a healthy satellite, anything other than 000 indicates issues) etc. Subframe 1, after the TLM and HOW words, carries clock correction, satellite week (Week number in reference to 6th January 1980 which has been designated as Week 0) and satellite health ( 000 indicates that all parameters OK i.e. Following the TLM and HOW words, each subframe has a different information payload. Broadly speaking, the HOW word apart from other things that it does, plays an important part in the receiver determining when the signal left the satellite. It also carries the subframe number (1-5). The first two words in each subframe are (a) the TLM or Telemetry word which is used for synchronization and testing for the beginning of each subframe and (b) the HOW or the Handover Word which contains a GPS time counter that is incremented at the start of each subframe indicating the start time for the next frame.
one-way communication from satellite to GPS receiver much like a TV broadcasting tower and TV sets receiving signals). The GPS is a passive system in that the GPS receivers just receive codes and messages broadcasted from GPS satellites permitting them to determine their position without transmitting anything back to the GPS satellites (i.e. More holistically stated, the GPS provides Positioning, Navigation and Timing (PNT) services.
Using Global Positioning services provided by the GPS constellation wherein each satellite is identified by a unique SVN or PRN number, GPS receivers can accurately compute their 3 Dimensional position (Latitude, Longitude and Altitude) on earth. Other Global Navigation Satellite Systems (GNSS) are the Russian GLONASS (live & fully operational), the European Union’s Galileo (live but at the time of writing, yet to achieve full operational capability) and the Chinese BeiDou (live but at the timing of writing, yet to achieve full operational capability).
This arrangement ensures that a minimum of 6 satellites are always visible in the sky from any place on earth, day or night. As many are aware, GPS is a currently a constellation of 32 satellites (minimum 24 are required for global coverage) operated by the USA that orbit the earth at 20200 km in circular orbits oriented in six planes (Figure 1). The Global Positioning System or the GPS is in wide use today.
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