Complex sentence: Such systems have existed for many years using mechanical clock-based systems (watchman clocks/guard tour clocks/patrol clocks) but modern systems are based on handheld data loggers and RFID sensors.
Split into simpler sentences: Such systems have existed for many years using mechanical clock-based systems (watchman clocks/guard tour clocks/patrol clocks). Modern systems are based on handheld data loggers and RFID sensors.