RF RTLS
RTLS solutions, based on IEEE 802.11 (Wi-Fi) - the tag-to-access point air interface, have location accuracy and resolution ~20 feet that depends on the AP installation density and the supporting Wi-Fi infrastructure. There are three relatively straightforward location-determining techniques in use:
Probably RSSI
method is more susceptible to environmental conditions than TDOA.
First two need an experimental confirmation of efficient transponder placement for each object type. Batteries last ~ 5 years for a repeat rate of 5 min.
CHALLENGES
& ISSUES
·
The
multipath wave propagation that is common in building environments greatly
lowering accuracy of all three methods
·
A
simplified Time of Arrival technique requires the Transponder and the Receiver’s
clocks synchronization
·
For
Time Difference of Arrival (aka Three Dimensional Hyperbolic Positioning) a multipath
propagation, noise and EMI/RFI are resulting in inaccurate intersections of the
hyperbolas
·
The
RSS requires installation of many 802.11 WLAN access points (AP)
·
TDOA
and RSS systems need often calibration and their position estimation is not
quite reliable
·
Angle
of Arrival method requires a complex set of antenna arrays with locating accuracy
proportional to number of antenna elements
·
The
calculated location may appear entirely faulty
·
Systems
can indicate incorrect location of the object real physical presence or
·
show
an unreal movement
Ultra-Wideband Based RTLS
UWB RTLS
occupy very wide spectrum from 3 to 10 GHz. They use a very short impulse to
transmit data, so most reflected parasitic RF signals, having a longer pathway,
arrive after the main signal. More often than not, this capability allows for
overcoming a multipath problem. However, high multipath resilience comes
together with low noise immunity because a receiver is sensitive to any
interference in wide band.
UWB RTLS systems
have high, ~ 1-foot location accuracy, high read rates and detection range of
up to 100 feet. Transponders are practically insensitive to their placement and
can have a battery life up to 8 years at minimum repeat rate 1 per second. UWB
RTLS are capable of localizing of more than 1000 transponders in 1 second.
CHALLENGES
& ISSUES
·
Worldwide
UWB RTLS licensing is not available yet
·
UWB
Technology for commercial applications is relatively new and immature
Sound (Ultrasound) Based RTLS
Ultrasound
RTLS is usually a Radio–Ultrasonic Hybrid using time-of-flight (TOF) method. It
utilizes a significantly lower sound propagation speed than RF wave and has a
superior to a radio-only system accuracy performance.
CAPABILITY
·
100%
room-level or sub-room level location accuracy
·
High
immunity to electromagnetic noise and interferences
CHALLENGES
& ISSUES
·
Method
is typically applicable only to short-range line-of-sight systems
·
TOF
systems depends on ultrasound speed that vitiates with air temperature and
humidity
Optical (Infrared) Based RTLS
Infrared (IR) based positioning systems use short-range transmissions of modulated IR light to send identity data from a tagged object to a fixed transceiver in a particular known location. It is relatively easy to focus light beam and direct it to assigned position. A transceiver linked to Wi-Fi detects a tag presence. This combination of IR and RFID or Wi-Fi helps to overcome limits associated with transmission range.
Battery life expectancy for 3-10 seconds flash rate is 1 year for badges and 2 years for tags.
CHALLENGES
& ISSUES
·
IR
System has significant costs of installation
·
100% room-level or sub-room level location accuracy
·
High immunity to electromagnetic noise and interferences
· IR is safe and accepted worldwide