What does all of this stand for?

Question

I looked at a diagram of the 777 and it labeled all of the antennas on the aircraft. There is one called SATCOM. What does that stand for, and what does it do? Also, why do I only see it on some 777's and not all of them?

Here are some others I would like to be identified:

TCAS

HF

RA

DME

Here is where I got the labels from: http://www.geocities.com/khlim777_my/b777antenna2.jpg

Answer 1

Traffic collision avoidance system: gives a audiovisual warning that there is traffic nearby. If that traffic comes too close it will give both planes audiovisual instructions on how to avoid each other.

HF: High frequency. Your normal radiofrequencys are in the Very High Frequency range. But in order to communicate over larger distances the High Frequency is used mostly over the Atlantic and in Africa. VHF just won't reach that far. HF is more prone to disturbances tough. I cannot remember the frequency range of HF but it was in the first post I believe

RA: Radio altimeter. An altimeter which measures the distance to the ground, a bit like a bat. It sends out a tone which will be bounced back to the receiver and by measuring the time between sending and receiving you'll know what the altitude is.

RA (2): it can also mean Resolution Advisory: A resolution advisory is a function of TCAS where the system will tell the pilots of both conflicting aircraft how to avoid each other. For example, the system will call out in cockpit one: "CLIMB", and in cockpit two "DESCEND" so that the aircraft will avoid each other.

DME: Distance measuring equipment. Your aircraft sends a package of random data towards the dme and at the same time starts the timer. The dme receives the data and holds it for some seconds and than sends a reply version of that data back. Your aircraft's dme receiver is expecting that precise form of data and stops the timer when it is received. From that the receiver can calculate the distance to the DME station (mostly built in with VOR's). But the receiver can also calculate the time to the station and your groundspeed to the station.

ADF: Automatic direction finder: When you have an NDB (non directional beacon) on the ground an adf will show an arrow as to where the beacon is situated from your plane

for example if you are flying to the west, and the beacon is to the north of your aircraft, the needle will indicate 90° to the right, pointing to the station. You can use these beacons to navigate or to fly certain approaches.

Answer 2

SAT COM = satellite communications

TCAS = terrain and collision avoidance system

HF = high frequency

RA = radio altimeter

DME = distance measuring equipment

ADF = automatic direction finder

Answer 3

The Traffic alert and Collision Avoidance System (or TCAS) is a computerized avionics device which is designed to reduce the danger of mid-air collisions between aircraft

High frequency (HF) radio frequencies are between 3 and 30 MHz

Distance Measuring Equipment (DME) is a transponder-based radio navigation technology that measures distance by timing the propagation delay of VHF or UHF radio signals

ADF- automatic direction finder.

SATCOM's satellite-based broadband communication systems enable worldwide high-speed Internet, voice and video capabilities on aircraft, vehicles and vessels. A leader in satellite technology, the company provides off-the-shelf communications systems for defense, commercial and corporate aviation, Search and Rescue, and tracking and messaging applications.

Answer 4

Comair has done a good job.

RA is Radar altimeter. Radar( Radio actually) waves are transmitted downward and the time for them to get recived again is measured. Then altitude is calculated.