watches in North America remained robust. Eco-Drive models were well received, generating a third of Citizen's North American revenues by 2000. In 2002 the VITRO technology (Eco-Drive VITRO) came on the market, where the solar cells were no longer even slightly visible under the dial. During the mid-2000s, wristwatch sales improved for Citizen thanks to further development of the Eco-Drive line and integration of radio-controlled timing with the 2002 Eco-Drive line. Later specialized tool watch designs were introduced like the Promaster Eco-Drive Professional Diver 1000M Titanium BN7020-09E in 2017. This is a very large watch, the titanium watch case has a diameter of 52.2 mm and thickness of 22 mm and features a helium release valve, designed for mixed gas saturation diving at great depths. Eco-Drive Concept Models Since 2009, Citizen has developed Eco-Drive Concept Models as technology demonstration and marketing tools. These Eco-Drive Concept Models are generally shown at exhibitions and produced in limited editions. The Concept Model 2011 was the Eco-Drive SATELLITE WAVE that has a movement that can receive time synchronization signals from GPS satellites. This makes radio-controlled timing possible in remote areas that are not serviced by land based radio time signal stations. In 2012 Citizen announced the Eco-Drive RING Concept Model. This watch features a ring-shaped solar cell surrounding the watch case sidewall. In 2018 Citizen announced it developed the Caliber 0100 Eco-Drive prototype autonomous high-accuracy quartz watch movement which is claimed to be accurate to ± 1 second per year. Key elements to obtain the high claimed accuracy are applying a for a watch unusual shaped (AT-cut) quartz crystal operated at 223 or frequency, thermal compensation and hand selecting pre-aged crystals. Besides that AT-cut variations allow for greater temperature tolerances, specifically in the range of , they exhibit reduced deviations caused by gravitational orientation changes. As a result, errors caused by spatial orientation and positioning become less of a concern. The Caliber 0100 movement in 2018 was not available for sale to the public, the technology inside the movement will be introduced in future models. In March 2019 three limited edition wrist watch models with the Caliber 0100 movement were announced to become available for sale with deliveries expected around the 2019 fall. Recent history According to Citizen, by 2011 80% of their wristwatches featured Eco-Drive, and the company saw Eco-Drive type watches as the focus of new generations of watches. In 2012 Citizen