Operations

Introduction
Operation concept
On-board recording
Data transmission
Mission scenario
The Envisat orbit

Introduction

Envisat flies in a sun-synchronous polar orbit of about 800-km altitude. The repeat cycle of the reference orbit is 35 days, and for most sensors, being wide swath, it provides a complete coverage of the globe within one to three days. The exceptions are the profiling instruments MWR and RA-2 which do not provide real global coverage, but span a tight grid of measurements over the globe. This grid is the same 35-day repeat pattern which has been well established by ERS-1 and ERS-2.

In order to ensure an efficient and optimum use of the system resources and to guarantee the achievement of the mission objectives Envisat reference mission operation profiles are established and used for mission and system analyses to define the instrument operational strategies, the command and control, and the data transmission, processing and distribution scenarios.

Mission and operation requirements

• Sun-synchronous polar orbit (SSO): Nominal reference orbit of mean altitude 800 km, 35 days repeat cycle, 10:00 AM mean local solar time (MLST) descending node, 98.55 deg. inclination.
• The orbit is controlled to a maximum deviation of +/- 1 km from ground track and +/- 5 minutes on the equator crossing MLST.
• Recording of payload data over each orbit for low bit rate (4.6 Mps) on tape recorders or solid state recorder (SSR).
• High rate data (ASAR and MERIS) to be accessible by direct telemetry or recording on SSR.

System operation concept

Global low rate mission and data recovery strategy

General operation concept

All instruments, with the exceptions of the ASAR imaging modes and of the MERIS full-resolution mode, are operated as part of the global mission. The global mission operation strategy is intended to provide the maximum coverage of the Earth components (atmosphere, ocean, ice, land) relevant to each payload instrument. This is based on:

• continuous operation of the low-rate instruments around the orbit (with the exception of MERIS, which is limited by sun-illumination conditions);
• on-board recording of all instrument data;
• data-recorder playback at least once per orbit in order to ensure availability of fast delivery products within less than three hours from observation
• systematic processing of all acquired data.

A nominal data recovery scenario is defined, when both Artemis and the on-board recorders are available, which ensures the recovery of the full low-rate global data set. The dumping of the recorders will be equally shared (seven consecutive orbits each) between the Kiruna X-band payload data handling station and the Ka-band Artemis user Earth terminal attached to the ESRIN payload data handling station. Backup scenarios are defined in case of temporary or permanent unavailability of Artemis, which make optimal use of on-board recorders and of the ESA ground segment.

Processing and data distribution strategy

Low-rate data is systematically processed on the ground and disseminated to users in near real time (NRT). NRT users register as "subscribing" users for the systematic reception of these products. All data is reprocessed off-line. Nominal off-line distribution is performed on physical media to registered users. Small products is available for retrieval on line, with the possibility for users to extract partial data sets.

Regional mission operations and data recovery strategy

Definition

The regional mission includes the full-resolution mode of MERIS and all modes of the ASAR, with the exception of the wave mode. The following sections provide the basic concepts ruling the operations of the regional mission.

ASAR general operation strategy

ASAR offers, by exploiting the combinations of polarisations and incidence angles, 37 different and mutually exclusive operating modes in high-, medium- (wide swath mode), and reduced- (global monitoring mode) resolution. These modes will be operated mainly in response to user requests. Wave mode is also mutually exclusive with respect to all the other modes. It is a low-rate mode operated systematically over oceans as part of the global mission.
Global monitoring and wave modes are recorded systematically when operated. ASAR high- and medium-resolution imaging modes are either transmitted on a real time link (direct X-band or via Artemis Ka-band link) or recorded on the on-board solid state recorder for ground data recovery. The high- and medium-resolution data are acquired only when required to satisfy either a background mission scenario and/or user requests.

ASAR processing and data distribution

All ASAR high-rate data acquired by ESA facilities is systematically processed in near real time to generate medium-resolution products (around 150 m resolution) and browse products. Browse products are available on-line. High-resolution products are processed in near real time or off-line, upon user request. All medium- and high- resolution products are delivered to users on request either in near real time on a dissemination channel or off line on physical media.

