Robinson, 23, of Hewlett Close, Chippenham, followed his victim across a car park making lewd suggestions to her before launching the violent assault.
He pushed his victim to the ground and punched her hard to the face before pulling down her trousers and underwear.
And the attack only stopped when two vehicles entered the car park and their headlights illuminated Robinson astride the 36-year-old victim.
Lynne Henderson said the driver of one of the cars recognised the attacker and saw him running off doing up his flies as he went.
But when he was questioned by police the sex pest claimed he was a Good Samaritan coming to the rescue of the victim after seeing another man attack her.
Miss Henderson told Swindon Crown Court today how the incident took place at about 4.30am on Sunday December 28, 2008.
She said the victim was walking through the Bath Road car park in Chippenham from a shelter where she had been staying to look for work at one of the fruit stalls.
As she went across the dark, deserted car park Robinson approached and started to talk to her asking her to perform a sex act on him.
He then pulled her to the ground asking to have sex with her for nothing to which she said she wasn’t a prostitute.
Robinson held her down and calling her a whore and a tart landed a powerful blow to her face then removed some of her clothing.
Miss Henderson said two cars pulled into the car park and saw the two on the ground in their headlights and recognised Robinson as he fled, shouting abuse at the victim.
When he was questioned later that day he told police he heard screaming from the car park and went in to see a man standing over a woman.
The man had his trousers down, he said, so he kicked him causing him to run off and was crouching over the victim when the cars arrived startling him.
Robinson was charged with attempted rape, which he denied.
But on the morning of his trial he pleaded guilty to sexual assault before a jury could be sworn in.
Jailing him for a year judge Euan Ambrose said “This was a serious sexual assault. It was on a woman alone in the middle of the night.
“She was subjected to violence and she was a vulnerable person; she is a homeless person.”
He also told him that he must register as a sex offender for ten years as a result of the conviction and sentence.