Introduction to Geographic Information Science. Last Lecture. Today s Outline. Geography 4103 / GNSS/GPS Technology

Transcription

1 Geography 4103 / 5103 Introduction to Geographic Information Science GNSS/GPS Technology Last Lecture Geoids Ellipsoid Datum Projection Basics Today s Outline GNSS technology How satellite based navigation and positioning works Range measurements Time delay Positional error Differential GPS for accuracy improvement 1

2 Learning Objectives GNSS, GPS Principles and types of systems Terms such as differential GPS, range, positional error GPS/GNSS The Global Positioning System (GPS) or Global Navigation Satellite System (GNSS) are satellitebased positioning systems Used for navigation, tracking, measuring positional information Source: 4 Main GPS/GNSS Systems US DoD NAVSTAR Operational since 1978; global since 1994 Clinton removes Selective Availability in 2000 accuracy improves from ~100m ~15m Russian GLONASS: Global Navigation Satellite System Operational since the mid 1970s, limited coverage Global coverage restored in Sep 2010 EU s Galileo projected to be fully active in(?) 2020 Better coverage in high northern latitudes than NAVSTAR (+) China s BeiDou (former COMPASS) 15 of 35 satellites in orbit; projected global coverage 2020 See updated specs table 2

3 GPS Segments Constellation of orbiting satellites Receiver to record transmitted satellite data and processes 3D coordinates Facilities for tracking, communication, gathering, analysis, Satellite Segment Altitudes: 20,000km for NAVSTAR 19,200km for GLONASS 23,600km for Galileo Stations in the GPS Control Segment 3

4 User Segment Users with a device that records data transmitted by each satellite and processes the signal to obtain three dimensional coordinates Trimble CMT Inc NAVSTAR/GLONASS/GALILEO compatibility == GNSS GPS Specifications Satellites circle the earth twice daily Four to eight satellites should be visible from any unobstructed location. Each satellite continuously broadcasts signals on two carrier frequencies, L1 and L2 ( carrier signals ). Modulated to produce coded signals (using the L1 and/or L2 signals): C/A code (L1) and P (military signal on L1 and L2), How does GPS work? Position is estimated based on range distances Travel time: difference between transmission time and reception time for each code fragment Range = speed of light * travel time = c(t 1 t 0 ) 4

5 Coded (or C/A code) Measurements GPS receiver & satellite generate same code at exactly the same time Carrier phase measurements using partial waves How many satellites are required to determine your exact location? Position Calculation Range measurements combined with satellite location data define spheres of possible locations 5

6 GPS & Relativity Perimeter Institute of Theoretical Physics u4 1:13 5:45 Positional Uncertainty Errors in range measurements and satellite location (ephemeris) introduce errors Creates a range of uncertainty around the GPS receiver position Can mitigate error using multiple position fixes if receiver is stationary (does not work for moving receivers). Sources of Error Source Typical Range Error (m) Satellite clock error 1 Satellite position error (PDOP) 1 Receiver error 1.5 Atmospheric/Ionospheric effects 4 Total 7.5 Multipath - additional potential error due to reflectance 6

7 Example of Multipath Error Source: PDOP Range Uncertainty range uncertatinty A function of satellite geometry range uncertainty range uncertainty positional dilution of precision Positional Dilution of Precision Ratio of the volume of the tetrahedon created by the four most widespread satellites to the volume of the ideal tetrahedron Is it better to have high PDOP or low PDOP? Satellite Spacing! 7

8 GPS - Differential Correction Increase accuracy by reducing range errors by estimating timing errors using a base station 1. Base station set up at known position 2. Collect data simultaneously 3. Calculate new roving points from known base station and signal difference Differential Correction Same conditions, same satellites (proximity) Two Types of Differential GPS Post processed - Highest accuracy, but a posteriori Real-time - Lower Accuracy - Better navigation - Requires a transmitting base station - Requires comm n link (FM radio or internet/cell phone) US Coast Guard established GPS radio beacons called Continuous Operation Reference Stations (CORS) 8

9 Wide Area Augmentation System (WAAS) Real-time diff. GPS developed for aircraft navigation. Based on a network of ground reference stations. Accuracy improves from ~13m to <2m Real time satellite position Blue = not monitored by WAAS Green = all WAAS corrections available Yellow = WAAS corrections available excluding ionospheric Geostationary satellites = 133, 135, 138 Tracking Navigation Field digitizing GPS Applications 9

10 Rhinos GPS chip inserting in horn used to monitor Rhinos in South Africa Alerts sent via cell phone if Rhino runs, sleeps too long, or ventures outside the park Walrus Tracking GPS Applications Tracking 10

11 GPS Applications - Navigation GPS Applications - Navigation On the ground Airborne GPS Applications Field digitization (carried by car,boat,helmets, hand,etc.,) 11

12 Field Digitization Combine: - GPS receiver, - Range pole, - Laser rangefinder, - Electronic compass GPS Data & ArcGIS Two ways to import GPS data into ArcGIS Upload points to a file and import directly Install and use provided GPS software and create ArGIS readable spatial layers GPS Summary Space based positioning system with satellite, control, and user segments Carrier phase (mm cm) and code phase (m) positioning higher carrier phase accuracy achieved after differential correction Range measurements from 4 or more satellites to estimate position Range errors from ionosphere, atmosphere, system, and receivers Satellite geometry affects position accuracy low PDOPs are better 12

13 GPS Summary We can also average, and differentially correct to improve accuracy Many flavors of differential correction real time, post processed, WAAS Positional accuracies higher in open than sub canopy or in canyons Main applications in GIS are for field digitizing 13