Human Factors and Operations Issues in GPS and WAAS Sensor Approvals: A Review and Comparison of FAA and RTCA Documents
Executive Summary
This report is the culmination of the first task in a project to evaluate human factors and operations issues associated
with the integration of Class C Global Positioning System (GPS) sensors and Class Beta GPS/Wide Area Augmentation System
(WAAS) sensors into navigation systems in low-end transport category aircraft. The objective of the project is to provide
aircraft certification specialists with information that will help them in the evaluation of GPS and GPS/WAAS sensors within
integrated navigation systems. The first task of the project was to compare the requirements of the various FAA and
industry documents currently used in the approval of GPS and GPS/WAAS sensor equipment and installation. The impact of
the use of required navigation performance (RNP) standards on approval also was evaluated.
This document describes both the GPS and GPS/WAAS navigation systems and equipment classifications. The major difference
between GPS and GPS/WAAS systems is improved accuracy, integrity, continuity, and availability of GPS/WAAS systems (through
the use of differential corrections) over GPS systems. Class C GPS sensors, together with an integrated navigation system,
may be approved as supplemental means of navigation, while GPS/WAAS Class Beta sensors, when combined with an integrated
navigation system, are expected (in the near future) to be approvable by the FAA as a primary means of navigation.
The GPS and GPS/WAAS equipment approval process is described. Five approval documents 1) TSO-C129A, 2) RTCA DO-208, 3)
RTCA DO-229, 4) AC 20-130A, and 5) RTCA 192-96 and three supporting documents were reviewed. The review served two main
purposes: 1) evaluation of the approval process and the identification of problems within the process and the documents,
and 2) a detailed comparison of the requirements of the different documents for GPS and for GPS/WAAS sensors.
The evaluation of the documents and discussion with FAA personnel revealed potential areas for improvement of the approval
documentation. Some of the documentation is difficult to understand and apply. Individuals at different Aircraft
Certification Offices (ACOs) as well as manufacturers and operators may have different interpretations of the same
information. At the time this document was prepared, not all of the FAA policy documentation regarding GPS/WAAS equipment
was available. With GPS equipment, the documentation is difficult to use because it requires a great deal of cross
referencing. Another problem is the distinction between technical standard order (TSO) authorization of equipment and
approval of installations. TSOs are intended only to address equipment requirements, and cannot address installation
requirements. TSO-C129 specifies requirements that apply to the multisensor system with which the equipment is to operate.
This ensures that sensors provide an acceptable interface, but it also places a burden on the applicant who must use an FMS
in order to demonstrate compliance. In many cases this situation is further complicated by the fact that the applicant is
not the manufacturer of the FMS. In addition, some ACO engineers interpret the equipment interface requirements of TSO-C129
as installation requirements and are unsure how to approve equipment (in accordance with a TSO) independent of an installation.
The comparison of the requirements specified in the documents is presented in table form and major differences are
discussed. The comparison revealed a high level of similarity between GPS and GPS/WAAS sensor requirements. While many
requirements are the same, they are organized and worded differently, so the only way to identify common requirements is
to carefully read each document. There were a number of differences between the GPS and GPS/WAAS requirements that did
not appear to be specifically related to conceptual differences between the systems. Rather, many of the differences may
be due to the increased availability of information at the time of the GPS/WAAS publication (1996) than at the time of the
GPS publication (1991). One difference in requirements that was due to technological differences between the two systems
was the treatment of accuracy and integrity requirements, including alerting and annunciation of accuracy or integrity problems.
The impact of required navigation performance for area navigation (RNP RNAV) on GPS and GPS/WAAS approvals also was
considered. RNP RNAV is a concept intended to enable user-preferred trajectories and to promote free-flight by specifying
minimum accuracy and integrity requirements for these operations. GPS and GPS/WAAS manufacturers may want to meet RNP RNAV
requirements as well as GPS equipment requirements. A comparison of GPS and GPS/WAAS requirements with RNP RNAV
requirements (as described in RTCA 192-96) revealed a few requirements unique to RNP RNAV approvals though GPS/WAAS
requirements include requirements that are equivalent. Other than a requirement for parallel offsets and additional
holding capabilities, equipment that meets the human factors and operations requirements of RTCA DO-229 for GPS/WAAS should
meet the requirements of RNP equipment. Equipment approved to TSO-C129 rather than TSO-WAAS will have to be evaluated more
carefully to ensure that all of the additional requirements of RTCA 192-96 have been met.
This review showed that the documentation and process for GPS and GPS/WAAS Class C sensor approvals is somewhat
complicated. Generally, the documentation is thorough, but in some cases is difficult to interpret and difficult to
cross-reference. There are efforts underway to account for some of the difficulties related to use of the documentation.
First, TSO-C129 is being updated (TSO-C129B) to refer to the newer requirements of RTCA DO-229. Second, a checklist for
approval of GPS/WAAS equipment is under development.
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