The Geostationary Lightning Mappers (GLM) for the Geostationary Operational Environmental Satellite (GOES) GOES-R series will, for the first time, provide hemispherical lightning information 24 hours a day from longitudes of 75 and 137 degrees west. The first GLM of a series of four is planned for launch in November, 2016. Observation of lightning patterns by GLM holds promise to improve tornado warning lead times to greater than 20 minutes while halving the present false alarm rates. In addition, GLM will improve airline traffic flow management, and provide climatology data allowing us to understand the Earth’s evolving climate.
The paper describes the method used for translating the pixel position of a lightning event to its corresponding geodetic longitude and latitude, using the J2000 attitude of the GLM mount frame reported by the spacecraft, the position of the spacecraft, and the alignment of the GLM coordinate frame relative to its mount frame. Because the latter alignment will experience seasonal variation, this alignment is determined daily using GLM background images collected over the previous 7 days. The process involves identification of coastlines in the background images and determination of the alignment change necessary to match the detected coastline with the coastline predicted using the GSHHS database.
Registration is achieved using a variation of the Lucas-Kanade algorithm where we added a dither and average technique to improve performance significantly. An innovative water mask technique was conceived to enable self-contained detection of clear coastline sections usable for registration. Extensive simulations using accurate visible images from GOES13 and GOES15 have been used to demonstrate the performance of the coastline registration method, the results of which are presented in the paper.
The geoCARB sensor uses a 4-channel slit-scan infrared imaging spectrometer to measure the absorption spectra of
sunlight reflected from the ground in narrow wavelength regions. The instrument, which is to be hosted on a
geostationary communication satellite, is designed to provide continual monitoring of greenhouse gas over continental
scales, several times per day, with a spatial resolution of a few kilometers. The paper discusses the image navigation and
registration (INR) of the geoCARB optical footprints on to the earth’s surface.
The instrument acquires data in a step and stare mode with 4.08 s stare time and 0.34s step time on 1016 footprints
spaced by 2.7 km at nadir in the NS direction along the slit, which is stepped in 3 km EW increments. Knowledge of the
instrument line of sight is obtained through use of a dual-head star tracker system (STS), high-precision optical encoders
for the scan mirrors, a GPS receiver, and a highly stable common optical bench to which the instrument components, the
scan mirror assembly, and the heads of the STS are kinematically mounted.
While attitude disturbances due to jitter and solar array flex affect spatial resolution, we show that the effect on INR is
negligible. GeoCARB performs a star sighting every 30 minutes to compensate for its diurnal alignment variation
relative to the STS, enabling a 1 sigma INR accuracy of 0.38 and 0.51 km at nadir in the NS and EW directions,
respectively. Coastline identification may be used to improve accuracy by 6%, while an additional 20% improvement is
achievable through identification of systematic errors via extensive post-processing. The paper quantifies all error
sources and describes how each of them affects overall INR accuracy.
The geoCARB sensor uses a 4-channel push broom slit-scan infrared imaging grating spectrometer to measure the absorption spectra of sunlight reflected from the ground in narrow wavelength regions. The instrument is designed for flight at geostationary orbit to provide mapping of greenhouse gases over continental scales, several times per day, with a spatial resolution of a few kilometers. The sensor provides multiple daily maps of column-averaged mixing ratios of CO2, CH4, and CO over the regions of interest, which enables flux determination at unprecedented time, space, and accuracy scales. The geoCARB sensor development is based on our experience in successful implementation of advanced space deployed optical instruments for remote sensing. A few recent examples include the Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) on the geostationary Solar Dynamics Observatory (SDO), the Space Based Infrared System (SBIRS GEO-1) and the Interface Region Imaging Spectrograph (IRIS), along with sensors under development, the Near Infared camera (NIRCam) for James Webb (JWST), and the Global Lightning Mapper (GLM) and Solar UltraViolet Imager (SUVI) for the GOES-R series. The Tropospheric Infrared Mapping Spectrometer (TIMS), developed in part through the NASA Instrument Incubator Program (IIP), provides an important part of the strong technological foundation for geoCARB. The paper discusses subsystem heritage and technology readiness levels for these subsystems. The system level flight technology readiness and methods used to determine this level are presented along with plans to enhance the level.
We have developed performance simulations for a precision attitude determination system using a focal plane star
tracker on an infrared space telescope. The telescope is being designed for the Destiny mission to measure
cosmologically distant supernovae as one of the candidate implementations for the Joint Dark Energy Mission. Repeat
observations of the supernovae require attitude control at the level of 0.010 arcseconds (0.05 microradians) during
integrations and at repeat intervals up to and over a year. While absolute accuracy is not required, the repoint precision is
challenging. We have simulated the performance of a focal plane star tracker in a multidimensional parameter space,
including pixel size, read noise, and readout rate. Systematic errors such as proper motion, velocity aberration, and
parallax can be measured and compensated out. Our prediction is that a relative attitude determination accuracy of 0.001
to 0.002 arcseconds (0.005 to 0.010 microradians) will be achievable. Attitude control will have a jitter of around 0.003
arcseconds and stability/repeatability to around 0.002 arcseconds.
