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Airbus satellite design passes important project milestone, preparing for industrialized manufacturing concept

Airbus has successfully completed the preliminary design review (PDR) for its system concept for the second-generation Galileo navigation satellites. During this important milestone, Airbus’ proposed preliminary design and the customer’s system requirements have been fully reviewed and agreed upon. Galileo is managed and funded by the European Union.

This milestone paves the way for further verification, acceptance and qualification at the equipment and module levels. Verification at the payload level is already in full swing, with the critical design review (CDR) for the satellite structure due shortly.

In parallel, the Airbus site in Friedrichshafen, on Lake Constance, is preparing for an industrialized production line for six second-generation Galileo satellites. The satellite integration center is being upgraded to meet requirements for these satellites.

Airbus is bringing to the project more than 200 highly skilled space engineers. The first Galileo second-generation satellites are expected to launch in 2024.

The second-generation Galileo satellites will make the Galileo service more accurate, secure, dependable and adaptable. Weighing 2.3 tons, each satellite is designed to operate for about 15 years. The all-electric medium-Earth-orbit (MEO) platform from Airbus reuses building blocks from the company’s telecoms and Earth observation programs. The flexible and modular navigation payload is also based on telecom elements for beam forming and signal generation.

MetaGeo has launched a geographic information system (GIS) platform to enable organizations of all sizes to host, analyze, find and share 3D map datasets among any internet-capable devices.

The platform processes location-based map or sensor data from the real world, combines it into a single 3D virtual environment, and streams it to any device or mapping platform.

The affordable and easy-to-use platform can load data from multiple sources: satellites, drones, mobile devices, public and crowdsourced repositories, internet of things (IoT) sensor data, 3D models and topographic maps.

The data is then processed by the MetaGeo platform into a 3D world and streamed to any internet-connected device, enabling live collaboration between the office and field via mobile or AR device. A plug-in software development kit (SDK) allows for third-party tools to scale and fit user needs.

Applications include academia, architecture, engineering, construction, energy, natural resource management, environmental monitoring, utilities and public safety. Uses include planning and managing construction sites, organizing layouts of events, creating maps for public safety, and visualizing inspection imagery from drones and mobile devices.

Pix4D is now offering a real-time kinematic (RTK) rover for use with iOS devices.

The Pix4D viDoc RTK handheld rover attaches to iOS devices to bring RTK accuracy to terrestrial scanning on iPhones and iPads.

When paired with the PIX4Dcatch mobile app, the viDoc rover can replace survey tools such as RTK GNSS rovers and terrestrial scanners, the company said.

Together, the products create a workflow that turns iPhones or iPads into an accurate terrestrial scanning device, with centimeter-accurate RTK positioning from an existing NTRIP network.

The tools can be used to 3D model small areas or structures.

By Peter Steigenberger, Steffen Thoelert, Sergei Yudanov and Markus Ramatschi

The Japanese QZS-1R satellite was launched on Oct. 26, 2021, from the Tanegashima Space Center in Japan. It serves as a replenishment for QZS-1, the first spacecraft of the Japanese Quasi-Zenith Satellite System (QZSS) in orbit since September 2010.

QZS-1R joins the current QZSS constellation of three satellites in inclined geosynchronous orbit (IGSO) and one geostationary satellite. These four Block I satellites transmit the L1C/A signal at 1575.42 MHz.

QZS-1R, as well as future QZSS satellites, are able to transmit the new L1C/B signal. L1C/B is based on the same family of gold codes as L1C/A, but uses a binary offset carrier (BOC) modulation instead of the binary phase-shift keying (BPSK) and a different PRN range (203–206).

Compared to BPSK, the BOC modulation adds a square wave subcarrier with a frequency of fsc = 1.023 MHz that equals the chipping rate of the ranging code. This subcarrier shifts the peak spectral energy from the center frequency fL1 to fL1 ± fsc to reduce interference with the GPS L1C/A signals.

During in-orbit testing (IOT) from late November until early December 2021, QZS-1R transmitted L1C/A and L1C/B signals intermittently. FIGURE 1 shows a spectrum of the L1-band transmissions of QZS-1R recorded on Nov. 25 with the 30-meter dish antenna of the German Space Operations Center in Weilheim, Germany, as well as a spectrum of QZS-2 recorded in July 2017.

During IOT, QZS-1R had an extremely low maximum elevation of 0.8° in Weilheim. Due to technical restrictions for such low elevations, QZS-1R had to be observed with a sidelobe of the 30-meter antenna. As a result, the respective observations are much more noisy than the QZS-2 reference data.

