Ground Segment - Vegetation System
Adapted from P. HENRY - JF. REULET (VG-NT-0-54-CN (GB) 1997-07-28)
- SYSTEM OPERATOR
- SPOT4 OPERATIONS CONTROL CENTRE (CMP)
- VEGETATION X-BAND IMAGERY RECEIVING STATION (SRIV)
- VEGETATION IMAGE PROCESSING CENTRE (CTIV)
- VEGETATION IMAGE QUALITY CENTRE (QIV)
- REFERENCE STATION FOR L-BAND VEGETATION IMAGERY RECEPTION (SRVL)
The prime missions of the VEGETATION system is to supply, on an operational basis, accurate radiometric data with a spatial resolution of 1 km quantifying certain basic characteristics of the planet's vegetation cover.
The system comprises a payload carried by the Spot 4 satellite and a complete ground segment for payload programming and control plus the reception and processing of radiometric data gathered by the VEGETATION instrument.
The ground segment entities are located at Toulouse (south-west of France), Kiruna (far northern Sweden) and Mol (in Belgium). Wherever possible these entities use or share existing facilities forming part of the Spot 4 ground segment. The interoperation of the VEGETATION and Spot 4 systems is illustrated in the diagram on the following page.
The VEGETATION system offers two modes of operation:
- global mode using onboard recording followed by recorder playback and X-band downlinking of radiometric data for all land masses to a single receiving station,
- regional mode using direct (i.e. real-time) L-band downlinking of radiometric data to regional receiving stations within view of the satellite.
Note that the two modes can be used at the same time.
This document deals solely with operational exploitation in the global mode and the entities involved in this mode of operation.
The VEGETATION ground segment comprises three sites, each under the responsibility of a different organization:
- French space agency CNES for the centres monitoring and programming the VEGETATION payload,
- Swedish space agency SSC for the station receiving global-mode data,
- Flemish institute for technological research VITO for the centre processing and producing global-mode VEGETATION image products.
System management and operation demand considerable technical and administrative coordination. These tasks have been assigned to VITO, the Proxy for the Exploitation Entity. In the remainder of this document, VITO shall be referred to as the "VEGETATION system operator".
The missions assigned to the VEGETATION system operator are:
- to ensure the operational exploitation of the VEGETATION mission,
- to interface with the Spot 4 satellite operator,
- to interface with the commercial operator ("distribution entity"),
- to investigate and process system-level corrections and changes and to coordinate their implementation,
- to organize technical boards and reviews dealing with system exploitation and supervise board and review activities,
- to keep the Programme Steering Committee informed regarding the operational exploitation of the VEGETATION system.
The VEGETATION system operator shall also support the Integrated Project Team (IPT) with the implementation of engineering changes required to ensure programme continuity ( in view of VEGETATION 2).
The general organisation of the production entity is outlined in red in the following overall organisation scheme
The CMP's main mission is to monitor and generate commands controlling the operation of the orbiting Spot 4 satellite. Satellite telemetry is received and commands uplinked via the CNES S-band TT&C network.
With regard to the VEGETATION payload, the CMP:
- monitors payload and equipment operation,
- generates and manages telecommands (uplinked via the CNES S-band TT&C network),
- performs operational programming in accordance with mission requirements.
System exploitation : Basic principles
The CMP is located on CNES premises in Toulouse. The centre is designed to operate 365 days a year. The number of operators on duty is organised to coincide with satellite passes over the nearby CNES S-band TT&C station at Aussaguel (06:00 to 14:00 and 16:00 to 24:00 - TBC).
The CMP comprises the Spot 4 satellite control centre (CCS), the Spot 4 housekeeping management centre (CGS) and the VEGETATION programming centre (CPV). The team of operators controlling the entire system operates all of these facilities.
CMP activities follow a cyclical pattern determined by satellite passes over the main Spot receiving station at Aussaguel, near Toulouse. The five or six passes per day over the Aussaguel station are used to receive housekeeping telemetry and to transmit telecommands.
The VEGETATION payload is programmed once each day after the predicted orbit for the next day has been computed. Programming instructions for the VEGETATION payload are uplinked to the satellite at the same time as those for Spot 4's main payload (i.e. the HRVIR imaging instruments) during evening passes (between 21:00 and 24:00 TU).
