Handheld GPS receivers are becoming increasingly popular among all manner of outdoor enthusiasts and leisure users

GRINGO (GPS RINEX Generator) is a program which has been developed at the Institute of Engineering Surveying and Space Geodesy (IESSG) at the University of Nottingham, to record the pseudorange and full carrier phase data from 12-channel Garmin handheld GPS receivers, in standard RINEX (Receiver INdependent EXchange) format. It offers owners of these receivers the possibility of post-processing to an accuracy of approximately 5 m (with pseudoranges) or even a few centimetres (with carrier phase), without having to invest in separate DGPS receiving equipment or expensive survey-grade receivers. They retain the benefits of an inexpensive receiver with a user-friendly interface and powerful navigation features, but gain the possibility of improved accuracy if needed. This accuracy could be of use to all manner of navigation, mapping and GIS applications, where the accuracy achievable with standalone GPS is insufficient. A number of experiments have been carried out to assess the accuracy of positioning with Garmin raw measurement data. For example, a zero baseline test, with two Garmin receivers attached to a single static antenna, has shown that under ideal conditions, sub-centimetre accuracy can be achieved with carrier phase measurements. Recently a new project considered the use of Gringo for the rapid production of a new orienteering map. This paper will present the background to the Gringo software and its operations, and will describe the recent survey and results. INTRODUCTION Handheld GPS receivers are becoming increasingly popular among outdoor enthusiasts and other leisure users. Public awareness of GPS has risen and prices have dropped to the point where these devices are now common in highstreet consumer electronics shops. With a growing market place and increasing turnover, manufacturers have been able to increase the performance and capabilities of handheld GPS devices, and many now sport an impressive array of features, such as the ability to apply received differential corrections in real-time, and built-in and/or uploadable mapping. In May 2000, the US Department of Defense removed their artificial degradation of the GPS signals, known as Selective Availability (SA), and overnight the horizontal accuracy of GPS improved from a specified 100 m (2drms) to somewhere around 10 m – we all had a free upgrade. Nevertheless, there are many applications for which even this accuracy is not sufficient, and the technique of differential positioning, or DGPS, which prospered under SA, can still provide a useful improvement to the stand-alone accuracy. DGPS compensates for common error sources such as atmospheric delays, and can give accuracies of better than 5 m. The use of carrier phase measurements can improve the performance still further, either by smoothing the basic range measurements, or through the use of interferometric processing techniques to yield relative coordinates with an accuracy of a decimetre or better. Until recently, users wishing to make use of these techniques have had a couple of options open to them. Firstly, receivers which accept DGPS corrections as input can use these corrections to compensate for common error sources in real-time, and yield accuracies of the order of 5 m. However, this requires the user to carry additional receiving equipment in order to pick up the DGPS broadcasts, if they are available, and feed them into the GPS receiver. Alternatively, board-level and survey-grade GPS receivers provide access to the raw GPS observables, namely the pseudoranges and the carrier phase measurements. Depending on the type of receiver, these raw measurements can either be recorded in internal memory, or logged to an attached computer, and then postprocessed in conjunction with data from reference receivers. This option, by virtue of the access to the carrier phase observations, provides the highest level of accuracy, typically a few cm, but usually at a cost. Besides being significantly more expensive and bulky than handheld receivers, survey-grade receivers do not usually come equipped with user-friendly navigation firmware or features such as built-in base mapping, while board-level receivers are designed and sold to be used by Original Equipment Manufaturers (OEMs) or as the basis of a custom-built system, in which the user adds keypad, screen and logging interfaces according to particular requirements. WHAT IS GRINGO? GRINGO (GPS RINEX Generator) is a program which has been developed at the Institute of Engineering Surveying and Space Geodesy (IESSG) at the University of Nottingham, to record the pseudorange and carrier phase data from a particular range of handheld GPS receivers, in standard RINEX (Receiver INdependent EXchange) format. It offers owners of Garmin 12-channel receivers the possibility of post-processing to an accuracy of approximately 5 m (with pseudoranges) or 10 cm (with carrier phase), without having to invest in separate DGPS receiving equipment or expensive survey-grade receivers. They retain the benefits of an inexpensive receiver with a user-friendly interface and powerful navigation features, but gain the possibility of improved accuracy if needed. Figure 1 – GRINGO Splash Screen HOW IS IT DONE In addition to industry-standard protocols for DGPS input (RTCM) and coordinate