I'm studying wireless navigation systems, especially GBAS. I have gone through some ICAO documents regarding GBAS. From what I have read, FAS data is included in Message Type 4. Closer examination of FAS data block tells that there is no block ID or sequence number which suggests that FAS data is not sent in chunks.

My question is, how does the on board equipment plot an entire accurate approach path? Since FAS data block has just the coordinates for LTP and FPAP.

I'm not a pilot so my understanding of operational details regarding GLS is pretty basic.


The best reference is the ICAO GBAS Guide. Unfortunately, it doesn't have diagrams to help understand it.

The Final Approach Segment (FAS) is a geodesic path to the runway. It is horizontally aligned with the runway center line and has a glide path angle that descends to the Threshold Crossing Height (TCH) directly above the Landing Threshold Point (LTP). This can be seen in the following figure:

FAS Definition
Source: SlideShare

All the necessary data is contained in the type 4 message as the ICAO table below shows:

FAS Data Block
Source: ICAO

There is one Type 4 message for each approach supported by the GBAS ground station.

The LTP/FTP latitude, longitude, and height are coded in the data block. The FPAP is coded as a delta Lat/Lon from the LTP/FTP. Horizontally, the FAS will pass through both of these points and thus be aligned with the runway center line.

The vertical component is defined by the Threshold Crossing Point (TCP which is the LTP vertically raised by the TCH) and the Glide Path Angle (GPA).

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    $\begingroup$ Thank! That helps a lot. So Type 4 messages defines the center line (LTP and FPAP) and the rest of the flight path until touchdown is dynamically calculated by the on-board equipment. Is this correct? Also, when the pilot enters the desired channel number, the equipment just receives 1 set of co-ordinates and TCH and GPA related data, right? $\endgroup$ – student1092 Jul 7 '18 at 13:26
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    $\begingroup$ Right. the LTP, FPAP, TCH, and GPA allow the GLS receiver to compute the flight path. The channel number does two things. First, the channel number is run through a hash to set the frequency for the VDB receiver to enable receiving the GBAS data. The receiver receives all the data broadcast from the ground station. The channel number is further used to select the appropriate Type 4 message from the data stream. $\endgroup$ – Gerry Jul 7 '18 at 20:50

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