The Spot 4 satellite control centre (CCS) handles all real-time telemetry reception and telecommand transmission duties when the satellite is within view of a station linked to the CNES S-band TT&C network. In particular, CCS receives all housekeeping telemetry, displays and checks critical telemetry parameters, and archives the housekeeping telemetry for later analysis, should this prove necessary.
As a payload on board the Spot 4 satellite, VEGETATION telemetry and telecommands are processed and archived by the CCS in the same way as those for other satellite subsystems.
The Spot 4 housekeeping management centre (CGS) performs all off-line processing for all onboard subsystems with the one exception of the preparation of programming for the VEGETATION payload.
Given the duration of each pass within view of the Aussaguel station, the CMP does not create telecommands in real time. All telecommands and memory load sessions are prepared in advance by the CGS, then relayed to the CCS. The CCS organises these telecommands and memory load sessions into what are known as "command plans". The CCS then ensures that the command plans are transmitted to the satellite. Similarly, the ground processing of housekeeping telemetry, to monitor the health of onboard equipment, is not carried out in real-time, but rather in off-line mode. Some parameters are monitored after each satellite pass, others on a daily or weekly basis.
In addition to housekeeping and the preparation of telecommands, the CGS also monitors the operation of the onboard software and manages the VEGETATION payload's equipment configuration. The mass memory, the most complex part of the VEGETATION payload, is monitored particularly closely.
The section of the CGS in charge of orbit determination calculations supplies the VEGETATION programming centre (CPV) with the orbit data it requires for payload programming.
On the CPV's behalf, the CGS manages communications with the VEGETATION X-band imagery receiving station (SRIV) and provides a range of general services. These include services relating to the "agenda" software used for scheduling processing operations, archive utilities, access to housekeeping data, access to interface files, logbook functions, and so forth.
The VEGETATION programming centre (CPV) is a component of the Spot 4 operations control centre (CMP). Note, however, that the CPV is dedicated solely to the VEGETATION payload.
- constant review and management of all mission-specific programming needs (global coverage, regions of special interest, etc.), payload-specific programming needs (calibration, etc.) and system constraints (memory capacity, equipment operating times, satellite manoeuvres, SRIV outages, etc.),
- generation of payload programming on the basis of these needs and constraints,
- formatting of programming instructions as Spot 4 telecommands,
- monitoring of VEGETATION workload and generation of statistics on operating time logged by various items of payload equipment,
- monitoring of the entire "image loop", which is to say, comparison of programming with the imagery actually acquired then processed by the VEGETATION image processing centre (CTIV) to ensure quick response in the event of an anomaly,
- scheduling of SRIV activities to accommodate memory dump sessions and generation of interface files to be sent to the SRIV,
- archiving of all data relating to VEGETATION programming.
The CPV has two operational modes:
- "operational" mode covering all phases of payload programming from the reception of programming requests to the generation of programming instructions ready to be uploaded to the satellite,
- "feasibility study" mode allowing the CPV operator to run a preliminary feasibility study and to fine tune the programming to ensure the best possible match between user requirements and the available onboard resources (memory capacity, SRIV availability, etc.). This mode does not result directly in the generation of programming instructions.
VEGETATION X-BAND IMAGERY RECEIVING STATION (SRIV)
The VEGETATION X-band imagery receiving station (SRIV) is the only one equipped to receive VEGETATION imagery downlinked by the payload's X-band telemetry transmitter. The SRIV station's main functions are:
- reception and recording of VEGETATION global-mode imagery downlinked by the X-band image telemetry transmitter,
- preprocessing of X-band image telemetry data, performing data consistency checks and formatting (for global inventory),
- forwarding of all preprocessed data to the CTIV,
- generation and forwarding of reception and processing reports to the CMP and the CTIV for operational monitoring and production planning,
- performing of engineering tests characterising and validating the X-band downlink.
The VEGETATION X-band imagery receiving station (SRIV) is located in Kiruna, in far northern Sweden. It is part of the ESRANGE complex set up by SSC. Given SPOT''s near-polar orbit, there are a large number of passes each day over the SRIV. The SRIV receiver is switched on for each pass for which a memory dump has been scheduled. This means that the SRIV must operate every day of the year, receiving 4 to 5 sessions per day in summer and 3 to 4 per day in winter.