The pointing calibration and reference sensor (PCRS) of the Spitzer Space Telescope (SST) consists of two 4 by 4 pixel visible band detectors plus associated electronics per redundant side. Located in the cryogenic multi instrument chamber of the science telescope system, these detectors, which have a field of view of 40 by 40 arcsec each, define the telescope coordinate frame. Employing guide stars in the range from 7 to 10 magnitude, the PCRS is used, among other things, for measuring the alignment of the active external autonomous star tracker twice per day. To ensure adequate accuracy, the standard radial position error of the guide stars needs to be less than 120 milliarcsec (mas) out to J2009.5. Initial selection of guide star candidates is performed using the Tycho, Tycho-2, Hipparcos, and Tycho Double Star catalogs. To obtain sufficient depth, the 2MASS PSC, USNO A2.0, 2MASS XSC, and PGC catalogs are used for computing the disturbing effect of neighboring objects. In addition, the DSS is employed for determining the perturbing effect of the sky background for each of the guide star candidates. The paper describes the guide star requirements, the methodology used for computing star position error due to neighboring objects and sky background, and the staged development approach that resulted in a ground-based catalog with 196,087 high-quality guide stars for the PCRS.
Two redundant AST-301 autonomous star trackers (AST) serve as the primary attitude sensors for JPL's space infrared telescope facility (SIRTF). These units, which employ a 1553B interface to output their attitude quaternions and uncertainty at a 2 Hz rate, provide a 1 σaccuracy of better than 0.18, 0.18, and 5.1 arcsec about their X, Y, and Z axes, respectively. This is a factor 5.5 better than the accuracy of the flight-proven AST-201 from which the trackers were derived. To obtain this improvement, the field of view (FOV) was reduced to 5 by 5 degrees, the accurate Tycho-1 and ACT catalogs were used for selecting the 71,830 guide stars, star image centroiding was improved to better than 1/50th of a pixel, and optimal attitude estimation was implemented. In addition, the apparent direction to each guide star in the FOV is compensated for proper motion, parallax, velocity aberration, and optical distortion. The AST-301 employs autonomous time-delayed integration (TDI) to achieve image motion compensation (IMC) about its X axis that prevents accuracy degradation, even at rates of 2.1 deg/s, making it actually suitable for use on spinning spacecraft. About the Y axis, a software function called "image motion accommodation" (IMA) processes smeared images to maximize the signal to noise ratio of the resulting synthetic images, which enables robust and accurate tracking at rates tested up to
0.42 deg/s. The AST-301 is capable of acquiring its attitude anywhere in the sky in less than 3 seconds with a 99.98% probability of success, without requiring any a priori attitude knowledge. Following a description of the 7.1 kg AST-301, its operation and IMA, the methodology for translating the night sky test data into performance numbers is presented, while, in addition, the results of tests used to measure alignment stability over temperature are included.
The SIRTF requires a visible light sensor at its focal plane to 1) calibrate the alignment between the externally mounted star trackers and the telescope boresight; 2) to establish the correspondence between the telescope coordinate system and the absolute J2000 reference frame; and 3) to provide starting attitudes for high accuracy absolute offset maneuvers. The Pointing Calibration and Reference Sensor (PCRS) functions as the primary absolute attitude reference for the SIRTF telescope. It measures the J2000 position of Tycho Catalogue stars to an accuracy of 0.14 arcsec 1-s per axis. To accurately measure Tycho objects, we have selected a silicon PIN photodiode operating in the Johnson V band, which we use with a cryogenic readout developed for the MIPS instrument on SIRTF. The PCRS employs a 4 by 4 Si:PIN detector array, using the outer rings for acquisition and the inner four pixels for precise measurements. Operation in the SIRTF focal plane presents us with several unique problems. Since the detector thermally links directly to the cryostat helium bath, it must operate at a temperature of 1.4K. Additionally, the power dissipation must be less than 0.1 mW to minimize the impact on helium lifetime. We describe low temperature characterization of Si'PIN detectors and readouts to verify their operability in the PCRS environment. Since the beryllium optics of the SIRTF telescope are diffraction limited only at 6.5 microns and longward, they yield a complicated point spread function at visible wavelengths. We present operational solutions to these and other challenges that allow the PCRS to meet its accuracy requirements with minimal impact on the rest of the SIRTF mission.
