iGPS-BT User Manual

FCC Notices
This device complies with Part 15(b and c) of the FCC Rules. Operation is subject to the following conditions: This device may not cause harmful interference. This device must accept any interference received, including interference that may cause undesired operation.
CAUTION: Change or modification not expressly approved by the party responsible for compliance could void the users authority to operate this equipment.

Class B

This equipment has been tested and found to comply with the limits for a Class B digital device pursuant to Part 15 of the FCC Rules. These limits are designed to provide reasonable protection against harmful interference in a residential installation. This equipment generates, uses, and can radiate radio frequency energy and, if not installed and used in accordance with the manufacturers instruction manual, may cause interference with radio communications. However, there is no guarantee that interference will not occur in a particular installation. If this equipment does cause harmful interference to radio or television reception, which can be determined by turning the equipment off and on, you are encouraged to try to correct the interference by one or more of the following measures: Reorient or relocate the receiving antenna. Increase the separation between the equipment and the receiver. Connect the equipment into an outlet on a circuit different from that to which the receiver is connected. Consult the dealer or an experienced radio/television technician for help.
FCC ID# Q7M-IGPS-BT CE Notices
This device has been tested and found to comply with CE marking according to the R&TTE Directive, 99/5/EEC. The test standards are listed below:
CE/LVD CE/EMC Radio Spectrum
EN60950: 1992+A1+A2+A3+A4+A11 EN301 489-17 V 1.1.1: 09-2000 EN301 489-1 V 1.3.1: 09-2001 EN300 328-1 V 1.3.1: 12-2001 EN300 328-2 V 1.2.1: 12-2001
iGPS-360 Receiver and Bluetooth Dock

iGPS-BT Dock

iGPS-360 Receiver

Power Jack

On/Off Switch
Bluetooth Status Indicator (Internal Blue LED) Power Jack Plug either the AC battery charger (optional, in PXT02) or the DC charging adapter (included) into the Power Jack to charge the battery. Refer to III. Battery charging instructions. iGPS-BT will need to be charged for approximately 2-3 hours before initial use.
On/Off Switch Turns the iGPS-BT on or off. Blue LEDs will light up when powered.
Bluetooth Status Indicator (Internal Blue LED) Blinking: Searching for Bluetooth signal Steady: Bluetooth communication established, ready to use.

II. iGPS-BT Setup

1. The iGPS-BT will need to be charged for two to three hours before initial use. 2. Turn on the iGPS-BT. The status indicator will blink as it searches for a Bluetooth connection. 3. Launch the Bluetooth configuration software on your mobile device. (This should be provided by the manufacturer of your Bluetooth device) 4. Have the Bluetooth configuration software search for any Bluetooth devices present. It should detect the iGPS-BT and list it as an available device (a Pharos iGPS-BT icon should be displayed). 5. Select the Pharos iGPS-BT device. Depending on your Bluetooth software, this should bring up a screen that will ask for a PIN Code or Security PIN. Enter the following PIN code: IMPORTANT

PIN Code: 12345678

(Please memorize this code for future use) 6. After the PIN number is entered, a connection between your mobile device and the iGPS-BT should be established. The blue LED on the iGPS-BT dock will change from blinking to a steady glow. 7. Once a connection is established, check the properties of the iGPS-BT in the Bluetooth configuration software and make a note as to what number COM port is assigned to it. The navigation software that will be used will require this COM port number. For more information regarding the iGPS-BT setup, please visit our Support Product Support Bluetooth Setup page at www.pharosgps.com. Hints & Tips GPS receiver must have a direct view to the sky. GPS receiver does not work indoors. If possible, make sure the GPS is stationary when getting the initial satellite position lock. Allow 1-2 minutes at initial start-up for the GPS receiver to acquire a satellite lock for tracking. Make sure that the communication (COM) port is configured correctly in the navigation software being used to ensure proper functioning of the GPS system.
III. Battery charging instructions
The Pharos Bluetooth Dock (BT Dock) for the GPS receiver includes a rechargeable and replaceable Li-Polymer battery. The optional Power Accessory Kit (PXT02), which contains a spare battery, battery charging holder, and an AC battery charger can be purchased from Pharos. Below are four possible ways to charge the battery.

