Before cell phones became ubiquitous, residency audits often focused on credit card receipts, static documents (driver's license, voter's registration, periodical mailing addresses, etc...) and travel records. Now that virtually every person has a cell phone, the data generated by cell phones has become an additional and in many cases, primary manner in establishing a person's location throughout each day.
Cell phones perform their wide and growing variety of functions by connecting to a set of radio antennas called “cell sites.” Although cell sites are usually mounted on a tower, they can also be found on light posts, flagpoles, church steeples, or the sides of buildings. Cell sites typically have several directional antennas that divide the covered area into sectors.
Cell-site Location Information
Cell phones continuously scan their environment looking for the best signal, which generally comes from the closest cell site. Most modern devices, such as smartphones, tap into the wireless network several times a minute whenever their signal is on, even if the owner is not using one of the phone’s features. Each time the phone connects to a cell site, it generates a time-stamped record known as cell-site location information (CSLI). The precision of this information depends on the size of the geographic area covered by the cell site. The greater the concentration of cell sites, the smaller the coverage area. As data usage from cell phones has increased, wireless carriers have installed more cell sites to handle the traffic. That has led to increasingly compact coverage areas, especially in urban areas.
Wireless carriers collect and store CSLI for their own business purposes, including finding weak spots in their network and applying “roaming” charges when another carrier routes data through their cell sites. In addition, wireless carriers often sell aggregated location records to data brokers, without individual identifying information of the sort at issue here. While carriers have long retained CSLI for the start and end of incoming calls, in recent years phone companies have also collected location information from the transmission of text messages and routine data connections. Accordingly, modern cell phones generate increasingly vast amounts of increasingly precise CSLI. It is unclear whether or which wireless carries do store historical GPS information.
Authorization to Obtain Cell Phone Location Records
In the criminal context, according to the U.S. Supreme Court's recent decision in Carpenter v. United States, a Government authority must obtain a warrant to obtain your cell phone location records. Courts have permitted location data derived from WiFi hotspots, however, to be obtained by authorities without a warrant. In the civil context, including tax audits, this information can often be obtained with a subpoena or consent.
Even if a taxing authority is authorized to obtain location data from a Cell Phone provider, each provider appears to maintain different types of data and sometimes, only for very limited time periods (T-Mobile (2 years), AT&T (5 years), Verizon (1 year)). Recent public reports indicate that AT&T provides the most granular level of data, followed by Verizon and then T-mobile. This is subject to change at any time. Unfortunately, because Cell Phone providers generally use Cell towers to determine location data, this can result in false locations when you travel close to city, state or country borders. These relatively short retention periods also suggest that this data will not be available in many cases from these carriers to assist taxpayers that need to obtain this data.
Accuracy Issues
The accuracy of location data from a single cell tower varies from a few blocks to several square miles, although by triangulating between multiple towers, more accurate location information is often available. A cell phone's GPS chip can provide location information accurate to 5-10 feet. Issues can arise with using cell phone records to establish a taxpayer's location. For example, call forwarding, WiFi calling via cell phone and VPNs, signal boosters, older devices, devices without updated operating systems and stale location posts can all produce inaccuracies in cell phone records.
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