GPS Performance

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GPS Performance in Southern Hardwood Forests
Pete Bettinger Warnell School of Forestry
and Natural Resources University of Georgia
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Introduction
In forests, vegetation plays a significant role
in obstructing signals and introducing error
into the system through the multipath effect of
signals being redirected from obstructive
surfaces. More multipath can be found, and
lower SNR values realized, in areas under a
forest canopy. These effects may be reduced by
antenna design, processing techniques related to
the data collected, and other methods, however
the improvement to data quality of an individual
receiver in high multipath environments is often
unknown.
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Introduction
Manufacturer's stated accuracy of GPS receivers
are usually ambitious. We have embarked on a set
of studies to determine the accuracy of various
receivers and antenna configurations in a
forested environment. Our goal is to better
understand the behavior of data positioning
as well as to better understand the capabilities
of GPS receivers.
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Introduction
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Introduction
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Introduction
Two recent developments in GPS technology - WAAS
and DGPS - may help improve the accuracy of
GPS-determined positions. These are on-the-fly
differential correction processes, as opposed to
traditional differential correction, which can be
performed after data has been collected.
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Introduction
The Wide Area Augmentation System (WAAS) was
begun in 1994 as a joint project between the
United States Department of Transportation and
the Federal Aviation Administration (FAA). WAAS
was meant to provide service for all classes of
aircraft in all phases of flight in the United
States. It is now available for use in a
variety of hand-held GPS receivers. As of
September 28, 2007, WAAS service is now available
to users throughout Canada and Mexico. The cost
to provide the WAAS signal is about 50 million
per year.
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Introduction
WAAS consists of A monitoring network
Processing facilities Geostationary
satellites Reference stations (34) Central
data processing sites The reference stations
collect measurements from the GPS and WAAS
satellites so that near real-time differential
correction can be made given ionospheric delay
information that is determined.
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Introduction
WAAS is said to improve basic GPS accuracy to
approximately 7.6 meters vertically and
horizontally, at least 95 of the time. Actual
performance measurements of system at specific
locations have shown it typically provides
better than 1.0 meters laterally and 1.5 meters
vertically throughout most of the U.S. In
forested conditions, however, this may not hold.
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Introduction
Differential Global Positioning System (DGPS) is
an enhancement GPS that uses a network of fixed
ground-based reference stations to broadcast the
difference between their positions as indicated
by the GPS satellites and their known fixed
positions. DGPS can refer to any type of
ground-based augmentation system. Static mode
DGPS is where the rover and the base station
remain in fixed places (are stationary).
Kinematic mode DGPS is where the base station
remains in a fixed position, yet the rover moves
from unknown location to unknown
location. According to the US Coast Guard, 47
countries operate systems similar to the US
NDGPS.
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Introduction
The U.S. Coast Guard DGPS system consists of two
control centers and 86 remote broadcast sites in
2007 - many more are planned. The Coast
Guard DGPS became fully operational in
1999. The accuracy is said to be about 10 m.
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Introduction
Lower-population areas, and areas away from
waterways, most notably the Rocky Mountains,
Texas, West Virginia, and Alaska, have poor
coverage by the Coast Guard's ground-based
DGPS.
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Introduction
Both WAAS and DGPS require dual frequency
capability in GPS receivers (one frequency is a
signal from the GPS satellites, the other
frequency is the signal from the WAAS or DGPS
satellites or beacons).
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Introduction
GPS technology changes rapidly. Advances in all
aspects of GPS technology require continual
review of this technology and its effect on
positional accuracy. The potential cost savings
made available from an evaluation of a range of
receivers might encourage forest managers to more
readily apply this technology to their
day-to-day operations.
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Introduction
Three general categories of GPS
receivers Survey-grade GPS receivers are
reported to be capable of providing
sub-centimeter positional accuracy, at a cost of
10,000 dollars or more per unit.
Mapping-grade receivers, which in some cases
can provide sub-meter accuracy, range from about
1,500 to 5,000 or 10,000. These are
frequently used in forestry applications.
