A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended:GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains or under water.:A GPS signal contains three different bits of information - a pseudorandom code, ephemeris data and almanac data. This information identifies the satellite, its current status, date and time as well where it should be at any given time.:GPS refers to the U.S. Department of Defense's GNSS (Global Navigation Satellite System), also known as NAVSTAR. There are other GNSSs around the world including the Russian GLONASS, the Chinese Compass and the EU Galileo system.:The first GPS satellite was launched in 1978.:A full constellation of 24 satellites was achieved in 1994.:Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.:Transmitter power is only 50 watts or less.:It takes between 65 and 85 milliseconds for a signal to come from a GPS satellite to a receiver on earth.:World financial markets set their clocks to the highly accurate atomic clocks on board GPS satellites.