Aerospace and Defense

Public Private Perplexity

High up in one corner of a trophy hall in the castle of the Bavarian royal dynasty in Berchtesgaden, Germany, hangs a poignant scene of inextricable conflict and death: the heads of two mighty stags, their antlers locked in combat.

Our tour guide translated the story behind this tableau. On October 14, 1735, a hunting party from the castle discovered the pair of struggling animals. Unable to separate the entangled antlers and with one deer’s neck already broken, the hunters had to shoot the other stag.

High up in one corner of a trophy hall in the castle of the Bavarian royal dynasty in Berchtesgaden, Germany, hangs a poignant scene of inextricable conflict and death: the heads of two mighty stags, their antlers locked in combat.

Our tour guide translated the story behind this tableau. On October 14, 1735, a hunting party from the castle discovered the pair of struggling animals. Unable to separate the entangled antlers and with one deer’s neck already broken, the hunters had to shoot the other stag.

A quiet murmur fell over the tour group, a post-conference excursion of delegates from the Munich Satellite Navigation Conference, as we reflected on the brute intransigence of nature. Then a voice spoke up from the back of the crowd: “That’s what happens when you have a PPP.”

Public Private Partnership or PPP, the now-notorious solution to a temporary impasse in Europe’s Galileo program. How quickly shibboleth can turn into epithet.

The laughter that followed the delegate’s wry comparison of death throes and thwarted politics drew strength from the discussions we had heard in the three previous days.

Prolonged negotiations have broken down between public patrons of Galileo and the private consortium seeking to complete and operate the European GNSS. The two sides have not come to terms over several major elements of risk-sharing associated with the program. The consortium has failed to incorporate a Galileo Operating Company (GOC), which would manage the system infrastructure as a profit-seeking venture under the oversight of the European GNSS Supervisory Authority (GSA).

“The negotiations are far more difficult than anyone anticipated,” Matthias Ruete, director general of energy and transport for the European Commission (EC), told his Munich summit audience.

Added Pedro Pedreira, executive director of the GSA, an EC entity that took over public sector responsibilities for Galileo at the beginning of the year, “The consortium is not able to present its own [unified] terms, let alone negotiate those terms.”

For their part representatives of the private consortium, while complaining that engineering changes and an elusive business model derived from the Galileo infrastructure were complicating the situation, acknowledged the new stalemate and their own internal disunity.

As this issue of Inside GNSS went to press, the problem was on its way to the council of European transport ministers meeting on March 22.

In its nearly 14 years of evolution, Europe’s GNSS program has met many such obstacles and, ultimately, always found a way through or around them. But never, as Ruete commented to me, have the challenges needed to be dealt with at such a high level.

And perhaps never with such urgency in the context of a global surge in GNSS expansion and modernization. China’s proposed new Compass system, the U.S. GPS III initiative, and restoration of Russia’s GLONASS are all scheduled (perhaps wishfully) to take place before or about the same time as Galileo is now expected to reach full operational capability around 2010–12.

PPP, a solution in the political environment of 1999, has now created a larger problem than the one it solved. Consequently, the public sector has a large responsibility for ensuring that it brings sufficient resources to resolve the current situation.

For years many in Europe have started referring to PPP as meaning, “Public Pays Private,” referring to the practical necessity for public subsidy — overt or covert — of an infrastructure that will ultimately pay for itself in the tax revenues generated by user equipment, services, and applications and not solely from revenues derived directly from the system itself.

For their part, the individual companies comprising the consortium need to resist the temptation of short-term competitive maneuvers or opportunistic national interests and reach an accord with the GSA.

Otherwise, without a two-sided exercise of self-discipline, it won’t matter which of these entangled entities gets its neck broken and which gets shot.

glen@insidegnss.com

GNSS Marketplace

I like the marketplace.

I really do.

I love the energy, the innovation, the diversity, the pricing mechanism of demand curves, the buyer-seller feedback loops, the promotional hoopla, the whole deal.

I like the marketplace.

I really do.

I love the energy, the innovation, the diversity, the pricing mechanism of demand curves, the buyer-seller feedback loops, the promotional hoopla, the whole deal.

In the rather remote town where I live, we have a Saturday Market that’s more than 30 years old. Every week a bunch of home-grown entrepreneurs set up their booths with pottery, tie-dyed shirts, fresh blueberries, metal sculptures, Guatemalan tamales, jewelry, everything imaginable. There’s colorful banners waving and musicians playing and aging hippies dancing.

And — just to let you know how far we are outside not merely the D.C. Beltway but outside the whole North American Free Trade Agreement zone — all the products sold in the market must be grown or made by the people selling them.

Of course, the market can’t do everything.

It apparently can’t produce enough bird flu vaccine to protect a nation or distribute enough food in the right place to prevent hunger in the midst of plenty. And one thing many people wouldn’t have expected the market to do was produce a multitude of global navigation satellite systems.

But it has, with GPS, GLONASS, Galileo, and perhaps even Beidou waiting in the wings.

And thank goodness for the marketplace, because it’s apparently going to take not merely a village, but a world to raise up a new generation of robust, modernized GNSS.

Fortunately, we have redundant programs, not merely redundant satellites. If one GNSS is delayed or changes its plan, another one is there to keep the parade moving forward. So, when Galileo sets itself hurdles, such as agreeing on not merely a concession contract but a business model as well, there’s GPS with its simple taxpayer-driven approach to keep things moving. And when GPS loses its way amid the engineering changes and chain-of-command silos of the military-industrial complex, there’s GLONASS speeding up its schedule in a race to the future.

Hopefully, by the time GLONASS encounters its next economic or technical or political challenge, Galileo and GPS will be back on track.

Yes, we definitely are in another “two-steps-forward-one-step-back” phase of GNSS development.

In an industrious and cooperative surge of activity, we have seen three sets of draft specifications reach fruition in the last few weeks: publication of a joint recommendation for design of new civil signals on GPS and Galileo, the Galileo Interface Control Document, and the GPS L1C interface specification.

Although further discussion and revisions are inevitable, these anxiously awaited items will provide guidance to GNSS product designers and encouragement to users.

Meanwhile, however, we once again have to wonder: what’s going on — or, more to the point, not going on — with the GPS program? In a short span of time, we’ve seen new delays in the first launch of a GPS Block IIF satellite (originally planned for 2005), a possible yearlong delay in GPS III, and a missed deadline for awarding an important contract on modernized user equipment for the Department of Defense (DoD) community.

Those developments underscore the need for implementing a recommendation in the Defense Science Board Task Force on GPS report: coordinate DoD responsibilities for GPS by designating a single focal point within the Office of the Secretary of Defense.

Currently, as with many other defense programs, the GPS program has three separate decision-making channels for acquisition, budget, and command. What in a well-managed organization might serve as supporting columns for a common endeavor have turned into self-contained silos sheltering processes, personnel, and purposes that become laws unto themselves. These parallel, non-convergent functions are producing asynchronous, discontinuous results.

Deputy Secretary of Defense Donald England has reportedly kicked the Positioning, Navigation, and Timing (PNT) Executive Committee meeting schedule into high gear; perhaps he’s the man to do the same for the PNT program itself.

glen@insidegnss.com

Speaking with Authority: Galileo’s Lead Agency in a Changing World

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.

Two for One: Tracking Galileo CBOC Signal with TMBOC

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.

Enhancing the Future of Civil GPS: Overview of the L1C Signal

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.