GPS Introduction

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GPS (Global Positioning System): 

GPS (Global Positioning System)

Basic Concept: 

Basic Concept 28 Satellites orbiting the earth +/- 11,000 miles up (6 Paths, sun-synchronous, each satellite orbits earth 2x/day) Receivers communicate with Satellites Communication (signal delay) determines distance Distance from multiple satellites determines location Requires very precise clocks Satellites use atomic clocks (0.000000003 seconds) Receiver’s clock corrected by satellites More satellites, widely spaced provides better results Longer duration of data capture provides better results

Accuracy (Potential): 

Accuracy (Potential) +/- RT-DGPS DGPS $ Recreational/Pocket Unit 100-500 Mapping Grade (GeoExp 3) 4,500 Surveyor Grade 11-50,000 * w/external add-on Radio Beacon unit ** w/10 minutes of data & Post-Processing: 0.3 with 30 minutes of data 0.1 with 45 minutes of data 0.01 (centimeter accuracy) with 48+ hours of data 0.001 (millimeter accuracy?) Horizontal location, in meters (Vertical Accuracy typically half as good as horizontal) 5-20 100-500 5-20 5-20 2-5* 2-5 1 0.5** 5 m : 11,000 miles ~ 1 mm : 2.2 miles… (the Empire State Building ~ ¼ mile)

What you get for your $: 

What you get for your $ waterproof - backlight display - battery life - screen size - split screen zoom more memory - storage of points, tracks, areas, offsets store/load maps/charts - streets, hydro accessories: remote antenna options, RT-DGPS, PALM and Laptop connection, Laser Range Finder download data to PC ability to perform RT-DGPS ability to post process (DGPS) ability to enter attribute data (features), as well as to append to existing data files vehicle navigation electronic compass barometric altimeter (more accurate than GPS for altitude, if calibrated) WAAS compatible velocity vertical profiles of tracks alarms (anchor drift, arrival, off-course) fish locators, tide charts, calendars, celestial information


Applications Military (DoD) – civilian uses now exceed military Space Travel (NASA) Agriculture Resource and Asset Management Environmental & Forestry Mining & Engineering Oil & Gas Construction Factory Automation Fishing (Recreational & Commercial) Mapping & GIS Survey Public Safety – Emergency Management – Crime Prevention Utilities Timing & Synchronization (banking, telecommunications) LBS - Location Based Services (cell phones, wireless web) Transportation: FAA (WAAS), Marine, Rail Vehicle Security (Fleet Management)

Other Applications…: 

Other Applications… Cell Phones (E911, October 2001…) On Board Vehicle Navigation Systems Vehicle Tracking Systems (beyond fleet management) - Rental Car Companies - Family/Friends vehicle location - Crime: Stolen cars; Criminal tracking - Accident notification systems - GPS-measured Tolls – variable taxation based on congestion (UK) Child/Senior/Pet Safety Tracking Systems - Teddy Bears, Backpacks, Wristwatches - Implants… (health status w/location) Parole, Probation Tacking Systems Package/Assesst Tracking Systems Bridge stuctural monitoring Sports and Broadcasting (Skiers, NASCAR, Sailboat races) Golf Courses (distance to next hole…) Geo-Caching (GPS scavenger/treasure hunts – global or local scale) Beer Bottle GPS etc, etc,






X? 2D-Satellites

2D-3 Satellites: 

2D-3 Satellites

Poor GDOP: 


Good GDOP: 


Good-Bad GDOP: 

Good-Bad GDOP


PDOP vs. GDOP PDOP = Position Dilution of Precision User tolerance setting for acceptability of signal quality (a “PDOP mask”) Typically set from 4 – 6 (< 4 is excellent, > 8 poor) GDOP = Geometric Dilution of Precision Estimate of satellite conditions for a given location & time Given in distance units (meters or feet) PDOP * GDOP = Overall estimate of accuracy (PDOP of 4) * (GDOP of 30’) = (Accuracy of +/- 120’) PDOP - GDOP often used interchangeably Also: HDOP, VDOP, TDOP, RDOP… (horizontal,vertical, time, relative) In all cases, smaller is better

