Global Navigation Satellite Systems Engineering, Policy, and Design
After several false starts in the previous months and a multi-year delay in the overall GPS III architecture development, the GPS Wing (formerly the GPS Joint Program Office) announced on July 12 the release of a request for proposal for the development and production of the GPS Block IIIA satellites.
By Inside GNSS
GSA Executive Director Pedro Pedreira (left) and Guiseppe Viriglio, ESA director of telecom and navigation, at signing of accordHaving abandoned — for the time being at least — attempts to attract private investment to the creation of Galileo’s infrastructure, European GNSS leaders are working to shape a Plan B that can gain support from the program’s extensive group of stakeholders.
By Inside GNSS
Sometimes things don’t go as planned.
That certainly is the situation facing the European GNSS Supervisory Authority (GSA) today as the new lead agency for Galileo affairs.
A meltdown of the public private partnership (PPP) that was supposed to build and operate Europe’s GNSS has thrust an unexpected — and unscripted — role into the hands of the GSA almost at the very moment that the new organization first stepped onto the public stage.
As originally envisioned, the GSA — a European community agency operating under the aegis of the European Commission (EC) — had only to “conclude a concession contract with whichever consortium is selected on completion of the development phase of Galileo.”
Not “negotiate” a contract — that was the task of GSA’s predecessor, the Galileo Joint Undertaking (GJU) — but merely sign off on the deal. And then monitor the contract’s implementation on behalf of its public sponsors while taking up a full suite of other tasks.
Instead, on May 11, the GSA Administrative Board delivered its opinion that progress in negotiations with the consortium of eight companies seeking the 20-year concession contract was not making “relevant progress at the level” needed to ensure a timely completion of the Galileo project.
That conclusion by the GSA board, and a set of alternative courses of action, will now go to the European Union (EU) Transport Ministers Council in early June.
At the center of the storm are the GSA and its executive director, Pedro Pedreira, who took up his responsibilities in July 2005. Before his GSA appointment, Pedreira was serving as director of business development at Portugal Telecom, having spent more than 20 years in the satellite communications field.
Still unfamiliar to many in the GNSS community, the GSA has a €420 million budget for 2007, including €40 million in this year’s R&D allocation from the EC’s 7th Framework Program. An administrative board with representatives from the EU’s 25 member states oversees the authority. Unlike many European Community institutions, however, the board only requires two-thirds majorities for its decisions, which should enable it to act quickly and powerfully.
Although he has been at the GSA nearly two years, it was only upon the “liquidation” of the GJU in January that Pedreira and his organization became truly visible. During the interim, his focus was on building a staff with “critical mass” — now numbering about 50 — and preparing for the authority’s role as the lead agent for supervising implementation of Galileo and monitoring compliance of a Galileo Operating Company (GOC) with the concession contract.
In a series of exclusive conversations with Pedreira and key GSA staff members in March and May, Inside GNSS explored the agency’s mission and the implications of a potentially strategic change of direction for the Galileo program and the authority’s role.
European sources close to the concession negotiations have told Inside GNSS that the leading alternate approach to a PPP is an outright public takeover of the project now and issuance of a new tender for a private operator once most or all of the Galileo space and ground infrastructure is built.
Pedreira would not discuss the specifics of any of the proposed alternatives before they are presented to the transport ministers in June. However, he acknowledged that “the original mandate [to the GSA] was based on a certain model – PPP.” Depending on the decision reached by the transport ministers, “The Council may look at the governance of the program and adjust the mandate of the GSA.”
More, and Then Less
As laid out in a July 12, 2004, EU Council regulation establishing the GSA, the first order of business for the authority was to “conclude a concession contract with whichever consortium is selected on completion of the development phase of Galileo and take steps to ensure compliance by that consortium with the obligations — in particular the public service obligations — arising from the concession contract.”
Because at the time it went out of business the GJU had only managed to thrash out a “head of terms” agreement — essentially, the chapter headings and outline of points for an eventual contract — it initially appeared that GSA would be saddled with a lengthy negotiation with the private consortium. Instead, by the time of the Munich Satellite Navigation Summit in early March, Pedreira and EC Director-General for Transport and Energy Matthias Ruete were decrying the failure of the consortium to incorporate, appoint a CEO, and finish talks on the concession contract.
At a March 22 meeting, the transport ministers gave the consortium until May 10 to meet a series of milestones, leaving it to the EC, “assisted by GSA and ESA [the European Space Agency], to assess progress in the concession negotiations and to submit alternative scenarios, also assessed for costs, risk, and affordability,” in time for their June council meeting.
“The council in March noted its previous decision to implement the project with PPP,” says Pedreira. “But we could have a different geometry of partnership [with the private sector]. It could have a different shape.”
So, what happens to GSA if the transport council (and probably the Ministers Council — heads of state of the 25 EU members) drops or delays implementation of the PPP concept and goes for an public sector–only plan?
“It’s too soon to see how to adjust forms of the current organization,” says Pedreira. “The concession has taken a considerable amount of our resources. We have been giving a priority to the concession [since GSA was established].”
Indeed, supporting the GSA concession effort, headed by Carlo des Dorides who had served as chief negotiator with the GJU, was at the top of several other GSA administrators’ agendas.
In a March interview, Gian Gherado Calini, head of market development, told Inside GNSS that his group had two main tasks: first, the concession and working with des Dorides to identify size and growth of those markets supported by services operated by the concession. Second came downstream markets and creating conditions for them to succeed.
