Dana Goward, President of the Resilient Navigation and Timing Foundation, introducing Brad Parkinson and Matteo Luccio, GPS World EIC. (Image: RNTF)

On December 5, in Houston, Texas, at a gala event to celebrate the 50^th anniversary of GPS hosted by the Resilient Navigation and Timing Foundation, Matteo Luccio, Editor-in-Chief of GPS World, interviewed Brad Parkinson.

Here are two excerpts from the interview:

How does GPS today differ from the design that came out of the Lonely Halls meeting 50 years ago this past September?

Well, I’m very proud of what happened because, to my knowledge, there is no fundamental difference. Basically, that fundamental design has held up. … As a matter of fact, I still have one of the old Trimble handhelds, it’s called an EnsignGPS. It was one of those little devices that got shipped to the Iraq War. The other day, I pulled it out, batteries were kind of crummy, I got those squared away and went out, sure enough and navigated. I probably hadn’t pulled it out in at least 20 years. The point of the story is that evidently it still works.

What do you consider the most significant impact of GPS on society?

Well, the most significant impact is also probably the most perilous: kids today just take it for granted. They know where they are.

Watch the full interview below. 

<p>The post Celebrating 50 years of GPS: An evening with the father of GPS first appeared on GPS World.</p>

Matteo Luccio

When Boston Light — an 89 ft-high, white lighthouse on Little Brewster Island in Boston’s outer harbor — opened in September 1716, it was the first one in the Thirteen Colonies. Sally Snowman, who has been its keeper for most of the past two decades, is the last official lighthouse keeper in the United States. Contemplating the horrible trips across the Atlantic on merchants’ galleons, when many gale-tossed passengers despaired of ever setting foot on land again, she recently commented: “Imagine what they felt when they spotted the light.” See Dorothy Wickenden’s article “Last Watch” in the November 6, issue of my favorite magazine, The New Yorker. Of the roughly 850 lighthouses currently in the United States, Wickenden reported, only about half serve as active aids to navigation and the U.S. Coast Guard has automated all of them. “The rest,” Wickenden wrote, “have been made obsolete by GPS.” Yet, she pointed out, even hardheaded ship captains and pilots say that “lighthouses still have a place.”

When Snowman retires at the end of this month, it will mark the end of an era that lasted more than three centuries. This month also marks the 50th anniversary of the approval of Navstar GPS (as it was originally called) by the Defense Systems Acquisition Review Council (DSARC) of the U.S. Department of Defense. Three months earlier, at the meeting now remembered as Lonely Halls (see my editorial in the September issue), Brad Parkinson and his team had made the key decisions about the system’s architecture, including the number of satellites, their orbits, and what kinds of signals to use.

In this month’s issue, we revisit how, after initial opposition, the U.S. armed forces adopted GPS; how the civilian/commercial GPS (now GNSS) industry was born; and how surveyors reacted to this disruptive new technology.

To answer the first question, I asked Gaylord Green, who was on Parkinson’s team and later led the GPS Joint Program Office, to write his recollections on the subject. I also interviewed Marty Faga, whose long and distinguished career included four years as both Director, National Reconnaissance Office and Assistant Secretary for Space, U.S. Air Force. Faga passed away on October 19. To answer the second question, I turned to Charlie Trimble, who in 1978 co-founded the company named after him and was its CEO until 1998. To answer the third question, I chose Dave Zilkoski, who earned a master’s degree in geodetic science in 1979, the year after the first GPS satellite was deployed, while working for the National Geodetic Survey, of which he was later the director for about three years. Many readers of this magazine also know Zilkoski as the regular contributor to one of our four digital newsletters, Survey Scene.

This issue’s cover story also focuses, in part, on the 50th anniversary of GPS, as seen by three large players in the aerospace industry: Spirent, BAE Systems, and Northrop Grumman.

