While an ordinary global positioning system (GPS) receiver uses information carried by radio signals from a constellation of GPS satellites to calculate its position, NASA's BlackJack GPS receiver looks at the radio waves themselves. By making precise measurements of how the signals are distorted or delayed along their way, the BlackJack provides a new way to study Earth's gravity field and atmosphere.

The first of these experimental receivers is flying on a German scientific satellite, Challenging Minisatellite Payload (CHAMP), launched in July. The second is onboard an Argentine satellite called SAC-C, launched on November 21. Both instruments were designed and built at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

"Like any other GPS receiver, the BlackJack can calculate where it is in space and time, but comparing the BlackJack to an ordinary GPS receiver is like comparing a home camcorder to a professional studio camera," said JPL engineer Thomas Yunck, manager of the GPS Observatories Office. While a typical GPS receiver can determine its position to about 20 meters (22 yards), the BlackJack can pinpoint the position of its host satellite continuously with an accuracy of 2 to 3 centimeters (.8 to 1.1 inch). This flight data can be used to improve computer models of the Earth's gravitational field, which in turn can help reveal different Earth properties, including the structure and evolution of the deep interior, the movement of surface ice and atmospheric mass, and ocean circulation.

In a more dramatic departure from conventional GPS use, BlackJack receivers track the radio signals slicing through the earth's atmosphere as the GPS satellites appear to rise and set. This information will help scientists construct detailed images of the ionosphere and will provide precise profiles of atmospheric density, pressure, temperature, and moisture for climate studies and weather prediction. The BlackJacks are also equipped with small down-looking antennas to attempt to receive GPS signals that reflect off the oceans. This highly experimental technique could one day be used to map the subtle variations of the ocean topography, derive ocean circulation patterns, sense the roughness or state of the ocean surface and estimate surface winds.

The SAC-C and CHAMP experiments are early attempts to assess the engineering feasibility of such measurements. Both experiments are designed to last about 3 years.

JPL is a division of the California Institute of Technology in Pasadena.

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