At that time, Hermann Ebner, head of the largest GSA unit, the Technical Department, put support for the concession process at the top of his list as well. A GSA technical task force had completed an assessment of design risks, and the department handled design revisions proposed by the consortium and kept a running tab on the changing cost figures associated with the program.
Even the security section had a role through the PACIFIC project to size the potential markets for the publicly regulated service (PRS), an encrypted signal designed for public safety, law enforcement, and possibly military applications.
Still Plenty to Do
Although the concession headed the list of GSA responsibilities, it is far from the only task given the agency by the 2004 regulation.
“Many aspects of GSA role are independent of the procurement model,” Pedreira says, ticking off some of the others that are top of mind: Galileo security, frequency coordination, management of R&D programs, and integration of the European Geostationary Navigation Overlay Service (EGNOS, essentially a satellite-based augmentation system) into the Galileo infrastructure and operations.
In the matter of market development, for instance, Pedreira points out, “The business plan of the concession internalized only a fraction of the public activity [in application markets].” The Open Service, from which an overwhelming portion of Galileo market revenues will come, was not part of the concession’s mandate. That represents, in Calini’s words, “a gold mine” of potential new services and products.
Getting Technical. Meanwhile, the EC 7th Framework Program has allocated a total of €350 million over the next seven years for R&D projects under the GSA’s control, plus any still-uncompleted FP6 projects taken over from the GJU. The first calls for tenders on FP7 projects this year will target applications, receiver development, and Galileo implementation, says Ebner.
Another large item on Ebner’s agenda, regardless of GSA’s partner in moving Galileo forward, is system definition and development. On May 11, the agency published announcements for a system definition and performance head, a space segment implementation officer, and a ground mission segment implementation officer.
Other major tasks for the GSA Technical Department include managing the Galileo signal Interface Control Documents (ICDs) and frequency coordination. At its March 21 meeting, the GSA Administrative Board issued notice of an intention to proceed with implementing the multiplexed BOC waveform that will serve as the basis not only for Galileo Open Service signals but the new GPS civil signal at the L1 frequency (L1C).
Unlike the GJU, GSA can sign contracts and handle international agreements previously overseen by other EC departments. It has taken over from the GJU the responsibility for keeping track of projects by the People’s Republic of China, Israel, and other co-investors in Galileo.
Keeping Galileo Safe. Nearly untouched by the success or failure of the concession is the GSA’s role regarding security for the Galileo system — including space, ground, and user segments. “Galileo is the first EU space program for which security was needed,” says Olivier Crop, the agency’s PRS officer.
The GSA has created a System, Safety, and Security Committee (3SC), which will be a key player in EU decisions on Galileo. The authority also will be responsible for establishing a Galileo Security Center charged with helping protect the system’s critical infrastructure, controlled signals, and PRS-capable user equipment.
EC policy on PRS, which was only approved for inclusion in Galileo in 2004, lets every member nation decide whether they want to allow use of PRS within their own “sovereignty domain.” Each country controls access to its own receivers, but operations in other countries or throughout the EU generally requires approval of the European Council.
EGNOS. The 2004 council regulation also entrusted the GSA with “managing the agreement with the economic operator charged with operating EGNOS and of presenting a framework on the future policy options concerning EGNOS,” which is largely complete and in provisional operation.
In 2004, with the incorporation of EGNOS into the concession, Pedreira says, came the recognition “that the consortium could not tackle [operating EGNOS] as soon as hoped. There was a need to go for an open tender on EGNOS economic operation.”
EGNOS, in fact, was a subject of discussion at the GSA board’s May 11 meeting. “There are many aspects to settle,” says Pedreira — issues involving ESA and the aviation organizations that are co-owners of EGNOS with the EU. “We will need to transfer assets to GSA to proceed with an open tender” for a service provider to implement early operation of EGNOS.
“At the working level, GSA has very good, very intense relations with ESA, especially on EGNOS and IOV,” he adds.
The GSA is also charged with responsibilities during the in-orbit validation (IOV) phase of Galileo’s development, although ESA is in charge of the technical side of things. “It would be surprising if the council went for a solution without taking note of the progress on the IOV phase and making best use of the assets and investment made to date.”
Perhaps most significantly, under the 2004 council regulation the GSA owns the tangible and intangible assets created during the development and implementation phases of the program. In other words, the agency is the legal guardian of the public interest in Galileo.
Overhanging that role, of course, is what the GSA’s political masters — initially, the transport ministers and, ultimately, the member states — decide to do about Galileo as a whole.
By Inside GNSS
On-going discussions are taking place between U.S. and European Union (EU) experts concerning the future GPSIII L1C and Galileo E1 OS civil signals. An agreement on a common power spectral density (PSD) known as multiplexed binary offset carrier (MBOC) recently emerged as a solid candidate to replace the current baseline: BOC(1,1).
On-going discussions are taking place between U.S. and European Union (EU) experts concerning the future GPSIII L1C and Galileo E1 OS civil signals. An agreement on a common power spectral density (PSD) known as multiplexed binary offset carrier (MBOC) recently emerged as a solid candidate to replace the current baseline: BOC(1,1).
In order to comply with the MBOC PSD, two candidate implementations, known as time-multiplexed BOC (TMBOC) and composite BOC (CBOC) modulations, have been proposed. If fully exploited, these implementations will provide improved performance but require a more complex receiver architecture than a BOC(1,1) receiver.
