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

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.