A. Charging the battery at home with AC battery charger:
1. Charging the battery in the BT Dock with or without GPS receiver Plug the DC plug of the AC charger (optional) into the DC jack on the BT Dock then plug the AC charger into the AC outlet. When the LED indicator turns green, the battery is fully charged.
2. Charging a spare battery in the battery charging holder (in PXT02) Plug the DC plug of the AC charger into the DC jack on the battery charging holder (make sure the battery is properly placed in the holder) then plug the AC charger into the AC outlet. When the LED indicator turns green, the battery is fully charged.
B. Charging the battery in the car (or at home) with DC charging adapter:
3. Charging battery while using the BT GPS in the car Plug the DC plug from the DC charging adapter into the DC jack on the BT Dock then make sure the car charge is on and is connected to the DC charging adapter. When the LED indicator turns green, the battery is fully charged. Note: You can charge both PDA and iGPS-BT at home by plugging the PDA AC charger (comes with your PDA) instead of the car charger into the DC charging adapter.
4. Charging a spare battery in the battery charging holder (in PXT02) in the car (or at home) Plug the DC plug from the DC charging adapter into the DC jack on the battery charging holder (make sure the battery is properly placed in the holder) then make sure the car charge is on and is connected to the DC charging adapter. When the LED indicator turns green, the battery is fully charged. Note: You can charge both PDA and the spare battery at home by plugging the PDA AC charger (comes with your PDA) instead of the car charger into the DC charging adapter.
Battery Precautions (w: WARNING!) 1.w 2. 3.w 4.w 5.w 6. 7.w 8.w Reverse charging is not acceptable. Charge before use. The cells/batteries are delivered in an uncharged state. Do not charge/discharge with more than our specified current (400mA). Do not short circuit the cell/battery, permanent damage to the cell/battery may result. Do not incinerate or mutilate the cell/battery. Do not solder directly to the cell/battery. The life expectancy may be reduced if the cell/battery is subjected to adverse conditions like: extreme temperature, deep cycling, excessive overcharge, or over-discharge. Store the cell/battery uncharged in a cool dry place. Always discharge batteries before bulk storage or shipment.
Note: Please follow the instructions whenever you use or dispose of your battery pack. a. b. c. d. e. f. g. Use provided charger only. Do not incinerate the battery. Do not disassemble or modify the battery. Do not allow metal objects to contact or short circuit the battery terminal. Avoid exposure to excessive heat (> 60oC or 140), moist, or caustic environments. Stop using the battery whenever there are unusual conditions (for example: deformed, discolor, peculiar smell, leakage, etc.) Must be recycled or disposed of according to the local waste disposal agency.

IV. iGPS-BT GPS Receiver Specifications
Performance Chipset Antenna Frequency Sensitivity Channels Acquisition time Reacquisition Position update Accuracy SiRF StarIIe/LP Integrated patch antenna L1, 1575.42 MHz -140 dBm (typical) 12 Channel all-in-view tracking Cold Start: 60 sec Warm Start: 40 sec Hot Start: 5 sec 0.1 sec 1 Hz Position: 10 meters 2D-RMS, SA off Velocity: 0.1 meter/second, SA off Time: 1 microsecond synchronized to GPS time Datum: WGS-84 720mAh Li-Polymer rechargable/replaceable battery AC battery charger optional, DC charging adapter (works with car charger in car or PDA AC charger at home) included. Constant current charging circuit included in both AC and DC battery charger. 6 hours minimum continuous use with full charge > 6 hours in trickle power mode Communicate with Host Platform via Bluetooth(Class2) Serial Profile NMEA-0183 (V2.3) standard 4800 bps GGA, GSA, GSV, and RMC Programmable Programmable Frequency band: 2400-2483.5 MHz Data rate: up to 721 Kbps Security: data encryption up to 128-bits Typical range: up to 32 feet (10 meters) 60 mm x 85 mm x 25 mm 85 grams -20C to 60C (-4F to 140F) Altitude < 20 km Velocity < 900 km/h Acceleration < 3g
Electrical Power Battery Battery charger
Operation time Interface Connection Protocol Data rate NMEA message WAAS/EGNOS Trickle power Bluetooth spec
Physical Dimension Weight Environmental Operation temperature Dynamics
Specifications are subject to change without prior notice.

