# 3 Time Difference of Arrival method Time Difference

3 Range-Based Localization Scheme:

Range-Based Localization 1,2 scheme is used to calculate the distance between an anchor
node and unknown node using parameters such as angle, time  and signal strength. In this method the
locality of sensor node estimation is accurate, but the hardware implementation
cost is high.

3.1
Angle of Arrival:

Angle
of Arrival(AoA) Localization Scheme 8 is based on measuring the angels of the
signal received from anchors. Antennas are used to compute the angle of arrival
of each signal at  the receiver. This
scheme is an inaccuracy, and this type of measurement is inefficient due to
additional hardware cost and size of nodes. This method is not advisable for
all kinds of applications because of more cost and less accuracy. In the Fig.
2. Shows angle of arrival.

3.2
Time of Arrival Localization

Time
of Arrival(ToA) Localization Scheme 9,10 is used to determine the distance
between two sensor nodes. In this method the distance between anchor node and
unknown node is determined using the formula D=SP*T; where D is the
distance between two nodes and SP is propagation speed of signal.
Fig. 3. Indicates ToA scheme.

.3
Time Difference of Arrival method

Time Difference of Arrival method 9,10 determines the time difference
between two signals. In this method the beacon nodes should be synchronized,
the clock time must be same. In Fig. 4. T1 is the time taken to transmit
signal from anchor2 to unknown node, T2 is the time taken to
transmit signal from anchor1 to unknown node and T3 is the time
taken to transmit signal from anchor3 to unknown node.Time difference between
Anchor1  and Anchor2 is
calculated as ToDA21 = T2-T1 ; Time difference
between Anchor2  and Anchor3
is calculated as ToDA31 = T3-T1.

Received Signal Strength Indicator

Signal Strength Indicator (RSSI) localization scheme is used to determine
location of unknown node  with help of
signal strenght received from anchor.If received singal strenght is high then
the  anchor node is near to unknown node.
If the recived signal strenght is low which indicates anchor node is
farway.RSSI is a range-based localization scheme in which the accuracy is good
but cost is high because additional hardware is required to measure the signal
send by anchor node and receiver side. In Fig. 5. three anchor nodes are
present at (x1,y1), (x2,y2) and (x3,y3)  and unknow node is located at (x,y). The
received signal strenght is  r1,r2
and r3.

4 Range-Free Localization Scheme:

Range-free
localization 3,4,5  method is used because of hardware equipment
confinements and power requirements and this method produces locality
information with economical manner.

4.1 Centroid Localization Scheme:

Centroid
Localization Scheme is a range-free localization scheme in which does not
requires any additional hardware and exchanging information cost when compared
with range-based localization scheme. Localization  algorithm is simple and low cost the sensor
node localizes itself by calculating the centroid of position all the nearby
beacon nodes4, 6.

The algorithm works as follows: Deploy  all sensor nodes in the required wireless
sensor field ,anchor nodes send is locality information periodically to all
other blind nodes, which contains the anchor node id and its 2D locality
information {ID,Loctioninfo(A(x,y))} which can be obtained with the help of GPS
device. All blind nodes collects locality information about the nearby beacon
mode.

Now all the
blind nodes {B(x1,y1),B(x2,y2),
to calculate their geographical coordinates with  the help of anchor nodes {A(x1,y1), A(x2,y2),…  ,A(xn,yn)} using
centroid formula.In Fig. 6. A(x1,y1),
A(x2,y2) and A(x3,y3) represents anchor
nodes and location of  the blind node b(x,y)
can be estimated using the centroid localization scheme. The equation (1) is
used to calculate the node b(x,y) position.

x=

; y=

———————————–(1)

Fig. 6. Centroid Localization Scheme