Payload - Vegetation System
An independant add-on package
The VEGETATION package is designed as an add-on payload independant of its host satellite bus. In addition to the components required for imaging purposes, the package includes a solid-state recorder, the X band and L band telemetry subsystems and a computer for managing the instrument's functional modes.
Monitoring of the cameras' radiometric performances is ensured by a dedicated on-board calibration device.
|Transmit frequencies (MHz)||X
|Number of bits per pixel||10|
|Bit rate (kbps) L band||510|
|X band telemetry rate (Mbits/s)||3400|
|Design lifetime||>5 years|
|Storage capacity (Gbits)||2.25|
- One Engineering/Qualification/Flight model
- Specific requirements for parts procurement
Main specification : distortion / registration
- Telecentric lenses with spheroparabolic optics
- Inter-camera adjustment and stability requirements
- Ultra wide angle optics (101°)
- On-a-line Short Wave Infrared detector
- 10 bit analog-digital converter
- X band Solid State Power Amplifier hybrid
- Flight software in ADA language
- Harness density
At the core of the VEGETATION payload lies, of course, the optical imaging instrument, pioneering (with POLDER) the space application of CCD linear arrays on a wide angle (101°) instrument.
The instrument collects radiation reflected by the earth's surface to convert it into usable electrical signals. Silicon linear arrays are used for spectral band B0 (Blue), B2(Red) and B3(Near Infrared), while InGaAs linear array is used for the Short Wave Infrared spectral band. Each array features 1728 individual CCD detectors. The output voltage signals are fed to a multiplexer (beforehand SWIR voltages are adjusted for dark current values), then to a single analog to digital converter(ADC). All sensors are processed by a single high precision 11 bit ADC, thus completely by-passing the problem of electronic interband calibration.
The instrument consists of a number of functional components:
Ray diagram for typical telecentric optics. Note that entrance pupil width varies with viewing angle and that the exit beal is always perpendicular to the focal plane.
Thanks to a spheroparabolic first lens and to the twelve other lenses, the four high performance telecentric optics -one for each spectral band- allow quasi constant illumination and spectral response through the field of view and guarantees a raw distortion lower than 1 pixel, which, corrected by a mathematical law,leads to a residual 100 millipixel.
Each spectral band has its own optics with the associated detector unit in the focal plane. Each detection unit contains:
- an interference filter for spectral selection, actually the same for the last three spectral bands as for the High Resolution Instrument (HRVIR) of SPOT,
1728-detector linear array:
- improved HRVIR detectors for visible and NIR bands,
- one line InAsGa 3000-detector for SWIR band (only 1728 are used)
- a low noise amplifier and CCD follower unit
- a titanium structure,
- a SWIR dedicated, closed loop active thermal control concept using a nearby Detection Interface Unit.
Electronic video units
These twin, cold redundant units are capable of:
- digitizing the reference level and the video level,
- filtering values measured over 11 bits on the reference and video levels, through appropiate digital processes,
- subtracting those values after truncation from 11 to 10 bits, on a single output.
Power supply unit
This unit is composed of two redundant converters, delivering 25 voltages each, and of the SWIR thermal regulation circuit which guarantees a ± 0.05°C detector temperature stability.
To meet the registration requirements, a highly stable structure optically positions all 4 cameras optical axes to within 5 arc second. This is achieved through the optical bench's structural design, consisting of a carbon fibre panel fitted with titanium inserts and decoupled from equipment bay deformations by three flexible blades. The stability is also due to the closed loop active thermal control concept which keeps thermal stability to within 1°C for the cameras and the optical bench, 0.5°C for the VIS-NIR detectors and °0.05°C for the SWIR detector.
These performances were met in the Structural & Thermal model tests.
To meet the stringent radiometric requirements, a stable on board calibration system periodically monitors the instrument's responses, giving the updated corrective coefficients.
The radiometric reference's high stability relies on appropriate design and materials (carbon fibre bar), of the lighting bulb and of the thermal concept. The calibration procedure, based on a dedicated optical configuration consists in:
- illuminating the cameras' focal planes with 2 redundant light sources,
- scanning the 101° field of view in 2 minutes typically, through a step by step motor.
This is the payload brain and memory, dialoging with the Central Process Unit for reading the programming message and delivering the housekeeping messages to the satellite;
Onboard management, which enables execution of the planned programme by initiating and monitoring the onboard electronics, consists of:
onboard computer (UGB):
based on the use of a DATA 1750A processor and of a central memory, exclusively made of Random Access Memories (RAM) with a capacity of 64k × 16-bit words each. The UGB computer provides coupling with the satellite's data bus, TM/TC interfacing with the payload's other units and storage of the VEGETATION flight software and of the programming message.
hinging on a 1 Hz rated main task, onboard software manages communications and operation modes, executes the programming message by operating the critical units and functions, controls acquisition and monitoring actions, more particularly the thermal control algorithm.
Mass Memory (MdM):
with a capacity of 2.25 Gbits, the Mass Memory stores the formatted image data at a rate of 412 kb/s, and reads the recorded data at an average rate of 3.4Mb/s, with the possibility of simultaneous recording and reading. Mass Memory is structured around a 10-page memory stack composed of 8 columns × 32 Mbit (DRAM random access memory component).
Calibration heaters control (BCMR)
This unit is needed to monitor the calibration device as well as the control heaters' power levels, which are computed every second by the onboard software. To meet the radiometric requirements, the calibration motor is monitored by microsteps in sine/cosine intensity, with 4 selectable speeds. For the same reason, the BCMR delivers a highly stable power to the calibration lamp :accuracy of 0.0003 during calibration (less than 4 minutes) and .005 during 4 years.
Image data processing (SETI)
Interfacing with almost all units, SETI processes images from the instrument, to send images either to the Mass Memory or to L-band image transmission. Processing consists in a sequencing of data, a post accumulation (10 bit encoded arithmetic average), formatting (4 band format to memory, 5 band format to L band Image Telemetry -TMI- unit), scrambling and encoding to the TMI units.
Image data Telemetry
Each of the 2 units TMI-X band and TMI-L band consists of 2 BPSK modulators, 2 converters and of one RF switch on the Solid State Power Amplifier (SSPA) output.
The TMI-X unit delivers to the antenna the signals directly received from SETI. A specific feature of VEGETATION was the development of the SSPA-X amplifier.
As Prime Contractor for the VEGETATION payload, Alcatel is responsible for the overall project management and system enginering, as leader of an industrial partnership:
- SODERN for detection boxes and detection chain,
- CERCO for the optics (lenses and calibration),
- SEXTANT (with DASSAULT for the Mass Memory) for the electronic video, mass memory and processor units,
- THOMSON for VIS,NIR and SWIR CCDs
- ETCA for distribution and power supply,
- SAAB ERICSSON SPACE for image data processing, telemetry units and L band antenna,
- LABEN for calibration and thermal heaters control units,
- SPACEBEL Informatique for onboard software,
- ALCATEL BELL for EGSE,
- SPACEBEL Instrumentation for vacuum tests and OGSE
- AMOS for MGSE
- and finally Alcatel for structure, thermal control, calibration device, harness, integration and tests.
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