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Session 2548

Generating Low Cost Serial Waveforms For Global Positioning System (GPS) Applications

John D. Cremin

Parks College of Engineering and Aviation

Saint Louis University

Abstract

The purpose of this paper is to present a number of techniques for generating serial waveforms associated with the Global positioning System (GPS) in a laboratory environment. The incentive for this project was the need for simulating GPS data in a lab environment. The waveforms of interest are the output data signals provided by GPS receivers and used by applications such as navigation computers, autopilots, street atlas software, and Differential GPS (DGPS), a system for improving the position accuracy of GPS receivers.

Introduction

A previous paper 1described the use of low cost GPS receivers to teach interfacing. One of the formats described was the National Marine Electronics Association (NMEA) format. Examples of the format using a Magellan Trail Blazer XL (TBXL) NMEA interface were presented. The paper also described a simulation for the NMEA GPS messages. In addition to the NMEA interface for outputting navigation data, the TBXL has an interface for accepting differential corrections2 to the internal position data. This interface uses an RTCM SC104, (Radio Technical Commission Maritime) standard for DGPS data broadcast by the US Coast Guard and other services, some of them commercial. In order to explore other applications for the GPS receiver, documentation of the interface characteristics for the NMEA, ARINC 429 and the RTCM interfaces is required.

To understand where these interfaces occur in the larger system context, Figure 1 was generated. The TBXL falls in the category labeled Airborne/Marine GPS Receivers. Using signals from the GPS satellites and error correction signals from an Airborne/Marine DGPS Receiver, the TBXL can internally correct errors in the measurement of range to the satellites, which the TBXL uses to triangulate its position. The range errors, which can be corrected by the DGPS capability in the TBXL, are errors detected by the DGPS Ground System shown in Figure 1. The DGPS calculates range errors by comparing the ranges to the GPS satellites using its own GPS receiver and then calculating the ranges from the surveyed location of the receiver on the ground. The ranges from the surveyed site to the GPS satellites can be calculated because the locations of the satellites are transmitted along with the information used to determine range by the GPS

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Cremin, J. D. (1999, June), Generating Low Cost Serial Waveforms For Global Positioning Systems (Gps) Applications Paper presented at 1999 Annual Conference, Charlotte, North Carolina. 10.18260/1-2--7692