stretched out and/or the constellation to stop short of 30 satellites. Russia’s GLONASS program is unaffected by budget pressures as long oil prices do not fall dramatically below the $80 level. China’s Compass program is not likely to be subjected to delays due to funding even if the Chinese economy slows dramatically. However, economic weakness does cause delays in Japan’s QZSS system and India’s IRNSS system.

    Government budget pressures on both sides of the Atlantic, which are greatest in the recession scenarios, could make resolution of the MBOC patent dispute on the common GPS-Galileo civil signal more difficult and drawn out, adding uncertainty and delaying efforts to take advantage of the common signal.

    The impact of economic weakness on private R&D funding for user equipment and services could be substantial in all countries. The private GNSS investment climate is favored by low interest rates, rapid technological change in the industry and in information technology generally, by the evolution of several GNSS systems, and by the growth of markets in developing countries. However, with economies slowing, investment risk remains high.

    In the United States, investment in GNSS product and production process development is hampered by political/policy uncertainty, including satellite deployment, spectrum issues, and European licensing demands. Capital investment and merger and acquisition incentives depend significantly on prospects for scheduled tax increases on capital gains and dividends, and for investment and R&D tax credits, but the composition of tax revisions is not predictable in the present political climate. In Europe, private investment is adversely affected by recession and uncertainty about the economic and policy outlook.

    Business costs decline in the recession scenarios as demand for materials weakens from many industries and the labor market loosens. Company borrowing costs remain low from low interest rates but can rise because of higher risk premiums from lender concerns about the health of borrowers. Costs start to increase in the recovery scenario.

    Percentage swings in profits are much greater than those in revenue, and some firms move from profit to loss when economic conditions deteriorate. Profits fall sharply in the recession scenarios as effects of weakening demand on revenue and unit costs greatly exceed the benefits of lower input costs.

    Prices of products such as chips, antennas, and receivers that have been declining over time fall more rapidly in recession. In the early stages of recession, inventories can pile up, but production cutbacks are incomplete at first because of uncertainty about demand. This contributes to declining profits. More extensive cutbacks that follow are insufficient to offset the allocation of fixed costs over a smaller production base for most companies. Competition intensifies as companies adjust inventories and vie for a shrinking market or one that is growing less rapidly than expected.

    Mergers and acquisitions tend to be most prevalent at the ends of the economic spectrum. When the industry is in recession, some companies merge to obtain cost savings. In early stages of recovery, it is less expensive for companies to acquire existing assets and companies than to build new. When times are good, mergers often occur because the value of the more successful acquirer’s stock is high relative to the stock of the acquired company, and because of a desire to obtain scarce technology and talent. There may be greater interest in bringing a product that has had a limited market to the acquiring company’s larger customer base when the market is growing more rapidly. Over the last century, merger booms in the United States have largely occurred during stock market booms. Initial public offerings of stock also are more frequent during periods of generally high stock prices.

    Mergers and acquisitions can permanently alter the structure of the industry, leading to fewer, more dominant players and redefining customer, partner, and supplier relationships. Some acquisitions may increase pricing power in the long run. More GNSS companies will be owned by firms providing instrumentation, information technology, and other products. Some companies such as Trimble and Hexagon have strategies of making numerous strategic acquisitions; their pace of acquisitions may not vary as much with business conditions as those of more opportunistic acquirers.

    Prices of stocks in companies in the industry tend to move with trends in overall stock markets, but also reflect specific industry developments such as product cycles, technology shifts, and sources of competition. For example, some companies that have thrived with GPS may not be the same ones that are most successful in offering GPS+GLONASS receivers to industry. Some European companies may get a head start in making user equipment that takes full advantage of Galileo. However, a slow product market may give some suppliers a chance to catch up in product development.

    The shift from consumer receivers to smartphones has reduced the stock prices of consumer receiver manufacturers such as Garmin and TomTom. The Navteq division of Nokia and the TeleAtlas division of TomTom that supply maps have had to face great pressures from new sources of competition from Google, Microsoft, Apple, and others just when they had to deal with economic slowdowns.

