1. Sensor Network-Based Countersniper SystemGyula S, Gyorgy B, Gabor P, Miklos M, Branislav K, Janos S, Akos L, Andras N, Ken F Presented by Vikram Reddy

2. Outline • Problem and solution • Architecture • Middleware • Conclusion • Limitations

3. Problem and challenges • To locate snipers in urban environments. • Work with constraints of the urban environment • Multipath effects • Poor coverage due to shading effect of buildings • Limitations of existing systems • Require direct line of sight • Rely on muzzle flash that can be suppressed • Centralized system not tolerant to sensor failure • Cost effectiveness

4. Solution • Use an ad-hoc wireless sensor network-based system • Utilize many cheap sensors for • good coverage of direct signal • tolerance to failures • Detect via acoustic signals like muzzle blasts and shockwaves

5. PinPtr - System Architecture • Ad-hoc wireless network of inexpensive sensors • Sensors can • detect muzzle blasts and acoustic shockwaves • Measure their time of arrival (TOA) • Message routing service delivers TOA to a base station • User Interface through base stations or PDAs • System field tested at the US Army McKenna MOUT (Military Operations in Urban Terrain) facility at Fort Nenning, GA

6. Custom Sensor Board and Mica2 Mote PinPtr Application Middleware • Time synchronization • Message routing • Data aggregation Operating System • Tiny OS (UC Berkeley) • Task scheduling • Radio communication • Clocks and timers • I/O • Power management Hardware • Mica Mote • Microcontroller • Multichannel receiver • 4KB RAM, 128KB Mem • Extension Interface • Acoustic Sensor Board • 3 acouastic channels • FPGA • Signal Processing • Measure TOA

7. Time synchronization • Flooding Time Synchronization Protocol (FTSP) • Synchronize local clock to clock of selected root node • Time stamping broadcasted radio message multiple times at sender and receiver nodes • Compensates for clock skew by linear regression • Time stamps made when sending/receiving individual bytes thus reducing uncertainties of encoding/decoding and interrupt handling times • Final error corrected value embedded into message before end of transmission • Estimate global time by synchronizing with nodes a level above • Less communication overhead

8. Time Synchronization • Alternate algorithm • Power conservation • Does not require continuous radio broadcast to synch time • Use data packets for time synch • Each node adds an age = (prev age) + (time pkt resent – time pkt received) • Time of event = T(recv) - age

9. Routing • Gradient-Based best effort converge-cast protocol • Assign a root node • Route data from all nodes to the root node • Each node rebroadcasts data packets upto 3 times • Data packets reach the root through multiple paths • Fast and robust • Does not guarantee message delivery • Has significant message overhead

10. Sensor Localization • Localization of a node refers to the problem of identifying its spatial co-ordinates in some co-ordinate system. • Passive acoustic sensor localization • Use external acoustic sources • In sniper scenario estimate sensor location through shots rather than sniper location with sensors • Produce shots at known locations and known times

11. Other Services • Signal Detection - Angle of Arrival and Time Difference of Arrival – Not practical because exact orientation of the board must be known. - Uses Time of Arrival and data fusion. • Sensor Data Fusion

12. Performance Results • Experimental setup • 56 motes • 20 different known shooter positions were used • 171 shots were fired

13. Performance Results • Shooter localization error • Elevation info eliminated for 2D • 3D errors are more as sensors were mostly positioned on the ground • Error Sources • Time Synchronization errors • Sensor localization errors

14. Performance Results • Sensor Density • Effects signal detection • Increases shooter localization error

15. Future Work and Conclusion • Future Work • Provide power management • Support multiple shots with sensor fusion algorithm • Use post-facto time synchronization to conserve power • Use system in other Concepts of Operations • Reconnaissance missions • Protect convoy routes • Work on sensor self-localization techniques • Conclusion • PinPtr provides a counter-sniper system • Provides efficient algorithms for time synchronization and shooter/sensor localization • Good experiment to reassure actual deployment

16. Limitations/Discussion • Since PinPtr does not employ shockwave signals and relies on muzzle blast it may not work when silencers are used • Deployment of sensors in an actual urban environment is not trivial • Does not provide for power conservation hence battery life can be an issue since these systems typically need to be available always • Cannot deal with multiple shots fired by multiple snipers • No self-localization performed hence cannot dynamically configure with change in number of sensors