Satellite Delay Question

davestar_delux · 6 May 2008 at 15:27

I got asked this at an interview the other week:

Say a news studio has a live satellite feed from a reporter in a far away place and there is a delay of a few seconds between the two people communicating, what would cause this?

My best guess is there is a small amount of latency for data to reach the satellite and this is multiplied by protocol handshaking and stuff.

I don't think much error checking is done on real-time or video protocols IIRC and I don't think there would be lost packets like there could be on Internet communication.

Anyone else know anything more in depth about satellite communications? Just for my own curiosity now really.

J.B · 6 May 2008 at 15:28

Could be a processing delay if its being converted from analogue to digital and back again

Draeger · 6 May 2008 at 15:47

Also, as far as I am aware, satellite broadband means the users must have L.O.S. ( Line of sight ) between sateliites for optimum efficiency.

Weather patterns, trees and other geographical factors can affect the latency severely.

dave-lew99 · 6 May 2008 at 16:16

The delay is mostly caused by the physical distance - sattelites are a LONG way away and the signal can only travel so fast.

Added to the fact there will be at least some buffering each end to cope with a signal which may not be entirely stable.

Don't forget the effect is more pronounced if two people are talking to each other becuase each has to wait for the other's response (ie it is twice the actual delay).

Coping with delay in live broadcasting is an absolute nightmare, the way i've done it is to use a landland phone to estimate the delay over the broadcast signal then have the remote end set their radio clock back by so many seconds - it worked perfectly, but is by no means a reliable system!

box · 6 May 2008 at 16:23

I agree with dave, the main contributing factor to the delay will be the physical distance. For a communications satellite, we're talking a geo-stationary orbit, so roughly 35,000km above the equator. And assuming you're transmitting from GB, this will become around 84,000km. That's a 280ms delay for each leg of the journey. So if you've got 3 satellite hops between you and the recipient of your signal, that's 1.68s delay already. And as dave said, if you're talking to someone, you might as well make that 3.3s delay...seems about right.