    Application Sector Impacts

    Both business and consumer demand for user equipment decline in the global recession scenarios. In the muddling through scenario, consumer demand for receivers and smartphones is saturating. Commercial demand continues at a moderate pace, spurred by opportunities for multi-constellation equipment. Demand from both businesses and consumers improves in the rebound scenario.

    Recession scenarios adversely impact demand for GNSS equipment for survey and construction around the world. A U.S. recession would reverse the mid-2012 fledgling start of a housing recovery, but increased spending on infrastructure would raise public construction spending. In the rebound scenario, U.S. private construction picks up along with other investments. Greater construction spending increases demand for survey and construction applications, with public construction heaviest on road paving and building, and private construction heavier on energy and other engineering construction projects. Telecommunications and information technology are encouraged as part of the emphasis on infrastructure.

    A severe outcome for the European economy in the Europe-led global recession scenario stalls growth. Demand for equipment to take advantage of Galileo is slow in the next 2–3 years. In the rebound scenario, European stimulus has only limited impacts on construction because of financial constraints and an overhang of supplies from overbuilding and weakened demand. Financial problems of regional and local governments, for example in the United States, Germany, and Spain, adversely impact construction, especially in recession scenarios. Demand for GIS systems depends both on construction and on government use and is especially sensitive to economic and government budget conditions.

    Economic rebound raises commodity prices, increasing demand for agricultural and mining GNSS equipment. In a stronger U.S. subsidy-cutting environment and/or if there are large declines in commodity prices from economic weakness, demand for GNSS agricultural equipment is reduced. Demand for GNSS mining equipment is closely aligned with the behavior of commodity prices, which are very sensitive to economic conditions.

    Demands for aviation and marine systems are subject to cyclical influences in both transportation and recreation uses. Demand for scientific uses is heavily influenced by government budgets.

    In the rebound scenario, the shift from consumer receivers to smartphones is accelerated as more households are able to afford data plans, and more businesses take advantage of mobile connectivity. In the recession scenarios, receiver markets become saturated more quickly as demand ebbs. Some consumers switch to smartphone use of GPS where it is free, to avoid the cost of purchasing receivers. Nevertheless, smartphone use of GPS grows less rapidly because of a slower shift from unconnected phones to connected smartphones. Purchase of new or upgraded vehicle GNSS systems is more cyclical than the already highly cyclical demand for vehicles, and is further impacted in recession by the availability of phone-based alternatives. Location-based services continue to grow rapidly in all scenarios, with the rate of growth moderated by conditions in the various economies.

    Conclusion

    The overall outlook is cautious in the face of large potential threats and uncertainties. However, the industry has weathered many storms before, and its long-term outlook remains strong.


    Irving Leveson of Leveson Consulting is an economist and strategic planner who has worked extensively on GNSS markets, benefits, and financing. He previously served as director of economic studies of the Hudson Institute and senior vice president and director of research of Hudson Strategy Group. He received his Ph.D. from Columbia University.

     

  • Optimizing Small Antennas for Body-Loading Applications

    By Oliver Leisten and Viktor Knobe.

    Styling for consumer usage has progressively miniaturized of the antenna package to tiny dimensions compared to a free-space wavelength, even as devices with these miniscule antennas are designed to work close to the absorbent tissues of the user’s body and in the electromagnetic maelstrom of city street levels. GNSS antennas have responded with significant advances.

    The selection of the GNSS antenna, especially for small portable wireless devices, demands careful consideration of how it will interact with its expected environment. A physical appreciation can explain how many impairment factors can actually have a common cause: often the effect of human body-loading. This explanation starts with a counter-intuitive foundation: though the GNSS receiver does not transmit signals, for the sake of clarity we invoke the law of reciprocity and proceed with the conceptual thinking that the antenna is radiating outwards. This gives us a basis for understanding the causal physics of how the antenna shares energy with the immediate environment.

