By Brad Parkinson.
Position, navigation, and time (PNT) are essential enablers for warfighter capabilities. They are used in virtually every weapons system of the Department of Defense. The GPS system has become the ubiquitous provider of this military service. In addition, GPS is the backbone of scores of civil applications that have provided startling improvements in safety, productivity, and convenience.
Credit for this achievement should go to the thousands of developers, researchers, and operators. In particular, Air Force Space Command under the leadership of Gen. Willie Shelton has consistently recognized its global stewardship for GPS, the stealth utility.
That said, the job is far from over. New threats, needs, and challenges must be met. The essential overarching goal is PNT Assurance. While GPS is an outstanding system, there are still areas for improvement. In providing PNT assurance, what should be the highest priorities for those improvements? Of course an answer to this question could involve many aspects or dimensions. The GPS Independent Review Team (IRT) focused on a number of attributes it designated as The Big Five.
Instead of the Big Five, for the purpose of this discussion, I would like to examine three key attributes. These could be applied to GPS or any other, alternative, PNT system.
I call these three essential attributes the Three As. They are:
I will discuss each briefly and then add some improvement goals for each attribute. I call these improvements my personal Druthers.
Availability of Position, Navigation and Time
Without assured PNT availability, the warfighter cannot depend on the effectiveness of his weapon systems. Neither can civilian users count on their attendant benefits. To achieve GPS availability, the first requirement is adequate satellite geometry. Fewer than four satellites in view implies that the user will not have a PNT solution. A military user in the middle of a desert does not stress this geometry problem. More difficult is warrior support in mountainous or urban terrain. The steep mountains of Afghanistan can cause availability outages exceeding 10 hours per day for the currently specified 24-satellite constellation. The Department of Homeland Security has similar challenges in urban areas. Many effects-based studies have shown that 30 active satellites plus three spares are the knee in the availability curve.
A 30-satellite constellation plus three spares (optimally distributed) greatly increases availability for the sky-challenged user. Special Operation Forces in mountainous areas or Army forces in villages have precision location and can promptly designate fleeting targets of opportunity. A 30-satellite constellation assures civilian emergency service providers that they can meet their obligations in domestic urban canyons.
There are two new GNSS programs being developed that emulate GPS, named Galileo and Compass. They have made similar availability calculations and both are nominally sized at 30 satellites or more.
To maintain GPS as the gold standard, I therefore propose my first druther:
Druther One. The Department of Defense (DoD) should define the GPS constellation to be 30 satellites plus 3 spares distributed in an optimal manner.
The second aspect of availability is that the user must be able to receive the signal. Independent advisory groups have repeatedly called for increased interference-resistant solutions for the last 14 years. The technical solutions to produce virtually jam-impervious receivers are well-known. More than 33 years ago, the GPS Joint Program Office, allied with a creative program at Wright Patterson Air Force Base, demonstrated over 100 DB of J/S or anti-jam (AJ) resistance. This is enough resistance to defeat any jammer less than 1 kW in effective power. The techniques included deep integration with inertial units, controlled reception pattern antennas (CRPA), and averaging using low-phase noise clocks. To counter the problem of blinking jammers, the CRPA should be beam steering rather than null steering. This leads us to:
Druther Two. The installed GPS user equipment in both commercial and military aircraft should be able to fly directly over a 1 kW jamming source with no effect.
This is readily achievable with technology we understand. We need not employ high anti-jam techniques in all receivers; however, both the DoD and the Federal Aviation Administration (FAA) need to focus on GPS jamming resistance as a requirement. That said, the developers and manufacturers still must focus on affordability for these AJ solutions (see below).
To ensure availability, and to discourage the use of enemy jammers, the U.S. government should deploy augmentation, that is, backup systems. Recently, psuedolites (ground-based transmitters of GPS ranging signals) have become a focus for augmentation. I remain deeply skeptical concerning psuedolites in a fluid battlefield situation. Psuedolites do not perform well for attributes two and three: affordability (including operational complexity and support structure) and accuracy.
Alternatively, low-cost or navigation-grade inertial units are potentially viable augmentations, and the FAA is investigating enhanced versions of distance-measuring equipment (DME) and tactical area navigation systems (TACANs). In addition, a recent study highlighted the value of an enhanced long-range navigation (eLoran) system with its high-power, low-frequency signal. These augmentation alternatives deserve further study.
Spectrum Threats. Federal Communications Commission- (FCC-) licensed jammers are an emerging threat to GPS. Somehow, a myth has grown up that the GPS band is underutilized, and that additional services should be licensed in adjacent frequency bands. With well over a billion users, the GPS spectrum is definitely not underutilized.
An example of the licensing threat is the FCC tentative approval for high-powered, terrestrial, communication transmitters in the band immediately adjacent to GPS. This band had previously been reserved for quiet communication signals from satellites (including GPS corrections). Extensive independent testing has shown that high-powered terrestrial transmitters would have an immediate and devastating effect on military receivers, aviation and commercial receivers, including those used for precision applications such as farming. Fortunately this threat has been, at least temporarily, postponed. Many inquire why GPS is so fragile that it cannot tolerate high-powered transmitters in adjacent bands. Unfortunately, because the proposed 15 kW transmitters/jammers are not those of an enemy, we cannot bomb them. An enemy jammer of such magnitude would not get off so lightly. This leads to:
Druther Three. Ensure the Federal government, particularly the FCC, maintains the frequency bands adjacent to GPS as a quiet neighborhood as they are now.
Affordability of the PNT System
All Federal discretionary programs are under enormous budget pressure. With the threat of sequestration, the DoD is particularly susceptible. The doomsday budget may be rapidly approaching.