MERIS general operation strategy

MERIS operates at full resolution (FR with 300 m resolution at nadir). The data are averaged on board to produce a separate data stream at reduced resolution (RR with 1200 m resolution at nadir). The two data streams are available in parallel on board.  MERIS RR is systematically operated and recorded on board for the duration of the sun-illuminated segment.
MERIS FR is either transmitted on a real time link (direct X-band or via Artemis Ka-band link) or recorded on the on-board solid state recorder for ground data recovery. The FR data is acquired only when required to satisfy either a background mission scenario and/or user requests. MERIS is nominally operated with a fixed set of bands. Alternative band sets might be used for experiment campaigns of a few weeks duration.

MERIS processing and data distribution

MERIS RR data acquired at ESA facilities is systematically processed in near real time to generate MERIS RR Level 1b and 2 products as well as browse products. Browse products are available on-line.

Upon user request MERIS FR data is processed and distributed either in near real time (dissemination link) or off-line (physical media).

MERIS FR data browse images are systematically available since RR data are always acquired in parallel with FR data and browse products are systematically produced for all received RR data. Both RR and FR data are available for off-line processing and distribution. Near real time delivery of MERIS full-resolution products is also available.

Regional mission data recovery

For operation and data recovery of ASAR high- and medium-resolution data and MERIS full resolution, the following nominal strategy will be applied:

• data over Europe will be acquired directly via X-band links (Kiruna and Matera coverage),
• data outside Europe will be acquired whenever possible via Artemis direct transmission to the UET located at ESRIN,
• data outside direct coverage of the ESA X-band stations and Artemis, will be recorded on board using the SSR and acquired via deferred dump to one of the ESA stations via X- or Ka-band links,
• data requested by a station operator or by the distributing entities on behalf of a station operator will be transmitted in X-band for acquisition by the corresponding station,
• if data to be acquired by a station operator are also needed as part of ESA archive, then in parallel with X-band direct data transmission, ESA will acquire the data via combined use of the SSR, X- or Ka-links as appropriate.

In case of unavailability of Artemis, whether temporary or permanent, the regional mission data acquisition will not be affected. The time required to recover the data could restrict in some orbits the provision of three hour NRT services, but would still preserve potential delivery of products to users within less than a day for data acquired over any site in the world.

On-board recording capability

Overview

The on-board recording system is composed of two solid state recorders (SSR) with 70 Gbits capacity each, and one tape recorder (TR), 30 Gbits capacity as back up for low rate data recording.

On-board data system

Main technical characteristics of the SSR's

The two SSR's allow parallel recording of the low rate global data with MERIS FR or ASAR HR data. For each SSR, the three memory areas can be dumped separately at either 100 Mbps or 50 Mbps (half RF channel) via the X-band or Ka-band RF channels.

The recording capacity of a solid state recorder (SSR) degrades from 70 Gbits at begin of lifetime (BOL) to 60 Gbits at the end of lifetime (EOL). The memory is partitioned dynamically in three areas, one per input-data stream, which allows the allocation of capacity according to the needs per input. Dummy source packets (filling data) are removed before recording, allowing more efficient use of the capacity: recording of 9,000 sec of global low bit rate (LBR) within 30 Gbits of SSR memory.

All input and output data channels of the SSR are equipped with redundant interfaces.

Tape Recorder

There is only one tape recorder (TR) onboard ENVISAT capable of recording the low-rate global data. The recorder allows to record up to 6500 seconds, which is slightly more than one full orbit of LR data recording. The TR playback dump is performed at 50 Mbps (half-RF channel) via X- or Ka-band links. The full TR dump (10 minutes duration) is compatible with the nominal pass visibility duration of an X-band receiving station.

On ground data recovery (data dump)

Two selectable data dump rates are usable on X-band and/or Ka-band RF-channels:

• 50 Mbps (half RF channel) dump of 70 Gbits in 23 min 20 sec.
• 100 Mbps (full RF channel) dump of 70 Gbits in 11 min 40 sec. (for ASAR HR data only).

Each of the three memory areas can be separately controlled with respect to data dump time and data rate.

Operation strategy and mission impact

In the nominal scenario, one SSR will be used for the global mission, for low rate data recording. The second SSR will be used for the regional mission (ASAR HR and MERIS FR), with data dump via ARTEMIS to the user Earth terminal (UET). Note that 30 Gbits memory allows five minutes of ASAR HR, equivalent to a stripe 2,000 km long, or 20 minutes of MERIS full resolution recording.

