Presentation on theme: "Nigel Shaw National Park Service Boston, MA (617) Dennis Skidds"— Presentation transcript:
1GPS Fundamentals and Field Mapping University of Rhode Island January 28, 2014 Nigel ShawNational Park ServiceBoston, MA(617)Dennis SkiddsNational Park ServiceKingston, RI(401)
2GPS Fundamentals1. How & Why GPS Works 2. Sources of Positional Error 3. Reducing Positional Error 4. Features and Attributes 5. Documentation and Archiving
3What is GPS? A navigational tool Locating a single pointNavigating between pointsA data collector for mapping and surveyingTracking changing locational informationCollecting coordinates of features for use in GISCollecting information (attributes) about features for use in GISA high precision instrument for researchMeasuring volcano swelling, glacial retreatAn tool for search and rescueIdentifying probable paths, finding accessible areasJust Plain FunGeocachingHikingFinding out where your dog goesDifferent countries sponsor different GPS constellations
4How GPS WorksGPS works by triangulating your position on the earth, based on satellite signalsThere are four components:SatellitesSignalsReceiversMathematics
5Satellites & SignalsU.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.SATELLITES:28 SVs currently in constellation, 2 spares not broadcasting.Conceiveably 6 visible always, everywhere (barring ground conditions)Precisely controlled by DOD, each has an atomic clock & radio transmitterConstantly transmitting id, position and time via radio waves (so an open system)(Also transmitting “almanac” (ephemeris) w/orbit information, health, for all SVs)id, position and time contained in pseudo-random code
6Receivers & Mathematics The receiver picks up the signal from the satellitesDetermines how long the signal took to reach the receiver’s location(by comparing time stamp for when the signal was sent from the satellite with the receiver’s record for when it was received)Calculates the distance to the satellite(speed x time = distance)RECEIVERS:Rover units, such as Trimble Geo3. There are many kinds, more on this later.Receiver reads pseudo-random code to get SV id and position & time signal sentDerives distance by calculating time change since signal sent
7Time * Speed = Distance Signal leaves satellite at time “X” Signal is picked up by receiver at time “X + k”Trilateration formula:Velocity x Time = DistanceVelocity = speed of lightTime = derived by Rover from Time Stamp in signal(For example, if a GPS radio signal leaves the satellite at precisely T+ 0 nanoseconds (billionths of a second), travels at 186,000 miles per second, and arrives at the receiver at precisely T+ 645,160 nanoseconds later, then it traveled 12,000 miles.(12, ,000 miles per second = milliseconds, or 645,160 nanoseconds.))Each satellite transmits a message which essentially says, "I'm satellite X, my position is currently Y, and this message was sent at time Z.”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.
8Signal From One Satellite 6Signal From One SatelliteThe receiver is somewhere on this sphere.
9Signals From Two Satellites 7Signals From Two SatellitesReceiver is on the overlap of the two spheres
10Three Satellites (2D Positioning) 8Three Satellites (2D Positioning)Receiver is on one of these two points
11Four Satellites Receiver is one known point 9Four SatellitesReceiver is one known pointMore about early navigation methods:Video on using a parallel ruler and compass rose to determine direction:
12GPS Fundamentals1. How & Why GPS Works 2. Sources of Positional Error 3. Reducing Positional Error 4. Features and Attributes 5. Documentation and Archiving
132. Sources of Positional Error a. Internal System Errorb. Selective Availabilityc. Signal Interferenced. Satellite Geometrye. User Innocence
142. Sources of Positional Error a. Internal System Error 112. Sources of Positional Error a. Internal System ErrorSatellite clock errorsOrbital deviationsThese errors affect the values used in the equation Time * Speed = DistanceTime * Speed = DistanceClock errors affect the time.Orbital deviations affect the distance. (If the satellite is not on its assigned path then the distance between it and the receiver is not correctly calculated)
152. Sources of Positional Error b. Selective Availability Inaccuracy introduced to the US system by the US Department of Defense for national security purposesSignals from the satellites are deliberately mistimedResults in average error of 30 meters, but can be as high as 200 metersSet to zero on May 1, 2000 to support commercial use of GPS. Could be ramped up again, but this is unlikely.
162. Sources of Positional Error c. Signal Interference 122. Sources of Positional Error c. Signal InterferenceIonosphere & Troposphere (attenuate)Electromagnetic Fields (attenuate)Multipath (bounce)Receiver Noise (attenuate)
172. Sources of Positional Error d. Satellite Geometry Good Satellite GeometryPoor Satellite GeometryNN
182. Sources of Positional Error e. user innocence Using rover unit’s precision filters incorrectlyOverriding precision filters (impatience)Poorly chosen feature settings in data dictionaryQuestionable field techniquesWrong planet (e.g. forgetting to save features, updates)
19Effects of GPS Positional Error 13Effects of GPS Positional ErrorStandard Positioning Service (SPS ):Satellite clocks: < 1 to 3.6 metersOrbital errors: < 1 meterIonosphere: 5.0 to 7.0 metersTroposphere: 0.5 to 0.7 metersElectromagnetic fields unpredictableReceiver noise: 0.3 to 1.5 metersMultipath: unmeasurableSelective Availability: 0 to 100 metersUser error: Up to a kilometer or moreErrors are cumulative and you must pay attention to PDOP, EHE!
