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Vertical Profile A line or curve, or series of connected lines and/or curves in the vertical plane, defining an ascending or descending flight path either emanating from or terminating at a specified waypoint and altitude, or connecting two or more specified waypoints and altitudes. In this sense, a curve may be defined by performance of the aeroplane relative to the air mass. Vertical Profile Angle Error The difference in degrees that the current aircraft flight path angle makes with the vertical profile. Vertical Rate Deviation Threshold A monitoring criteria parameter that determines whether a vertical altitude deviation non-conformancy exists. Very High Frequencies The set of frequencies between 30 and 300 MHz. Very High Frequency Aeronautical Mobile Services VHF radio (voice) will remain in use in many continental and terminal areas. However, increasing use will be made of VHF Data links with data eventually being used more than voice. The present frequency band in which the aeronautical VHF communication system is operating is 118-137 MHz. Very High Frequency Data Link VDL provides packet data communications to aircraft and ground users provided by a VDL sub network that comprises airborne 'VHF data radios (VDRs), VHF ground stations and connectivity to routers on the aircraft and the ground. Very High Frequency Omni directional Range Test A ground facility which emits a test signal to check VOR receiver accuracy. Some VOT's are available to the user while airborne, and others are limited to ground use only. Very High Frequency Omni-directional (radio) Range 1- Sophisticated NDB which sends out a signal with a phase-shift dependent on the direction the signal is sent. This allows for more precise navigation and also corrects navigational problems due to wind.