The VERA passive radar (known as Věra in Czech) is an electronic support measures (ESM) system that uses measurements of time difference of arrival (TDOA) of pulses at three or four sites to accurately detect and track airborne emitters. It is reportedly able to detect military "invisible aircraft". The manufacturer is , based in Pardubice. Mode of operation The deployed system typically comprises a central site (containing the signal processing equipment and an ESM receiver) and two or three side sites containing only an ESM receiver. The side sites relay the signals received to the central site over a point-to-point microwave link. The central site uses the known propagation delay from the side sites to estimate the TDOA of the pulses at each site. The TDOA of a pulse between one side site and the central site locates the target on a hyperboloid. A second side site provides a second TDOA and hence a second hyperboloid. The intersection of these two hyperboloids places the target on a line, providing a 2D measurement of the target's location (no height). A third side-site provides a third hyperboloid, which, when intersected with the line provided by the other two side sites, provides a full 3D location of the target. This process is known as multilateration. The height of a target with an SSR Mode C or Mode S transponder can also be provided by decoding the Mode C or Mode S response directly, which is the normal mode of operation for the civilian VERA-AP system. System accuracy System accuracy is typically better than a microwave surveillance radar, and is a function of the deployment geometry, the inherent timing accuracy of the central site, the bandwidth of the pulse being detected and the signal-to-noise ratio. Wider separations of the side sites from the central site provide better accuracies – but at the expense of a reduced area of common coverage. Typically side sites are deployed 15–40 km from the central site, providing a reasonable compromise of coverage and accuracy. The system is generally line-of-sight limited, with a nominal range of 450 km, the normal radio horizon. Detection of targets is within a sector of approximately 120 degrees, although IFF and SSR targets are detected using a dedicated omnidirectional antenna, and hence may be seen over 360 degrees. Up to 200 targets can be automatically tracked simultaneously, with an output rate adjustable from 1 to