Polar-orbiting Spacecraft Research Articles

Updated radiometric and wavelength calibration of the Juno ultraviolet spectrograph

Juno is a spacecraft in polar orbit around Jupiter, and one of its objectives is to explore the Jovian magnetosphere. Charged particles trapped along the Jovian magnetic field lines may generate auroral emission. The Juno magnetospheric payload is designed to measure the charged particle distributions in the Jovian magnetosphere while remote sensing the related UV-auroral emissions. The ultraviolet spectrograph on Juno (Juno-UVS) is a photon-counting imaging spectrograph designed to observe Jupiter’s prominent far-UV auroral emissions in the 68 to 210 nm spectral range. This work presents an improvement on the wavelength and radiometric calibration of Juno-UVS. Ground calibration measurements were reanalyzed by expanding the spectral database ultimately used to produce the instrument spectral pixel size map. This significantly improves the wavelength assignation, up to 4 nm at the longer wavelengths, compared with the previous calibration. Dedicated Juno-UVS observation sequences were made to evaluate the slit-dependent radiometric calibration, which was revised accordingly. This work highlights a slight slit-dependent variation in the radiometric calibration. An additional radiometric correction was established at wavelengths greater than 160 nm using some of UVS’s brightest recorded stellar spectra.

Latitudinal Beaming of Jupiter's Radio Emissions From Juno/Waves Flux Density Measurements

AbstractThe observations from the Juno spacecraft in polar orbit of Jupiter provide for the first time a complete view of Jupiter's radio emissions from all latitudes. Characterizing the latitudinal distribution of radio emissions' occurrence and intensity is a useful step for elucidating their origin. Here, we analyze for that purpose the first 3 years of observations from the Waves experiment on the Juno spacecraft (mid‐2016 to mid‐2019). Two prerequisites for the construction of the latitudinal distribution of intensities for each Jovian radio component are (a) to work with absolute flux densities and (b) to be able to associate each radio measurement with a specific radio component. Accordingly, we develop a method to convert the Juno/Waves data in flux densities and then we build a catalog of all Jovian radio components over the first 3 years of Juno's orbital mission. From these, we derive occurrence and intensity distributions versus observer's latitude and frequency for each component; these will be the basis for future detailed studies and interpretations of each component's characteristics and origin.

A Persistent Depletion of Plasma Ions Within Jupiter's Auroral Polar Caps

AbstractRegions poleward of Jupiter's main auroral ovals are largely devoid of 0.01 to 46 keV/q ions. We report ion measurements from the plasma instrument on the Juno spacecraft in polar orbit around Jupiter that characterize the presence or absence of structured ion fluxes poleward of the main ovals. We approximate geometric centers of published north and south Jovian main ovals and develop a probability metric for observation of plasma ions as a function of angular distance from these centers to the magnetic foot points of the observations, in the north and south separately. Using data from Juno's first 26 perijoves, at Jovicentric distances of 1.06 to 12 RJ, this metric shows a systematic decrease within both north and south main ovals, in the probability of observing plasma ions as the chosen centers are approached. This is consistent with previous reports of large electrostatic potential drops above Jupiter's polar caps.

3D radar wavefield migration of comet interiors

Imaging the interior structure of small planetary bodies facilitates a deep understanding of their origin and evolution, thus addressing fundamental questions about the formation of the Solar System. We show that high resolution 3D imaging of their interior structure is possible using radar waves that reflect from internal discontinuities of dielectric properties. A radar imaging mission at a comet nucleus would have the benefit of orbiting all around a finite and transparent body, collecting echoes that derive only from the target, and processing them collectively in phase.As is the case in the medical field, imaging a comet nucleus requires its illumination from multiple directions, which can be accomplished with a spacecraft in slow polar orbit around the studied object. Long acquisition time leads to a dense acquisition array resembling that of conventional synthetic aperture radar systems, but completely surrounding the nucleus (4π steradians). Acquisition with an orbiter at large distance from the comet nucleus (5× the mean diameter) results in relatively coarse data sampling relative to the radar wavelength, thus enabling 3D imaging with a short (<90 days) mission.Radar migration is performed using techniques adapted from terrestrial exploration seismology. Wavefield migration identifies interior reflectors by time reversal of monostatic radar data redatumed to the known comet surface for computational speed-up. This technique is applicable to nuclei of arbitrary shape and interior complexity, and it is complementary to wavefield tomography tasked with constraining physical properties in-between the interior interfaces. Migration resolution is identical in all directions (range and cross-range) and it is equivalent to the range resolution of the radar system. Least-squares migration enhances resolution further by deconvolution of the radar wavelet from the reflectivity image.

