Have you ever wondered what does the channel numbers on the SBAS approach charts stand for?
When you select a PBN approach, we find WAAS approach type in United States, or EGNOS in Europe, amongst others. Below the approach procedure designator, i.e. RNAV (GPS) Y 04L in KJFK, a WAAS with CH 77519 appears underneath. Just below channel numbers there is a combination of letters and numbers.
If we remember, WAAS is system which augments GPS signal in other to improve GPS position. To do that, a geostationary satellite system broadcast a correction signal to aircraft. In U.S.A is WAAS. Then, it’s not necessary to place any ground station at airports to broadcast this correction signal to correct aircraft GPS position like in GBAS system. But why there is a channel on the approach chart though?
When the system was designed, the channel number was considered as an optional equipment capability which allowed to pilot to use a 5-digit number to select a specific approach instead of using the menu method as we usually do to choose from our navigation database. These numbers correspond to only one type of approach and they’re unique.
Below channel numbers, there is a combination of 4 letters and numbers. This is the approach identifier. In conventional radionavaids, when an ILS or VOR is selected, there is Morse code to aurally ident it. In this case, is visually identified to verify the selection and extraction of the correct approach chart from the navigation database. The first letter corresponds to augmentation system used: W for WAAS or E or EGNOS. The next two numbers are used for runway designator. When there are parallel runways, lest say LEFT, CENTER or RIGHT, letter A, B or C are added at the end. So, in KJFK, for the approach RNAV (GPS) Y 04L, our approach ident will be W04A.