In-flight Calibration Methods Research Articles

In-Flight Calibration of Lorentz Actuators for Non-Contact Close-Proximity Formation Satellites with Cooperative Control

The non-contact close-proximity formation satellite (NCCPFS) is one of the technical solutions to improve the attitude performance, consisting of a payload module (PM) and a support module (SM). The non-contact Lorentz actuator (NCLA), as the core components of the NCCPFS, directly affect the attitude control performance of the entire satellite. In order to ensure the ultra-high attitude pointing performance and stability of the PM, an in-flight calibration method for the NCLAs based on minimum model error (MME) algorithm and Kalman filtering (KF) with cooperative control strategy is proposed in this article. In this method, the NCLAs generate a periodic nominal torque that causes the attitude of the PM to be periodically deflected. This periodic torque also reacts on the SM, and the SM counteracts this periodic torque through the flywheel to realize the cooperative tracking relative to the PM. Then, the gyroscope data are substituted into the MME algorithm to obtain the angular acceleration of the two modules, and the KF algorithm is adopted to observe the actual output torque of the NCLAs to complete the in-flight calibration of the NCLAs. Numerical simulation results show that the accuracy of the proposed calibration algorithm can reach about 8%, which proves the effectiveness of the proposed method.

A new system to measure the gradient vector of the magnetic field on unmanned aerial vehicles – data processing and field experiment

Abstract We present a novel airborne magnetometer system deployed on an unmanned aerial vehicle (UAV) that is capable of measuring the horizontal gradient of the three components of the magnetic field. The system consists of two three-component fluxgate magnetometers (FGM) that are mounted on a transverse horizontal boom. The sensor attitude is determined with a low-cost inertial measurement unit. The estimation of the magnetic field components as well as its gradient is extremely sensitive to sensor movement and sensor rotation and requires sophisticated data processing and corrections. Here, we present four specific calibration and correction procedures we consider essential to achieve sufficient accuracy. First, we present a new in-flight calibration method for an FGM gradiometer. Second, we introduce a procedure that corrects for rotation-induced noise in the FGMs that has not been described previously in the literature. In a third step, we correct for mechanical vibrations, which induce high-frequency noise in the data. Finally, the gradient of each component is mathematically rotated into the geographical coordinate system. The performance of the system is evaluated on a test site where several metal objects of known magnetization were placed on the ground surface. For the first time, we show the gradients of magnetic field components measured on a UAV. The gradients agree with the results of a forward simulation within a few nT m−1. The accuracy will be sufficient for many practical applications, such as geological mapping, ore exploration, and the search for metallic bodies.

Fluxgate Magnetometer Offset Vector Determination Using Current Sheets in the Solar Wind

Abstract The zero offset of the fluxgate magnetometer (FGM) on board the spacecraft varies slowly with time, therefore the FGM needs regular in-flight calibration. There are abundant physical phenomena in the solar wind, such as waves, mirror mode structures, and current sheets. Several in-flight calibration methods have been developed based on the properties of Alfvén waves or mirror mode structures instead of current sheets. Here, we develop a method to determine the zero offset O using current sheets in the solar wind, which is based on an assumption that the magnetic field in the normal direction of the current sheet is zero. The nonzero magnetic field in the normal direction is the projection of O , therefore we can obtain the component of O in the normal direction. In the offset cube, the zero offset is expected to be in a plane, which is referred to as the optimal offset plane (OOP). Each current sheet can obtain an OOP in the offset cube. At least three nonparallel OOPs must be used to determine the zero offset. We test our method by using data from the Magnetospheric Multiscale mission and find that our method is able to determine the zero offset. Our method can simultaneously use Alfvén waves, mirror mode structures, and current sheets to determine the zero offset, thus it might be a useful tool to perform the in-flight calibration of the FGM for solar wind monitors.

