EnVision Definitions - EnVision

The closeness of agreement between a measurement and the accepted reference value. The term accuracy, when applied to a set of measurements, involves a combination of random components and a common systematic error or bias component, and can be expressed as follows:

Based on ISO standard 3534-1 (ISO, 1993), and ISO/EC Guide 98-3:2008 – Section 4.2 for uncertainty.

An acquisition is the result of a single self-consistent measurement, i.e. a single spectrum (with a given spectral and spatial resolution). This spectrum is obtained by processing one or several half-frames (for LR/HR channels) with the following operations: half-frame stacking, deviant pixel rejection, pixel binning.

The difference between the expectation of measurements and the accepted reference value. The bias is the total systematic error. There may be one or more systematic error components contributing to the bias.

Scalar product of the radiometric error vector ΔS, with the gain vector G, for a given spectral interval:

ESRA = Σ_k G_k×ΔS_k

In the case of VenSpec-U, ΔS_k is dimensionless (radiance factor). It could also be expressed in spectral radiance or brightness temperature (would potentially be implemented for VenSpec-H and VenSpec-M in the future). Therefore, the retrieval accuracy is specified in ESRA.

The spectral grid is chosen consistently with the Instrument Spectral Response Function width.

Generalized inverse of the linearized forward model. It links spectral features δy_k with retrieval bias δx: δx = Σ_k G_k×δy_k. This yields:

Instrument

A unit or collection of units provided by an instrument consortium.

Launch strategy

Currently two launch strategies into the same interplanetary transfer trajectories are considered per launch date: the spacecraft is either inserted directly by the launcher into the interplanetary transfer trajectory (direct escape strategy) or by the spacecraft's own propulsion system out of a highly elliptical Earth orbit (HEO strategy). Consequently, the actual launch date differs in fact several weeks as the HEO strategy has to account for the additional time spent in Earth orbit between launcher separation and Earth escape sequence.

Measurement resolution

Spatial sampling (see below).

Venus mesosphere (also called stratosphere in some literature), reaching from about 50 km (pending on latitudinal position) up to about 100 km.

Mixed statistical interpretation

Defined in ECSS-E-ST-60-10C as the variation across both time and
over the statistical ensemble.

The science operation phase, defined as 6 cycles (SciRD v2.4).

An observation is a set of acquisitions having a similar (or at least linked) context and set of parameters. It is related to a sequence of operations and actions performed at instrument level to obtain a consistent set of data, typically on a dayside half-orbit.

Based on EnVision document VES-RP-LAT-0104.

Payload

The part of the system performing the measurements required to achieve the science requirements. The Payload consists of multiple instruments. Venus mesosphere (also called stratosphere in some literature), reaching from about 50 km (pending on latitudinal position) up to about 100 km.

Platform

The part of the Space Segment performing all the functions required to support the operations of the payload and more generally, of the Spacecraft.

Precision

The closeness of agreement between independent test results obtained under stipulated conditions. It depends only on the distribution of the random errors. It is computed as the standard deviation of the measurements.

Based on ISO standard 3534-1 (ISO, 1993).

Precision (see above).

Safe mode

Safe mode is defined such that

• uninterrupted power supply as required for spacecraft safety is provided

• a thermally safe attitude is maintained

• permanent up and downlink communication with the ground (ESTRACK 35m ground stations) is guaranteed, enabling the spacecraft to transmit to ground the critical event log and essential HK TM.

Note: The critical event log must be downlinked within one non-occulted communication period over an orbit around Venus.

The solar RAAN is the angle of the Sun direction with respect to the
orbital plane. It is also known as the “Beta” angle

Space Segment

The flying part of the System, also named Spacecraft or Spacecraft System and composed of the Payload and the Platform.

The FWHM size of the spatial point-spread function of the instrument projected at the altitude of the measurement at, below or above the Venus surface.

Geometrical horizontal or vertical sampling by the instrument or the Venus sub-surface, surface or atmosphere. Typically 2x smaller than the spatial resolution.

Survival mode

The Survival mode is defined such that

• uninterrupted power supply as required for spacecraft safety is provided

• a thermally safe attitude is maintained

• enables the spacecraft to signal to ground (ESTRACK 35m ground stations) that the spacecraft is in Survival mode

• enables the spacecraft to be commanded by ground (ESTRACK 35m ground stations).

System

The assembly of Space Segment, Ground Segment and Launch Vehicle which allows the fulfilment of the mission requirements.

Bias (see above).

Venus Thermosphere reaching from 100 km to about 140 km altitude.

Venus Troposphere reaching from 0 km up to about 50 km altitude (pending on latitudinal position).

The closeness of agreement between the average value obtained from a large series of measurements and an accepted reference value. The measure of trueness is in the document expressed in terms of bias.

Based on ISO standard 3534-1 (ISO, 1993).

Venus cycle (or cycle)

The time needed by Venus to perform a rotation around itself (Venus “day”). A cycle is a bit longer than a Venus sidereal day due to the tiny regression of the lines of nodes caused mainly by Venus J2. A cycle is approx. 243.37 days (Earth days).

Note : A Venus revolution (Venus “year”) is defined as the time needed by Venus to perform a revolution around the Sun. A Venus revolution corresponds to ca. 224 Earth days

The ground / launch campaign shall start after the Flight Acceptance
Review (FAR) of the system and last until launcher lift-off.