Nevertheless, the different spectral characteristics of L1C/B and L1C/A can be clearly seen in FIGURE 1: L1C/B has two maxima at 1574.4 and 1576.5 MHz due to the BOC modulation, whereas the BPSK L1C/A signal has one maximum at the center frequency of 1575.42 MHz.

GNSS receivers of the International GNSS Service (IGS) started to track L1C/A, L1C, L2C and L5 signals of QZS-1R on Nov. 17. Aside from the regular PRN code J04, test signals using the non-standard code PRN J06 were intermittently transmitted by QZS-1R during the IOT and tracked by these receivers.

Based on the public specification of the new L1C/B signal, Javad GNSS developed a prototype firmware that enabled tracking of this signal during the early transmissions. This firmware was installed on a Javad TRE-3 receiver operated by GFZ German Research Centre for Geosciences at its IGS station WUH200CHN in Wuhan, China.

FIGURE 2 illustrates the noise and multipath characteristics of different QZS-1R pseudorange measurements. It is based on the so-called multipath linear combination of L1 pseudorange and L1/L2 carrier-phase observations covering a six-hour data arc. RMS values were computed for 5-degree elevation bins for each pseudorange signal. While the individual signals were tracked on different days of the IOT and the associated results have to be interpreted with care, the data indicate a very similar ranging performance of the legacy C/A signal and the new C/B signal. Best results are obtained with the L1C signal, which uses both a higher signal power and an advanced modulation with superior multipath suppression.

QZS-1R will resume continuous transmission of L1C/A as soon as declared healthy. The transition from L1C/A to L1C/B is planned for 2023-2024, when an operational QZSS constellation of seven satellites is reached. The launches of the IGSO satellite QZS-5, the geostationary QZS-6, and the quasi-geostationary QZS-7 are all planned for 2023.

Also see Directions 2022: Now 3 years old, QZSS hits its stride.

Manufacturers

GNSS data used in this article were collected with a Javad GNSS TRE-3 receiver. The spectral overviews were captured with a Rohde & Schwarz FSQ26 signal analyzer.

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Peter Steigenberger is a senior scientist at the German Space Operations Center of the German Aerospace Center (DLR), where he conducts research in the field of new satellite navigation systems.

Steffen Thoelert is an electrical engineer at DLR’s Institute of Communications and Navigation. His research activities focus on signal-quality monitoring and satellite payload characterization.

Sergei Yudanov is a senior firmware developer at Javad GNSS, Moscow. His main field of activity is GNSS signal processing.

Markus Ramatschi is a senior scientist at the Helmholtz Centre Potsdam, GFZ German Research Centre for Geoscience. He is operating a global GNSS reference station network.

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Further Reading

Cabinet Office, Quasi-Zenith Satellite System Interface Specification: Satellite Positioning, Navigation and Timing Service, IS-QZSS-PNT-004, Jan. 25, 2021.

Ramatschi M., Bradke M., Nischan T., Männel B. (2019): “GNSS data of the global GFZ tracking network,” vol 1. GFZ Data Services. https://doi.org/10.5880/GFZ.1.1.2020.001

Thoelert S., Hauschild A., Steigenberger P., Montenbruck O., Langley R. (2017), “QZS-2 signal analysis, QZS-3 launched.” GPS World 28(9): 10–14,

By Satoshi Kogure
Director, National Space Policy Secretariat Cabinet Office, Japan/QZSS Strategy Office

At 02:19:37 UTC on Oct. 26, 2021, a new satellite in the QZSS constellation — QZS-1R — was launched from the Tanegashima Space Center in Japan. It is the fifth satellite in the constellation and the replacement of the first satellite, launched in September 2010. 

As of December 2021, initial on-orbit testing (IOT) and tuning of the precise orbit determination (POD) function in the ground control segment was ongoing. QZS-1R is the first QZSS satellite that will transmit the L1 C/B signal, splitting the power spectrum at the L1 center frequency by adopting BOC modulation on the existing C/A signal, to mitigate interference into the GPS L1 C/A signal. C/B signal transmission was verified during the IOT phase. QZS-1R will transmit the C/A signal continuously until QZS-5, 6 and 7 are launched and the noise floor increased. 

The launch of QZS-1R was a milestone toward a sustainable national infrastructure for Japan. The Japanese government’s Cabinet Office (CAO) is trying to establish more secure positioning, navigation and timing (PNT) services by deploying seven satellites for the QZSS constellation. It will add three satellites to the current four around 2023. 

