In-orbit Calibration Research Articles

Magnetic properties of CHAMP and their effects on in-orbit calibration

Abstract. CHAMP has so far been the most successful magnetic field mission. For achieving that, special effort had to be invested in building a magnetically clean spacecraft. The magnetic moment of the spacecraft is about 1 Am2. In this article we introduce a new method that allows the determination of the magnetic moment during the mission from the average current strength of the magneto-torquers. In order to achieve precise field vector data, the readings of the fluxgate magnetometer are routinely calibrated against the absolute Overhauser measurements. A reanalysis of all the magnetic field data is performed which takes also into account small disturbances from the power system. Uncertainties of the final magnetic field data are estimated to be of the order of 0.1 nT.

Digging supplementary buried channels: investigating the notch architecture within the CCD pixels on ESA's Gaia satellite

The European Space Agency (ESA) Gaia satellite has 106 CCD image sensors which will suffer from increased charge transfer inefficiency (CTI) as a result of radiation damage. To aid the mitigation at low signal levels, the CCD design includes Supplementary Buried Channels (SBCs, otherwise known as `notches') within each CCD column. We present the largest published sample of Gaia CCD SBC Full Well Capacity (FWC) laboratory measurements and simulations based on 13 devices. We find that Gaia CCDs manufactured post-2004 have SBCs with FWCs in the upper half of each CCD that are systematically smaller by two orders of magnitude (<50 electrons) compared to those manufactured pre-2004 (thousands of electrons). Gaia's faint star (13 < G < 20 mag) astrometric performance predictions by Prod'homme et al. and Holl et al. use pre-2004 SBC FWCs as inputs to their simulations. However, all the CCDs already integrated onto the satellite for the 2013 launch are post-2004. SBC FWC measurements are not available for one of our five post-2004 CCDs but the fact it meets Gaia's image location requirements suggests it has SBC FWCs similar to pre-2004. It is too late to measure the SBC FWCs onboard the satellite and it is not possible to theoretically predict them. Gaia's faint star astrometric performance predictions depend on knowledge of the onboard SBC FWCs but as these are currently unavailable, it is not known how representative of the whole focal plane the current predictions are. Therefore, we suggest Gaia's initial in-orbit calibrations should include measurement of the onboard SBC FWCs. We present a potential method to do this. Faint star astrometric performance predictions based on onboard SBC FWCs at the start of the mission would allow satellite operating conditions or CTI software mitigation to be further optimised to improve the scientific return of Gaia.

Absolute Radiometric Calibration of Earth Radiation Measurement on FY-3B and Its Comparison With CERES/Aqua Data

The Earth Radiation Measurement (ERM) instrument onboard FengYun (FY)-3B satellite observes the Earth's atmosphere through a narrow scanning field of view (NFOV) and a wide nonscanning FOV. For each field of view, the measurements are made from two broadband channels: a total waveband channel covering 0.2-50 μm and a shortwave (SW) band covering 0.2-4.3 μm. The validation to the ERM calibration was carried out by comparing the unfiltered longwave (LW) and SW radiances from ERM with those from Clouds and Earth's Radiation Energy System (CERES) flight model 3 onboard Earth Observing System Aqua satellite. While the ERM LW and SW radiances have a good correlation with CERES data, there is a systemic bias between the two data sets. A spectral correction is made for the ERM data using the CERES data. After the correction, the error of the ERM LW radiance is reduced from -3.00 to -0.60 W/sr · m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . For the SW radiance, the bias is reduced from 6.00 to 4.00 W/sr · m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Based on the ERM in-orbit calibration data, the stability of the ERM LW radiometric response is analyzed, and it is shown that the gains are stable with a variation of less than 1.5% during its first 9 months in orbit. However, the gains at the SW channels have larger changes and exceed 3%. These drifts might be caused by the detector degradation. Also, the NFOV scanner at the SW channel is no longer working after its 8 months in orbit.

