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Session E6: GPS Modernization, Space-Based PNT Services, and Constellation Status

Military Space Service Volume (MSSV) Improvements with a Potential GPS IIIF Magneto-Electric Dipole Array (MEDA) Antenna
Erik Lier, Chuck Frey, Mark Crews, Graeme Ramsey, Lockheed Martin Space; David J. Goldstein, The Aerospace Corporation
Location: Ballroom D
Alternate Number 2

GPS signal transmissions consist of two regimes, the Terrestrial Service Volume (TSV) and the Space Service Volume. The GPS Military Space Service Volume (MSSV) is the set of M-Code transmissions serving users in geosynchronous (GEO) orbit. Proliferation of M-Code receivers led Lockheed Martin (LM) to design GPS satellite transmit antenna that would improve signal availability at GEO. LM also sought to design the antenna to reduce the risk and magnitude of multipactor, a phenomenon in which electron resonance inside the antenna amplifies during operation and can damage or destroy the antenna as well as upset the signal.
In this paper we present a new L-band Military Earth Coverage (MEC) antenna with greatly increased Military Space Service Volume (MSSV) coverage and improved Geometric Dilution of Precision (GDOP) over the heritage GPS MEC antenna. This offers enhanced Position, Navigation, and Timing (PNT) service to GEO satellites in the MSSV region from ±14? to ±90?(from antenna nadir) while satisfying all MEC TSV requirements. The Magneto-Electric Dipole Array (MEDA) consists of 6 dual-polarized magneto-electric dipole elements with a 90-degree hybrid in front of each dipole, and a symmetric 6-way coaxial power splitter. MEDA was originally developed for the GPS IIIF combined EC/MEC antenna with approximately 700W radiofrequency (RF) power needed for the proposed Concentric Array Based Antenna (CABA) configuration (US Patent No. 10,749,252). Surrounding these elements is a multibeam Regional Military Protection (RMP) Active Electronically Steerable Array (AESA). CABA was in part developed under the APRICoTS program (AFRL Contract No. FA9453-20-C-2003) from 2019-2020 and presented at JNC in previous years.
The MEDA antenna design reduces the risk and cost of antenna multipaction by relying on simulation tools instead of laboratory testing only. All major parts of the MEDA antenna are designed to enable multipaction simulation by Spark3D (https://www.3ds.com/products-services/simulia/products/spark3d/). The US satellite community uses this unique simulation tool to evaluate the RF breakdown power level in a wide variety of passive devices, including cavities, waveguides, microstrip, stripline and antennas. MEDA Spark3D simulations with 700W input power have demonstrated enough margin to avoid multipaction testing according to the “standard handbook ANSI/AIAA S-142-2016” (https://doi.org/10.2514/4.104602.001), which greatly reduces recurring test costs. In contrast, the current GPS III MEC array requires multipaction screening for every antenna produced. It is a program goal to be able to remove screening events for future L-band GPS IIIF antennas to decrease program cost, technical risk, and manufacture times.
When compared to the heritage MEC antenna MEDA also provides reduced group delay pattern variation, improved cross-polarization or axial ratio (average AR under 0.5 dB) over the earth Field of View, and higher gain from lower insertion loss. There are credible benefits to enabling enhanced MSSV capabilities through GPS IIIF. LM’s MEDA antenna offers the opportunity of improved performance as well as reduced manufacturing costs and test schedule.
This paper shows simulated coverage in percentage of the entire MSSV, i.e. the percent of GEO GPS satellite receivers that can detect L1 and L2 GPS signals. It measures GDOP from GEO receiver locations for various GPS constellation configurations, and compares MEDA performance versus heritage MEC antenna performance. Since receiver sensitivity has improved over the years, we discuss and propose new and lower SSV Received Signal Strength (SRSS) threshold requirements based on state-of-the-art receiver technology. We also compare two different GEO GPS satellite receiver antenna designs for best performance.
References: Standard Handbook ANSI/AIAA S-142-2016, https://doi.org/10.2514/4.104602.001



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