Guaranteeing Positioning, Navigation, and Timing for the United States - Gene McCall*
Tuesday, August 1st, 2017 @ 12:33AM
A special amendment to the defense budget for 2018, calls for allocating $10 million to the Departments of Defense, Homeland Security, and Transportation. The funds would be used to develop a plan based on results of an ongoing study, “to devise a demonstration plan for a GPS backup and then actually do the demo.” The lawmakers stressed that “an independent backup is very, very important to U.S. security.” And according to Rep. John Garamendi (D-Ca), and Frank LoBiondo (R-NJ), the sponsors of the amendment, the new backup is likely to be the eloran.
Mr. Garamendi mentioned the eloran could be used for a self-driving car in a tunnel. But the error in eloran is larger than the diameter of a tunnel. Even current GPS accuracy is not adequate for self-driving cars.
There is a way of providing a more accurate and secure information than what the eloran can offer. And there is no one better to present this new method than Dr. Gene McCall, who served as the Chief Scientist with Air Force Space Command, the Chairman of the United States Air Force Scientific Advisory Board and the Chairman of the Global Positioning System (GPS) Independent Review Team, (IRT), who is a Fellow of the American Institute of Aeronautics and Astronautics (AIAA) and at Los Alamos National Laboratory. Dr. McCall writes:
For most, the words, positioning, navigation, and timing, and its abbreviation, PNT, describe concepts that are esoteric. Associated with the words are the systems, concepts, and acronyms that provide these functions which are essential for modern civilization. These include Global Positioning System, or GPS, long range navigation, or eloran, or Global Navigation Satellite System, GNSS.
Most are familiar with the GPS (Global Positioning System), which they associate with the device that they mount on the windshield of their automobile or as an application designated a navigator on their mobile phone. Almost everyone refers to it as, “my GPS.” Few realize that the GPS is not performing the navigation function. Rather, the GPS is only providing positioning information to software in the navigator, and it is this software which is using the positioning information, along with stored digital maps and well-tested algorithms to calculate the navigation instructions. When the users claim that the GPS made a mistake, they are, almost certainly, incorrect, if the error amplitude is more than a few meters. Also, if the address being sought is not available in the navigator memory, the error, certainly is the fault of the companies that designed and sold the navigator. The GPS, itself, is aware of no addresses. Perhaps the map was wrong, or the algorithm or the user interpretation of the information was wrong. Seldom does the GPS provides incorrect information.
Another positioning application that depends on the GPS is surveying. Anyone who has had a property surveyed in recent years will, almost certainly, find that the survey was done with a specialized GPS survey receiver that provides accuracy at the millimeter level. The collected information, usually, requires post processing to reach the finest of resolutions. However, survey receivers are not useful for time-sensitive applications, such as vehicle navigation.
GPS navigators are widely used in farming. It is used for guiding plowing tractors and for controlling sowing of seeds. Field management applications also use GPS measurements to control irrigation to optimize crop yield. Crop dusters use it for controlling insecticide applications.
But how accurate is GPS positioning? Is it accurate enough for the future? The FAA routinely conducts worldwide surveys of GPS accuracy. They have found that the accuracy is, on average, about 4 meters, or, approximately, 13 feet. Not bad for a system providing information 20, 000 kilometers away from the user. Certainly, this is accurate enough for ordinary driving navigation, or, even, cross country air navigation, but it is not accurate enough for landing an airplane under instrument conditions.
Anyone who has arrived at a destination airport in bad weather in recent years has, very likely, been aboard an airplane that has used the FAA Wide Area Augmentation System (WAAS), whose operation depends on the GPS. The WAAS provides correction information throughout the United States and most of North America.
WAAS reduces the error to approximately one meter for receivers that are equipped to use the information. WAAS has the disadvantage that it depends on geosynchronous satellites to transmit the correction information, and those satellites are not always visible at extreme northern latitudes, such as in parts of Alaska. Accurate timing would facilitate better farming, too. Corn planting should be controlled to an accuracy of, at least, a row spacing, and other plants could benefit from centimeter accuracy in depositing seeds and harvesting crops. Earth moving equipment now uses the GPS to control the depth and lateral position of scrapers and diggers. Millimeter accuracy is often required. Because the current GPS accuracy is not adequate for some important applications,
specialized differential and reference system enhancements for the GPS have been built to provide accuracy at this level
In the future, the demands for better accuracy are almost certain to increase. An important application on the horizon is the control of driverless automobiles. Certainly, no passenger would feel safe in an automobile whose position could be determined to an accuracy of no better than 13 feet, even though we all may know drivers who are no better than that.
The obvious conclusion is that we need better accuracy from our national positioning system. It is unlikely that the GPS system of the future can provide these improvements without additional complex, and expensive, enhancements. A solution will be suggested below.