Matteo Luccio | Editor-in-Chief
mluccio@northcoastmedia.net

<p>The post Lighthouses on land and in the sky first appeared on GPS World.</p>

1976: The first military GPS five-channel receiver built in one of several programs that studied the feasibility of GPS. The receiver weighed more than 270 pounds and had seats for two operators. (Image: Rockwell Collins/Smithsonian)

Half a century ago, on December 22, 1973, Deputy Secretary of Defense William P. Clements, on the recommendation of the Defense Systems Acquisition and Review Council, directed the entire Department of Defense — through the Navstar GPS Joint Program Office, under the spectacular leadership of  Col. Bradford Parkinson — to proceed with the GPS program. While this magazine mostly focuses on the present and the future, we occasionally pause to remember how it all began.

In the following articles, we are lucky to benefit from the long memories of four gentlemen who were there. Read the full articles.

“Lost in the desert, they demanded GPS” by Gaylord Green

“From ‘We don’t need it’ to ‘We can’t live without it'” by Martin Faga

“They used GPS even before it was fully built” by Dave Zilkoski

“GPS: The birth of the commercial GPS industry and how it changed the world” by Charles R. Trimble

<p>The post The early days of GPS: How it was adopted by the US military and surveyors first appeared on GPS World.</p>

Image: USAF

GPS turns 50 this year, marking five decades of transforming the world in ways that have profoundly impacted society. Since its approval as a program on December 17th, 1973, GPS has revolutionized the way we navigate and comprehend our world, often in ways few realize.

To honor this achievement, a special event will be held at the South Shore Harbor Resort and Conference Center in Houston, Texas, on December 5, at 6:00PM. This event aims to be a historic tribute to GPS’s journey and its impact on the global community.

At the special event, Matteo Luccio, editor in chief of GPS World, will lead an engaging discussion with Brad Parkinson, the original chief architect of GPS, shedding light on the system’s early days, its far-reaching impacts on humanity, and exciting prospects for the future.

Members of the press, federal employees, Resilient Navigation Timing Foundation members, PNT Advisory Board members, and presenters may attend the event for free. Others can secure their attendance for $75, which includes an optional one-year membership in the RNT Foundation.

To reserve your spot, RSVP at inquiries@RNTFnd.org no later than November 27.

The President’s National Space-based Positioning, Navigation, and Timing Advisory Board, which advises the government on GPS and related issues, will meet the following two days in the same location. Members of the public are welcome and encouraged to attend. Click here for more information on that event.

Check back to watch a recording of the interview!

<p>The post Celebrating GPS: An evening with the father of GPS first appeared on GPS World.</p>

Brad Parkinson

We, of the PNT universe, have been hearing a rather continual message of doom from the media regarding the fragility of the GPS (or GNSS) signals. In a way, they are right. The received GPS signal is 1/10th of 1 millionth of 1 billionth of a Watt. It can be susceptible to jamming and spoofing.

In response, the U.S. government has sponsored major studies and some competitive tests of techniques to augment or possibly replace GPS. I applaud such queries, but also would strongly advocate more balance in efforts to increase robustness of positioning, navigation and timing (PNT).

Specifically, I argue for increased emphasis on well-known techniques that can greatly toughen GNSS receivers to both jamming and spoofing. Some of these techniques are deliberately denied to civil users by government policy.

Background

The PNT Advisory Board (PNTAB) is a panel of national experts who report to the PNT ExCom. The ExCom is comprised of the deputy heads of the nine U.S. government departments with the largest stakes in PNT. The PNTAB has a starkly simple and well-stated goal:

To meet its overarching goal, the PNTAB has developed a three-legged strategic framework, known as “PTA”: “We must protect, toughen and augment GPS to ensure that it continues to provide economic and societal benefits to the nation.”

Most current U.S. government efforts have been focused on the third of the PNTAB strategic legs: augmenting the GPS system. These system augmentations include: modernized Loran (eLoran), fiber-optic distribution of time, and ranging to low-Earth-orbit (LEO) satellites (particularly the swarms of communications satellites). In general, these system augmentations offer no hope of being equivalent to GPS in terms of availability and accuracy.

