1. Department of Earth and Space Sciences, University of Washington, Johnson Hall Room 070, Box 351310, 4000 15th Avenue NE, Seattle, Washington 98195‐1310, crowellb@uw.edu
2. Department of Earth Sciences, University of Oregon, 100 Cascade Hall, Eugene, Oregon 97403‐1272
3. Global Navigation Satellite System (GNSS) Research Center, Wuhan University, 129 Luoyu Road, Wuchang District, Wuhan 430079, China