View Full Version : What does the IRS have to do with the Satcom?

1st Oct 2010, 17:52
Have you ever noticed that the Satcom does not work as long as the IRSs are down or aligning? At least this is the case on B747/B744. I do not know about other airplanes but I was told it's the same on the Airbus.
For what does the Satcom need the IRS?

1st Oct 2010, 17:54
To align the aerial?

1st Oct 2010, 19:28
To tell the Satcom controller where it is so it can contact the right satellite.

1st Oct 2010, 23:00
thats correct, the aircraft/satcom controller needs the actual position of the aircraft to communicate with the satellites above a certain transmission rate. It can be compared with some kind of directional antenna.

2nd Oct 2010, 04:16
In the several fleets I maintain nothing, one of them is not even equipped with IRU's. I assume the possibility of a "valid" signal to prevent abuse of the system in your company or a different system.

2nd Oct 2010, 07:58
The aircraft (primarily the Satelite Data Unit) requires the geo position, the azimuth and eleveation of the aircraft (typically this is obtained from an INU/IRU or ADIRU) so that the beam steering unit (or in the case of a mechanically steered system, the antenna control unit) can line up on the available comsats at any given time. Airline usage policy etc is stored in the ORT (Owner Requirements Table) programmed into the SDU.
Hope this helps.

Dude :O

2nd Oct 2010, 09:46
Steering the antenna/e (electronically or otherwise) to point at the satellites, increases the strength of the transmitted/received signal. These steerable antennae are often called High Gain antennae.

Some Satcom systems have a Low Gain and a High Gain antenna (or two). The low gain antenna can be used for data transmission. IRS alignment is not required for Low Gain antennae. The High Gain Antenna can be used for data and voice comms.


2nd Oct 2010, 10:01
Agreed NSEU. Jeepers I forgot about LG antennas, you are quite right of course..

Dude :O