In-orbit Measurements Research Articles

Self-calibration method of the bias of a space electrostatic accelerometer.

The high precision space electrostatic accelerometer is an instrument to measure the non-gravitational forces acting on a spacecraft. It is one of the key payloads for satellite gravity measurements and space fundamental physics experiments. The measurement error of the accelerometer directly affects the precision of gravity field recovery for the earth. This paper analyzes the sources of the bias according to the operating principle and structural constitution of the space electrostatic accelerometer. Models of bias due to the asymmetry of the displacement sensing system, including the mechanical sensor head and the capacitance sensing circuit, and the asymmetry of the feedback control actuator circuit are described separately. According to the two models, a method of bias self-calibration by using only the accelerometer data is proposed, based on the feedback voltage data of the accelerometer before and after modulating the DC biasing voltage (Vb) applied on its test mass. Two types of accelerometer biases are evaluated separately using in-orbit measurement data of a space electrostatic accelerometer. Based on the preliminary analysis, the bias of the accelerometer onboard of an experiment satellite is evaluated to be around 10-4 m/s2, about 4 orders of magnitude greater than the noise limit. Finally, considering the two asymmetries, a comprehensive bias model is analyzed. A modified method to directly calibrate the accelerometer comprehensive bias is proposed.

Planning constraints of low grazing altitude GEO-LEO laser links based on in-orbit data

Part of the operational concept for laser communication networks such as the European data relay system is planning constraints. In addition to hard constraints such as angular range of the actuators, there is also the atmosphere, which has a gradual increasing impact as the distance between laser beam path and earth decreases. The shortest distance between the laser beam path and the surface of the earth is called “grazing altitude.” The atmosphere impacts spatial acquisition as well as communication performance. In-orbit measurement data of geostationary orbit–low-Earth orbit (GEO-LEO) links between two laser communication terminals (LCT) developed by TESAT-Spacecom, Germany, in the frame of Alphasat TDP1 optical GEO-relay demonstration program are presented. Planning constraint guidelines are formulated based on the observed influence. TESAT-Spacecom’s LCTs use a highly sensitive and high-performance coherent transmission technology and a beaconless spatial acquisition system.

Initial Irish light pollution measurements and a new Sky Quality Meter-based data logger

In the Celtic Tiger years, Ireland underwent an explosive growth in housing and an increase in light pollution, nationally. We outline work undertaken towards the first ever ground-based survey of light pollution in the island of Ireland and illustrate this with some representative data taken between 2009 and the present. The aim of our work is to establish a database to serve as a reference to monitor future changes, to compare with in-orbit measurements, to study the potential impact on the environment and to permit the calculation of the effects of future changes in lighting usage and patterns. We also report the development of a microcontroller-based data-logging instrument based on a Unihedron Sky Quality Meter-LR detector, which can be used by amateurs or can serve as a stand-alone instrument for night-sky brightness surveys.

In-Orbit Measurement of SET and DD Effects on Optical Wireless Links for Intra-Satellite Data Transmission

In-orbit measurements of two experimental optical-wireless data links on board a polar LEO (Low Earth Orbit) spacecraft are presented. The effects of single event transients on the bit error rate, as well as those of displacement damage on the optoelectronic components being used, were measured. The results are consistent with those obtained from proton irradiations carried out in ground facilities.

Atmospheric ultraviolet and red-infrared flashes from Universitetsky-Tatiana-2 satellite data

Millisecond ultraviolet (240–400 nm) and red-infrared (610–800 nm) flashes were detected in the nighttime atmosphere with the scientific payload installed onboard the Universitetsky-Tatiana-2 micro-satellite. Flashes with various numbers of photons, from 1020 to 1026, were detected within the atmospheric area 300 km in diameter observed by the detector. The flashes differ in duration and temporal profile: from single short flashes ∼1 ms in duration to flashes with a complex profile more than 100 ms in duration. Different global geographic distributions are observed for flashes with different numbers of photons. Flashes with fewer than 1022 photons are distributed uniformly over the Earth’s map. Flashes with more than 1022 photons are concentrated near the equator and above the continents. Series of flashes were observed in one turn of the satellite when flying not only over thunderstorm regions but also over cloudless ones. The flash number distribution has been derived from the ratio of the numbers of red-infrared and ultraviolet photons. As applied to discharges in the upper atmosphere, whose glow is dominated by the emission in the first and second positive systems of molecular nitrogen bands (1PN2 and 2PN2), this distribution is equivalent to the flash altitude distribution in the atmosphere. The observed ratio of the numbers of photons in red-infrared and ultraviolet flashes agrees with the calculated one for electric discharges at altitudes higher than 50 km. In-orbit measurements of the charged particle flux (with a threshold energy for electrons of 1 MeV) provide no evidence for a synchronous occurrence of an ultraviolet flash and a burst in the particle flux in the orbit.

