Accurate Location Starts with Accurate Data

Co-Author Darryl Sladden

We recently wrote about the three elements required for more effective location based services—location accuracy, refresh rate and system latency. Today we’ll take a close look at the first of those elements—location accuracy.

As with any type of computing, the quality of the output is directly related to the quality of the input. There are several factors that influence the precision of the location data.  Here are several factors in order of importance that influence the quality of location:

AP Density—The number of installed access points in the venue are the biggest contributor to location data quality. It makes sense. More access points means more points of reference, fewer dead spots, and greater capacity to track devices. However, there is a point of diminishing returns after which additional APs are not helpful. If this is the case in your network, you can move these APs into monitor mode.

AP Placement—Closely following AP density, the placement of those access points has a huge impact on location data quality. Placement should start at the perimeter of the space then move inward. Placement should be as non-linear as possible yet avoid dead spots caused by elevator shafts, large open atriums, and wiring closets. During set up, it’s best to define these areas as exclusion zones as one can safely assume that no one will or should be moving through elevator shafts, wiring closets or the second floor of an open, three-story atrium.

It’s also important that access points be placed as close to their mapped location as possible. At a minimum, they should be placed on the correct floor. Ideally, they should be within three feet or less of their actual location and their correct heights and antenna orientation should be recorded in PI

RSSI Signal— The density and placement of the access points should enable the RSSI signal to be stronger than -75dbm by four APs throughout the entire venue.

Chirps and Probes—While the wireless network has the greatest influence on location accuracy, the signal intervals of RF tags and mobile devices also impacts accuracy. For the greatest accuracy, RF ID tags should be set to chirp on all three channels—1, 6, and 11—every 30 seconds.

Multi-floor deployment – In a venue with multiple floors, access points should be aligned along a common vertical axis. Third floor directly above the second floor access point, and so on.

Tracking mobile devices – Locating mobile devices is more challenging. Generally, device manufacturers set probe rates at intervals of 90 seconds or more to save battery life. Reducing the probe rate can profoundly reduce battery life. In addition, many mobile devices randomize their MAC address, making it nearly impossible to get a consistent location fix. The best way to ensure that mobile devices gets good consistent location is to have them associated with the network, sending traffic and utilizing Cisco Fastlocate capabilities of the network.

Zones—You can improve location data accuracy by defining zones within the venue. The general rule of thumb is to define a zone that is twice the size as your desired level of accuracy. Need accuracy to within ten meters? Then make the zone no larger than 20 meters by 20 meters. The zone size should also take into account the average probe rates of your typical clients and their associated user move rate.

Calibration—Location accuracy can be calibrated. While it is a time-consuming process, it can result in the highest degree of accuracy. Essentially, the calibration process involves placing a mobile device in a known location using its probe to validate the location.

You can decide the required level of accuracy and calibrate to accommodate. For instance, if room-level accuracy is required, you simply calibrate the perimeter of the space by probing devices just inside the wall. Then, on a second pass, probing devices just outside the space.

If a higher level of accuracy is required, you can walk through the entire venue, probing a new location every few feet. Clearly this can be very time-consuming for very large spaces. But in applications where location accuracy needs to be pinpoint, the effort may be worth it.

Each of the elements above contributes to the overall location accuracy.  Please follow the steps described above in the order shown to achieve the best location possible, with location calibration being an optional step.