CDR Analysis and Cell Tower Mapping

A call detail record (CDR) is a data file generated by a cellular carrier documenting the metadata of a communications event. It typically includes the originating and receiving phone numbers, date and time, call duration, call type, and identifiers for the cell site that served the device. CDRs do not contain the content of communications. They are produced automatically by carrier infrastructure for billing and network management and are retained under carrier-specific policies, typically ranging from one to seven years depending on the carrier and record type.

CDR records identify which cell tower sector handled a communications event. That sector covers a geographic area that may range from under one square mile in dense urban environments to over ten square miles in rural or suburban areas. The record tells you a phone was within the coverage footprint of a sector, not where within that sector it was located. GPS uses satellite triangulation to pinpoint coordinates within meters. CDR and cell site location information are area indicators, not coordinate systems, and should never be presented to a jury as equivalent in precision to GPS data.

A cell tower sector is a directional slice of radio coverage served by one antenna on the tower. Most towers have three sectors, each covering roughly 120 degrees of arc, together forming 360-degree coverage. The azimuth is the compass bearing toward which the antenna faces, not the direction to any connected device. A sector with an azimuth of 0 degrees faces north. When a phone connects to a sector, the connection indicates the device was somewhere within the general coverage arc of that antenna, which can encompass multiple neighborhoods or square miles depending on environment.

No. A CDR record shows that a phone connected to a particular tower sector during a communications event. The coverage area of that sector may encompass multiple neighborhoods, roadways, or geographic zones. Additionally, phones do not always connect to the nearest tower: signal load balancing, physical obstructions, multipath propagation, and network conditions can cause a device to connect to a more distant tower. An expert must account for all of these variables and ideally conduct radio frequency drive testing before drawing location inferences from CDR data.

In Carpenter v. United States, 585 U.S. 296 (2018), the Supreme Court held in a 5-4 decision that the government's acquisition of seven or more days of historical cell site location information constitutes a Fourth Amendment search requiring a warrant supported by probable cause. The court rejected the third-party doctrine as a basis for warrantless access to CSLI held by carriers. The ruling applies to historical CSLI and has been applied and refined by lower courts to related location data types. The good-faith exception may apply to records obtained before June 22, 2018.

Carriers may retain specialized measurement data under designations including Timing Advance, Per Call Measurement Data (PCMD), Real Time Tool (RTT), Round Trip Delay, and Network Event Location System (NELOS) records. These datasets use signal propagation timing to estimate a device's distance from a tower along an arc within a sector. However, carriers themselves publish disclaimers stating that this data is not equivalent to historical GPS and carries inherent inaccuracies. Availability and retention periods vary significantly by carrier and network generation.

CDR and cell site records are obtained from carriers by subpoena, or where constitutionally required under Carpenter, by search warrant. The subpoena should specify the phone number, date range, and each specific record type sought, including voice CDR, SMS records, data connection records, specialized location measurement data (PCMD, RTT, TA), and the cell site reference file listing tower coordinates and antenna parameters. Early engagement of a forensic expert to draft precise subpoena language helps ensure all relevant data classes are captured before carrier retention windows expire.

Common methodological errors in CDR expert testimony include: overstating the precision of tower connections as placing a phone at a specific address; assuming without validation that phones always connect to the nearest tower; misidentifying azimuth direction as the bearing to the device rather than the direction the antenna faces; presenting sector coverage cones as hard geographic boundaries rather than probabilistic approximations; and failing to conduct radio frequency drive testing to validate coverage assumptions. Attorneys should ask experts about each of these issues during Daubert hearings and cross-examination.