Protection of U. S. PNT Capabilities – No New GPS Satellites, Please! *
By Gene H. McCall - Los Alamos National Laboratory Fellow(retired)
Wednesday, April 25th, 2018 @ 6:45PM
GPS capabilities can be protected against high power jamming sources to make them unjammable or practical purposes. Accuracy can be, simultaneously, improved. GPS III satellites are unnecessary and provide too little improvement over current capabilities.
The, somewhat, whimsical title, essentially, states the conclusion of this paper. Adding features and power to GPS satellites is exactly the wrong way to assure PNT capabilities for the nation. Also, no backup to GPS is needed. Protection is the answer.
Who is in Charge?
The question points to a significant problem in the program.
Most of the funding goes to support the space portion of the GPS program. In line with usual government practice, the one who controls the money controls the program. The old trite saying applies:
The golden rule in government is that he who has the gold makes the rules.
Thus, the program is controlled by the space component, but the use of the capability occurs on the ground. This situation should be fixed.
The Cultural Dichotomy
The disconnect between the users and the providers of the capabilities of the Global Positioning System results from what I refer to as the cultural Dichotomy. I will attempt to describe the situation.
In space, power is extremely limited and costly. Attitude control of a satellite uses both power and fuel. Fuel exhaustion ends the life of a satellite.
Satellite engineers think and discuss at length before increasing the power used by a feature by, even, one dB, about 25 percent. A factor of two, 3 dB, is likely to cost millions in a typical satellite. And, a 10 dB change is, almost, unheard of. On the earth, however, terrestrial communication engineers, typically, consider one dB to be in the roundoff noise, three dB may involve only the replacement of a few low-cost devices and the use of a heavier gauge wire. Of course, there are many details involved in the decisions made in both places, but the difference in mindset certainly exists.
Thus, the engineer designing the GPS III satellite can proudly state: “I am giving you 20 dB, a factor of 100, in additional jamming protection” –The government is buying factors of a hundred, or so, when they need a factor of a billion, to make a real difference — The user then replies, “Thank you very much, but it costs me 300 million dollars, and you are still a factor of 16 million short”. Both walk away shaking their heads at the lack of understanding evinced by the other.
An article on positioning, Navigation, and timing (PNT) usually begins by asserting the value of PNT capabilities to any modern nation, and by noting that the United States, in particular, is very dependent on those capabilities. Then, the author equates PNT capabilities to the performance of a system of satellites, which provide signals that enable PNT capabilities. Further, the signals are said to be weak enough to be susceptible to interference and jamming. Some system configurations are, even, susceptible to spoofing. That is, incorrect signals can cause the PNT system to generate measurements that are believed to be correct, although they are not. The result of such reasoning is, usually, that the author claims that the system vulnerability demands the creation of another PNT system that can replace, or, back up, the original system if the original system becomes unavailable.
As an emergency solution, the replacement system can be allowed to produce less accurate PNT results if the results are considered acceptable according to criteria defined by a designated government agency. Some government sources state a requirement for timing accuracy of one microsecond, compared to universal coordinated time(UTC). This is an accuracy that is poorer than that used by early users of GPS in the 198os, before the system was operational. There is a mention of a 100 ns accuracy for regional, or local, areas, whatever that means, but even that requirement is more than a factor of ten poorer than current values. Neither of the requirements is appropriate for the 21st century. Even the current performance of 8 ns in time, 3.35 meters horizontal accuracy, and 4.68 meters vertical accuracy are inadequate as we move into the era of driverless automobiles, which need better position accuracy, and increased dependence on cyber technologies, which need more, and more, accurate timing. The controlling organization, the National Executive Committee for Space-Based PNT(PNT excom) is composed of high-level bureaucrats, and position in the government is more important than technical knowledge.
The usual suggestion is that eloran can be a suitable backup system for our current PNT system, but I find that suggestion, at best, absurd. One severe consequence of the failure of our current PNT system, the global positioning system (GPS) is that almost all of the instrument approaches, so important to our national transportation system, would disappear.
The important question is, certainly, whether we can extract ourselves from this predicament. I believe that we can.
First, drop the implication that the main characteristic of an adequate PNT system is its space component. The idea that satellites are the most important part of PNT systems severely limits our thinking. No doubt, the fact that government expenditures for the space assets associated with our current PNT system are very high tends to force us into that opinion. The romantic appeal of dealing with objects that fly in space may, also, have an effect. Certainly, the global coverage of satellite systems and their ability to produce useful signals at the surface of the earth is a valid consideration, though.
But, given our current state of knowledge about space, and PNT, technologies, we should consider space as, simply, another commodity. Just decide what one wants from space assets in term of performance, assuming that one’s desires are consistent with technological capabilities. Write a large-enough check, and eventually, the specified system will appear. The most important feature of this process, though, is deciding which capabilities are important, and which are not.
As an example from GPS, it is observed that the addition of the L5 signal to the IIF satellites enabled some important characteristics. The so-called enhancements offered by the planned features of the GPS III satellite are, practically, useless (More about this below).
After placing the space needs in their proper perspective, can we guarantee adequate PNT performance for the nation? I believe that we can. The next change to our ways of thinking about PNT is to downgrade the importance of replacement, or backup, system to our current PNT system, which is, almost always, assumed to be GPS. Instead of moaning about the vulnerability of our current system, we should concentrate on the possibility of generating a system that is adequately protected from adverse effects, more accurate than our existing system, and is reliable enough that backup is unnecessary. Or, in other words, it forms its backup.
