Air Ground Communication

Air Ground Communication

**Frequency Blocking

Frequency blocking is a phenomenon that occurs when two or more stations transmit on the same frequency or when a station makes an unnecessary transmission that lasts for too long. In the former case, the result is a signal that is a mix of all transmissions and the receiving stations hear a garbled message which is often completely unreadable. This frequency coupling function is available in most modern VCSs.

Types of frequency blocking:

1) Partial Overlap

2) Complete Overlap

3) Prolonged blocking

Common reasons of frequency blocking:

1) Lack of radio discipline

2) Coincidence

3) Callsign confusion

4) Stuck microphone button

5) A pilot making a PA announcement but forgetting to appropriately switch the equipment

**Frequency Congestion

A single radio frequency is capable of handling a limited number of radio messages in a specified time. The maximum number of messages will be determined by the length of each message and its response. Ideally, a pilot should be able to transmit a message at any time of his/her choosing, and receive an immediate reply. As radio traffic increases above the ideal, the frequency becomes congested. The pilot must wait for a break in transmissions to pass a message and may have to wait for a response from the ATCO, who has to judge different priorities.

Typically, The pilot takes a frequency change intended for another aircraft, resulting in loss of communication as one of the typical scenario.

**Frequency Change

The process of changing frequency offers many possibilities for communication failure if the pilot subsequently selects the wrong frequency.

Frequency change occurrences are often of short duration because the pilot realises on checking in that he/she is on the wrong frequency: either the frequency is silent, in which case the pilot returns to the previous frequency, or it is active, in which case the controller directs the pilot to the correct frequency.

Frequency change occurrences often happen in areas of high density air traffic, especially during climb and descent, where many frequency changes are required as the aircraft is passed from one agency to another. Since these occasions coincide with periods of high pilot work-load, there is an enhanced likelihood that an error in copying the frequency or in setting it correctly will go undetected.

Frequency change errors are equally common in low-workload situations, perhaps because the pilots are more relaxed, or concentrating on some routine issue. Moreover, it may take longer to detect loss of communication than when working a busy frequency.

**Frequency Coupling

Frequency coupling is a method for air-ground voice communication in which more than one frequency is used but stations using one of the frequencies can receive messages sent on the other(s).

Sometimes it is necessary for a single ATS sector or unit to use more than one frequency simultaneously. Examples of such situations include:

1) Temporary collapsed area control sectors due to sector closure. Generally, when a sector is closed and its airspace is absorbed by another sector, the adjacent sectors and units are informed, that ATS will be provided on a new frequency.

2) Temporary combining of a tower and approach unit during less busy hours. 

3) Provision of service to aircraft with different communication capabilities. While 8.33 kHz communication channel spacing is a standard for ATS communications there are still aircraft that are not equipped with such radios (e.g. some general aviation aircraft which can only use 25 kHz channel spacing and some state aircraft that are only UHF equipped).

There are two basic methods to use multiple frequencies simultaneously:

a) Separate Use : 

b) Frequency Coupling

Sources:

Frequency Blocking

Frequency Congestion

Frequency Change

Frequency Coupling

Frequency coupling- Articles