In the mid-1970s, JPL evaluated several techniques for determining atmospheric water vapor effects on radiometric range. These experiments allowed the signals between spacecraft and the Deep Space Network antennas to be properly calibrated. One of the experiments was the Scanning Microwave Inversion Layer Experiment (SMILE). In May 1974, this test was conducted in El Monte, California, with a radiosonde suspended beneath a weather balloon. When the balloon reached 10,000 feet (about 3 km) it began measuring absolute pressure, ambient temperature, and relative humidity, then radioed the results to ground receivers.