October 28, 2011
As the Department of Defense's (DoD's) Global Positioning System (GPS) satellites reach the end of their service lives, the department plans to replace them with ones that can counter deliberate interference by generating stronger signals. Analysis by the Congressional Budget Office (CBO) indicates that an alternative approach—namely, improving military receivers to retain the GPS signal even in the presence of such jamming—would be less expensive than DoD's plan for upgrading its constellation of GPS satellites. Furthermore, the alternative would yield benefits almost a decade earlier than DoD's plan. However, the improvements to military receivers could make them larger and heavier (and thereby less useful to personnel operating on foot) until they could incorporate the substantial gains that have been achieved in miniaturization in other applications.
The GPS uses a constellation of at least 24 satellites, each of which transmits precise data on the time and its location. Receivers—both military and civilian—use the data transmitted by the satellites to calculate their own position; information from a minimum of 4 satellites is required to determine a position accurately in three dimensions. Since 1995 (when GPS became fully operational), the U.S. military has come to rely on it to precisely locate both enemy and friendly forces. However, because the GPS signal from space is very weak by the time it reaches Earth (like the light from a 25-watt lightbulb shining 12,500 miles away), the system can easily be swamped by interference.
In 2000, DoD initiated plans to reduce the system's susceptibility to intentional interference. As a first step toward providing some protection against jamming, DoD decided that GPS satellites would transmit additional signals, available only to military users, each of which covered a wider range of frequencies than those already being transmitted. Those signals, called M-code signals, are more difficult for enemy jammers to overwhelm and can improve the ability of military receivers to operate in the presence of jammers. Ten satellites capable of transmitting M-code signals were already in orbit as of August 2011.
To maintain the constellation as existing and new satellites reach the end of their service lives, DoD plans to launch a total of 50 satellites through 2030 at an average rate of 2 to 3 satellites each year starting in 2012. The department has already purchased—but not yet launched—10 of those GPS satellites capable of transmitting M-code signals. DoD plans to acquire 40 more satellites—known as GPS III—that are capable of transmitting stronger M-code signals than existing satellites over the next 10 to 15 years.
DoD plans to develop and purchase the new satellites in three phases. In the first phase, DoD plans to acquire 8 GPS IIIA satellites capable of emitting M-code signals that are three times stronger than those transmitted by current GPS satellites. The first IIIA satellite is scheduled to be launched in 2014. In the second phase, DoD plans to acquire 16 GPS IIIB satellites with M-code signals that are five times stronger than those of current satellites. For the final phase, the department's plan calls for an initial purchase of 8 GPS IIIC satellites, which will be equipped with a special antenna capable of focusing the M-code signals in a "spotbeam"; however, CBO assumes that the department would need to purchase an additional 8 IIIC satellites in order to have enough IIIC satellites in orbit to take advantage of the IIIC's advanced capabilities. Those satellites will transmit signals with the same strength as IIIB satellites and will be able to use the spotbeam to illuminate an area with a diameter of 600 miles on the Earth's surface with signals 100 times stronger than those of current GPS satellites. In addition, IIIC satellites will be equipped with high-speed cross-links, which will allow continuous data updates. As a result, those satellites will be able to provide more accurate data to receivers, enabling a user's location to be determined within 6 inches, instead of 10 feet (using current satellites) or 3 feet (using IIIA and IIIB models). After the 16th IIIC satellite is launched in 2030, the entire constellation should be composed of GPS III satellites, 16 of which will be IIICs.
Over the next 15 years, DoD also plans to develop software to control the M-code signals and the new GPS III satellites and to develop and purchase receivers that are capable of processing the M-code signals. Although 10 satellites capable of transmitting the harder-to-jam M-code signals are currently in orbit (the first one since 2005), no users have been able to benefit from them because DoD does not have the ability to monitor or control the signals, nor has it fielded receivers to process the signals. DoD plans to have a new control system fully in place by the end of 2016. To make the entire planned system functional, however, additional control capabilities, such as being able to update satellite data transmissions continuously when IIIC satellites enter the constellation and to control their spotbeam antenna, will need to be developed. Moreover, to make the planned system useful, M-code-capable receivers will need to be fielded as well. DoD's current plan envisions fielding the first such receivers in 2017, but because the various armed services now field more than 400,000 GPS receivers, it may be 2030 before all units are fully equipped.
If the satellites and receivers perform as planned, the combination of all of the upgrades proposed by DoD would enable military receivers to operate in the presence of much stronger jamming signals than they can withstand today. For example, the effective range of a 10-watt jammer trying to cause a military receiver within the spotbeam of a GPS IIIC satellite to lose the GPS signal would be reduced by 96 percent, shrinking from 55 miles to about 2 miles.