PLEIADES ABSOLUTE CALIBRATION : INFLIGHT CALIBRATION SITES AND METHODOLOGY

Abstract. In-flight calibration of space sensors once in orbit is a decisive step to be able to fulfil the mission objectives. This article presents the methods of the in-flight absolute calibration processed during the commissioning phase. Four In-flight calibration methods are used: absolute calibration, cross-calibration with reference sensors such as PARASOL or MERIS, multi-temporal monitoring and inter-bands calibration. These algorithms are based on acquisitions over natural targets such as African deserts, Antarctic sites, La Crau (Automatic calibration station) and Oceans (Calibration over molecular scattering) or also new extra-terrestrial sites such as the Moon and selected stars. After an overview of the instrument and a description of the calibration sites, it is pointed out how each method is able to address one or several aspects of the calibration. We focus on how these methods complete each other in their operational use, and how they help building a coherent set of information that addresses all aspects of in-orbit calibration. Finally, we present the perspectives that the high level of agility of PLEIADES offers for the improvement of its calibration and a better characterization of the calibration sites.

SENSOR CALIBRATION OF THREE-LINE CCD SCANNERS ON ZY-3

Abstract. ZY-3 is the first Chinese civilian high-resolution stereo mapping satellite. ZY-3 is equipped with three-line scanners (nadir, backward and forward views with 2.5 m and 4.0 m resolutions, respectively) and an additional multispectral scanner. In order to improve the mapping and positioning accuracy, sensor in-orbit calibration is a crucial procedure especially for high-resolution mapping-enabled satellites. The main focus of this paper is to describe the sensor calibration for ZY-3 sensors. Based on the experiences for other satellite sensor calibration, the following approach is proposed for ZY-3 sensor calibration in orbit: i) the initial internal geometric relationships of ZY-3 imaging sensors are established using the laboratory measurement and validation results; ii) assuming noise and errors are introduced once ZY-3 is launched, a further investigation is conducted to detect those changed geometric relationships and their associated parameters, and detect and reduce highly correlated parameters. A self-calibration method is applied to identify and correct those parameters. Geometric calibration field data is used together with simulated ZY-3 data to perform the self-calibration; iii) the newly obtained parameters are used to establish the final geometric relationships model of ZY-3 imaging sensors, and with the aid of ground control points and check points from the geometric calibration field, those parameters are to be validated, and the position accuracy is also assessed by applying ZY-3 image data. Although promising results were obtained using ZY-3 simulated data, the proposed approach for ZY-3 imaging sensor calibration is an ongoing research work and needs to be fine tuned in the coming months.

Improvements in Calibration of GSO Scintillators in the Suzaku Hard X-Ray Detector

Abstract Improvements of the in-orbit calibration of GSO scintillators in the Hard X-ray Detector aboard Suzaku are reported. To resolve an apparent change in the energy scale of GSO, which appeared across the launch for unknown reasons, consistent and thorough re-analyses of both pre-launch and in-orbit data have been performed. With laboratory experiments using spare hardware, the pulse-height offset, corresponding to zero energy input, was found to change by $\sim\ $ 0.5% of the full analog voltage scale, depending on the power supply. Furthermore, by carefully calculating all of the light outputs of secondaries from activation lines used in the in-orbit gain determination, their energy deposits in GSO were found to be effectively lower, by several percent, than their nominal energies. Taking both of these effects into account, the in-orbit data agree with the on-ground measurements within $\sim\ $ 5%, without employing the artificial correction introduced in previous work (Kokubun et al. 2007, PASJ, 59, S53). With this knowledge, we updated the data processing, the response, and the auxiliary files of GSO, and reproduced the HXD-PIN and HXD-GSO spectra of the Crab Nebula over 12–300 keV by a broken power-law with a break energy of $\sim\ $ 110 keV.

Engineering Design and Error Analysis of HY-2 Radar Altimeter In-orbit Calibration

Radar altimeter is one of the major payloads on microwave remote sensing.An altimeter will be embarked on the upcoming HY-2 satellite.In-orbit instabilities will occur in the radar system due to ageing effects,temperature floats,and space radiations and so on.Internal calibrations are applied to solve this problem.In this paper the authors describe the tasks of internal calibration and compare the two main calibration schemes,and choose the second for engineering design.Then the authors design the attenuation and delay parameters of the calibration channel.The authors also analyze the error sources occurring in calibration channels.

