BEIJING, Nov. 27 -- The three satellites launched this year for China's indigenous satellite navigation system are sending twice as many signals as their predecessors, said the system's designer, after completing tests on the new units.

The 18th and 19th satellites for the Beidou Navigation Satellite System (BDS), which is being developed as an alternative to U.S.-operated GPS, were sent into space on July 26, and the 20th on Sept. 30.

While they are less than half the weight of earlier generations, the new satellites' output is greater, matching the best around the world, said the China Academy of Space Technology in its latest newsletter.

After tests of their orbits and key technology, they are working as intended and in all weather, according to the academy.

The 18th and 19th BDS satellites are the first that can communicate with each other, helping with distance measurements, said Wang Ping, chief engineer on the project.

China began to build the BDS in 1994, two decades after the United States developed GPS. China plans to complete a constellation of 35 satellites, achieving global coverage, by 2020.