The primary function of the GPS is to provide accurate timing. Position information is determined by measuring the time delay of a signal transmitted from satellites which are in known positions and then solving a set of equations to determine position. To determine latitude, longitude, altitude, and time, measurements of transmission delays from four satellites simultaneously are needed. The FAA has measured GPS average, worldwide, timing accuracy of 8 nanoseconds. Light and radio signals travel, approximately, 2.4 meters (7.9 feet) in 8 nanoseconds. Thus, timing accuracy is consistent with the position error of 4 meters when important uncertainties are taken into account.
Indeed, eight nanoseconds of timing accuracy is better than any routinely encountered by the average American. Why is such accuracy important?
Here are a few examples:
Cellphone systems use a modulation method essentially the same as the one used by the GPS to transmit information. For the transmitted information to be intelligible, the transmitter and the receiver must produce information bits at the nearly the same instant of time. The signal used to synchronize the bits, in most of the world, is GPS time. Without GPS timing information, cell phones in much of the country will become unusable. This had happened in cases when a minor GPS failures were amplified by receiver design oversights. Also, in general, faster timing can be used to generate additional communication channels. Cyber security and offense are, even now, require higher speed and accuracy.
Accurate timing is necessary for synchronizing computer networks, such as the internet.
The nation’s power grid is dependent on accurate timing, too. A separate section of the grid must be synchronous in phase to reduce power losses and, even, damage to equipment.
Precision timing is a must for stock exchanges. The time at which a purchase, or a sale, is made can determine the selling or buying price of shares of stock at a different time. When transactions were made by human hands, a time accuracy of a few seconds was adequate. Now, however, computers both sell and buy shares of stock and their time accuracy can determine whether a particular trader is a winner or a loser. Currently, time accuracy in the microsecond range is adequate, but transaction speeds increase regularly. It is clear, too, that if an unscrupulous trader could gain control of a stock exchange’s timing, he/she could shift the flow of money into his/her direction. The simple act of jamming, or interrupting the function could cause chaos.
Other, scientific applications depend on accurate timing to the limit of GPS accuracy. Before the GPS was fully operational because only a few satellites were in orbit, GPS timing was used to detonate underground nuclear weapon tests at a precise time and to synchronize measuring equipment around the world.The need for additional precision and speed appears to be unending. The GPS of today is barely adequate for today’s needs, and a system of the future should provide the possibility for better accuracy in both positioning and timing.
The need for additional precision and speed appears to be unending. The GPS of today is barely adequate for today’s needs, and a system of the future should provide the possibility for better accuracy in both positioning and timing.
The category omitted in the discussion above is navigation. Although included in the designation, PNT, the GPS does not provide navigation instructions. An analogy is the north star. It, and the other stars, known as navigator stars, have provided references for navigators for millennia. They do not, however, directly instruct the observer. Systems, such as the GPS, Galileo, Bei dou, and Glonass are similar, as mentioned above. Therefore, the following discussion will be limited to positioning and timing.
Jamming and Interference
The discussion above showed that PNT capabilities are essential functions for a modern society. Because the GPS satellites are at a distance o 20,200 kilometers above the earth and the signals are generated by low power transmitters the signals at the earth’s surface are very weak. In fact, the signal strength is below that of thermal noise. The only reason that the signals are usable at all is that the signals are modulated onto their carrier wave using a very clever scheme called spread spectrum modulation. This technique permits some channels to use the same frequency band without interfering with one another. Then, the signals are processed in a way that, effectively, increases their power by a factor of approximately twenty thousand. The processed signal amplitude is adequate for normal use in GPS navigator devices, but it is still sensitive to interference, either natural or human generated.
GPS jammers, though illegal for public use, can be easily purchased on the internet for, approximately, $100. These jammers plug into a vehicle’s cigarette lighter outlet and can jam GPS receivers out to ranges of more than two miles. They appear to be used, mostly, by professional truck and taxi drivers who want to block their employer’s tracking devices. While their sale and use, is illegal, prosecutions are rare. A Chinese website is now offering higher power jammers with power around 250 watts for prices in the range of $10-15,000. This jammer could jam the GPS civil signal out to a range of 150 kilometers, nearly 100 miles.
In an attempt to determine the extent of the intentional jamming problem, jammer detectors have been established at forty sites in Europe. Over one year, detectors have detected 124,100 cases of jamming from 17,000 distinguishable jammers. According to The Economist, the New York Stock Exchange is jammed five to ten minutes every day. News reports from Russia have claimed that the Russian government is installing 20,000 GPS jammers on cell phone towers throughout Russia to protect the country against American cruise missiles.