On reception by the SRIV station, the VEGETATION data are decommutated, recorded, then preprocessed. Once these operations have been completed, the data are forwarded to the CTIV.
If the link with CTIV is unavailable, the SRIV station has data storage facilities to store then forward the data when the link to CTIV becomes available.
The SRIV station is an add-on to the SRIS-K station receiving imagery from SPOT 4's HRVIR imaging instruments and is operated by the same staff.
The SRIV station comprises five main subsystems performing five functions:
- antenna pointing and signal acquisition,
- data reception and recording,
- data formatting for transfer to the CTIV ("inventory" function),
- exchanges with other VEGETATION ground segment entities,
- testing and performance monitoring.
The antenna pointing and signal acquisition subsystem is the same as that used by the SRIS-K station to receive HRVIR image data. It can be used with any of three antennas, including two for the VEGETATION payload, and shared data acquisition equipment.
The data reception and recording equipment is dedicated solely to the VEGETATION payload. The functions performed include the separation of the HRVIR and VEGETATION signals, demodulation, bit synchronisation, differential decoding and frame synchronisation, then data decryption, time-coding and recording.
The so-called "inventory" feature is a software function, again dedicated to the VEGETATION payload. This software checks the quality and consistency of the in-coming data, deletes any duplicate data strings, including intentional repetitions, rearranges the image by continuous data segments (as acquired by the VEGETATION instrument), then formats the data for transfer to the CTIV.
To manage its exchanges with the CMP, the SRIV station uses already existing facilities offered by the SRIS-K station. Exchanges between the SRIV station and the CTIV pass via a dedicated high-data-rate link.
The test subsystem also monitors station performance and the quality of the X-band downlink. Performance monitoring will be continuous throughout the SRIV station's useful life.
VEGETATION IMAGE PROCESSING CENTRE (CTIV)
The VEGETATION image processing centre (CTIV) processes VEGETATION image data and generates standard products for VEGETATION data users. The CTIV is dedicated solely to the VEGETATION programme. Its main functions are:
- to compile and manage the archive of all VEGETATION data received by the SRIV station,
- to automatically process all in-coming data and produce global overview products,
- to produce image products ordered by data users by processing either overview products or archive data,
- to forward finished products to users or to the VEGETATION image quality centre (QIV),
- to provide, subject to the commercial operator's approval:
- a VEGETATION data archive query/browsing service (i.e. a catalogue access service) linked to SPOT Image's HRVIR archive,
- a service to help users formulate accurate acquisition requests by compiling a suitable technical file plus a service for making finished products available to users,
- a service to manage the user list, production requests, and to forward billing data to the commercial operator.
The VEGETATION image processing centre (CTIV) is located on VITO premises in Mol, Belgium. It is designed to operate seven days a week and to be staffed for eight hours each day.
On the upstream side, activity at the CTIV is paced by the arrival of data from the SRIV station and weather data ; on the downstream side, by the deadlines for the delivery of finished products.
Image data are received following each memory dump over the SRIV station, which is to say at regular times each day. There are four or five such dump sessions per day. Each dump is processed as soon as the automatic processing system receives it. The only manual operations interrupting the automatic processing cycle involve the setting of ground control points (to model the VEGETATION instrument's imaging geometry) and quality control at various processing stages.
Automatic processing is performed 24 hours a day.
The production and recording of CTIV products are scheduled to tie in with the daily collection times for mail, courier service, etc., as the case may be.
The CTIV production scheme, starting with the reception of data from the SRIV station and ending with the generation of CTIV image products, can be schematically represented as follows:
Image data received by the SRIV station are automatically preprocessed, whether or not they correspond to products requested by users. Preprocessing involves:
· geometric modelling of data strips using location data, time and date codes and attitude data supplied by the satellite plus measurements based on ground control points (GCPs) set by an operator using a GCP workstation,
· radiometric and atmospheric corrections, plus data strip conversion to the required map projection,
· for each emerged land point, compilation of daily data and compilation of ten-day data (with the selection of the best pixel recorded during these 10 days).
Standing or "a priori" orders, reflecting the needs of subscribers, are received by the CTIV before the onboard instrument acquires the corresponding imagery. Data output by the automatic processing system are immediately processed to fill such orders thus ensuring fast delivery to the users concerned. Additional processing to fill standing orders involves:
· projection or re-projection onto the map reference system specified in the order,
· formatting (where necessary) and recording on magnetic media or CD-ROM.
