Participatory sensing

Another form of mobility in sensor networks is participatory sensing[2]. In participatory sensing, users make up the sensor nodes such that each user of the sensor network carries a mobile sensing device and gathers data which is shared among all users. Even though this approach is not applicable to all sectors, but it does have a number of advantages which can prove very useful for some applications. One such advantage is the mobility of nodes without any extra costs to the mobile node. Another advantage is that the mobile nodes can be recharged by their owners as part of another device. For example a PDA could run an application to sense noise pollution using its on-board microphone and when the user recharges the PDA, the sensor node is being recharged in fact. Hence the excessively strict and careful consideration of power consumption is eliminated.

Participatory sensing allows people carrying everyday mobile devices to act as sensor nodes and form a sensor network with other such devices. A large number of mobile phones, PDAs, laptops and cars are equipped with sensors and GPS receivers, which are potential candidate devices for participatory sensor nodes. The network of such devices can interact like a conventional sensor network to collect and share local data or be part of a larger scale network, for example the sensor network could be queried through internet.

Another example where participatory sensing could be useful would be the measurement of air pollution using sensing devices mounted on vehicles. Since all vehicles have power supplies and a majority also have a GPS unit, the sensing device can be simplified to a great extent. (Some cars even have sensors which sense air pollution.) Also considering vehicles cover a big area and a large number of vehicles are constantly on the move in different locations - particularly in city areas - a huge amount of up-to-date data can be gathered which closely represents the physical world.

Participatory sensor networks have the potential to allow users to request specific data from specific locations at specified date and times which will be provided by participants who meet those requirements. This type of requests does not have to be limited to human-mobile devices but can also include a large set of already deployed and managed static sensor networks which allow users to freely query them. So people can form their own participatory sensor networks or be part of a hybrid sensor network which is made up of human-mobile sensor nodes and static sensor nodes which allows one to gather a richer set of results. An optimistic vision is to have so many sensor nodes that the sensor network would become a real-time always-on sensor network which would represent the physical world very closely. This network could then be queried for various phenomena in one's locality and return rich sets of results with minimum delay.

Various aspects of participation in such networks are yet to be explored particularly in the area of privacy concerns. E.g. if a mobile device can be associated with a person, it can be queried for sensing particular physical phenomena and assuming the mobile device provides geo-coded data a person can easily be tracked and monitored.

Another aspect which has not been fully explored is how easily participatory sensing can be implemented without disturbing the flow of everyday life for the participants using their current mobile devices. For example if participants' mobile phone would send data on regular basis through its WiFi interface, and as a result draining the battery quickly, users would find it very inconvenient and a disruption to their life. Another example would be where the users were charged for packets of data that was sent using the GPRS feature of their mobile phone handsets. This is only an example attempting to highlight the point that users' priorities must be respected. So a solution to this arbitrary problem would be to use the mobile phone handset's wireless connectivity facility to send data only upon acquiring a free internet connection so no extra charge would incur and also only attempt this when the connection is obtained through the Bluetooth interface of the device, in which case the battery consumption would be much lower compared with a WiFi connection.

The vision for Participatory sensing applications is to allow users to go on about their everyday life while performing data collection tasks without any extra effort. Also with necessary means available to users, they should be able to deploy their own participatory sensing applications and encourage other people to join their network and create a community based sensor network. These networks could then be queried for data, perhaps through internet.