Researchers Studying PASS Alarm Interference

Tests developed by researchers at the National Institute of Standards and Technology may make Personal Alert Safety Systems more useful and reliable. PASS devices are worn by firefighters; the systems detect motion and activate an alarm if the wearer has been motionless for too long. But there is interference between PASS systems with wireless alarm capability and radio-frequency identification (RFID) systems, so NIST developed test methods to evaluate how well the PASS technology works under realistic conditions. The methods can test interference in other wireless devices, such as radios, local area networks, and urban search and rescue robots, according to the agency.
 
PASS devices sense movement and activate an alarm if a firefighter remains motionless for too long. Newer PASS systems also have a wireless link connecting incident command base stations and portable units, allowing emergency recall signals to be sent to firefighters or “firefighter down” alarms to be sent to the base. Because firefighters also may carry RFID tags for location tracking, or may be in warehouses or other buildings using RFID inventory systems, there is potential for significant interference.
 
“Every wireless device will fail given strong enough interference,” says NIST project leader Kate Remley. “The question is the level at which the device fails. Our goal is to develop lab-based test methods to quantify the level of interference at which PASS units fail so we can help ensure they operate reliably.” The NIST researchers shared their findings at the 2011 IEEE Electromagnetic Compatibility Symposium in Long Beach, Calif., on Aug. 17. They measured the interference between “frequency hopping” PASS and RFID systems operating in similar frequency bands and found that, when signals are weak due to environmental or other conditions, a portable PASS unit’s reception of an alarm from its base station can be delayed or fail, even without interference, and becomes more likely to fail in the presence of moderate RFID interference. Strong interference caused longer, variable delays that sometimes lasted longer than one minute, which the researchers defined as signal failure. They also found that an RFID system can be less reliable when the PASS unit is nearby.
 
The NIST tests involved measuring the total output power of each system in a test chamber and then isolating the systems in different labs for the interference tests. The portable PASS device and RFID tag and reader were placed in a test chamber, while the PASS base station was in a separate room. Researchers evaluated performance at various levels of signal strength and interference.
 
NIST is working with the National Fire Protection Association, which will consider adopting the NIST tests as part of revised PASS performance standards. An NFPA technical committee on electronic safety equipment will soon consider the wording of a draft standard, and after a public comment period, the standards could be approved by 2013. At that point, manufacturers would need to show that their PASS devices pass the tests.
 
The research is supported by the Department of Homeland Security.

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