Tag: navigation and timing

eLoran: Part of the solution to GNSS vulnerability

Opposite and complementary

Though marvelous, GNSS are also highly vulnerable. eLoran, which has no common failure modes with GNSS, could provide continuity of essential timing and navigation services in a crisis.

GPS fits Arthur C. Clarke’s famous third law: “Any sufficiently advanced technology is indistinguishable from magic.” Yet, it also has several well-known vulnerabilities — including unintentional and intentional RF interference (the latter known as jamming), spoofing, solar flares, the accidental destruction of satellites by space debris and their intentional destruction in an act of war, system anomalies and failures, and problems with satellite launches and the ground segment.

Over the past two decades, many reports have been written on these vulnerabilities, and calls have been made to fund and develop complementary positioning, navigation and timing (PNT) systems. In recent years, as vast sectors of our economy and many of our daily activities have become dependent on GNSS, these calls have intensified.

A key component of any continent-wide complementary PNT would be a low-frequency, very high power, ground-based system, because it does not have any common failure modes with GNSS, which are high-frequency, very low power and space-based. Such a system already exists, in principle: it is Loran, which was the international PNT gold standard for almost 50 years prior to GPS becoming operational in 1995. At that point, Loran-C was scheduled for termination at the end of 2000.

However, beginning in 1997, Congress provided more than $160M to convert the U.S. portion of the North American Loran-C service to enhanced Loran (eLoran). In 2010, when the U.S. Loran-C service ended, its modernized and upgraded successor was almost completely built out in the continental United States and Alaska. During the following five years, Canada, Japan, and European countries followed the United States’ lead in terminating their Loran-C programs.

Today, however, eLoran is one of several PNT systems proposed as a backup for GPS.

The National Timing Resilience and Security Act of 2018 required the Secretary of the U.S. Department of Transportation (DOT) to “provide for the establishment, sustainment, and operation of a land-based, resilient, and reliable alternative timing system” as a backup to GPS. In January 2020, the DOT awarded contracts to 11 companies to demonstrate their technologies’ ability to act as a backup for GPS. Of these companies, two were working on eLoran projects.

Technical advisers to the federal PNT Executive Committee have been advocating and recommending that the government implement eLoran for the past 11 years. Yet, while the U.S. government announced in 2008, and again in 2015, its intention to build an eLoran system, it has not done so yet.

Not Your Grandfather’s Loran

In the 1980s, I used Loran-C to navigate on sailing trips off the U.S. East Coast. It had an accuracy of a few hundred feet and required interpreting blue, magenta, black and green lines that were overprinted on nautical charts. The system was a modernized version, launched in 1958, of a radio navigation system first deployed for U.S. ship convoys crossing the Atlantic during World War II. Its repeatability was greater than its accuracy: lobster trappers could rely on it to return to the same spots where they had been successful before, though they may have had some offset from the actual latitude and longitude.

By contrast, eLoran has an accuracy of better than 20 meters, and in many cases, better than 10 meters. It was developed by the U.S. and British governments, in collaboration with various industry and academic groups, to provide coverage over extremely wide areas using a part of the RF spectrum protected worldwide. Unlike GNSS, eLoran can penetrate to some degree indoors, under very thick canopy, underwater and underground, and it is exceptionally hard to disrupt, jam or spoof.

Unlike Loran-C, eLoran is synchronized to UTC and includes one or more data channels for low-rate data messaging, added integrity, differential corrections, navigation messages, and other communications. Additionally, modern Loran receivers allow users to mix and match signals from all eLoran transmitters and GNSS satellites in view.

Finally, eLoran can be used for integrity monitoring of GPS — and vice versa. “Think of a resiliency triad, consisting of GNSS (global), eLoran (continental), and an inertial measurement unit, a precise clock, or a fiber connection,” said Charles A. Schue, CEO of UrsaNav. “It is extremely difficult to jam or spoof all three sources at the same time, in the same direction, and to the same amount.”

For the eLoran system to cover the contiguous United States, between four and six transmission sites could provide overlapping timing coverage, and 18 transmission sites could provide overlapping positioning and navigation.
U.S. Developments

The INVEST in America Act authorizes $157 million for the Department of Homeland Security to conduct research in five separate areas, one of which is positioning, navigation and timing resiliency; however, none of this money is for eLoran per se. The regular DOT appropriation for next year has $17 million for PNT-related research, $10 million of which is for “GPS Backup/Complementary PNT Technologies Research.” However, neither of these bills has yet been finalized, let alone passed into law, so they may change.