Although the planned upgrades to GPS satellites will not increase the strength of civilian signals and will not improve the performance of civilian receivers in the presence of interference, other planned improvements will benefit both military and civilian users. In particular, GPS IIIA satellites will transmit signals that will enable both types of users to determine their position to within 3 feet, compared with the 10 feet that is possible with signals from current satellites. And once enough IIIC satellites enter the constellation, positioning within 6 inches will be possible for all users, according to DoD.
CBO estimates that it will cost DoD roughly $22 billion from 2012 to 2025 to modernize the GPS. That total would include the cost from 2012 onward to develop and purchase the 40 GPS III satellites (including $3.6 billion for the additional 8 IIIC satellites), to develop the software and capability needed to control those satellites and their transmissions, and to develop and purchase hundreds of thousands of military receivers capable of receiving and interpreting the M-code signals.
The Government Accountability Office and the Defense Science Board have reviewed DoD's plan to modernize the GPS and raised several concerns, particularly regarding the plan's focus on improving the satellites rather than the receivers and the plan's lack of coordination in terms of the timing for various capabilities. CBO has developed options by which it explores those concerns.
CBO examined three options that would provide antijamming improvements to military users sooner and at a lower cost than DoD's plan. Those options focus more on improving the performance of receivers in a jamming environment and less on strengthening the signal that the GPS satellites transmit. CBO focused on the handheld receivers used primarily by the Army and the Marines because they are the most widespread throughout the services. (Despite their designation, most such receivers are mounted, sometimes permanently, in various military vehicles.)
- Option 1 would improve current military GPS receivers by fitting them with better antennas and by adding inertial navigation systems.
- Option 2 would capitalize on a DoD research and development program by enabling current GPS receivers to integrate information received via the Iridium commercial communications satellite network.
- Option 3 would include the improvements of both Option 1 and Option 2.
Three items are common to all of the options. First, under each option, DoD would purchase 40 GPS III satellites (the same number as in DoD's plan) but confine those purchases to the IIIA model. Second, under DoD's plan as well as all the options, DoD would continue to develop the ground control system, enabling it to control current M-code-transmitting GPS satellites (IIR-M and IIF) as well as the newer GPS IIIA satellites. And third, DoD would develop and purchase M-code-capable receivers in the same numbers under its plan and all three options. By CBO's estimates, the total cost for those three common items is $17.9 billion from 2012 through 2025—which is the amount to maintain, modernize, and control the GPS constellation through 2030 and to field military receivers that can take full advantage of the
All three options would cost less to carry out than DoD's plan and would yield military receivers with greater antijamming capability earlier. CBO's options would not yield similar benefits for civilian users as DoD's plan, and they would forgo some improvements in accuracy for all users offered by the GPS IIIC satellites. Because all three options would cancel the IIIB and IIIC portions of the GPS III program, they would save more than $4 billion on satellite and ground control costs from 2012 through 2025, CBO estimates. Those savings would be partly offset by the cost to make improvements to the receivers under the three options, yielding net savings of approximately $2 billion, $3 billion, and slightly more than
$1 billion, respectively, for Options 1, 2, and 3.
Option 1. Improve the Capabilities of Military GPS Receivers
Option 1 would augment military receivers to provide users with a better ability to keep track of their location in jamming environments. The improvement would come from new antennas—capable of rejecting signals from jammers—and from the integration of very small inertial navigation systems, which would reduce location errors introduced by interference and enable users on the move to determine their position accurately even after losing the GPS signal entirely.
By increasing the level of noise that receivers could tolerate and still be able to detect and process the GPS signal, those augmentations to receivers could reduce the effective range of a wideband noise jammer by 97 percent. (The effective range of a 10-watt jammer would be decreased from 55 miles to about 2 miles, which means that the jammer would need to be within 2 miles of the receiver to have an effect.) Because the hardware for the improvements in this option has already been developed, modifications to existing receivers could begin almost immediately, and a significant number of improved receivers could be in the field by 2018. The additional signal power from the IIIA satellites and the antijamming capabilities of M-code receivers would further enhance the overall capability of military receivers to operate in the presence of jamming. By 2026 (the point at which about half the force could be equipped with M-code-capable receivers), the combined improvements under this option would reduce the range of a 10-watt jammer by 99.3 percent, to 0.4 miles.
Option 2. Enhance GPS Using the Iridium Satellite System
Option 2 would capitalize on a DoD initiative to use an existing satellite constellation in low-earth orbit to provide military GPS receivers with information that would enable them to operate better in a jamming environment. The program—known as High Integrity GPS, or iGPS—would rely on the commercial Iridium satellite communications network to relay data to modified military GPS receivers, allowing them to more easily pick up and maintain signals from the GPS satellites located in medium-earth orbit.
