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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)