The Launch and Early Operations Phase (LEOP) shall start upon
removal of the umbilical from the launcher and end at the completion of launcher dispersion trajectory correction and acquisition of safe attitude and nominal communication with ground.

The S/C commissioning phase starts at the end of the LEOP and ends
with all spacecraft units switched on and verified.

The EES (also called High Elliptical Earth Orbit or HEO) phase exists
only for launch scenarios, where the spacecraft is not reaching Earth
escape velocity at launcher separation (direct escape). During the EES phase, the spacecraft remains several revolutions in orbit around Earth, and performs a sequence of manoeuvres by its own propulsion system, consisting of Perigee Raising Manoeuvres (only for sone launch dates), Apogee Raising Manoeuvre, Inclination Correction Manoeuvre and Earth Escape Manoeuvre, where it is injected on the interplanetary transfer trajectory by its own propulsion system. The sequence details are described in the CreMA document.

The interplanetary transfer covers the phase between reaching Earth
escape velocity and performing the Venus Orbit Insertion (VOI) burn and may include in some scenarios a Deep Space Manoeuvre and/or an Earth Swing By and a B-plane targeting manoeuvre.

The aerobraking phase starts from an initial aerobraking orbit with a
walk-in and ends with a walk out followed by a science orbit acquisition burn. During the aerobraking phase, the apocenter of the spacecraft is successively decreased using atmospheric drag whilst the pericenter altitude is controlled within an acceptable aerobraking corridor.

The instrument commissioning phase starts after reaching the science orbit and is foreseen for commissioning (and potentially calibrating) the scientific instruments before start of nominal science phase.

Refers to the nominal science operations phase starting after instrument commissioning in Venus orbit and ends with decommissioning after 6 full Venus cycles. In phase A, also an extended science operations phase (EOP) was discussed, which is currently not part of the analysed baseline.

The disposal phase starts at end of the nominal science phase
(potentially end of extended science phase if applicable) and ends with the passivation of all S/C elements. The spacecraft will then be passively decaying thanks to atmospheric drag and other environmental disturbances and entering Venus' atmosphere (uncontrolled atmospheric entry).

Aerobraking pass sequence

The sequence of operations and tele-commands to be carried out to achieve an aerobraking pass during the pericentre passage.

An aerobraking corridor is a domain, usually expressed in terms of heat
flux and/or heatload as function of time and/or orbital period, such that if the SC is within the domain, it will remain within a safe environment with reasonable probability over the full aerobraking duration, while fulfilling the requirement on the maximum aerobraking duration.

AOC manoeuvre

Attitude and Orbit Control  (AOC) Manoeuvres;  includes small OCM orbit correction burns  and burns to achieve reaction wheel off-loadings

Ascending / Descending Arc

The ascending arc is the arc of the orbit where the S/C is traveling in
North direction (VZ>0 in VME frame),
The descending arc is the arc of the orbit where the S/C is traveling in
South direction ( VZ<0 in VME frame)

The Aerobraking corridor safety limit corresponds to the heat flux profile
which would need to be followed by the S/C to reach the qualification
temperature for the sizing exposed elements of the SC.

Autonomously performed by the spacecraft to respond to contingencies during aerobraking operations.

Small emergency manoeuvre that raises pericentre height by a few kilometres with the aim of reducing the encountered atmospheric density. This manoeuvre is typically triggered by too high reading from sensors related to aerodynamic flux, such as accelerometers or thermistors.

Manoeuvres executed typically during in-orbit phase to compensate for accumulated perturbations (station-keeping) or to change the orbit of a spacecraft.​​​

Big emergency manoeuvre that raises pericentre height out of the atmosphere and interrupts aerobraking operations, typically triggered by a safe-mode transition.

Stochastic manoeuvres performed typically during interplanetary phase (for fine targeting of a swing-by or a planetary orbit insertion) to correct for  the accumulated trajectory errors.

Sequence of manoeuvres that progressively reduce the pericentre height dipping into the atmosphere. These manoeuvres are executed during the transition from non-aerobraking operations to aerobraking (at the beginning of aerobraking phase or to resume aerobraking operations after an interruption due to solar conjunction or an execution of a pop-up manoeuvre).

Single manoeuvre executed at the end of aerobraking phase that raises the pericentre height "out of the atmosphere" (out of the atmosphere corresponds to a height for which the atmospheric density is sufficiently low so that the spacecraft can control its attitude in normal mode, i.e. with reaction wheels).

Raw instrument data

Original data from the payloads (depacketized, decompressed, and reformatted instrument data), including instrument housekeeping data, instrument health data, calibration data, instrument sampling information, etc.

ESA, immediate delivery to instrument teams

Raw Spacecraft data

Selected spacecraft housekeeping data, platform health information, Attitude and Orbit Control System (AOCS) information, orbit reconstruction, satellite tracking data, etc.

ESA, immediate delivery to instrument teams

Calibrated data

Data converted to physical units, which makes values independent of the instrument, e.g. calibrated spectra, calibrated reflectivities, calibrated brightness temperatures, calibrated polarimetric variables, etc.

Instrument Teams, delivery to ESA and public release within 6 months

Derived data

Results that have been distilled from one or more calibrated data products (e.g. surface emissivity maps, atmospheric trace gas concentrations, gravity field maps, particle size distributions, etc.). Supplementary data, such as calibration tables or tables of viewing geometry, used to interpret observational data should be classified as “derived” if not easily matched to one or the other data categories.

Instrument Teams, delivery to ESA for archiving and distribution (timeline to be agreed)