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GPS Fundamentals and Field Mapping University of Rhode Island January 28, 2014 Nigel Shaw National Park Service Boston, MA (617) 223-5065

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Presentation on theme: "GPS Fundamentals and Field Mapping University of Rhode Island January 28, 2014 Nigel Shaw National Park Service Boston, MA (617) 223-5065"— Presentation transcript:

1 GPS Fundamentals and Field Mapping University of Rhode Island January 28, 2014 Nigel Shaw National Park Service Boston, MA (617) Dennis Skidds National Park Service Kingston, RI (401)

2 1. How & Why GPS Works 2. Sources of Positional Error 3. Reducing Positional Error 4. Features and Attributes 5. Documentation and Archiving GPS Fundamentals 2

3 3 What is GPS? A navigational tool –Locating a single point –Navigating between points A data collector for mapping and surveying –Tracking changing locational information –Collecting coordinates of features for use in GIS –Collecting information (attributes) about features for use in GIS A high precision instrument for research –Measuring volcano swelling, glacial retreat An tool for search and rescue –Identifying probable paths, finding accessible areas Just Plain Fun –Geocaching –Hiking –Finding out where your dog goes Different countries sponsor different GPS constellations

4 4 How GPS Works GPS works by triangulating your position on the earth, based on satellite signals There are four components: –Satellites –Signals –Receivers –Mathematics

5 5 Satellites & Signals U.S. GPS satellites are controlled and operated by the Air Force as an open system (available for civilian use). 31 GPS-dedicated satellites in orbit plus 3-4 decommissioned residuals as back-up. Aim for 24 available 95% of the time. At least 4 satellites are always within view of any point on earth (provided terrain or structures do not block the signals). Flying at a medium earth orbit with an altitude of approximately 20,200 km. Each satellite circles the earth twice a day. Satellites constantly transmit their locational information, and time data via radio signals which travel at about the speed of light.

6 6 Receivers & Mathematics The receiver picks up the signal from the satellites Determines how long the signal took to reach the receivers location (by comparing time stamp for when the signal was sent from the satellite with the receivers record for when it was received) Calculates the distance to the satellite (speed x time = distance)

7 7 Time * Speed = Distance The amount of Time the signal spent traveling (k) multiplied by the Speed at which it traveled (speed of light) = the Distance between the satellite and the receiver. Signal leaves satellite at time X Signal is picked up by receiver at time X + k

8 8 Signal From One Satellite The receiver is somewhere on this sphere. 6

9 9 Signals From Two Satellites Receiver is on the overlap of the two spheres 7

10 10 Three Satellites (2D Positioning) Receiver is on one of these two points 8

11 11 Four Satellites Receiver is one known point 9 More about early navigation methods: Video on using a parallel ruler and compass rose to determine direction:

12 1. How & Why GPS Works 2. Sources of Positional Error 3. Reducing Positional Error 4. Features and Attributes 5. Documentation and Archiving GPS Fundamentals 12

13 13 2. Sources of Positional Error a. Internal System Error b. Selective Availability c. Signal Interference d. Satellite Geometry e. User Innocence

14 14 2. Sources of Positional Error a. Internal System Error Satellite clock errors Orbital deviations These errors affect the values used in the equation Time * Speed = Distance 11

15 15 2. Sources of Positional Error b. Selective Availability Inaccuracy introduced to the US system by the US Department of Defense for national security purposes Signals from the satellites are deliberately mistimed Results in average error of 30 meters, but can be as high as 200 meters Set to zero on May 1, 2000 to support commercial use of GPS. Could be ramped up again, but this is unlikely.

