1: Infrared satellite system picks up heat signatures of hostile ballistic missiles launched towards Nato target.
2: Information is transmitted to ground stations for processing.
3: Processed information is then sent to Nato command and control network.
The command and control network relays information to sensor and weapons systems in the region.
Once the missiles' engines burn out, the infrared satellite can no longer detect them.
1: Long-range sensors such as the US AN/TPY-2 high-resolution radar and the Dutch sea-based Air Defence and Command Frigate (ADCF), continue to track the missile to help command system calculate options for destroying them.
2: Information is constantly shared among the sensors and weapons systems.
Command system has the option of shooting down the hostile missiles while in the upper or lower layers of the atmosphere. As tracking continues, greater accuracy is achieved.
Lower-layer shooter systems include the German or Dutch Patriot battery systems connected to the Nato network.
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Missile defence has been a point of tension between Nato and Russia in the past, but President Medvedev's presence at the summit is being seen as a strengthening of ties.
Nato's new "strategic concept" is the third revision that the organisation's mission statement has undergone since the Berlin Wall came down. Hence Secretary General Anders Fogh Rasmussen's quip about this being "Nato 3.0".
But is this a speedy new software version with added bells and whistles or an attempt to re-package an older product for very different market conditions?
Nato's much-vaunted new missions - specifically defence against cyber-warfare and ballistic missile attack - look a little bit like being a case of "Back to the Future" since they hark back in many ways to Nato's traditional role - the defence of its own territory and populations.
On crucial (and divisive) issues like the fate of Nato short-range or battlefield nuclear weapons in Europe, the new strategic concept offers only limited guidance.
No doubt a full-scale nuclear review will be needed to determine the role of these weapons - which many critics see as outdated - and to ascertain the broader circumstances in which they might be withdrawn.
"It offers a role for all of our allies. It responds to the threats of our times," Mr Obama said, according to the Reuters news agency.
"Tomorrow we look forward to working with Russia to build our co-operation with them in this area as well, recognising that we share many of the same threats," Mr Obama said.
Last month, President Medvedev said Russia was considering whether to join a missile shield.
In November 2009, President Obama announced the US was scrapping plans for a missile shield based in Poland and the Czech Republic which had infuriated Russia.
Nato Secretary General Anders Fogh Rasmussen and Mr Obama both called for the speedy ratification of the Start treaty, which Russia and the US signed in April.
The agreement would reduce both countries' nuclear arsenals and allow each to inspect the other's facilities.
Any delay "would be damaging to security in Europe," Mr Rasmussen said.
The treaty is currently before the US Congress, where some Republican lawmakers are resisting ratification, saying they need further reassurance about America's nuclear deterrent capability after Start.
The Obama administration currently needs eight Republican votes in the Senate to reach the 67 out of 100 needed to ratify the treaty.
But that number will increase to 14 when the new Congress convenes in January, as the Republicans won more Senate seats in the mid-term elections.
Afghan debate
On Saturday, the discussion will focus on Afghanistan, where President Obama has said Nato is "moving to a new phase".
Nato Secretary General Anders Fogh Rasmussen described the agreement as "an historic moment"
He said it was the beginning of "a transition to Afghan responsibility that begins in 2011 with Afghan forces taking the lead for security across Afghanistan by 2014".
Afghan President Hamid Karzai, who is scheduled to address the summit on Saturday, has said he wants Nato to hand back control of the country's security by the end of 2014 - a deadline the US has described as realistic but not set in stone.
The BBC's Paul Wood in Kabul says the last thing Nato wants is for its military campaign to end with parts of Afghanistan in the hands of warlords and an opium mafia.
Nato generals do not use the term "victory" any more.
But our correspondent says if the violence is at a level that can be managed by Afghan forces, Nato will consider it has done its job - and the troops will start coming home.
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