For GPS, the most visible segment is spacecraft. Many advocate dual-launch capability, for GPS launches. Launch costs are roughly half the cost of a satellite on orbit. Thus, dual launch could eliminate 25 percent of the cost for this capability. Of course, the real issue is the total cost of a satellite operationally deployed on orbit. A triple-launch capability, or satellite size reductions compatible with more affordable space launch vehicles, will help reduce this total on-orbit cost. This leads us to:
Druther Four. Total on-orbit GPS satellite cost should be less than $175 million.
The GPS program office recently initiated an affordable satellite design study to reduce satellite cost. The affordable satellite should broadcast all GPS signals, with no extra payloads except a laser reflector (a small passive device, added for accuracy).
Additionally, the radio frequency (RF) chain should be improved to create greater efficiency with either gallium nitride power amplifiers or traveling wave tube amplifiers (TWTAs). With the 30+3 orbital configuration, military power should be specified at a 15° Earth mask angle (rather than the standard 5°), which would significantly reduce the amount of RF power required. With an affordable 30+3 SV constellation, users should easily lock on to four, full-power satellites above a 15 degree elevation mask. No flex-power capability need be included since the advantages of the few DB that flex power offers are more easily obtained with user equipment modifications. The net result of these modifications could produce a reduction of approximately 75 percent in the power needs of an operational GPS satellite. Such reductions generate significant savings in satellite weight and cost, as well as making dual or triple launch much more easily achievable.
The military GPS user equipment (UE) program has come under considerable and warranted criticism because military UE does not afford the user the flexibility nor ease-of-use found in less-expensive commercial and/or civil GPS receivers. The current UE program office initiative to demonstrate the advanced design of front-end chips seems a good initial step. In addition to demonstrating representative military applications, the JPO should develop a simple, intuitive, GUI interface similar to existing commercial handheld devices such as Apple, Magellan, Trimble, Garmin, or TomTom. Further, to attain affordable jam resistance, the CRPA costs must be reduced using digital electronics and commercial practices.
This background leads to:
Druther Five. The military GPS user equipment (UE) program should include front-end interfaces conversant with the best commercial devices including small handheld receivers.
Druther Six. The AJ program should leverage modern advances in commercial digital electronics, producing more affordable CRPAs and using the state-of-the-art micro-electromechanical systems (MEMS).
Additionally, the GPS Control Segment should re-examine current and future requirements, particularly those related to training the relatively inexperienced military cadre. A shift to a more permanent, technically-sophisticated, civilian cadre is probably warranted, retaining a military operational commander to direct the essential warfighter capabilities.
In this discussion, accuracy includes bounded inaccuracy: limiting the probability of errant weapons and inaccurate positioning.
For the military, weapons delivery accuracy is usually parsed into three contributors:
- target location error (TLE),
- weapon location error (WLE), and
- weapon guidance error (WGE).
All three components can be affected by GPS accuracy. Focusing on the Special Operations, Army, and Marine operators, the TLE today is limited by the ability of the target designator to determine azimuth. To ensure weapon delivery accuracy is 5 meters or better, we need:
Druther Seven. The DoD should develop and deploy an affordable azimuth-determination device for forward observers with an accuracy that is better than one milliradian.
For GPS, accuracy and bounded inaccuracy is a combination of geometry and user ranging error for all users. Druther One assures the geometry for virtually all users, but it bears repeating here:
Druther Eight. The GPS operational on-orbit constellation size requirement should be set at 30 satellites plus 3 spares. This repeat of Druther One greatly improves both accuracy and availability for many users.
Further improvements can be made in the inherent GPS ranging error through more accurate and sustainable atomic reference systems (clocks) and more accurate measurement of GPS satellite positions (ephemeris) by the user segment. This leads to:
Druther Nine. The GPS program office should pursue a vigorous effort to improve spacecraft atomic reference systems (clocks) and provide retroreflectors onboard all operational GPS satellites.
This will prove particularly beneficial to all users because long-range ephemeris accuracy and clock predictions will improve significantly.
As a longtime participant and observer of the GPS program, I would like to submit this wish list (see sidebar) of druthers to government decision-makers. In particular, if the Department of Defense were to act on these requests, I would regard it as a wonderful Christmas present for all users. Hopefully it will be for an immediate Christmas rather than a Christmas in the indefinite future, which I may not be around to see.
Thank you for your attention.
Brad Parkinson’s Wish List
Availability of PNT
1. The DOD should define the GPS constellation to be 30 satellites plus 3 spares distributed in an optimal manner.
2. The installed GPS user equipment in both commercial and military aircraft should be able to fly directly over a 1 kW jamming source with no effect.
3. Ensure that the federal government, particularly the FCC, maintains the frequency bands adjacent to GPS as a quiet neighborhood.
Affordability of PNT
4. Total on-orbit cost of a GPS satellite should be less than $175 million.
5. The user equipment program must include front end interfaces conversant with the best commercial devices including small handheld receivers
6. The AJ program should leverage modern advances in commercial digital electronics, producing more affordable CRPA’s and using the state-of-the-art MEMS.
Accuracy, Bounded Inaccuracy
7. DoD should develop and deploy an affordable azimuth determination device for forward observers with an accuracy that is better than one milliradian.
8. The GPS operational constellation requirement should be set at 30 satellites plus 3 spares.
9. The GPS program office should pursue a vigorous effort to improve spacecraft atomic reference systems (clocks) and provide retroreflectors on all operational GPS satellites.
Bradford w. Parkinson was the original chief architect, advocate and Program Director for GPS. His numerous awards include the Draper Prize, sometimes considered the Nobel for engineering.
He adds, “All thoughts are mine, and should not be assumed to be the views of the GPS Independent Review Team, the Department of Defense, or any GPS manufacturer.”