Prior to the late 1990’s, ground-based or terrestrial navigation aids were used as the sole means to provide pilots with a navigation capability for all phase of flight – departure, enroute, arrival and approach.
A few of these aids have long since disappeared or have become obsolete and are no longer in use. Examples include the Radio Range and the Microwave Landing System (MLS).
Today, a few devices still remain and continue to serve as the backbone of the air navigation network throughout the world. All of these devices require a physical installation on the ground complete with electrical power, monitoring equipment and maintenance.
The simplest of these devices is a Non-directional Beacon (NDB). An NDB is a radio station broadcasting an electronic signal on a specific frequency. This device is unsophisticated and simply provides a crude navigation signal. From a pilot’s perspective, all it does is provide a bearing to the signal source. There is no indication of how far an aircraft is from the station and the only way of knowing is when station passage occurs and the needle reverses direction on the flight instrument called the Automatic Direction Finder. Many airports own and operate these devices; NAV CANADA also operates a number of these devices at airports throughout Canada.
A Very High Frequency Omni Range (VOR) is a more sophisticated device. The navigation signal from it provides a pilot with a more accurate picture of his position with respect to this device. It too provides the pilot with a bearing to the transmission site but is more accurate and can be integrated into other flight instruments.
A Distance Measuring Equipment (DME) is a device that provides the aircraft with a distance (in nautical miles) from its transmitter. This information if often combined with either the NDB or VOR based position to give both a bearing and distance to a navigation aid. This provides the pilot with a more complete picture of where the aircraft is.
The instrument approach procedures that use NDB or VOR with or without a DME are non-precision approaches. The approach minimums – how low an aircraft can descend during the procedure – varies but is usually never less than 400 to 500 ft above the airport.
The Instrument Landing System (ILS) is a sophisticated device that provides precise navigation guidance to an aircraft landing on a runway. The lateral guidance is provided by a "localizer" and the vertical guidance is provided by a "glide slope". The ILS has been in existence for decades and is the gold standard of landing navigation aids. ILS installations are expensive – there is the capital cost of the hardware, the site preparation work, the maintenance of the equipment and the site, and the flight checking of the navigation signal using an aircraft equipped with sophisticated flight checking equipment.
The instrument approach minimum from an ILS varies with the ILS Category. Category I is generally 200 ft above the runway or ½ sm visibility – this is typically found at most airports. In some cases the minimums are higher to due a host of reasons such as terrain (Kelowna) or lighting (Iqaluit). In order achieve the full 200/ ½ expensive approach lighting is required. Category I ILS can be flown by any qualified pilot and aircraft without special training or equipment maintenance.
Category II and III ILS are in an entirely different league. While the approach minimums are reduced to as low as “Zero-Zero” (no ceiling or visibility),it comes with an extremely high cost. The ground installation and lighting requirements are significant. The aircraft equipage, equipment maintenance and crew training are also significant. For these reasons, Category II and III ILS installations are found only at the largest airports in the world and are flown by sophisticated aircraft and professional pilots.