Given that the GPS signal is extremely weak and highly susceptible to jamming and interference, a backup PNT system is needed. The system usually mentioned, as in the latest amendment to the Defense budget, is eloran, which is a modernized version of the long-range navigation(loran) system of World War II. This possibility, and what I suggest as a far better solution will be discussed below.
The eloran system operates at a low frequency in the band from 90 to 110 kHz at high power. The signals are projected from multiple transmitters acting together in groups called chains. Studies have shown that, while jamming is not impossible, it is so difficult that the system can be categorized, in a practical sense, as un-jammable.
Unlike the GPS, the eloran is subject to interference from adverse weather, though this is considered a minor difficulty. The primary drawback is that the spatial resolution is only, approximately 20 meters, and the time resolution is only about 20 nanoseconds. Also, eloran offers no altitude measurement. While the timing resolution is adequate for some current applications, it is marginal. Further, there appears to be no opportunity for significant improvement in the future. The signal, though, is rather effective in penetrating building structures, and it can be used for some indoor navigation applications.
Effective, noise-free detection of GPS signals depends on the user’s knowing exactly, the form of the transmitted signal. The user can then compare the received signal to a replica of the known signal inside the receiver to discriminate against unwanted noise or interference. The limit on this process is the existence of the navigation message.
Each satellite broadcasts information about its status, including its orbital or ephemeris information, and clock calibration information. The information is modulated onto the ranging signal and is uploaded to the satellite from a ground station at least once a day. Thus, the actual transmitted signal differs from the published ranging signal. Until this information is extracted and included as part of the replica, complete processing enhancement cannot be done. The information in the navigation message is, however, essential for accurate determination of position and time. One might refer to the necessity of extracting the navigation message from the ranging signal as the Achilles Heel of the GPS.
The first question asked by a student or a newcomer to the theory of GPS operation is, “why not transmit the navigation message over a different channel?” The suggestion has been made many times, but the idea has been strongly rebuffed by the GPS program office. Once, a few years ago, when I suggested this to the, then, chief engineer of the GPS program, he rejected my idea because ‘the policy was not to transmit information outside the satellite links.’ My reply was, probably, not publishable. I never heard from him again.
In the discussion below, I will show the value of ignoring program office policy and doing, precisely, that. Cooperation from the GPS program office could be helpful, but it is not required.
It should be noted, too, that the satellite ephemeris information is a part of the navigation message. Lack of knowledge of the exact positions of the GPS satellites is the major factor limiting the accuracy of the GPS. If accurate, and timely, ephemeris information can be supplied to the user, and accuracy can be noticeably improved.
Perhaps, the next relevant question is, given that we know how to make substantial improvements in the accuracy and how to substantially reduce the vulnerability to jamming and interference without rebuilding or modifying the existing system, why is it not being done?
The answer is the usual one that applies to most neglected government programs: politics, lack of inadequate or specific funding, inefficient procurement system, and bureaucratic inertia. There is a way of providing a more accurate information.
The role of eloran
Once eloran is operational, it will serve as a, rather crude, backup for the GPS, but its true value lies elsewhere.
The eloran navigation signal consists of a series of eight pulses spaced in time by one millisecond. There is room in the signal for a ninth pulse, but it will, usually, be ignored by the eloran receiver. It has been shown, however, that the ninth pulse can be modulated in time position and phase to transmit information. The modulation process was demonstrated at a rate of, approximately, 250 bits per second. It was used to transmit information needed by the FAA WAAS in a part of Alaska, where the geosynchronous satellites used by the WAAS were obscured by mountainous terrain. Work done at the Coast Guard Academy has shown that rates as high as 4,000 bits per second can be obtained.
Using both the signaling and the positioning and timing capability of eloran, coupled with the capabilities of the GPS, a total system with capabilities exceeding those of either system can be generated.
The eloran system consists of some stations distributed throughout the United States. The loran system had stations in many other areas, as well. There will be, at least, coverage for the entire United States and, perhaps, for all of North America. The stations will communicate with, and be controlled by, a central control station.
Each station will be equipped with a GPS receiver that can monitor all satellites in view and a jammer detector. The station receiver location will be known precisely to an accuracy of a few millimeters. The station receivers will measure the range to each satellite in view and will detect each satellite’s navigation message. The range information, the navigation message, satellite time, and jamming information will be reported to the control station.
Each station, or, at least, a substantial fraction of them will have a two-way satellite time transfer station. These are, relatively, inexpensive units that can measure time to an accuracy of one nanosecond. Thus, time measured the station will be more accurate than GPS time. The time difference will be reported to the control station, and it will serve as an accurate measure of the satellite clock bias.
The control station will have accurate range information for the distance from several stations to each satellite in view. Using this information, the control station can calculate the orbits of all satellites in view over North America very accurately.
Given the available information, the control station will direct each station to use the eloran signaling capability to transmit the following information to every dual eloran/GPS receiver:
- GPS satellite navigation message to be used to construct a perfect satellite signal.
