For more than 30 years, Thomas S. Logsdon, M. S., has worked on the Navstar GPS and other related technologies at the Naval Ordinance Laboratory, McDonnell Douglas, Lockheed Martin, Boeing Aerospace, and Rockwell International. His research projects and consulting assignments have included the Transit Navigation Satellites, The Tartar and Talos shipboard missiles, and the Navstar GPS. In addition, he has helped put astronauts on the moon and guide their colleagues on rendezvous missions headed toward the Skylab capsule. Some of his more challenging assignments have centered around constellation coverage studies, GPS performance enhancement, military applications, spacecraft survivability, differential navigation, booster rocket guidance using the GPS signals and shipboard attitude determination. Tom Logsdon has taught short courses and lectured in 31 different countries. He has written and published 40 technical papers and journal articles, a dozen of which have dealt with military and civilian radionavigation techniques. He is also the author of 29 technical books on various engineering and scientific subjects. These include Understanding the Navstar, Orbital Mechanics: Theory and Applications, Mobile Communication Satellites, and The Navstar Global Positioning System