Navigation before satellites
Pilots have always needed to know where they are, and for much of the twentieth century they relied on a mix of methods. Early aviators followed railways, rivers and roads. Later, radio ranges and then beacons gave a direction to fly. In the postwar decades the standard civil systems were the non-directional beacon (NDB), which gives a bearing to a station, and the VHF omnidirectional range (VOR), which gives the aircraft’s bearing from a ground station, often paired with distance measuring equipment. For landing in poor weather, the instrument landing system (ILS) guided aircraft along a radio beam to the runway. Over oceans, aircraft used inertial navigation units and, for a time, long-range radio systems such as Omega.
These systems worked, but they had limits. Aircraft had to fly from one ground station to another, which meant that routes were often zigzags along the airways rather than straight lines. Each beacon had to be bought, maintained and checked by flight inspection, and some terrain blocked the signals. Our article on aircraft navigation aids covers the earlier equipment in more detail.
The first interest in satellites
Aviation interest in satellite navigation was present from the start of the GPS program. The accuracy and worldwide coverage were obvious advantages, especially over oceans and remote regions. After the loss of a Korean airliner in 1983, which is described in our piece on civilian GPS access, the case for civil use gained political backing.
The first airborne receivers were military and experimental. Civil use began in the late 1980s and early 1990s, when manufacturers began to sell receivers intended for aircraft. At first these were not approved as a primary means of navigation. Pilots flying under instrument rules were typically required to carry conventional equipment, and a GPS unit was treated as a supplement, a second opinion. Many general aviation pilots also took handheld units on board, even though they were not certified aircraft equipment, and used them as backups. Dates for the first certified units depend on the country and the type of approval, so no exact year is given here. The FAA allowed GPS as a supplemental means in the early and mid-1990s.
The concerns of regulators
Aviation is a safety-critical field, and regulators had several reasons for caution.
- Integrity: a pilot needs to be told quickly if the position is unreliable. The basic GPS service did not provide such a warning to users, so receivers had to include their own checks, a feature known as receiver autonomous integrity monitoring (RAIM).
- Availability: during the early years, the constellation was incomplete, so some times of day had poor coverage.
- Accuracy: with selective availability active, civil accuracy was deliberately limited, which was not good enough for precision approaches.
- Single-system dependence: a signal that could be jammed or lost was a concern if it became the only means of navigation.
Augmentation
To meet these needs, aviation authorities developed augmentation systems. The best known in the United States is the Wide Area Augmentation System (WAAS), which uses ground stations to measure GPS errors and sends corrections and integrity information through geostationary satellites. It was commissioned for aviation use in the early 2000s, and later it supported approaches that give vertical guidance down to low altitudes at airports that do not have an ILS. Other regions have similar systems, such as Europe’s EGNOS and Japan’s MSAS. Another approach for airports is a ground-based augmentation system, which uses a local transmitter to provide precision approach guidance.
From beacons to routes
The impact on flying procedures was greater than the impact on the cockpit hardware. Satellite navigation made area navigation (RNAV) practical, in which an aircraft can fly a direct route between any two points rather than from beacon to beacon. Authorities began publishing GPS-based approaches for airports, which meant that even small airports could have instrument procedures without a costly installation. Over time this developed into performance-based navigation, where the rules specify how accurately an aircraft must navigate rather than what equipment it must use.
As GPS became widespread, some ground-based systems were reduced. Omega was shut down in 1997, and the American LORAN-C network ended in 2010. Authorities in several countries have announced plans to reduce the number of VOR stations while keeping a backup network, because of concerns about relying on a single system. The pace and details of these plans vary by country and change often.
Other uses in aviation
GPS in aircraft is used for much more than showing a position. Position data feed automatic broadcast systems such as ADS-B, where aircraft transmit their own location to other aircraft and to ground receivers. This supports air traffic control and, in some areas, provides surveillance where there is no radar. GPS time also synchronizes avionics and ground systems. Airline operators use satellite positioning to plan efficient routes, and it supports monitoring of aircraft position over remote ocean areas.
Limits today
GPS signals are weak and can be blocked or interfered with. Aviation authorities and airlines therefore generally keep backup methods such as inertial navigation, some ground beacons and procedures for navigating without satellites. Reports of jamming and spoofing near conflict areas have led regulators to issue warnings to pilots.
The introduction of GPS to aviation was gradual rather than sudden. It started as a helpful extra in the 1990s, grew into an approved primary means for many operations and changed the structure of airways. It is best told as the story of a regulatory process, not just a single device.