Chapter X - Global Navigation Satellite Systems and Applications

GNSS

Global Navigation Satellite System - satellite-based location and navigational system with satellites pairs with ground-based receivers

• Surveying, navigation, shipping, tracking, military, and GIS (data collection)

• Inexpensive, wide-ranging applications, accurate, and user-friendly

NAVSTAR GPS

(Navigation System using Timing and Ranging) (Global Positioning System) First deployed and most widely used GNSS

Began in 1973 and fully up and running by 1994

Funded/operated by U.S. military: first, military support and second, civilian uses free to all

GLONASS System

Russian system intended for independence from GPS (military applications)

Begun in 1982 and first up and running by 1995

Increasingly used internationally/nationally by consumer applications (phones)

3 Main Segments of a GNSS

Satellite, Control, User

Paths for a GNSS Signal

Satellite → Master station → Ground Antennas

Satellite → Monitor Stations → Master Station → Ground Antennas

Satellite → Receiver

Control Segment of GNSS

Tracking, communications, data gathering, integration, analysis, and control facilities

User Segment of GNSS

The receiver

Carrier Signal VS Coded Signal

Carrier signal is good for high precision and high accuracy positioning (ambiguity, sensitive to noise and signal interruptions)

Coded signal is easier to acquire and track; enables real time position (lower accuracy)

Trilateration

Determining location (relative or absolute) through the measurements of ranging or distances, using the intersection of multiple spheres projected out from a signal source

How to Compensate for Ionosphere and Atmosphere

Variations in ionosphere and atmosphere are location and time specific

Dual frequency receivers compensate by comparing changes between frequencies

• Non-code so expensive, slower positional fixes

Limit satellites to those above 15° horizon in order to reduce fraction-based errors

Multipath Errors

Reflection and refraction causes signal to travel a further distance, erroneous ranging data

• Common in urban settings with tall buildings (reflection)

• Steep valleys or topographic barriers as well as forested cover in natural settings

DOP

Dilution of Precision, a measure of satellite geometry, with lower values indicating better geometry and lower errors

PDOP is positional DOP

Ideal Satellite Geometry

One overhead and three satellites spaced at 120° intervals

WAAS

Wide Area Augmentation System, satellite-based augmentation system operated by the Federal Aviation Administration (FAA)

• Widely available for commercial receivers including recreational-grade GPS receivers

• Designed primarily for aircraft navigation over North America

GDGPS

Global Differential GPS, a high accuracy GPS augmentation system, developed by the Jet Propulsion Laboratory (JPL) to support the real-time positioning, timing, and orbit determination requirements of the U.S. National Aeronautics and Space Administration (NASA) space missions

Datum

Viewing tracks/waypoints on a map/screen requires projection

GPS Data Characteristics

All record location in Geographic Coordinates (latitude and longitude, no projection)