A new launch date for 5C would be fixed early next week, explained Jean-Yves Le Gall, the CEO of launcher company Arianespace.
"There is an anomaly on software. The launch will be delayed a few days", a period of the order of a week, he told the AFP news agency.
The Skynet satellites match the sophistication of the very latest civilian platforms used to pass TV, phone and internet traffic - but also are "hardened" for military use and will resist attempts to "jam" them, for example.
Malcolm Peto explains how Skynet 5 will aid UK military communications
"These satellites have technology that allows them to pinpoint communications on to particular regions of the world with absolute precision, and without interference," explained Malcolm Peto from Paradigm Secure Communications, the company set up to run Skynet.
"You know the areas where British forces operate, and you can imagine the type of interference attempted. We can avoid that very simply, very clinically," he said.
The new Skynet constellation will allow the British Army, Royal Navy and Royal Air Force to pass much more data, faster between command centres. The bandwidth capacity of Skynet 5 is two-and-a-half times that of the old satellite constellation, Skynet 4.
The new system will enable British forces to make use of next-generation weapons systems, such as the recently introduced Reaper drones.
These unmanned aerial vehicles (UAV) are deployed over Afghanistan but are remotely piloted by RAF personnel in the US.
Skynet 5C was built in response to high satellite insurance costs
This is made possible only by Skynet's ability to handle the drones' real-time video feeds.
Other battlefield applications would include surgeons, unsure how best to treat badly injured soldiers, being able to send scan information back to the UK for a second assessment; and then using video conferencing to discuss cases.
Skynet 5 is the largest PFI (Private Finance Initiative) so far delivered to the UK's Ministry of Defence (MoD). Under the PFI, the British military buys guaranteed capacity from its commercial supplier (Paradigm), which is free then to sell any spare capacity to friendly governments.
These third-party sales earn money for Paradigm and the MoD in what is a "shared gain" arrangement. The launch of the Skynet 5C satellite should provide substantial extra revenue potential.
When the PFI deal was originally set up, the MoD was expecting to receive two satellites; but the high cost of spacecraft insurance in the early 2000s prompted project chiefs to put that money into building an extra platform instead.
"In effect we've self-insured," said Mr Peto. "We've used the money we would have spent on insurance to build a third satellite.
"When we launch [Skynet 5C], we will have three very capable satellites to provide communications, not only for British forces but for our third-party market which makes this whole project viable."
Skynet will support reconnaissance video gathered by unmanned drones
Paradigm hopes the money it earns over the course of its contract (which runs to 2020) will be sufficient for it to reinvest in further spacecraft with enhanced payloads.
If this is achievable, Mr Peto says, it will provide UK forces with better equipment, much faster than traditional procurement routes.
As is normal for Ariane flights, Skynet 5C will ride into space with a co-passenger - on this occasion, a Turkish TV satellite.
"We'll be first out; the separation of 5C from the rocket occurs about 30 minutes after launch," explained the Skynet project manager, Patrick Wood, from manufacturer EADS Astrium.
"We'll pick it up via a ground station we're using in South Africa. The first thing we'll do is send a command that should be sent straight back to us, telling us the receive electronics are working. We can then send a sequence of commands to start waking up the satellite's systems."
It will take about a week to move 5C to its initial operational position 36,000km over the Atlantic Ocean, at 17.8 degrees West.
Friday's launch window in Kourou opens at 2152 GMT.
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
1. Improved technologies, including a solar 'sail', lengthen the platforms' operational lives to at least 15 years
2. The satellites are 'hardened' against interference. A special receive antenna can resist attempts at jamming
3. Each spacecraft has four steerable antennas that can concentrate bandwidth onto particular regions
4. The system gives near-global coverage, providing 2.5 times the capacity afforded by the previous system
5. Each spacecraft is a 3x4x4.5m box and weighs just under 5 tonnes; the solar wings once unfurled measure 34m tip to tip
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