Location:Low Cost, Single-Frequency GPS For Surveying

Abstract
This study presents a cost-effective precise positioning system, which utilizes a low-cost single-frequency OEM GPS receiver and examines the potential use of the system in surveying applications. Several static sessions were carried out in the Istanbul Technical University Ayazaga Campus, Turkey, to examine the accuracy of the proposed system as a function of occupation time, baseline length, and antenna configuration in differential mode. Collected data were processed by the Bernese scientific software and the Leica Geo Office (LGO) commercial software package. It is shown that the system can achieve a positioning accuracy in the order of one decimetre or better for short baselines. The same accuracy level was achieved when a relatively longer baseline of 51 km was processed with the Bernese GPS software. This, however, required a relatively longer occupational time. Such initial results are encouraging, as they meet the required accuracy for a number of surveying and GIS tasks while reducing the cost dramatically.

1. INTRODUCTION
GPS has been used in a wide range of applications that require different accuracy and reliability levels. As a result of the improvements in GPS positioning techniques over the last two decades, positioning accuracy in the order of a few centimetres can now be routinely achieved. This, however, requires the availability of expensive geodetic-grade GPS receivers, which are available at about $5,000 and $15,000 for single and dual-frequency units, respectively. More recently, a number of researchers have investigated the potential use of low-cost (less than $1,000) single frequency receivers, with carrier phase capability, in geodetic applications. For example, Masella et al. (1997), Rizos et al. (1998), Masella (1999), Alkan et al. (2005), Söderholm (2005), Saeki and Hori (2006), Alkan et al. (2006), investigated the performance of low-cost systems in static and/or kinematic modes. They obtained a differential positioning accuracy in the range of 1.5 m (for a 20 km baseline) to a few centimetres in the post-processing mode. The use of the low-cost system in the RTK mode was also investigated by Masella et al. (1997), Masella (1999) and Saeki and Hori (2006).

This paper examines the potential use of a low-cost system, namely Garmin GPS 25-HVS series OEM GPS sensor board, for surveying applications. Such a receiver board is available for a few hundred dollars. The Garmin GPS 25-HVS is a single-frequency receiver, which outputs raw pseudorange and carrier-phase data in the Garmin proprietary format. A series of static tests were carried out to assess the performance of the low-cost system. Various baseline lengths (from approximately 152 m to 51 km) and occupation times (from 30 minutes to 3 hours) were used in the field trials. Collected data were first converted to RINEX format using a computer program developed for this purpose and then processed by both the Bernese scientific GPS software and the Leica Geo Office (LGO) commercial software. It is shown that positioning accuracy at the decimetre- to centimetre-level is possible with such a low-cost system, which meets the requirements of a number of surveying and GIS tasks.

2. SYSTEM DESCRIPTION
The Garmin GPS 25-HVS series OEM GPS sensor board was selected as a low-cost GPS receiver. This single-frequency receiver is a differential-ready 12 parallel-channel receiver, which tracks and uses up to twelve satellites with a default data rate of 1 Hz (programmable from 1 second to 15 minutes). The GPS 25 series offers a compact profile that includes some features such as position, velocity, and time, receiver and satellite status, pulse per second (PPS) timing output, differential GPS capability and raw measurement output for both pseudorange and phase data (i.e. code and carrier phase on L1). The size of the receiver is 46.5 mm ? 69.9 mm ? 11.4 mm and weighs about 38 grams. More details about its usage and specifications can be found in Garmin (2000). Fig 1 shows the low-cost Garmin OEM GPS receiver.

Fig 1. Schematic Depiction of Low-Cost Positioning System Used in the Study.

The Garmin GPS 25 receiver can output raw data through RS 232 serial interface and should be connected to a data logger, a laptop for example. The receiver board has two serial data output ports, one of which provides phase data output while the other normally provides positioning and other information in the NMEA 0183 format. DOS-based Garmin software was used to receive, display and log the raw data (both pseudorange and carrier phase). The program also performs almanac, position and time information upload as well as almanac and ephemeris information download (Fig 2). The system was designed so that the receiver can be used with either low-cost navigational or geodetic GPS antennas.

Fig 2. Data Logging Display with Garmin Software.

Collected GPS data sets in the Garmin proprietary format were converted to standard RINEX format, which can be imported and processed by the Bernese and Leica Geo Office software packages. For this purpose, a RINEX conversion software named GARRIN was developed for the project (Alkan et al., 2005). This software is running on a Windows-based PC and converts the collected data into a standard RINEX observation file.