MICROSCOPE On-ground and In-orbit Calibration

The MICROSCOPE mission, to be launched in 2011, will perform the test of the universality of free fall (Equivalence Principle) to an accuracy of 10−15. The payload consists of two sensors, each controlling the free fall of a pair of test masses: the first for the test of the Equivalence Principle (titanium/platinum), the second for performance verification (platinum/platinum).

Final TerraSAR-X Calibration Results Based on Novel Efficient Methods

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> TerraSAR-X is a satellite mission for scientific and commercial applications operating a highly flexible X-band synthetic aperture radar (SAR) instrument with a multitude of different operation modes. As product quality is of crucial importance, the success or failure of the mission depends essentially on the method of calibrating TerraSAR-X in an efficient way during commissioning the entire system in a restricted time. Only then, product quality and the correct in-orbit operation of the entire SAR system can be ensured. This paper describes the in-orbit calibration method for TerraSAR-X and dedicated activities performed during the commissioning phase as well as final results derived from all calibration procedures. </para>

Introduction

Today we are living in an era of challenging space missions. They will improve our knowledge and understanding of our planet, our solar system, our galaxy and the universe enormously. A very challenging class of missions is aiming at the understanding of the universe and its underlying fundamental principles of physics thus combining astronomy, astrophysics and fundamental physics. But for all these missions the same conditions arise: (a) They are scientifically and technically a new generation of scientific satellites because of an expected improvement of measurement accuracy by several orders of magnitude. (b) The dynamics of the satellite is very closely linked to the scientific measurement instrument. (c) The processing of the measurements needs a long time. Keeping that in mind it becomes clear how challenging these missions are, and the questions arise: How can one make sure that the measurements that are taken today are not affected by undetectable errors which may disturb or even falsify the final result? and How can we prove that spacecraft and instrument which have been developed for these missions will really provide the expected performance? The workshop “Satellite Dynamics: Modelling, In-Orbit Calibration and Data Processing” held from May 21st to May 23rd 2008 in Bremen is one element of the EU-project “First Look”. It showed current developments and methodologies for monitoring scientific data quality, allowed discussions about approaches for scientific quality monitoring, processing techniques and lessons learned.

Polarimetric SAR Internal Calibration Scheme Based on T/R Module Orthogonal Phase Coding

Two important aspects of internal calibration of polarimetric synthetic aperture radar (SAR) are discussed, i.e., individual transmit/receive module (TRM) calibration and system gain calibration. The system has a general structure utilizing a phased array antenna composed of dual-channel TRMs. TRM gain and phase calibration is carried out using orthogonal phase coding (OPC). The signal of the individual TRM is phase-encoded according to a set of orthogonal codes to be separated from the composite calibration signal. OPC uses 1 bit of a digital phase-shifter for encoding, without the need for additional encoding hardware. Performance of the method is examined. Calibration results are developed both theoretically and through simulation in case of TRM amplifier or phase-shifter failure. Zero-padding is used to eliminate calibration error of the first TRM. A crosstalk model is proposed to investigate the effect of imperfect isolation between the two polarization channels of each TRM, and a way to reduce this error is also given. At last, system path gain variation is measured utilizing the internal calibration loop. The OPC method has an accuracy of 0.2 dB for gain and better than 2deg for phase, with 10-dB signal-to-noise ratio and perfect isolation between the two polarization channels. The error due to imperfect isolation is usually small and can be ignored. The simple way to detect TRM malfunction is verified through simulation, and it is also in accordance with TerraSAR-X in-orbit calibration outcomes. The proposed OPC method is shown to be an effective way of internally calibrating TRMs of a phased array antenna.

