RTCA Airborne GPS Antenna Testing and Analysis for a New Antenna Minimum Operational Performance Standards (MOPS)

During mid-year 2004, RTCA Special Committee SC159 established a working group (WG 7) to develop new airborne antenna Minimum Operational Performance Standards (MOPS), at the time, primarily for the purpose of including the GPS L5 signal reception. The Terms of Reference for this new working group also included updating the existing L1 MOPS in order to close some deficiencies, especially for more demanding aviation applications such as Precision Approach. This included adding requirements such as limits on group delay variations that had not been specified in the earlier MOPS. It was determined that the working group should use measured performance of existing MOPS and ARINC 743 certified antennas as a guide for defining the new minimum performance standards as well as the performance of combined L1/L5 prototype antennas, and that this performance should be determined via article testing. Initially, prototype L1/L5 passive antennas, similar to existing L1/L2 antennas, were furnished by an antenna vendor and tested when mounted on the standard 4 foot diameter flat ground plane. Unfortunately, the antenna gain pattern performance did not meet expectations, even at the L1 frequency. Thinking that this may have been influenced by the dual patch L1/L5 design, L1-only passive antennas were procured from two different antenna vendors, and tested. They did not meet expectations, either. As a result, development of the L1/L5 MOPS was put on hold, and more emphasis was put on updating the L1 MOPS. More extensive antenna testing was conducted on existing antennas, especially those designed specifically for air-transport aircraft (ARINC 743). The intent was, if possible, to specify requirements consistent with the installed base so that existing antennas in most installations would not have to be replaced. Also, requirements were updated to support testing of active antennas and to support requirements for a new WAAS receiver MOPS. Approval of the new MOPS was obtained from RTCA Special Committee SC-159 in October 2006, and then approved for official publication by the RTCA Program Management Committee in December 2006. This paper presents the results of extensive L1 antenna testing and MOPS development for active L1 airborne antennas by RTCA SC159 Working Group 7. New rolled-edge ground planes are introduced for minimize edge scattering effects on gain pattern testing. To accommodate active antennas, the concepts of gain-tonoise temperature ratio (G/T) and relative minimum-tomaximum gain ratio are introduced, including requirements and test procedures/results, followed by a brief sensitivity analysis for using alternate G/T test procedures. New group delay requirements are also presented along with test verification of those requirements.