    We can visualize the basic radiating action of the antenna by recognizing that it is a resonant component. We must consider what exactly is in resonance, because the antenna designer has two different design options. In the self-resonant configuration, the antenna can be considered to be resonating autonomously, forming the entire dipole of the antenna within the antenna body. Here, dimensions and topological structure act in conjunction with reflecting and absorbing features surrounding it to define where and how the antenna radiates.

    In the second or probe antenna case, a larger radiating space can be configured by resonating the antenna with the housing together. The antenna typically forms a monopole counterpoised by currents and voltages in the housing. Here, the topology of the radiating system (antenna and housing) acts in conjunction with the near environment to define the radiation pattern.

    The value of distinguishing these two configurations is clearly reflected in the contrast between their behaviors with regard to radiation efficiencies in different uses. We conducted an experiment with three example antennas. Each antenna was installed in as common a package format as was practically feasible to model the top portion of a slim-line demonstration platform, with dimensions typical of consumer devices and containing a conductive chassis 55 millimeters wide. Obviously, a probe antenna must be installed in a chassis in order to function, and this directed the experimental approach to be structured around a similar-housing methodology.

    The probe antenna was a small metal and ceramic chip, and we compared its performance with a small microstrip patch antenna mounted horizontally in a broader but otherwise similar housing, and a hexafilar antenna mounted in an identically dimensioned housing. Strictly, the microstrip antenna is a single terminal element, but it can be considered as self-resonant as the resonance fields are very tightly constrained. Figure 1 plots the radiation efficiencies for benign free-space conditions (without body-loading) together, as frequency responses.

    Source: GPS
    Figure 1. Frequency response of radiated efficiency in unloaded (free-space conditions) and mounted in similar housings (ground-plane width 55mm).

    In benign open-field conditions the probe antenna has excellent efficiency performance and superior bandwidth compared to the two self-resonant configurations. Conversely, the self-resonant antennas (patch and hexafilar) have similarly narrow frequency-response bandwidths and lower efficiencies. We will show how it is important to repeat the test for realistic use scenarios that determine how close the antenna will be juxtaposed to the user’s biological tissues before concluding that the probe antenna is the best solution.

    Antenna studies have shown that the bandwidth reduces very rapidly as the resonant volume of the antenna reduces. This accounts for the reduction in bandwidth shown in Figure 1 for the self-resonant antennas (microstrip patch and hexafilar) with respect to the probe antenna (chip). In the case of the probe, the resonant structure is the entire metal chassis of the device (in this case the circuit-board ground-plane) so that the resonant volume of the resonating system is much larger than those of the self-resonant structures.

    To analyze the behavior of antennas in different use scenarios, it helps to consider the nature of resonance in antennas: open fields, with equal time average amounts of electric and magnetic field energy oscillating in space. These fields, induced by the time-varying voltage potentials and currents in the antenna, can launch a radiating wave into space because time-varying electromagnetic fields can carry or displace energy. We need to appreciate that this volume is where the so-called reactance fields exist, where field oscillations function as a sort of pump that propagates the electromagnetic wave. The antenna induces those fields in a configuration that manages the propagation of waves in useful directions and with desired polarization.

    Any invasion of the reactance field region will disrupt this process and cause impairment. Whilst obstruction of the radiating fields far away from the antenna will just cause a masking effect, a similar obstruction in the reactance-field region can disrupt the basic process of generating radiation. The density of fields in the reactance field region is much higher than would be implied by the straightforward application of the inverse square law.

    Use Near the Body

    We evaluated the antenna types, installed in packages as thin as test antenna dimensions allow, to draw conclusions as to how they might operate in slim-line consumer devices held close to the user’s body. Figure 2 shows CAD diagrams of the three antennas installed in their respective test packages.

    Source: GPS
    Figure 2. Antenna test housings for the chip antenna (left), patch antenna (middle) and hexafilar antenna (right). The housings were constructed to have a height of 26mm, a width of 60mm and a depth of 11 mm for the chip antenna and the hexafilar antenna and of 20.5mm for the patch antenna. In all cases the horizontal width extent of the printed circuit board (with continuous copper ground-plane on at least one side) was set at 55mm.