3D radar wavefield tomography of comet interiors

Answering fundamental questions about the origin and evolution of small planetary bodies hinges on our ability to image their surface and interior structure in detail and at high resolution. The interior structure is not easily accessible without systematic imaging using, e.g., radar transmission and reflection data from multiple viewpoints, as in medical tomography. Radar tomography can be performed using methodology adapted from terrestrial exploration seismology. Our feasibility study primarily focuses on full wavefield methods that facilitate high quality imaging of small body interiors. We consider the case of a monostatic system (co-located transmitters and receivers) operated in various frequency bands between 5 and 15 MHz, from a spacecraft in slow polar orbit around a spinning comet nucleus. Using realistic numerical experiments, we demonstrate that wavefield techniques can generate high resolution tomograms of comets nuclei with arbitrary shape and complex interior properties.

The impact of satellite sensor viewing geometry on time-series analysis of volcanic emissions

Abstract Time-series analysis techniques are being increasingly used to process satellite observations of volcanic gas emissions and heat flux, with the aim of identifying cyclic behaviour that could inform hazard assessment or elucidate volcanic processes. However, it can be difficult to distinguish cyclic variations due to geophysical processes from those that are artefacts of the observation technique. Here, we conduct a comprehensive investigation into the origin of cyclicity in volcanic observations by analysing daily, global satellite measurements of volcanic SO2 loading by the Ozone Monitoring Instrument (OMI) and thermal infrared anomalies detected by the Moderate Resolution Imaging Spectroradiometer (MODIS). We use a fast Fourier Transform (FFT) multi-taper method (MTM) to analyse multiple phases of activity at 32 target volcanoes, utilising measurements obtained from three NASA satellite instruments (Aura – OMI, Aqua – MODIS and Terra – MODIS), and identify a common cycle (period of ~ 2.3 days), which is not considered to be of volcanic origin. Based on the presence of this cycle in multiple satellite datasets, we attribute it to variations in viewing angle during the 16-day orbit repeat cycle of sun-synchronous satellites maintained in Low Earth Orbit (LEO). A 5-point averaging correction procedure is tested on satellite observations from Kilauea volcano, Hawaii, and is found to reduce the impact of higher frequency cycles and reveal the presence of longer-period geophysical signals. In addition to the identification of a signal common to different measurement techniques, an underlying cyclical pattern was found in the OMI SO2 observations (periods of ~ 7.9 and 3.2 days) generated by the OMI Row Anomaly (ORA). We conclude that identification of the presence and magnitude of non-geophysical cyclic behaviour, which can suppress natural cycles in time-series data, and implementation of appropriate corrections, is crucial for accurate interpretation of satellite observations. The use of data averaging to suppress non-geophysical cycles imposes limits on the length of natural cycles that can be confidently identified in moderate resolution satellite observations from polar-orbiting spacecraft.

Geodetic constraints from multi-beam laser altimeter crossovers

The round-trip travel time measurements made by spacecraft laser altimeters are primarily used to construct topographic maps of the target body. The accuracy of the calculated bounce point locations of the laser pulses depends on the quality of the spacecraft trajectory reconstruction. The trajectory constraints from Doppler and range radio tracking data can be supplemented by altimetric “crossovers”, to greatly improve the reconstruction of the spacecraft trajectory. Crossovers have been used successfully in the past (e.g., Mars Orbiter Laser Altimeter on Mars Global Surveyor), but only with single-beam altimeters. The same algorithms can be used with a multi-beam laser altimeter, but we present a method using the unique cross-track topographic information present in the multi-beam data. Those crossovers are especially adapted to shallow (small angle) intersections, as the overlapping area is large, reducing the inherent ambiguities of single-beam data in that situation. We call those “swath crossovers”. They prove particularly useful in the case of polar-orbiting spacecraft over slowly rotating bodies, because all the non-polar crossovers have small intersection angles. To demonstrate this method, we perform a simplified simulation based on the Lunar Reconnaissance Orbiter (LRO) and its five-beam Lunar Orbiter Laser Altimeter. We show that swath crossovers over one lunar month can independently, from geometry alone, recover the imposed orbital perturbations with great accuracy (5 m horizontal, < 1 m vertical, about one order of magnitude smaller than the imposed perturbations). We also present new types of constraints that can be derived from the swath crossovers, and designed to be used in a precision orbit determination setup. In future work, we will use such multi-beam altimetric constraints with data from LRO.

