Tuesday, October 1, 2002
JPALS: Not Just LAAS in Navy Uniform
The seagoing Joint Precision Approach and Landing System for the U.S. Navy provides much more than GPS differential accuracy corrections. It uses data link to give pilots a plethora of data from a host of sources.
When the U.S. Department of Defense opted for the Joint Precision Approach and Landing System (JPALS) in the mid-90s, most observers understood that this would be the military’s version of the GPS-based Local Area Augmentation System (LAAS), which is being developed for the Federal Aviation Administration (FAA). And to a certain extent, it will be. When deliveries commence around 2010 to the Army, Air Force, Marine Corps and Navy, land-based JPALS installations will closely resemble the FAA system.
But the seagoing JPALS will be a horse (or a LAAS) of a different color. One of the biggest differences will be its data links. For, as development has evolved, carrier-based JPALS has become a generic term applied to a wider data link environment than just the automatic landing portion.
Land-based units will use the conventional, single VHF data link to transmit local differential GPS accuracy corrections to landing aircraft. But aboard an aircraft carrier, accuracy corrections will be just one of many elements in a complex data link environment. The Navy is expanding the environment around the basic JPALS to monitor and control the entire operating envelope of each of its aircraft, from before takeoff to after landing.
In fact, the Navy’s seagoing JPALS will be the centerpiece of a dedicated, data link-based, communications, navigation and surveillance/air traffic management (CNS/ATM) system, which will be aboard each of its 12 carriers. The Navy needs such a capability to provide safety, airspace management and, of course, surveillance protection against adversaries, as the vessel moves away from the mainland and across oceans, often towards unfriendly territory.
In a way, it will be like picking up a complete FAA air route traffic control center (ARTCC) from the mainland, along with all its radars and infrastructure, and shoehorning it into an aircraft carrier. And since the carrier’s raison d’etre is to extend military air power in all weather, you could even say that the seagoing JPALS’ ultimate purpose is to thread the tip of an autolanding aircraft’s arrester hook through an imaginary 9-square foot (0.83-square meter) box centered precisely 14 feet (4.3 meters) above the pitching and rolling stern of a carrier in very low visibility, by day or night.
What’s more, the aircraft are mostly F-14 Tomcats and F/A-18 Hornets, to be joined in the future by unmanned combat air vehicles (UCAVs) and Joint Strike Fighters (JSFs). The Navy’s JPALS will be a far cry from its shore-based siblings.
Easier Said Than Done
JPALS got its marching orders in August 1995, when the Pentagon’s Joint Requirements Oversight Council (JROC) issued its "mission need statement." This document called for "a rapidly deployable, adverse weather, adverse terrain, survivable, maintainable and interoperable precision approach and landing system, on land and at sea, that supports the warfighter when ceiling and visibility are limiting factors."
While that sounds easier said than done, it precisely dovetailed with Navy warfighting doctrine, which dictated that future aircraft carrier operations would be highly automated and highly secure but as accident-free as possible. Catapult launches and deck landings would be fully automatic. Communications, navigation and surveillance would employ secure data link transmissions. And continuous data link monitoring of each aircraft, from before launch to after landing, would greatly enhance overall flight safety.
For such totally integrated activity, it was not sufficient to simply bolt the newest landing guidance system to the carrier deck, which essentially has been the practice to date. While effective, the Navy’s current AN/SPN-41 and -46 instrument and automatic carrier landing systems (ICLS and ACLS) are virtually stand-alone aids. They are not tightly integrated into the overall carrier task force command and control structure. And neither the microwave scanning beam SPN-41 nor the radar-based SPN-46 is accurate enough to meet autoland requirements.
Navy planners, however, saw that the military JPALS, developed by Raytheon, could become the core element of the command and control networks of its future carrier task forces, although a substantial amount of development would be required. The project is now in the component advanced development phase. When the total system reaches operational status at the end of the decade, its research and development costs are expected to have topped $500 million, and production and maintenance costs through 2025 are forecast to reach around $570 million.
What will the Navy get for its money? Essentially, the service will receive a secure, low probability of intercept (LPI), wideband UHF data link network on each of its aircraft carriers. Each carrier-based network will integrate all aspects of communications, navigation and surveillance, plus air traffic management, out to a radius of 200 nautical miles (nm). All signals will have high update rates, with high integrity and fault tolerance. The network will use covert, antijam technologies and "featureless" spread spectrum transmissions to try to prevent adversaries from determining the sender’s location. It will support complementary levels of data links over three concentric radii of 20, 50 and 200 nm around the carrier.
