Differential Barometry in Personal Navigation J. Parviainen, J. Kantola, J. Collin Tampere University of Technology P.O.Box 553, 33101 Tampere Finland Abstract- In many personal navigation applications accurate altitude information is required. Finding a correct floor in a tall building or precise guidance in multi-layer intersection is not possible with two-dimensional navigation system. Satellite navigation systems provide altitude information, but accuracy is not necessarily sufficient for this kind of situations. Barometers can be used to improve the accuracy and availability of the altitude solution. The objective of this study was to find relevant error sources of barometer based altitude in the context of personal navigation. MEMS barometer data were collected in several scenarios where different disturbances affect the pressure reading. To separate error sources, reference barometer at known altitude was used. The results show that barometers in differential mode provide highly accurate altitude solution, but local disturbances in pressure need to be taken into account in the application design. I. INTRODUCTION As location based services become increasingly popular, accurate 3D position must be provided to the service application. In many situations, the navigation solution from a satellite-based system is not available, or accuracy of the solution is wanting. The lack of visible satellites, and the bad quality of observations is a common problem in urban canyons and even more severe in indoor navigation. In vertical direction, the positioning availability and accuracy can be improved by adding a barometer to the system. Current Micro- Electro-Mechanical Systems (MEMS) technology enables barometers to be integrated with satellite navigation units. Barometers have their own difficulties as changes in ambient air pressure are not necessarily due to changes in altitude. Thus, the pressure sensor must be constantly calibrated, and preferably a reference barometer is needed. The possibility to aid GPS with a barometer was well known at the beginning of the nineties, e.g. [1], [2], and [3]. However, because of the size and the cost it was not feasible to use them as a part of personal navigation. The idea of using differential barometry with GPS to seismic measurements was presented in [4]. In the publication it was noticed that in the forested areas positioning accuracy can be increased when the results were obtained by post processing the data. The use of barometer to obtain solution with less than four GPS satellites was represented in [5]. It was found that when the result is fed in to the Kalman filter, the acceptable accuracy can be achieved with less than four satellites. The work described in this paper was carried out in the project Future GNSS Applications and Techniques (FUGAT) funded by the Finnish Funding Agency for Technology and Innovation (Tekes). In [6], the accuracy of GNSS positioning with MEMS pressure sensor was studied. Reference barometer information was obtained via auxiliary TCP/IP server connection. The pressure altitude was used with the GPS pseudoranges in the least square iteration process. In the paper evaluation was carried out using both indoor and outdoor measurements. Horizontal accuracy was not increased but in vertical direction the accuracy was noticeably improved. The objective of [7] was to find working settings for data delivery between the reference and measuring barometer. Also the accuracy of GPS/barometer positioning was discussed with and without the use of reference barometer. The reference data delivery was done using internet (TCP/IP). Recent studies in the use of MEMS barometer to increase the accuracy of GNSS/INS vertical channel are presented in [8], [9] and [10]. These papers state that MEMS barometers can be integrated successfully with all kinds of sensors. In this paper, the aim is to estimate the magnitude of different disturbances in barometer measurements and their effects on height solution. This analysis contains the influences of change of speed, ventilation, car fan, and distance to the reference barometer. This research was done for the purposes of various applications, such as estimating the profile of the road in the direction of movement and guiding the user to a correct floor in a tall building. Such applications require meter- level accuracy, and thus even small disturbance sources need to be studied. The research was carried out using low-power MEMS barometers. These sensors are small and can easily be integrated with a GNSS receiver. Used pressure sensors are able to sense altitude changes of even a few centimeters. The measurements were carried out with a ‘rover’ barometer, while another barometer, ‘base’, was kept at a constant altitude and used as a reference. The results were obtained by post processing these measurements using the base barometer data and GPS/DGPS solutions. II. PRESSURE ALTITUDE As it is commonly known, pressure measured by barometers can be converted to altitude information. A popularly used term for this height is pressure altitude. However, barometric altimeters give no absolute height information without the proper knowledge of local sea level pressure. The normal air pressure on the sea level is approximately 101300 Pa (1013 hPa), but it varies locally depending on the weather. If the weather conditions are not changing, near the Earth’s surface a difference of 100 Pa in pressure equals approximately 8 meters in altitude difference [11]; as the altitude increases the pressure decreases. 148