“These are very complex systems, with five- to seven-year sales cycles,” pointed out Schue, “and the process is even slower now due to the pandemic. With adequate funding, eLoran signals could start becoming available in the contiguous United States within a year of a service contract being signed. We should recall that GPS — as, indeed all of the GNSS — was brought online gradually as satellites were developed and launched into space. There should be no expectation that any other nationwide system would be available at the flip of a switch instead of through gradual implementation.”

the former Loran-C transmission antenna at Værlandet, Norway. (Photo: UrsaNav)

International Developments

Loran-C and eLoran operate internationally. Saudi Arabia, China and Russia continue to operate Loran-C or Chayka systems. In October 2020, a Chinese paper described how the nation is expanding Loran to its west to cover the whole country to protect itself from disruptions of space-based services. A previously published report made it clear that they are upgrading or have upgraded from Loran-C to eLoran. South Korea has an ongoing project to upgrade its Loran-C to eLoran. It also seems the project will ensure that the South Korean system will be useable on its own, even if the Russian and Chinese systems with which it normally cooperates are not available for some reason, according to Dana Goward, president of the Resilient Navigation and Timing Foundation.

The United Kingdom is still committed to eLoran, and operates one station that has been used as an alternative time reference to GNSS. “However, as the sole station still transmitting in that area of Europe it’s of no use for positioning,” said Nunzio Gambale, CEO of Locata Corporation. “Unfortunately, the EU’s shutdown of their old Loran sites seems to have been completed, and no EU-based Loran sites remain operational. Their actions leave scant hope for Loran’s resurrection any time soon as an alternative to GNSS positioning in Europe. That’s a shame, because eLoran has beneficial PNT characteristics that other alternate technologies will struggle to replicate.”

A deck officer on a ship takes a relative bearing using a pelorus. Loran-C was developed in large part for maritime navigation. (Photo: aytugaskin/iStock/Getty Images Plus/Getty Images)

Advocacy

“There is fairly good agreement across the PNT community that there is no sole solution [to GPS vulnerabilities],” Schue said. “It needs to be a system of systems.”

The PNT community, he said, is working with Congress and the administration “to move ahead with actual RFPs to start the contracting process — instead of continuing to admire the problem.” UrsaNav, NextNav, OPNT and other companies and organizations “are working together as best as we can to tell the federal government that we all believe in a system-of-systems approach and that there ought to be some tangible forward motion.”

While DOT has the lead on providing PNT resiliency, it and the departments of Defense and Homeland Security need to cooperate on this, Schue argued. “Many, if not all, of the other departments — such as Commerce, Energy, State, Interior and Agriculture — also have a stake.”

GNSS will remain for a reason. “Unless a new national terrestrial PNT system moves the game forward for many markets, it’s just far too easy to remain with the GNSS system, which is fundamentally free,” Gambale said. “That’s a really difficult price point to compete with, unless you’re delivering significant new value to the market.”

The time to act is now. “This issue has been studied to death for more than 20 years,” Goward said. “There are technologies ready to deploy. It is time for action. A failure of national PNT will be catastrophic.”

November 3, 2021
NextNav demos GPS-free PNT network for critical infrastructure

The trial for the U.S. Department of Homeland Security showcases the precise, resilient timing capabilities of NextNav’s TerraPoiNT service in the event GPS is unavailable.

NextNav has successfully demonstrated the timing precision and resilience of its terrestrial positioning, navigation and timing (PNT) system, TerraPoiNT, in a recent evaluation by the Science and Technology Directorate of the Department of Homeland Security (DHS S&T).

The trial tested the timing redundancy of the TerraPoiNT system in a number of scenarios, including instances of GPS outages, spoofing and jamming. It validates TerraPoiNT’s capabilities as a terrestrial, GPS-free network capable of powering critical national infrastructure in the event of GPS failure.

During a simulated 72-hour GPS outage, the TerraPoiNT service was able to deliver a timing accuracy better than 50 nanosecond in urban and semi-urban environments, successfully meeting timing requirements for various applications including 5G networks, the synchronization of the power grid, and more.

In addition, TerraPoiNT provided precise timing and redundancy utilizing two alternate absolute timing sources — atomic clock (Cesium/Rb) and LEO satellite (Satelles).

“GPS is critical infrastructure, but it has its limitations,” said Ganesh Pattabiraman, co-founder and CEO of NextNav. “In working with DHS S&T, we’ve validated that TerraPoiNT can serve as an important backup to GPS and ensure the resilience and continuity of our nation’s most critical systems, including next-generation telecommunications networks, financial services, and power grids.”

Satelles STL

STL from Satelles was one of two alternate absolute timing sources for the trial. Available today on a global basis, STL is a service that provides alternative PNT independent of GPS, supporting PNT-reliant applications such as 5G communications networks, high-frequency trading in financial markets, and electrical grids throughout the United States and around the world.

“Satelles applauds NextNav for conducting a successful field demonstration of its resilient PNT service, and we were delighted to have played an instrumental part in the exhibition,” said Christina Riley, vice president of Commercial PNT.