This will give QZSS an independent PNT capability and enhance GNSS performance as well as robustness, and cover a broader area in the Asia Pacific region. CAO is still investigating the future of the QZSS constellation, including its final configuration and how to provide assured PNT services corresponding to future user requirements. However, it is thought that the full operational capability for Japan at minimum may be declared after the completion of the initial seven-satellite constellation. 

Today, QZSS is providing ranging signals on L1C/A, L1C, L2C and L5 for all users able to acquire and track those signals. Those signals have the same RF properties and almost the same message format as the corresponding GPS signals — they are interoperable. 

In addition, a unique characteristic of QZSS is that it transmits error correction messages available in Japan on separate channels — L1S, L1Sb and L6 — from those used to broadcast its ranging signals. Messaging functions are also provided through QZSS L1S and S-band two-way communication links on QZS-3 in support of disaster mitigation and relief operations in Japan. 

CAO launched the QZSS operational services using a four-satellite constellation on Nov. 1, 2018. Its first three years of operation have provided much knowledge to improve their performance. The averaged signal-in-space user ranging error, a 95 percentile daily statistic, has been improved and achieved less than 1.0 meter, while the specification is 2.6 meters; the best daily value in the evaluation period (Aug. 31, 2020 to Sept. 1, 2021) was less than 0.5 meter for QZS-1, 2 and 3. 

This remarkable improvement was shown on the Centimeter Level Augmentation Service (CLAS). According to the original design of CLAS, transmitted error corrections were for only 11 satellites in the GPS, QZSS and Galileo constellations. After two years of initial operation, we updated the ground control segment for CLAS to increase the number of augmented satellites from 11 to a maximum of 17. This increase in the number of satellites with error corrections leads to excellent improvement of CLAS performance in more challenging user environments such as urban and mountainous areas.  

To improve the service performance further and measure new observables for satellite orbit clock estimation, inter-satellite ranging and two-way ranging functions between tracking station and satellite will be developed for QZS-5 to -7 and following satellites. The ground control segment will also be updated.

Multi-GNSS ADvanced Orbit and Clock Augmentation (MADOCA) precise point positioning (PPP) will be implemented as a practical service no later than 2024. It is aiming to provide decimeter-level PPP service with broadcast of globally available satellite orbit and clock error corrections as well as code-phase and carrier-phase biases. 

PPP has a well-known disadvantage: long convergence time. By using the marginal L6D channel on QZS-5 to -7, the ionospheric delay correction for wide area will be distributed. CAO will try to evaluate how such ionospheric correction could reduce the initial convergence time for the PPP calculation. In an experiment planned in collaboration with Asian Pacific countries, regional stations in the nationwide CORS network will be used for generating such corrections. 

Early or Emergency Warning Service (EWS) is also expanding its service coverage into the region. The common EWS format is being jointly investigated by India, the European Union and Japan under the UN-ICG framework. The QZSS EWS for the Asia Pacific region through the L1S signal on QZS-1R, 2, 3 and 4 will be established after completion of a ground segment update around 2024. 

Also see First Transmission of L1C/B by QZS-1R.

The PureL5 Customer Evaluation System is being tested by California and Chinese companies

OneNav has announced the commercial availability of its pureL5 GNSS digital IP core.

The pureL5 digital IP core’s architecture enables it to directly acquire and track L5 signals from GPS, Galileo, BeiDou, QZSS and GLONASS without any L1 aiding. This eliminates the entire L1 RF chain, saves space on the printed circuit board, and simplifies the RF front-end and antenna subsystem in smartphones, wearables and trackers.

The pureL5 digital IP core’s massively parallel array processor searches the entire 1-millisecond L5 code space in parallel, delivering 1 second time to first fix (TTFF). The pureL5 digital IP core is 0.28mm² in the 3-nm semiconductor process and consumes 4.7 mW of power in 1-Hz tracking mode.

OneNav has delivered the pureL5 digital IP core register-transfer level (RTL) to its first system-on-chip (SOC) customer. IP core RTL verification and physical implementation are complete, and oneNav’s SOC licensee will tape out in the first quarter of this year. The pureL5 digital IP core RTL is available for customer licensing and shipment now.

Customer Evaluation System. OneNav’s pureL5 Customer Evaluation System is being tested by companies in California and China. The system is available for smartphone and wearable OEMs and SOC providers who want to evaluate oneNav’s pureL5 in the field and the lab.