In-orbit measurements of spacecraft microvibrations for satellite laser communication links

Angular microvibrations of platform jitter on the optical inter-orbit communications engineering test satellite are measured in space during ground-to-satellite laser communication links. The microaccelerations are measured by the onboard accelerometers at a sampling rate of 2048 Hz. The angular microvibrations are estimated from the measured microaccelerations using the tracking characteristics of the laser communications terminal and the conversion factor on the basis of microvibration data obtained from ground-based tests. The power spectral density (PSD) of the satellite microvibrations is analyzed by using the fast Fourier transform analysis and the data length is examined according to the frequency resolution of the PSD. The in-orbit measurements of the PSDs are compared with those obtained from the ground test. The angular microvibrational base motion is estimated and a PSD up to 1024 Hz is additionally provided as a database of the real measurement results with previously obtained in-orbit measurements. The measured results will contribute to the angular jitter estimation and the design of a tracking control loop for space laser communication systems in the future.

Metop-GRAS in-orbit instrument performance

The GRAS instrument on the Metop-A satellite provides more than 600 radio occultation measurement profiles per day. The instrument is characterized by its wide antenna coverage, high signal-to-noise ratio and an ultra-stable clock reference. The conventional dual-frequency tracking of GPS signals is under dynamic atmosphere conditions complemented by open loop tracking with sampling of the signal at a 1 kHz rate, providing an unprecedented view of the signal spectral environment. This paper presents the instrument performance as derived from analysis of in-orbit measurement data. We show that the noise figure is low enough to enable mapping of external radio noise variations over the earth’s surface. An error propagation model is presented to relate instrument characteristics to bending angle performance. This model is also used to illustrate the relation between filter bandwidth, resolution and measurement noise. The Doppler model, guiding open loop measurements, is found to be accurate to better than 20 Hz with a possibility for improvement to 10 Hz. The high performance at low altitudes enables the presence of surface reflections at the −20-dB level to be identified in more than 50% of the occultations. The potential performance improvements for next generation receivers are discussed.

High precision refraction measurements by solar imaging during occultation: results from SOFIE

A new method for measuring atmospheric refraction angles is presented, with in-orbit measurements demonstrating a precision of +/-0.02 arcsec (+/-0.1 microrad). Key advantages of the method are the following: (1) Simultaneous observation of two celestial points during occultation (i.e., top and bottom edges of the solar image) eliminates error from instrument attitude uncertainty. (2) The refraction angle is primarily a normalized difference measurement, causing only scale error, not absolute error. (3) A large number of detector pixels are used in the edge location by fitting to a known edge shape. The resulting refraction angle measurements allow temperature sounding up to the lower mesosphere.

Performance of the Charge-Injection Capability of Suzaku XIS

Abstract A charge-injection technique is applied to the X-ray CCD camera, XIS (X-ray Imaging Spectrometer) aboard Suzaku. The charge transfer inefficiency (CTI) in each CCD column (vertical transfer channel) is measured by the injection of charge packets into a transfer channel and subsequent readout. This paper reports on the performances of the charge-injection capability based on the ground experiments using a radiation-damaged device, and in-orbit measurements of the XIS. The ground experiments show that charges are stably injected with a dispersion of 91eV in FWHM in a specific column for the charges equivalent to an X-ray energy of 5.1keV. This dispersion width is significantly smaller than that of the X-ray events of 113eV (FWHM) at approximately the same energy. The amount of charge loss during transfer in a specific column, which is measured with the charge-injection capability, is consistent with that measured with the calibration source. These results indicate that the charge-injection technique can accurately measure column-dependent charge losses, rather than the calibration sources. The column-to-column CTI correction to the calibration source spectra significantly reduces the line widths compared to those with a column-averaged CTI correction (from 193eV to 173eV in FWHM on an average at the time of one year after the launch). In addition, this method significantly reduces the low-energy tail in the line profile of the calibration source spectrum.

