Demonstration system for using shipboard-relative GPS

GPS World, April, 2005 by Kathleen Boseley, Jim Waid

This article presents the Honeywell H-764 Advanced Configurable Embedded GPS/Inertial (EGI)--or ACE--demonstration system design, developed in support of the shipboard relative GPS (SRGPS) program, now known as the Sea Based Joint Precision Approach and Landing System (JPALS). Emphasis is placed on the system architecture and the integrity design with additional discussion of SRGPS development phases, the relative navigation Kalman filter design, and a review of the program's status.

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The United States Department of Defense (DoD) JPALS Program seeks to meet the DoD's need for an all-weather, anti-jam, combat-ready, Category II/III aircraft precision approach and landing system. JPALS must be fully interoperable among all the branches of the service with a large number of different platforms while meeting a wide range of landing scenarios. It must also be compatible with planned civil system updates incorporating the same GPS-based technology.

Sea Based JPALS is a specific subcategory of JPALS being developed to provide navigation for autonomous shipboard landings in zero-visibility conditions. Shipboard landing is considerably more demanding than a typical land-based approach and landing. As a result, the requirements are also more stringent and difficult to meet. Higher accuracy and improved continuity of function are key elements in the Sea-Based JPALS design that will assist the military in completing this critical and challenging operation. Due to the more demanding requirements, Honeywell and the U.S. Army and U.S. Navy are investigating the suitability of an inertially-aided kinematic carrier phase differential GPS (DGPS) solution.

The U.S. Air Force, U.S. Navy, and U.S. Army are using the Honeywell H-764 ACE as a demonstration system for Sea Based JPALS operations because of its direct architecture lineage to the majority of multi-service aircraft. For Sea Based JPALS a new relative navigation solution is being designed that integrates inertial data from both the ship and the aircraft with GPS carrier phase measurements from both platforms. A unique (with respect to land-based) feature of shipboard approach and landing is the fact that the touchdown point and the runway itself are both moving with respect to the earth. This complexity introduces the need to define the approach and landing path dynamically. For this reason, we are developing a "relative" solution for the Sea Based JPALS demonstration program. The position of the aircraft in absolute space (traditionally computed with differential GPS (DGPS) correction based systems) is not our primary concern. We are interested in defining the aircraft's position and velocity with repect to the moving runway and touchdown point as accurately as possible. The solution of interest becomes the relative vector defining the spatial separation of the aircraft and the touchdown point on the aircraft carrier.

To implement the relative navigation solution design and to comply with the stringent accuracy requirements, modifications to the hardware are required. Changes to the hardware configuration include the integration of an embedded 24-channel GPS sensor as well as an additional system processor that will be dedicated to relative solution processing. This article presents the Sea Based JPALS risk-reduction demonstration system EGI design, emphasizing the hardware and software changes required to accommodate the dual-processor architecture and the 24-channel GPS sensor. We will cover system requirements, architecture design considerations, solution integrity, system modeling, simulation and test, and a general review of the program's status.

Sea-Based JPALS

Multiple civil and military systems currently exist that provide precision approach and landing for aircraft. Table 1 provides a partial list of existing systems.

As evident from Table 1, none of these standards is currently being employed by both military and civilian users for precision approach landings. During the early 1990s, the U.S. Federal Aviation Administration (FAA) discontinued its MLS deployment plans in favor of GPS-based precision approach and landing. With that decision, the development of performance standards became the focus of working groups in both the United States and Europe. The U.S. military quickly initiated a parallel effort defining their next-generation landing system. One of the goals for the evolving system was to develop a system that would be common to all branches of the military, and JPALS was born.

JPALS Background

In the mid-1990s, the DoD recognized the need for a system to enable its aircraft to land on any suitable surface worldwide during peaceful and hostile times under any weather conditions. The JPALS program began in May 1996 and is now in the technology development (TD) phase, aimed at defining and solidifying the requirements for the system development and demonstration (SDD) phase targeted to begin in 2006.

Honeywell has been extensively involved with the airborne system aspects of the Sea Based JPALS program since 2001. During this time, Honeywell's tasks have been organized as phases including the execution of various trade studies (Phase 1A), the definition of requirements (Phase 1B) and the design and eventual building and testing of three Sea Based JPALS EGI units for demonstration purposes (Phase 1C). The 24-month Phase 1C effort has been segregated into two phases, the soon-to-be-completed Phase 1C1 and Phase 1C2, which is scheduled for completion in December.