Terrestrial optical interferometers have produced the first resolved images of main-sequence stellar surfaces by using long co-phased baselines and correcting for atmospheric errors using fringe tracking and closure phases. These stellar surface images are helpful for understanding the formation, dynamical structure, and evolution of stars, and also for evaluating the potential habitability of exoplanetary systems. However, the astronomical targets suitable for ground based interferometric imaging remain limited due to restricted baseline diversity, atmospheric absorption in the ultraviolet and much of the infrared, and shot noise limits on the fringe tracker when observing faint objects through tropospheric turbulence. A spaceborne formation flying optical interferometer could potentially image dimmer targets with longer baselines, but must still contend with disturbances like propulsive stationkeeping and attitude control system noise. This work simulates on-orbit fringe tracking controller performance under representative disturbances for a small three-spacecraft Michelson interferometer, and evaluates the effects of imperfect fringe tracking on the measured visibilities. We investigate fringe tracker design options, including the choice of optical path delay estimator(s), number of spectral channels, and integration time. Assuming 10cm subapertures, we compute limiting target magnitudes and the maximum baseline beyond which noise dominates the interferometric visibility measurements. Based on this analysis, we identify design parameters for a cost effective demonstration mission that could complement ground-based stellar surface imaging capabilities.
Astronomical space interferometers have the potential to achieve milliarcsecond resolution via formation-flying collectors hundreds of meters apart. The collectors’ role is to transfer the starlight beams to a combiner that coherently interferes them. One challenge is controlling optical distances within a fraction of a wavelength while maneuvering the spacecraft to maintain their relative position. Since measuring relative position is much easier than controlling it, we propose long compact delay lines that significantly relax formation flying requirements. We present a proof-of-concept demonstrating an optical 4-m free-space delay in the lab. The delay line utilizes four high-reflectance mirrors in a configuration that fits within a 10cm x 20cm footprint suitable for a CubeSat. We also describe a visible-laser metrology approach that controls the optical path across the 4-m range. The delay line and metrology system would be part of the combiner spacecraft. Such an arrangement will not only relax the relative positioning requirements but also enable a two-spacecraft (total) interferometer that would make a technology demonstration mission more feasible in the near future.
Space interferometers could, in principle, exploit the relatively stable space environment and ease of baseline reconfiguration to collect measurements beyond the limitations of ground-based interferometers. In particular, a two-element interferometer could provide excellent uv-plane coverage over a few tens of low Earth orbits. One of the challenges for free-flying interferometers is controlling the optical path distance with subwavelength accuracies despite the collectors flying up to hundreds of meters apart. We consider two approaches: an artificial in-orbit laser guide star (LGS) that provides a phase reference for the space interferometer and fringe tracking on the science target itself. The two approaches (LGS versus no LGS) would require different image processing techniques. In this work, we explore image processing with LGS phase residuals due to global positioning system (GPS) uncertainties. We use GPS uncertainties from the Gravity Recovery and Climate Experiment Follow-On mission to simulate image retrieval with a 300-m baseline laser-guided space interferometer. This is done by fitting the slowly varying phase errors of complex visibility measurements. We also consider a 40-m baseline interferometer with visibility(-modulus)-only measurements. In this case, we simulate the bias in visibility due to fringe tracking in the presence of parasitic forces acting on the spacecraft. We then use a modified version of the hybrid input–output phase retrieval algorithm for image reconstruction. We conclude that under our optimistic assumptions, both approaches could enable general imaging of a few large stars even with CubeSats, although an LGS would significantly improve the best resolution obtainable.
Space interferometers could, in principle, exploit the relatively stable space environment and ease of baseline reconfiguration to collect measurements beyond the limitations of ground-based interferometers. In particular, a two-element interferometer could provide excellent uv-plane coverage over a few tens of low-Earth orbits. One of the challenges for free-flying interferometers is controlling the optical path distance with sub-wavelength accuracies despite the collectors flying up to hundreds of meters apart. This work considers two approaches: an artificial in-orbit laser guide star (LGS) that provides a phase reference for the space interferometer and fringe tracking on the science target itself. The two approaches (LGS vs. no LGS) would require different image processing techniques. In this work, we explore image processing with LGS phase residuals due to GPS uncertainties. We use GPS uncertainties from the GRACE-FO mission to simulate image retrieval with a 300 m baseline laser-guided space interferometer. This is done by fitting the slowly varying phase errors of complex visibility measurements. We also consider a 40 m baseline interferometer with visibility(-modulus)-only measurements. In this case, we simulate the bias in visibility due to fringe tracking in the presence of parasitic forces acting on the spacecraft. We then use a modified version of the Hybrid Input-Output phase retrieval algorithm for image reconstruction. We conclude that under our optimistic assumptions, both approaches could enable general imaging of a few large stars even with CubeSats, although an LGS would significantly improve the best resolution obtainable.
The Advanced Baseline Imager (ABI) is a passive imaging radiometer on-board National Oceanic and Atmospheric Administration’s (NOAA) Geostationary Operational Environmental Satellites-R (GOES-R) series. Its bands 7 to 16 are categorized as infrared (IR) bands, sampling within a spectral range of 3.9 to 13.3 μm in mid-wave infrared (MWIR) and long-wave infrared (LWIR) regions. ABI provides variable area imagery and radiometric information of Earth’s surface, atmosphere, and cloud cover. All of the IR bands are calibrated on-orbit in reference to an internal blackbody. While the ABI aboard the GOES-16 satellite has been working properly, an anomaly with GOES-17 ABI’s cooling system, specifically its loop heat pipe (LHP) subsystem, prevents heat from being efficiently transferred from the ABI electronics to the radiator to be dissipated into space. As a consequence, the heat accumulates inside the instrument, so the temperatures of its key components for IR calibration, including the focal plane modules (FPMs), scan mirrors, and blackbody, cannot be maintained at their designed operational levels. As an example, the temperatures of MWIR and LWIR FPMs, where IR detectors are located, are currently operated at a baseline temperature of ∼20 K warmer than the design and vary by as many as 27 K diurnally. This causes severe degradation to the data quality of ABI IR Level 1b radiance and subsequent Level 2+ products during the hot period of the day. Significant progress has been made to mitigate the effects of the LHP anomaly to optimize the IR performance of GOES-17 ABI. We summarize the efforts made by NOAA’s GOES-R Calibration Working Group, working collaboratively with other teams, to evaluate and alleviate the negative impacts of warmer and floating FPM temperatures on ABI IR calibration, and assess the IR performance accordingly.
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