16 Sources of Positional Error c. Signal Interference Ionosphere & Troposphere (attenuate) Electromagnetic Fields (attenuate) Multipath (bounce) Receiver Noise (attenuate)

17 17 N Poor Satellite Geometry Good Satellite Geometry N 2. Sources of Positional Error d. Satellite Geometry

18 18 Using rover units precision filters incorrectly Overriding precision filters (impatience) Poorly chosen feature settings in data dictionary Questionable field techniques Wrong planet (e.g. forgetting to save features, updates) 2. Sources of Positional Error e. user innocence

19 19 Effects of GPS Positional Error FStandard Positioning Service (SPS ): –Satellite clocks:< 1 to 3.6 meters –Orbital errors:< 1 meter –Ionosphere:5.0 to 7.0 meters –Troposphere:0.5 to 0.7 meters –Electromagnetic fieldsunpredictable –Receiver noise:0.3 to 1.5 meters –Multipath:unmeasurable –Selective Availability:0 to 100 meters –User error:Up to a kilometer or more FErrors are cumulative and you must pay attention to PDOP, EHE! 13

20 1. How & Why GPS Works 2. Sources of Positional Error 3. Reducing Positional Error 4. Features and Attributes 5. Documentation and Archiving GPS Fundamentals 20

21 21 GPS Theory 3. Reducing Positional Error TechniqueProblems Addressed AUse Ephemeris DataClock Errors Orbital Errors BUse Differential CorrectionAtmospheric Error Selective Availability CRover Unit SettingsSignal Interference Satellite Geometry Atmospheric Error User Innocence DMission PlanningSatellite Geometry Signal Interference Saves time in the field ETune Field TechniquesSignal Interference User Innocence Multipath

22 22 3. Reducing Positional Error a. Use Ephemeris Data Orbital path and exact time are pre-programmed for each satellite. Deviations from the set path are usually caused by gravitational pull and solar radiation pressure. Data on these deviations are constantly transmitted to the control station on earth and then relayed to all other satellites. In this way each satellite gets deviation data for all satellites. This is called ephemeris data and it is transmited to the rover receivers along with the satellites positional data. The receivers use it to correct for orbital path errors. Enabling the rover unit to regularly access this ephemeris data from the satellites will significantly reduce the effects of orbital and clock errors. Ephemeris data is relayed approximately once each hour by each satellite.

23 23 3. Reducing Positional Error b. Differential Correction Differential correction addresses selective availability (if in effect) and internal system error. It may also compensate, to a degree, for atmospheric interference. does not address signal static, multipath, EM fields or lack of planning. can be run in real-time or post-processed. uses 2 GPS receivers, rover and base. The base station is at an established stable point with known coordinates (w/in 300 km of rover field site). works on the assumption that the 2 receivers will have the same conditions & errors b/c they are relatively close. 13

24 24 (continued) Differential Correction The base unit is set up on a known point It measures signal attenuation (error) by calculating the correct timing given the base stations known location. That is, it runs the calculation the rover uses backwards, solving for correct time using known distance (i.e. known location): Distance / Speed = Time (i.e. duration of time for signals travel) Base and rover files are compared Correction factor applied to rover files

25 Base Station x+30, y+60 x+5, y-3 x+0, y+0 x+(30-5) and y+(60+3) x+25, y+63 Post-processed Differential GPS Receiver x-5, y+3 14 Base Correction Calculation (posted to Internet) Reported Base Location: Actual Base Location: Reported Receiver Location: Correction downloaded and applied to reported receiver location: Corrected Receiver Location: 25

26 Base Station x+30, y+60 x+5, y-3 x+0, y+0 x+(30-5) and y+(60+3) x+25, y+63 x-5, y+3 Real Time Differential GPS DGPS Receiver Receiver 15 Reported Base Location: Reported Receiver Location: Base Correction Calculation (broadcast) Actual Base Location: Correction received and applied to reported receiver location: Corrected Receiver Location: 26

27 27 Sources of Differential Correction Data Post-Processed Differential Correction Real-time Differential Correction Continuously Operating Reference Stations (CORS) (uploaded to Internet) Wide Area Augmentation System (WAAS) (aka: SBAS, Satellite-based Augmentation System) (broadcast in assigned frequency) Your own local base station (downloaded from base station receiver) National Differential Global Positioning Service (NDGPS) (low frequency broadcast) your own local base station with radio broadcast (maximum 5 km) All methods are not equal in the degree to which they can correct field data. The results for any one system can vary depending on the distance to the correction source and other factors beyond the users control.

