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

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. 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. 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.
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 transmitter
Constantly 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

6. Receivers & Mathematics
The receiver picks up the signal from the satellites
Determines 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 sent
Derives distance by calculating time change since signal sent

7. Time * Speed = Distance Signal leaves satellite at time “X”
Signal is picked up by receiver at time “X + k”
Trilateration formula:
Velocity x Time = Distance
Velocity = speed of light
Time = 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.

8. Signal From One Satellite6
Signal From One Satellite
The receiver is somewhere on this sphere.

9. Signals From Two Satellites7
Signals From Two Satellites
Receiver is on the overlap of the two spheres

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

11. Four Satellites Receiver is one known point9
Four Satellites
Receiver is one known point
More about early navigation methods:
Video on using a parallel ruler and compass rose to determine direction:

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

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

14. 2. Sources of Positional Error a. Internal System Error11
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
Time * Speed = Distance
Clock 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)

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. 2. Sources of Positional Error c. Signal Interference12
2. Sources of Positional Error c. Signal Interference
Ionosphere & Troposphere (attenuate)
Electromagnetic Fields (attenuate)
Multipath (bounce)
Receiver Noise (attenuate)

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

18. 2. Sources of Positional Error e. user innocence
Using rover unit’s 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)

19. Effects of GPS Positional Error13
Effects of GPS Positional Error
Standard 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 fields unpredictable
Receiver noise: 0.3 to 1.5 meters
Multipath: unmeasurable
Selective Availability: 0 to 100 meters
User error: Up to a kilometer or more
Errors are cumulative and you must pay attention to PDOP, EHE!

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

21. GPS Theory 3. Reducing Positional Error
Technique
Problems Addressed A
Use Ephemeris Data
Clock Errors
Orbital Errors B
Use Differential Correction
Atmospheric Error
Selective Availability C
Rover Unit Settings
Signal Interference
Satellite Geometry
User Innocence D
Mission Planning
Saves time in the field E
Tune Field Techniques
Multipath

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 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.

23. 3. Reducing Positional Error b. Differential Correction13
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.

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 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 compared
Correction factor applied to rover files

25. Post-processed Differential GPS14
Reported Base Location:
x+30, y+60
x+5, y-3
Reported Receiver Location:
Base Correction Calculation (posted to Internet)
Base Station
x-5, y+3
Receiver
Correction downloaded and applied to reported receiver location:
Actual Base Location:
x+(30-5) and y+(60+3)
x+25, y+63
x+0, y+0
Corrected Receiver Location:

26. Real Time Differential GPS15
Real Time Differential GPS
Reported Base Location:
x+30, y+60
x+5, y-3
Reported Receiver Location:
Base Correction Calculation (broadcast)
Base Station
x-5, y+3
DGPS Receiver
Receiver
Correction received and applied to reported receiver location:
x+(30-5) and y+(60+3)
x+25, y+63
Actual Base Location:
x+0, y+0
Corrected Receiver Location:

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 user’s control.

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. 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
Geo-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.

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. 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. 3. Reducing Positional Error c. Settings on Rover Units
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)
Rover Settings - forcing quality data collection
PDOP
SNR
Elevation
Other
Recommended settings are covered in hand-out.

33. 10

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. Autonomous* GPS Under Canopy
*no differential correction
Garmins – 20 meters
Trimble – 8 meters
Watch Out

36. 3. 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.

37. 3. Reducing Positional Error e. Field Techniques16
3. Reducing Positional Error e. Field Techniques
Start

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

39. GPS Fundamentals 4. Features and Attributes
GIS feature types and attributes are handled differently by different types of GPS.
Type of GPS
Attribute 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.

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

41. GPS 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’T
LOSE IT!!

42. You Are Here
You are here