However, augmentations have the advantage of either being less vulnerable to interference, or highly proliferated in case of satellite outages. As supplements, or in an emergency, they can perform a very valuable role, but with nowhere near the equivalent performance of normally operating GNSS, which can routinely provide worldwide, 24/7 precisions better than decimeters in the dynamic real-time kinematic (RTK) mode. In the United States, GPS also offers continuous, real-time integrity assessments courtesy of the FAA¹. Europe has a similar, compatible integrity system called EGNOS, and there are other regional system augmentations.

In summary, the current PNTAB assessment regarding these substitutes is:

“No current or foreseeable alternative to GNSS (primarily GPS) can deliver the equivalent accuracy (static down to millimeters) and worldwide, 24/7 availability.”

Toughening User Equipment

Toughening, the second leg of our assurance strategy, includes all aspects of GPS enterprise vulnerability — satellites, ground control and user equipment. For this article, I am focused on toughening the user equipment. I would argue that we have largely under-emphasized, or been prohibited by national policy from using, well-known and widely available user equipment toughening technology.

The main vulnerabilities of GPS receivers are jamming and spoofing of the received signals. Familiar anti-jam (A/J) methods can substantially overcome the inherent weakness of GPS signals to defeat deliberate jamming and spoofing. As I outline here, such measures can reduce a jammer’s effective radius by a factor of more than 100 and reduce the effective jammer area by a factor of 10,000 compared to the unprotected receiver.²

Thus, these methods are also deterrents, because they can render ineffective such hostile (or possibly inadvertent) acts. Further, the technology that provides this significant toughening is available now or will be within a few years, rather than the many years required by some alternative, system-level augmentations.

Toughening techniques (A/J improvements) are traditionally calibrated as the improvement in the amount of jamming that can be tolerated, measured by the jamming-to-signal power ratio (J/S) expressed as decibels (dB). However, for this discussion, I will also use a different, more intuitive, measure. This metric is the Denial Radius Reduction Ratio (DRRR):

DRRR = (radius of jammer denial after J/S measure applied)/(jammer radius without improvements)

For example, a 15-dB improvement in J/S would lead to a DRRR of 0.178.³ In other words, the 15-dB improvement has reduced the denial radius to about 18% of the line-of-sight radius that would be denied to an untoughened receiver. Note that the simultaneous use of techniques is generally multiplicative. For example, simultaneously applying technique #1 with a DRRR₁ of 0.5 and technique #2 with a DRRR₂ of 0.3, would result in a DRRR_1&2 of 0.3 *0.5, or an overall DRRR of 0.15. This is the advantage of using this metric to describe the A/J improvements. ⁴

Baseline Case

For our basis for comparison, we will consider the L1 C/A signal in full accuracy (State 5) tracking mode and a 1-kW noise jammer.^5,6 For this situation, the line-of-sight jammer could deny GPS to a radius of about 560 kilometers. A discussion of the lower accuracy State 3 tracking is included below.
It is useful to consider toughening techniques in four major categories.

Toughening Category 1: Signal Processing. With L1 C/A, GPS receivers can improve jamming resistance, albeit with loss of ranging (tracking) accuracy, by using code tracking mode – State 3. This reduces a line-of-sight jammer’s denial radius (DRRR) to about 0.29 (a 10.7 dB improvement).

Toughening Category 2: Inertial Components and Very Stable User Clocks. This includes miniature micro-electromechanical (MEMS) components up to high-grade inertial measurement units (IMUs) and quartz to chip-scale atomic clocks (CSACs). These techniques enable narrower tracking filters and longer averaging, as well as allowing navigation through regions when GPS is denied. The range of DRRRs is 0.40 down to 0.10. We will use a nominal value of 0.18 (a 15-dB improvement).

Toughening Category 3: CRPAs. Controlled reception pattern antennas (CRPAS) are digital, multi-element, phase-steered antennas. They represent well-understood and available technology; they have been used in large surface-search radar systems for many decades.⁷ They can be used in null-steering or beam-steering modes.⁸ The number of antenna elements could range up to dozens. Potentially, they could produce DRRRs down to .01 — that is, a 99% reduction of jammer radius to 1% of the unprotected GPS receiver value.

