With the GAGAN fully deployed with APV 1/1.5 (Approach with Vertical guidance), expected by end of year 2014, after the GAGAN RNP 0.1 is activated (expected anytime this month), GPS approaches with qualified equipment, and in some cases qualified air crew, are instrument precision approaches. The present GNSS 27 at Cochin is a non-precision approach. (see chart on the left)
The major difference lies in three aspects: accuracy, integrity, and vertical guidance. With GAGAN, GNSS accuracy is further enhanced, leading to greater confidence in the approach. With integrity (pilots getting a warning should the performance degrade), confidence in the system is further enhanced, allowing not just operators, but the regulator to approve instrument precision approaches. The precision is because of the enhanced GPS accuracy and integrity, which allow the aircraft to descend on a glide slope generated with the help of the GNSS’s vertical guidance.
The chart clearly shows that lateral guidance is provided by the GNSS, and vertical guidance by a barometric system: the altimeter. This non-precision approach has a minimum descent height(MDH) of 430 feet, and a “decision height” of 410 feet, though vertical guidance is not precision.
With an APV, the approach becomes precision, with minimums between 200ft and 250ft. The approach is now precision, and the decision height is similar to ILS CAT I. In case the vertical performance of the GNSS degrades, it becomes an LNAV / VNAV approach, with minimums as published in the chart.
Such approaches can be very quickly published at many airports, without the need for a costly ILS system. This will allow many operators to exercise an APV at airports, leading to higher flight safety in one of the most critical phases of flight: the approach. In addition, operator can fly into an airfield even in weather conditions that will prohibit non-precision approaches, if an APV approach is published at that airfield, no matter how remote or deserted it may be.