- An additional message containing current clock offset and orbital information(ephemeris) for each satellite.
- Jamming information for each station. A jammed station will have its information ignored, and an alarm will be sounded. The station, itself, can be directed to use the anti-jam measures described below, and appropriate authorities will be notified.
An update rate for the clock and ephemeris information of, approximately, once every three minutes will enable a position accuracy of 10 centimeters, and a time accuracy of better than two nanoseconds for every dual receiver.
Each receiver will be capable of constructing an exact satellite signal for all satellites in view. Therefore, the correlation time for each signal can be made as long as necessary to discriminate against jamming signals. During the correlation time, a method for compensating for the receiver is necessary.
The methods to be used are:
- Coherent initial detection of all satellites in view.
- Ultra tight coupling of a MEMS(Micro ElectroMechanical System) solid state inertial system or differential eloran to the GPS system.
- Increased correlation time.
The use of these measures will permit a receiver to operate within one kilometer of a one-kilowatt jammer. I claim that this makes the receiver, for practical purposes, unjammable.
A further feature of the dual receiver is that natural noise and weak signals are treated as jamming. Thus, navigation is enabled indoors or in other weak signal areas.
Spoofing, where a receiver is tricked into using false satellite signals for navigation information will not be possible if the integrated eloran/GPS system is in use.
Spoofing is done by supplying false satellite signals to a receiver. The integrated system will require that the same false signals be supplied to every station receiver that could, in principle, view the false satellite, and to the receiver of interest. Otherwise, the false signal will be, simply, ignored, or treated as a jammer.
The anti-spoofing encryption system now used by military receivers will be unnecessary. The so-called Y-code can be replaced by the previously used P-code to give all users access to a 10 MHz ranging code, which will improve anti-jam performance. Also, the expensive SAASM chip, which decrypts the Y-code in military receivers will no longer be required.
Eloran is a system which has internationally accepted specifications. The processes described in this paper do not change or effect those specifications for the stations participating in the integrated system. In fact, the options for other nations are expanded. Nations choosing to participate in the U. S. system, using the GPS can easily do so by coupling their stations to the U. S. control station. Nations which have their satellite systems can use the methods described here to provide assurance for their systems. Thus, Europe could have an eloran/Galileo integrated system, China – an eloran/beidou system, and Russia – an eloran/Glonass system, etc. The method is international, even if the details are not.
It may appear that the measures described above will be very expensive. It is not. Given that the eloran system is likely to be deployed as a misguided attempt to provide a backup system for the GPS, most of the costs will be incurred in eloran construction.
A major cost saving factor is that it will not be necessary to deploy the expensive GPS III satellites. Replacement satellites of the IIF type will be adequate. It is claimed that GPS III provides some anti-jam features, but they are trivial compared to those described above.
Most likely the integrated system will provide adequate accuracy and availability to replace the WAAS instrument landing system, but that will have to be verified.
How to Proceed
Few people, certainly not this author, would recommend that the government contract for a complete design and construction without prototype testing, even though that seems to be the accepted procedure for many aircraft and satellite programs, which have incurred significant cost and schedule overruns.
Therefore, it is recommended that the integrated system program is begun as a research and development demonstration program, using the following steps:
- Bring into operation two, or three, overlapping eloran stations while developing the signaling method and equipping the stations with the necessary GPS receivers, time transfer receivers, and control station communication software.
- Concurrently, develop a dual receiver to be tested when initial station construction is complete.
- Upon completion of steps 1, and 2, conduct jamming, and spoofing tests.
- If steps 1, 2, and 3, are completed successfully, design, and build, the entire integrated system.
Of course, the construction of the entire eloran system can proceed while the integrated system development is being done. If the integrated system development is not successful, the eloran fallback system will still exist.
A way of providing an assured PNT capability, better performance than the current GPS system and an upgrade path for the future is achievable. The possible cost appears reasonable, given that a U. S. eloran system will be built. Cost savings from the elimination of high-cost programs, such as GPS III, will, no doubt, more than balance the cost of the integrated system.
With the exception of the eloran/GPS ultratight coupling, all the necessary technologies have been demonstrated, and there is a demonstrated substitute for the coupling option.
Can it be done? Certainly!
Will it be done? Only time and politics will tell.
There has been some motion toward, at least, evaluating the problem with the insertion of $10M in the 2018 defense budget, at least the House version, for the Department of Transportation to address the problem. Unfortunately, terms, such as backup for the GPS, are still being used, even though the idea is not useful. Acquisition tasks, such as the development of requirements, which will guarantee an obsolete system are, also, rather disturbing.”
* Dr. Gene McCall is a member of the American Center for Democracy’s Economic Warfare Institue’s Advisory Board.