NextNav’s selection of STL to help demonstrate their own technology’s operation in the absence of GPS was a natural fit. That’s because earlier this year the U.S. National Institute of Standards and Technology (NIST) confirmed STL as an accurate and reliable source for the wide-area delivery of Coordinated Universal Time independent of GPS/GNSS.

Previous TerraPoiNT evaluations

The successful trial builds on recent evaluations of TerraPoiNT conducted by independent bodies. Earlier this year, the Department of Transportation (DOT) evaluated 11 alternate PNT solutions, in which each was rigorously tested across applications and scenarios. As a result of the evaluation, the DOT named TerraPoiNT the best and only performing solution across all PNT categories.

Spartacus. In June, NextNav entered into a definitive merger agreement with Spartacus Acquisition Corporation in a transaction that would result in NextNav being listed on the Nasdaq. The transaction is expected to close late in the third quarter of 2021 or early in the fourth quarter of 2021, subject to satisfaction of customary closing conditions.

Image: kanawatvector/iStock/Getty Images Plus/Getty Images

August 16, 2021
Controversial GAO report on DOD nav webinar June 15

On May 10, the Government Accountability Office (GAO) released a report on navigation efforts n the Department of Defense (DOD). Among its findings were that the department regularly fails to establish validated navigation and timing requirements for systems and instead defaults to the capabilities of GPS. Also, that alternate positioning, navigation and timing (PNT) efforts are not prioritized, and sometimes actively opposed, by forces within DOD.

One of the report’s more startling recommendations is that the department change its focus from GPS as the central pillar of its PNT strategy going forward in favor of resilient technologies.

The Institute of Navigation (ION) and Resilient Navigation and Timing Foundation (RNTF) are hosting a webinar to discuss the report with two of its authors on June 15 at 11 a.m. ET. Two members of the Senate Armed Services Committee staff have also been invited as panelists.

After a panel discussion moderated by Dana A. Goward, president of the RNT Foundation, attendees will have the opportunity to ask questions of the panelists.

The event is open to the public, but attendees must register in advance.

June 4, 2021
A matter of time: Ensuring precise time and synchronization for critical infrastructure

Critical infrastructure services such as telecommunications, utilities, transportation and defense are of national strategic importance. The U.S. Cybersecurity and Infrastructure Security Agency (CISA) lists 16 such sectors considered vital for security. Presidential Policy Directive 21 (PPD-21): Critical Infrastructure Security and Resilience advances a national policy to strengthen and maintain secure, functioning and resilient critical infrastructure.

Together, positioning, navigation and timing (PNT) are necessary for the functioning of a nation’s critical infrastructure. However, ubiquitous use of GPS as the primary source of PNT information introduces vulnerabilities. CISA, through the National Risk Management Center, works with government and industry partners alike to strengthen the security and resiliency of the national PNT ecosystem in the U.S. In early 2020, Executive Order (E.O.) 13905 on Strengthening National Resilience through Responsible Use of Positioning, Navigation, and Timing (PNT) Services was signed to strengthen, through policy promotion, the responsible use of PNT services by government and infrastructure operators.

The following is a review of cost considerations and exploration of the three key elements for critical infrastructure that help to strengthen PNT, focused on synchronization and precise timing: redundancy, resiliency and security.

Evaluating Cost and Location

It is often hard for operators to justify the resiliency, redundancy and security costs associated with deploying these capabilities at every layer of the architecture. New timing and synchronization solutions and design choices are leading to the right cost structures to deliver robust and reliable solutions.

The dilemma between cost and solution type is typically related to which deployment location is considered. With the evolution of technologies such as the migration from SDH/TDM to Ethernet and the development of LTE/4G and 5G in mobile, the number of aggregation offices and, above all, of network access sites at the edge has exploded. This inevitably leads to devices becoming much smaller, typically 1U-rack mountable devices, and with a cost in line with the much smaller size of edge base stations (small cells and gNodeBs).

Operators are left with the question: What is the best way to provide redundancy, resiliency and security in this environment? There are two core levels to consider — the architecture level and design level.

Exploring Redundancy

Redundancy at the architecture level can be engineered with core functions at both ends of a deployment (east/west) with dual paths for directional redundancy and high-performance capabilities for efficient high-accuracy time transfer over the long haul for cost-effective distribution. The virtual Primary Time Reference clock (vPRTC) architecture is such an architecture-level solution.

Redundancy can also be considered in the device itself, where the design choices are critical. Smaller devices cannot realistically be cost-effectively designed with modular hardware redundancy. The innovation here is to offer software redundancy, so a distributed, low cost, efficient and high-performance distributed solution can be deployed. A hardware module is typically expensive for two reasons: cost, and because the redundant module takes the space of another module, typically for input and output ports.

