Understanding the GPS Frame Structure: A Detailed Guide

The message frame in GPS typically has a duration of 0.6 seconds. It contains 240 bits of data along with 60 parity bits for error detection.

The GPS frame structure is composed of 5 subframes. Together, these subframes carry 300 bits, divided into 240 data bits and 60 parity bits.

The frame frequency is approximately 50 Hz, translating to a data rate of 50 bits per second (bps). Each satellite transmits these 5 subframes within a 30-second interval.

The GPS frame begins with a telemetry word, acting as a preamble. This preamble provides key details about the transmitting satellite. The GPS system time, referred to as the ‘Z count,’ is conveyed within the handover word (HOW).

The HOW is followed by eight data words, each accompanied by parity bits.

• Subframe 1: Carries information necessary for correcting the satellite’s clock.
• Subframe 2 & 3: Provide ephemeris data, crucial for determining the satellite’s precise position.
• Subframe 4 & 5: Send Almanac data, offering a less precise but still valuable overview of the entire GPS satellite constellation.