Recreation-grade receivers generally provide
the least accurate positional information -
between 3m and 10m accuracy under optimal
conditions - and range in price from 100 to
about 1,000. Recreation-grade receivers have
become popular among many outdoors enthusiasts,
and this popularity has likely influenced the
wide variety of inexpensive GPS receivers
available on the market today.
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Introduction
Extrex 100
GPS Rino 650
GPS 60 200
GPS 72 130
Magellan Triton 300 150
Magellan eXplorist 100 120
Magellan eXplorist XL 400
Magellan eXplorist 600 350
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Introduction
Brunton Atlas 400
Delorme Mapping GPS 400
Lowrence iFinder 100 to 300
Bushnell Onix 200 200
Mio P550 PDA 300
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Introduction
Trimble GeoXH 5,300
Trimble GeoXM 2,600
Trimble Recon 1,000
Trimble Juno ST 650
Leica GS20 Professional data mapper 4,800
Magellan Promark 3 3,000
CMT MC-GPS 2,500
CMT March-II-E 2,500
CMT PDA GPS 1,300
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Introduction
Research was conducted at the GPS test course
facility in Whitehall Forest, Athens, Georgia.
Three OPUS benchmarks were established within
two kilometers of the field course. Position
determination of twenty-seven permanent monuments
was then made using standard surveying
techniques. Each survey point consists of a
brass survey cap mounted on a 0.6 m rebar post,
which is encased in concrete. Survey points
were established under a range of topographical
and forested conditions, and positional
accuracy is known to lt 2 cm.
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Introduction
Twenty-four of the points were classified as
under full canopy (not influenced by forest
edge). Aspect ranges from North to Southeast,
while slopes vary from 2 - 40. Forest
conditions changed nominally over these
positions, a general pattern of species
gradation occurs, changing from bottomland
hardwood forest with a larger component of beech
(Fagus grandifolia) and related species to an
upland dominated by oak and hickory (Quercus
spp., Carya spp.) with some remnant shortleaf
pine (Pinus echinata) still present.
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Introduction
A laser-level tripod stand was designed and used
to allowed all receivers to be placed directly
over each monument, at a height ranging from
1.3 m to 1.5 m, which varied based on ground
conditions.
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Study 1
Study 1 A Comparison of GPS Performance in a
Southern Hardwood Forest Exploring Low-Cost
Solutions for Forestry Applications
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Methods
GPS receivers studied Mapping-grade Trimble
ProXR Recreation-grade Garmin Etrex Garmin
Map 60C Thales Mobile Mapper Conditions Uppe
r slope, mid-slope, lower slope positions Leaf-on
time of year (summer) Leaf-off time of year
(winter)
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Results
Leaf-on RMSE
Lower Slope 14.2 10.7 21.0 13.9 31.5 8.1 3.1
Mid- Slope 10.6 10.5 15.9 12.1 31.0 8.4 3.0
Upper Slope 9.4 7.4 13.7 12.5 22.4 5.6 3.1
Receiver Etrex Etrex with WAAS Map 60C Map 60C
with WAAS Thales Mobile Mapper Trimble
ProXR Trimble ProXR diff. corrected
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Results
Leaf-off RMSE
Lower Slope 6.3 6.0 32.4 18.3 32.6 8.9 2.0
Mid- Slope 5.5 4.7 31.9 14.6 19.2 7.5 2.0
Upper Slope 7.7 6.6 26.1 8.8 21.3 5.7 2.1
Receiver Etrex Etrex with WAAS Map 60C Map 60C
with WAAS Thales Mobile Mapper Trimble
ProXR Trimble ProXR diff. corrected
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Results
General trends Positional accuracy increased as
slope position increased. However, improvements
with WAAS correction were found across the slope
positions studied. Change in RMSE with
increasing number of position fixes was not as
predictable as one would have hoped. Mapping-grad
e receiver outperformed the other receivers on
all slope conditions. Differentially corrected
(post-processed) data was better than
WAAS corrected data (on the fly).
