By Jonathan Amos
Science reporter, BBC News, Kourou
The UK is due to continue the upgrade to its military satellite communications system with the launch of a new spacecraft on Friday.
The launch pad was swept by wind and rain on Friday morning
The Skynet 5B platform will ride into orbit atop an Ariane 5 rocket from the Kourou spaceport, in French Guiana.
It will join the 5A satellite lofted successfully in March and which is already handling secure traffic for UK forces in Iraq and Afghanistan.
The £3.6bn Skynet project represents the UK's single biggest space venture.
The investment includes replacing and updating control centres, and the major antennas and terminals used by military ships, land vehicles and planes to communicate through the satellites.
The 5B spacecraft is waiting on the launch pad in Kourou for a flight timed to start at 1904 local time (2204 GMT).
"We've had an incredibly smooth launch campaign so far, and now we're in the final stages," said Patrick Wood, who has led the development of both spacecraft for manufacturer EADS Astrium.
"There are a series of umbilical connections and we communicate with 5B night and day. We're doing final battery charging, making sure it is optimised for launch," he told BBC News.
Skynet 5 will allow the Army, Royal Navy and RAF to pass much more data, faster between command centres.
The bandwidth capacity is two-and-a-half times that of the previous satellite constellation, Skynet 4.
Although they look much like any of the other modern commercial telecommunications spacecraft launched from Kourou, the 5s incorporate new technologies specially prepared for military use.
Four steerable antennas give them the ability to focus bandwidth onto particular locations where it is most needed - where British forces are engaged in operations.
The spacecraft have also been "hardened" to withstand any interference - attempts to disable or take control of the satellites - and any efforts to eavesdrop on their sensitive communications.
They each have advanced receive antennas that enable the spacecraft to selectively listen to signals and filter out attempts to "jam" them.
They will also resist attempts to disrupt them with high-powered lasers.
"Hardened doesn't mean armour-plated, because the cost of launching a spacecraft like that would be phenomenal and the amount of useable payload in it would be very small," explained Mr Wood.
"But Skynet 5 does incorporate electronic - and some physical - protection that will help the spacecraft in some types of adverse environment."
The benefits of the upgrade should become evident when the UK starts using Predator unmanned aerial vehicles (UAVs) to support troops in Afghanistan.
These aircraft will stream real-time video of Taleban activity and, with the capacity of Skynet 5, those pictures can be fed back to the UK quickly for further analysis, if necessary.
The UK military increasingly talks about NEC - Network Enabled Capability. It describes modern commanders taking rapid decisions based on superior information - enabling them to deploy equipment and personnel far more effectively than has been possible in the past.
The abundance of information has to be managed, however.
Tony Skinner, from Jane's Defence Weekly, said British commanders had admitted they were still learning how best to handle what is becoming a flood of information.
"As some of these new systems are coming online, and the communication bearers increase capacity, the armed forces have to work out the human processes, standards and protocols needed to use the information efficiently," Mr Skinner explained.
"Also, your systems have to be interoperable, especially working in a coalition environment. It's all very well taking a video stream from a drone but can you share that with friendly forces, other bases and even back to the UK?"
The cost of the Skynet project has raised eyebrows, not least because it has been financed through a private company with City money.
Paradigm Secure Communications won a contract with the Ministry of Defence to supply satellite services to UK forces.
The contract called for two new spacecraft and, because of the way the financing deal was put together, will allow for an in-orbit spare, Skynet 5C to be launched next year.
The MoD is currently struggling to tie down the details of another major PFI (Private Finance Initiative) to overhaul the RAF's mid-air re-fuelling tankers, but Paradigm managing director, Malcolm Peto, is in no doubt the novel procurement route is good value.
"This is the way to do it," he said.
"Other European militaries and procurement agencies have been sceptical that we could put this satcom network and services together.
"But with one satellite already in orbit, people are starting to take notice. With two in orbit we will have nothing left to prove."
Skynet 5A looks over the Atlantic Ocean. 5B will occupy a position above the Indian Ocean. Together, they will provide coverage from the eastern US seaboard to east of China.
Friday's Ariane rocket will also loft a Brazilian telecommunications spacecraft.
1. Skynet 5 overhauls satellite communications for UK forces
2. The largely autonomous satellites talk to two UK ground stations
3. Skynet 5 supports high-bandwidth applications, such as UAV video
4. Antennas and terminals are upgraded to make best use of Skynet
5. New battlefield networks, such as Cormorant, feed into the system
6. System gives commanders access to more information, faster
The satellites are 'hardened' against interference. A special receive antenna (1) can resist attempts at jamming
Each spacecraft has four steerable antennas (2) that can concentrate bandwidth onto particular regions
The system gives near-global coverage (3), providing 2.5 times the capacity afforded by the previous system
Each spacecraft (4) is a 3x4x4.5m box and weighs just under 5 tonnes; the solar wings once unfurled measure 34m tip to tip
Improved technologies, including a solar 'sail' (5), lengthen the platforms' operational lives to at least 15 years