cut oil production in 2019.
Gunners from the Royal Artillery brought Giraffe anti-aircraft radars but they did not deploy with missiles.
A defence source said the arrival of Starstreak anti-aircraft weapons would bring "a significant change in capability".
The missile launchers are mounted on all-terrain Alvis Stormers, 13-ton tracked light tanks which can reach 50mph.
A source said: “The Saudis want us there because they like the UK standing with them.”
Gunners from the Royal Artillery deployed "in the last few weeks" with two Stormer light tanks with mounted Starstreak missile launchers to shoot down Iranian drones.
Defence Secretary Ben Wallace said: "This is a purely defensive capability."
Up to six all-terrain Alvis Stormers and 60 personnel are due to deploy this year.
High-tech Starstreak missiles can hit targets the size of a tennis ball travelling at the speed of sound.
They zoom at over three times the speed of sound and and split into three darts to puncture armour before warheads detonate.
The Belfast-made missiles can be shoulder-launched or mounted on a vehicle.
They are not "fire-and-forget" missiles like some other anti-aircraft systems.
Instead they are guided to targets by an operator on the ground, which makes them lethally immune to counter measures.
Gunners from 16 Regiment Royal Artillery flew out to Riyadh in Feb 2020 on what was originally supposed to be a short term mission.
They will be replaced by soldiers from their sister regiment 12 Royal Artillery, based on Thorney Island, Hants.
At first the deployment was kept under wraps as Saudi ruler Mohammed bin Salman was accused of killing thousands of civilians in the war in neighbouring Yemen .
The CIA also said he ordered the murder of journalist Jamal Khashoggi who was cut up with bone saws in a Saudi consulate in Turkey.
In March Britain said it was sending Stormers and Starstreaks to help Ukrainian forces repel Russian invaders.
Dramatic footage showed them using a Starstreak to take down a Russian Mi-28N chopper over Luhansk in April.
Ukraine is now calling for more of those and other weapons to take on Putin's war machine.
And it wants the West to send modern long-range artillery as it is now heavily outgunned in the fight for the Donbas region.
Britain has given Ukraine identical Starstreak missiles which can hit objects the size of a tennis ball travelling at the speed of sound.
Terrified Russian pilots have refused to fly sorties ahead of their troops in Ukraine over fear of being shot down, western spooks have claimed.
Wallace said Britain had bought 20 heavy self-propelled artillery guns for Ukraine and long-promised multiple launch rocket launchers are due to arrive within days.
Britain has also sent Ukraine thousands of NLAW antitank missiles, hundreds of Javelin anti tank weapons , armoured vehicles
The Starstreak is the fastest surface-to-air missile in the world One was used to shoot down a Russian helicopter over Ukraine in April The missiles can be fired from a Stormer light tank Credit: Alamy
We pay for your stories! Do you have a story for The Sun news desk?