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Study 2
Multipath mitigation under a forested
canopy Using a choke-ring antenna
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Methods
GPS receivers studied Mapping-grade Trimble
ProXR Topcon Choke ring antenna Conditions Up
per slope, mid-slope, lower slope
positions Leaf-on time of year (summer) Leaf-off
time of year (winter)
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Results
Leaf-on RMSE
Lower Slope 6.5 3.1 2.8 0.3
Mid- Slope 8.1 2.8 2.0 0.2
Upper Slope 5.6 3.0 2.0 0.2
Receiver Trimble ProXR Trimble ProXR diff.
corrected Choke ring Choke ring diff. corrected
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Results
Leaf-off RMSE
Lower Slope 8.7 1.9 2.8 0.3
Mid- Slope 7.3 1.9 2.1 0.2
Upper Slope 5.7 1.9 2.4 0.2
Receiver Trimble ProXR Trimble ProXR diff.
corrected Choke ring Choke ring diff. corrected
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Results
General trends ? Positional accuracy increased
as slope position increased. Differentially
corrected (post-processed) data was better than
raw data collected on the fly. Change in RMSE
with increasing number of position fixes was
not as predictable as one would have hoped.
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Results
General trends The choke ring antenna more
effectively mitigated signal degradation. Real-tim
e RMSE with the choke-ring configuration was
about as good as the RMSE associated with
differentially-corrected ProXR data. a) One
might assume that 3.5 m of inaccuracy is
associated with other factors than multipath
(uncorrected choke-ring data - differentially
corrected choke-ring data) b) Similarly,
multi-path results in 5-12 m of inaccuracy
depending on slope and canopy position and time
of year. (uncorrected ProXR data - uncorrected
choke-ring data)
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Study 3
A Comparison of GPS Performance in a Southern
Hardwood Forest Exploring Real-time Data
Accuracy of Forestry Receivers
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Methods
GPS receivers studied Mapping-grade Trimble
ProXH Trimble GeoXH Garmin 17 HVS TDS
Nomad Recreation-grade Garmin Etrex Trimble
Juno Conditions Random positions Leaf-on time
of year (summer)
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Results
Leaf-on RMSE95
Lower Slope 2.3 3.5 11.6 6.5 12.9 12.4
Mid- Slope 9.1 2.5 6.1 17.9 6.2 9.8
Receiver Trimble ProXH Trimble GeoXH Garmin 17
HVS TDS Nomad Trimble Juno Garmin Etrex
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Results
General trends ?- Positional accuracy increased
as slope position increased (for
most situations). ? Change in RMSE with
increasing number of position fixes was not as
predictable as one would have hoped. While the
order of units tested at each site was randomly
determined, the RMSE results are curious, and
require further examination. These were one-time
measurements, typical of operational
work. Repeated measurements, as what were
collected in the other studies, may temper some
of the results, but then the study will not be
reflective of typical operational work.
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Study 4
A test of the static and dynamic accuracy of
ScoutPak
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Methods
GPS receivers studied Mapping-grade ScoutPak
GPS system Conditions A rather long
course Leaf-off time of year (November)
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Results
Leaf-off CEP50
All Slopes 1.3 1.2 1.7
Attempt Run 1 Run 2 Run 3
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Results
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Results
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Results
Overall Observations 1. With a mapping-grade
GPS receiver, real-time positional accuracy may
be 4-15 meters under tree canopies. 2. WAAS
does slightly improve real-time position
estimates. 3. Post-process differential
correction significantly improves position
estimates. With a mapping-grade GPS receiver,
3-5 m accuracy may be obtained. 4. Positional
accuracy changes with slope position. 5. Error
levels do not necessarily change (get smaller)
with more position fixes. 6. Multi-path accounts
for about 5-12 m of error, depending on slope
and canopy position and time of year. 7.
Time of year matters.
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http//warnell.forestry.uga.edu/Warnell/Bettinger/
GPS/UGA_GPS.htm
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http//warnell.forestry.uga.edu/Warnell/Bettinger/
GPS/UGA_GPS.htm
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