When the President of the United States is travelling in a highly protected motorcade, the convoy must maintain secure communications with the White House and Pentagon – the US military's headquarters in Washington DC.
The President gets to enjoy the comforts of Cadillac One – codenamed 'Stagecoach' by the Secret Service, or 'The Beast' by outsiders – which is essentially a tank with a limousine interior, Cadillac styling, and a Chevrolet pick-up chassis, with a long trail of vehicles following.
Look towards the back of the motorcade and you'll often spy a black Ford F-350 Super Duty sporting a modified six-door SUV body and a large satellite dome antenna on the roof.
Codenamed 'Roadrunner' by the Secret Service, this vehicle is the White House Communications Agency's (WHCA) way of ensuring a secure connection between the President and their military commanders.
Created by company The Armoured Group, Roadrunner has what's formally known as a 'Sensitive Compartmentalised Information Facility' built into the rear of the vehicle, referred to as a 'skiff' or 'vault'. Or to put it another way, it's a very private phone booth with a trusted signals operative at the helm.
Much in the same way a home modem routes the internet to a laptop over wifi, Roadrunner uses powerful military-grade technology to securely connect Stagecoach with the Pentagon and White House.
It acts as a mobile command and control vehicle, providing encrypted phone, radio, and video services via a private network of defence satellites, as well as providing radio frequency, electromagnetic, and heat countermeasures.
Theoretically, a nuclear attack could be ordered from the President and coordinated using Roadrunner.
It's more than a modem on wheels, with the ability to detect incoming projectiles and unmanned aerial drones thanks to its highly sophisticated radar systems.
Roadrunner also has a number of traditional countermeasures built into it, with run-flat tyres and armour to protect it against attack.
Coming into service in 2018, the Ford version replaced the previous Chevrolet Suburban used for many years. However, the WHCA has been known to use anything from plain white vans to motorhomes for the same job ever since the vehicle was added to the Presidential motorcade in the 1970s.