In addition to enabling military receivers to process data from the Iridium satellites, this option would integrate inertial navigation systems into the receivers in a procedure similar to that in Option 1 (but would not incorporate the improved antennas of that option). The extent of the improvements in military receivers' capability resulting from this option would be somewhat greater than that from Option 1 and could be realized just as quickly—by 2018. When combined with the contributions from IIIA satellites and M-code-capable receivers, the total improvement would reduce the range of a 10-watt jammer by 99.7 percent, from 55 miles to about 0.14 miles, by 2026. Unlike Option 1, however, this option would have the added advantage of improving the accuracy with which military users could determine their position.
Option 3. Combine Options 1 and 2
Option 3 would combine the enhancements in Options 1 and 2. Specifically, it would provide military receivers with improved antennas and inertial navigation systems and also enable them to incorporate data relayed by the iGPS network. The combined improvements to military receivers with M-code capability would reduce the effective range of a typical 10-watt noise jammer by more than 99.9 percent, from 55 miles to 0.025 miles (or slightly more than 130 feet) when all the components of this option were fielded.
Advantages and Disadvantages of the Options
Compared with DoD's plan, the options would yield greater improvements in reception and would yield improvements sooner. Under DoD's plan, the full benefit of the increased M-code signal power of the IIIC satellites would not be fully realized until 2030, when the 16th IIIC satellite could be in orbit. The earliest benefits would probably come once the constellation of 18 GPS III satellites—comprised of 8 IIIA and 10 IIIB satellites—is in orbit, scheduled for 2022, but only small numbers of military receivers capable of processing the stronger M-code signals would be in the field then. While the IIIC satellites were being placed in orbit, the benefit of their stronger signals would be unavailable to users until sufficient numbers of M-code-capable receivers were fielded, possibly no earlier than 2026.
In contrast, the technologies included in CBO's options—those for improved antennas for GPS receivers, small inertial navigation devices, and iGPS—have already been developed. The fielding of ancillary devices to augment existing military GPS receivers could begin in a few years, with appreciable numbers of improved receivers in the field by 2018. Consequently, the options could increase the military's antijamming capability eight years before large numbers of M-code receivers could be in the hands of military users under DoD's plan.
Additional advantages of Options 2 and 3 come from augmenting the GPS constellation with the Iridium satellites in low-Earth orbit. That fuller coverage would virtually ensure that receivers had a line of sight to at least one satellite, even in mountainous terrain and urban settings where tall buildings block the view of the sky. In addition, because data can be received and updated frequently, receivers using iGPS can determine their position with almost the same accuracy as would be possible using data from IIIC satellites—but the receivers would have that ability several years earlier.
The options would have several disadvantages when compared with DoD's plan, however. All of the options would require hardware additions to existing receivers: an improved antenna and integrated inertial navigation system for Option 1; a module to interpret data relayed from Iridium satellites and an inertial navigation system for Option 2; and all of the above for Option 3. Each of those hardware devices might not be much larger or heavier than a typical military handheld receiver, which is about six inches long and weighs about a pound, but when combined, they would add bulk and requirements for additional power. Although that added weight might not prove too onerous for military personnel in a vehicle, plane, or ship, it could prove troublesome for those on foot. The current trend in miniaturization has made it possible to integrate such devices into military receivers designed specifically for use in munitions (such as cruise missiles or small guided bombs) or in the confined space of very small unmanned aerial vehicles. But, designing and integrating such miniaturized devices for and into existing receivers would take time and could entail costs not included in CBO's estimates.
Another disadvantage common to the options is that they would forgo the improvements offered by the IIIC satellites, so military users would not benefit from the increased power of the M-code signals within the spotbeam. Those signals, which would be roughly 30 times stronger than those transmitted by IIIA satellites, could be advantageous for users who could not handle the added weight and power needed for improved antennas or who could not take advantage of the iGPS program. Forgoing the IIIC satellites would also mean the loss of the ability to determine position to within about 6 inches for civilian users under all of the options and for military users under Option 1. In those cases, GPS users would have to rely on the less accurate signals from the IIIA satellites, allowing them to determine their position to within about 3 feet. The addition of iGPS in Options 2 and 3 would allow military—but not civilian—users to determine their position to within 8 inches—almost as accurately as would be the case under DoD's plan. That increased accuracy might not be important for most applications, but it could be useful when items need to be located precisely, such as in the case of land mines or unexploded ordnance.
A final disadvantage shared by Options 2 and 3 is the dependence on the commercial Iridium satellite constellation and support network, which is not controlled by DoD. The future of the Iridium constellation cannot be guaranteed by the government, at least not without a cost that CBO has not included in its estimates. Although apparently financially healthy at the end of 2011, the Iridium communications system has had financial setbacks in the past. The risk that DoD might need to infuse funds into the system in the future is a disadvantage of relying on iGPS to enhance the ability of GPS receivers to operate in a jamming environment.