Hardware module redundancy often leads to a tradeoff between adding redundancy and losing capabilities, such as a choice between 10-gigabyte Ethernet (GE) support or multi-band GNSS or other compromises if redundancy is enabled. On the other hand, with software redundancy no tradeoff is necessary. Redundancy can be introduced while preserving all existing capabilities; no inputs or outputs are eliminated, no multi-band GNSS capability is eliminated. Redundancy is introduced via a software upgrade; therefore, it does not remove any hardware. Hardware redundancy, however, means duplicating an existing module with a similar module inside the device; this new module takes the slot of an existing module, and the function of that existing module is lost when it is removed from the unit.

Figure 1 depicts a commonly deployed redundancy use case with two aggregation routers using virtual router redundancy protocol (VRRP).

Figure 1. Example of redundancy connectivity between the active and standby units. (Image: Microchip)

Software redundancy is a dual-unit scheme based on two reasonably priced devices, one active and the other on standby. It is more cost-effective for two reasons. First, it does not involve a costly device design with expensive hardware modules. Second, each unit (passive and active) keeps all of its capabilities compared to a hardware redundant design, which involves duplication of modules in the device, thus reducing the existing possible capabilities to host the redundant module.

Software redundancy provides total redundancy of the whole device because the active and standby units are the same. One hundred percent of the capabilities are redundant, including oscillator, GNSS receiver, ports and input/outputs. A hardware module is only redundant for its own features, not the rest of the unit.

Leveraging Resiliency

Resiliency at the architecture level is key to engineering the network so grandmasters in the deployment can be connected to each other. Some grandmasters are connected to GNSS as their source of time and frequency. It is key to connect these systems to other 1588 grandmasters to enable assisted partial time support (APTS) and to leverage key innovation such as automatic asymmetry correction (AAC).

AAC is a key (patented) differentiator in a resilient design that enables calibration of the different paths a PTP flow may use to/from upstream grandmasters, thus allowing for a backup in case GNSS fails at the location of a grandmaster. A backup path to an upstream grandmaster can guarantee uninterrupted and precise time and phase operation. This architecture makes sure that GNSS can be backed up by IEEE 1588 Precision Time Protocol (PTP) when GNSS is interrupted, with the best path being utilized.

The alternative architecture choice is virtual PRTC (vPRTC), which enables operators to leverage redundancy and resiliency via a chain of high-performance boundary clocks using PTP over long distances for high accuracy, typically over optical networks. This architecture reduces reliance on GNSS and uses PTP as its primary source of time and phase.

Figure 2 depicts an optical network deployment with a dedicated optical timing channel (OTC) for high-accuracy distribution of phase over long distances.

Figure 2. Optical network deployment with OTC. (Image: Microchip)

Resiliency at the device level starts with the right choice of an oscillator, from OCXO to atomic clock (Rubidium) — and is dependent on the location, use case and respective requirements for timekeeping holdover performance. Also, the choice of GNSS receiver is key. Some typically support a single frequency, yet ionospheric phenomenon can create significant time delays during cyclical events such as solar storms. To mitigate such delays, a multi-band GNSS receiver is required.

Figure 3 depicts a comparison between single-band and multi-band time delays due to ionospheric effects and shows how multiband clearly mitigates the time error as highlighted in red.

Figure 3. Comparative ionosphere phenomenon. Source: https://www.gsc-Europa.eu/system/files/galileo_documents/Galileo-OS-SDD.pdf. (Image: Microchip)

GNSS satellites transmit time information in several frequency bands. The delay difference between signals at different frequencies provides information about ionospheric impact on the absolute delay. This enables multi-band GNSS receivers to compensate for delay variations of radio signals transmitted from the satellite to the receiver. Embedding a multi-band receiver mitigates these time delays, which is critical for applications requiring Primary Reference Time Clock class B (PRTC-B),40 ns, as well as enhanced PRTC (ePRTC) 30 ns.

These device design choices are equally important. The GNSS receiver can be embedded inside the unit on the main board, or it can be offered as a hardware module, often at an additional cost, and may impact and replace an existing module that needs to be ripped and replaced. It may be preferable to have the unit enabled with a multi-band receiver and have the multi-band capability turned on via a license as opposed to offering a multiband option on a hardware module, as this becomes a tradeoff with other important capabilities.

Evaluating Security

Security is of utmost importance. Authentication and authorization via standard mechanisms such as Terminal Access Controller Access Control System + (TACACS+) and Remote Authentication Dial-In User Service (RADIUS) provide the benefit of a standard security framework. In addition, two-factor authentication (2FA) is an extra layer of protection used to ensure the security of accounts beyond just a username and password.

Also, it is key to provide Secure Shell (SSH) extensions with various levels of security profiles to offer more granularity for the types of users and related access rights and limitations. Offering high-security profiles provides for the definition and enforcement of the most stringent access rules to the system. Scripting vulnerabilities and relevant Common Vulnerabilities and Exposures (CVE) need to be addressed to make sure all potential security holes are being reviewed and addressed.

