You can triangulate 802.11 devices in a similar manner to GPS. You need at least 3 access points to do it though. Signal attenuation due to distance is indestinguishable from signal attenuation due to physical barriers. To do it, you need fairly complicated software algorithms, and need to give them an awareness of the physical location - what sort of walls are where etc.
If you are looking to locate a device, another alternative is to use directional antenna and an appropriate search pattern to determine where the device is.
This generally falls in the too hard and not worth it basket for home use.
iainw: You can triangulate 802.11 devices in a similar manner to GPS. You need at least 3 access points to do it though. Signal attenuation due to distance is indestinguishable from signal attenuation due to physical barriers. To do it, you need fairly complicated software algorithms, and need to give them an awareness of the physical location - what sort of walls are where etc.
If you are looking to locate a device, another alternative is to use directional antenna and an appropriate search pattern to determine where the device is.
This generally falls in the too hard and not worth it basket for home use.
Ianw, don't think this would work for an 802.11 device. Whilst you could triangulate WIFI using a directive antenna, GPS does not use that method, it uses a time source. I don't think that a WIFI router generates the accurate time signal that would be needed to use a method similar to GPS.
Ianw, don't think this would work for an 802.11 device. Whilst you could triangulate WIFI using a directive antenna, GPS does not use that method, it uses a time source. I don't think that a WIFI router generates the accurate time signal that would be needed to use a method similar to GPS.
Cheers Mike
Mike,
You are correct in that the exact algorithm used is slightly different, and accuracy is not quite as high, however it can, and is regularly done.
802.11 tracking uses one of two methods, TDA (Time Difference on Arrival), and RSSI (Recieved Signal Strength Indication). It requires a lot more work to set up than GPS, as you first have to have a site with appropriately dense AP coverage. This means that everywhere you want to track a device must be able to see at least 3 AP's spaced out in two dimensions relative to the device (ie not all in a straight line). You then need to input a map of the site, and provide it with information about the RF barriers. This is typically done during the site survey process, as you use the same information for determining AP location and predicted RF coverage.
You then need to tell the application where on this map the AP's are located, and it can then calculate device location based on the RSSI or TDA from three different AP's.
Accuracy on wifi alone is around 10m. The only system using wifi alone (AFAIK) is the RTLS engine built in to Motorola's RFS6000 controller. Commercially dedicated applications such as WhereNet, Ekahau, and Aeroscout then use UHF RFID excitors to provide greater accuracy in proximity to key locations - usually the borders of differing zones. Using these excitors, you can recieve greater accuracy around entry and exit points.
Applications include tracking children at Legoland, tracking assets and healthcare professionals within a hospital, tracking security personal within a campus...
Thanks iainw - good feedback. I can see how RSSI can be implemented, but I did not think that TDA would be viable (like it is in say GSM etc) because of the lack of an accurate time source. But your suggesting that some Wireless devices do include an accurate time base from which the TDA can be generated/determined.
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See below for Aeroscout's comments on TDA (they call it TDOA)
Location AlgorithmsAeroScout’s Wi-Fi RFID technology uniquely utilizes both of the two most popular RTLS algorithms for determining location
AeroScout is the only vendor that calculates both RSSI and TDOA-based location, coming from the same tag signals. Unlike vendors who promote just one of these algorithms, AeroScout recognizes that each fills a specific need, and offers tags that can smoothly transition between the two. This means that AeroScout tags can roam seamlessly between environments and continue to be located, without switching frequencies or suffering a loss of accuracy.
Received Signal Strength Indication (RSSI): The RSSI algorithm compares the strength of Wi-Fi RFID signals to an expectation of that strength, in order to determine accurate positioning. RSSI is ideal for indoor asset tracking, where receivers (access points) are often already installed for data and/or voice services. RSSI is typically used in RTLS systems that utilize 2.4 GHz or 915 MHz.
Time Difference of Arrival (TDOA): The TDOA algorithm measures the time of arrival of a signal from multiple points, and compares those times of arrival to determine the accurate source of the signal. Because time of arrival is related in a linear manner with distance, the TDOA algorithm is ideal for asset tracking in wide open spaces such as outdoors or open indoor areas (hangars, manufacturing facilities, etc.) TDOA is typically used in RTLS systems that utilize 2.4 GHz or UWB.
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#30617 15-Feb-2009 15:40
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