Sensor networks are prone to failures and are hard to debug. This is 
particularely true for failures caused by incorrect interaction of multiple 
nodes. We propose a mechanism called passive distributed assertions (PDA) that 
allows developers to detect such failures and provides hints on possible 
causes. PDA allow a programmer to formulate assertions over distributed node 
states using a simple declarative language, causing the sensor network to emit 
information that can be passively collected (e.g., using packet sniffing) and 
evaluated to verify that assertions hold. This passive approach allows us to 
minimize the interference between the application and assertion verification. 
Further, our system provides mechanisms to deal with inaccurate traces that 
result from message loss and synchronization inaccuracies. We implement PDA on 
the BTnode platform and evaluate it using an extensive case study.