Unfortunately, the U.S. government does not allow more than three-element CRPAs to be manufactured or sold for civil use. ⁹ This is due to some very old International Traffic in Arms Regulations (ITAR). For our nominal example, we will assume the restriction has been relaxed and use a CRPA of about 20 elements, which should produce a DRRR of 0.06 (a 25-dB improvement).

Toughening Category 4: Signal Alternatives. This category includes alternative modulations at 1575 MHz (L1C, Galileo or other GNSS) and alternative frequencies (L5, L2, Galileo). Note that the modern signals generally offer significantly improved signal-processing toughening as well as increased power.

Using L1C in State 3 compared to L1 C/A in State 5 would yield a DRRR of 0.10. (a 20.3-dB improvement). The L1C international signal should be operational on GPS by mid-decade. The L5 signal, at 1176 MHz, is clearly the most capable of the civil GPS signals in terms of jam resistance. L5 also should be declared operational by mid-decade. As the use of LEO communication satellites matures, their use may also fit this category.

Summary of Receiver Toughening Options. Quantification of the selected, nominal receiver augmentations are summarized in FIGURE 1 for both full accuracy (State 5, centimeter-level accuracy in RTK) and for less accurate code tracking (State 3, meter-level accuracy). These results are shown with a logarithmic scale to accommodate the wide range of denial radii.

Figure 1. Effect of receiver augmentations on accuracy for both State 5 and State 3. (Image: Brad Parkinson)

The example shows that a 1-kW hostile jammer’s denial radius¹⁰ can be reduced by a factor of about 100, using the conservative example augmentations of inertial and CRPAs. Because area is proportional to radius squared, the effective denial area of an augmented receiver would be 1/10,000th of the unaugmented receiver, using the example values.

Reverting to code-only (State 3) tracking, it enables operating through higher levels of jamming, albeit with less ranging precision. All these receiver augmentations and tracking techniques would also offer a significant defense against any attempt to spoof (deceive) the position measurement. Again, none of these techniques are new; we demonstrated the capabilities at the original GPS Joint Program Office in 1978, more than 40 years ago. Today, many competent manufacturers are offering toughened GPS receivers with combinations and variations of these techniques.

GPS jamming tests at White Sands have caused aircraft interference, which could be largely avoided with toughened receivers. Here, M-code is tested on Joint Light Tactical Vehicle platforms in 2020. (Photo: Joe Bullinger/U.S. Navy)

Meeting Increasing Threats

Threats of both jamming and spoofing seem to have accelerated. Devices to perform these illegal acts are freely advertised on the internet. In fact, we read of incidents both in the United States and abroad.¹¹ Near White Sands Missile Range in New Mexico, there have been GPS air traffic control outages due to authorized military operational jamming exercises. Such interruptions could be largely avoided if more robust (toughened) GPS receivers, with the enhanced jam resistance techniques outlined here, were in use.

News reports also highlight the spoofing issue. Hardening against this threat is also a task for toughening. A serious spoofing sequence usually starts with a strong jamming signal to cause the user’s receiver to break lock, followed by a strong false GNSS signal that causes false lock by the receiver. Using the false signal leads to a false position, of course. The first line of defense is to avoid the break-lock threat. Failing this, numerous self-check and authentication schemes can be used to avoid false positions.

A conclusion is that avoiding the break-lock jamming is a first line of defense against a spoofing attack. Of course, the toughening techniques to avoid this are the main subject of this paper. One well-known expert has stated that, for a well-designed receiver, a spoofing attack might deny the measurement of position, but should never cause false PNT. I will leave further discussion of spoofing to other authors.
Returning to disruptions of service in general, some have suggested many interference occurrences have gone unreported, because the typical user would not know where to make such a report. To remind the reader, the official reporting center is online at www.navcen.uscg.gov/?pageName=gpsUserInput.

