type of approach is similar to the ILS localizer approach, but with less precise guidance. Non-precision approaches and systems Non-precision systems provide lateral guidance (that is, heading information), but do not provide vertical guidance (i.e., altitude and/or glidepath guidance). VHF omnidirectional range (VOR) Tactical air navigation (TACAN) Non-directional beacon (NDB) – ground-based transmitter for aircraft equipped with an Automatic Direction Finder (ADF). Simplified directional facility (SDF) Satellite navigation systems, such as the American Global Positioning System (GPS). LNAV and LNAV/VNAV approaches require Receiver autonomous integrity monitoring (RAIM) which detects problems with GPS satellites. LPV (Localiser Performance with Vertical guidance) and LP (without vertical guidance) do not require RAIM since they utilise SBAS correcting signal like Wide Area Augmentation System (WAAS) or European Geostationary Navigation Overlay Service (EGNOS). Required navigation performance (RNP) – a system that utilizes on-board performance monitoring through the aircraft's flight management system Localizer Localizer type directional aid (LDA) Surveillance radar approach (SRA) – also known in some countries as an ASR approach Airport surveillance radar (ASR) – military designation for SRA VDF approach -- these days rare, but based on the air traffic controller determining the direction of arrival of an airplane's VHF radio transmission, and guiding the pilot based on this information. Can also exist in a charted form wherein controller-guided VDF navigation is combined with dead-reckoning by the pilot. Precision approaches and systems Precision approach systems provide both lateral (heading) and vertical (glidepath) guidance. Ground controlled approach (GCA) GBAS landing system (GLS) Instrument landing system (ILS) Joint Precision Approach and Landing System (JPALS) Microwave landing system (MLS) Precision approach radar (PAR) Transponder landing system (TLS) Basic concepts Decision height or altitude In a precision approach, the decision height (DH) or decision altitude (DA) is a specified lowest height or altitude in the approach descent at which, if the required visual reference to continue the approach (such as the runway markings or runway environment) is not visible to the pilot, the pilot must initiate a missed approach. (A decision height is measured AGL (above ground level) while a decision altitude is measured above MSL (mean sea level).) The specific values for DH and/or DA at a given airport are established with intention to allow a pilot sufficient time to safely re-configure an aircraft to climb and execute the missed approach procedures while avoiding terrain and obstacles. While a DH/DA denotes the altitude at which a missed