Spacecraft carrying optical communication lasers can be treated as artificial stars, whose relative astrometry to Gaia reference stars provides spacecraft positions in the plane-of-sky for optical navigation. To be comparable to current Deep Space Network delta-Differential One-way Ranging measurements, thus sufficient for navigation, nanoradian optical astrometry is required. Here we describe our error budget, techniques for achieving nanoradian level ground-base astrometry, and preliminary results from a 1 m telescope. We discuss also how these spacecraft may serve as artificial reference stars for adaptive optics, high precision astrometry to detect exoplanets, and tying reference frames defined by radio and optical measurements.
The SIM Lite Astrometric Observatory is to perform narrow angle astrometry to search for Earth-like planets, and global
astrometry for a broad astrophysics program, for example, mapping the distribution of dark matter in the Galaxy. The
new SIM Lite consists of two Michelson interferometers and one star tracking telescope. The main six-meter baseline
science interferometer observes a target star and a set of reference stars. The four-meter baseline interferometer (guide-1)
monitors the attitude of the instrument in the direction of a target star. The Guide-2 telescope (G2T) tracks a bright star
to monitor the attitude of the instrument in the other two orthogonal directions. A testbed has been built to demonstrate
star-tracking capability of the G2T concept using a new interferometric angle metrology system. In the presence of
simulated 0.2 arcsecond level of expected spacecraft attitude control system perturbations, the measured star-tracking
capability of the G2T testbed system is less than 43 micro-arcsecond during single narrow angle observation.
The Space Interferometry Mission Light (SIM-Lite) is a new mission concept to perform a micro-arcsecond narrow-angle
astrometry to search approximately 50 nearby stars for Earth-like planets, and to perform a global astrometry with
an accuracy of six micro-arcsecond position and parallax measurements. The SIM-Lite consists of two Michelson
interferometers and one telescope. The main six-meter baseline science interferometer observes a target star and a set of
reference stars. The four-meter baseline interferometer (guide-1) monitors the attitude of the instrument in the direction
of a target star. A Guide-2 telescope (G2T) tracks a bright star to monitor the attitude of the instrument in the other two
orthogonal directions. To demonstrate the concept of the G2T, we have developed a testbed using brassboard optics
built for the SIM project. The G2T testbed consists of a 35 cm siderostat, a beam compressor, and a fast steering mirror
(FSM) in closed loop with a CCD based pointing sensor. A heterodyne laser angle metrology system is used to monitor
angular positions of the FSM with required accuracy of 20 micro-arcsecond during SIM-Lite narrow-angle observation
time. We present the concept of the testbed architecture and preliminary test results of the angular metrology (aMet)
system.
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