3. Implementation of dynamic RSSI during loading process
Since the pallet accompanying sensor nodes make a simple straightforward movement during the loading process
before transportation, the dynamic spatial RSSI filter is suitable to be applied for this application. In the
implementation test we added a simple spatial RSSI filter in the RSSI-Demo protocol to collect the unfiltered and
filtered RSSI values from the anchor nodes simultaneously. Using the both RSSI readings the position of the moving
sensor node is calculated and compared.
3.1. Test setting
The practical test is carried out in an empty TEU (Twenty-foot-Equivalent-Unit, 6058u 2488u 2591mm3
) truck
(Fig 2a). The telosb sensor nodes by Company Crossbow are used for this test. Twenty anchor nodes are attached to
the wall of the truck in upper and lower layers. In this test only the lower layer is used, i.e. the distance measurement
and localization are two-dimensional. The freight with the accompanying sensor node is pushed straightforward into
the container along the x-axe (Fig 2b). The sensor node receives the RSSI-value from ten anchor nodes. The
frequency of the packet sending is 16 packets/S.
3.2. Test results
The dynamic RSSI filter calculates the mean value of every 20 RSSI readings. The direct RSSI records and the
filtered ones from a single anchor node are illustrated in Fig. 3a. The unfiltered RSSI values show that the receiver
node collects during the movement always the signal with the extreme fluctuating strength. The spatial filter
smoothes the outliers and the filtered RSSI show if the receiver moves to or departs from an anchor. With more
anchors in a row (Fig. 3b) the movement of the receiver is able to be estimated. Figure 3b shows that the moving
sensor has past the anchor sensor 4. Using these RSSI curves simple cell based localization can also be achieved.
The numerical position of the moving sensor node was calculated using Multilateration with the RSSI from ten
anchor nodes. The localization results in X- direction with filtered and unfiltered RSSI are compared to each other in
Fig. 4. The maximum localization error is about 0.8 m. Compared to the scale of the container ( 6058u 2488 mm2
)
and the Euro pallets (1200 u 800 mm2
) the accuracy reaches the requirement of the application.