Plus, evolving jamming and spoofing threats need to be part of the precise time security strategy and implementation via monitoring of signals and consistency checks and remediation. Automatic gain control (AGC) and other metrics can be leveraged to provide thresholds with interpretation of results, as well as mitigation actions when encountered.

Final Decision Making

To ensure continued performance, it is critical to make the right architecture choices. A thorough network engineering study should include the locations where grandmaster units need to be deployed and their performance and accuracy requirements. These steps will guide which types of precise time and synchronization devices need to be selected

In addition, network planners and synchronization engineers should pay careful attention to design choices such as fanless devices versus devices that require a fan, modular hardware redundancy versus software redundancy, and the related advantages in terms of cost and tradeoffs — as well as similar choices regarding embedded or modular GNSS.
These choices can lead critical infrastructure operators to deploy redundancy, resiliency and security at all layers.

For architecture choices and solutions, visit vPRTC..White papers on this topic and others are also available. Additional information on devices and redundancy software schema is here.

Eric Colard is head of Emerging Products, Frequency & Time Systems at Microchip. He leads the product line management for Microchip’s TimeProvider 4100 and Integrated GNSS Master solutions for the telecom, utility and other industries.

March 22, 2021
New 2-book set explores 21st-century PNT
By Jade Morton,
Guest Author

After more than five years of hard work by 131 authors from 18 countries, the new book set Position, Navigation, and Timing Technologies in the 21st Century (PNT21) is finally ready to meet readers.

Published by Wiley-IEEE Press, PNT21 offers a uniquely comprehensive coverage of the latest developments in the field of PNT by world-renowned experts. The two-volume set contains 64 chapters organized into six parts.

Position, Navigation, and Timing Technologies in the 21st Century
Integrated Satellite Navigation, Sensor Systems, and Civil Applications
Y. Jade Morton, Frank van Diggelen, James J. Spilker Jr. and Bradford W. Parkinson, editors; Sherman Lo and Grace Gao, associate editors
Publisher: Wiley-IEEE Press
Hardcover Publication Date: January 2021
Vol. 1: ISBN: 978-1-119-45841-8, 1288 Pages
Vol 2: ISBN: 978-1-119-45849-4, 912 Pages

Volume 1 focuses on satellite navigation systems, technologies, and applications. It starts with a historical perspective on GPS and other related PNT development.

Part A consists of 12 chapters on fundamentals of and latest developments in global and regional satellite navigation systems (GNSS and RNSS), the need for their coexistence and mutual benefits, signal quality monitoring, satellite orbit and time synchronization, and satellite- and ground-based augmentation systems that provide information to improve the accuracy of navigation solutions.

Part B contains 13 chapters on recent progress in satellite navigation receiver technologies such as vector processing, assisted and high sensitivity GNSS, precise point positioning (PPP) and real time kinematic (RTK) systems, direct position estimation techniques, and GNSS antennas and array signal processing. Also included are the challenges of multipath-rich urban environments, handling spoofing and interference, and ensuring PNT integrity.

Part C finishes the volume with eight chapters on satellite navigation for engineering and scientific applications. A review of global geodesy and reference frames sets the stage for discussions on the broad field of geodetic sciences, followed by a chapter on GNSS-based time and frequency distribution. One chapter each is dedicated to severe weather, ionospheric effects and hazardous event monitoring. Finally, comprehensive treatments of GNSS radio occultation and reflectometry are provided.

This simplified block diagram of a modern GNSS receiver — one of many illustrations in the book set — appears in Chapter 14, “Fundamentals and Overview of GNSS Receivers,” by Sanjeev Gunawardena and Y. Jade Morton. See excerpt below. (Image: Wiley-IEEE Press)

Volume 2 addresses PNT using alternative signals and sensors and integrated PNT technologies for consumer and commercial applications. An overview chapter provides the motivation and organization of the volume, followed by a chapter on nonlinear estimation methods which are often employed in navigation system modeling and sensor integration.

Part D provides seven chapters devoted to using various radio signals-of-opportunity transmitted from sources on the ground, from aircraft, or from low Earth orbit (LEO) satellites for PNT purposes.

In Part E, eight chapters cover a broad range of non-radio frequency sensors operating in passive and active modes to produce navigation solutions, including MEMS inertial sensors, advances in clock technologies, magnetometers, imaging, lidar, digital photogrammetry, and signals received from celestial bodies.

A tutorial-style chapter on GNSS/INS integration methods is included in Part E. Also included are chapters on the neuroscience of navigation and animal navigation.

Finally, Part F presents a collection of contemporary PNT applications such as surveying and mobile mapping, precision agriculture, wearable systems, automated driving, train control, commercial unmanned aircraft systems, aviation, satellite orbit determination and formation flying, and navigation in the unique Arctic environment.

