ABSTRACT
This chapter examines the definitions of time and how the different definitions are related. It explains the differences between the various time systems and explores the fields used in Global Positioning System (GPS) navigation messages. Time information can be found in many places in telemetry and command system. The time difference between two events is easier to measure at high resolution than absolute time, particularly when comparing measurements made in differential mode versus those made in absolute mode. The chapter discusses the Inter Range Instrumentation Group, the National Institute of Standards and Technology, and the National Aeronautics and Space Administration standard formats that allow the expression of time in varying degrees of precision and, in some cases, the specification of control signals for use with the time code. The GPS has to revolutionize how people think of measuring time and distance. The GPS satellite basically broadcasts a known signal format on a periodic basis.
