Historical Photo of the Month - February 2008

Explorer Telemetering
Photograph Number D2000_0918_C1_2

In JPL's satellite data reduction center, telemetered data from Explorer I was printed out on graph paper and referred to as a Sanborn trace, named after the equipment that amplified and recorded the signal. These data were recorded February 2, 1958 at the Earthquake Valley tracking station -- one of four located around the world in California, Florida, Nigeria, and Singapore. The signal was received by the tracking stations and transmitted to JPL for processing.

This printout displays about 2 1/2 minutes worth of data. The left side of this image shows data recorded soon after the satellite first became detectable, so there is a lot of noise. As the satellite approached and its signal strengthened, the noise disappeared. The lines also show the tumbling motion the satellite assumed soon after injection, turning approximately once every seven seconds.

Each transmitter included four channels of data. In this printout from Explorer I's low power transmitter, channel two showed the spacecraft skin temperature, three was the nosecone temperature, four was data from the micrometeorite erosion gauges, and five indicated the cosmic ray count. The measurements made by instruments and sensors on the Explorer satellite were translated by circuits into varying tones for each of the four channels.

For more information about the history of Explorer, contact the JPL Archives for assistance. You can also check the Explorer web site for stories relating to the 50th anniversary of the Explorer launch. [Archival and other sources: Photo albums, photo indices, Dr. George Ludwig, Dr. Henry Richter, various Explorer publications.]