Time of arrival(TOA) Prepared By Sushmita Pal Roll No Dept.-CSE,4 th year

Location-awareness is crucial and becoming increasingly important to many applications in wireless sensor networks. An efficient principled approach to localize a mobile sensor is using time of arrival (TOA) and angle of arrival (AOA) information employing multiple seeds in the line-of-sight scenario.

Robust and distributed internal algorithms of lower cost are required for sensor positioning problems due to the low power of wireless sensor network. Common ranging techniques are i) Receiver Signal Strength Indicator (RSSI) techniques are used to translate signal strength into distance. ii) Time Of Arrival (TOA) iii) Time Difference Of Arrival (TDOA) iv) Angle Of Arrival (AOA)

ii) Time Of Arrival (TOA) method tries to estimate distances between two nodes using time based measures. iii) Time Difference Of Arrival (TDOA) is a method for determining the distance between a mobile station and nearby synchronized base station. iv) Angle Of Arrival (AOA) method allows each sensor to evaluate the relative angles between received radio signals.

TOA is a conventional scheme for which at least three TOA measurements must be obtained from three line-of-sight (LOS) seeds (i.e., reference nodes). In order to estimate the position of a moving target sensor in most environments, incorporating angle information may help tackle the localization problem in addition to distance measurements.

Thus, an AOA-aided TOA localization scheme may be employed to make the position estimation possible. In general, the localization problem can be solved by the joint AOA/TOA positioning scheme using a single seed.

Mobile location with TOA information at a single base station is first proposed in After that Deng and Fan introduced an TOA location algorithm with multiple base stations.

In this approach, the speed of the mobile station is assumed to be very low and the relative movement between the base station and the mobile station is not considered.

So the definition of TOA is as follows Definition: Time of arrival (TOA or ToA), sometimes called time of flight (ToF), is the travel time of a radio signal from a single transmitter to a remote single receiver.

By the relation between light speed in vacuum and the carrier frequency of a signal the time is a measure for the distance between transmitter and receiver.

Louvros and Kotsopoulos introduced two hybrid TOA/AOA techniques ---- i) Enhanced Time of Arrivals (E-TOA) ii) Enhanced Angle of Arrival (E-AOA) in order to optimize the location positioning estimations.

Time of arrival (TOA) Example: GPS Uses a satellite constellation of at least 24 satellites with atomic clocks Satellites broadcast precise time Estimate distance to satellite using signal TOA Trilateration

Time of arrival # Basic idea: O Measure time it takes for a signal to travel from sender to receiver O Multiply by signal propagation speed # Need sender and receiver to be synchronized, and exact time of transmission known # Exact transmission time hard to determine # Time Difference of Arrival (TDOA) O Measure TDOA at two receivers O Can obtain difference in distances between the two receivers and the sender # Signal speed not necessarily constant O Local beacons and measurements to estimate signal speed # Apparently distance measurement to within centimeters achievable

Difference between TDOA and TOA Compared to the TDOA technique, time of arrival uses the absolute time of arrival at a certain base station rather than the measured time difference between departing from one and arriving at the other station. The distance can be directly calculated from the time of arrival as signals travel with a known velocity.

Time of arrival data from two base stations will narrow a position to a position circle; data from a third base station is required to resolve the precise position to a single point. Many radiolocation systems, including GPS, use ToA.

Ways of synchronization As with TDOA, synchronization of the network base station with the locating reference stations is important. This synchronization can be done in different ways.

The Different Ways are as follows:  With exact synchronous clock on both sides. Inaccuracy in the clock synchronization translates directly to an imprecise location.  With two signals which have different frequency, hence different speed. Sound ranging to a lightning strike works this way (speed of light and sound velocity).

 Via measurement to or triggering from a common reference point.  Without direct synchronization, but with compensation of clock phase differences.

USRP2: Universal Software Radio Peripheral 2 Localization system setup with three USRP2 sensors and a transmitter

In order to properly localize with ToA, there must be at least three sensors. When the distances from three different sensors are known, the location can be found at the intersection of the three circles created around each sensor with the radius being the distance calculated.

Time of Arrival

Drawbacks: i)Imperfect measurements create a region of uncertainty between each of the sensors in which the transmitter might be contained.

ii) Since ToA relies on the difference between the time of arrival and time of departure, all receivers and transmitters must be synchronized so there is no error in the difference due to clock offsets. This may prove to be a problem, especially considering the high speed at which the signals travel.

iii) Also, as with any time sensitive systems, there is also the possibility of significant hardware delays that must be accounted for to calculate the correct distances. These are the significant drawbacks of this scheme.

Calculating Distance using Timestamps Every ToA/TDoA approach to wireless localization relies on determining the time of flight of a packet. Three different approaches to this problem have been identified.

1.Synchronized BS and MS, one-packet ToA This approach uses a single packet sent from the BS to the MS containing the time it was transmitted, relying on BS-MS synchronization to eliminate clock-related drift (citation needed). Since the receiving MS knows when the packet arrived and that it is synchronized with the BS, the distance travelled can be calculated using the following formula: d toa = C * t toa Distance calculation from time difference

d toa = C * t toa Distance calculation from time difference This simple equation is used to calculate distance based on the time it took a signal to go from the transmitter to the receiver when d toa is the distance between the transmitters and receiver, c is the speed of light (approximately 3*106 m/s), and t toa is the time difference

Summary: Time of arrival (ToA), uses the travel time from the transmitter to the receiver, or time- of-flight (ToF), to measure the distance between the two. In order to properly localize with ToA, there must be at least three sensors. When the distances from three different sensors are known, the location can be found at the intersection of the three circles created around each sensor with the radius being the distance calculated.

Bibliography Google.com and pdfs of different researches