Increased complexity and associated higher costs, however, might be detrimental for a GNSS receiver manufacturer that would like to use MBOC, but with limited modifications to their receivers — particularly for those companies producing equipment for mass consumer markets. This article aims at evaluating a new CBOC receiver architecture using locally generated TMBOC-like signals that will result in a simpler architecture comparable to a BOC receiver.
The normalized MBOC PSD includes the whole of GPSIII L1C or Galileo E1 OS civil signals, which means both their data and pilot components.
Because the MBOC is defined only in the frequency domain, a variety of compliant temporal modulations can be used. In the literature, two different modulations were proposed to implement the MBOC:
• TMBOC, which multiplexes in the time domain BOC(1,1) and BOC(6,1) sub-carriers and seems likely to become the main candidate used by the future GPSIII L1C signal, and
• CBOC, which linearly combines BOC(1,1) and BOC(6,1) sub-carriers (both components being present at all times), and appears to be the leading candidate for the Galileo E1 OS signal.
The Additional Resources section lists papers by G. Hein et al, J. Betz et al, and J.-A. Avila-Rodriguez et al, which introduce and discuss TMBOC and CBOC in detail. (Available in the downloadable PDF, above.)
The philosophy behind these two modulations is very different, and, although they would theoretically produce equivalent tracking when used with a TMBOC or CBOC receiver (considering pilot and data channels), they can result in different performances in other configurations (for instance, considering the pilot channel only).
A major difference between the TMBOC and CBOC modulations is that the CBOC sub-carrier, as the weighted sum of two squared-wave sub-carriers, will have four different levels. Consequently, this means that an optimal CBOC receiver has to generate a local replica that also has four levels, resulting in a local replica encoded on more than just one bit. This could complicate the CBOC receiver architecture and might prove detrimental to the widespread use of this modulation for certain types of receiver, if retained as the Galileo E1 OS modulation.
This article describes an innovative technique that only requires a 1-bit local replica, very similar to a TMBOC waveform, to track CBOC signals. This method is particularly interesting because, despite its simple implementation, it has only limited losses in tracking performance with respect to traditional CBOC or TMBOC receivers. Moreover, it shows excellent performance when compared to the previous GPS/Galileo L1 baseline signal, the BOC(1,1).
The first part of this article describes the possible CBOC and TMBOC candidates for Galileo E1 OS and GPS III L1C modulations. The second part looks at the traditional tracking performances of these two modulations in terms of thermal noise and multipath-induced errors.
Finally, we introduce the new 1-bit tracking technique and compare it against optimal CBOC and TMBOC tracking in terms of tracking noise and multipath resistance.
Conclusions
Following the US/EU MBOC agreement, the current main candidates for the GPSIII L1C and Galileo E1 OS have been introduced. In particular, the pilot channels have been analyzed with their use of the new CBOC and TMBOC modulations.
Although adding a very small amount of BOC(6,1) to the previous BOC(1,1) baseline, it has been shown that the tracking performances of these future signals are significantly improved compared to pure BOC(1,1) tracking. In particular, tracking noise is reduced by 2.4 to 3 dBs in terms of equivalent C/N0, and multipath mitigation is significantly improved.
Focusing on the CBOC modulation, its multi-level waveform could result in more challenging receiver architecture. In order to keep a simple receiver design to receive a CBOC signal, a new tracking technique, referred to as TM61, has been proposed to allow tracking of the CBOC modulation with a 1-bit only locally generated replica. This method uses time-multiplexing of BOC(1,1) and BOC(6,1) sub-carrier on the same model as the TMBOC modulation.
A preferred implementation of TM61 is the use of a pure BOC(1,1) sub-carrier for the prompt correlators and a pure BOC(6,1) sub-carrier for the early and late correlators (a DP discriminator being assumed). This yields a much simpler receiver architecture since it requires only pure sub-carriers with no-multiplexing (different from TMBOC receivers), 1-bit local replicas (unlike a CBOC local replica) and a minimum of correlators. Please note it is also possible to use another implementation of the TM61 tracking methods with time-multiplexing.
In its preferred implementation, TM61 brings only a slight post-correlation SNR degradation (about 0.35 dBs for the selected CBOC main candidate for Galileo pilot channel), enabling good phase tracking. TM61 code tracking noise performance is degraded with respect to traditional CBOC tracking by approximately 2.4 dBs. However, this has to be put into perspective considering the substantial reduction in receiver complexity with TM61 and the fact that thermal noise might not be the main source of error for many applications.
Finally, the TM61 tracking technique has been demonstrated to provide, in its preferred implementation, a better multipath resistance compared to traditional CBOC tracking. In any case, the use of TM61 to receive a CBOC signal has been shown to significantly outperform the traditional reception of a pure BOC(1,1) with equivalent power, thus supporting the use of the modernized CBOC signal. Consequently, it seems to be a very good tracking technique for implementation in future CBOC receivers.
For the full article, including graphs, figures, and additional resources, download the PDF above.
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The Global Positioning System is undergoing continual modernization, providing ongoing improvements for users worldwide. Although various enhancements in system features have been under development since the mid-1990s, modernization first benefited civil users when Selective Availability — a security-motivated technique for “dithering” the open L1 signal to reduce positioning accuracy — was set to zero in May 2000.
The Global Positioning System is undergoing continual modernization, providing ongoing improvements for users worldwide. Although various enhancements in system features have been under development since the mid-1990s, modernization first benefited civil users when Selective Availability — a security-motivated technique for “dithering” the open L1 signal to reduce positioning accuracy — was set to zero in May 2000.