In-Flight Calibration of Visible and Near-Infrared Imaging Spectrometer (VNIS) Onboard Chang’E-4 Unmanned Lunar Rover

The Visible and Near-infrared Imaging Spectrometer (VNIS) is one of the scientific payloads onboard the Yutu-2 lunar rover of the Chang’E-4 mission. Based on Acousto-Optic Tunable Filter (AOTF) dispersion, a moving platform and a calibration unit, the VNIS was designed for lunar surface observations of the far side of the Moon for composition exploration and spectral experiments in the wavelength range of 0.45–2.4μm. Large differences in radiance caused by the discrepancy between the laboratory and lunar environment could affect the spectral shape and features. Therefore, in-flight calibration of the VNIS is critical for the interpretation of the acquired spectra. To precisely map the digital output to the spectral radiance, a calibration unit was installed on the VNIS to facilitate radiometric calibration measurements on the lunar surface. After the elimination of stray light and the shadow effect, the measured radiance of the calibration unit was derived by digital multiplication of the pre-flight coefficients. Considering the observation geometry, Moon-Sun distance, and the bi-directional reflectance factor (BRF) of the calibration panel, the theoretical radiance at the entrance pupil of VNIS was calculated. Using linear regression, in-flight calibration of the coefficients was performed and used to correct the spectral data. The performance of the spectrometer was evaluated using 12 lunar-day data. It was determined that the coefficients gradually converged and the variation was less than 1%, indicating that the in-flight calibration was robust. The proposed in-flight calibration method facilitated the acquisition of reliable data for lunar composition interpretation.

In-flight calibration of the spaceborne fluxgate magnetometer in the Martian magnetosheath

In-flight calibration of the zero offset is crucial for ensuring high-precision measurement of the spaceborne fluxgate magnetometer. Tianwen-1 is China’s first Mars mission, and its orbiter will remain out of the solar wind for tens of days each year. Previous in-flight calibration methods might not be suitable for this orbiter during such a period. Recently, a new method was proposed by<xref ref-type="bibr" rid="bWang2022b">Wang GQ (2022b)</xref>, which we refer to as the Wang method II for ease of description. Here, we test the performance of this method in the Martian magnetosheath by using magnetic field data measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We find that the accuracy of the Wang method II is affected by the number of magnetic field subinterval events, the eigenvalues of the minimum variance analysis for each event, and the position of the spacecraft in the magnetosheath. The estimated zero offset varies over a period of ~27 days and has a 57.3% probability of accuracy within 2.0 nT. After being smoothed with a temporal window of 27 days, the zero offset has a 48.4% (99.3%) probability of accuracy within 1.0 (2.0) nT. Our tests suggest that the Wang method II provides an option for the Tianwen-1 orbiter to perform in-flight calibration when the orbiter remains out of the solar wind for an extended period of time.

The Operational Inflight Radiometric Uniform Calibration of a Directional Polarimetric Camera

The directional polarimetric camera (DPC) on-board the GF-5A satellite is designed for atmospheric or water color detection, which requires high radiometric accuracy. Therefore, in-flight calibration is a prerequisite for its inversion application. For large field optical sensors, it is very challenging to ensure the consistency of radiation detection in the whole field of view in the space environment. Our work proposes a vicarious in-flight calibration method based on sea non-equipment sites (visible bands) and land non-equipment sites (all bands). Combined with environmental parameters and radiation transmission calculations, we evaluated the radiation detection accuracy of the 0° to 60° view zenith angle of the DPC in each band. Our calibration method is based on the single-day normalized radiance data measured by the DPC. Through data selection, enough calibration samples can be obtained in a single day (the number of desert samples is more than 5000, and the number of calibration samples of the ocean is more than 2.8×106). The measurements are compared with the simulation of 6SV VRT code or look-up tables. The massive amount of data averages the uncertainty of a single-point calculation. Although the uncertainty of a single sample is significant, the final fitting of the curve of the variation in the radiometric calibration coefficient with the observation angle can still keep the root mean squared error at approximately 2–3% or even lower, and for visible bands, the calibration results for both ocean sites and desert sites are in good agreement regarding the non-uniformity of the sensor.

항공 시스템 기반의 통신신호 방향 탐지를 위한 비행 보정 기법

Direction-finding equipment with multiple antennas are used to estimate the direction of a signal emitted by a source; they can be used to rescue a victim or locate a specified source. During direction finding, reflection waves are present and signal distortion is observed depending on the external shape and material of a system that incorporates the direction-finding equipment and multiple antennas. Therefore, to accurately estimate the azimuth of the signal source and develop the direction-finding equipment, a calibration should be performed to reflect the influence of the antenna arrangement(layout) and system contour. In this paper, we describe an in-flight calibration method to develop direction-finding equipment to locate communication signals using an aviation system, and we analyze the direction-finding performance when applying phase calibration data obtained through the in-flight calibration.