We have the technology needed to build such a system, and the entire system could be built for considerably less than the costs associated with fielding the almost trivial performance improvements associated with GPS III satellites and required to support unnecessary, and failed, multi-billion dollar software programs, such as OCX.
What is needed?
At this point, many authors observe that satellite signals are weak and that interfering with them is an easy task. Therefore, the next task must be to propose alternatives to existing systems. Almost never asked are few important questions such as:
- Weak in what sense?
- Is there a way to enhance the signals enough to compensate for the shortfalls?
- How should we respond to the perceived vulnerabilities?
I hope to convince the readers that these questions can be answered in a satisfactory way. First, we need to calibrate our thinking with actual data, not emotional responses. A report supported by much useful information was published by the Congressional Budget Office in 2011. They show that after spending nearly $16Billion for GPS III satellites and $4Billion for a control system, the resulting system still has inadequate protection from jamming. I agree. We all know, too, that the price will increase beyond 2011 estimates. The control system, OCX, has already increased to $5.4Billion in 2017, and it is, now, estimated at $6B. In the end, no doubt, the cost will exceed $20Billion. At a time of severely limited funding for the military, and the Air Force, in particular, neither the Service nor the country can afford the bill. Especially, since we could field an adequately protected system for a fraction of the cost.
I have tried to avoid too much technical detail, but some numbers are needed to support my assertions.
First, let us address the weak signal concern, which relates to interference.
I will use the GPS L1C/A signal as a reference. If it can be protected, the others follow automatically. The received signal power at L1C/A will be taken as -158.5 dBW, or, 1.4 10−16watts. The power at which a receiver is jammed, depends strongly on the receiver type, but I will use, as a typical value of the protection needed, the power ratio between the normal signal level times the processing gain of 42 dB compared to the power of a one kilowatt jammer at a range of one kilometer. As a practical matter, I will assume that a receiver that can withstand a one-kilowatt jammer at one kilometer can be classified as unjammable. Assuming that all the power is used to jam only one channel will produce a conservative estimate. Hitting the target value within, approximately, 3 dB should be sufficient. Plus, a kilowatt jammer is likely to be expensive, and heavy. It is no longer a device that can be plugged into an automobile cigarette lighter outlet. And, a jammer at that power is easily detected, and located, making it easily disabled by first responders in a civil region, or targeted in a war zone. How much protection is needed?
Starting with a signal power of -158.5 dBW and a processing gain of 42 dB, the reference signal is -116.5 dBW or 2.12 10−12watts. The effective antenna area of a unity gain antenna at the L1 frequency of 1575 MHz is λ/4π= 2.89 10−3m2The power received from the one-kilowatt jammer is -66.4 dBW or 2.3 10−7watts. Thus, the protection needed is 50.1 dB, or, a factor of 1.03 105, more than one-hundred thousand times.
If the standard 42 B processing gain is replaced by a technology that replaces it, but offers more protection, the protection needed is 50.1 plus 42 = 92.1 dB. As a round number, to account for the effects mentioned above, I will use 90 dB as the protection needed. That is a factor of 109, or 1 billion. Can we do it? Yes, we can, using demonstrated technologies. I have been told that the Air Force achieved 114 dB, more than enough. The Air Force demonstration may have used a nulling antenna(CRPA), which would make the protected receiver rather expensive, and would only function properly for a single jammer. It can be done, however, with a standard antenna[11, 12]
How does that compare to some of the antijam features touted for GPS III?
It is said that the GPS III basic signal will be 3 dB, a factor of two, stronger than earlier satellites. Thus, the remaining antijam factor(AJ) becomes 87 dB. Only a factor of five-hundred million left. The M-code supplied to the military has a component that covers the entire surface of the earth that is visible to the satellite. The code has a chipping rate of 5.11 MHz which, other things being equal, offers an additional 7 dB over L1C/A. The code uses an antenna separate from the one used for the other codes. This feature is exploited in the satellite design to project a spot beam containing only the M-code, and having a power of -138 dBW at the surface of the earth, a gain of 20 dB, or, a factor of 100. But, how much does it, really contribute? Take the M-code advantage over C/A to be 10 dB, rather than the 7 dB mentioned above. That is ten times the protection offered by the processing gain of the L1 channel. So, the total protection in the spot beam, available to a very limited set of users is 42+10+20 = 72 dB, still short by a factor of, about 16 million. (The government is buying factors of a hundred, when they need a factor of a billion, to make a real difference).
Consider the practical result, however. The Russians are installing Pole-21 jammers, believed to project a jamming power of, approximately, 20 watts, on cellphone towers throughout Russia. Given the typical distance between cellphone towers, the M-code spot beam does not give sufficient protection to provide reliable navigation throughout Russia. It does, however, substantially increase the cost of the GPS III satellites. It also provides a complex problem to the control system. Continuing to point at a specific point on the earth from a satellite moving along its orbit is not a simple problem. Plus, could the spot beam track a penetrating hypersonic weapon? The difficulties are endless. There is, also, the question of how much satellite fuel is used in the process.
The beamwidth of the spot beam is, only, approximately, 10 milliradians. For proper operation, the pointing accuracy needs to be, approximately, 2 milliradians, about a tenth of a degree. Possible? Probably. Low cost? Not in our wildest dreams!
The principle useful feature of the GPS III satellite to most users is the pair of signals on the L5 channel, the I5 and Q5 signals. These signals are, however, also transmitted by the IIF satellites. They are in a safety-of-life frequency band, they provide 10 dB of AJ, and increased accuracy, over the C/A code, and they can be acquired and tracked, nearly error-free.
- This article is exclusive to ACD.