Received signal strength indication (RSSI) location principle uses received signal strength to determine the location of the transmitting device.
A wireless device sends a signal out and that signal is received by a gateway or access point. Multiple access points will pick up the signal and from that a received signal strength is determined by each access point.
The further away the access point to the transmitting device the weaker the signal. In comparison, the closer the transmitting device is to an access point the stronger the received signal strength.
Location is determined by receiving the signal from multiple access points and taking the different measurements to make an approximation of location by triangulation or trilateration, as it is more technically known.
The approximate location can then be positioned on a map. For example, an asset or a worker wearing a Wi-Fi RTLS tag can be accurately located using this method.
All radio technology, including others like BLE, suffer from the same inherent issue – signal strength is not linear to distance. This is especially true after fifteen to twenty metres.
At this range the signal strength drop off becomes exponential; a small signal strength change can result in a ten- or twenty-metre shift in location accuracy.
The challenge with RSSI is the amount of infrastructure you need to maintain sub-ten-metre accuracy.
Indoor or built up processing areas often have an advantage over large outdoor areas when using RSSI location. For example, walls between different rooms help provide a substantial difference in signal strength. The signal strength received by the access point in the room where the transmitting device is located will be much stronger in comparison to the other rooms.
The same cannot be said for outdoor environments. To maintain a high level of accuracy using Wi-Fi triangulation requires huge investment.
Sub-ten-metre location accuracy would require an access point every twenty to twenty-five metres. When you consider the size of a facility like a tank farm, a refinery, or large petrochemical plant, then this would result in potentially thousands of access points.
The cost of installing that number of access points is huge, especially as many areas are also hazardous environments and would require certified enclosures. It would also require long cable runs and the associated installation costs that come with that work.
RSSI location is a perfectly good solution in areas where you would typically see dense Wi-Fi, such as dense metal infrastructure on processing plants. These areas typically need an access point every twenty metres to ensure good connectivity for Wi-Fi devices such as tablets.
A hybrid solution allows you to utilise multiple location technology. In large outdoor environments, GPS can be used to maintain location accuracy. In these scenarios, Wi-Fi is only used for GPS location data backhaul and therefore there is no need for access points every twenty metres. This approach significantly reduces the level of Wi-Fi infrastructure required for accurately locating personnel or assets in outdoor environments.
For site-wide Industrial Internet of Things (IoT) functionality, you should consider the connectivity required for your RTLS requirements alongside the other uses of wireless networking, such as other sensor backhaul, data connectivity, ePermitting, video collaboration or applications like VOIP.
To achieve sub-ten-metre location accuracy across an entire site using RSSI would require a dense wireless network. RSSI is well suited to indoor or built up environments where you typically see dense Wi-Fi infrastructure for other connectivity reasons. However, to maintain the same level of accuracy in large outdoor spaces typically found on processing plants like refineries and petrochemical works, would require significant capital expenditure. Hybrid technology provides a more cost-effective method that achieves a high degree of accuracy regardless where you are on site.
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