The Johns Hopkins Gazette: October 12, 1998
Oct. 12, 1998
VOL. 28, NO. 7


APL's Small Antenna Points The Way To Deep Space

By Pamela Houghtaling
Applied Physics Laboratory
Johns Hopkins Gazette Online Edition

APL's Space Department has demonstrated that it can reach deep space with a small-aperture antenna, setting the stage for low-cost monitoring of future missions from the laboratory. Currently, the tracking of spacecraft in deep space requires the larger, more sensitive antennas of NASA's Deep Space Network, which are heavily utilized by the many deep-space missions. In July, however, the Space Department was able to observe the NEAR spacecraft signal using APL's less costly 10-meter antenna.

Kate Flaherty, who led the Space Department effort, and Glen Baer, manager of APL's Satellite Communications Facility, worked with the Jet Propulsion Laboratory to get accurate predictions of NEAR's position and downlink frequencies. Henry DeWitt modified the software at the SCF so that its 10-meter antenna could be pointed at NEAR. The SCF 10-meter antenna is normally used to track the Earth-orbiting MSX satellite. Tim Nalepa, Pat Moon and summer intern Alicia Singer worked with Flaherty to do the actual monitoring of NEAR.

Kate Flaherty and summer intern Alicia Singer discuss the tracking of NEAR at APL's ground station.

NEAR's location was approximately one astronomical unit from the APL ground station--the distance from the Earth to the sun. The spacecraft's high gain antenna was pointed toward the Earth for its regularly scheduled communication with the DSN.

As Flaherty explains, it was difficult to pinpoint such a low-level signal with a small-aperture antenna. However, using a spectrum analyzer and JPL's frequency predictions, the APL team was able to match the right frequency, power level and rate of frequency change that clearly identified NEAR.

APL's ability to observe NEAR's weak signal is a milestone in the advancement of a new approach to spacecraft monitoring known as beacon monitor technology. Developed by JPL, this technology is aimed at reducing the cost of mission operations by shifting the routine monitoring of spacecraft away from the larger DSN antennas to smaller-aperture antennas.

A future "smart" spacecraft equipped with an automated onboard monitoring system could evaluate its own condition and then relay the information down to a ground station through one of four frequency tones. Ranging from normal to emergency, the tone frequencies would indicate whether communications with the DSN are required to transmit more detailed data on the spacecraft's condition and the urgency of the need.

Flaherty points out that it's expensive to use the larger antennas, especially to collect the spacecraft's health data. "It's overkill in a sense," she says. "In reality, you don't need much information for the day-to-day monitoring of a deep space probe. While the 10-meter antenna can't receive a lot of data, it can receive a simple health message."