PureL5 GNSS Features

• Smaller footprint than L1+L5 hybrids, simplifying implementation in highly space-constrained devices such as 5G smartphones and wearables
• Lowers bills of material (BOM) cost and simplifies the RF front-end and antenna subsystem by eliminating the entire L1 RF chain
• No L1 aiding required: directly acquires L5/E5/B2 with 1-second TTFF
• Less software complexity, simplifies RF coexistence engineering
• Better interference resiliency
• Scalable IP signal processing core is semiconductor process-node independent
• Multi-constellation L5: Beidou, Galileo, GPS, QZSS, GLONASS.

Net Insight’s sync solution becomes fully PTP-standard compliant with synchronization module for 5G and other mission-critical networks

Net Insight has selected Meinberg’s precision time protocol (PTP) software stack — Precision TimeNet — to implement full PTP functionality in all of its platforms.

The Precision TimeNet solution offers a GNSS-independent delivery of high-accuracy timing across any IP vendor network, which can significantly reduce the cost and rollout times of 5G and other mission-critical networks.

In 2021, Meinberg also delivered a synchronization module to Net Insight’s Nimbra MSR 300 series, providing full PTP IEEE 1588v2 interoperability and GNSS integration for 5G networks. The new module is part of the Nimbra Time Node, an important component of the Precision TimeNet solution.

Net Insight licensed the PTP stack from Oregano Systems, owned by Meinberg, to deliver network synchronization for both media and 5G networks. Meinberg leverages Net Insight’s network synchronization capabilities to serve customers across the telecom, fintech, government, and power telecom industries. The expansion into a strategic technology partnership means that both companies will utilize their expertise in time synchronization to deploy solutions that remove the challenges of reliable precision timing over any IP network.

“The shift to IP is accelerating, making precision timing key to the successful deployment of new applications,” said Heiko Gerstung, managing director of Meinberg. “Net Insight’s Precision TimeNet offers a unique solution on the market that we see a strong and growing need for, across multiple industries. We’re excited to be working with Net Insight, a leader in mission-critical IP transport, to drive innovation and enable our customers to benefit from GNSS-independent time synchronization.”

“Net Insight has been developing time transfer for nearly two decades, delivering industry-leading time accuracy and resilience over IP networks,” said Per Lindgren, CTO and co-founder at Net Insight. “When expanding our synchronization business into new markets, integrating with the IEEE 1588 PTP standard was key to enhancing our interoperability. Teaming up with Meinberg, a leader in time and frequency synchronization, was the obvious choice.. We’re excited that our joint expertise in IP networking and time synchronization will enable us to reinvent precision timing for our customers.”

By Julian Thomas
Managing Director, Racelogic

Driving simulators are commonly used by vehicle manufacturers to expedite the test and development process of their many electronic systems. This not only saves the considerable time and expense of using a real car on a test track, but it is, of course, significantly more environmentally friendly.

LabSat simulators are used by many leading technology companies and car manufacturers to develop and verify the performance of their new products containing GNSS receivers. These tests are performed using either a pre-recorded or an artificially generated RF signal. This RF signal contains the combination of multiple satellite signals, which are decoded by the GNSS engine, tracking the artificial satellites as though they were real. Static or moving scenarios can be generated, and the user can select parameters to suit their own application, such as time, date and available constellations.

Recently, an automotive LabSat customer had a specific requirement to synchronize a GNSS receiver with the real-time trajectory data generated by one of their driving simulators. This was for a hardware-in-the-loop test rig where a human driver would navigate a route around a virtual test track, while the normal electronic systems reacted as if the vehicle were being driven around a real environment.

The challenge in this customer’s application was that the time delay between the trajectory coming from the simulator and the generation of the corresponding GNSS signals had to be less than 100 ms. This low latency was necessary to achieve realistic synchronization between the driver’s inputs and the resulting output from the GNSS-based device under test.

Traditionally, low-latency real-time simulators use bulky expensive hardware that relies on power-hungry field programmable gate arrays (FPGAs) to create the necessary satellite signals. However, due to the inevitable tick of Moore’s Law, and with some clever optimizations, your entry-level desktop PC now packs more than enough punch to simulate multiple constellations and signals with very low latency.

Using a standard PC to do the heavy lifting means that the hardware required to output the simulated signal is much easier to obtain, can be a lot simpler, and is considerably more cost effective. For example, an 8-core, 3-Ghz Intel i7 processor can generate the signals from 20 satellites in real-time, which normally is sufficient to simulate all but the most complex scenarios.