Subsequently, other improvements in accuracy have been obtained through enhancements to the capabilities and operation of the control and space segments, still based on the original set of GPS signals.
The launch of the IIR-14(M) (modernized replenishment satellite) in 2005 began a new era with transmission of the L2 civil (L2C) signal, along with the modernized military M-code signal. A third civil signal, called L5, will be transmitted from Block IIF satellites.
All the while, improvements in monitoring, satellite technology (for example, the on-board atomic clocks) and operations yield continuing increases in accuracy. The United States plans to continue providing these capabilities free of user fees. It will continue to complement this pricing policy by providing free and open signal descriptions and other technical information needed for development of receivers and services using civil signals.
In the meantime, development of the next generation of satellites, called GPS III, and a modernized control segment (OCX) continues, which will lead to greatly enhanced capabilities beginning early in the next decade. An integral part of the GPS III capabilities being developed is a new civil signal, called L1C, which will be transmitted on the L1 carrier frequency in addition to current signals.
Approximately one year ago, the U.S. Air Force released the initial draft of Interface Specification IS-GPS-800, describing L1C. Novel characteristics of the optimized L1C signal design provide advanced capabilities while offering to receiver designers considerable flexibility in how to use these capabilities.
The development of L1C represents a new stage in international GNSS: not only is the signal being designed for transmission from GPS satellites, its design also seeks to maximize interoperability with Galileo’s Open Service signal. Further, Japan’s Quazi-Zenith Satellite System (QZSS) will transmit a signal with virtually the same design as L1C.
L1C has been designed to take advantage of many unique opportunities. Its center frequency of 1575.42 MHz is the pre-eminent GNSS frequency for a variety of reasons, including the extensive existing use of GPS C/A code, the lower ionospheric error at L1 band relative to lower frequencies, spectrum protection of the L1 band, and the use of this same center frequency by GPS, Galileo, QZSS, and satellite-based augmentation system (SBAS) signals for open access service and safety-of-life applications.
Other unique opportunities that the L1C design leverages include advances in signal design knowledge, improvements in receiver processing techniques, developments in circuit technologies, and enhancements in supporting services such as communications. The L1C design has been optimized to provide superior performance, while providing compatibility and interoperability with other signals in the L1 band.
L1C provides a number of advanced features, including: 75 percent of power in a pilot component for enhanced signal tracking, advanced Weil-based spreading codes, an overlay code on the pilot that provides data message synchronization, support for improved reading of clock and ephemeris by combining message symbols across messages, advanced forward error control coding, and data symbol interleaving to combat fading.
The resulting design offers receiver designers the opportunity to obtain unmatched performance in many ways.This article will give an overview of the L1C signal design, highlighting the features that will benefit receiver designers and, ultimately, end users. The following section provides background on L1C and its design process, from its beginnings in 2003.
Subsequent sections then provide an overview of the signal structure, details of the signal’s spreading codes and overlay codes, spreading modulation, data message structure, and encoding and decoding of message information.
Summary of Benefits
Although more complete details are provided in IS-GPS-800, we will outline the most significant characteristics here.
L1C has been designed with unique, innovative, and powerful new features to enhance its robustness for all users, especially in difficult environments.
The signal structure alone, with the spreading code and the overlay code, provides exact GPS time, modulo 18 seconds. Alignment to the spreading code provides bit synchronization and alignment to the overlay code provide frame synchronization, making these receiver functions simple and robust.
For high-precision (e.g., survey) use, the pilot carrier removes the half cycle phase ambiguity, and the larger RMS bandwidth of the new spreading modulation has the potential to improve tracking performance, especially multipath mitigation. With the combination of improved carrier tracking of the pilot component, segmentation of clock and ephemeris in the data message, and FEC design, an autonomous navigator can demodulate the satellite clock and ephemeris whenever the signal can be tracked.
The improved cross-correlation of the new codes will also improve the performance of high-sensitivity receivers. Performance will also improve as a result of the new message format that allows code combining across satellites for the TOI and code combining of the near constant sub-frame 2 ephemeris data across multiple frames. International collaboration and outreach have assisted in producing a truly international signal with capabilities that will serve users for decades to come.
For the full article, including figures, graphs, and additional resources, download the PDF above.
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The U.S. Air Force has awarded Lockheed Martin Company a $6 million contract to develop and integrate a demonstration payload that will temporarily transmit an L5 civil signal on a modernized GPS Block IIR (GPS IIR-M) satellite.
By Inside GNSS
President Bush’s appointment of retired U.S. Coast Guard Vice Admiral Thomas Barrett as Acting Deputy Secretary of Transportation has brought a new co-chairman to the U.S. Space-Based Positioning, Navigation, and Timing Executive Committee (PNT EXCOM).
By Inside GNSS
A convergence of developments over the past few months has brought Europe’s Galileo program to the most critical passage of its history — at least, since final approval of the GNSS initiative by the European Space Agency (ESA) and the European Union (EU) in 2003 and 2004, respectively.
By Inside GNSS
The U.S. Air Force has awarded two $50 million contracts to Boeing and Lockheed Martin to execute a system design review for the next-generation GPS space segment program, GPS Block III.
The contracts come on the heels of both companies successfully completing system requirements reviews in November 2006. Those reviews, part of a $10 million follow-on order to a Phase A Concept Development Contract awarded in 2004, assessed Boeing’s and Lockheed’s ability to mitigate development and delivery risks associated with building the Block III satellites.
By Inside GNSS
Successful launch of the second modernized Block IIR satellite, IIR-15(M2), on September 25 and scheduling of another IIR-M launch on November 14 underlines recent progress in the GPS program.