Table of Contents

Volume 1: Satellite Navigation Systems, Technologies, and Applications
- Part A: Satellite Navigation Systems
- Part B: Satellite Navigation Technologies
- Part C: Satellite Navigation for Engineering and Scientific Applications
Volume 2: Integrated Navigation Systems, Technologies, and Applications
- Part D: Position, Navigation, and Timing Using Radio Signals-of-Opportunity
- Part E: Position, Navigation, and Timing Using Non-Radio Signals-of-Opportunity
- Part F: Position, Navigation, and Timing for Consumer and Commercial Applications
Collective Goal. Because of the diverse authorship and topics covered in PNT21, the chapters were written in a variety of styles. Some offer high-level reviews of progress in specific subject areas, while others are tutorials. A few chapters include links to MatLab or Python example code as well as test data for readers who desire hands-on practice.

The collective goal is to appeal to industry professionals, researchers and academics involved with the science, engineering and application of PNT technologies. The website pnt21book.com provides downloadable code examples, data, homework problems, select high-resolution figures, errata and a way for readers to provide feedback.
Jade Morton is a professor at the University of Colorado Boulder and director of the Colorado Center for Astrodynamics Research (CCAR).

Jade Morton is a professor at the University of Colorado Boulder and director of the Colorado Center for Astrodynamics Research (CCAR).

Excerpt from PNT21

14.1 Anatomy of a GNSS Receiver

Irrespective of the receiver type, the functionality of all GNSS receivers can be broken down into three major blocks: RFFE, baseband processor (BBP), and system processor (SP). In the literature, the term “baseband processor” may be used to refer to the combination of both the BBP and SP defined here. The general anatomy of a GNSS receiver is shown in Figure 14.3.

The RFFE converts the signals induced at one or more antennas into digitized sample streams. Depending on the application and market segment, data rates for these streams may be as low as 0.4 Mbytes/s (e.g. L1 band sampled at 3.5 MSPS and 1-bit sampling in an asset tracking device) to greater than 3 GB/s (e.g. L1 and L2 bands sampled at 60 MSPS and 16 bits across seven elements in an anti-jam military GPS receiver).

The BBP performs digital signal processing to acquire and track GNSS signals present in the digitized sample streams to produce raw GNSS observables for each visible satellite. These observables include time of transmission (TOT), accumulated Doppler Range (ADR), signal quality metrics such as carrier-to-noise density ratio (C/N0), in-phase and quadrature prompt correlator output (I/Q), and raw symbols of a GNSS signal’s broadcast navigation message (which are subsequently decoded). In addition, modern receivers typically perform varying degrees of situational awareness processing to monitor in-band interference such that a level of confidence can be assigned to these raw observables. Some advanced receivers have the ability to identify spoofing signals. Depending on the application, situational awareness outputs may be as rudimentary as the automatic gain control (AGC) voltage used to adjust front-end amplification or as sophisticated as spectrogram, histogram, and sample statistics for all streams evaluated at full sample precision.

The BBP also contains a counter that is driven by a digital clock signal that is phase-locked to the receiver’s reference oscillator. This counter is the basis for the receiver’s clock and is used to generate time-of-reception (TOR) epochs. Raw observables for all satellites in view that lead to range measurements are computed with respect to TOR epochs. Since the receiver clock is based on its reference oscillator, it drifts with respect to GNSS system times. Although possible, the frequency bias, drift, and drift rate of the reference oscillator are typically not adjusted to align with GNSS system time because dynamic adjustment of the oscillator can lead to instabilities. Instead, these parameters are estimated and used to drive a separate adjustable-rate counter that compensates for the reference oscillator errors. This forms the basis for GNSS disciplined oscillators.

It is possible to partition all baseband processing into two categories: sample processor (SMP) and reduced-data processor (RDP). The SMP performs high-rate but simple and algorithmically regular operations which largely comprise multiply-accumulate operations performed at the sample rate. The SMP may also contain configurable timers and pulse/event generators that determine sample processing intervals, as well as output precise timing pulses that are synchronized down to the nanosecond level with respect to GNSS system times (timing accuracy and precision are dependent on the application and market segment). The RDP performs low-rate but algorithmically complex operations. Some representative software functions running within the RDP are illustrated in Figure 14.3.

Bidirectional communications occur between the SMP and RDP at regular timed intervals corresponding to a kilohertz rate. This rate is easily handled by all modern microprocessors. Since these SMP/RDP transactions are time critical, the RDP runs either bare-metal code (i.e. no operating system) or a real-time operating system. The operations within the BBP are inherently parallel and largely independent of each other at the signal processing level. Some coupling occurs, for example‚ in code-carrier aiding, inter-frequency aiding (see Chapter 15), inter-satellite aiding (referred to as vector tracking, described in Chapter 16), and multi-element processing. However, this coupling is typically implemented at higher levels of abstraction. Modern multi-band and multi-constellation receivers are capable of tracking hundreds of GNSS signals simultaneously. To facilitate this highly complex command and control structure – which also needs to be dynamically scalable and adaptive depending on the number of satellites in view, environmental conditions‚ and operating modes – the control architecture is typically layered (i.e. hierarchical). Control at the individual signal acquisition and tracking layers is performed using simple configurable finite state machines (FSMs) whose state transitions are based on signal condition indicators such as code lock, phase lock, C/N0, and code-carrier divergence (CCD). These FSMs operate independently but are typically managed at a high level by the SP.

