The VHF Omni-range (VOR) provides approach information (Horizontal Guidance) for airfield use and navigation information for all aircraft in the coverage area. This information is either modulated onto a VHF carrier or calculated by the aircraft receiver from signal characteristics. Aircraft utilizing the VOR system can obtain their azimuth (location) in relation to the VOR facility; station identification in the form of Morse code; and Voice transmissions, which provide current information about field conditions. The VOR Transmitter Group is housed in an environmentally controlled shelter with the antenna mounted on the roof; the roof serves as a grounding plane, or counterpoise. Unlike the ILS, the VOR radiates in all directions (omni-directional) so locating the equipment shelter in a separate building near the antenna is not feasible. Additionally, unlike the TACAN, the VOR is highly sensitive to surrounding terrain and simply elevating the antenna (as is done in TACAN only installations) is not a complete solution. Therefore, the VOR (or VORTAC if the systems are co-located) facility is usually a circular building directly underneath the counterpoise, making it invisible in the pattern.