In-Flight Calibration of GF-1/WFV Visible Channels Using Rayleigh Scattering

China is planning to launch more and more optical remote-sensing satellites with high spatial resolution and multistep gains. Field calibration, the current operational method of satellite in-flight radiometric calibration, still does not have enough capacity to meet these demands. Gaofen-1 (GF-1), as the first satellite of the Chinese High-resolution Earth Observation System, has been specially arranged to obtain 22 images over clean ocean areas using the Wide Field Viewing camera. Following this, Rayleigh scattering calibration was carried out for the visible channels with these images after the appropriate data processing steps. To guarantee a high calibration precision, uncertainty was analyzed in advance taking into account ozone, aerosol optical depth (AOD), seawater salinity, chlorophyll concentration, wind speed and solar zenith angle. AOD and wind speed were found to be the biggest error sources, which were also closely coupled to the solar zenith angle. Therefore, the best sample data for Rayleigh scattering calibration were selected at the following solar zenith angle of 19–22° and wind speed of 5–13 m/s to reduce the reflection contributed by the water surface. The total Rayleigh scattering calibration uncertainties of visible bands are 2.44% (blue), 3.86% (green), and 4.63% (red) respectively. Compared with the recent field calibration results, the errors are −1.69% (blue), 1.83% (green), and −0.79% (red). Therefore, the Rayleigh scattering calibration can become an operational in-flight calibration method for the high spatial resolution satellites.

Phase Residual Estimations for PCVs of Spaceborne GPS Receiver Antenna and Their Impacts on Precise Orbit Determination of GRACE Satellites

In-flight phase center systematic errors of global positioning system (GPS) receiver antenna are the main restriction for improving the precision of precise orbit determination using dual-frequency GPS. Residual approach is one of the valid methods for in-flight calibration of GPS receiver antenna phase center variations (PCVs) from ground calibration. In this paper, followed by the correction model of spaceborne GPS receiver antenna phase center, ionosphere-free PCVs can be directly estimated by ionosphere-free carrier phase post-fit residuals of reduced dynamic orbit determination. By the data processing of gravity recovery and climate experiment (GRACE) satellites, the following conclusions are drawn. Firstly, the distributions of ionosphere-free carrier phase post-fit residuals from different periods have the similar systematic characteristics. Secondly, simulations show that the influence of phase residual estimations for ionosphere-free PCVs on orbit determination can reach the centimeter level. Finally, it is shown by in-flight data processing that phase residual estimations of current period could not only be used for the calibration for GPS receiver antenna phase center of foretime and current period, but also be used for the forecast of ionosphere-free PCVs in future period, and the accuracy of orbit determination can be well improved.

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.

Calibration and alignment of metrology system for the Nuclear Spectroscopic Telescope Array mission

A metrology system to measure the on-orbit movement of a ten meter mast has been built for the Nuclear Spectroscopic Telescope Array (NuSTAR) x-ray observatory. In this paper, the metrology system is described, and the performance is measured. The laser beam stability is discussed in detail. Pre-launch alignment and calibration are also described. The invisible infrared laser beams must be aligned to their corresponding detectors without deploying the telescope in Earth’s gravity. Finally, a possible method for in-flight calibration of the metrology system is described.

An In-Flight Calibration Method for Near-Real-Time Humidity Measurements with the Airborne MOZAIC Sensor

Abstract A new in-flight calibration (IFC) method is described for the humidity sensor flown routinely since 1994 on the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) program’s aircraft. The IFC method corrects the potential drift of the sensor offset at zero relative humidity, which is the critical parameter in determining the uncertainty of the measurements. The sensor offset is determined from the measurements themselves as obtained during periods when the aircraft is flying in the lower stratosphere at or above the hygropause, where the H2O mixing ratio reaches well-defined minimum values of about 5 ppmv and the contribution of atmospheric H2O to the sensor signal is minimal. The selection of stratospheric data is achieved with the help of potential temperature, which can be calculated in situ from measured temperature and pressure. The IFC method is capable of providing humidity measurements in near–real time with an uncertainty of ±8% RH at the surface and ±7% RH in the upper troposphere. For validation, the IFC method was applied to 5 yr of archived raw signals from the MOZAIC aircraft. The resulting humidity data are in good agreement (within 2% RH) with the original MOZAIC data that used monthly pre- and postflight calibrations of the sensor. The standard deviation of the differences varies with altitude between ±4% and ±6% RH, which is comparable to the accuracy of the MOZAIC laboratory calibrations. Compared to MOZAIC operation based on monthly calibrations in the laboratory, the use of IFC will substantially reduce the efforts for maintenance and thus will enable operation of the sensor on a large fleet of in-service aircraft for near-real-time measurements of humidity in the troposphere. Because the IFC method will not work on aircraft that never enter the lower stratosphere, for example, aircraft that fly exclusively regional routes or in the tropics, regular offline calibrations will remain important for such aircraft.