IIR-15(M2), also identified by its space vehicle number (SVN58) and pseudorandom code number (PRN31), will be placed into orbital plane A, slot 2. The U.S. Air Force has designated the satellite to be launched in November as GPS IIR-16/M3, PRN15/SVN55.
By Inside GNSS
The term “Correlator” is often used in discussions of GPS and GNSS receiver design. It has been used to describe devices as simple as a single exclusive OR gate through to complete “baseband” chips that include a microprocessor.
Most usually, and in this article, the term describes the hardware or software that produces all of the required correlation data for a single signal from a specific GNSS satellite signal. This is also sometimes termed a “channel.”
The term “Correlator” is often used in discussions of GPS and GNSS receiver design. It has been used to describe devices as simple as a single exclusive OR gate through to complete “baseband” chips that include a microprocessor.
Most usually, and in this article, the term describes the hardware or software that produces all of the required correlation data for a single signal from a specific GNSS satellite signal. This is also sometimes termed a “channel.”
With the open GPS civil signal at the L2 frequency (L2C) now becoming available on Block IIR-M satellites, receiver designers have the opportunity to work with a markedly different GNSS signal resource. The first IIR-M spacecraft (designated SVN53/PRN17) was launched September 25, 2005, and the second is scheduled to go into orbit on September 14, 2006. (IIR-Ms also transmit the new GPS L1/L2 military (M-code) signal, but we will not treat this issue here.)
Against that historic backdrop, then, this article examines some of the novel elements of the L2C signal and its implications for GNSS receiver correlators. Our focus will be on a technically challenging aspect of receiver operation: initial acquisition of the signal and its processing by the correlator.
But first we will review some of the key functional aspects of GNSS correlators and some of the signal parameters that affect their operation.
. . . We can more easily explain the role of a correlator if we examine its two functions separately. In a GNSS receiver, correlation is used in two distinct activities:
Acquisition. Before the receiver knows whether it can receive a certain satellite’s signal, it must “search” for it using the correlation in an ordered but relatively indiscriminate way. Effectively, many correlation trials are run for each of many code delays and Doppler frequency offsets.
Tracking. Once acquired, the receiver must still despread the received signal in order to receive data and measure pseudoranges. Several correlators are usually used to keep the local code as closely aligned to the received code as possible. To do this, a “delay-locked loop” is used, with the correlators operating within the loops, some typically ahead of the received code (“early”) and some behind (“late”).
In other words, correlation is used both to “get” the signal and to “keep” it. These actions should be considered quite separately. In this article, we concentrate on the acquisition process.
. . . Various signal parameters affect correlation. These are listed for the three GPS signals in Table 1. Both L2C and L5 have dataless sub-signals which are time-multiplexed (CM and CL) and in quadrature (I5 and Q5), respectively. Both use longer codes than L1, while L5 has a higher chipping rate. L5 also has the added complication of Neuman-Hoffman codes, which will not be further discussed here.
. . . Correlation Signal “Shape” – the L1 civilian C/A-code signal is a single BPSK modulation. Despite the fact that the L2C signal is also BPSK, it introduces another layer of complexity by having the data-carrying and dataless signals multiplexed in time. Typically, the shorter (20-millisecond) data-carrying CM code will be acquired first, then the receiver would hand over to the longer (1.5-second) dataless CL code for tracking.
. . . The beauty of the exploitation of the circular convolution is that the code in the data does not need to be well aligned with the stored data— the whole point of acquisition, after all, is to perform this alignment. So, if the CM code was chopped up into pieces smaller than 20 milliseconds, this circular convolution would no longer reflect the time domain correlation, and a receiver would need to have many stored codes.
. . . Many GPS chipsets are optimized for operation in mobile telephone handsets and, as such, are aimed at minimizing the drain on handset batteries. For such applications, the large L2C acquisition overhead presents a serious problem . .
Conclusion
In this article, we have primarily examined the implications of signal acquisition for the new L2C signal. In a typical L2C-only receiver, significantly more effort is required to acquire the signal than is the case for L1 C/A code: more than 200 times higher in the hardware case and more than 500 times higher in the software case. However, because the long CL code does not carry any data, it can be used for the long integrations required for acquisition in a weak-signal environment.
For the complete story, including figures, graphs, and images, please download the PDF of the article, above.
Acknowledgments
The author wishes to acknowledge the useful suggestions made by Eamonn Glennon of Signav Pty Ltd.
Okay, now it’s official: The GPS Block III contract award has been delayed from Fiscal Year 2006 (FY06) to Fiscal Year 2007 (FY07).
By Inside GNSS
GPS has had enormous benefits to the economy and society that go well beyond military and civil aviation applications – that is becoming ever more widely understood. What has been more open to discussion are the civilian non-aviation benefits of further U.S. efforts at GPS modernization, particularly the introduction of additional signals.
GPS has had enormous benefits to the economy and society that go well beyond military and civil aviation applications – that is becoming ever more widely understood. What has been more open to discussion are the civilian non-aviation benefits of further U.S. efforts at GPS modernization, particularly the introduction of additional signals.
In an effort to define and measure civilian benefits, the U.S. departments of commerce and transportation commissioned some economic analyses of civil signal modernization. Particular emphasis was placed on the value of the L2C signal centered at 1227.60 MHZ, which recently began broadcasting from the first modernized GPS Block IIR-M satellite. This article is an outgrowth of that effort.
