Posted by Oleg
Kamenetsky

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Using Wi-Fi signals is a great way to accurately position connected devices throughout the world in a battery-efficient way. If done correctly, this technology can provide precise location for a wide range of connected devices, including the smallest and most resource-constrained ones. Given the proliferation of Wi-Fi access points (APs) and advances in the technology in recent years, there have also been some key challenges that must be taken into consideration before using Wi-Fi for location. 

Mobile Access Points in Phones, Cars, Trains, Planes

Skyhook's hybrid positioning system relies largely on Wi-Fi access points that it does not own or control, that can (and do) move around. Most of the data Skyhook receives is incredibly noisy, because Wi-Fi signals often don't behave as expected once put into use. APs have become very mobile; cars, trains and airplanes all have routers to provide connectivity for users. In addition, when residences or businesses relocate, they often bring their routers with them.
 
This high level of mobility poses a challenge when trying to use routers to ascertain location. How can you determine if the AP you see is stationary? Can an AP still be used to determine location if its location may have changed? Skyhook takes a disciplined approach to these types of cases, identifying all APs that move, using data intelligently to augment its Wi-Fi positioning system. While these APs don’t provide data that can definitively determine a location, they do provide other vital information that can be used in a variety of ways.

large_devices_skyhook_precision_location

Access Points in a Wide Area

Some APs, while static, can be seen over a much wider range than average. For instance, APs in tall buildings can often be seen hundreds of meters away. Since signal strength is our main source of distance qualification, it is important that APs in unique locations such as tall buildings are properly identified and treated differently, to maintain accuracy.
 

MAC Address Collapsing

A media access control address, better known as a MAC address, is (generally) a unique identifier for a Wi-Fi access point (AP). These addresses are broadcasted by the AP to help identify it out in the wild, and therefore, can be used to accurately position all types of devices. However, not all Wi-Fi APs provide the unambiguous data that we need to accurately locate devices.
 
With major advances in technology, many APs include multiple MAC addresses for different frequencies and radio signals. While this is not an issue in and of itself, it can make a Wi-Fi scan appear to have much more data than it actually contains. All the MAC addresses point to one location and do not help in positioning a device via triangulation. Improper handling of this scenario can cause heavy weighting towards one AP, while discounting other APs seen in the scan.
 
To overcome this challenge, Skyhook employs a process called MAC address collapsing. This process is used to filter out MAC addresses that are associated with only one router. These multiple MAC addresses are generally one ‘nibble’ different from the another, for example: 1A:2B:3C:4D:5E vs 1A:2B:3C:4D:5F. To avoid the effects of over-weighting one router, Skyhook eliminates all MAC addresses that are one 'nibble' apart, keeping only one address to serve as the identifier for the AP. Other MAC addresses that are more than one nibble apart will remain.  
 

Locally Administered MAC Addresses

Occasionally, data can point to 2 different locations. Cell data can point to one set of latitude and longitude coordinates, while Wi-Fi data can point to another location. In some instances, Wi-Fi data alone can point to two distinct locations. Determining the correct location in these instances can be extremely complicated. Sometimes location can be resolved based on confidence intervals of the MAC addresses in the area, however, this method does not always resolve to a precise location. In addition, mobile country code (MCC) data can help pinpoint to a specific region or country, minimizing larger errors. 
 

Quarantine

Skyhook’s network is intelligent, self-healing and self-learning, keeping bad location data from infecting its system through a quarantine process. Location readings are never taken at face value; every inaccuracy, outlier, and anomaly is questioned in an effort to deliver the most reliable location in real-time.
 

Skyhook Quarantines APs That:

  • Display any of the anomalies described above and are deemed to provide unreliable information
  • APs like mobile Wi-Fi hotspots that move frequently
  • APs that cover large areas like college campuses, which often hinder the ability to precisely position devices
  • Non-unique MAC addresses
  • Inaccurate OUIs
These APs are unhelpful in positioning devices and are therefore isolated and removed from the data set, allowing Skyhook to provide the most accurate and precise location services.
 
Additionally, Skyhook addresses questions about issues like mobile APs and “what happens when somebody takes their Wi-Fi access point and moves?” To account for this issue, Skyhook has developed a number of methods, algorithms, and heuristics for dealing with ‘moved’, ‘misbehaving’ or ‘pathological’ signals. In particular, Skyhook is able to discover when APs move based on spatiotemporal clustering of observations. Location data is recomputed as new observations come in, and APs are repositioned if new trustworthy clusters are seen. Similarly, Skyhook has developed methods to find pathological signals. A ‘Pathological’ signal is one that is not trustworthy for use in computing a location. Some of the categories of pathological Wi-Fi APs include:
  • Mobile APs
  • Ubiquitous APs
  • wifi_signal_skyhook_precision_locationBig Coverage APs

In today’s world, mobile Wi-Fi APs are the most prolific in the ‘pathological’ categorization. This encompasses APs found across cars, airplanes, buses, trains, and more. It also includes APs such as mobile Wi-Fi (Mi-Fi) devices and smartphones being used as Wi-Fi access points.

The Bottom Line

With the proliferation and mass adoption of Wi-Fi, the sheer number of active APs is staggering. Skyhook has a database of over 5 billion Wi-Fi access points, all of which provide valuable and reliable information to locate devices. By continuously applying proprietary algorithms to qualify these access points, Skyhook ensures that APs currently in the database can be relied upon for location accuracy, and any new APs identified are thoroughly vetted, so the information they provide is equally beneficial.
 
   

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