The Electric Antennas for the STEREO/WAVES Experiment

The STEREO/WAVES experiment is designed to measure the electric component of radio emission from interplanetary radio bursts and in situ plasma waves and fluctuations in the solar wind. Interplanetary radio bursts are generated from electron beams at interplanetary shocks and solar flares and are observed from near the Sun to 1 AU, corresponding to frequencies of approximately 16 MHz to 10 kHz. In situ plasma waves occur in a range of wavelengths larger than the Debye length in the solar wind plasma λ D ≈10 m and appear Doppler-shifted into the frequency regime down to a fraction of a Hertz. These phenomena are measured by STEREO/WAVES with a set of three orthogonal electric monopole antennas. This paper describes the electrical and mechanical design of the antenna system and discusses efforts to model the antenna pattern and response and methods for in-flight calibration.

PARASOL in-flight calibration and performance

Since 18 December 2004, the PARASOL satellite is a member of the so-called A-train atmospheric orbital observatory, flying together with Aqua, Aura, CALIPSO, CLOUDSAT, and OCO satellites. These satellites combine for the first time a full suite of instruments for observing aerosols and clouds, using passive radiometer complementarily with active lidar and radar sounders. The PARASOL payload is extensively derived from the instrument developed for the POLDER programs that performs measurements of bidirectionality and polarization for a very wide field-of-view and for a visible/near-infrared spectral range. An overview of the results obtained during the commissioning phase and the reevaluation after one year in orbit is presented. In-flight calibration methods are briefly described, and radiometric and geometric performances are both evaluated. All algorithms are based on a panel of methods using mainly natural targets previously developed for POLDER missions and adapted or redeveloped in the PARASOL context. Regarding performances, all mission requirements are met except for band 443 (not recommended for use). After one year in orbit, a perfect geometrical stability was found while a slight decrease of the radiometric sensitivity was observed and corrected through an innovative multitemporal algorithm based on observations of bright and scattered convective clouds. The scientific exploitation of PARASOL has now begun, particularly by coupling these specific observations with other A-train sensor measurements.

Multi-band imaging camera and its sciences for the Japanese near-earth asteroid mission MUSES-C

In this paper we present current development status of our Asteroid Multi-band Imaging CAmera (AMICA) for the Japan-US joint asteroid sample return mission MUSES-C. The launch of the spacecraft is planned around the end of 2002 and the whole mission period till sample retrieval on Earth will be approximately five years. The nominal target is the asteroid 1998SF36, one of the Amor-type asteroids. The AMICA specifications for the mission are shown here along with its ground-based and inflight calibration methods. We also describe the observational scenario at the asteroid, in relation to scientific goals.

In-flight calibration of the POLDER polarized channels using the Sun's glitter

The spaceborne sensor Polarization and Directionality of the Earth Reflectances (POLDER), launched on the Japanese platform Advanced Earth Observation Satellite (ADEOS) on August 17, 1996, is a new instrument devoted to multispectral observations of the directionality and polarization of the solar radiation reflected by the Earth-atmosphere system. Polarization measurements are performed in three channels, centered at 443, 670, and 865 nm. As POLDER has no onboard calibration system, in-flight calibration methods have been developed. The authors address in this paper the calibration of the polarization measurements. The method uses the sunlight reflected within the Sun's glitter. While the radiance of the Sun's glitter depends strongly on the sea surface roughness, its intrinsic degree of polarization depends only on the observation geometry, which is specially convenient for calibration purposes. However, the degree of polarization measured at the satellite level is affected by the atmosphere. The proposed calibration scheme allows the authors to take into account the influence of the atmosphere on the degree of polarization measured in some viewing direction within the glitter pattern by using the radiance measured in the same viewing direction and in another direction far from the glitter. The expected accuracy is about 0.5% in the near-infrared channel 865 nm and about 2% in the visible channels, in terms of percent polarization. The method has been applied successfully to measurements achieved over ocean areas with the airborne version of the POLDER instrument.

