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Quality of low-cost GPS for geodetic and navigation applications
Volker Schwieger
head of department,
measurement and positioning techniques,
iagb habilitation, university of Stuttgart, Germany
Email:
The paper defines the term low-cost GPS and gives an overview of the different low-cost GPS receiver classes and the respective applications
In general the global satellite navigation systems (GNSS) are seen as a global problem solver as indicated by the name of the most famous representative the Global Positioning System (GPS). It is expected that the GNSS will play an even more important part in the concert of positioning techniques due to the development of the non-military European satellite system Galileo that will deliver integrity services.
At present GPS is used for precise geodetic applications covering the cm up to the mm level, for geodata acquisition for different GIS related applications and for mass market applications like car navigation or location based services covering the accuracy level of up to some m. Phase measurements are required for precise applications in geodesy and navigation, code data is sufficient for mass market applications. GIS related geodata acquisition often reverts to code measurements supported by phase data. These different measurement techniques require different receiver classes and therefore different financial investments (e.g. Schwieger and Gläser 2005).
Independently of the receiver class one important and non-solvable problem is the shadowing of signals leading to the non-availability or falsification of GPS signals e.g. in urban canyons or forests. Another restriction is the financial effort for the purchase of high precise GPS receivers. This is valid especially for small engineering office and even more important for engineering offices in developing countries due to the difficulties to get money for investments. This paper will show possibilities and give examples for the improvement of the three quality accuracy criteria, reliability and availability for low-cost GPS. In this paper reliability stands for the percentage of available positions without gross or systematic errors.
Low-Cost GPS
In general a surveyor or a geodesist uses GPS receivers in relative mode and processes the phase information to get the highest possible accuracy for the estimated coordinates. Consequently high quality and therefore expensive GPS receivers have to be purchased. These coordinates may be of static or - in the case of a moving object - kinematic nature. In the second case the coordinates sequence is called a trajectory.
For geodata acquisition the accuracy requirements are below the ones for classic geodetic fields. They depend on the application; e.g. cadastre demands for precise phase measurements and mapping of roads for navigation purpose demands for code measurements only. In general GPS receivers for geodata acquistion are equipped with a special software allowing to acquire digital object-related data from the field and to transfer it via a defined interface to a GIS (e.g. LEICA GS20, Trimble GeoXT and THALES Mobile Mapper). These receivers are less expensive than the precise ones mentioned above, but nevertheless they require an investment of some thousand Euros. Both receiver classes may not be called low-cost, due too the required financial investment effort.
In this paper a GPS receiver costing less than 1000 Euro shall be defined as low-cost receiver. This definition covers handheld receivers (e.g. Garmin eTrex or Magellan SporTrak), as well as Original Equipment Manufacturer boards (OEM boards) or chip sets (e.g. SiRFstarIII or Garmin GPS 15). These boards are often integrated into positioning modules of navigation systems or driver assistance systems. Other low-cost GPS receivers, so-called GPS Mouse, (e.g. T-Mobile NaviGate BlueKit or GPS Mouse GM-200) may be combined with a mobile phone, a navigation system or a computer on different integration levels. All these receiver types use code measurements for the realtime-estimation of the position. Sometimes the phase information is used to smooth the positions. Almost all low-cost GPS receivers use filter algorithms to smooth the positions respectively the trajectories. In general no details about these algorithms are available for the users and the scientific community. Table 1 gives an overview about the different low-cost GPS receiver classes.
For typical low-cost applications the separation into static and kinematic positioning is quite difficult to be done. Most of the application fields like car navigation, fleet management or driver assistance systems are kinematic per definition. Another interesting field of low-cost GPS is the positioning for Location Based Services (LBS). In this case the difficulty to distinguish between static and kinematic positioning arises due to the fact that the user may move or do not move while the coordinates are determined. In any case the estimation of the position is realized using the data of one measurement epoch only. That's why the separation between the two modes is of little importance for low-cost GPS, if it used in a standard application.
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