Corrections for Temporal Discontinuities in the OPI

Abstract The longest record of precipitation estimated from satellites is the outgoing longwave radiation (OLR) precipitation index (OPI), which is based on polar-orbiting infrared observations from the Advanced Very High Resolution Radiometer (AVHRR) instrument that has flown onboard successive NOAA satellites. A significant barrier to the use of these data in studies of the climate of tropical precipitation (among other things) is the large bias caused by orbital drift that is present in the OLR data. Because the AVHRR instruments are deployed on the polar-orbiting spacecraft, OLR observations are recorded at specific times for each earth location for each day. Discontinuities are caused by the use of multiple satellites with different observing times as well as the orbital drift that occurs throughout the lifetime of each satellite. A regression-based correction is proposed based solely on the equator crossing time (ECT). The correction allows for separate means for each satellite as well as separate coefficients for each satellite ECT. The correction is calculated separately for each grid box but is applied only at locations where the correction is correlated with the OLR estimate. Thus, the correction is applied only where deemed necessary. The OPI is used to estimate precipitation from the OLR estimates based on the new corrected version of the OLR, the uncorrected OLR, and two earlier published corrected versions. One of the earlier corrections is derived by removing variations from AVHRR based on EOFs that are identified as containing spurious variations related to the ECT bias, whereas the other is based on OLR estimates from the High Resolution Infrared Radiation Sounder (HIRS) that have been corrected using diurnal models for each grid box. The new corrected version is shown to be free of nearly all of the ECT bias and has the lowest root mean square difference when compared to gauges and passive microwave estimates of precipitation. The EOF-based correction fails to remove all of the variations related to the ECT bias, whereas the correction based on HIRS removes much of the bias but appears to introduce erroneous trends caused by the water vapor signal to which these data are sensitive. The new correction for AVHRR OLR works well in the tropics where the OPI has the most skill, but users should be careful when interpreting trends outside this region.

An Overview of a New Chinese Weather Satellite FY-3A

FengYun-3A (FY-3A), the first satellite in the second generation of the Chinese polar-orbiting meteorological satellites, was launched at Taiyuan, China, launching center on 27 May 2008. Equipped with both sounding and imaging payloads, enabling more powerful observations than the first generation of the FY-1 series, FY-3A carries 11 instruments. Two of them are the same as those on FY-1C/D, while the others, whose spectral bands cover violet, visible, near-infrared, infrared, and microwave spectral regions, are all newly developed. FY-3A instruments can be used to detect and study weather, clouds, radiation, climate, atmosphere, land, ocean, and other environmental features. FY-3A check out took about 5 months following its launch; FY-3A has been operational since January 2009. The plan for the future FY-3 series is to operate two polar-orbiting spacecraft—one in the morning and the other in the afternoon orbit—with different payloads for each spacecraft. This orbit configuration will be further coordina...

恒星センサを用いた地球指向衛星姿勢決定系のフィールド試験

Earth-pointing and polar-orbiting spacecraft, the Advanced Land Observing Satellite (ALOS), has a precision attitude determination system exploiting measurements of a precision star tracker and an inertial reference unit, and controls its attitude, based on the attitude estimates. The star tracker provides the positions and magnitudes of stars. The attitude control system's computer identifies stars which enter the star tracker's FOV and move across it, and applies the extended Kalman filter. The ALOS precision attitude determination system consisting of the star tracker and the onboard computer was tested at the Usuda Deep Space Center in the real night sky environment. The tests using a two-axis motion table demonstrated end-to-end performance, capability, and robustness of the attitude determination system. This paper presents the overview and results of the real sky tests. It also proposes test settings for Earth-pointing spacecraft and a method for alignment error assessment and calibration. Limit of performance due to the test site and configuration are also evaluated.

