In 1999, the U.S. National Weather Service initiated a program to upgrade its existing network of upper air observation stations with a new generation system employing a GPS receiver with radiosondes transmitting in the 1680 MHz band. This system, called the Radiosonde Replacement System (RRS), is the first to use GPS with radiosondes transmitting in the 1680 MHz frequency band. Previously, only radio direction finding systems without GPS used this band. The new radiosonde measures pressure, temperature, humidity, and wind better than any radiosonde that is currently available. Development of a 1680 MHz GPS radiosonde with improved performance presented numerous design and testing challenges. The new radiosonde is described together with the approaches taken to meet the challenges.