GPS Masks: PDOP, Elevation, SNR: 

GPS Masks: PDOP, Elevation, SNR Allow the user to control the quality of the data accepted at the time of data collection (unacceptable readings are filtered out) Elevation Mask: Sets minimum elevation above horizon for satellites to be used. The lower on the horizon a satellite is the more atmosphere the signal must pass through, thus the greater the potential for signal diffraction (inaccurate estimations of time/distance), as well as greater chance of multi-path errors. Also, with Differential Correction, insures that all satellites used are visible to base station as well as the field receiver. SNR (Signal to Noise Ratio) Mask: (higher is better, stronger signal) Filters out signals with excessive noise, using only those satellites with low noise (more accurate). SNR ranges from 0-35; 10-15 is typical, less than 5 is generally considered unusable. PDOP Mask: Allows the recording of positions only when there is acceptable satellite geometry. Typically considers both quantity and quality of satellites (e.g., 4 satellites with good precision, or 6 with reasonable precision, or 8 with average precision)

Sources of Error : 

Sources of Error Satellite Atomic Clock Errors (corrected periodically) Satellite Orbit (Position) Errors (corrected periodically) Earth’s ionosphere (charged particles) Earth’s troposphere (moisture) Receiver Noise (local conditions, radio interference) Multipath Errors (bounce off buildings, etc.) Local Weather (moisture in air, lightning) Poor Satellite Geometry (GDOP) Receiver Clock Errors (corrected by 4th + Satellites) 1.5 m 2.5 m 5.0 m 0.5 m 0.3 m 0.6+ m Typical amount of Error (per Satellite) Satellite Atomic Clock Errors (corrected periodically) Satellite Orbit (Position) Errors (corrected periodically) Earth’s ionosphere (charged particles) Earth’s troposphere (moisture) Receiver Noise (local conditions, radio interference) Multipath Errors (bounce off buildings, etc.) Local Weather (moisture in air, lightning) Poor Satellite Geometry (GDOP) Receiver Clock Errors (corrected by 4th + Satellites) Beyond quality of equipment/size of antennea, etc.

Selective Availability (SA): 

Selective Availability (SA) Intentional degradation of signal quality by DoD for security reasons. Spawned numerous ‘work-around’ technologies Turned off in May, 2001, recognizing civilian need for better quality GPS signal (while reserving the option to reinstate it should the need arise)

Differential Correction: 

Differential Correction Compare GPS data file from Rover file (handheld unit) with a data file from a Base Station (at a known coordinate) for the exact same time period. Relies on the fact that receivers located relatively close together, will record similar errors from the same constellation of satellites. Uses the apparent “error” of the base station file to correct the corresponding error of the Rover file.

Differential Correction 2: 

Differential Correction 2

Differential Correction: 

Differential Correction Requires local Base Station (w/in 100 miles) No effect on multi-path and/or receiver errors Requires “post-processing” (back in the lab) OR can be done on-the-fly using Real-Time DGPS But we’re not going to do that… Can improve accuracy by up to 20 m. (50-90%) Need better data – longer recording period, better GDOP More Base Stations near coasts (navigation)

Data Dictionaries: 

Data Dictionaries Created with Pathfinder Office (in the lab) and transferred to the GeoExplorer 3 before using in the field. Allows creation of custom fields (attributes) and field values (defined lists of possible attribute values) for feature collection. So, for a database of TREES, you might create a data dictionary with: Species List (fir, pine, alder, etc) Type List (Either Deciduous or Conifer) DBH Number field (Enter size in inches) Date Auto-generated date field (day-month-year) Or, for a database of STREETS, you might create a data dictionary with: Name Text field (Enter Street Name) Type List (Ave, St., Way, Place, Circle, etc) Type List (Arterial, Residential, Highway, Private Surface List (Paved, Gravel, Dirt) #Lanes Number field (Enter number of Lanes) Bike Yes/No (as to existence of Bike Lanes) But we’re not going to do that either…

Other Navigation Systems: 