In addition, the U.S. Federal Communications Commission (FCC) has repurposed a portion of the spectrum adjacent to the main GNSS L1 frequency (1575 MHz). The agency is converting the license holder’s original authorization to transmit a weak space-transmitted signal into a much stronger terrestrial system, potentially with thousands of transmitters. Extensive testing of civil GPS receivers by the U.S. Department of Transportation demonstrated that the planned repurposing will interfere with many existing receivers. Some observers call this disruption “legal jamming.”

Such a new spectrum use could have grave impacts on those existing receivers, notably aviation (especially helicopters and UAVs) and first providers. On the other hand, installing toughened replacement receivers would make the users virtually immune to this threat.

So, this begs the question: If the receiver toughening techniques are so effective, why are they not more prevalent?

Barriers to Adoption

Let’s examine the potential resistance to more extensive use of receiver augmentations.

Knowledge. This involves underestimating the threat to PNT and not understanding that toughening techniques are available. As mentioned above, threats to the fragile GNSS signals are growing.

There seems to be little interest in the U.S. government to monitor and suppress interference in the United States. Internationally, the reported incidents continue to increase.¹² It is also reported that certain European aircraft manufacturers have installed advanced, deeply integrated inertial systems with civil GNSS receivers to defeat or “flywheel” through radio-frequency threats (particularly in the Middle East).

As this threat trend continues, GPS manufacturers and users must realize that many of these solutions will take time to authorize, implement and install. It appears that the media are not aware that not only are the toughening techniques outlined here feasible, but many manufacturers have product offerings that address these threats. Having off-the-shelf solutions will give the PNT user the opportunity to retrofit and defeat such threats.

Cost. The cost for a receiver to revert from State 5 to State 3 is zero, and all receivers that use Code 5 (for example, RTK) would naturally have this built in. Regarding use of other frequencies (such as L5) and modulations (L1C) rather than the original L1 C/A, there is some small cost associated, including the additional antenna for L5. Note that all modern cellphone chips, such as Qualcomm’s, have this capability — including integrated carrier-phase measurements — in a chip that is estimated to cost about $5. A potential barrier is that the L5 and L1C signals are not yet declared operational, but these newer GPS signals should be operational within about five years.

The costs of many inertial components (accelerometers and gyros) have plummeted in the last few decades with the proliferation of MEMS devices, particularly into cellphones and automobiles. Their power consumption has also decreased while their performance has steadily improved. Full IMUs are much more expensive, but are already installed on many commercial aircraft. Robust toughening with inertial sensors can be achieved, but requires deep integration and careful engineering.

Depending on their complexity, CRPA antennas can be a costly receiver augmentation. Very high-speed (330 MHz is available), 16-bit, A-to-D converters are at the heart of most of these phased-array devices. Some are priced at about $150 each. Applications with a high premium for PNT availability in the face of interference — such as commercial aircraft and cargo ships — should find them affordable. Aircraft manufacturers have resisted retrofitting existing aircraft with larger diameter CRPA antennas because of costs. For some of these applications, integration costs can be more than the costs of the receiver itself, particularly if not included in the original manufacture.

As the yearly sales of fully toughened receivers increase, the economies of scale should significantly reduce unit costs. Each application will make its own determination of affordability, based on risk.

Government restrictions. Civil use of CRPAs with four or more elements is restricted by ITAR. These are well-meaning restrictions on technologies that could be used against the United States by hostile military forces. Unfortunately, the phased-array antenna techniques are not only well understood and tested, but relatively inexpensive components are widely available on the open world market. In particular, the restriction on the number of CRPA elements for civil use should be completely removed. All potential enemies are well aware of the beam-steering method and have ready access to the parts to build them. Thus, the restriction is only harming civil users without affording any apparent improvement in general military posture.

Certified aviation receivers need approval for deep integration of inertial systems and multi-element CRPAs. (Photo: JasonDoiy/iStock/Getty Images Plus/Getty Images)

Gaining permission: FAA flight certifications. To be used in commercial aircraft operations, navigation equipment must be certified by the U.S. Federal Aviation Administration (FAA). Current, certified GPS aviation receivers have rudimentary toughening techniques, but gaining approval for deep integration of inertial systems and multi-element CRPAs must be completed. It is gratifying to hear that work is underway to do this.