In-orbit calibration of the lunar magnetometer onboard SELENE (KAGUYA)

Abstract The high-sensitivity fluxgate Lunar MAGnetometer (LMAG) is mounted on SELENE (KAGUYA) to investigate the near-surface electromagnetic environment and the evolution of the Moon through magnetic field observation. To avoid possible electromagnetic interferences, a triaxial fluxgate sensor (MGF-S) is installed at the far end of a 12-m-long mast. It is critical for the accurate observation to monitor MGF-S alignment in orbit, and thus we have calibrated the sensor alignment by measuring the known magnetic fields generated by the sensor alignment monitor coil (SAM-C) wound onto the mast canister. In-orbit calibration of the MGF-S alignment was performed twice each revolution during the initial check-out phase of the satellite. It is concluded that there is no systematic difference in the sensor alignment between the day-side and night-side. Applying a new technique based on the Davis-Smith method to the observed magnetic field data when KAGUYA was exposed to the solar wind, a zero offset of each axis was quickly and stably determined every month. As a result, LMAG has been calibrated with an accuracy that is sufficient for detection of the lunar magnetic anomaly at an altitude of 100 km and for high-resolution electron reflectometry.

Analysis on Effect of Space Microwave Emission Environment on In-Orbit Calibration of Chang’E-1 Microwave Sounder

Chang'E-1 Lunar Microwave Sounder(CELMS) is the first attempt to sound lunar soil by passive microwave sensors.Microwave emission from cosmic background is used as low-temperature reference for the in-orbit calibration of CELMS,which is usually assumed 2.7 K.But when the calibration antenna of CELMS is pointed to the space background for cold-sky calibration,some celestial bodies near the moon,such as the sun,the earth and some other radio sources with larger angular breadth from the moon and stronger radiation in microwave band,like the galactic center,Cygnus,Taurus etc.,may enter the calibration antenna at some certain time.These additional microwave emissions can be the main factor affecting calibration accuracy of CELMS.In this paper,the orbit analysis of CE-1 lunar satellite is provided.The incident angles and microwave emission contributions of each celestial body entering the view of the calibration antenna are calculated according to the distribution of these microwave radiation sources.Some results of the effect of the space microwave emission environment on the calibration accuracy of CELMS are obtained.Based on these results,it can be concluded that the biggest effect is from the radiation of the sun,followed by the earth and the galactic center.All these factors can lead to an effect with a magnitude of 10 K.

The Flat Target Transformation

This paper describes a practical method to calibrate the microwave imaging radiometer with aperture synthesis (MIRAS) in orbit using external targets. Together with the in-orbit validation of the antenna patterns, the method ensures the generation of valid and calibrated visibilities from raw measurements with minimum impact from instrument errors, particularly antenna errors. It has been possible to devise this strategy only after the derivation of the Corbella equation, which correctly describes how MIRAS works. The proposed in-orbit calibration and validation approach is fully based on this equation. In a markedly different way to the point target response method used to calibrate other imaging systems, the technique put forward is essentially based on measuring a ldquoflatrdquo target such as the cold sky near the Galactic poles. This is why it has been called the ldquoflat target transformationrdquo.

Strategies for in-orbit calibration of drag-free control systems

Drag-Free Satellites (DFS) are a class of scientific satellite missions designed for research on fundamental physics as well as geodesy. They consist, basically, of a small inner satellite (test mass) located in a cavity inside a larger satellite, the normal one. The Drag-Free Attitude Control System (DFACS) is the most complex technology on-board these satellites. This key technology allows the residual accelerations on experiments on board the satellites to be significantly reduced. In order to achieve this very low disturbance environment (for some missions < 10 −14 g) the drag-free control system has to be optimized. This optimization process is required because of uncertainties in system parameters that demand a robustness of the control system. This paper will present approaches for in-orbit calibration of drag-free control systems. The discussion includes modeling, with scale factors and cross couplings, possible excitation signals, comparison of different parameter identification/estimation methods as well as simulation results.

Hard X-ray emission of the Earth's atmosphere: Monte Carlo simulations

We perform Monte Carlo simulations of cosmic ray-induced hard X-ray radiation from the Earth's atmosphere. We find that the shape of the spectrum emergent from the atmosphere in the energy range 25-300 keV is mainly determined by Compton scatterings and photoabsorption, and is almost insensitive to the incident cosmic ray spectrum. We provide a fitting formula for the hard X-ray surface brightness of the atmosphere as would be measured by a satellite-borne instrument, as a function of energy, solar modulation level, geomagnetic cut-off rigidity and zenith angle. A recent measurement by the INTEGRAL observatory of the atmospheric hard X-ray flux during the occultation of the cosmic X-ray background by the Earth agrees with our prediction within 10 per cent. This suggests that Earth observations could be used for in-orbit calibration of future hard X-ray telescopes. We also demonstrate that the hard X-ray spectra generated by cosmic rays in the crusts of the Moon, Mars and Mercury should be significantly different from that emitted by the Earth's atmosphere.