Other Navigation Systems GLONASS (GLObal NAvigation Satellite System) Russian – 10 satellites Galileo (European Union GPS) – Civilian based initiative (2006? 2008?) WAAS (Wide Area Augmentation System) – FAA (2003?) 3 m., North America, near airports Loran-C (LOng RAnge Navigation) - Coast Guard, Radio navigation, 50 m, North America… Russia, Europe Many now used in combination w/GPS (augmented systems) Etc. NDGPS (Nationwide Differential GPS) – DOT, Railroad (2002?) Auto Navigation – GPS + GIS + Inertial Sensors (Dead Reckoning)

The Future…: 

The Future… Augmented systems (using WAAS, Galileo, etc…) Incorporation with other technologies (Cell phones, PDA’s, Cars) Components the size of a credit card, available for under $10.00 Dual (or Tri) Frequency GPS receivers Track L1 and L2 (& L5?) satellite signals for greater accuracy. (GeoExplorer 3 is single frequency) LBS (Location Based Services) – advertising, via cell phone, mobile ads (busses, taxis), service location/lookup, etc. Transponders: tracking and finding your pet/child/car/package. Enhanced GPS – Ability to use low-quality satellite data (typically filtered out by masks) for estimating positions under canopy, inside buildings/cars, etc. Additional Satellites, Ground support (augmentation)

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GeoExplorer 3: 

GeoExplorer 3

Sys-Data-Nav Buttons: 

Sys-Data-Nav Buttons Status Nav Data GPS SYS File Setup Function TAB Road Compass Chart New Update Map View information about the satellites that the GeoExplorer 3 is tracking and their relative positions in the sky. See your current position. View information about the GeoExplorer 3 hardware, accessories and external connections. Create and edit data dictionaries and feature settings. Edit configuration. Reset factory defaults. Create a new data file or open an existing one. Collect new features and attributes. Update features and attributes View features and select them for update. Navigate to features and waypoints using the Road. Navigate to features and waypoints using the Compass. Navigate to features and waypoints using the Chart.

Sys Button Screens: 

Sys Button Screens

Sys-GPS (Skyplot) Screen: 

Sys-GPS (Skyplot) Screen

Sys-Status Screen: 

Sys-Status Screen

Sys-Setup Screen: 

Sys-Setup Screen

Data-File Screen: 

Data-File Screen

Data-File Screen: 

Data-File Screen

Data-Map Screen: 

Data-Map Screen

The “Keyboard”: 

The “Keyboard”

GeoExplorer 3 Notes: 

GeoExplorer 3 Notes Rover Files: Contain Feature(s) collected in the field Automatically generated: R101810A.ssf R 10 18 15 A . ssf = Rover, October 18th, 10 am, ‘A’ file Greenwich Mean Time (note that GMT is 7 or 8 hours later than the west coast of the USA depending on Daylight Savings: 7 in the summer, 8 in the winter) A = First rover file for that hour (B = 2nd rover file, etc) .ssf = Trimble Rover file extension Features: Points Lines Areas (polygons) Can have multiple features (points, lines or areas) within a single Rover file. Can be ‘paused’ while recording positions. Can be updated at later.

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GeoExplorer 3: 

GeoExplorer 3 ~ 10 hours of battery life 10 MB of data storage

GPS – Ellipsoid - Datum: 

GPS – Ellipsoid - Datum GPS uses the WGS84 (World Geodetic System of 1984) as mathematical surface (model) of the earth WGS84 is for all practical purposes the same as the Geodetic Reference System of 1980 (GRS80) that was used for the North American Datum of 1983 (NAD83) horizontal datum. Elevations are referenced to Height Above Ellipsoid (HAE)

Code vs. Carrier Phase: 

Code vs. Carrier Phase Standard GPS uses ‘Code’ phase – comparing ‘pseudo-random’ code to determine distance (amount of time out of sync between satellite’s code and receiver’s code = distance satellite’s code had to travel) Carrier phase uses the Code phase to get close, then uses the actual carrier frequency wave pattern (that which carries the pseudo-random code) to increase the precision. For post-processing (Differential Correction), we want to use Carrier phase recordings to get the best results. Carrier phase also stores information from each satellite individually, allowing later comparison of the base station’s readings (error estimation) for each separate satellite recorded. Consequently, Carrier phase recordings use far more memory for storage than Code phase does.

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