Chaos and its control in the pitch motion of an asymmetric magnetic spacecraft in polar elliptic orbit

We study the pitch attitude dynamics of an asymmetric magnetic spacecraft in a polar almost circular orbit under the influence of a gravity gradient torque. The spacecraft is perturbed by the small eccentricity of the elliptic orbit and by a small magnetic torque generated by the interaction between the Earth’s magnetic field and the magnetic moment of the spacecraft. Under both perturbations, we show that the pitch motion exhibits heteroclinic chaotic behavior by means of the Melnikov method. Numerical methods applied to simulations of the pitch motion also confirm the chaotic character of the spacecraft attitude dynamics. Finally, a linear time-delay feedback method for controlling chaos is applied to the governing equations of the spacecraft pitch motion in order to remove the chaotic character of initially irregular attitude motions and transform them into periodic ones.

Poleward moving auroral forms (PMAFs) revisited: responses of aurorae, plasma convection and Birkeland currents in the pre- and postnoon sectors under positive and negative IMF &amp;lt;I&amp;gt;B&amp;lt;sub&amp;gt;y&amp;lt;/sub&amp;gt;&amp;lt;/I&amp;gt; conditions

Abstract. Using five case studies, we investigate the dynamical evolution of dayside auroral precipitation in relation to plasma convection, classifying it by the IMF By component and position with respect to noon. Auroral observations were made by meridian scanning photometers (MSPs) and an all-sky camera (ASC) in Ny Ålesund, Svalbard at 76° MLAT, while the spatial structure of the ionospheric plasma convection is inferred from SuperDARN radars and ion drift observations from spacecraft in polar orbit. The IMF configuration of major interest here is one pointing southward and with a dominant east-west component. Our emphasis is on the auroral phenomenon of PMAFs (poleward moving auroral forms), which are ionospheric signatures of pulsed reconnection at the magnetopause. We distinguish between PMAFs/prenoon and PMAFs/postnoon. These two activities are found to be separated by an auroral form around noon with attenuated emission at 630.0 nm. We document for the first time that this "midday gap aurora" appears in the form of a midday auroral brightening sequence (MABS). We study the PMAF activity consisting of an initial brightening phase and the later stages of PMAF evolution in relation to plasma convection cells, flow vorticity, and precipitation boundaries in the prenoon and postnoon sectors for both By polarities. Flow channels (PIFs) associated with PMAFs are strengthened by polarization effects at auroral boundaries. Addressing the implications of our proposed, extended perspective on dayside auroral morphology under southeast/west IMF for M-I coupling associated with pulsed magnetopause reconnection (FTEs), we draw inferences on the MLT-dependent geoeffectiveness (Birkeland current/auroral intensity) of magnetopause FTEs (subsolar region versus flanks).

A Wide Field Auroral Imager (WFAI) for low Earth orbit missions

Abstract. A comprehensive understanding of the solar wind interaction with Earth's coupled magnetosphere-ionosphere system requires an ability to observe the charged particle environment and auroral activity from the same platform, generating particle and photon image data which are matched in time and location. While unambiguous identification of the particles giving rise to the aurora requires a Low Earth Orbit satellite, obtaining adequate spatial coverage of aurorae with the relatively limited field of view of current space bourne auroral imaging systems requires much higher orbits. A goal for future satellite missions, therefore, is the development of compact, wide field-of-view optics permitting high spatial and temporal resolution ultraviolet imaging of the aurora from small spacecraft in low polar orbit. Microchannel plate optics offer a method of achieving the required performance. We describe a new, compact instrument design which can observe a wide field-of-view with the required spatial resolution. We report the focusing of 121.6 nm radiation using a spherically-slumped, square-pore microchannel plate with a focal length of 32 mm and an F number of 0.7. Measurements are compared with detailed ray-trace simulations of imaging performance. The angular resolution is 2.7±0.2° for the prototype, corresponding to a footprint ~33 km in diameter for an aurora altitude of 110 km and a spacecraft altitude of 800 km. In preliminary analysis, a more recent optic has demonstrated a full width at half maximum of 5.0±0.3 arcminutes, corresponding to a footprint of ~1 km from the same spacecraft altitude. We further report the imaging properties of a convex microchannel plate detector with planar resistive anode readout; this detector, whose active surface has a radius of curvature of only 100 mm, is shown to meet the spatial resolution and sensitivity requirements of the new wide field auroral imager (WFAI).