See ARTEMIS for more information.

Data transmission

The satellite data transmission system allows:

• up to two X-band links operating at a time
• up to two Ka-band links operating at a time
• X- and Ka-band transmitters operate independently, therefore up to two X-band links and two Ka-band links can be operated simultaneously.

ASAR HR operation, with real time data transmission, requires use of one full RF channel at 100 Mbps. MERIS FR operation, with real time data transmission, requires use of half an RF channel (50 Mbps). Therefore MERIS FR can share an RF channel with a simultaneous TR dump or an SSR dump at 50 Mbps. When there is no need for a full RF channel, then the RF channel is operated at half rate (50 Mbps) using BPSK modulation instead of the nominal 100 Mbps QPSK modulation.

The down link RF channel relaying the LR global data will be operated only when in visibility of ESA stations or ARTEMIS for dump of the recorded data. This measure needs to be taken to preserve satellite energy and lifetime of the corresponding on board transmitter.

Mission scenario

Global mission scenario

The global mission operation includes all Envisat instruments which have global coverage objectives and implies:

• continuous operation of the low-rate instrument around the orbit (For MERIS, sun-illumination constraints limit the observation to about 43.5 minutes per orbit);
• on board recording of all instrument data;
• playback at least once per orbit to ensure availability of FD products within less than three hours from observation;
• systematic processing of all acquired data.

The global mission strategy is defined in the High Level Operation Plan (HLOP) approved by the programme participants. The corresponding detailed operation requirements for the FOS and PDS are defined in the Reference Operation Plan (ROP).

Regional mission scenario

• the ASAR high-rate operations in wide-swath mode and narrow-swath modes
• the MERIS full-resolution (FR) mode

Data rates implies real time transmission or recording on SSR. Data acquisition is based on user requests plus a background operation scenario. Strategy for handling user request is based on the Data Policy and the HLOP. The result is implemented via detailed ROP rules. Data processing to high resolution product is performed on user request.

Data recovery assumes combined use of:

• SSR
• ESA X-band stations
• Ka-band link via ARTEMIS
• X-band national and
• foreign stations having an agreement with ESA.

Simultaneous operation of Ka- and X-band channels will be possible.

The Envisat orbit

Measurement patterns orbital parameters

Orbit selection

The factors to be considered for the selection of the orbit for each instrument include:

Some of these are fundamental and have an impact on the overall concept of the system. In particular, the selection of a sun-synchronous orbit is of primary importance and has driven the physical configuration of the spacecraft. The gross altitude range, within some tens of kilometres of 800 km, is also critical to the design. After that, there is a certain degree of freedom in the choice of parameters.  Many of the choices were examined during the ERS-1 mission preparation and the concept of the multidisciplinary orbit, with a 35-day cycle, evolved. Envisat flies this same high-inclination, sun-synchronous, near-circular orbit with the same ground track. The only difference is in the MLST being 10:00 instead of 10:30.

Orbit maintenance

The orbit maintenance requirements are that the deviation of the actual ground track from the nominal one is kept below 1 km and that the mean local nodal crossing time matches the nominal one to better than five minutes. The orbit maintenance strategy aims for minimum disturbance of the payload operation.  In-plane manoeuvres are used for altitude adjustment to compensate for the effects of air-drag. This altitude decay controls the ground-track repeatability, mainly in the equatorial regions. The frequency of these manoeuvres is determined by the rate of orbital decay, which is in turn determined by the air density, and this is a function of solar activity. The nominal rate for these in-plane manoeuvres is nominally twice a month. They will not interrupt the operations of most sensors.  Out-of-plane corrections are used to correct the steady drift of inclination mainly caused by solar and lunar gravity perturbations. The solar wind also influences the inclination, but its contribution is typically one order of magnitude smaller than the one given by solar and lunar gravity. Inclination drift degrades the ground-track maintenance at high latitudes. The drift rate does not depend on air density and corrections are required every few months. As they are out-of-plane they require a 90 degree rotation of the spacecraft, to align the thrusters with the required thrust direction, and so these manoeuvres will be performed in eclipse to avoid the risk of optical sensors viewing the sun.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry