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to provide broadcasts at all points within the coverage area that adhere to the specified minimum and maximum data
broadcast rates and field strengths, without exceeding the receiver’s ability to adapt to transmission-to-transmission
variations in signal strength in a given slot. To avoid receiver processing issues concerning lost or duplicated messages, all
transmissions of the Type 1 or Type 101 message, or linked pair of Type 1 or Type 101 messages for a given measurement
type within a single frame need to provide identical data content.
7.12.4.2 One example of the use of multiple antennas is a facility with two antennas installed at the same location but
at different heights above the ground plane. The heights of the antennas are chosen so that the pattern from one antenna fills
the nulls in the pattern of the other antenna that result from reflections from the ground plane. The GBAS ground subsystem
alternates broadcasts between the two antennas, using one or two assigned slots of each frame for each antenna. Type 1 or
Type 101 messages are broadcast once per frame, per antenna. This allows for reception of one or two Type 1 or Type 101
messages per frame, depending on whether the user is located within the null of one of the antenna patterns. Type 2 and 4
messages are broadcast from the first antenna in one frame, then from the second antenna in the next frame. This allows for
reception of one each of the Type 2 and 4 messages per one or two frames, depending on the user location.
7.13 Definition of lateral and vertical alert limits
7.13.1 The lateral and vertical alert limits for Category I precision approach are computed as defined in Appendix B,
Tables B-68 and B-69. In these computations the parameters D and H have the meaning shown in Figure D-8.
7.13.2 The vertical alert limit for Category I precision approach is scaled from a height of 60 m (200 ft) above the
LTP/FTP. For a procedure designed with a decision height of more than 60 m (200 ft), the VAL at that decision height will
be larger than the broadcast FASVAL.
7.13.3 The lateral and vertical alert limits for APV procedures associated with channel numbers 40 001 to 99 999
are computed in the same manner as for APV procedures using SBAS as given in Attachment D, 3.2.8.
7.14 Monitoring and maintenance actions
7.14.1 Specific monitoring requirements or built-in tests may be necessary and should be determined by individual
States. Since the VDB signal is critical to the operation of the GBAS broadcast station, any failure of the VDB to
successfully transmit a usable signal within the assigned slots and over the entire coverage area is to be corrected as soon as
possible. Therefore, it is recommended that the following conditions be used as a guide for implementing a VDB monitor:
a) Power. A significant drop in power is to be detected within 3 seconds.
b) Loss of message type. The failure to transmit any scheduled message type(s). This could be based on the failure to
transmit a unique message type in succession, or a combination of different message types.
c) Loss of all message types. The failure to transmit any message type for a period equal to or greater than 3 seconds
will be detected.
7.14.2 Upon detection of a failure, and in the absence of a backup transmitter, termination of the VDB service should
be considered if the signal cannot be used reliably within the coverage area to the extent that aircraft operations could be
significantly impacted. Appropriate actions in operational procedures are to be considered to mitigate the event of the signal
being removed from service. These would include dispatching maintenance specialists to service the GBAS VDB or special
ATC procedures. Additionally, maintenance actions should be taken when possible for all built-in test failures to prevent loss
of GBAS service.
ATT D-29 23/11/06
Annex 10 — Aeronautical Communications Volume I
7.15 Examples of VDB messages
7.15.1 Examples of the coding of VDB messages are provided in Tables D-7 through D-10. The examples illustrate the
coding of the various application parameters, including the cyclic redundancy check (CRC) and forward error correction
(FEC) parameters, and the results of bit scrambling and D8PSK symbol coding. The engineering values for the message
parameters in these tables illustrate the message coding process, but are not necessarily representative of realistic values.
7.15.2 Table D-7 provides an example of a Type 1 VDB message. The additional message flag field is coded to
indicate that this is the first of two Type 1 messages to be broadcast within the same frame. This is done for illustration
purposes; a second Type 1 message is not typically required, except to allow broadcast of more ranging source corrections
than can be accommodated in a single message.
　
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