Any civil solution for the United States must expand integrity monitoring beyond GPS to include all GNSS, and must be operationally included in the FAA’s integrity monitoring with WAAS.

Recommendation

In describing resistance to interference, I have introduced the idea of DRRR – Denial Radius Reduction Ratio. Also, I have used a 1-kW white-noise jammer as a standard threat for calculating the denial radius of various GPS receiver configurations. My recommendation is that equipment manufacturers specify their receiver offerings by stating their equipment’s denial radius against a “standardized” 1-kW EIRP white-noise jammer.

Summary

Media reports of interference to GPS may be accurate, but they generally do not recognize that available toughening techniques can largely defeat those interference threats. While exploring systems-level replacements or augmentations (such as LEO ranging or Loran) is worthwhile, GPS (or GNSS) still offers the greatest capability in combined terms of accuracy, integrity and coverage.

The goal of all PNT providers — GPS operators, certifiers and manufacturers — should be assured PNT, with the expected accuracy and availability. The described toughening techniques to do that have been known for decades, but have not been generally adopted by many critical civil users. Many manufacturers do offer civil products under existing government constraints.

The purpose of this article was to describe and advocate the solutions available to increase the robustness and toughening of civil GPS receivers. For example, readily available toughening augmentations for civil receivers can reduce the denial radius of interference by 99% or more. This implies that any denied area would be squeezed down to 1/10,000th of that experienced by an unaugmented receiver.

The payoff is high, and should be affordable to many high-end, safety-of-life users. Therefore, a renewed focus on toughening of GPS receivers is overdue. We discussed barriers to rapid adoption but, more than the specifics, it is crucial to fully and urgently embrace the goal of toughening receivers, particularly removing the ITAR restriction on antennas.

Opinions expressed in this article are those of the writer and should not be construed as the official position of the PNTAB or any U.S. government organization.

Notes

1. While not a part of the U.S. Department of Defense’s GPS operation, the FAA’s integrity signal (WAAS) is a GPS-type signal directly available and being used by almost all modern GPS receivers, including cellphones.

2. This is the ration of Denial Radius between, for example, unaugmented L1 C/A in State 5 and augmented L1C in State 3. Please see later footnote and graph.

3. Calculated as 10 ^(–15/20)

4. Of course, the more traditional dB measure of jammer resistance can (in most cases) be simply summed to estimate the total effectiveness. The use of DRRR gives a more intuitive calibration, particularly for non-technical persons who may not be at all familiar with dBs.

5. State 5 is the tracking mode that provides full accuracy; it requires tracking both the PRN code and the reconstructed carrier. It is required for RTK positioning, which is usually used for automatic control of machines or vehicles. It is most vulnerable to interference. Less vulnerable is State 3 tracking, which only provides code tracking, with precisions of perhaps a few meters.

6. Deliberate jamming using “matched spectrum” GPS-like modulations have also been employed in the Middle East. The toughening techniques described are also generally applicable, with appropriate rescaling. The matched spectrum is fundamentally used to improve the jammer spectrum efficiency.

7. See Michael Jones, “Anti-jam systems: Which one works for you?”, posted on gpsworld.com on June 14, 2017, for a survey of manufacturer offerings at that time. Named companies generally continue to offer improved, jam-resistant products. 

8. Phased-array antennas, by their nature, distort phase and would probably have to be calibrated for precise operations such as RTK. Fortunately, we understand that this problem has been reportedly addressed and largely solved by the U.S. Navy’s JPALs program.

9. See U.S. Munitions List at www.ecfr.gov/current/title-22/chapter-I/subchapter-M/part-121.

10. The denial radius results shown can be easily scaled for weaker or more powerful jammers. The scaling goes as simply the square root of the power ratio of a different size jammer to the 1-kW example. A 10-watt jammer is 1/100th the power of the example. The denial radius would then be one tenth of the example, which is the square root of 1/100.