    Consumer devices have drawn antenna technologies from traditional GNSS applications as well as from terrestrial mobile telephone origins. The overall evolution combines adaptation of the circularly polarized technologies (multi-filar and microstrip patch) into smaller body-loaded platforms with insufficient space for effective ground-planes, together with adaptation of the art of low-cost cellular-telephone embedded antenna technologies that were never developed for circular polarization. Taking our three solutions in their embedded test platforms, we can appraise their body-loaded efficiencies by testing them juxtaposed to a phantom head, providing a means of assessing impairment due to body-loading.

    The phantom head in the loading experiment was filled with a tissue simulating liquid conforming to requirements for specific energy absorption measurements according to CENELEC and IEEE procedures. Comparing the antenna efficiencies for open-field conditions (Figure 1) and body-loaded conditions (Figure 3), reveals impairment to antenna efficiency in all three cases, with the most severe loss of approximately 80 percent by the chip antenna.

    Source: GPS
    Figure 3. Combination of FFT-based acquisition with FDAF.

    The self-resonant antennas suffered less impairment: approximately 30 percent reduction for the patch and 65 percent for the hexafilar antenna. The probe’s significant loss of efficiency is typical of this class of antennas, as the resonant fields are heavily loaded by the phantom head. The peak efficiency for this chip antenna has tuned downwards in frequency as the dielectric loading effect of the head-phantom introduced a regime of net higher relative dielectric constant (εr) into the resonance field region of the antenna system.

    By contrast, the self-resonant antennas did not tune down in frequency as they were brought into proximity with the phantom head. This indicates that the resonance fields were not offered to the dielectric materials of the head phantom to an extent that materially changed the relative dielectric constant (εr).

    Nevertheless, there is a significant difference between the impairment that develops between the patch and hexafilar cases as body-loading is applied, with the hexafilar solution losing more radiation efficiency than the patch antenna. There are two explanations for this difference.

    The first is that the patch housing is simply larger, with a greater depth required to accommodate the patch antenna horizontally at the top of the device housing. On average this larger housing size spaces the resonant fields further from the phantom and from the lossy simulated head tissues.

    The second explanation offers an insight into the symbiotic relationship between the hexafilar antenna and the demonstration platform’s vertically orientated housing. The horizontal ground-plane required for the patch antenna is inconvenient from the style and total integration cost point of view, but also ineffective as a ground-plane as it lacks sufficient width in a device styled to minimize depth. In this scenario the patch antenna is not getting much reflection uplift from the ground-plane; therefore there is little impairment when the device is body-loaded.

    The hexafilar solution is designed to benefit from reflective uplift from the vertically disposed ground-plane of the device. This property is convenient for device packaging because it allows the hexafilar antenna to be integrated at a device corner. The installation of a large and effective vertically oriented ground-plane for the hexafilar case is, by contrast, highly convenient and potentially more cost-effective. When the device is not body-loaded, reflections from the vertically disposed ground-plane uplift the gain and efficiency of the hexafilar antenna. The important advantage over the chip antenna (which is also convenient for space-constrained designs) is that for the self-resonant hexafilar antenna, the frequency of resonance does not change for open-field and body-loaded cases.

    Polarization, Pattern, Positioning

    Sufficient data has now been presented to make an antenna selection on the basis of efficiency and styling. The probe antenna in the guise of a chip antenna provided the highest radiation efficiency in free-space, comparable radiation efficiency to the hexafilar antenna in a body-loaded use scenario, and the small physical size supports compact product designs. However, for GNSS applications we must consider wave polarization, especially if there is multipath scattering. GNSS systems employ right-hand circular polarization (RHCP) and ideally should use antennas with hemisphereically omni-directional antennas. The zenith gain of a circularly polarized antenna is expected to be 3dB higher than that of a linearly polarized antenna of the same efficiency.