V. Precautions

Position fixing may not be available when this receiver is used near highvoltage wire, electronics equipment that generates electrical noise, or mobile phones in operation at 1.5 GHz. The noise from PDA or PC may deteriorate the GPS receiver performance. Note that some glass material containing metal, such as heat-ray protection glass, screen out signals from GPS satellites. GPS satellites are under the control of the U.S. Department of Defense. Therefore, services provided for general consumers are subject to change without prior notice. Pharos shall not be liable for any losses caused by such changes. Do not touch the connectors of the Bluetooth Dock with fingers or insert foreign substances; otherwise, a failure may be caused. Do not put this receiver at a place exposed to direct sunlight for a long period of time; near heating equipment; a place that can become hot, or a humid place, such as a bathroom. Do not wet this receiver; otherwise a failure may be caused. If you use it in the rain, fog, or snow, be careful not to wet it. Do not drop this receiver, apply strong impact to it, or put it on a surface that vibrates; otherwise a failure may be caused. Use a soft cloth to wipe dirt off this receiver. If dirt is severe, dampen a soft cloth with diluted neutral detergent, securely wring it out, and wipe off the dirt. Do not use a spray-type detergent; highly volatile solvent, such thinner and benzene; or a chemical cloth. Otherwise, deformation or discoloration may be caused.

VI. Package contents

PT200: Bluetooth Pocket iGPS NavigatorTM iGPS-360 receiver iGPS-BT Dock with Li-Polymer battery DC charging adapter Car charger for PDA Vent mounted PDA Holder Ostia Software and street level maps Travel carrying case Belt case Bluetooth iGPS Receiver iGPS-360 receiver iGPS-BT Dock with Li-Polymer battery AC battery charger Belt case

PT100:

Note: Contents may vary- see the box cover or contents list. Note: New products may not be listed here.
VII. Quick Start Guide for Ostia Navigation Software(included in PT200)
1. Install the Pharos Ostia & MapFinder Software
Insert the CD-ROM. If Auto-Run is enabled for your CD-ROM, the Setup program will run automatically. If not, run the Setup program on the CD-ROM. Select Install Ostia to install the navigation software to your Pocket PC and follow the on-screen instructions. Select Install MapFinder to install the map loading utility to your Desktop/Laptop PC and follow the on-screen instructions.

SYNCHRONIZATION CABLE

2. Install Map Data
Insert map CD, MapFinder will appear. Type a city name or zip code in the search field, then click "Search. Double click on the appropriate result (to the right). -OR- Double click sections directly on the map to select multiple maps. Right click on the map to bring up the command menu and choose where you want the maps to be transferred to. You will have the option to copy the maps directly to your Pocket PC or to your PC hard drive for later use. If you choose to extract maps to your PC hard drive, you can use MapFinder later to transfer the maps to your Pocket PC or Memory Card.
Installing Map Data to Internal Storage