Reverse Trajectory Approach to Computing Ionospheric Currents to the Special Sensor Ultraviolet Limb Imager on DMSP

The special sensor ultraviolet limb imager was developed by the U.S. Naval Research Laboratory and is deployed on the Defense Meteorological Satellite Program (DMSP) F16 polar-orbiting spacecraft. The instrument is experiencing a level of noise that is, at times, interfering with its proper operation. The noise is correlated with the spacecraft chassis potential. The potentials about DMSP and the resulting ionospheric current entering the instrument were computed to determine if the noise could be due to this current. In order to obtain results of sufficient accuracy, it is necessary to use a reverse trajectory technique that effectively integrates over the thermal distribution of incident ions. The reverse trajectory technique is described in detail. Once the resistance between the mirror surface and the chassis is taken into account, the current computed using this approach shows the same dependence on the chassis potential as the observed noise, which supports the conjecture that ionospheric ions are responsible for the noise

Multi-instrument mapping of the small-scale flow dynamics related to a cusp auroral transient

Abstract. In this paper we focus on flux transfer events (FTEs) and poleward moving auroral forms (PMAFs) in the cusp region, combining data from the EISCAT Svalbard radar, SuperDARN HF radars, ground-based optics, and three low-altitude polar-orbiting spacecraft. During an interval of southward interplanetary magnetic field the EISCAT Svalbard radar tracked a train of narrow flow channels drifting into the polar cap. One 30-60 km wide flow channel surrounded by flow running in the opposite direction is studied in great detail from when it formed equatorward of the cusp aurora, near magnetic noon, until it left the field-of-view and disappeared into the polar cap. Satellite data shows that the flow channel was on open field lines. The flow pattern is consistent with field-aligned currents on the sides of the flow channel; with a downward current on the equatorward side, and an upward current on the poleward side. The poleward edge of the flow channel was coincident with a PMAF that separated from the background cusp aurora and drifted into the polar cap. A passage of the DMSP F13 spacecraft confirms that the FTE flow channel was still discernable over 15 minutes after it formed, as the spacecraft revealed a 30–40 km wide region of sunward flow within the anti-sunward background convection. From the dimensions of the flow channel we estimate that the magnetic flux contained in the event was at least 1 MWb. This data set also shows that Birkeland current filaments often seen by low-altitude spacecraft in the cusp/mantle are really associated with individual FTE events or a train of FTEs in progress. As the region 0 or cusp/mantle current represents the statistical average consistent with the large-scale flow pattern, we therefore introduce a new term – FTE currents – to denote the unique pair of Birkeland current sheets that are associated with individual meso-scale FTE flow disturbances. The poleward moving auroral forms (PMAFs), often referred to in the literature, are the optical signature of the upward FTE current. Keywords. Magnetospheric physics (Current systems; Magnetopause, cusp and boundary layers) – Ionosphere (plasma convection)

ASTER observations of thermal anomalies preceding the April 2003 eruption of Chikurachki volcano, Kurile Islands, Russia

Chikurachki volcano (Northern Kurile Islands Chain, Paramushir Island 50° 20′N, 155° 27′E; elevation 1816 m, stratovolcano) has been in a state of unrest for over twenty years. Its most recent eruption that began in April 2003 was preceded by an eruption between January and May 2002. Thermal infrared images from the Japanese–United States' Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER-5 TIR channels, 8–12 μm, 90 m/pixel, Noise Equivalent Delta Temperature [NEΔT] ∼0.1–0.3 K; onboard the U.S. National Aeronautics and Space Administration's Terra polar-orbiting spacecraft) of a snow-covered Chikurachki Volcano taken in January 2003 show muted evidence of thermal activity. ASTER thermal infrared (TIR) images from February 2003, however, indicate warmer areas within the summit crater. Average whole-pixel temperatures of adjacent summit slopes are in the range of 250–252 K, while many summit-crater whole pixel temperatures are ∼2–6 K greater. A two component Planck solution for the warmer pixels yields a solution of 275–277 K for 10–15% of the Chikurachki summit crater and 285–295 K for 25–35% of a prominent “hotspot” on its flank. An interpretation of this enhanced pre-eruption heat flow is the presence of surface melt water. The detection in ASTER data of such subtle precursory heat-flow enhancement, even retrospectively, raises important issues for remote monitoring of “dormant” snow-capped volcanoes, especially those that threaten nearby populations, like Mt. Rainier.