The SP takes the raw signal observables produced by the BBP and transforms them to the standard GNSS receiver measurements. These measurements include pseudorange (PR), accumulated Doppler range (ADR), carrier phase (CP), carrier Doppler, and C/N0. All modern GNSS receivers also compute position, velocity, and time (PVT) at configurable rates (1 to 100 Hz depending on the receiver type). The SP encodes these in one or more industry-standard data formats for distribution. These formats include Receiver Independent Exchange Format (RINEX), the National Marine Electronics Association (NMEA) format, the Radio Technical Commission for Maritime Services (RTCM) format, and vendor-specific proprietary binary formats.

The SP also performs all high-level functions that include receiver initialization, channel management, and user interface functions. Unlike the BBP, the operations within the SP are generally not time critical. In modern GNSS receivers, the SP is often an embedded computer running an advanced non-real-time operating system. It may also support modern data interfaces (wired USB and Ethernet, or wireless/cellular connectivity) and an advanced graphical user interface with touchscreen support. While too numerous to mention, representative software processes running within the SP are illustrated in Figure 14.3.

Although not shown in Figure 14.3, modern receivers (or the navigation system to which they are interfaced) may also support aiding from external sensors such as inertial measurement units (IMUs), magnetometers, inclinometers, barometers, wheel sensors, RADAR, lidar, infrared (IR), and electro-optical (EO) sensors. This external aiding to GNSS can occur at three levels: loose coupling (position level), tight coupling (measurement level), or ultra-tight coupling (sampled signal processing level). GNSS aiding using various non-GNSS sensors is described in Chapters 43–51 in Volume II, Part E.

As shown in Figure 14.3, a stand-alone GNSS receiver contains battery-powered low-power circuitry to keep track of absolute time while it is turned off. A real-time clock (RTC) driven by a low-power crystal oscillator accomplishes this task. In some cases, this crystal may be the same as the reference oscillator. Knowledge of absolute time, along with the last known location and previously decoded almanac/ephemeris data stored in the receiver’s non-volatile memory, allows it to estimate satellites in view and their Doppler offsets, thereby significantly reducing the TTFF: the time needed to acquire satellites and produce the initial PVT solution. In the case of modern military receivers such as M-Code, or subscription-based services such as the Galileo Public Regulated Service (PRS), the receiver must acquire the cryptographically generated spreading code that may never repeat. In this case, the initial time uncertainty has a significant impact on the acquisition search space and consequently the computational resources consumed by the acquisition engine as well as power consumption. The TTFF can be dramatically reduced when absolute time, the satellites in view, their Doppler frequencies, and ephemerides are sent to the receiver from a nearby reference station via a communications link. This describes the basis of Assisted GNSS (A-GNSS) technology, covered in Chapter 17 of this book.

In some respects, the reference oscillator can be considered the single most important component that affects GNSS receiver performance. Although the PVT solution estimates the deterministic components of the reference oscillator’s frequency error (i.e. short-term bias, drift, and drift rate), the stochastic component cannot be estimated and hence represents additional dynamics that must be tracked (i.e. in addition to satellite motion, user motion, satellite clock motion, and any ionospheric scintillation and multipath). The bandwidth of the carrier tracking loops must be increased to accommodate this close-in phase noise of the reference oscillator. This in turn increases the variance of the range measurements. The reference oscillator is also the only “moving part” in the receiver since it is based on the resonance of a quartz crystal or microelectromechanical systems (MEMS) structure. In addition to microphonics, which are small phase variations that may occur within the RFFE due to external forces (particularly if the RFFE comprises large discrete components), these forces couple through the resonating element leading to shock and vibration sensitivity [6]. Similarly, thermal expansion of the crystal as well as analog components in the RFFE due to changing ambient temperature, unless appropriately compensated or isolated, causes temperature sensitivity. The frequency synthesizer in the RFFE multiplies the oscillator phase noise and dynamics by the ratio of the synthesizer output frequency to the oscillator fundamental frequency, thus placing a significant short-term stability requirement on the reference oscillator. Oscillator short-term stability limits the coherent integration time, which is proportional to the processing gain. Hence, the quality of the reference oscillator directly impacts the recever’s attainable sensitivity (i.e. the minimum observable signal levels) as well as the rate at which it can output statistically independent measurements. Oscillator effects are covered in detail in Chapter 47.

