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.