Our LabSat SatGen software has been continuously developed and optimized during the past 15 years, so it did not take us long to enable the reception of an NMEA trajectory stream with a latency of less than 100 ms. We then streamed this simulated data via USB to our LabSat Real-Time, which generated a corresponding RF signal that can be connected directly to the RF input of any modern GNSS engine.

Using a PC to generate the signals does not mean a loss of fidelity, with the resulting output achieving a repeatable position of less than 10 cm, while the trajectory data can be received at up to 100 Hz.

The resulting solution can take trajectory data from any kind of simulator that has an API to obtain real-time data, such as many popular off-the-shelf driving and flight software simulators, and use this to provide a real-time signal that can be utilized by the GNSS device under test.

Our future development roadmap includes synthesizing external signals, such as CAN-based sensors or inertial measurement units, and then synchronizing these signals with the incoming trajectory. With the amazing power of a modern PC, we are finding that this kind of complex simulation is now much more cost effective and easier to achieve.

Letter to the Editor

February 2022

In November’s issue of GPS World, Editor-in-Chief Matteo Luccio opined that eLoran is part of the solution to GNSS vulnerability.

In January’s issue, he listed 10 questions from a PNT expert perhaps unfamiliar with eLoran.

These are important questions that must be asked of any technology, especially one under consideration to augment and back up our essential, but very weak and vulnerable, GNSS signals.

Yet the expert’s concerns pale in comparison to the essential questions about GNSS and PNT facing the United States and the West.

While I look forward to answers to the “10 questions” as a part of our ongoing professional dialogue, there are two important points of context we all need to keep in mind.

A Broad Consensus

First, Mr. Luccio’s assertion about eLoran being a part of the solution is more than reasonable. It also has a lot of impressive support from a wide variety of authoritative sources.

In 2008 and 2015, after much study each time, the U.S. government decided on and committed to building eLoran systems. Also, the U.S. government-sponsored National Space-based Positioning, Navigation and Timing (PNT) Advisory Board recommended eLoran in 2010 and 2018 as a part of securing the nation’s critical PNT capability.

In 2021, the U.S. Department of Transportation told Congress that wide-area terrestrial broadcast was a necessary part of a national PNT architecture. They later commented that infrastructure required per coverage area would be a key selection criterion for that broadcast technology. In other words, a system like eLoran.

Overseas, support for Mr. Luccio’s statement on eLoran is even stronger.

• The United Kingdom has long endorsed eLoran and operates an eLoran transmitter as a timing reference.
• Russia operates Chayka, a version of Loran.
• Available information points to Iran’s terrestrial PNT system being a form of Loran or eLoran.
• China and South Korea have long had Loran-C systems, and both are in the process of upgrading to the eLoran standard.

Each of these countries has publicly announced that it operates Loran/eLoran as a matter of national security in case space-based systems are jammed or destroyed, and to generally avoid overdependence on space-based PNT signals.

So, Mr. Luccio’s assertion was not at all revolutionary. Given all the studies, recommendations and existing uses, it would be surprising if he did not consider eLoran a part of the solution.

The Important Questions

Second, modern keying, encryption, authentication and other tech advances will help make all PNT technologies much safer and more resilient than they would have been decades ago, Loran and eLoran included.

Yet all will still have their strengths and weaknesses.

The most important questions we must ask are about how to establish the right level of national PNT security. These include:

• What is the right combination of technologies and systems with different delivery and failure modes that complement and reinforce GNSS and each other?
• How can the systems be efficiently and effectively implemented?
• How can the services they provide be easily accessed and widely adopted to ensure all parts of society are protected?

Countries such as China have answered these questions and are well down the path to implementation and wide adoption. Their robust national PNT architectures support easier rollout of 5G, rural broadband and other systems. They also serve as solid tech infrastructure upon which to build myriads of technologies and applications yet to be conceived.

Those nations not so advanced must accelerate their efforts. Otherwise, they must resign themselves to perpetually coping with GNSS vulnerabilities, including the possibility of attacks, and an eventual second or third place in the world because of their shortsightedness.

Dana A. Goward, President
Resilient Navigation and Timing Foundation

A U.S. Secretary of Defense once predicted that navigation would eventually be based on inertial devices that were set at the factory, and then always knew where they were forever after. Recently published research has reported on steps in that direction. However, according to navigation expert Brad Parkinson, the outlook is not as bright as some might think.

RNT Foundation President Dana A. Goward recently discussed the issue with him.

