What is Time Difference of Arrival (TDoA)?

Time Difference of Arrival (TDoA) is a method used for locating the position of a transmitter based on the differences in propagation times of signals received by multiple receivers. It relies on the principle that signals travel at finite speeds, and by measuring the differences in arrival times of these signals at different points, one can triangulate the transmitter's position. TDoA can also be used to identify and track multiple objects simultaneously, measure the speed of moving objects, and detect & locate the source of radio interference.

Working of TDoA

Figure 1: 4 receivers being used to create 3 hyperbolic curves for geolocation of a target 

TDoA operates on the basis of geometric trilateration. Instead of measuring the absolute time of arrival of a signal, TDoA focuses on the relative differences in arrival times at different receivers. It requires at least three receivers to determine the position of a transmitter in two-dimensional space and four receivers for three-dimensional localization.

When the transmitter sends out a signal, receivers at known locations receive the signal. Each receiver measures the time it takes for the signal to arrive. Using the known distances between receivers and the differences in arrival times, the system calculates the transmitter's position.

Figure 2

For TDoA geolocation, a minimum of three receivers connected to omnidirectional antennas are placed in a spatially separated network. These receivers form pairs (e.g. A&C and B&C in Figure 2) to measure the difference in signal arrival times from the transmitter and generate probable geolocation along a hyperbolic curve (as shown in Figure 2). When there are three receivers, they form two pairs and generate two such hyperbolic curves. The point where these curves intersect gives the highest probable site of the transmission. In 2D TDoA, only the longitude and latitude of transmissions are calculated and there is no indication of the third dimension i.e. altitude. Extending TDOA geometry into three dimensions requires a fourth receiver to create a third hyperbolic curve to define the height. The point where all three cross gives us the location and altitude.

Figure 3: Intersection of 3 hyperbolic curves to locate an aircraft using 3D TDoA

TDoA is used in a real-time location system (RTLS) to accurately calculate the location of tracked entities, such as tracking tags affixed to personnel or key assets, in real-time.

TDoA can provide accurate and reliable positioning information in diverse environments. Unlike Global Positioning System (GPS) or other GNSS systems, TDoA can function indoors and in areas where line-of-sight to satellites is obstructed. This makes it beneficial for applications where precise localization is essential, but GPS signals are unreliable or unavailable. TDoA systems can also be implemented using existing infrastructure such as Wi-Fi networks or cellular towers, especially indoors, reducing the need for additional hardware and infrastructure investment.

While TDoA offers numerous benefits, it also faces certain challenges and limitations. Signal reflections and multipath propagation can introduce errors in time measurements, leading to inaccuracies in localization using TDoA. Precise synchronization between receivers is critical for accurate TDoA measurements. Any timing discrepancies can compromise the accuracy of the system. These systems also require sophisticated algorithms to be developed for signal processing, time synchronization, and position estimation.

Time Difference of Arrival (TDoA) represents a powerful technique for localization and positioning in wireless communication systems. Its ability to provide accurate positioning information in various environments has made it essential for various localization applications. As technology advances and algorithms improve, TDoA is expected to play an increasingly significant role in shaping the future of wireless communication and location-based services.