April 28th 05, 11:33 PM
http://messenger.jhuapl.edu/news_room/press_release_4_27_05.html
MESSENGER Antenna Gains from Teamwork
April 27, 2005
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."
MESSENGER Antenna Gains from Teamwork
April 27, 2005
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."