Map Data

Docking Station
Installing Map Data to Memory Card

Memory Card Map Data

PLEASE NOTE: IN EITHER CASE MAPS SHOULD RESIDE IN THE \My Documents DIRECTORY OF THE POCKET PC OR MEMORY CARD. MAPFINDER WILL AUTOMATICALLY CREATE THE \My Documents FOLDER.
Create a Route Using the Pharos Pocket GPS Navigator
Start your vehicle in a safe area where as much of the open sky is in view (GPS does not work indoors), then proceed with the following steps: On the PDA start running the Pharos Ostia program. Click File then Open and select the map(s) you want to use. In the Find menu, select your destination using Recent Destination, Contacts, Favorites, Address, Intersection, or Point of Interest. Click the Happy Face icon then click Yes to Enable GPS. Wait approximately 60 seconds for a red arrow to appear indicating your current position on the map. In the Find menu click New Route to calculate a route from your current position to the selected destination. The calculated route will be highlighted on the map in light blue. Start driving along the blue route with the advisory of the voice/sound prompts and on-screen graphics. A Favorite can be used to find a specific Latitude and Longitude point on the map which can then be used to find the nearest street that can be selected as the Origin or Destination.

Additional Tips

If you should miss a turn or make a wrong turn, the voice prompt will inform you that You are off route. Simply press the Pocket PCs Action Button (Refer to your PDA manual to identify the Action Button) to have a new route calculated from your current position to the selected destination. To have the program automatically generate a new route when you go off route, access the Tools/Options menu, and then check Auto Re-route. Under Tools"/Options, check "Heading Up" to choose the vehicles forward direction as the orientation of the map on the PDA display. This option will display a compass in the bottom, right-hand corner of the map. North up is the default map orientation. In the View menu, click GPS Info to view Compass Information (Lat., Lon., Alt., Time, Speed, and Distance) about the current position of the iGPS receiver. In the View menu, click Text Directions to view turn-by-turn text directions for your route with street names and distances between each.
Please refer to the Ostia Users Manual for detailed instructions.

VIII. Troubleshooting

For any troubleshooting assistance, please check the Frequently Asked Questions section at www.pharosgps.com.

IX. Warranty

The iGPS-BT Receiver is warranted by Pharos to the original purchaser to be free from defects in material workmanship under normal use for a period of one year from the date of purchase. During the warranty period, and upon proof of purchase, the product will be repaired or replaced (with the same or a similar model) at Pharos' option without charge for parts or labor. This warranty will not apply if the product has been misused, abused, or altered. To obtain warranty service, you must take or send the product, postage paid, with a copy of your sales receipt or other proof of purchase and the date of purchase to Pharos at 411 Amapola Avenue, Torrance, CA 90501, USA. Prior approval and RMA number must be included for your warranty return to be accepted.

X. Return Policy

Our goal is for every customer to be satisfied with each purchase, and we will make every effort to resolve any issues you may have. All non-defective returns must be shipped with the original packaging with all original components within the first 30 days from purchase. There is a 15% restocking fee on all non-defective returns. Shipping charges are not refundable. The customer is responsible for returning items to Pharos at their own expense. All defective returns for either repair or replacement will be processed and returned to the customer within 3 business days of receipt. For items beyond the 1year warranty, a repair charge of $35 to $50 or a replacement charge of $95 to $150 will be due. For items sent to us after 90 days from purchase, an $8.50 shipping and handling fee will be due upon completion of repair or replacement. Every return, defective or not, MUST have prior written approval from Pharos. The RMA number MUST be marked on the outside of the package otherwise it will be rejected. The RMA form must also be included in the box with the customer's information and reason for return. Proof of purchase must also be attached.

