Monday, May 17, 2004

"Visualisation of Surveillance Coverage by Latency Mapping"

A paper by Patrick Chisan Hew in 2003 from the Defence Science and Technology Organisation in Australia.

Hew describes a technique for visualizing information about surveillance coverage by mobile sensor platforms.

There are three key concepts that explain the approach:

1. Swath - This a space region R covered by a mobile sensor over a time interval T. That is, the union of all regions covered by the sensor in the time interval T.

2. Scan History - The value {1,0} of a point or region over a history indicating the instantaneous coverage by a sensor. This reflects whether the given point or region is covered by a sensor and for how long during the history. 1 if the point or region is instantaneously covered and 0 otherwise.

3. Latency Mapping - A visualization method where the regions of space that have a short time since they were last covered give a strong color or intensity and regions with a long time since they were last covered give a weak color or intensity. In addition there is a gradient of color in between the extrema of durations.

The use of latency mapping and latency history gives one the operational ability to visualize the sensor coverage of one's network or resources. This allows the ability to make judgements or design decisions.

What is useful about this method is that the details of the sensor modality and environmental conditions are abstracted from visualization. However, this is also a drawback because the details are not made available of how difficult it is to monitor a target or how costly it is to employ a sensor. In addition, the sensors, environmental conditions, and the target characteristics need to be modeled to give good specifications for coverage capability of each sensor.

Sunday, May 16, 2004

"Surveillance Coverage of Sensor Networks under a Random Mobility Strategy"

This is a paper by Kesidis, Konstantopoulus, and Phoha about the surveillance coverage of mobile sensors in a 2d or 3d space. They assume the sensors move in a Brownian motion strategy and that the sensors have a uniform spherical surveillance coverage around each device.

They give a mathematical formulation to calculate the following quantities about their strategy:

1. Contact time between sensor nodes

2. Time-until-detection of slow moving objects

3. Effects of mobility variance and sensor density

This is a good mathematical starting point to make proofs about surveillance coverage. However, you need some knowledge about set theory and probability to understand their formulation.

There are some limitations however. This strategy uses random mobility which makes things easy to formulate, but it's not clear whether this formulation will apply to deliberative surveillance strategies. In addition, the spherical sensor coverage of mobile sensors may not be a good model of our system. One would need different shapes such as cones to account for a camera surveillance network.

Further research will give us insight into what other methodologies we can use.