    If a GNSS terminal is equipped with an omni-directional but linearly polarized antenna, it can receive circularly polarized signals from all directions (albeit with a spatial average 3dB polarization loss). However, the positioning performance of such a terminal will be compromised because a linearly polarized antenna cannot discriminate between RHCP or LHCP, and reflections change the direction of spin of the circularly polarized wave.

    More color to the subjects of polarization, pattern, and consequential GNSS accuracy can be gained by focussing on the operation of the dielectric-loaded hexafilar antenna, as an example of a small antenna. Figure 4 shows the measured RHCP and LHCP elevation patterns of an exemplary small hexafilar antenna. These are excellent examples of the signature cardiod pattern shapes of good circular polarization antennas, but they point in opposite boresight directions. A dipole rotating anti-clockwise (viewed from above) in a plane would simultaneously excite a RHCP cardiod elevation pattern in the upwards direction and an oppositely directed, but otherwise similar, LHCP cardiod pattern downwards. If the antenna has no ground-plane and the dipole rotation is planar, the power of the upward RHCP and downward LHCP responses are equal. However, the dielectrically-loaded hexafilar antenna is a synthesis of a small travelling-wave upwardly spiralling dipole, emulating the axial-mode of a helical antenna. As the electrical size of such an antenna is increased, the area of the upwardly directed RHCP pattern progressively increases, and the area of the downwardly directed LHCP pattern progressively reduces. The antenna’s dielectric core enables this right-to-left discrimination within dimensions that are very much smaller than a free-space wavelength of the GNSS signal.

    Source: GPS
    Figure 4. RHCP and LHCP elevation for small dielectrically loaded hexafilar antenna (with no ground-plane).

    We can describe the polarization sorting behavior of the small dielectrically loaded antenna in figure 4 as follows. GNSS signals direct from the space vehicles will arrive in the directions of the upper hemisphere of the patterns where the highest sensitivity of the antenna to RHCP is deployed. GNSS signals bounced from a reflective object may also arrive in these upper hemisphere directions, but with reversed polarization: LHCP. In these directions the antenna has a very much lower sensitivity to LHCP, and the GNSS receiving process will accord a low value on these signals that as a result of the low antenna gain will be assessed as relatively noisy.

    Signals that arrive at the antenna from directions in the lower hemisphere will certainly have reflected from the ground surface (assuming that the antenna is held upright). These reflected left-hand polarized signals may have been attenuated by absorption losses of materials present on ground surfaces and also reduced in GNSS receiver process weighting by the antenna’s discrimination in favor of RHCP.

    RHCP and LHCP Gain

    Whilst appraisal of antenna patterns is certainly the most important method for assessing the performance of antennas in different use scenarios, it is nevertheless difficult to report accurately because the three-dimensional data-set is inevitably complex. To provide a meaningful physical basis for discriminating performance between the test antennas for open-field and body-loaded, we propose a single parameter: cross-pole rejection at zenith as one which is directly relevant to GNSS accuracy in a multi-path environment. Figure 5 plots the right hand and left hand comparative frequency responses for open-field and body-loaded use scenarios. Table 1 summarizes these responses.

    (a)

    Source: GPS

    (b)

    (c)

    Source: GPS

    (d)

    Source: GPS
    Figure 5. RHCP and LHCP frequency responses at the zenith direction for conditions of free-space and body-loading. From top to bottom: a) open-field conditions and RHCP, b) open-field conditions and LHCP, c) body-loaded conditions and RHCP, and d) body-loaded conditions and LHCP.
    Source: GPS
    Table 1. RHCP to LHCP gain ratio at the zenith direction (θ=0, φ=0) at GPS L1 center frequency (1.575.42 GHz).

    In open field, the chip antenna does not have a gain advantage for right-hand versus left-hand polarization and also suffers the highest impairment in gain when body-loading is applied. In this test there is an advantage in favor of RHCP gain for the body-loaded test scenario, but we presume this depends on the mounting position of this particular probe antenna on the test device. Perhaps a mounting position towards the left of the assembly might have incurred a disadvantage of similar magnitude?