Normal or "a posteriori" orders generally concern data or products already archived by the CTIV (i.e. already acquired by the onboard instrument). The first step is thus to retrieve the original data from the archive for further processing. From this stage on, the data are processed in the same way as standing orders (i.e. extraction or region of interest, projection, formatting, recording).
Normal order processing is generally of a lower priority than standing order processing.
The CTIV's computerised management system performs a host of tasks, including:
· making of backup copies of all important data and data retrieval when required,
· processing of all types of anomalies (image data anomalies, software errors, hardware failures, etc.),
· record keeping covering details of all events describing the work performed by the CTIV (logbook),
· management of communications with the SRIV station, the QIV, the commercial operator and users for product delivery via the Internet and dedicated links.
All CTIV communications with the outside world are protected by a firewall-type server which effectively filters all exchanges.
In accordance with the product specifications, the CTIV produces the following types of products:
· VGT-S: image product covering all or part of a global overview,
the overview being compiled from all data strips acquired during a given
period. The method used for overview compilation (synthesis) must ensure
the coverage of all land masses world-wide while minimising the effects
of cloud cover.
VGT-S1: The data strips acquired in a single day are compiled to yield a "daily overview".
VGT-S10: The data strips acquired over ten consecutive days are compiled to yield a "ten-day overview".
Each pixel of a VGT-S product represents a ground area of approximately 1 x 1 km2. The pixel brightness count is the ground area's reflectance (corrected for atmospheric effects). Each pixel brightness count is encoded using 16 bits yielding 12 significant bits. A map of computed normalised difference vegetation index values (NDVI image plane) is also supplied.
VGT-S10 products are also available.
VGT-S10.4: VGT-S10 product in which each pixel represents a ground area of 4 x 4 km2.
VGT S10.8: VGT-S10 product in which each pixel represents a ground area of 8 x 8 km2.
All of these products can be delivered on CD-ROM, tape cartridge, or on-line (Internet or dedicated link). Each product is produced in response to a precise order. When ordering, the user specifies the required system of mapsheet lines, map projection, image planes (spectral bands, NDVI), and selection of auxiliary data.
The CTIV also produces "browse" products designed to give users a quick, general appreciation of VEGETATION data and to help them select which products they need or assess their coverage and image quality requirements before they order. The CTIV will produce a master of two types of browse CD-ROMs which will then be reproduced and distributed by the commercial operator.
· Browse BS10 CD-ROM: This product comprises, on a single CD-ROM disc, about six months' periodical synthesis, subsampled (16 x 16 km²), with three spectral bands, and encoded using 3 x 8 bits per pixel.
The CTIV also produces special products for the QIV that are much closer to the raw instrument imagery as regards both geometry and brightness counts (radiometric values). These products are used for instrument calibration and for routine image quality monitoring throughout the payload's orbital life.
VEGETATION IMAGE QUALITY CENTRE (QIV)
The VEGETATION image quality centre's main missions include:
radiometric and geometric calibration of the VEGETATION instrument and determination of the parameters to be supplied to processing centres for the correction of raw image data,
regular monitoring of image quality to check that all quality parameters meet user specifications,
creation and maintenance of the GCP database used by the CTIV to model the VEGETATION instrument's imaging geometry,
expert appraisals of anomalies involving image quality, including proposed corrective actions and follow-up to ensure that these are correctly implemented,
validation, in image quality terms, of the L-band image telemetry system.
The QIV is located on CNES premises in Toulouse. Its operation is closely coordinated with that of the SPOT 4 image quality centre (QIS). The QIV also has direct access to SPOT 4 HRVIR data. The QIV's mission is compatible with operation during normal office hours.
During the operational exploitation of the VEGETATION system, QIV operations will be governed by:
- scheduled activities that can be planned in advance (generation of monthly image correction parameters, half-yearly checks on the performance of the VEGETATION payload, etc.),
- unscheduled activities including expert appraisals and the analysis of anomalies detected during routine monitoring or in response to a request by the production entity.
The QIV plans in advance the payload programming required for its routine activities (instrument calibration using the onboard calibration lamp, calibration in complete darkness, ocean data strip, etc.) then forwards this information to the VEGETATION system operator so the necessary telecommands can be added to the mission programming.
