pete bettinger warnell school of forestry and natural...

GPS Performance in Southern Hardwood ForestsPete BettingerWarnell School of Forestry

and Natural ResourcesUniversity of Georgia

6th Southern Forestry and Natural Resources GIS Conference

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 areasunder 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.


Introduction

Manufacturer's stated accuracy of GPS receivers are usually ambitious.

We have embarked on a set of studies to determine the accuracy ofvarious receivers and antenna configurations in a forested environment.

Our goal is to better understand the behavior of data positioning aswell as to better understand the capabilities of GPS receivers.


Introduction


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 opposedto traditional differential correction, which can be performed afterdata has been collected.


Introduction

The Wide Area Augmentation System (WAAS) was begun in 1994as 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.


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 givenionospheric delay information that is determined.

Introduction


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.


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.

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.

Introduction


Lower-population areas, and areas away from waterways, most notably the RockyMountains, Texas,West Virginia, and Alaska, have poor coverage by the Coast Guard's ground-based DGPS.

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).

Introduction


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.


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.


Introduction

Extrex~$100

GPS 60~$200

GPS 72~$130

GPS Rino~$650

Magellan Triton 300

~$150

Magellan eXplorist 100

~$120

Magellan eXplorist XL

~$400

Magellan eXplorist 600

~$350


Brunton Atlas ~$400

DelormeMapping

GPS ~$400

LowrenceiFinder

~$100 to 300

BushnellOnix 200

~$200

Introduction

Mio P550 PDA~$300


Introduction

Trimble GeoXH~$5,300

Trimble GeoXM~$2,600

Trimble Recon~$1,000

TrimbleJuno ST

~$650

CMT MC-GPS~$2,500

CMT March-II-E~$2,500

CMT PDA GPS~$1,300

Leica GS20Professionaldata mapper

~$4,800

Magellan Promark 3

~$3,000


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 < 2 cm.

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.

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.

Introduction


Study #1

Study #1: A Comparison of GPS Performance in a Southern Hardwood Forest: Exploring Low-Cost Solutions for Forestry Applications


Methods

GPS receivers studied:

Mapping-grade: Trimble ProXR

Recreation-grade: Garmin EtrexGarmin Map 60CThales Mobile Mapper

Conditions:

Upper slope, mid-slope, lower slope positionsLeaf-on time of year (summer)Leaf-off time of year (winter)


Results

Receiver

EtrexEtrex with WAASMap 60CMap 60C with WAASThales Mobile MapperTrimble ProXRTrimble ProXR diff. corrected

LowerSlope

14.210.721.013.931.58.13.1

Mid-Slope

10.610.515.912.131.08.43.0

UpperSlope

9.47.4

13.712.522.45.63.1

Leaf-on RMSE


Results

Receiver

EtrexEtrex with WAASMap 60CMap 60C with WAASThales Mobile MapperTrimble ProXRTrimble ProXR diff. corrected

LowerSlope

6.36.0

32.418.332.68.92.0

Mid-Slope

5.54.7

31.914.619.27.52.0

UpperSlope

7.76.6

26.18.8

21.35.72.1

Leaf-off RMSE


Results

General trends:

Positional accuracy increased as slope position increased.

However, improvements with WAAS correction were found across theslope positions studied.

Change in RMSE with increasing number of position fixes was notas predictable as one would have hoped.

Mapping-grade receiver outperformed the other receivers on all slopeconditions.

Differentially corrected (post-processed) data was better than WAAScorrected data (on the fly).


Study #2

Multipath mitigation under a forested canopy:Using a choke-ring antenna


Methods


Mapping-grade: Trimble ProXR

Topcon Choke ring antenna

Conditions:

Upper slope, mid-slope, lower slope positions

Leaf-on time of year (summer)Leaf-off time of year (winter)


Results

Receiver

Trimble ProXRTrimble ProXR diff. corrected

Choke ringChoke ring diff. corrected

LowerSlope

6.53.1

2.80.3

Mid-Slope

8.12.8

2.00.2

UpperSlope

5.63.0

2.00.2

Leaf-on RMSE


Results

Receiver

Trimble ProXRTrimble ProXR diff. corrected

Choke ringChoke ring diff. corrected

LowerSlope

8.71.9

2.80.3

Mid-Slope

7.31.9

2.10.2

UpperSlope

5.71.9

2.40.2

Leaf-off RMSE


Results

General trends:

Positional accuracy increased as slope position increased.

Differentially corrected (post-processed) data was better than rawdata collected on the fly.



Results

General trends:

The choke ring antenna more effectively mitigated signal degradation.Real-time RMSE with the choke-ring configuration was about as goodas the RMSE associated with differentially-corrected ProXR data.

a) One might assume that 3.5 m of inaccuracy is associated with other factorsthan multipath

(uncorrected choke-ring data - differentially corrected choke-ring data)

b) Similarly, multi-path results in 5-12 m of inaccuracy depending on slope andcanopy position and time of year.

(uncorrected ProXR data - uncorrected choke-ring data)


Study #3

A Comparison of GPS Performance in a Southern Hardwood Forest: Exploring Real-time Data Accuracy of Forestry Receivers


Methods


Mapping-grade: Trimble ProXHTrimble GeoXHGarmin 17 HVSTDS Nomad

Recreation-grade: Garmin EtrexTrimble Juno

Conditions:

Random positionsLeaf-on time of year (summer)


Results

Receiver

Trimble ProXHTrimble GeoXHGarmin 17 HVSTDS Nomad

Trimble JunoGarmin Etrex

LowerSlope

2.33.5

11.66.5

12.912.4

Mid-Slope

9.12.56.1

17.9

6.29.8

Leaf-on RMSE95


Results

General trends:

- Positional accuracy increased as slope position increased (for mostsituations).


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 reflectiveof typical operational work.


Study #4

A test of the static and dynamic accuracy of ScoutPak


Methods


Mapping-grade: ScoutPak GPS system

Conditions:

A rather long courseLeaf-off time of year (November)


Results

Attempt

Run #1Run #2Run #3

AllSlopes

1.31.21.7

Leaf-off CEP50


Results


Overall Observations:

1. With a mapping-grade GPS receiver, real-time positional accuracy maybe 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 andcanopy position and time of year.

7. Time of year matters.

Results


http://warnell.forestry.uga.edu/Warnell/Bettinger/GPS/UGA_GPS.htm


pete bettinger warnell school of forestry and natural...

Documents