Ranging signal characteristics and minimum usage

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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° mask angle. 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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