20GPS Fundamentals1. How & Why GPS Works 2. Sources of Positional Error 3. Reducing Positional Error 4. Features and Attributes 5. Documentation and Archiving
21GPS Theory 3. Reducing Positional Error TechniqueProblems AddressedAUse Ephemeris DataClock ErrorsOrbital ErrorsBUse Differential CorrectionAtmospheric ErrorSelective AvailabilityCRover Unit SettingsSignal InterferenceSatellite GeometryUser InnocenceDMission PlanningSaves time in the fieldETune Field TechniquesMultipath
223. 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 satellite’s 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.
233. Reducing Positional Error b. Differential Correction 133. Reducing Positional Error b. Differential CorrectionDifferential 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.
24(continued) Differential Correction The base unit is set up on a known pointIt measures signal attenuation (error) by calculating the correct timing given the base station’s 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 signal’s travel)Base and rover files are comparedCorrection factor applied to rover files
25Post-processed Differential GPS 14Reported Base Location:x+30, y+60x+5, y-3Reported Receiver Location:Base Correction Calculation (posted to Internet)Base Stationx-5, y+3ReceiverCorrection downloaded and applied to reported receiver location:Actual Base Location:x+(30-5) and y+(60+3)x+25, y+63x+0, y+0Corrected Receiver Location:
26Real Time Differential GPS 15Real Time Differential GPSReported Base Location:x+30, y+60x+5, y-3Reported Receiver Location:Base Correction Calculation (broadcast)Base Stationx-5, y+3DGPS ReceiverReceiverCorrection received and applied to reported receiver location:x+(30-5) and y+(60+3)x+25, y+63Actual Base Location:x+0, y+0Corrected Receiver Location:
27Sources of Differential Correction Data Post-Processed Differential CorrectionReal-time Differential CorrectionContinuously 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 user’s control.
28Continuously 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.
29WAAS Wide Area Augmentation System Geo-stationary satellites broadcasting differential correction data for use by GPS receivers in real timeDesigned 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 secondsGeo-stationary satellites broadcast differential correction data for use by GPS receivers, providing an accuracy of 3-5 meters worldwide and 1-2 meters in North America.Accuracy in North America is enhanced with data from a network of ground-based reference stations that is used to calculate small variations in the satellite signals and send corrections back to the every 5 seconds.Designed esp. for aircraft so they can use GPS for all phases of flight, including approach/landing.
30High Precison WAAS Coverage (as of December, 2010) Advantages1-2 meters real-time accuracy in North America.No additional receiver neededInexpensiveDisadvantagesProblems under canopySatellites are geo-stationary over equator so coverage further north can be problematic.
31NDGPS: 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
323. Reducing Positional Error c. Settings on Rover Units Forcing quality data collectionPositional Dilution of Precision (PDOP) maskMeasures quality of GPS calculations,Based on the geometry of the visible satellitesLow PDOP=High AccuracySignal to Noise Ratio (SNR) maskReject noisy/attenuated signals (high SNR=good)Elevation maskReject signals from satellites low on the horizon (travel through more atmosphere, may not be visible to base station)Everest Multipath Rejection (ProXR & XH only)Rover Settings - forcing quality data collectionPDOPSNRElevationOtherRecommended settings are covered in hand-out.
34Achievable AccuracyThese 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
35Autonomous* GPS Under Canopy *no differential correctionGarmins – 20 metersTrimble – 8 metersWatch Out
363. Reducing Positional Error d. Mission Planning 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.
373. Reducing Positional Error e. Field Techniques 163. Reducing Positional Error e. Field TechniquesStart
38GPS Fundamentals1. How & Why GPS Works 2. Sources of Positional Error 3. Reducing Positional Error 4. Features and Attributes 5. Documentation and Archiving
39GPS Fundamentals 4. Features and Attributes GIS feature types and attributes are handled differently by different types of GPS.Type of GPSAttribute HandlingTrimblee.g. GeoExplorer, GeoXT, XH, XM, ProXRS, XT, XHUser 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.Garmine.g. GPS 76 series, Etrex series, GPS3+, and othersGarmins 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.
40GPS Fundamentals1. How & Why GPS Works 2. Sources of Positional Error 3. Reducing Positional Error 4. Features and Attributes 5. Documentation and Archiving
41GPS Fundamentals 6. Data Documentation & Management A GPS user’s 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 DON’TLOSE IT!!