28 28 Continuously Operating Reference Stations (CORS) A network of independently owned and operated ground-based stations coordinated by the National Geodetic Survey (NOAA). Over 1800 stations in 200 different organizations (including URI). Differential Correction data for each reference station is posted hourly on the CORS website.

29 29 WAAS Wide Area Augmentation System Geo-stationary satellites broadcasting differential correction data for use by GPS receivers in real time Designed especially for aircraft to use GPS for all phases of flight, including approach/landing. Provide an accuracy of 3-5 meters worldwide, 1-2 meters in N.America. Accuracy in N. America is enhanced with data from a network of ground- based reference stations used to calculate small variations in the satellite signals and send corrections back to the every 5 seconds

30 30 High Precison WAAS Coverage (as of December, 2010) Advantages 1-2 meters real-time accuracy in North America. No additional receiver needed Inexpensive Disadvantages Problems under canopy Satellites are geo-stationary over equator so coverage further north can be problematic.

31 31 NDGPS: National Differential Global Positioning System Coverage Live radio transmission of differential correction data from a land-based network of reference stations managed by US DOT (w/ Coast Guard & Army Corps of Engineers) Initially designed for marine use and expanded to nationwide coverage during 1990s <1 m accuracy close to reference station & degrades 3 m at 400 km distance from reference station

32 32 Forcing quality data collection –Positional Dilution of Precision (PDOP) mask Measures quality of GPS calculations, Based on the geometry of the visible satellites Low PDOP=High Accuracy –Signal to Noise Ratio (SNR) mask Reject noisy/attenuated signals (high SNR=good) –Elevation mask Reject signals from satellites low on the horizon (travel through more atmosphere, may not be visible to base station) –Everest Multipath Rejection (ProXR & XH only) 3. Reducing Positional Error c. Settings on Rover Units

33 33 10

34 34 Achievable Accuracy These are the best practical accuracies with these units. Achieving these levels of accuracy may require using specific settings and ancillary equipment. Trimble 6000 Series m

35 35 Autonomous* GPS Under Canopy Garmins 17 – 20 meters Trimble 3 – 8 meters *no differential correction

36 36 Mission Planning focuses on figuring out where the satellites will be at specific times. This tells you where and when you can be most effective at collecting data. With ephemeris data you can calculate times and locations of desirable PDOP values as well as how features like mountains or buildings may affect satellite visibility. 3. Reducing Positional Error d. Mission Planning

37 37 Start Reducing Positional Error e. Field Techniques

38 1. How & Why GPS Works 2. Sources of Positional Error 3. Reducing Positional Error 4. Features and Attributes 5. Documentation and Archiving GPS Fundamentals 38

39 GPS Fundamentals 4. Features and Attributes GIS feature types and attributes are handled differently by different types of GPS. Type of GPSAttribute Handling Trimble e.g. GeoExplorer, GeoXT, XH, XM, ProXRS, XT, XH User defined features and linked attribute tables. These are then assigned in the field ass the data are collected. Data is later exported to GIS in a separate step. Garmin e.g. GPS 76 series, Etrex series, GPS3+, and others Garmins only allow attributes for waypoints. Typically a naming convention or linking code will allow the data to be associated with an attribute table developed in GIS. 39

40 1. How & Why GPS Works 2. Sources of Positional Error 3. Reducing Positional Error 4. Features and Attributes 5. Documentation and Archiving GPS Fundamentals 40

41 GPS Fundamentals 6. Data Documentation & Management A GPS users responsibilities include: 1.documenting data quality and processing steps; 2. archiving the data; 3. using this documentation, along with the GPS data itself, to create full metadata for each data product. Remember: Without such metadata the work has no long term value. Please DONT LOSE IT!! 41

42 42 You are here You Are Here


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