Update on Swarm Field-Aligned Current Dual (FAC_AC) data issue

<p>As a follow-up of the news published on <a href="https://earth.esa.int/eogateway/news/swarm-field-aligned-current-dual-fac-ac-data-missing-from-7-november-2021">23 November 2021</a> and&nbsp;on <a href="https://earth.esa.int/eogateway/news/update-on-swarm-field-aligned-current-dual-fac_ac-data-missing-from-7-november-2021">16 December 2021</a>, we want to update Swarm users about the issue affecting Field-Aligned Current Dual (FAC_TMS_2F) data: we can confirm that such data are affected by the reduced longitudinal separation between Swarm Alpha and Charlie consequent to orbits variations in the period from July 2021 to January 2022, and thus they are not scientifically meaningful.</p>

<p>For this reason, all FAC_TMS_2F data from 1 July 2021 to 18&nbsp;January 2022 have been removed from the relevant folder in the <a href="https://swarm-diss.eo.esa.int/#swarm%2FLevel2daily%2FLatest_baselines%2FFAC%2FTMS%2FSat_AC">Swarm dissemination server</a> and will not be recovered.</p>

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Star Camera Assembly (SCA) precisely determines the two GRACE satellite's orientation by tracking them relative to the position of the stars.

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The Laser Retro-Reflector (LRR) is mounted on the Earth-facing panel of Envisat close to the RA-2 antenna to support satellite ranging for precise orbit determination and RA-2 range measurement calibration. Laser tracking provides the distance between the spacecraft and the station and is assimilated in precise orbit determination. It will be used extensively during the commissioning phase and regularly during the mission to verify the stability of the positioning system.

Laser Retro-Reflector consisting of four corner cubes mounted in a small pyramid on the underside of the spacecraft. They provide a means of tracking the GRACE satellites from the ground for backup and orbit verification.

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Precise orbit determination relies on the data from the GPS receivers providing full global coverage of orbit data. The alternative method of Satellite Laser Ranging (SLR) is used as a supporting and independent means for orbit determination. Its limitation is the rather poor global and temporal coverage, but the SLR data is free from ambiguities and directly related to the terrestrial reference frame.

Each of the Swarm satellites is equipped with a Laser Retro-Reflector (LRR) of novel design for external calibration and validation of the onboard GPS receiver. A basic requirement for LRR is to enable the worldwide SLR station network to track the satellite with high accuracy and with a sufficiently high link budget under both night and daytime ranging conditions.

The PRARE (Precise Range and Range-Rate Equipment) instruments, carried aboard ERS-1 and ERS-2, were developed for precise satellite range determination leading to higher-accuracy altitude measurements.

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A spectrometer is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the spectral components are somehow mixed. In visible light a spectrometer can for instance separate white light and measure individual narrow bands of colour, called a spectrum, while a mass spectrometer measures the spectrum of the masses of the atoms or molecules present in a gas. The first spectrometers were used to split light into an array of separate colours. Spectrometers were developed in early studies of physics, astronomy, and chemistry. The capability of spectroscopy to determine chemical composition drove its advancement and continues to be one of its primary uses. Spectrometers are used in astronomy to analyse the chemical composition of stars and planets, and spectrometers gather data on the origin of the universe. Hyperspectral radiometer—An advanced multispectral sensor that detects hundreds of very narrow spectral bands throughout the visible, near-infrared, and mid-infrared portions of the electromagnetic spectrum. This sensor’s very high spectral resolution facilitates fine discrimination between different targets based on their spectral response in each of the narrow bands. A device that is designed to detect, measure, and analyse the spectral content of incident electromagnetic radiation. Conventional imaging spectrometers use gratings or prisms to disperse the radiation for spectral discrimination.

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Interferometric radiometers are passive imaging instruments for imaging radiation emission from a target scene.

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By combining different radar images of the same location – a technique known as interferometry – tiny millimetre-scale ground motion can be identified.

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A technology demonstration that uses laser interferometry to measure fluctuations in the separation between the two GRACE spacecraft.

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The accelerometer serves to measure all non-gravitational accelerations on the GRACE satellite due to air drag, solar radiation pressure or attitude control activator impulses initiated by the attitude and orbit control system (AOCS).

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The Accelerometer measures the Swarm satellite's non-gravitational acceleration in its respective orbit. The instrument helps scientists to study factors that cause non-gravitational accelerations such as air drag, winds, Earth albedo, and direct solar radiation pressure on the spacecraft.

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The Advanced Technology Microwave Sounder (ATMS), a cross-track scanner with 22 channels, provides sounding observations needed to retrieve profiles of atmospheric temperature and moisture for civilian operational weather forecasting as well as continuity of these measurements for climate monitoring purposes. Like the long heritage of its predecessors, ATMS combines all the channels of the preceding AMSU-A1, AMSU-A2, and AMSU-B sensors into a single package with considerable savings in mass, power, and volume.[Source: NPP home page, http://npp.gsfc.nasa.gov/atms.html].

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The SSTI (Satellite to Satellite Tracking Instrument)is a state-of-the-art GPS receiver that has been designed to operate in a low-Earth orbit environment. The objective is to provide the SST-hl (Satellite-to-Satellite Tracking - high/low) contribution to the gravity field recovery, by the simultaneous tracking of up to 12 GPS satellite signals. In addition, SSTI provides data for precise orbit determination; it is also used for real-time on-board navigation and attitude-reference-frame determination. The SSTI instrument is based on the Lagrange architecture, a flight-proven device of Laben, Milan, Italy, a unit of Thales Alenia Space, Italy

SSTI

GAP (GPS Attitude and Positioning Experiment) instrument objective is to measure S/C velocity and attitude as well as TEC (Total Electron Content) of the ionosphere. The relative phase delay of signals in both the L1 and L2 bands from a GPS satellite occulted by the limb ionosphere provide large-scale (1000's of km) information on how the total electron content responds to magnetospheric perturbations. GAP consists of two components:
- GAP-A: Three single-frequency (L1) GPS receivers and four patch antennas. This package is being used to provide an accurate absolute time reference, spacecraft 3-D attitude, and post-processed spacecraft position and velocity.
- GAP-O: The objective is to provide ionospheric tomography observations.

Update on Swarm Field-Aligned Current Dual (FAC_AC) data issue

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