11. “Ships have reported an increasing number of cases of significant GPS interference and jamming in recent months. The geographic areas with more than one reported incident include the eastern and central Mediterranean Sea, the Persian Gulf, and multiple Chinese ports.” (Source: www.gard.no/web/updates/content/30454065/gps-interference-and-jamming-on-the-increase).

“North Korea is using radio waves to jam GPS navigation systems near the border regions, South Korean officials said. The broadcasts have reportedly affected 110 planes and ships and can cause mobile phones to malfunction.” (Source: www.bbc.com/news/world-asia-35940542).

12. “Reports of GPS outages submitted by pilots from the cockpits of commercial flights show that disruptions to the navigation system, which was created and is maintained by the U.S. government, are now standard occurrence on the flight routes between North America and Europe and the Middle East, according to data from the European Organization for the Safety of Air Navigation, known as Eurocontrol.” Fortune Magazine, Nov. 1, 2020.

<p>The post Toughen GPS to resist jamming and spoofing first appeared on GPS World.</p>

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

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.

At the ION GNSS+ 2021 conference in St. Louis, Missouri, the annual meeting of the Satellite Division of the Institute of Navigation, Brad Parkinson bestowed Lakshay Narula with the division’s Bradford W. Parkinson Award for his Ph.D. thesis “Towards Secure & Robust PNT for Automated Systems” at the University of Texas at Austin. The award honors Parkinson, known as the “father of GPS,” for his leadership in establishing both GPS and the Satellite Division of the ION. Narula is now an applied scientist at Amazon Lab126 in Sunnyvale, California, where he researches robust navigation and state estimation methods for robots, from self-driving cars to aerospace applications. (Photo: ION)

<p>The post The true value of inertial navigation: An interview with Brad Parkinson first appeared on GPS World.</p>

We asked Brad Parkinson, the “Father of GPS” and a GPS World Editorial Advisory Board member, what the new U.S. administration’s priorities should be to make positioning, navigation and timing (PNT) more resilient. For more answers from board members, see below.

Brad Parkinson

Protect the Spectrum. Reverse FCC authorization for relatively high-powered Ligado transmitters that have been proven to degrade GPS and other GNSS operation for thousands of PNT users. All U.S. government departments and major user groups affected have pleaded with the FCC to reverse this terrible decision. There is little benefit from it to the American public.

Protect the rapidly evaporating and self-proclaimed Gold Standard of GPS. The GPS satellite designs are showing their age. They need to go to multiple launch (three at a time) and revert to simpler designs without the spot-beams and other weighty add-ons that greatly increase complexity and cost. The Chinese have added to BeiDou (a) inter-satellite precision ranging and wide-band communications, (b) geosynchronous satellites, probably with good spot-beam acquisition aids, and (c) a WAAS-like correction directly on the satellites, which may have accuracies down to real-time kinematic (RTK, perhaps a few centimeters). Also, they claim their basic accuracies to be better than GPS (it might be true!) — I think they already have operational retro-reflectors.

Allow and encourage export of the basic and quickest fix to jamming and spoofing for high-value PNT users. More than 40 years ago, we demonstrated, in hardware, a high anti-jamming receiver that could fly directly over a 10 kW GPS jammer and not be affected. We know that high-gain, digital beam-steering antennas will create close to immunity, but our manufacturers will not move this way because we cannot sell or use them on the international market.  These devices, combined with inexpensive inertial components and the newer signals, would make PNT virtually immune to current threats of interference — both jamming and spoofing.

Move the military focus from alternative PNT techniques to seriously upgrading their receivers and useful signals. No current or reasonably anticipated alternative can provide the accuracy (3D), availability or integrity of GPS. The new M-code and L1C signals have been in the queue for about 20 years. (Loran for ground operations probably is very vulnerable to direct attack in a fluid battlefield operation. Loran’s main value is to distribute time and for maritime users.) In those 20 years, we now have cellphone chips costing less than $5 that can listen to about 200 ranging signals and process RTK, as well as use all the corrections available (WAAS, EGNOS, etc.). Such capability cannot be found in military receivers. The Defense Department must improve its acquisition strategy in terms of both speed and competition, and ncorporate existing civil capability into military user equipment.