A Feature-Based Approach to Satellite Precipitation Monitoring Using Geostationary IR Imagery

Abstract This paper describes a new geostationary infrared satellite precipitation monitoring methodology that draws on techniques from machine vision to develop a mathematical representation of cloud shapes and textures and the form of their embedding meteorological context. Quantitative feature descriptions derived from Geostationary Operational Environmental Satellite (GOES) band-4 infrared imagery were translated into 15-min precipitation estimates through the use of an artificial neural network. The network was trained using 3-hourly ground-radar data in lieu of a dense constellation of polar-orbiting spacecraft such as the proposed Global Precipitation Measurement (GPM) mission. The technique has been tested using data from the Texas–Florida Underflights Experiment (TEFLUN-B) and has demonstrated significant advantages over existing satellite precipitation monitoring techniques when applied to a summer precipitation regime in Florida.

Autonomous behavioural algorithm for space applications

AbstractThe purpose of this paper is to present a new approach in the concept and implementation of autonomy for autonomous spacecraft. The one true ‘artificial agent’ approach to autonomy requires the spacecraft to interact in a direct manner with the environment through the use of sensors and actuators. Rather than using complex world models, the spacecraft is allowed to exploit the dynamics of its environment for cues as to appropriate actions to take to achieve its mission goals. The particular artificial agent implementation used here has been inspired by studies of biological systems. The so-called ‘cue-deficit’ action selection algorithm considers the spacecraft to be a non-linear dynamical system with a number of observable states. Using optimal control theory a set of rules is derived which determine which of a finite repertoire of behaviours the spacecraft will perform. A simple model of a single imaging spacecraft in low polar Earth orbit is used to demonstrate the algorithm.

Low Altitude Image of Particle Acceleration and Magnetospheric Reconfiguration at Substorm Onset

We compare meridional distributions of auroral (0.3-20 keV) and energetic (30-300 keV) particle fluxes measured by two low-altitude polar-orbiting spacecraft which crossed the auroral oval before and shortly after the sudden substorm onset. 3-4 min after the substorm onset a very strong particle energization was observed with the particle flux increased by a factor 100-1000 at energies up to >300 keV. It was found in the middle of auroral oval, poleward of the isotropic boundary of energetic electrons detected prior to the substorm onset. Interpreting the variations of Energetic Particle (EP) flux and anisotropy (ratio of precipitated to trapped particle flux) in terms of particle scattering in the equatorial current sheet, we were able to characterize the configuration of equatorial magnetotail. The major acceleration/precipitation region in the center of auroral zone was inferred to be confined in the region of dipolarized magnetic field in the magnetotail. Further poleward (tailward in the magnetotail) the isotropic electron distributions indicated the presence of current sheet with very stretched magnetic field lines. Based on magnetic flux conservation in the flux tube, we estimate the width of transition region between dipolarized region and current sheet to be ∼0.5RE (about the gyroradius of plasma sheet proton in the equatorial magnetic field ∼5 nT). Such sharp transition may be formed by fast Earthward flow in the current sheet colliding with the dipolarized magnetic shells. An evidence supporting this interpretation is that the explosive westward electrojet was found at latitudes occupied by the current sheet but poleward of the dipolarized region. We also show another event in which the dipolarized region expanded up to the poleward boundary of auroral oval in 7 minutes after the substorm onset.

One‐ to two‐month oscillations in SSMI surface wind speed in western tropical Pacific Ocean

The 10‐m wind speed over the ocean can be estimated from microwave brightness temperature measurements recorded by the Special Sensor Microwave Imager (SSMI) instrument mounted on a polar‐orbiting spacecraft. Four‐year (1988–1991) time series of average daily 1° × 1° SSMI wind speeds were analyzed at selected sites in the western tropical Pacific Ocean. One‐ to two‐month period wind speed oscillations with amplitudes statistically significant at the 95% confidence level were observed near Kanton, Eniwetok, Guam, and Truk. This is the first report of such an oscillation in SSMI wind speeds.