The analysis focused on the value of signals at more than one frequency for precision non-aviation use by business and government. It considered how utilization of the second civilian signal and its benefits would evolve in the coming decades as the L2C constellation expands and as additional signals become available from GPS and other GNSSes.
In the study, projections were developed under four scenarios — with the “moderate benefit”scenario seeming most likely — that reflect combinations of developments, including the strength of markets, the timing of L2C signal availability, the timing of Galileo availability, and complementary and competitive relationships with augmentations.
The main findings of the study are:
In addition to the domestic benefits examined, L2C will undoubtedly have important international benefits.
This article presents in more detail how we defined the problem, approached the study, and arrived at those conclusions.
The L2C Evolution
L2C, together with the present L1 C/A-code signal and the future modernized civil signal L1C, will provide an alternative to augmented single frequency GPS for precision users. Separate investigations have outlined the incremental benefits of L1C (See sidebar, “The L1C Studies,” at the end of this article)
L2C signals can be used for both horizontal and vertical measurement and positioning along with L1 C/A as satellites become available over more areas and in more times of the day. The first satellite can be used for improved timing. L2C also can be used in configurations of three or more frequencies in combination with the forthcoming GPS L5 signal and with signals from Galileo and GLONASS.
At various times in each signal’s deployment and development of markets, other signals will, to varying degrees, provide complements to L2C and competitors to it. L2C has its greatest potential to generate benefits for dual frequency applications until alternative signals are widely utilized, and for long-term use in applications taking advantage of three or more frequencies.
The L2 signal is currently being widely used for augmentations, and the new signals can be used in that way along with the existing constellation. However, L5’s use as a competitor to L2C and as a partner to L2C in multiple frequency implementations primarily depends on the launch timeline for satellites carrying the L5 signal since L5, centered at the 1176.45 MHz frequency, is not currently in service. Plans call for its implementation on the GPS Block IIF satellites, with the first IIF now expected to be launched in 2008.
L2C deployment requires a commitment to operational capability. Decisions will be required as to launch dates and signal activation for each successive satellite containing the signal. The L2C benefits study is intended to contribute to decisions about L2C deployment with consideration of alternative scenarios informed by quantitative and qualitative analysis.
To explore the implications of L2C evolution, we make projections about the numbers of U.S. precision users, incremental benefits, and user costs, based on examination of applications and available evidence on value of benefits, and consider how these can unfold over the period 2006–2030.
The analysis focuses on precision users of L2C who use two or more frequencies, although we do include estimates for supplementary multiple-frequency users and single-frequency users. However, the estimates of these types of use are more conjectural and do not contribute much to the overall value of benefits.
Benefits net of user costs are measured according to the widely accepted economic productivity approach, which includes productivity gains and cost savings. This comprehensive approach is more appropriate than one that measures benefits simply by expenditures on equipment and services.
Incremental benefits and user costs are defined to include all differences in outcomes from what would be expected in the absence of L2C.
Signal Advantages and Availability
The L2C signal, scheduled to be the first of the modernized civil GPS signals, is intended for civilian purposes other than aviation and safety-of-life. It will provide greater accuracy and robustness and faster signal acquisition than the current L1 C/A-code signal.
Higher signal power and forward error correction will improve GPS mobile, indoor, and other uses.
The L5 signal that will arrive within a few years will be in a protected aeronautical radionavigation system (ARNS) band intended for aviation and other safety-of-life uses and will have broader applications.
Multiple signals will allow many users to obtain greater precision and availability at lower cost than achievable with proprietary augmentation systems. However, signal combinations combined with public and private augmentations for even greater precision and reliability will support applications with some of the greatest potential benefits.
Combined use of L2C with L1 C/A and L5 will also enable some precision users to achieve even greater reliability and accuracy. Although available simulations differ on the size of benefits of three signals over two, many professionals expect important advantages from such “tri-laning” techniques.
The U.S. Air Force launched first satellite containing the L2C frequency on September 25, 2005, and the signal became available on December 16. Going forward, two to four Block IIR-M satellites are expected to be launched each year. With six to eight satellites anticipated to be available by about December 2007, users will be able to access at least one single satellite with L2C at almost all times. Eighteen L2C-capable satellites (including the Block IIF generation) will be available by about 2011 and 24 L2C signals, around 2012. (These statements are based on official 2005 launch schedules and are subject to revision.)
The first L5 launch is scheduled for March 2008. L5 does not have a GPS signal in use at its frequency, so it will not be usable to any great extent until a large part of its constellation is available. In contrast, L2 is in place to transmit the military P(Y) code and the carrier signals of the satellites are currently being used along with L1 C/A for higher-accuracy applications.
Consequently, the L2C signal can be used immediately as a second frequency. The GPS signal L1C, which is being planned now for implementation on the GPS III satellites scheduled for launch beginning in 2013, will be able to be used immediately, even for single frequency use, without augmentation because it is at the same frequency as the L1 C/A-code.
Using Multiple Frequency GPS
Many private and government precision applications could potentially benefit from multiple frequency GPS.
For example:
Scope of Benefits and Costs
Incremental benefits — those that arise because of the availability of L2C— include far more than the comparison of multiple frequency with augmented single frequency use. Companies adopting GPS in the future may even skip single-frequency options and instead choose multiple-frequency equipment (incorporating L2C) over non-GPS alternatives. Large candidate markets include construction, agriculture, and other applications where technological alternatives exist.
In some organizations, dual-frequency GPS will be the catalyst for extensive changes in systems that will occur earlier than if dual frequency GPS had not been adopted.