Statistically Optimized In-Flight Estimation of GPS Carrier Phase Multipath for LEO Satellite Attitude Determination

: Measurement of the GPS carrier phase difference between multiple antennas on a satellite offers a fast low-cost system for the determination of its attitude. However, an acceptable system requires mitigation of the error due to carrier phase multipath. The UoSat12 low Earth orbit satellite was employed to investigate this effect for in-orbit measurements. The disparity between GPS measurements and independent computed reference phase differences constituted the raw data for constructing a multipath error map. Processing found substantial errors—an estimated multipath error lying between peaks of – 43.9 mm and +42.6 mm with an rms value of 9.8 mm. Starting from conventional spherical harmonics and least square estimation, a novel method eliminates the effect of line bias variation and also determines the actual map error, i.e., the difference between true and estimated multipath. Selective optimization of choice of harmonics minimizes this error, which here is estimated to be 1.0 mm (1 sigma).

Radiometric and spectral performance and calibration of the GHz bands of EOS MLS

This paper describes radiometric performance and prelaunch radiometric and spectral calibrations of the GHz component of the Microwave Limb Sounder (MLS) experiment on NASA's Aura spacecraft. Estimated systematic scaling uncertainties (3/spl sigma/) on limb port radiances are /spl sim/0.5% from radiometric calibration and /spl sim/0.5% to /spl sim/1% from spectral calibrations. Operational noise performance is consistent with prelaunch expectations, and in-orbit measurements to date indicate no changes in noise characteristics, and no observable calibration drifts. Spectral baseline has remained stable to /spl sim/20 mK since launch. Refinements to calibrations based on in-flight data are discussed, and radiometric calibration algorithms are described.

Effect of Space Environment on Spacecraft Surfaces Control in Sun-Synchronous Orbits

Sixteen years of in-orbit measurements of thermal-control coating properties on sun-synchronous SPOT spacecraft are reported and analyzed. Their nonmonotonous evolution is shown to be due mostly to a succession of contamination and contaminant erosion by atomic oxygen phased with the solar cycle. A careful quantitative analysis of the different spacecraft walls and of their time evolution was performed via a semi-empirical model. The major outcome of the model was to indicate the role of the solar UV radiations in enhancing the deposition through chemical bonding. Further UV polymerization of contaminant layers over time was also presumed to be responsible for the decreasing efficiency of the erosion by atomic oxygen.

The CHAMP-only earth gravity field model EIGEN-2

The German small geoscientific satellite CHAMP flies around the Earth since July 2000 in a highly inclined orbit with an altitude of initially 450 km and meanwhile at about 400 km. From the gravitational orbit perturbations, exploiting GPS-CHAMP satellite-to-satellite tracking and on-board accelerometer data over a time span of altogether six months, a new long-wavelength global gravity field model, called EIGEN-2, has been derived in a German/French effort. Thanks to CHAMP's dedicated orbit configuration, continuous GPS tracking and in-orbit measurement of non-gravitational satellite accelerations, the new CHAMP-only solution provides the geoid and gravity with an accuracy of 10 cm and 0.5 mGal, respectively, at a half wavelength resolution of 550 km. This is an improvement by almost one order of magnitude compared to any multi-satellite pre-CHAMP satellite-only gravity field model.

In-orbit measurements of short term attitude and vibrational environment on the Engineering Test Satellite VI using laser communication equipment

Morio ToyoshimaNational Space Development Agency ofJapan2-2-1 SengenTsukuba, Ibaraki 305-8505JapanE-mail: toyoshima.morio@nasda.go.jpKenichi ArakiCommunications Research Laboratory4-2-1 Nukui-KitamachiKoganei, Tokyo 184-8795JapanE-mail: araki@crl.go.jpAbstract. Angular microvibrations of platform jitter on a three-axisattitude-stabilized satellite are measured in space using an onboard la-ser communication terminal. Stable optical tracking control allows themeasurement of relative angular variations from the reference line ofsight of the optical link. The tracking accuracy in this measurement isless than 1 mrad rms. The angular variation and the drift rate on thesatellite are measured at a sampling rate of 500 Hz, and these data aredownlinked to a ground station via a satellite-to-ground optical commu-nication link. The power spectral density of the satellite microvibration iscalculated by a Fourier transform analysis. The frequency spectrum ofthe gimbal mirror’s angular variation is also obtained when the coarsetracking control is operational. The rms value agrees with the residualcoarse tracking error estimated before launch of the satellite. These re-sults will be useful for the future design of a tracking control loop foroptical communication systems.

