Optical Ground Station 1 (OGS1) is the first of a new breed of dedicated ground terminals to support NASA’s developing space-based optical communications infrastructure. It is based at NASA’s Optical Communications Telescope Laboratory (OCTL) at the Table Mountain Observatory near Wrightwood, CA. The system will serve as the primary ground station for NASA’s Laser Communications Relay Demonstration (LCRD) experiment. This paper presents an overview of the OCTL telescope facility, the OGS1 ground-based optical communications systems, and the networking and control infrastructure currently under development. The OGS1 laser safety systems and atmospheric monitoring systems are also briefly described.
A technology demonstration of free space optical communication at interplanetary distances is planned via one or more future NASA deep-space missions. Such demonstrations will "pave the way" for operational use of optical communications on future robotic/potential Human missions. Hence, the Deep Space Network architecture will need to evolve. Preliminary attempts to model the anticipated future mission set and simulate how well it loads onto assumed architectures with combinations of RF and optical apertures have been evaluated. This paper discusses the results of preliminary loading simulations for hybrid RF-optical network architectures and highlights key mission and ground infrastructure considerations that emerge.
The SIM-Lite astrometric interferometer will search for Earth-size planets in the habitable zones of nearby stars. In this
search the interferometer will monitor the astrometric position of candidate stars relative to nearby reference stars over
the course of a 5 year mission. The elemental measurement is the angle between a target star and a reference star. This is
a two-step process, in which the interferometer will each time need to use its controllable optics to align the starlight in
the two arms with each other and with the metrology beams. The sensor for this alignment is an angle tracking CCD
camera. Various constraints in the design of the camera subject it to systematic alignment errors when observing a star of
one spectrum compared with a start of a different spectrum. This effect is called a Color Dependent Centroid Shift
(CDCS) and has been studied extensively with SIM-Lite's SCDU testbed. Here we describe results from the simulation
and testing of this error in the SCDU testbed, as well as effective ways that it can be reduced to acceptable levels.
In the course of fulfilling its mandate, the Spectral Calibration Development Unit (SCDU) testbed for SIM-Lite produces
copious amounts of raw data. To effectively spend time attempting to understand the science driving the data, the team
devised computerized automations to limit the time spent bringing the testbed to a healthy state and commanding it,
and instead focus on analyzing the processed results. We developed a multi-layered scripting language that emphasized
the scientific experiments we conducted, which drastically shortened our experiment scripts, improved their readability,
and all-but-eliminated testbed operator errors. In addition to scientific experiment functions, we also developed a set of
automated alignments that bring the testbed up to a well-aligned state with little more than the push of a button. These
scripts were written in the scripting language, and in Matlab via an interface library, allowing all members of the team to
augment the existing scripting language with complex analysis scripts. To keep track of these results, we created an easilyparseable
state log in which we logged both the state of the testbed and relevant metadata. Finally, we designed a distributed
processing system that allowed us to farm lengthy analyses to a collection of client computers which reported their results
in a central log. Since these logs were parseable, we wrote query scripts that gave us an effortless way to compare results
collected under different conditions. This paper serves as a case-study, detailing the motivating requirements for the
decisions we made and explaining the implementation process.
The most stringent astrometric performance requirements on NASA's SIM(Space Interferometer
Mission)-Lite mission will come from the so-called Narrow-Angle (NA) observing scenario,
aimed at finding Earth-like exoplanets, where the interferometer chops between the target star
and several nearby reference stars multiple times over the course of a single visit. Previously,
about 20 pm NA error with various shifts was reported1. Since then, investigation has been under
way to understand the mechanisms that give rise to these shifts. In this paper we report our
findings, the adopted mitigation strategies, and the resulting testbed performance.
SIM-Lite missions will perform astrometry at microarcsecond accuracy using star light interferometry. For typical
baselines that are shorter than 10 meters, this requires to measure optical path difference (OPD) accurate to tens of
picometers calling for highly accurate calibration. A major challenge is to calibrate the star spectral dependency
in fringe measurements - the spectral calibration. Previously, we have developed a spectral calibration and
estimation scheme achieving picometer level accuracy. In this paper, we present the improvements regarding the
application of this scheme from sensitivity studies. Data from the SIM Spectral Calibration Development Unit
(SCDU) test facility shows that the fringe OPD is very sensitive to pointings of both beams from the two arms of
the interferometer. This sensitivity coupled with a systematic pointing error provides a mechanism to explain the
bias changes in 2007. Improving system alignment can effectively reduce this sensitivity and thus errors due to
pointing errors. Modeling this sensitivity can lead to further improvement in data processing. We then investigate
the sensitivity to a model parameter, the bandwidth used in the fringe model, which presents an interesting trade
between systematic and random errors. Finally we show the mitigation of calibration errors due to system drifts
by interpolating instrument calibrations. These improvements enable us to use SCDU data to demonstrate that SIM-Lite missions can meet the 1pm noise floor requirement for detecting earth-like exoplanets.
SIM Lite is a space-borne stellar interferometer capable of searching for Earth-size planets in the habitable zones of
nearby stars. This search will require measurement of astrometric angles with sub micro-arcsecond accuracy and optical
pathlength differences to 1 picometer by the end of the five-year mission. One of the most significant technical risks in
achieving this level of accuracy is from systematic errors that arise from spectral differences between candidate stars and
nearby reference stars. The Spectral Calibration Development Unit (SCDU), in operation since 2007, has been used to
explore this effect and demonstrate performance meeting SIM goals. In this paper we present the status of this testbed
and recent results.
Optical links from a spacecraft at planetary distance to a ground-based receiver presume a cloud free line of site (CFLOS). Future ground-based optical receiving networks, should they be implemented, will rely on site diversity of cloud cover to increase link availability. Recent analysis shows that at least 90% and as high as 96% CFLOS availability can be realized from a cluster comprised of 3-4 nodes. During CFLOS availability variations of atmospheric parameters such as attenuation, sky radiance and “seeing” will determine the link performance. However, it is the statistical distributions of these parameters at any given node that will ultimately determine the data volumes that can be realized. This involves a complex interaction of site-specific atmospheric parameters. In the present work a simplified approach toward addressing this problem is presented. The worst-case link conditions for a spacecraft orbiting Mars, namely, maximum range (2.38 AU) and minimum sun-Earth-probe (SEP) angle of 3-10° is considered. A lower bound of ~100 Gbits/day under the most stressing link conditions is estimated possible.