Ground-Based Augmentation

There’s no question that, over its past quarter century of operation, the U.S. Department of Defense (DoD) GPS system has revolutionized navigation for civil and military users worldwide. Apart from its use in guiding smart bombs, unmanned air vehicles (UAVs) and the like, the system’s remarkable accuracy allows civil surveyors to routinely measure distances to fractions of an inch, while geologists have been able to determine that India is slowly disappearing under the Himalayas towards China at a speed of around 5 inches (12 cm) per year.

Yet curiously, one seemingly obvious GPS application–that of safely guiding aircraft along precision landing approaches in low visibility–continues to elude researchers. For while the system can now be used to bring aircraft to "near Category [or Cat] I" limits of 250 feet above the runway, it still has not been certified to continue down to the full Cat I ceiling of 200 feet, or even lower, which remains the exclusive domain of ILS, developed more than 60 years ago. The issue here is not one of accuracy, which GPS clearly has in spades, but in the integrity, or trustworthiness, of its signals and their availability and continuity throughout the landing approach. It is in these areas that both FAA and DoD continue to conduct extensive tests.

Reaching these goals is now a DoD imperative since the agency has determined that differential GPS, under the Joint Precision Approach and Landing System (JPALS) program, will be the foundation technology for its aviation components, particularly those of the U.S. Navy. FAA’s testing of its somewhat similar local area augmentation system (LAAS) is considerably less urgent since, for reasons discussed later, it has downgraded its GPS precision landing guidance program to a research and development project. (Interestingly, FAA is increasingly using the International Civil Aviation Organization [ICAO] term, ground-based augmentation system, or GBAS, rather than LAAS.) Both JPALS and LAAS use the differential GPS correction technique for landing guidance, where an airport- or ship-based station analyzes satellite signals received by several nearby monitors to determine the local GPS errors. The information is then used to calculate correction signals data-linked up to receivers aboard approaching aircraft.

JPALS

Initiated in 1995, the JPALS project was intended to meet the needs of all four services in the U.S. military. Since then, there have been predictable interservice disagreements–and the occasional threat by one or another service to pull out. But gradually most differences have been ironed out, and the program now is firmly established. Nevertheless, it was necessary for Navy Capt. Barbara Bell, program manager for air traffic control and combat identification systems, to stress to a bidder’s conference late last year that "This is now a serious program."

And indeed it is. The seven-year, cost-plus, design and development effort, alone, is estimated to cost around $300 million, and the planned follow-on, five-year production phase could exceed $2 billion, according to industry observers. Eventually, DoD wants to field 157 airport-based and 288 ship-based JPALS stations, as well as 69 tactical stations and an unspecified number of manpack units, to support avionics installations across the more than 13,000 fixed-wing, helicopter and UAV assets operated by the four services.

Underlining the importance of JPALS to DoD, the project has been designated a potential Acquisition Category (ACAT)-1D program, nomenclature that typically applies to major programs like new aircraft or ships. One DoD official states that the project now has become one of the top 10 at the Pentagon, and noted, "Our present radars, TACANs, communication methods and shipboard landing systems just can’t support the needs of advanced warfighting technologies, such as UAVs and the Joint Strike Fighter." Echoing Capt. Bell, he says, "The program has never been more solid over its entire history."

A New ‘Function’

Correspondingly, therefore, the JPALS stations and their associated avionics aren’t regarded as simply new units to be placed at runway ends, bolted to ship’s decks or somehow squeezed into already crammed aircraft radio racks. In DoD parlance JPALS will introduce a new operational "function," which means, for example, that its avionics components and their integration throughout the airframe will be as critical as the aircraft’s present flight control and flight management systems, and almost as critical as its engines.

The main service driver for JPALS is the Navy, which requires an extremely accurate approach and landing system for its current aircraft carrier fleet and its future CVX carriers, while operating within a highly secure, nonvoice, communications data link environment. "Extremely accurate" here means that the landing aircraft’s arrester hook must pass through an imaginary box (3 feet [1m] on a side) at the stern, poised precisely 14 feet (4 m) above the centerline of the deck, in order to exactly catch the third of the four arrester cables stretched across the deck. To do this, the guidance accuracy must be within 6 inches (15 cm) of the ideal approach path, and with integrity assurance of no more than 3.6 feet (1.1 m) in 10 million landings.

This is a formidable target, made even more so by the fact that the carrier deck can be pitching, rolling, yawing and heaving as the aircraft approaches. Differential GPS, even with the survey-quality, relative kinematic phase tracking technique adopted by the Navy, could not achieve this performance. The error corrections uplinked to the approaching aircraft, therefore, are continuously augmented with inputs from the ship’s own inertial navigation system to compensate for deck motion.

The same technique will be employed on smaller Navy vessels, including future DDX destroyers, which will launch and recover helicopters and UAVs. In fact, the kinematic tracking data links, transmitted over the military’s encrypted GPS Y-Code, would be employed in all seagoing JPALS stations, but nontactical, land-based units also would transmit GPS differential corrections in ICAO standard civil data link formats.

At the same time, while JPALS will support fully automatic approaches and landings to extremely high accuracy and signal integrity levels, it will not be designed to meet the complete range of requirements called for in a civil Cat III installation designed for near-zero visibility operations.

