Williams have unveiled the car they hope will revive their fortunes after the worst season in their history.
The team, who dominated Formula 1 for much of the 1980s and 1990s, have undergone a major restructure aimed at ending years of steady decline.
"We are off to a truly fresh start," said team principal Frank Williams. "We have a new car, new driver line-up, new engine and new senior personnel."
The FW34 car features a similar 'platypus' nose to many of its rivals.
It also features a notably tightly packaged rear end, in a bid to optimise aerodynamic performance, although this was also a feature of last year's uncompetitive car.
Chief operations engineer Mark Gillan, one of the new faces at a senior level on the technical front, said: "We are really looking forward to this season and from a personal perspective I can't wait to get the FW34 on to the race track.
"It is imperative that we demonstrate a continuous improvement in our performance and deliver results throughout the season."
"I'm very excited to be back in action with the Williams F1 Team in Jerez," said Senna. "This is my ever first pre-season testing programme since I started in Formula One two years ago.
"I'm really motivated and looking forward to working hard and with everyone in the team."
Williams have replaced their Cosworth engine with the Renault, which is also used by world champions Red Bull.
The team have great history with Renault - they won four drivers and five constructors' titles together in the 1990s, with great names such as Nigel Mansell, Alain Prost, Damon Hill and Jacques Villeneuve.
"The team has had a good winter at the factory," added Williams. "We feel ready and strong for the fight."