In the L2C study, benefits are measured according to the “economic productivity approach,” which is superior to the expenditure/economic impact approach because:
L2C benefits can take both market and non-market forms, including increases in the productivity of business and government operations, user cost savings, benefits to the public through provision of public services and saving lives, and through improved health and environment.
Net benefits are benefits minus user costs. Incremental user costs include all additional costs that are expected with the availability of L2C, not simply the difference in costs between single- and dual-frequency receivers. These can take the forms of enhancements and accessories purchased when adding L2C capability (e.g. better displays, controllers and software) or costs associated with users upgrading to multiple frequency GPS from less sophisticated single-frequency GPS systems or non-GPS systems.
However, incremental user cost is net of savings from use of receivers with less proprietary technology and any reduced use of private augmentation subscription services.
Expenditures to develop the GPS system infrastructure (satellites and ground segment) are not included, however, because most represent nonrecurring, sunk costs. Moreover, if we added them to our L2C analysis, we would need to include benefits to aviation and military users as well as their associated equipment costs.
Scenarios
The analysis takes into account alternative conditions of timing and impact of alternatives through the use of scenarios. Projections of signal use and value of benefits are developed through the year 2030 under four scenarios: High Opportunity, Moderate Benefits, Diluted Benefits, and Opportunity lost.
These scenarios reflect combinations of developments, including the strength of markets, the timing of L2C signal availability, the timing of Galileo availability, and complementary and competitive relationships with augmentations. (See the sidebar, “L2C Benefit Scenarios” at the end of this article for details of assumptions behind each.)
Probabilities are not given for the scenarios because the likelihood of alternative Galileo delays cannot be evaluated quantitatively. Moreover, the diluted benefits and opportunity lost scenarios are significantly affected by U.S. GPS policy, which is also not predicted.
Estimates of GPS Users
The L2C study projections shown in Figure 1 are based on assumed rates of decline in prices for user equipment and services and increases in the number of users in response to price changes. Projections reflect assessments of market sizes and patterns of market penetration under each scenario. Allowance also is made for effects of economic growth on market size. Table 1 (to view tables and figures, please download the PDF of this article using the link above) provides a detailed breakdown of results by scenario.
Within each scenario, projections are made for precision L2C users of three or more frequencies, dual frequency precision users, multiple frequency supplementary users, and single frequency users of L2C.
The starting point for determining the number of high precision users is a widely relied–upon estimate of 50,000 high precision users worldwide in 2000. We assumed that the United States had 40 percent of precision users in that year.
The study further assumes that the number of U.S. high-precision GPS users will grow by 18 percent per year from 2000 to 2030. This projection is based on a rate of price decline for user equipment of 15 percent per year and a corresponding a 1 percent increase in users for each 1 percent decline in price. Finally, we include an assumption of general growth in the economy (i.e., independent of GPS receiver price) that adds 3 percent per year.
These assumptions and calculations produce a projection of U.S. high precision GPS users — those using augmentations, of 38,776 in 2004. The estimated number of U.S. high precision users of any signal or combination nearly doubles to 75,177 from 2004 to 2008 and reaches 145,752 in 2012 and 333,445 in 2017.
We computed the numbers of multi-frequency GPS users by applying an estimated percentage to the number of high-precision users for each scenario. The number of multi-frequency precision users adopting dual versus three or more frequencies was then calculated using projected values for the percent of each category. Finally, the number of L2C users was calculated based on projections of the percent of multiple frequency users that use L2C, constructed to reflect the dynamics of each of the scenarios.
Rapid growth is projected in the numbers of U.S. precision multiple-frequency L2C users. In the moderate benefits scenario, the number of L2C users reaches 64,000 by 2017, of which 35,000 are dual frequency users and 29,000 use three or more frequencies. The numbers of L2C users vary widely among scenarios.
Average Net Benefits per User
The study defines average incremental net value of benefits per L2C user as the incremental value of benefits per L2C user above the incremental user cost of equipment and services. Benefits largely reflect productivity gains and/or cost savings. Estimates reflect a review of available evidence ranging from formal studies to case histories and expert opinion across a wide range of applications.
Our research suggests that average annual incremental benefit per precision L2C user net of costs could reach the range of $8,000–$16,000 per year. This includes benefits across systems that are not attributable to specific numbers of users and non-market benefits, such as safety and environmental advantages, as well as market benefits associated with the value of goods and services transactions. Market benefits attributable to numbers of users are estimated at 60 percent of all incremental net benefits.
These are peak values after benefits have had an opportunity to rise with experience using the new signal. The values decline from their peaks as new users with lower benefits are attracted by declining costs and some high benefit users move to alternatives.
In considering the plausibility of these figures, consider that:
(This could result, for example, from avoiding 100 deaths due to industrial accidents or environmental impacts at a value of $4 million per incident.)
The present values of incremental user costs range among scenarios from $175 million to $514 million in year 2005 purchasing power.
Costs represent one eighth or less of the total value of benefits in each scenario.
Value of Benefits
Civilian net benefits per user are incremental, net of incremental costs, and derive from prospects for major areas of application. The patterns incorporate some high-value initial use, assume that higher benefit users switch earlier to newer signals, factor in a buildup of productivity gains with experience, and project lower values for late-entry users attracted by lower equipment prices as well as later increases in higher benefit users switching to alternative signals.
We calculate the value of civilian net benefits of L2C through multiplying civilian net benefits per user by the number of L2C users for the user type and scenario. Higher net benefit scenarios result from higher benefits per user and larger numbers of users.