An autonomous star tracker (AST) is a "starfield in, 3-axis attitude out" device capable of determining its attitude rapidly without requiring any a-priori attitude knowledge. Following attitude acquisition, the AST switches to a track mode where it outputs its attitude and rate at typically 5 Hz. The Lockheed Martin Advanced Technology Center has developed the first version of a line of reliable, highly accurate, low cost, ASTs. This "AST-201" is to be test flown on a sounding rocket and will function as a key attitude sensor aboard NASA's Small Spacecraft Technology Initiative (SSTI) "Clark" spacecraft scheduled for launch in 1996. The AST is of modular construction and comprises athermalized, radiation hard refractive optics, a frame-transfer CCD with a sensitive area of 512 by 512 pixels, camera electronics, a single board computer, an all-sky guide star database and a highly effective sunshade that allows operation to within 25 degrees from the Sun. Initial star identification is performed by a memory-efficient algorithm that provides an attitude acquisition success rate of better than 99.85% anywhere in the sky. The AST achieves its high accuracy through use of a moderate 8.8 by 8.8 degree field of view and by tracking 27 stars on average. On geostationary satellites a single AST can provide 3-axis attitude with greater precision, at lower cost, and with higher reliability than is possible with a combination of Earth sensors, fine Sun sensors, and a high performance gyro system. In addition, ASTs are devoid of angular range limitations, avoid Sun and Moon interference by proper orientation on the spacecraft, and enable rapid fault recovery, a capability especially important to geostationary missions where outages are usually costly. Two ASTs can provide a geostationary spacecraft with an attitude accuracy of 5 microradians (1 a). The paper contains a description of the AST, a summary of the functions enabled or improved by the device, real-sky AST test results, and accuracy statistics for an AST on a geostationary spacecraft, as obtained through realistic simulations.
Keywords: star tracker, autonomy, attitude determination, pattern recognition, geostationary satellites, spacecraft attitude control.
An autonomous star tracker (AST) is basically a `star field in, attitude out' device capable of determining its attitude without requiring any a priori attitude knowledge. In addition to this attitude acquisition capability, an AST can perform attitude updates autonomously and is able to provide its attitude `continuously' while tracking a star field. The Lockheed Palo Alto Research Laboratory is developing a reliable, low-cost, miniature AST that has a one arcsec overall accuracy, weighs less than 1.5 kg, consumes less than 7 watts of power, and is sufficiently sensitive to be used at all sky locations. The device performs attitude acquisition in a fraction of a second and outputs its attitude at a 10 Hz rate when operating in its tracking mode. Besides providing the functionality needed for future advanced attitude control and navigation systems, an AST also improves spacecraft reliability, mass, power, cost, and operating expenses. The AST comprises a-thermalized, refractive optics, a frame-transfer CCD with a sensitive area of 1024 by 1024 pixels, camera electronics implemented with application- specific integrated circuits, a compact single board computer with a radiation hard 32 bit RISC processor, and an all-sky guide star database. Star identification is performed by a memory- efficient and highly robust algorithm that finds the largest group of observed stars matching a group of guide stars. An important milestone has recently been achieved with the validation of the attitude acquisition capability through correct and rapid identification of all 704 true-sky star fields obtained at the Lick Observatory, using an uncalibrated prototype AST with a 512 by 1024 pixel frame-transfer CCD and a 50 mm f/1.2 lens that provided an effective 6.5 by 13.2 degree field of view. The overlapping fields cover 47% of the sky, including both rich and sparse areas. The paper contains a description of the AST, a summary of the functions enabled or improved by the device, an overview of the identification algorithm, results obtained with a realistic simulation program, a description of the true-sky star field identification method and a presentation of the results obtained. The AST tolerates the presence of bright objects as was demonstrated by a field that included Jupiter.
The attitude control and navigation systems of future advanced spacecraft will be characterized by a high degree of autonomy, very high accuracy, efficient commandability, and fast fault recovery. These characteristics are incompatible with the constraints of conventional star sensors which mandate a-priori definition of all onboard attitude fixes and work only if attitude uncertainties remain small. With the availability of accurate, anti-blooming capable CCDs, fast microprocessors, high density memory chips, and star pattern recognition algorithms, it is now feasible to fabricate miniature Autonomous Star Trackers (ASTs) capable of (1) determining their attitude rapidly and reliably while having no a-priori attitude knowledge, (2) autonomous attitude updating, and (3) providing their attitude at rates up to typically 40 Hz. In addition to providing the functionality needed for future missions, ASTs can also be exploited to improve the reliability, mass, power, and cost of spacecraft and reduce the cost of operating them.
This paper describes star identification schemes used in the past, it discusses a number of star pattern recognition algorithms, and provides the main characteristics of current CCD star trackers. A number of specific functions enabled or enhanced by an AST are described including fast attitude acquisition, rapid fault recovery, attitude safing, gyroless/cheap-gyro attitude control, autonomous target acquisition by astronomy telescopes, autonomous optical navigation, and precision pointing to terrestrial targets. The AST being developed at Lockheed uses a fast, memory-efficient, and highly robust star pattern recognition algorithm based on matching groups of stars. The algorithm, which is also applicable to star scanners, is described along with a realistic simulation program for testing its performance. It is shown that an AST with an 11.3 degree FOV diameter, a database of 4100 guide stars, a 25 MHz MC68030 class microprocessor, and 800 Kbytes of memory will be capable of determining its attitude in 0.45 seconds with a success rate greater than 99.98% when using an optimal guide star selection method. Compute time and memory are found to be inversely proportional to the FOV area. The paper also reports on AST development by other organizations in a number of countries.