Final Report: Detection, Prediction, Impact, and Management of Invasive Plants Using GIS
Comparing GPS Receivers: A Field Study
Kindra Serr, ISU GIS Training and Research Center, Campus Box 8130, Pocatello, Idaho 83209-8130 (giscenter@isu.edu) Thomas Windholz, ISU GIS Training and Research Center, Campus Box 8130, Pocatello, Idaho 832098130 Keith Weber, ISU GIS Training and Research Center, Campus Box 8130, Pocatello, Idaho 83209-8130 (webekeit@isu.edu) ABSTRACT This paper compares the precision and accuracy of five current global positioning system (GPS) receiversTrimble ProXR, Trimble GeoXT without WAAS, Trimble GeoXT with WAAS, Trimble GeoExplorer II, and an HP/Pharos receiver. Each of these receivers, along with other similar units, are frequently used today for data collection and integration within a geographic information system (GIS). To compare receivers, we conducted a field study of 15 established survey markers in the City of Pocatello, Idaho. The points were observed on ten different dates with equivalent settings (e.g., averaging and acceptable point dilution of precisionPDOP). Overall, the results indicate that the GeoXT is well suited where sub-meter accuracy is required while the Pharos receiver is a viable alternative for applications with accuracy requirements of +/- 10m and more. Keywords: GPS, co-registration, high resolution imagery
INTRODUCTION The use of GPS receivers has become wide spread over recent years. Many applications, from hunting to surveying, benefit greatly from these devices. The level of accuracy required from application to application varies greatly. It is important to recognize the grades of GPS receivers, namely consumer, mapping and survey grade, and their ability to accurately map features with or without differential correction. The accuracies of these receivers range from centimeter to several meters, making it necessary to evaluate how accuracy and precision can affect individual applications. When using a GPS receiver to collect field data, accuracy can be very important, especially when collecting data for use with high-spatial resolution imagery. Quickbird multispectral imagery, for example, achieves a resolution of 2.4 meters per pixel. In order to co-register corresponding ground sample locations within the correct pixel(s), an accurate GPS receiver is required. To ensure that each field observation is co-registered with the correct pixel, a GPS receiver must achieve an accuracy <50% of the pixel size (e.g., +/-1.2m @ 95% CI where Quickbird imagery will be used). The increased availability of less expensive, consumer grade GPS receivers, such at the HP/Pharos receiver used in this study, that are compatible with common GPS software, such as ESRIs ArcPad or Trimbles TerraSync, has raised concern about data quality. Many such receivers collect data that cannot be differentially corrected, increasing the margin of positional errors in the data collected. Consumer grade receivers are also unable to control the quality of PDOP during data collection, further increasing positional error. To assess the validity of these concerns, a field study was designed to calculate and compare the accuracy and precision of several GPS receivers. The goal of this study was to identify the receivers most appropriate for various research, remote sensing, and GIS applications. Similar studies have been conducted where GPS receiver accuracy has been investigated. Some studies compared receivers under various collection protocols. Studies conducted in Ridley State Park in Pennsylvania (McCullough 2002) and the Clackamas Test Network in Oregon (Chamberlain 2002) tested the capability of the Trimble GeoXT receiver in forested and clear areas with similar procedures and yielding comparable results in each study. Using internal and external receivers (antenna located within the receiver internal, antenna attached externally to receiver external), the studies experimented with WAAS and post-process differential correction techniques, but used higher PDOP masks (e.g., PDOP mask= 7.0) than used in this study (PDOP mask=5.0). Published studies comparing various GPS receivers are limited. One completed in the summer of 2000, compared the accuracy of five different GPS receivers under forest canopy cover with Selective Availability (SA) off (Karsky et.al. 2000). In this study, WAAS was not used because it was not yet available. Differential correction was performed on files that could be corrected and positions were taken at known points in forested areas with 1, 60, and 120 positions averaged for each point. None of the above studies mentions how often points were collected over time or how many times points were collected. Each study concluded the receiver tested was appropriate for their research purposes, whatever those may have been. Overall, previous studies have taken into account some of the aspects related to GPS receiver accuracy, but a comprehensive analysis was not completed. A study done in McDonald Forest, located in western Oregon, investigated the accuracy and reliability of consumer-grade GPS receivers under differing canopy conditions. Six different GPS receivers were evaluated for accuracy under three different canopies: open sky, young forest, and closed canopy. Although the collected data was unable to be differentially corrected, points were averaged and compared relative to the known location, allowing for the receivers accuracies to be compared to one another (Wing et. al. 2005). This evaluation did not include real-time correction, nor was it conducted over an extended period of time. In this paper we describe a field study comparing different GPS receivers to determine optimum applicability for various uses.