In-flight calibration of the Modular Optoelectronic Scanner (MOS)

Since March 1996 the Modular Optoelectronic Scanner (MOS) of the German Aerospace Research Establishment (DLR) has provided spectral radiance data of the atmosphere-surface system in 18 spectral channels and up to 420 pixels in a 200 km swath from a 817 km sun synchronous polar orbit. The MOS consists of two imaging spectrometers, MOS-A (four channels with a halfwidth Delta lambda 1.4 nm in the O2 A absorption band) and MOS-B (13 channels between 400 and 1010 nm with Delta lambda 10 nm), and a camera, MOS-C, at 1.6 mu m. Beside the onground laboratory calibration as the basis for calculation of the spectral radiance data, a mission of such long duration requires a periodic recalibration or at least a stability check of instrument properties in orbit to support the reliability of the remote data. In this paper the two in-flight calibration methods will be described and their capabilities will be demonstrated on the basis of the real data during MOS's first year in orbit: the relative checking of the instrument parameters with internal lamps and the absolute calibration with the extraterrestrial sun radiation.

In-flight calibration of NOAA AVHRR visible and near-IR bands over Greenland and Antarctica

A new method for in-flight calibration of NOAA AVHRR visible and near-IR bands is presented. The approach involves using calibrated NOAA-9 near-nadir reflectances over spatially and temporally uniform ice-surfaces from Greenland and Antarctica to produce reflectance calibration curves for AVHRR instruments in all orbits. The reflectance calibration curves consist of second order polynomial regressions of reflectance on solar zenith angle, derived from observations that are spatially uniform in all AVHRR channels over sub-regions of area 68 km by 68 km. By comparing reflectances from uncalibrated AVHRR instruments with these calibration curves, new channel 1 and 2 calibration coefficients are obtained with an accuracy of 5 per cent. The main advantages of this calibration method are: (1) calibration targets are large; (2) it can be applied over multiple years; (3) it is applicable for a wide range of solar zenith angles, and can therefore be used year-round. When calibration coefficients inferred from NOAA-11 (1994) and NOAA-14 (1995) observations over Greenland and Antarctica are compared with those from the formulae of Rao and Chen (1995, 1996), the two methods are in excellent agreement in channel 1 (within 3 per cent). In channel 2, they agree to within 4 per cent for NOAA-14, but are significantly different for NOAA-11 ( 9 per cent). When applied to NOAA-12 AVHRR observations for 1994-95, channel 1 and 2 calibration coefficients are 20 per cent and 35 per cent larger than prelaunch values, respectively.

Calibration of SCARAB-FM1: Methods and first results

The first flight model of SCARAB (SCAnner for RAdiation Budget), has been launched in February 94. Due to failures of on-board lamps just after the launch, the in-flight calibration strategy has been changed during the mission. Thanks to this new method, one year of data has been collected. This alternate method of calibration is so performant that it will become the main in-flight calibration strategy for the next model (that will be launched during the second trimester of 97). In this paper is firstly explained the original in-flight calibration method planned to fulfill the mission. The behaviour of the calibration module after the launch is analysed and a diagnosis of the failures is set. A synthesis of remaining resourses is made up. Ascertaining that some parameters are very stable during all the mission, the new calibration strategy is built. This method is based on the accuracy of the law linking gains to the temperature of the instrument, and also on the very high reliability of the on-board black body simulators. Performances of both methods are compared. Points to strengthen for the future model are also raised.

NOAA-11 AVHRR visible and near-IR inflight calibration

An inflight calibration method for NOAA-11 AVHRR visible and near-infrared bands using desert reflectance is discussed. The results show a decrease in sensor systems response of 22 percent and 32 percent for the visible and near-infrared bands, respectively, relative to the published preflight calibration. The relative accuracy of the present calibration to the calibration of NOAA-9, published elsewhere, is + or - 3 percent. The absolute accuracy is around + or - 6 percent. An approximate correction to the Normalized Difference Vegetation Index ratio suggests that an increase of 0.06 is required.