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US President Donald Trump will touch down in the UK on Tuesday for a Nato summit - the second visit he has made to Britain this year. What will the security operation involve and what hardware and staff will the president bring with him?

Whenever the US president arrives in the UK, a multi-million-pound security operation is brought into action.

Mr Trump's three-day state visit in June, which involved more than 6,300 officers, cost the Metropolitan Police £3.4m, according to figures released under the Freedom of Information Act. A previous four-day working visit in 2018 cost more than £14.2m.

Here are some of the incredible vehicles and entourage the president could be bringing with him this time around.

The president is likely to arrive in the UK on his customised, high-spec aircraft Air Force One.

Air Force One isn't actually a specific plane but instead refers to one of two specially adapted Boeing 747-200B series aircraft, which carry the tail codes 28000 and 29000.

Whichever of these aircraft the president is on board is referred to as Air Force One.

With its advanced avionics and defences, Air Force One is classed as a military aircraft, designed to withstand an air attack.

It can jam enemy radar and eject flares to throw heat-seeking missiles off course.

It is also capable of refuelling midair, allowing it to fly for an unlimited time - crucial in an emergency.

Air Force One is also equipped with secure communications equipment, allowing the aircraft to function as a mobile command centre.

There are 85 onboard telephones, a collection of two-way radios and computer connections.

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Inside, the president and his travel companions enjoy 4,000 sq ft of floor space on three levels, including an extensive suite for the president, a medical facility with an operating table, a conference and dining room, two food preparation galleys that can feed 100 people at a time, and designated areas for the press, VIPs, security and secretarial staff.

Several cargo planes, including C-17 Globemaster transport aircraft, carry the president's fleet of armoured vehicles and helicopters, usually landing in advance of his arrival.

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According to the Washington Post, the president is always accompanied by a military aide carrying an emergency satchel known as the "football", which contains the "gold codes" for launching the country's nuclear weapons and options for their use.

The military aide must be nearby the president at all times, as the commander-in-chief is in possession of personal identification codes required to order a strike.

They are carried on a plastic card known as the "biscuit", which can be read only when its opaque plastic covering is snapped in two and removed.

The presidential motorcade, which includes two identical limousines and other security and communications vehicles, are transported ahead of the president by United States Air Force transport aircraft.

On the ground, the president travels in Cadillac One - a bullish, enhanced limousine dubbed the "Beast" for obvious reasons.

The spare, decoy vehicle that accompanies it has the same Washington DC licence plates - 800-002.

President Trump's generation of presidential car debuted in 2018 - with the US Secret Service tweeting ahead of the UN General Assembly that it was "ready to roll".

But the service and vehicle's designers at General Motors have remained tight-lipped about the vehicle's special security features.

Weighing in at about nine tonnes (20,000lb) - with an armour-plated body and bulletproof windows (which don't all open) - the car is reported to have tear gas grenade launchers, night vision cameras and a built-in satellite phone.

Reinforced tyres surround steel-rimmed wheels, which mean the car can still be driven if the tyres are flat.

The passenger cabin is said to be sealed, to fend off a chemical attack, while special foam would surround the fuel tank in case of impact.

The vehicle also has extensive electronic equipment, Reuters reports.

The car can hold at least seven people and has a wide range of medical supplies on board, including - NBC News suggests - a fridge full of blood matching the president's blood type, in case of emergency.

But that's not all.

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When the president's on the move - you know about it.

Other vehicles in the cavalcade include a parade of police outriders, secret service backup vehicles, counter-assault and hazardous attack teams, an armoured SUV communications vehicle, known as Roadrunner, medics and the press corps.

The president could also bring a fleet of helicopters with him to the UK.

Among them Marine One, which, like Air Force One, isn't a specific aircraft but instead refers to any US Marine Corps aircraft carrying the president.

However, Marine One usually refers to one of the president's large Sikorsky VH-3D Sea Kings or the newer, smaller VH-60N White Hawks.

The specially adapted helicopters are known as "white tops" because of their livery and are fitted with communications equipment, anti-missile defences and hardened hulls.

It was Sea King versions that met the president at Stansted Airport and carried him to London, accompanied by tandem rotor chinook aircraft.

As a security measure, Marine One often flies in a group of identical helicopters acting as decoys.

It is also usually accompanied by two or three Osprey MV-22 escort aircraft, referred to as "green tops".

These tilt-rotor aircraft carry support staff, special forces and secret service agents, who are tasked with dealing with any mid-flight emergency.

The Ospreys, capable of vertical landings and high-speed flight, were heard circling around London during President Trump's last visit to the UK in 2018.

Staff are also transported around in CH-46s Sea Knight helicopters.

British forces' aircraft are also likely to be part of the security operation during his visit.

Secret service and special forces

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Some estimates put the number of people in Mr Trump's entourage for his UK visit in 2018 at 1,000, including more than 150 US secret service agents.

Staff included military communications specialists, White House aides, a doctor, a chef and members of the media.

Some 750 rooms were booked out to accommodate his entourage, according to Matt Chorley, of the Times newspaper.

For his 2019 state visit, the president was reported to have booked a floor of the Corinthia Hotel in Westminster for his family and entourage.

This time around Mr Trump will be in London and Hertfordshire between 2 and 4 December for the Nato summit.

He will also attend a reception at Buckingham Palace on 3 December, which will be hosted by the Queen.

By Lucy Rodgers, Dominic Bailey, Gerry Fletcher, Sandra Rodriguez Chillida and Irene de la Torre Arenas.