Real Time Control (RTC) for the Space Interferometry Mission will build on the real time core interferometer control technology under development at JPL since the mid 1990s, with heritage from the ground based MKII and Palomar Testbed Interferometer projects developed in the late '80s and early '90s. The core software and electronics technology for SIM interferometer real time control is successfully operating on several SIM technology demonstration testbeds,
including the Real-time Interferometer Control System Testbed, System Testbed-3, and the Microarcsecond Metrology testbed. This paper provides an overview of the architecture, design, integration, and test of the SIM flight interferometer real time control to meet challenging flight system requirements for the high processor throughput, low-latency interconnect, and precise synchronization to support microarcsecond-level astrometric measurements for greater than five years at 1 AU in Earth-trailing orbit. The electronics and software architecture of the interferometer real time control core and its adaptation to a flight design concept are described. Control loops for pointing and pathlength control within each of four
flight interferometers and for coordination of control and data across interferometers are illustrated. The nature of onboard
data processing to fit average downlink rates while retaining post-processed astrometric measurement precision and accuracy is also addressed. Interferometer flight software will be developed using a software simulation environment incorporating models of the
metrology and starlight sensors and actuators to close the real time control loops. RTC flight software and instrument flight electronics will in turn be integrated utilizing the same simulation architecture for metrology and starlight component models to close real time control loops and verify RTC functionality and performance prior to delivery to flight interferometer system integration at Lockheed Martin's Sunnyvale facility. A description is provided of the test environment architecture supporting the RTC path to flight.
One of the most critical technology requirements for the Space Interferometry Mission is that the difference in pathlength traveled by the starlight through each arm of the instrument be known with picometers of precision. SIM accomplishes this by using an internal laser metrology system to measure the optical path traveled by the starlight. The SIM technology program has previously demonstrated laser gauges with measurement accuracy below 10 picometers. The next challenge is to integrate one of these gauges into a full interferometer system and demonstrate that the system still operates at the required level. For SIM, the ultimate requirement is that the internal metrology system be able to give an accurate measure of the starlight internal path difference to about 150 picometers over its narrow-angle field, with a goal of 50 picometer accuracy. This accuracy must be maintained even as SIM's various active systems articulate the SIM optics and vary the SIM internal pathlengths.
The Microarcsecond Metrology Testbed (MAM) is a full single-baseline interferometer coupled with a precision pseudostar, intended to demonstrate the level of agreement between starlight and metrology phase measurements needed to make microarcsecond-level measurements of stellar positions. MAM has been under development for several years and is now producing picometers-level consistency that translates into microarcseconds-level performance. This paper will present an overview of the MAM Testbed, together with recent results targeting the 150 picometer performance level required by SIM.
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