The Effect of X‐Ray Absorption Fine Structure in Soft X‐Ray Astronomical Telescopes

Recent in-orbit measurements by high resolution soft X-ray telescopes have revealed low-level fine structure in target spectra that cannot be attributed to a celestial source. Ultimately, this can be traced to the ability of the new high spectral resolution silicon detectors to resolve X-ray absorption fine structure (XAFS) produced in the various detection subsystems. Based on measurements taken at the Daresbury Synchrotron Radiation Source (SRS) and the National Synchrotron Light Source (NSLS), we have modeled the full-up response function of the Joint European X-ray Telescope (JET-X), taking into account edge structure generated in the detectors, filters, and mirrors. It is found that unfolding celestial source spectra using a response function in which the detailed edge shapes are calculated from standard absorption cross sections leads to the generation of spectral artifacts at every absorption edge. These in turn produce unacceptably high values of χ2 in model fits for total source fluxes above ~4 × 104 counts. For JET-X, this corresponds to a source strength of ~0.4 millicrab observed for 105 s. Statistically significant linelike features are introduced into the derived source spectra with amplitudes as great as 10% of the source flux. For JET-X, these features rise above the 3 σ level for integral source exposures above ~5 × 104 source counts. The largest deviations in the residuals arise near 0.5 keV and 2.2 keV and are attributed to XAFS produced in the oxide surface layers of the CCD and the gold reflective surface of the mirrors, respectively. These results are significant for data interpretation tasks with the ASCA, JET-X, XMM, and Advanced X-Ray Astrophysics Facility (AXAF) telescopes.

Measurement of satellite communication parameters-transponder measurements

Measurement procedures and instrumentation for the in-orbit measurement of satellite communication parameters are presented. The measurement procedures are described by equations in which the unknown parameter is expressed by measurable or calculable quantities. The measurement error is evaluated on the basis of the parameter equation and component error estimates. The error estimate is considered useful for purposes of verifying the satellite design and for settling contractual arguments, should they arise.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

In-orbit measurements of Landsat-4 thematic mapper dynamic disturbances

This paper describes Landsat-4 sensors, spacecraft disturbances, and the methodology used to correct Thematic Mapper (TM) images. The discussion includes system requirements, mathematical modeling, in-orbit angular motion measurement, Fast Fourier Transformer (FFT) data analysis, and scan system correction. Emphasis is given to the cause and effect of the electromechanical disturbances beginning with the scan mirror and its interaction and sensitivity to externally and self-generated disturbances. Ground-testing results are compared with those obtained in orbit.

Cryogenic Telescope On The Infrared Astronomical Satellite (IRAS)

The Infrared Astronomical Satellite (IRAS)* has completed an unbiased all-sky survey at wavelengths from 10 to 100 Am. The design and performance of the focal plane array is described with emphasis on in-orbit measurements of the sensitivity and stability. In the four broad spectral bands centered at 12, 25, 60, and 100 Am, the system noise equivalent flux density (NEFD) values are, in Jy/Hz1/2, 0.03, 0.025, 0.046, and 0.21, respectively (Jansky = 10-26 W/m2/Hz). For point sources, a single scan at the survey rate of 3.8 arcmin/s yields limiting flux densities at the 3-sigma confidence level of 0.36, 0.30, 0.39, and 1.2 Jy. The dc stability of the junction field effect transistor (JFET) amplifiers and the excellent off-axis rejection of the tele-scope permit total flux measurements of extended infrared emission at levels below 106 Jy/sr. Response of the extrinsic silicon and germanium photo-detectors to ionizing radiation is described.

Satellite system measurements

This paper describes the measurement techniques employed to test the elements of a communications satellite system. Techniques for measuring the gain-to-noise-temperature ratio (G/T) of an earth station antenna are presented. The measurement of earth station polarization characteristics is discussed extensively in terms of the new requirement for frequency reuse on orthogonal polarizations. Methods for in-orbit measurement of spacecraft performance including antenna characteristics are included, as well as some typical experimental results.