exchange (NMEA), Garmin receivers use a proprietary data format to allow internal waypoints, tracks and other information to be exchanged with a computer or another Garmin receiver. Even before parts of this so-called Garmin Communications Protocol were officially published by Garmin, most of the important parts had been decoded and published on the internet by a small number of interested users of the Garmin receivers. A great deal of software, from free utilities to shareware and fully commercial programs, has been written to make use of the Garmin Communications Protocol. However, there is still quite a lot of information that the Garmin receivers output using this protocol, which is not documented by Garmin. According to Garmin, these 'undocumented protocols' are intended as engineering and manufacturing 'testing aids'. GRINGO's authors have deciphered parts of some of the undocumented protocols, which appear to contain the raw pseudorange and carrier phase measurements necessary for post-processing. GRINGO is a Windows program which decodes the relevant protocols and logs the raw data to a file using the widely accepted RINEX format. Users must connect their Garmin receiver to a serial port on their laptop computer, and run GRINGO in real-time to capture the pseudorange data as the measurements are generated and output (Figure 2). For users who do not have access to a suitable laptop for field use, a companion program has also been developed for the Psion Series 3mx PDA and the Psion Workabout mx. This companion program captures the necessary data from the Garmin receivers, in a format which GRINGO can later decode to produce a RINEX file. Figure 2 Data Logging screen from GRINGO POST-PROCESSING With the appropriate processing software, a RINEX pseudorange data file can be combined with a data file from another ('reference') receiver, to measure the vector between the receivers. If the coordinates of the reference receiver are known to a high accuracy, the coordinates of the Garmin receiver can be determined from pseudorange measurements to an accuracy equivalent to DGPS. With carrier phase measurements, the vector between the reference station and the Garmin receiver can be measured to an accuracy of 10 cm or better. Of course, such accuracies cannot be guaranteed, as many factors can influence the performance of a GPS receiver, and hence the precision of the decoded data. GRINGO comes with a Pseudorange and Phase Post-Processor (P4), which is optimised for the task of handling RINEX files of Garmin pseudorange and carrier phase data. P4 provides all the options necessary to compute stand-alone, DGPS or carrier phase positions. It provides a statistical and graphical analysis of the resulting positions (Figure 3), as well as providing details of the observations used in the computations (Figure 7, Figure 9). Figure 3 Positioning Results from P4 Post-Processor DOES IT WORK? A useful way of assessing the accuracy and precision of a GPS receiver is to carry out a 'zero baseline' test. This involves two receivers connected to a single antenna, independently recording measurements for later analysis. Since they share the same antenna, the derived baseline between the two receivers should be identically zero. The test is useful because a number of phenomena which contribute to errors in the raw measurements, such as atmospheric refraction, satellite ephemeris and multipath, should be common to the two receivers, and should cancel each other in the post-processing. The test therefore highlights the instrumental precision of the raw observables. A zero baseline test has been carried out using two similar Garmin receivers, connected to a single low-cost antenna via an antenna splitter (Figure 4). In this case, the exercise is a good test of GRINGO's decoding abilities, since any errors made by GRINGO on one receiver would not be cancelled by independent errors made on the other receiver. One receiver logged RINEX data directly to a laptop computer, while the other logged the necessary raw data to a Psion PDA (Figure 5), and GRINGO was used to process the raw data into a RINEX file. The exercise was carried out over 10 minutes, with observations recorded at 1-second intervals. Figure 4 Zero Baseline Experiment Figure 5 Logging Data to a Psion PDA Figure 3 illustrates the results achieved if one receiver is treated as a reference receiver and the other as a mobile receiver, ie each epoch of data is processed independently to give the track of the mobile. Since the shared antenna was static, each epoch of data should give the same (zero) result. The figure shows that the individual coordinate solutions for the mobile receiver have a mean of 1.1 m from the reference receiver, and that 95% of the results are within 1.4 m of the mean. Figure 6 illustrates the results achieved if the second receiver is treated as static, ie every epoch of data contributes to a single overall coordinate solution from the second receiver. The figure shows how the solution converges to a final position as successive epochs of data are added. The final position was 0.9 m from the reference receiver. Zero Baseline Test Static DGPS Solution (10 minutes of 1-second data) P4 Pseudorange and Phase Post-Processor © Copyright The University of Nottingham 1999 Longitude (metres from Reference) -0 .2 -0 .4 -0 .6 -0 .8 -1 -1 .2 La tit ud e (m et re s fro m R ef er en ce ) -0.8