Throughout the 200-nm radius, each aircraft’s inertial navigation/GPS system will be augmented by ship-to-air data link coverage to provide high-accuracy positioning, both relative to the carrier and to other aircraft in the area, such as tankers. Project officials sometimes describe this positioning system as "TACAN-like," referring to the current use of the UHF tactical air navigation (TACAN) system, which provides pilots with distance and bearing information from the carrier. However the inertial/GPS/data link combination will provide much more information and will replace TACAN, which has data limitations, along with inherent security vulnerabilities.
From the information data-linked up from the carrier, pilots will be able to monitor the whole air and surface scene on their multifunction cockpit displays of traffic information (CDTI). The information also will include the location of other aircraft, which have been detected by the surveillance radars of outlying carrier task force vessels and data-linked up. (The use of other vessels’ radars is particularly valuable when the carrier’s radars are shut down in a high-threat environment.) Targets detected by patrolling airborne warning and control system (AWACS) aircraft will be data-linked across, as well. All "friendly" CDTI targets will, of course, carry positive identification, friend or foe (IFF) tags. Each aircraft also will carry collision avoidance avionics, which will monitor the surrounding airspace out to 20 nm and, again, present their alerts on the CDTI.
Within about 50 nm of the carrier, manned and unmanned aircraft will come under two-way controller pilot data link communications (CPDLC) coverage of the vessel’s air traffic control (ATC) system. For security purposes, voice communications will be either passed through voice recognition "firewalls" or be digitally synthesized. The carrier’s ATC facility also will accurately track each aircraft via automatic dependent surveillance (ADS). Inside the 50-nm zone, an endurance management air traffic system (EMATS) data link will obtain each returning aircraft’s position, fuel state, weapons status and other factors. The EMATS link will determine the aircraft’s optimum time of arrival, landing priority and projected landing weight, and sequence it into a four-dimensional–latitude, longitude, altitude and time–arrival traffic stream of manned and unmanned aircraft. EMATS also will integrate the arrival and departure flows, although the latter will be less demanding.
At a 20-nm range from the carrier, the autoland level of data link operations kicks in. Uplinked data will include landing weather conditions, such as the all-important wind speed and direction over the carrier deck, plus the continuously updated GPS differential accuracy corrections similar to those required for a LAAS-like precision approach. The corrections will be similar but with much higher accuracy than those used in LAAS or the shore-based JPALS units.
To assure the exact positioning of the aircraft’s arrester hook within the very small area on the carrier deck, the Navy turned to the commercial survey industry’s real-time kinematic (RTK) GPS technique, which uses the carrier phase of the GPS signals to achieve accuracies within centimeters. The Navy requires horizontal and vertical accuracies of less than 15 cm (5.9 inches), with integrity assurance of no more than 1.1- meter (3.6-foot) error in 10 million landings.
Remember that imaginary 9-square foot box? The Navy has proved that accuracies of this type are possible in autolanding exercises with F/A-18s and other aircraft on carriers and land facilities, using a modified, prototype JPALS system in conjunction with RTK. The service also has demonstrated the system’s immunity to GPS jamming.
Carrier pilots, however, also must contend with the challenge of landing on a very short and very narrow runway that also happens to be rising, falling, rolling and yawing beneath them as they are about to touch down. And while doing this, they must also stay exactly on the center line, since other aircraft are parked just 20 feet (6.1 meters) off either wingtip.
But JPALS has the answer here, too. Real-time corrections for deck movement, derived from the carrier’s inertial navigation system, are continually uplinked to an aircraft, as it makes its final approach. The corrections are fed to the autoland system, which makes attitude adjustments all the way to touchdown–or, more precisely, to the point of placing the arrester hook exactly between the second and third arrester cables, four of which are stretched across the deck 40 feet (12.2 meters) apart. JPALS will bid farewell to the "bolter," that colorful expression used by naval aviators for an aircraft whose arrester hook misses the cables and is forced to make a missed approach. But in tomorrow’s Navy, bolters may simply be an undesirable impediment to 4D, high-speed traffic flows.
There are many industry contributors to the JPALS program–too many to mention individually. But, perhaps surprisingly, one organization that has played a leading role in the development and engineering of JPALS’ complex software is ARINC Inc., Annapolis, Md. (www.arinc.com). A major player in the sedate world of airline avionics, ARINC also has vast experience in aviation data links, from the earliest days of the airborne communications addressing and reporting system (ACARS) to future civil aviation digital data links. But it was the combined excellence of all members of the program team that caused the Pentagon’s Defense Standardization Program Office (DSPO) to present its 2001 DSPO Achievement Award to the JPALS program.