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The USA Navy (USN) Challenge Athena (CA) program, which provides two-way shipboard satellite communications (SATCOM) at data rates up to 1.544 Mbit/s (T1), utilizes the C-band frequencies 5925-6425 MHz Earth-to-space and 3700-4200 MHz space-to-Earth. The International Telecommunication Union (ITU) has allocated C-band for the fixed satellite service (FSS) and the point-to-point, line-of-sight microwave links of the fixed service (FS) on a co-primary basis throughout the world. CA is best described as a broadband maritime mobile satellite service (MMSS), a subset of the mobile services (MS). However, there is no ITU MS, let alone MMSS, allocation in C-band. Therefore, from a regulatory perspective, CA's operation must be done on a not-to-interfere basis, without protection from electromagnetic interference (EMI) caused by incumbent users of the band (i.e., the FS and FSS). But the USN is not alone with requirements for broadband services that must be provided throughout the oceans. Earth stations on vessels ("ESV" as they are referred to in the ITU) are also widely used by the cruise ship and offshore oil industries. Therefore, delegates representing several broadband maritime SATCOM service providers are cooperating with FS delegates in ITU Radiocommunication Bureau (ITU-R) Working Party 4-9S to study the potential for interference from ESV and to develop ITU-R Recommendations for ESV use of FS/FSS shared frequency bands. This paper discusses WP 4-9S ESV-on-FS interference avoidance studies, which are currently being conducted in preparation for World Radiocommunication Conference 2003 (WRC-2003). The methods described herein could be adapted to other situations where mobile satellite service (MSS) transmissions might interfere with FS and FSS systems.