Robert E. Wallis

An advanced synthesized ultra-stable oscillator for spacecraft applications

Current ultra-stable oscillator (USO) technology relies on highly precise quartz resonators that are selected based on the desired output frequency and stability. These constraints on the crystal specifications significantly increase the lead time and expense of each USO. Recent research and development efforts in USOs by The Johns Hopkins University Applied Physics Laboratory (JHU/APL) have focused on a frequency synthesized USO based on a standardized, fixed-frequency resonator. The result of these efforts is a synthesized USO that will provide a frequency reference for transponders and other on-board users on future space missions. The frequency reference is stable enough for radio-science and navigation applications (Allan deviation <1.5 /spl times/ 10/sup -13/ at /spl tau/ = 10 s), and is electronically adjustable to cover the entire deep-space communications band. This frequency agility allows in flight re-assignment of the transponder frequencies. The synthesized USO offers low mass and DC power consumption yet maintains world-class noise performance and frequency stability performance.

MESSENGER mission: first electronically steered antenna for deep space communications

The MESSENGER mission to orbit the planet Mercury poses significant design challenges for its deep space communication system. These challenges include a wide pointing range, tight packaging, and a high temperature environment. To meet these challenges, the spacecraft incorporates the first steerable phased array antenna flown for deep space communications. The invention of a method for achieving circular polarization in a high-temperature (+300/spl deg/C) environment has doubled the science return of the mission relative to its initially proposed implementation. Cross-strapping between the phased array antennas and solid state power amplifiers (SSPAs) enables both amplifiers to be turned on when sufficient power is available, enhancing the scientific return at the planet. A science return of 25 Gbits/year is achieved using only one SSPA in Mercury orbit. The science return increases to 100 Gbits/year if both SSPAs are used in the orbital phase.

Phased-array antenna system for the MESSENGER deep space mission

MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) is the latest NASA Discovery mission managed by The Johns Hopkins University Applied Physics Laboratory. MESSENGER is a mission to orbit the least explored terrestrial planet, Mercury. MESSENGERs high-data-rate downlink will use the first electronically scanned phased-array antenna for a deep-space telecommunication application. Measured results for the lightweight phased-array antenna and the high-efficiency solid state power amplifiers (SSPAs) are presented. Two functional amplifiers within the X-band SSPAs provide output power that is scalable from 11 to 15 watts without major design changes. Five different hybrid microcircuits, including high-efficiency HFET amplifiers and MMIC phase shifters have been developed for use within the SSPAs. A highly efficient packaging approach enables the integration of a large number of hybrid circuits into a SSPA with a mass less than 450 g. The hybrids and the hermetic package are generic and are suitable for a wide range of space applications beyond the MESSENGER program.

Implementation of an X-band phased-array subsystem in a deep space mission

The MESSENGER spacecraft, the first mission to the planet Mercury since 1975, will achieve Mercury orbit in 2011. The spacecraft uses two opposite-facing mission-enabling X-band (8.4 GHz) phased-array antennas to achieve high-rate downlink communications. The spacecraft orientation is constrained such that a preferred direction faces the Sun; rotation about the Sun-line is allowable. The main beam of each antenna is steerable in one dimension. These two degrees of freedom allow the main beam of the phased array to be pointed in any direction about the spacecraft. A novel system-level design requires many different subsystems of the spacecraft to interact together to achieve accurate beam-pointing, and thus, high-rate downlink data from Mercury to Earth

Spacecraft-level testing and verification of an X-band phased array

The MESSENGER spacecraft uses an X-band (8.4-GHz) phased array for high-rate downlink communications to meet mission data requirements yet still survive the extreme environment at the planet Mercury. To survive the solar intensity at the planet, the MESSENGER spacecraft uses a sunshade that must remain Sun-pointed; this restricts pointing of the spacecraft. The use of two phased-array antennas alleviates the need for a gimbaled high-gain dish. The RF signal is routed through on-board solid-state power amplifiers that control the phases of the signals fed to the phased arrays, thereby pointing without the need for any moving parts while maintaining a Sun-pointed attitude. Each phased array is composed of eight slotted waveguide sticks. This paper describes a method for a real-time, fast verification of the steering of the phased array during any phase of spacecraft-level testing (including thermal-vacuum) without the need to free radiate, which is specifically critical to a spacecraft during integration and test. This newly developed and implemented approach does not require near-field probing, in-line couplers, or extra flight mates and de-mates. Once the antennas are integrated onto the spacecraft, schedule constraints force the need for very quick verification methods. The technique described herein quickly samples the phase of the signal at each array element and, in conjunction with subsystem-level measurements, mathematically calculates the radiated antenna pattern. The phases within each array element are measured using innovative loop couplers that may simply be removed once testing is complete. These phases are combined using specifically designed software to calculate the far-field radiated pattern to verify pointing.

Advanced electronics developed for NASA's Mars Exploration Program

Six electronic technologies under development for the Mars Technology Program are discussed. The scope of this set ranges from electronic packaging to microprocessors to power supply to communication technologies. An overview (motivation, objective, approach) of these six projects is presented, along with a comparison to the state-of-art. The overarching goal is to develop and mature these technologies to a point where the Mars Exploration Program can consider their use in a specific flight mission.

Advances in Microwave Technology for the MESSENGER Mission to Mercury

The MESSENGER spacecraft, designed to orbit the planet Mercury, uses the first electronically scanned phased-array antenna for a deep-space telecommunication application. Two lightweight phased arrays, mounted on opposite sides of the spacecraft, provide the high-gain downlink coverage. Medium-gain antennas are used for uplink and downlink during cruise phase. The invention of a method for achieving circular polarization in a high-temperature (+300degC) environment has doubled the science return of the mission relative to the inherent linear polarization from a slotted waveguide array. Monolithic microwave integrated circuits and discrete heterostructure field effect transistors are integrated to provide the high-efficiency X-band solid-state power amplifier.