In-Orbit Performance of the Hard X-Ray Detector on Board Suzaku

Abstract The in-orbit performance and calibration of the Hard X-ray Detector (HXD) on board the X-ray astronomy satellite Suzaku are described. Its basic performances, including a wide energy bandpass of 10–600 keV, energy resolutions of $\sim 4 \,\mathrm{keV}$ (FWHM) at 40 keV and $\sim 11\%$ at 511 keV, and a high background rejection efficiency, have been confirmed by extensive in-orbit calibrations. The long-term gains of PIN-Si diodes have been stable within 1% for half a year, and those of scintillators have decreased by 5–20%. The residual non-X-ray background of the HXD is the lowest among past non-imaging hard X-ray instruments in energy ranges of 15–70 and 150–500 keV. We provide accurate calibrations of energy responses, angular responses, timing accuracy of the HXD, and relative normalizations to the X-ray CCD cameras using multiple observations of the Crab Nebula.

Equivalence principle test with microscope: Laboratory and engineering models preliminary results for evaluation of performance

MICROSCOPE is a space mission, scheduled for a launch in 2010, which aims to verify the equivalence principle (EP) with an accuracy of 10 - 15 , over a hundred times better than what has been realized on Earth today. The EP test is based on the measurement of the electrostatic forces to be applied on each test mass of two concentric inertial sensors, in order to maintain these two masses on the same gravitational trajectory. The instrument formed by these two sensors is called a differential accelerometer. On board the MICROSCOPE satellite, two instruments will embark: one for the test itself, and the other as a performance reference. The first of several mechanical models for this instrument have been manufactured. Each is designed to demonstrate a specific part of the development. Associated to the differential accelerometer, a performance software has also been developed. In order to reach the required performance, an in-orbit calibration phase is planned to improve the knowledge of the geometrical dissymmetries and orientation mismatches. Detailed finite element thermal models of the accelerometers have been realized and currently provide information on the behavior of accelerometer parts in response to the satellite thermal perturbations.

Denormalization of Visibilities for In-Orbit Calibration of Interferometric Radiometers

This paper reviews the relative calibration of an interferometric radiometer taking into account the experimental results of the first batch of receivers developed in the frame of the European Space Agency's Soil Moisture and Ocean Salinity mission. Measurements show state-of-the-art baseline performance as long as the system is capable of correcting the effect of orbital temperature swing. A method to validate internal calibration during in-orbit deep-sky views and to correct linearity errors is also presented.

Calibration of MICROSCOPE

The MICROSCOPE mission is planned for launch in early 2009. It aims to verify the Equivalence Principle to an accuracy of 10 −15, which is currently difficult to obtain on Earth because of the intrinsic limitations of the torsion pendulum and disturbing phenomena, like seismic activity. In space the experiment can take advantage of the quiet environment provided by a drag-free satellite. The instrument used for the test is a differential electrostatic accelerometer composed of two inertial sensors with test-masses made of different materials: one in Platinum–Rhodium alloy, the other in Titanium alloy. The space experiment will also benefit from a second differential accelerometer with both test-masses of the same material, which will be used as a reference instrument to characterise the disturbing signals and sensitivities. The in-orbit calibration of the instrument is mandatory to validate the space test and several procedures have been previously proposed, taking advantage of the satellite propulsion system or the a priori knowledge of natural in-orbit applied accelerations. Due to the actual configuration of the MICROSCOPE propulsion system, the possibility of accurate satellite manoeuvres is limited but sufficient. This paper presents the necessary compromise between the knowledge of satellite and instrument parameters and the calibration procedures. The scenario of the MICROSCOPE in-orbit calibration phase is finely defined in agreement with the required performances for the EP test accuracy.