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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008 
According to Li et al. (2006) WLAN based positioning is easily 
implemented in indoor environments, as its associated consumer 
hardware is the most readily available of all signal strength- 
based methods. It is also the most accurate method, as the signal 
strength (SS) displays high spatial variance, and WLAN 
chipsets are relatively easily programmed for this purpose. 
WLAN operates in the 2.4 GHz band, which is the only 
accepted ISM (Industrial, Scientific and Medical) band 
available worldwide license free. There are essentially two 
approaches to using WLAN for positioning: one uses a signal 
propagation model and information about the geometry of the 
building to convert SS to a distance measurement from the 
access point, followed by trilatération from multiple access 
points to provide the final position fixes. The second method of 
WLAN positioning is known as location fingerprinting. The key 
idea behind this approach is mapping of the location-dependent 
parameters of measured radio signals within the area of interest 
that is the received signal strength indicator (RSSI) at the access 
points. According to ibid, location fingerprinting consists of two 
phases, (1) training and (2) positioning. The objective of the 
training phase is to build a fingerprint database. The generation 
of the database starts with a selection of reference points (RPs) 
followed by measuring SS at these locations, and recording it in 
the database. With a sufficient number of reference points 
stored together with their SS characteristics, a mobile user can 
position himself/herself by comparing the measured SS with the 
reference data in the database using some search/matching 
algorithm. Naturally, the accuracy of the fingerprinting method 
increases with the increasing number of RPs. 
Technique/sensor 
Navigation 
information 
Typical accuracy 
Selected characteristics 
GPS/GNSS 
• Position coordinates 
• Velocity 
X,Y,Z 
V x , v y 
V z 
~10m 
(1-3 m DGPS) 
-0.05 m/s 
~0.2 m/s 
• Line-of-sight system 
• Results in a global reference system 
Pseudolites 
X,Y,Z 
V V V 
v X5 v y, v z 
Comparable to 
GPS 
• Line-of-sight system 
• Operate at GPS and non-GPS frequencies 
WLAN 
• Signal strength-based 
method 
• Fingerprinting method 
X,Y,Z 
X,Y,Z 
2-6 m 
1-3 m 
• Indoor positioning in a local system 
• Signal attenuation due to distance, penetration through 
walls and floors, and multipath 
• Interference from other users of 2.4GHz frequency band 
UWB 
X,Y,Z 
dm-level accuracy 
theoretically 
achievable at 10- 
20 m range ++ 
• Resistant to multipath fading 
• Strong signal penetration 
• Possible interference with GPS 
• Positioning approach similar to WLAN 
Mobile phone positioning 
X, Y 
50-300 m 
• Cell-ID positioning approach (lower accuracy range) 
• Time of arrival or difference in time of arrival used to 
derive range or range difference 
Dead reckoning system 
X, Y 
Z 
Heading cp 
20-50 m/1 km 
3 m 
1° 
• Relative positioning 
• Sensors require calibration 
Direction of motion 
• Digital 
compass/magnetometer 
Heading cp 
O 
i 
o 
o 
• Long term accuracy stability 
• Subject to magnetic disturbances 
• Sensitive to tilt 
• Gyroscope 
• Short term accuracy stability 
• Not subject to external disturbances 
• Subject to drifts 
• Should be calibrated when GPS is available 
Accelerometer 
‘han? &radi 
<0.03 m/s 2 
• Subject to drifts 
• Should be calibrated when GPS is available 
Digital barometer 
z 
1-3 m 
• Requires calibration by a given initial height to provide 
heights with respect to, for example, WGS84 ellipsoid 
Optical systems 
• Image based 
• Optical sensor network 
• Laser 
x, Y, Z 
X, Y (Z optional) 
X,Y, Z 
few meters 
few meters 
cm to dm 
• Line-of-sight system 
• Network approach is geometry-depended 
• Image overlap required for 3D 
• Local or global reference system 
Table 1. Typical sensors used in personal navigation: observables and their characteristics (Retscher and Thienelt, 2004; modified 
and extended); where X,Y,Z are the 3D coordinates, v x , v y , v z are the 3D velocities, cp is the direction of motion (heading) in the 
horizontal plane XY, a tan is the tangential acceleration and a,. ad is the radial acceleration in the horizontal plane XY, a? is the vertical 
acceleration. 
t+ See, Ni et al. (2007)