Take government actions to rapidly identify, shut down, and prosecute GPS jammers. Some believe this problem is much larger than recognized already. All cellphones should be required to report extraordinary spectrum noise levels or apparent attempts at spoofing. This should be fed to a dynamic national database, perhaps maintained by the Coast Guard. GPS users should have an automated way to find out whether there are substantial threats in their operating area.

Brad Parkinson is the Edward Wells Professor, Emeritus, Aeronautics and Astronautics (recalled) and co-director of the Stanford Center for Position, Navigation and Time at Stanford University.

Editorial Advisory Board PNT Q&A

Here are additional responses to the question from more GPS World Editorial Advisory Board members.

John Fischer

“We hope the new administration continues on the path established with the Executive Order last year for resilient PNT, supporting progress made by DHS and NIST in establishing resilient and cybersecure frameworks. It will be important for them to maintain an open market concept toward future innovative solutions and not mandate a particular PNT approach. Awareness of the criticality for trusted PNT in our mobile connected society is established and we must not lose this.”
John Fischer
Orolia

Jules McNeff

“Resilient PNT should be a national security priority. Its continuity is vital to both military and economic/social activities of all kinds. Its qualities of spatial awareness and synchronization enable the efficient functioning of the most sophisticated modern technologies in the physical and cyber worlds while also simply getting people and things from point A to point B on schedule. In that context, the elements which comprise resilient PNT should be protected from natural or hostile disruption.”
Jules McNeff
Overlook Systems Technologies

Greg Turetzky

“Truly resilient PNT requires combining multiple positioning technologies to maximize resiliency. However, the government’s influence in many of the augmentation technologies (sensors, vision, etc.) is limited. What the administration can do is make GPS itself more resilient by speeding up the launch and acquisition schedule of GPS Block III. The new signals, particularly at L5, are invaluable for improved resiliency to jamming and spoofing as well as providing a significant improvement in accuracy.”
Greg Turetzky
Consultant

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James Litton

James D. Litton, GPS pioneer and founder of NavCom Technology Inc., died over the weekend at his home in California with his family at his side. He was 89 years old.

Litton was an early contributor to the development of GPS user equipment. He also played a pivotal role in the GPS-driven transformation of global agriculture that has greatly benefited humanity.

Litton was the director of engineering at Magnavox Research Labs when researchers were working on using CDMA for range measurements, a precursor to the GPS system. He also worked on the original proposal for GPS Phase I.

Later, as general manager of Magnavox’s Marine and Survey Systems Division, he helped develop new and advanced commercial navigation and survey receivers for both the Navy’s TRANSIT system and the Air Force’s GPS.

His team developed the first microprocessor-based commercial satellite navigation receivers and the first commercial GPS survey software. This led to Magnavox eventually having more than a 90 percent share of the survey receiver market.
The firm eventually held more than two dozen patents for improvements in GPS technology.

In 1992, Litton left Magnavox to start a consulting business. Two years later, with Ron Hatch, K.T. Woo and Jalal Alisobhani, he founded NavCom Technology Inc. With Litton as CEO, NavCom became a significant player in the GPS marketplace. Among its achievements was development — under contract — of a single-frequency WAAS-capable GPS aircraft navigation receiver.

NavCom also began a relationship with Deere & Company, supporting more efficient and productive agriculture. This relationship was so successful that Deere purchased NavCom in 1999. Litton continued to lead the company and serve as part of Deere’s senior management team for eight more years.

In recognition of his many achievements to the field, Jim Litton was presented the Institute of Navigation’s Hays Award in 2006.

Among his many contributions, his impact on global agriculture might well have been his greatest, according to Brad Parkinson, the original chief architect for GPS.

“His work transformed agriculture into a data-driven, technological industry that was incredibly more efficient,” Parkinson said. “The cost savings and increases in productivity have impacted billions around the world.”

Jim’s family has created a memorial fund at Doctors Without Borders for those wishing to make a donation in honor of his life and many good works. Click here.

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