Satellite Communication or SatCom is just one of the ways to communicate to the ground systems. And this is one of the most expensive ways to do so. Did you ever use the phone on the airplanes which is also the remote of the IFE (In-flight entertainment system) and for which you were to swipe your credit card to pay USD 5 per minute as call charges? Remember? That phone is connected to SatCom and the $5 more or less go to the satellite communication company. Airlines don’t make money on your calls, it is/was just a service to te passengers. Anyways, let’s get back to our discussion.
There are various ways through which airplanes communicate to the ground. These are:
- Sattelite Communication System (SatCom)
- HF (high Frequency) Radio
- VHF (Very High Frequency) Radio
We don’t normally notice, neither we get an opportunity to notice that all airplanes have multiple antennas sticking to their bodies. These antennas can vary in size, so much so that one might be of a size of a cigarette case and one might be over 6 feet long and a feet high, e.g. Inmarsat antennas.
The most basic explanantion of this communication system is that it is very much similar to the home Wi-Fi. There is a uplink (e.g.my laptop), a datalink (the router) and a downlink (the server).
Primarily, the typical aircraft’s air-ground communication depends on something called ACARS which stands for Aircraft Communications Addressing and Reporting System. Some interesting details can be found at Wikipedia which states it as a digital datalink system for transmission of short, relatively simple messages between aircraft and ground stations via radio or satellite. The protocol, which was designed by ARINC to replace their VHF voice service and deployed in 1978, uses telex formats. SITA later augmented their worldwide ground data network by adding radio stations to provide ACARS service.
ACARS messages are transmitted over one of three air-ground subnetworks.
- VHF is the most commonly used and least expensive. Transmission is line-of-sight so VHF is not available over the oceans or other vast expansions of uninhabited surface, such as the Amazon.
- SATCOM is a fairly expensive service that provides (near) worldwide coverage. The Inmarsat satellite network does not cover the polar regions. The Iridium satellite network became usable for ACARS transport in 2007 and provides excellent coverage in the polar regions.
- HF is a more recently established subnetwork. Its purpose is to provide coverage in the polar regions where Inmarsat coverage is unreliable.
Obviously, the new communication methods or protocols like Iridium have been developed and the next Gen aircraft use them too. The iridium antenna is also very small in size which is very beneficial to airlines. Does size matter? Yes it does. The smaller an antenna is, the lesser drag is. Drag is the aerodynamic force that opposes an aircraft’s motion through the air. NASA has developed a website to teach school & college students about principles of aviation. I highly recommend this website for anyone interested in this subject.
Ohh, I din’t notice that it was too late and I have a lot to accomplish at work tomorrow. I will be adding few pictures & drawings to make these concepts more understandable. I will also be relating these concepts to Testing world to make them more interesting to our testing community.