DoD’s initial requirements will be to provide the equivalent of a civil Cat I system, with guidance to weather limits of a 200-foot ceiling and a half-mile visibility, although the eventual aim for carrier operations will be to achieve 100 feet and a quarter mile.

Autocoupled seagoing tests carried out on the USS Roosevelt, plus land-based tests at Holloman AFB, N.M., and elsewhere have proved the JPALS concept. But one important phase remains to be completed before the preliminary acquisition process can begin. This is the technology development phase, now under way at ARINC, which is at the final risk reduction stage. This will complete the outstanding signal integrity enhancements and define the system’s vital anti-jamming protection measures, without which both carriers and aircraft would be unacceptably vulnerable to adversary interference, as would the land-based tactical and manpack stations.

A draft request for proposals (RFP) covering the initial $300-million system design and development (SDD) phase is expected in August 2006, with a final RFP late in the year. Bids are expected in early 2007, and contract awards to one or more teams in the third or fourth quarter of 2007. Program officials will select a single contractor 12 to 15 months after the initial contracts are awarded. The SDD phase will produce six or more preproduction shipboard and ground stations, and various avionics packages, for extensive testing prior to the final, full-rate production award. This is currently expected in 2014 and is generally valued at around $2 billion, although some observers have pegged it at closer to $3 billion.

Industry sources so far have identified two potential bidding teams, although others may emerge before the final RFP is released in November. It is reported that ARINC may team with Raytheon and, possibly, Rockwell Collins, while Honeywell may team with Boeing and possibly Sierra Nevada Corp. All these players have powerful strengths in their specialized areas. At the same time, other companies, such as BAE Systems and the UK’s QinetiQ, are expected to be involved, along with many other firms with unique capabilities.

LAAS

For FAA the gestation of its GPS local area augmentation system has been a roller coaster ride of promise and disappointment since the effort began in the late 1980s to develop GPS into a certifiable civil landing aid. The promise has always been there, but efforts to meet the approval criteria have been consistently disappointing. There have even been some internationally embarrassing periods for the agency.

In the late 1970s FAA launched development of the microwave landing system (MLS) as the successor to ILS, which was seen as becoming obsolete. The United States then persuaded ICAO to adopt MLS as the future international replacement for ILS, and the world transition was planned to commence in 1998. Accordingly, several nations launched their own MLS development and manufacturing programs.

But in 1995, after a number of U.S. and overseas MLS installations were already in operation, FAA proposed to ICAO that MLS had now been superseded by the soon-to-be-available LAAS. Again, ICAO agreed, and the MLS transition was canceled. But today, more than 10 years later, a certified LAAS has yet to appear, and FAA has relegated the system to a research and development activity. Meanwhile, technology advances have ensured that ILS will escape obsolescence for many years to come.

Basically, the problem with LAAS lay, and still lies, in assuring the system’s integrity, and in its inability to warn the pilots without failure of the presence of hazardously misleading information in its guidance signals. Unquestionably, as with JPALS, these problems will be overcome. But unlike JPALS, LAAS must meet the much more stringent standards of Cat III (near zero visibility) landing guidance before the airlines will adopt it. Unfortunately, all LAAS development activity so far has been directed at achieving certification to the much less demanding Cat I level. This would provide no better service than its ILS equivalent, of which there are around 1,200 in the United States and several hundred overseas, and for which the airlines are already equipped.

There is, therefore, as a senior FAA official stated in 2005, "no business case" for Cat I LAAS. In fact, a recent FAA Satnav News publication stated that "FAA does not intend to resume development of a federal LAAS facility."

Nevertheless, Cat I is regarded as a necessary technical stepping stone to the ultimate Cat III system and, while efforts continue to achieve Cat I certification with a test system at Memphis and others in Australia, Germany and elsewhere, Cat III has become the Holy Grail for LAAS.

In the past most observers felt that achieving certifiable Cat III LAAS ground station performance would be an extremely challenging task–some would say an almost impossible one. But recent FAA studies suggest that the aircraft avionics could also play a valuable role. FAA GNSS Manager Leo Eldredge points out that FAA and Boeing are interested in leveraging the potential of today’s tightly integrated autoland, inertial reference and radio altimeter systems–sometimes called an aircraft-based augmentation system (ABAS)–to assist in achieving LAAS Cat III with a less complex LAAS ground facility.

"Already," Eldredge states, "Boeing’s implementation of ABAS is used to `smooth out’ occasional ILS vertical guidance irregularities, and use of that technology may eventually enable LAAS Cat III operations," using a 32.8-foot [10-m] vertical alert limit. "And that’s interesting," he notes, "because our original vertical alert limit target for a Cat I LAAS was just that, 10 meters." (The vertical alert limit defines the error boundary that the guidance signals, normally accurate to around 6.6 feet [2 m], must not exceed.) Eldredge stresses, however, that the vertical alert limit was only one of the essential requirements for Cat III; the others are integrity, continuity and availability.

No date has been forecast for Cat III certification. But it is not expected before sufficient GPS satellites transmitting on two civil frequencies–to eliminate ionospheric errors–and supplemented by Europe’s dual-frequency Galileo constellation, are in orbit after 2012.