All calibration data are forwarded directly to the QIV without any processing whatsoever by the CTIV.
When the QIV needs normal data strips it simply orders them from the CTIV like any other user. Most QIV needs are met by placing standing orders, or, more generally, by placing normal orders after viewing data strips accessible via the catalogue.
The operators manually control the way data received by the QIV is used and task sequencing in general.
The operation of the QIV is based on a "toolbox" approach. Automation and productivity requirements are met by flexibly chain-linking individual processing operations while preserving the modularity and upgradability of each module.
Radiometric calibration is based jointly on methods using onboard calibration devices and on measurements involving selected targets (clouds, ocean glitter, deserts, etc.). Some of the methods using actual measurements will take some time to refine. Consequently, they will only be included in operational radiometric calibration after a period of validation lasting several months.
Geometric calibration will be performed by a block triangulation workshop. The workshop's main duties will be to determine instrument alignment biases and to compile a GCP database from VEGETATION pieces of image. The GCP database will then be used by the CTIV to model the instrument's imaging geometry.
The algorithms for monitoring radiometric quality will be based on a combination of image statistics and calibration data.
The algorithms for monitoring geometric quality will be based partly on data generated by the block triangulation workshop and partly on image correlation calculations.
Once the GCP database has been compiled, the QIV will monitor its use (by the CTIV) on an operational basis. This will involve the daily reception and analysis of the results achieved by the CTIV in the course of routine production activities using the imaging geometry model based on the QIV's GCP database. The quality of the GCPs will be validated by statistical analysis. If necessary, the GCP database can be updated.
The QIV will also be equipped with a range of smaller tools for image quality assessment. These will be used to analyse images containing defects and, where appropriate, to propose algorithms to correct the problems giving rise to such defects.
Although it is not a production centre, the QIV's role in the larger VEGETATION system demands rigorous computerised management. Computerised management tasks include:
· record-keeping covering details of all events describing the work performed by the QIV (logbook),
· management of communications with the CTIV.
REFERENCE STATION FOR L-BAND VEGETATION IMAGERY RECEPTION (SRVL)
The SRVL station's main missions are:
to demonstrate the feasibility VEGETATION image data reception in the regional mode,
to validate the quality of the VEGETATION L-band image telemetry downlink and to monitor its operation during operational exploitation,
to deliver raw imagery to the QIV for validation,
to provide a contingency capability for continuing instrument calibration in the event of a failure affecting the main X-band image telemetry downlink and so ensure that image correction parameters continue to be supplied to regional receiving stations.
The SRVL station is located at Aussaguel, near Toulouse, immediately alongside the Toulouse space imagery receiving station (SRIS-T), one of the main stations receiving HRVIR data. The station benefits directly from facilities set up as part of the SPOT 4 project. In particular, the SRVL station is linked to the automatic station management system set up to remotely control and manage both the SRIS-T station and the SRIP (Station de Réception des Images via PASTEL) station for receiving imagery via PASTEL.
The SRVL station's main mission being to demonstrate the feasibility of VEGETATION image data reception in the regional mode and monitor engineering parameters, the current plan is to use the station once a month. Given also that the station is fully equipped for remote control and management, there will be no need for on-site operators, save for the recovery of L-band image telemetry.
The SRVL station comprises four main subsystems performing four functions:
- antenna pointing and signal acquisition,
- data reception and recording,
- remote control and management from the CMP,
- testing and performance monitoring.
The antenna pointing and signal acquisition subsystem is based on that used by HRPT stations, but modified to accommodate SPOT-type ephemeris data.
The data reception and recording equipment is dedicated solely to the VEGETATION payload. The functions performed include demodulation, bit synchronisation, convolutional and differential decoding, and frame synchronisation, then data decryption, time-coding and recording.A Gascon (Gestion Automatique des Stations de CONtrôle) workstation already installed for the remote control and management of the SRIS-T station is also used for the remote control and management of the SRVL station and to handle its exchanges with the CMP. As far as the Gascon workstation is concerned, the SRVL station is part of the SRIS-T station. This means that the SRVL station also benefits from other Gascon capabilities including remote control of antenna pointing, remote monitoring, and the recovery of technologic data received during each pass.
The test subsystem generates test data to validate the quality of the L-band downlink and monitor the operation of the SRVL station.