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Unformatted text preview: rovides an overview of the SPS ranging signal characteristics and the assumptions
made to arrive at the performance standards. The representative receiver characteristics are
used to provide a framework for defining SPS performance standards. They are not intended to
impose any minimum requirements on receiver manufacturers or integrators. Receiver
characteristics used in this standard are required to establish a frame of reference within which
the SPS signal performance characteristics can be described.
The SPS Performance Standard is the authoritative U.S. document that defines the level of
performance that the U.S. Government commits to provide to all civil users. Details on the
technical characteristics of the SPS L-band carrier and the C/A code are defined in ICD-GPS200C (current edition). The U.S. Government has established ICD-GPS-200C as the technical
definition of interface requirements between the GPS constellation and SPS receivers. ICD-GPS200C is the definitive document in matters concerning technical definition of GPS navigation
signal characteristics and navigation message processing protocols. 2.1 An Overview of SPS Ranging Signal Characteristics This section provides an overview of SPS ranging signal characteristics. SPS ranging signal
characteristics are allocated to two categories: carrier and modulation Radio Frequency (RF)
characteristics; and the structure, protocols and contents of the navigation message.
2.1.1 An Overview of SPS Ranging Signal RF Characteristics
The GPS satellite transmits a Right Hand Circularly Polarized (RHCP) L-band signal known as L1
at 1575.42 MHz. This signal is transmitted with enough power to ensure a minimum signal power
level of -160 dBW at the Earth's surface. This power level is defined for a representative SPS
receiver located near the ground and tracking with a 3 dBi linearly polarized antenna above a 5°
L1 is Bipolar-Phase Shift Key (BPSK) modulated with a Pseudo Random Noise (PRN) 1.023
MHz code known as the C/A code. This C/A code sequence repeats each millisecond. The
transmitted PRN code sequence is actually the Modulo-2 addition of a 50-Hz navigation message
and the C/A code.
2.1.2 An Overview of the GPS Navigation Message
Each GPS satellite broadcasts data required to support the position determination process.
Figure 2-1 provides an overview of the data contents and structure within the navigation
message. The data include information required to determine the following:
• Satellite time-of-transmission • Satellite position • Satellite health • Satellite clock correction • Propagation delay effects • Time transfer to UTC (USNO) • Constellation status
Page 7 October, 2001 GPS SPS Performance Standard Significant Subframe Contents
GPS Week Number, SV Accuracy and Health,
and Satellite Clock Correction Terms SUBFRAME 1 TLM HOW SUBFRAME 2 TLM HOW Ephemeris Parameters SUBFRAME 3 TLM HOW Ephemeris Parameters SUBFRAME 4 TLM HOW SUBFRAME 5 TLM HOW Almanac and Health Data for Sate...
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- Fall '09