GPS Fundamentals and Field
Mapping
University of Rhode Island
January 28, 2014
Nigel Shaw
National Park Service
Boston, MA
(617) 223-5065
nigel_shaw@nps.gov
Dennis Skidds
National Park Service
Kingston, RI
(401) 874-4305
dennis_skidds@nps.gov
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GPS Fundamentals
1. How & Why GPS Works
2. Sources of Positional Error
3. Reducing Positional Error
4. Features and Attributes
5. Documentation and Archiving
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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
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How GPS Works
• GPS works by triangulating
your position on the earth,
based on satellite signals
• There are four components:
–
–
–
–
Satellites
Signals
Receivers
Mathematics
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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
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light.
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)
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Time * Speed = Distance
Signal leaves satellite at time “X”
Signal is picked up by receiver at time “X + k”
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
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receiver.
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Signal From One Satellite
The receiver is
somewhere on
this sphere.
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Signals From Two Satellites
Receiver is on
the overlap of the
two spheres
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Three Satellites (2D Positioning)
Receiver is on one
of these two points
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Four Satellites
Receiver is one
known point
More about early navigation methods:
http://www.marinersmuseum.org/education/viking-ships
Video on using a parallel ruler and compass rose to determine direction:
http://www.uspowerboating.com/Home/Education/Navigation/Longitude___Latitude.htm
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GPS Fundamentals
1. How & Why GPS Works
2. Sources of Positional Error
3. Reducing Positional Error
4. Features and Attributes
5. Documentation and Archiving
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2. Sources of Positional Error
a. Internal System Error
b. Selective Availability
c. Signal Interference
d. Satellite Geometry
e. User Innocence
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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
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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.
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2. Sources of Positional Error
c. Signal Interference
Ionosphere &
Troposphere
(attenuate)
Electromagnetic
Fields
(attenuate)
Multipath
(bounce)
Receiver Noise
(attenuate)
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2. Sources of Positional Error
d. Satellite Geometry
Good Satellite
Geometry
N
Poor Satellite
Geometry
N
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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)
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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!
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GPS Fundamentals
1. How & Why GPS Works
2. Sources of Positional Error
3. Reducing Positional Error
4. Features and Attributes
5. Documentation and Archiving
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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
Atmospheric Error
User Innocence
D Mission Planning
Satellite Geometry
Signal Interference
Saves time in the field
E Tune Field Techniques
Signal Interference
User Innocence
Multipath
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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.
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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.
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(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
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Post-processed Differential GPS
Reported Base
Location:
x+5, y-3
Reported Receiver
Location:
x+30, y+60
Base Correction
Calculation (posted to
Internet)
x-5, y+3
Receiver
Correction downloaded and applied to
reported receiver location:
x+(30-5) and y+(60+3)
Corrected Receiver Location:
x+25, y+63
Base Station
Actual Base
Location:
x+0, y+0
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Real Time Differential GPS
Reported Base
Location:
x+5, y-3
Reported Receiver
Location:
x+30, y+60
Base Correction
Calculation (broadcast)
x-5, y+3
Receiver
DGPS Receiver
Base Station
Correction received and applied to
reported receiver location:
Actual Base
Location:
x+(30-5) and y+(60+3)
Corrected Receiver Location:x+25,
y+63
x+0, y+0
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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 distance @ 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.
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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.
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http://www.ngs.noaa.gov/CORS/GoogleMap/CORS.shtml
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 groundbased reference stations used to
calculate small variations in the
satellite signals and send corrections
back to the satellites @ every 5
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seconds
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.
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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 to @ 3 m at 400 km distance
from reference station
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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)
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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
0.1 - 0.3 m
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Autonomous* GPS Under
Canopy
*no differential
correction
• Garmins
17 – 20 meters
• Trimble
3 – 8 meters
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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.
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3. Reducing Positional Error
e. Field Techniques
Start
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GPS Fundamentals
1. How & Why GPS Works
2. Sources of Positional Error
3. Reducing Positional Error
4. Features and Attributes
5. Documentation and Archiving
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GPS Fundamentals
4. Features and Attributes
GIS feature types and attributes are handled differently by different types of GPS.
Type of GPS
Trimble
e.g. GeoExplorer,
GeoXT, XH, XM,
ProXRS, XT, XH
Garmin
e.g. GPS 76 series,
Etrex series, GPS3+,
and others
Attribute Handling
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.
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.
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GPS Fundamentals
1. How & Why GPS Works
2. Sources of Positional Error
3. Reducing Positional Error
4. Features and Attributes
5. Documentation and Archiving
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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!!
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You
YouAre
are
Here
here
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