The MESSENGER spacecraft launched toward Mercury last summer made deep space communications history by taking with it electronically-steered phased array antennas that will allow scientists to send back twice as much data about the planet than originally envisioned.
The development of MESSENGER's antenna system at The Johns Hopkins University Applied Physics Laboratory (APL) not only demonstrates engineering prowess, but it also illustrates what can happen when teams of varying expertise put their heads together to develop and test a unique, mission-critical system.
"This was a major engineering achievement by the APL Space Department," says Bob Bokulic, of the Space Department's Radio Frequency Engineering Group, who served as the lead engineer for MESSENGER's communication system during the developmental phases of the program. "But we had considerable help from other departments, including Technical Services and Air Defense."
Communicating from Mercury
MESSENGER - which stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging - posed significant challenges for communicating information from deep space.
The spacecraft has to keep a protective shade oriented toward the Sun, creating the need to steer a high-gain antenna beam in any direction. Mercury's proximity to the Sun also made it necessary to use materials able to withstand temperatures up to 300 degrees Celsius, or 572 degrees Fahrenheit. (Bokulic says previous antennas built for APL spacecraft have operated only to 100 C).
These challenges, plus packaging constraints and a strong desire to eliminate moving parts, led engineers to implement an electronically steered system.
MESSENGER's phased array antenna grew from technology APL developed for what is now known as the Missile Defense Agency. In the mid-1990s, the Space Department developed an antenna and its driving solid-state circuitry for MDA. That project was eventually cancelled, but the technical work significantly influenced the Lab's MESSENGER proposal.
Development and Testing
Once the antenna design was selected, much work was still needed to develop it for use in deep space. An early challenge involved matching the linearly polarized signal of the antenna with the circularly polarized design of NASA's Deep Space Network antennas.
"The mismatch in the original MESSENGER antenna meant that we would lose half of the received signal power and achieve only half of the possible science return," says Bokulic.
Bob Stilwell, an antenna design and analysis specialist, invented a key technique for achieving circular polarization from the slotted waveguide elements used in the phased array, which doubled the science return of the mission relative to its original implementation.
Engineers then developed a high-temperature soldering process to assemble the antenna. "We had to find a way to solder on these pieces known as parasitic monopoles in such a precise way, at a high temperature, without melting the materials," explains Bob Wallis, the lead engineer for development of the flight phased array system.
Wallis worked with mechanical engineers Mike Rooney and Andy Lennon of the Technical Services Department to get the job done.
"We decided that the best fabrication approach would be a furnace brazing operation," says Lennon. "The Lab didn't have an oven long enough to handle the waveguides, so we cobbled together our own tube furnace from an aluminum tube, some heater wire and some scavenged controller parts. We nearly melted the tube while tuning the power controller, but within just a few weeks we built a reliable heat-treating facility."
Amplifying the Signal
Sheng Cheng led the team that built the solid-state power amplifiers for the phased array antenna, which amplify and steer the microwave communication signals sent from MESSENGER to Earth.
Cheng worked with other engineers in the Space, Technical Services and Air Defense Systems departments to develop custom microwave circuits that enabled the small size of the amplifier. More than 200 of these circuits were manufactured to ensure enough parts for the flight hardware in an effort described as "Herculean" by Tom Krimigis, head emeritus of the Space Department and a co-investigator on the MESSENGER science team.
The phased array system still had to be tested in a Mercury-like environment before being integrated onto the spacecraft. Jack Ercol, lead thermal engineer for MESSENGER, tested the antenna using intense heat lamps at the NASA Glenn Research Center in Sandusky, Ohio.
Finally, the antenna was integrated with the spacecraft. "We had to find a way to test the steering of the phased array on the spacecraft without radiating into free space and causing a safety hazard," says Karl Fielhauer, lead engineer for the MESSENGER radio frequency communication system. Small "pick-up" antennas were inserted into each waveguide element to sample the phase of the radio frequency signal and enable the steering direction to be determined mathematically.
"This technique worked like a charm," says Fielhauer, "and the performance
of the antenna in flight is very close to that predicted from our pre-launch measurements."
MESSENGER Project Manager Dave Grant says completion of the phased array communications system was a remarkable engineering feat.
"It is all the more noteworthy that this development was carried out as part of a NASA flight program, with its associated cost and schedule pressures," he says. "And the system's in-flight performance has been outstanding."