METHODS The study area was located in the City of Pocatello and environs (Figure 1). Fifteen points were selected from known locations in Pocatello, Idaho. These points were obtained from the City of Pocatellos ground control database. Each was referenced in the field with permanent survey markers so the exact location could be re-located easily. Each point was visited ten times over a period of one month at approximately the same time each day (+/- 1 hr.). The points were selected for their accessibility and visibility to GPS satellite signals (avoiding vegetation or building interference). These criteria were followed to provide uniformity and the best operating condition for each GPS receiver, thereby verifying the precision and accuracy reported by the manufacturer and eliminating as much environmental influence as is possible in a field-based study. Data collection occurred on days where PDOP was within acceptable limits (<5.0). This was determined using Trimbles QuickPlan software. The location for each point was observed with the following GPS receivers: 1. 2. 3. 4. 5. Trimble GeoXT receiver with WAAS Trimble GeoXT receiver without WAAS Trimble GeoExplorer II Trimble ProXR HP iPaq with Pharos Navigation software and antenna
Points were collected in latitude/longitude (WGS84), the native reference system for GPS receivers. This was done to avoid any transformation errors that may occur during projection. Receivers did not collect data when the PDOP was >5.0 to reduce this type of error. Receivers averaged 120 positions per point each time a site was visited. The weather conditions on most collection dates were comparable and skies were relatively cloud free in all cases. After collection, each point file was differentially corrected using files from Idaho State University (ISU) GIS Training and Research Centers (GIS TReC) GPS Community Base Station, with the exception of the those points collected with the HP/Pharos receiver (the Pharos receiver does not collect the necessary information for differential correction through a base station). The base station was located on the ISU campus in Pocatello. The location of each point ranged from 1.5km to 12.6km away from the base station. Seven of the fifteen original points were then revisited and their location collected using a Leica survey-grade GPS receiver (+/-0.1m @ 95% CI); corrected in real-time using the ISU College of Technologys GPS CORS station (NGS 2005), also located on the ISU campus. These seven locations were used to assess the accuracy of the GPS receivers; where as all 15 locations were used to assess precision. In this study precision refers to the repeatability of a specific GPS receiver collecting locational estimates. The error value (i.e., precision) was based on a relative comparison among measurements (Equation 1 and 2) of the same unit on different days. Accuracy, however, is not a relative comparison, but an absolute comparison. In this case the error value (i.e., accuracy) was calculated (Equation 3) by comparing measurements of a single unit on different days to the known true location of the observed point. These points were collected independently (i.e., different observer, different base station, and well established GPS receiver accuracy) and corrected using the nearby (<12 km) CORS station in real-time. Thus, 150 samples were collected to calculate precision (15 points visited 10 times each) and 70 samples were collected to calculate accuracy (seven points visited 10 times each).

n 1 and Equation 1 Accepted true location based on the mean of observations per sampling site.

2 ( xi x ) i =1 n

n i =1
n and Precision of observations at 95% confidence.