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Goward: Dr. Parkinson, you are well known for your contributions as the chief architect of the Global Positioning System. But you have more than a passing familiarity with inertial systems also, is that right?

Parkinson: I do. Long before I was involved in radio navigation, I was the chief analyst for all the U.S. Air Force testing of inertial navigation systems. I earned my masters degree in Doc Draper’s Inertial Lab at MIT in 1961. I am a major advocate and defender of inertial systems. I also have in-depth understanding of their limitations.

Goward: Have you been following the recent media coverage about advances with inertial systems?

Parkinson: I enjoy reading about these advances in physics devices. At the same time, I am a little impatient with media articles that do not appreciate the differences between building a device that measures specific force (or senses rotation) and a working inertial navigation system.

Goward: What are some of the inherent limitations of these systems?

Parkinson: I find it interesting that some of the articles speculate they may be able to supplant GPS and other GNSS. There is no way an inertial navigation system, even with perfect gyros and force sensors, can provide its accurate position (say, better than 10 meters) after extended periods (hours to days). In fact, attaining better than 200 meters accuracy after a few hours will be very difficult in a moving vehicle.

Today, farmers require even greater accuracy from GPS. They routinely use GPS for row operations, with accuracies of a few centimeters. The economic value is indirectly measured by the farmer’s purchase of such equipment — the agriculture market for GPS equipment is well over a billion dollars a year. Thus, a general replacement for GPS must provide centimeter accuracies.

Goward: So, what is it about inertial systems that stands in the way of them becoming autonomous substitutes for GPS?

Parkinson: There are some very simple and fundamental reasons that inertial positioning systems cannot hope to deliver such capability.
First, force sensors are not accelerometers, because they cannot sense gravity. To find acceleration, one needs to add vector gravity to their outputs. But gravity, or g force, varies a lot at the micro-g levels, and the inaccuracies are fed to the double integration that produces position. Errors grow as time or time squared and, without outside reset, are essentially unbounded. The physics devices described in some of these articles are definitely instruments that Doc Draper described as “specific force sensors.”

What we loosely call g force, or just g, is actually the inverse of the reaction to maintain stationarity on Earth. G is defined to include the centrifugal force due to Earth’s rotation, which varies greatly as a function of latitude — the radius of the merry-go-round called Earth. Mountains and chasms affect the local g. Further, it is a vector quantity: its direction can change locally by many arc seconds. In other words, down does not generally point to Earth’s center. Gravity gradiometers might be of limited help, but they are very large and not made for dynamic environments.

In a nutshell, estimating acceleration requires calculating and adding gravity to the three-dimensional specific force sensor.

Second, to use these devices for extended navigation, coordinate frames would have to be defined and stable to milli-arc seconds. All instruments would have to have input axes and cross-axis sensitivity calibrated to corresponding levels. Generally, this problem is ignored in many lab projects.

Third, for inertial navigation sensors to work, they need to accurately know their initial position. Any initial velocity or position errors will grow as a function of time.

Fourth, the vertical position axis is inherently unstable and diverges exponentially.
Physicists have been enamored with instruments that can use atoms to sense specific force and rotation. While scientifically interesting, even if perfect they cannot overcome these challenges.

Goward: But there is still a role for inertial systems in navigation, isn’t there? How good are they, and what are some of the applications?

Parkinson: I suspect the best inertial systems of today (which are in nuclear submarines) can maintain an accuracy of about 0.1 nautical miles or about 200 meters for a few days. I am sure the real number is classified. These systems are very large, expensive and complicated. They rely on a very low acceleration environment and are periodically reset with GPS. Furthermore, they probably use gravity gradiometry to calculate the local variations in gravity to the first order. They do not calculate the vertical position, and use water density and knowledge of the local geoid to keep the vertical axis stable.

An aircraft with inertial can, to some extent, keep the vertical dimension errors bounded, provided it has knowledge from elsewhere of local sea-level barometer settings and by assuming adiabatic pressure variations.

I strongly support the inertial/GPS/directional antenna marriage for users who want assured PNT. Aviation is a good use case for this. Inexpensive inertial components (called micro-electromechanical systems, or MEMS) can improve the jamming resistance of the GPS receiver by 15 dB or more. This step alone can reduce the effective line-of-sight jammer denial area by more than 95%.

Goward: So, inertials can be a good part of the solution but are not necessarily the whole solution themselves.

Parkinson: Exactly. Despite what some media outlets might publish to lure in readers.