    The patch antenna has an excellent RHCP over LHCP advantage in open-field conditions, but this advantage diminishes when this solution is body-loaded. This is the least gain-impacted solution as presumably the horizontal ground-plane and much greater device width produce a relatively low body-loading impact.

    The most interesting result concerns the hexafilar antenna, for which the RHCP to LHCP advantage actually improved in the body-loaded test scenario. As this device had the same depth, one might have expected it to sustain a body-loading impairment similar to that of the chip antenna, but due to the self-resonant character of the hexafilar element the loss in gain (in this zenith direction) was actually only slightly greater than that of the patch antenna.

    The hexafilar element’s CP performance is distorted by the lack of circular symmetry of the vertical ground-plane; therefore in open field this direction has a relatively modest RHCP to LHCP gain advantage of about 5dB. However, when the device containing the hexafilar antenna solution is body-loaded, the re-radiation from reflections from the circuit-board are heavily damped by the phantom head. The radiating source is then predominantly the hexafilar self-resonant element that by design is not itself so significantly impacted by the body-loading scenario. This source is restored to a more autonomous circularly polarized form with an advantage of RHC versus LHCP gain in zenith direction, nearly 13.5dB.

    Walk Tests

    Free-space and body-loaded test data, together with arguments concerning polarization discrimination and multipath led to an hypothesis that the antennas with the best circular polarization performance should provide the highest GNSS positioning accuracy. We tested the three devices, worn against the lower torso where the body provides a relatively homogeneous dielectric medium, so that position data could be compared with a reference antenna mounted over a large overhead ground plane.

    Many walk tests were conducted around different routes in London, which collectively demonstrate the value of circular-polarization discrimination as a key enabler for accurate street-level position determination. One segment (Figure 6) in the vicinity of an iconic tall London building commonly known as the Gherkin showed that, though the circularly polarized antennas closely followed the path of the reference antenna, the linearly polarized chip antenna produced an error of as much as 200 meters. It is possible that the dominant reflector in this case is the Gherkin itself.

    Source: GPS
    Figure 6. Data, central London walk test.

    Conclusions

    The chip and hexafilar antennas could be integrated tightly into consumer device housings; both experienced gain uplift from the vertically disposed circuit-board ground-plane. The gain uplift from the chip antenna arose as the resonant volume of the device is enlarged as the device size is increased. The gain uplift from the hexafilar antenna arose as a result of constructive reflections from the circuit-board functioning as a vertical ground-plane.

    The patch antenna was not the most convenient from the styling point of view because the depth was dictated by the size of the horizontally orientated patch. Consequently the housing was significantly thicker than for the chip and hexafilar solutions, and the patch antenna was not receiving significant uplift from reflections from the housing because the depth limitation constrained the ground-plane to ineffective dimensions.

    In body-loaded tests, the chip and hexafilar antennas demonstrated roughly equal radiation efficiency, but the hexafilar provided a significant RHCP advantage. Higher right-hand circular gain was measured for the patch antenna; this was expected due to the greater depth of the housing to accommodate the patch antenna. Urban walk tests showed that the RHCP antennas provided the highest position accuracy.

    Whilst the hexafilar antenna did experience some uplift due to reflections from the device circuit board, these were negated when the device was body-loaded. However, the distorting effects of the device ground-plane were also lost, so that the antenna’s advantage of RHCP over LHCP was improved in the body-loading condition.

    The GNSS industry has advanced the miniaturization of polarization-controlled antennas for small body-loaded uses. This is gaining currency as enabling polarization diversity in 4G data-communication terminals.

    Manufacturers

    Sarantel SL1350 antenna was the hexafilar element under test.

    Position data for all four devices was measured with Telit SE868 evaluation kits using CSR (now Samsung) SiRFstarIV chipset.


    Oliver Leisten is chief technical officer and founder of Sarantel Limited, where Viktor Knobe worked as a student intern from Imperial College London.

     

  • Delorme at the ESRI International Users Conference

    GPS World magazine interviews at the ESRI show, talking with Adrian Smith of Delorme.