Equation 2

n n and Equation 3 While the accepted true location was based on independent, survey-grade GPS observations of control points, accuracy of tested GPS receivers was calculated as given above at 95% confidence
Spatial analysis of these points was conducted within the native WGS84 geographic reference system. Conversion from decimal degrees (WGS84) to meters was performed using ESRIs ArcGIS software. Resulting units are reported in meters. RESULTS The results of precision and accuracy calculations for the tested GPS receivers are given in Table 1. There is a slight difference in the magnitude of errors between x and y coordinates. Sum of squares was used to assess positional accuracy (i.e., x 2 , y 2 ). To assess the utility of each receiver for various applications we used sum of squares. Extreme values of individual point observations (100% CI) varied between individual receivers (Table 2). The largest error observed was recorded with the HP/Pharos unit (8.41m). Table 1- Results of GPS receiver precision and accuracy (in meters) at 95% confidence Precision Accuracy Precision Accuracy Sum Sum of of Squares x y x y Squares ProXR 0.38 0.46 0.59 0.46 0.78 0.91 GeoXT 0.43 0.59 0.73 0.53 0.77 0.93 GeoXT with WAAS 0.36 0.66 0.75 0.43 0.96 1.05 GeoExp II 1.96 2.90 3.50 2.02 3.25 3.83 Pharos 1.68 2.32 2.86 3.73 4.21 5.62
Table 2-- Proportion of extreme positional outliers (>0.5 and >1.0m thresholds) by receiver.
Limit ProXR GeoXT GeoXT with WAAS GeoExp II Pharos
>0.5 >0.5 >0.5 >0.5 >0.5

Counts

14% 16% 20% 68% 78%
>1 >1 >1 >1 >1

0% 1% 3% 37% 67%

DISCUSSION The calculated accuracies were all within manufacturer specified ranges. Table 3 lists manufacturer stated accuracies with accuracies reported in the results of this paper. Also given is the cost of each receiver provided by the manufacturer. Selecting a GPS receiver that has acceptable accuracy and a reasonable price is important. Generally, increased accuracy comes at higher expense as was demonstrated by this study. While purchasing a low cost receiver, such as the Pharos iGPS 360, may create less expense for an organization but accuracy is compromised. The best accuracy was achieved using the Trimble ProXR (+/- 0.5m @ 95% CI), but this accuracy comes with increased expense. Based upon this information, we conclude that accuracy and cost are directly linked. Higher accuracy results in higher receiver costs. Table 3-- Correlation between manufacturers stated accuracy, measured accuracy, and cost of receiver. Stated Accuracy (m) ProXR (Trimble 2005a) GeoXT (Trimble 2005b) GeoExplorer II (Trimble 2005c) Pharos iGPS 360 (Pharos 2005) 0.5 <1.0 2.0-5.0 <10.0 Calculated Accuracy (m) 0.91 0.93 3.83 5.60 Cost $8,490 (w/data logger) $4,295 $3,995 $300

In Table 1, we reported diminished accuracy when the wide area augmentation system (WAAS) was activated on the Trimble GeoXT receiver. We speculate that the cause for this performance decline was the lack of station coverage within our study area. WAAS uses approximately 25 ground reference stations that collect correction data for effects of the atmosphere, clock errors, and slight satellite orbit errors (ephemeris) (Figure 2). The closest ground station to our study area was the Elko, Nevada station, which is approximately 360 kilometers away (Figure 1). However, the Elko station was off-line at the time of this study, making the Great Falls, Montana station the closest active reference station (523 kilometers away). We assumed that the correction factor applied for the column of atmosphere near Great Falls departed from conditions in and around the study area therefore making the WAAS correction less reliable for our application. This was not anticipated nor is it expected for all applications. In general, outliers, or extreme values were within vendor specified ranges. The Pharos receiver had the greatest extreme values. Thus, where accuracy and precision are concerned, the more expensive receivers outperformed less expensive receivers. It should be noted that Pharos GPS receivers cannot mask for PDOP and do not collect files suitable for differential correction. As indicated in Table 1 the lack of the ability to differentially correct the data is reflected in the relatively large decrease in accuracy compared to its precision. The results reported for the Pharos receiver were effectively best-case scenarios, inferring that accuracy and reliability will quickly deteriorate under more realistic conditions (i.e., poor PDOP, obstruction, etc.)
Figure 1- The location of the Pocatello study area and WAAS stations. The achieved accuracy and precision may be attributed at least in part-- to pre-collection planning. To better ensure field conditions would satisfy the PDOP mask, Trimbles QuickPlan software was used to determine the optimum collection window. This procedure virtually guaranteed that the Pharos receiver, as well as the other receivers tested, would also collect data under ideal conditions. The use of receivers with the ability to implement a PDOP mask allowed us to monitor PDOP, thereby assuring the Pharos receiver was collecting data within the same specified PDOP parameters. A more realistic scenario, however, often requires the user to collect data completely independent of other receivers and planning software/tools. For example, if the only available receiver was a Pharos, PDOP could not be observed or masked, which would lead to reduced accuracy. For these reasons, the Pharos receiver cannot be recommended for any tasks requiring <10m accuracy, yet it is definitely a viable alternative for other applications, such as data collection for lower resolution imagery (i.e., Landsat).