The receiver intelligence process within the SP shown in Figure 14.3 performs functions such as determining what satellites are in view, how best to mitigate any in-band interference (as observed by the situational awareness indicators), dynamically adapting to varying operating conditions, determining the best set of range measurements to use for the PVT solution based on optimum satellite geometry and estimated range error metrics indicated by C/N0 (for signal blockage) and CCD fluctuations (for multipath and ionospheric effects), and many such highly complex decisions. Typically, these high-level functions occur at a lower rate such as 1 Hz or less. To a large degree, the level of sophistication and engineering embedded within the receiver intelligence block, as well as the other low-level control functions determines the receiver’s performance in the real world, as expressed by established figures of merit. These include measurement accuracy, update rate, TTFF, sensitivity, dynamics handling capability, multipath mitigation performance, interference detection and mitigation capability, receiver autonomous integrity monitoring, and fault detection and exclusion (see Chapter 23). In other words, for a given market segment and its associated SWaP-C constraints, the receiver’s hardware and available signal processing capabilities can only do so much. The rest, and quite often the attributes that distinguish it in the marketplace, lies within the hundreds of thousands of person-hours and centuries of combined experience baked into its sophisticated software/firmware.
February 1, 2021
Orolia to host industry discussion on PNT Executive Order

GPS jamming and spoofing threats are national priorities

Orolia will host an industry discussion online at 2 p.m. ET on May 7 on the Positioning, Navigation and Timing (PNT) Executive Order. Register here for the informal PNT Coffee Talk.

The PNT Coffee Talk event will be held online and is open to those interested in learning more and discussing this national priority from the industry and government perspectives.

The Executive Order, issued Feb. 12, is a federal initiative to protect critical infrastructure from GPS/GNSS jamming and spoofing threats, and other disruptions to critical PNT services. These threats can deny access to, or compromise, essential PNT data that critical infrastructure needs to operate.

This initiative, titled “Executive Order on Strengthening National Resilience through Responsible Use of Positioning, Navigation and Timing Services,” includes the following critical infrastructure: defense, homeland security, transportation, power grids, communications/mobile, precision agriculture, weather forecasting and emergency response.

Key actions include assessing jamming, spoofing and other PNT disruption risks across these critical infrastructure programs, developing customized PNT profiles to address risks, and deploying Resilient PNT technology to increase resilience.

Survey seeks feedback. Orolia asks those interested to share questions and comments and let the company know what you’d like to discuss during this PNT Coffee Talk event through an anonymous survey.

April 6, 2020
Working together for a more navigable world

“Diverse teams bring diverse ideas to the table, and that’s the best way to progress.”

So said Professor Sheila Rowan, the UK government’s chief scientific advisor to Scotland, opening the Royal Institute of Navigation’s 2019 International Navigation Conference. Professor Rowan’s comments set the scene perfectly. Success in navigation is no longer about just getting a fix, or even an accurate fix. To succeed as a system or application provider, diversity and collaboration are key, whether it be multiple disciplines and the skills that go with them, or a mix of ages, beliefs and backgrounds. So, what were some key messages to emerge from four days of working together?

John Pottle opens the 2019 International Navigation Conference sponsored by the Royal Institute of Navigation (RIN). (Photo: RIN)

More practical help for non-experts wanting to improve resilience in positioning, navigation and timing (PNT) is needed. The top request from delegates at the pre-conference short course was for more detailed and specific information on threats to PNT. Of particular interest were how to measure the impacts and test the merits of various mitigation approaches. In other words: how to assess risk? How to decide what steps to take?

User acceptance and regulatory/legal structures for driverless vehicles are greater challenges than the positioning and communications technology. In the UK and across Europe, projects are under way to evaluate good practices for so-called “beyond line of sight” drone flights. For driverless cars, while huge strides have been taken to enable secure and resilient absolute and relative positioning, much remains to be done. Practical issues were highlighted, such as over-cautious vehicles and a tendency for driverless cars to make occupants feel more travel sick. So work needs to be done to avoid a stressfully slow and sickly experience.

Skills and knowledge are changing — and education/training needs to, too. A major developed-world problem is that the experts with experience who have seen generation after generation of technology evolution are now in their later careers or retired. Because of the wealth of knowledge vested in these individuals — we can all think of some, I’m sure — organizations have tended to over-rely on them. A key theme of the conference closing plenary was that the community wants to do more to collaborate — that word again — to define training needs and figure out how to deliver the skills that are needed today and tomorrow.

The next couple of years bring fewer, bigger navigation conferences in Europe. The European Navigation Conference (ENC) 2020 takes place in Dresden, May 11–14, organized by the German Institute of Navigation, DGON. ENC2021 will be combined with the triennial global congress of the International Association of Institutes of Navigation (IAIN), Nov. 15–18, 2021, in Edinburgh, organized by the Royal Institute of Navigation.

Please save the dates — joining these events is rewarding and stimulating as we work together toward a more navigable world.

John Pottle is director of the Royal Institute of Navigation.

January 15, 2020