Measurement Journal Issue 6

While for consumers the potential for exciting new applications is high, the constant evolution in wireless technologies and increasing demand for multi-format devices creates unique test challenges. It is also spurring new test strategies in R&D and manufacturing. Today’s mobile devices require comprehensive veriﬁcation and test solutions for multiple wireless technologies. Flexible test environments that are easily adaptable to changing test requirements are essential, since dedicated test equipment does not scale well with increased integration and quickly becomes cost prohibitive. This article will highlight some of the test engineering challenges for GPS and introduce a cost-effective solution that meets the demands of today’s new test environment. Using its knowledge of the satellite positions and their C/A codes, the GPS receiver correlates the incoming GPS-satellite signals to identify and calculate the signal travel time from each satellite in view. It then calculates the distance to the satellite based on this travel time. The distance D to the satellite can be determined by D = D1 x c, where D1 is the calculated signal travel time and c is the speed of light (approximately 300,000 km/s). GPS receivers calculate their position using the signals from at least four GPS satellites. By calculating the distance from these satellites, the GPS receiver is able to solve for four unknowns: longitude, latitude, altitude, and time. Using a principle known as trilateration, it then uses geometry and trigonometry to determine its actual location. Understanding GPS technology Before characterizing the test challenges and solutions for verifying and testing GPS receivers, it is necessary to ﬁrst gain a clear understanding of the technology. The United States Department of Defense manages the GPS-satellite constellation that includes 24 orbiting satellites (32 satellites if you include backups and Block II/IIA/IIR/IIR-M satellites). The satellite orbits are arranged such that at least six satellites are always visible from any line-of-sight point on earth (Figure 1). Each satellite broadcasts navigation data that includes ephemeris data (precise orbital data for itself) and almanac data (coarse orbit and status information on all satellites). This data is transmitted using a distinct spread-spectrum code, coarse/acquisition (C/A), which is unique for each individual satellite (Figure 2). All GPS satellites are also equipped with onboard atomic clocks that allow GPS receivers to perform the precise time measurements required for calculating signal travel time. Figure 1. The orbiting GPS constellation includes 24 satellites L1 carrier 1.57542 GHz L1 signal C/A code 1.023 MHz Exclusive - or Multiplier Navigation data 50 Hz Figure 2. Satellite-signal block diagram for GPS link 1 (L1) frequency 17 Agilent Measurement Journal

Table of Contents Feed for the Digital Edition of Measurement Journal Issue 6

Measurement Journal Issue 6 Using a Few Words to Convey a Deeply-Held Spirit Charting a Unique Path to Technology Innovation Contents Announcing First Commercial GC/MS Metabolites Library Assessing Nonlinear Behavior Testing Combines Form and Function Testing Multiplay Networks Higher Sensitivity LC/MS Easing Military Radio Test Stepping Up Calibration Accuracy Increasing Spectrum Analyzer Bandwidth 3GPP Tool Supports TS 36 Standards The Olympic Story Spotlights Agilent's Continuing Contribution to Technology Innovation Ensuring a Level Playing Field in Competitive Sports GPS: Coming of Age Resolving Design Issues in HSPA Mobile Devices Achieving Accurate Results with Oscilloscope-Based Jitter-Analysis Software RF Handheld Testers Guarantee Traffic Stability Under Olympic-Sized Stress Conditions Utilizing LAN-Based Instrumentation to Measure Total Harmonic Distortion in Remote Facilities IMT-Advanced: 4G Wireless Takes Shape in an Olympic Year EPO Doping: A Speed Engine, Turbo-Charged by Chemistry

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