Rolex SA is a Swiss watchmaking manufacturer of high-quality, luxury wristwatches. Rolex watches are popularly regarded as status symbols and BusinessWeek magazine ranks Rolex No.71 on its 2007 annual list of the 100 most valuable global brands. Rolex is also the largest single luxury watch brand, producing about 2,000 watches per day, with estimated revenues of around US$3 billion (2003 figures). The first self-winding Rolex wristwatch was offered to the public in 1931 (so-called the "bubbleback" due to the large caseback), preceded to the market by Harwood which patented the design in 1923 and produced the first self-winding watch in 1928, powered by an internal mechanism that used the movement of the wearer's arm. This not only made watch-winding unnecessary, but kept the power from the mainspring more consistent resulting in more reliable time keeping. Rolex produced specific models suitable for the extremes of deep-sea diving, mountain climbing and aviation. Early sports models included the Rolex Submariner and the Rolex Oyster Perpetual Date Sea Dweller. The latter watch has a helium release valve, co-invented with Swiss watchmaker Doxa, to release helium gas build-up during decompression. The Explorer and Explorer II were developed specifically for explorers who would navigate rough terrain, such as the world famous Mount Everest expeditions. Another iconic model is the Rolex GMT Master, which was originally developed in 1954 at the request of Pan Am Airways to assist its pilots with the problem of crossing multiple time zones when on transcontinental flights (GMT standing for Greenwich Mean Time)