Figure 2- The location of WAAS stations across the United States. Blue indicates active, gold indicates passive, and red indicates communication failure.
A limitation of this study was that accuracy calculations were not based on continuously observed data, but rather on field sampling and revisiting a site over a period of time (i.e., one month). This study does, however, offer a comparison between various GPS receivers under similar research conditions. Reliable accuracy and precision of GPS receivers has become increasingly important concomitant with advances in high spatial resolution imagery. GPS receivers with accuracies of 2 to 5 meters, such as the Trimble GeoExplorer II, are unable to collect data that will reliably co-register within the correct 2.4 meter pixel of Quickbird imagery (Table 3) or other similar imagery. Depending on these types of project-dependent considerations it may be necessary to use a GPS receiver capable of achieving superior accuracy and precision. The Trimble GeoXT tested in this study is a viable receiver for applications requiring high accuracy. Although the Trimble ProXR achieved better results, the GeoXT offers a user friendly interface and compatibility with common GPS software, such as ESRIs ArcPad or Trimble TerraSync, effectively lowering the total cost of ownership by decreasing the time it would take to learn to use the receiver. Table 4Suitability of various GPS receivers for use with remote sensing imagery and GIS mapping products. Precision Trimble ProXR Trimble GeoXT Trimble GeoExplorer II Pharos 0.59 0.73 3.50 2.86 Accuracy 0.91 0.93 3.83 5.62 Applicable image resolution >1.8m >1.9m >7.7m >11.2m Effective Map Scale 1:1,075 1:1,100 1:4,524 1:6,639
CONCLUSIONS This study assessed four GPS receivers and determined both precision and accuracy at 95% confidence. While selection of the optimal GPS receiver is a project-dependent consideration, the data we present are important for GIS managers to help them 1) understand the differences in horizontal positional accuracy obtained from various GPS receiver types, 2) ensure co-registration of GPS-acquired features and satellite or aerial imagery, and 3) determine the appropriate GPS receiver to use to satisfy mapping scale requirements. LITERAURE CITED Chamberlain, K., 2002, Performance Testing of the Trimble GeoXT Global Positioning System Receiver, Draft Report Global Positioning System, United States Department of Agriculture Forest Service, October 2002, http://www.fs.fed.us/database/gps/mtdc/geo_xt/trimble_geoxt.pdf. Karsky, D., K. Chamberlain, S. Mancebo, D. Patterson, and T. Jasumback, 2000, Comparison of GPS Receivers under a Forest Canopy with Selective Availability Off, Project Report - Technology and Development Program, United States Department of Agriculture Forest Service, December 2000, http://www.fs.fed.us/database/gps/mtdc/gps2000/gps_comparison.htm. McCullough, M., 2002, Performance Testing of the Trimble GeoXT Global Positioning System Receiver, Draft Report Global Positioning System, United States Department of Agriculture Forest Service, November 2002, http://www.fs.fed.us/database/gps/mtdc/geo_xt/ridley_ck_geoxt_rich_mccollough.pdf. NGS, 2005, National Geodetic Survey Continuously Operating Reference Stations (CORS), http://www.ngs.noaa.gov/cgi-cors/corsage.prl?site=idpo.