Sensor-net system and sensor node

A sensor node connected through a wireless communication to a gateway node being in operation all the time, for intermittently operating, comprising:

• a processing unit, a storage unit, an RF transceiver unit, and a real time clock,
• wherein the processing unit issues a time request command on a regular basis or at the timing of connecting to the gateway node, and invokes its internal timer at the time when receiving a reply to the time request command from the gateway node;
• the processing unit, after issuing the time request command, when transmitting a next command transmission request to the gateway node, receives a reference time obtained by the gateway node at the time when receiving the time request command from the sensor node;
• the sensor node stops the timer at the time when receiving the reference time, calculates the active time of the timer from the difference between the timer stopping time and the timer activated time, and then sets the time of the real time clock by adding the reference time received from the gateway node to the timer activated time.