What is WAAS? Wide Area Augmentation System – comprised of a system of satellites and ground stations that provide GPS signal corrections, resulting in better position accuracy. WAAS GPS is better by an average of up to five times compared to standard GPS. A WAAS-capable receiver can give you a position accuracy of better than three meters 95 percent of the time. WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.
How is WAAS different from standard GPS? GPS WAAS systems are capable of producing much better position accuracy than standard GPS without augmentation. In order to provide the improved accuracy the avionics systems need to be WAAS capable. GPS WAAS systems enable pilots to use GPS as a primary means of navigation, including instrument precision approaches.
What is the difference between a “nonprecision” and a “precision” approach? Non-precision approaches only provide lateral guidance to the pilot. Precision approaches provide both lateral and vertical guidance to the pilot. GPS WAAS enables pilots to fly instrument approaches to landing.
What is the difference between Gamma 1, 2 and 3? There are 3 operational classes of GPS WAAS equipment. Gamma 1 equipment enables GPS primary navigation for oceanic, domestic en route, terminal, non-precision approaches, and departure operations. Gamma 2 equipment also allows for LNAV/VNAV precision approaches. Gamma 3 equipment enables LPV or AVP-II precision approaches.
What is LNAV?Lateral Navigation – LNAV approaches are non-precision approaches that provide horizontal deviation and guidance to the pilot. Pilots use barometric altimeters for vertical restrictions.
What is LNAV/VNAV? Lateral navigation with vertical guidance – The least accurate precision GPS approach. Typical decision heights are approximately 350 feet above the ground.
What is APV-I and APV-II? Approach operations with Vertical guidance – APV-I approaches provide vertical guidance precision of 20 meters 95% of the time. APV-II approaches provide vertical guidance precision of 8 meters 95% of the time. APV-I approach minimums are higher than APV-II approach minimums. APV-II approach minimums are as low as 250 feet above the ground.
What is LPV? Lateral precision with vertical guidance approaches - Used in approach publications to depict minimum decision altitudes for APV-I and APV-II approaches.
Can I use a C144 WAAS antenna on a Gamma 1 system? Yes. TSO C144 antennas are compatible with GPS WAAS Gamma 1 systems, allowing for primary navigation for all phases of flight with the exception of precision approaches.
Can I use a C144 WAAS antenna on a Gamma 2 or 3 system? Refer to the system manufacturer’s install manual. If the antenna has been approved for installation, it must also state that it is approved for Gamma 2 or Gamma 3 operations. A TSO C144 antenna can not be used to fly precision GPS approaches without a suitable backup such as ILS.
Can I use RTV or other sealants on the new Gamma 2 and 3 antennas? No. RTV contains silicone, which is known to degrade antenna performance at low angles of elevation. This degradation is not acceptable for pilots using GPS as a primary means of navigation
Can I place a gasket on one of the new Gamma 2 and 3 antennas? Gaskets can only be used if the antenna was specifically designed with a gasket. GPS WAAS antenna performance can be degraded substantially by even the slightest space between the antenna base and the aircraft.
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