At a 7 percent real (above inflation) discount rate, present values of total net civilian market benefits range from $9.6 billion to $1.4 billion dollars. Benefits under the moderate benefits scenario have a present value of $5.8 billion and those under the high opportunity scenario $9.6 billion. (Values are discounted using annual data to calendar year 2006. That essentially places the values at the middle of 2006.)
Nearly all of the incremental benefits of L2C stem from precision use of two or more frequencies. That is both because of moderate numbers of other types of users in these and their low benefits per user.
The timeframe in which other signals become available after L2C plays an important role in the size of estimated benefits. In the high opportunity scenario, for example, dual-frequency net benefits appear higher than benefits from use of three or more frequencies because the latter applications start later as additional frequencies become available.
In the other scenarios, benefits from applications using three or more signals are higher than dual-frequency benefits because the benefits of dual frequency remain as strong when competing frequencies become available.
New spending can encourage greater long run economic growth, especially when it is associated with new technology for widely usable infrastructure. The spending may induce others to innovate, invest in greater capacity, take risks and/or provide financing. While direct estimates of the size of long run economic multipliers are not readily available, analyses of determinants of growth suggest that effects are modest, perhaps adding 20% to market benefits. Because of the uncertainty surrounding such estimates, no allowance is made for growth multiplier effects in the estimates shown.
Cost-Benefit Analysis
The ratio of incremental civilian benefits to user costs is calculated by dividing the present discounted value of total incremental benefits (including net benefits and costs) by the present value of incremental costs. These are shown with a 7% real (above inflation) discount rate.
The ratios of benefits to costs range from a multiple of 20 in the high opportunity scenario to 9 in the opportunity lost scenario. It would be surprising if benefit/cost ratios were not high because only direct user expenses (and not system costs) are included to get a picture of incremental costs of each set of outcomes.
The moderate benefits scenario, which has a ratio of 20, is considered more likely than the others. Because of the interest in obtaining the greatest benefits, focusing on the present value of net benefits is appropriate for policy rather than using the benefit/cost ratio when all ratios are high.
As mentioned, changes in various factors could substantially affect the outcomes of L2C benefits and produce either an overstatement or an understatement of these. See the “Benefit Variables” sidebar at the end of this article for a listing of the most important factors.
Conclusions
Rapid growth is projected in the numbers of U.S. precision GPS users and in most scenarios for the numbers of high-precision multiple frequency L2C users. Substantial L2C benefits can occur along with availability of other signals and constellations, augmentations, and alternative technologies. While Galileo will compete with L2C, Galileo signals also can increase precision L2C use in multiple frequency applications, an alternative that will become increasingly affordable.
The economic productivity approach offers a means of considering benefits in a comprehensive way. Benefits and costs are incremental. They are defined to include all changes that occur as a result of the existence of L2C.
Defined comprehensively, benefits can encompass results from more extensive changes in equipment and systems and include both benefits that are attributable to specific numbers of users and those that may be incorporated in systems and spread over a broad population. They include both market and non-market benefits — those that are not bought and sold in markets, such as benefits to life, health, security and the environment.
User costs also are incremental, including all changes that occur with the availability of L2C, and are net of savings from moving to less sophisticated and less proprietary equipment.
Sidebar: The L1C Studies
Before the L2C study, important progress had already been made in understanding the benefits of additional GPS signals. These activities included the discussion of civilian applications in the report of the Defense Science Board Task Force on GPS, released last December, and the L1C Study undertaken by the Interagency GPS Executive Board in 2004. (See the “Additional Resources” section at the end of this article to find out how to obtain these studies on line.)
Upper limits of total benefits of L1C for the single year 2005 — including those obtained by single- and multiple-frequency users in private households, businesses, governments — were estimated at approximately $2 billion: $640 million for mobile and wireless location services, $62.5 million for information/data services, $990 million for “commercial GPS,” and $490 million for in-vehicle information and navigation services (telematics).
The L1C study approximated a “rough order of magnitude” dollar value of L1C applications based on 2005 spending by applying a “team consensus” for an assumed incremental benefit as a percentage of market value (revenue) for each of 13 user categories. Spending in user group categories was based on a compilation of trade estimates.
Sidebar: L2C Benefit Scenarios
The four scenarios developed to support the L2C benefits study, along with the assumptions underling each, include the following:
High Opportunity
Moderate Benefits
Diluted Benefits
Opportunity Lost
Sidebar: Benefit Variables
Overstatement could result from competition from other signals, from augmentations and from other technologies that is greater than anticipated. For example,
Understatement could result from
For figures, graphs, and images, please download the PDF of the article, above.
Acknowledgments
Steve Bayless, Tyler Duval, Jason Kim, Scott Pace, Mike Shaw, Tom Stansell, Dave Turner, Jack Wells, Rodney Weiher, and Avery Sen offered comments, guidance and assistance to the study. Many others contributed expertise through interviews.
Additional Resources
Kenneth W. Hudnut, and Bryan Titus, GPS L1 Civil Signal Modernization (L1C), Interagency GPS Executive Board, July 30, 2004, <http://www.navcen.uscg.gov/gps/modernization/L1/L1C-report-short.pdf>
U.S. Defense Science Board, The Future of the Global Positioning System, Washington, D.C.: Office of the Under Secretary of Defense For Acquisition, Technology, and Logistics, October 2005, <http://www.acq.osd.mil/dsb/reports/2005-10-GPS_Report_Final.pdf>
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