Remote Sensing Complex PRIRODA
In the second quarter of 1994 there will be launched and docked to the Russian space station MIR a dedicated remote sensing module named PRIRODA (russian for nature, fig. 1). Originally the launch was planned for the end of 1993, but due to energy problems on the MIR station it was delayed for half a year. The module provides the experimental basis for an extensive scientific program of the same name with the major goal of investigation and application of complex, multisensoral remote sensing methods for earth observation. Therefore the module carries a great variety of passive and active instruments from the visible up to the microwave range of spectra: nadir looking and scanning microwave radiometers "IKAR" and "Delta", synthetic apperture radar "Travers", a LIDAR "Alissa" (provided by France), high spatial resolution scanners in the VIS/NIR/TIR (MSU-E, MSU-SK), an infrared spectrometer (ISTOK), an imaging VIS/NIR spectrometer (MOS) as well as photographic and TV cameras. Actually it is planned to complete the instrumentation additionally with the high resolution stereo multispectral scanner MOMS-02 from Germany, which could be brought to the station by a PROGRESS transporter spacecraft after it has been flown on the US space shuttle's D2-mission in 1993. So, from the intrumental point of view, PRIRODA will be the most complex equipped remote sensing space platform up to this time. The lifetime of the mission is planned at minimum up to 1996 and depends mainly on the lifetime of the space station MIR itself.
Fig. 1: Remote sensing module PRIRODA for the space station MIR
The complexity of the space segment of PRIRODA finds its expression in the extensive scientific program which devides into four major tasks: land, ocean and atmosphere related investigations and ecological applications. The land program is oriented on investigations of the state and control of snow cover, soil characteristics, large river basins and inland water ressources. The ocean investigations devide into the subtasks monitoring of sea surface temperature, determination of windfield and sea roughness, investigations of oceanic processes with microwave methods, determination af water constituents and bioproductivity, interaction in the atmosphere-ocean system and its influence on intercontinental processes and investigation of the ice cover. Subjects of the atmospheric experiments are large scale processes over the ocean, the maritime atmophere in tropical regions, the lower stratosphere and the troposphere as well as the determination and monitoring of minor gases and aerosols. In the field of ecological applications whithin PRIRODA there are planned investigations of the anthropogenic influence on aerosols and trace gases, monitoring of ecological desaster areas and state control in different regions of the earth.
Contributions to the entirte program, which also includes extensive ground and airborne campaigns for verification of the results, are made from the states of the former Soviet Union (Russia as a main contribution provides the module itself, the launch vehicle and ground facilities and a major part of the intruments), Bulgaria, Chechoslovakia, Finnlandia, France, Germany, Italy, Poland, Romania, Switzerland and the USA. The program execution is supervised by an International Scientific Council and additional working groups on several detailed subjects (e.g. experiment planning and control, data processing and distribution etc.). The working groups are formed by specialists on the different instruments and the major scientific tasks and in addition by invited specialists from the countries involved in the program.
The PRIRODA module's telemetry systems is working independent from the space stations telemetry, including high volume tape storage. Up to now there are planned three ground receiving stations for the scientific data: two in Russia (near Moskow and in the Far East) and one station in Germany (near Berlin). All of the scientific data will be open and available free of charge for noncommercial (scientific) use. However there will be some restrictions on the access time: access to all experiment data immediately after receiving is guaranteed to the payload contributors, contributors to the official scientific program have immediate access to all data relevant for their proposed experiment. A free access to the data for non-members of the PRIRODA science team will be possible not earlier than half a year after receiving. For information of the user community the DLR will issue a monthly bulletin on the taken scenes, the state of the intruments and available ground truth data as well. Up to now there is no final decision on commercial aspects of PRIRODA data, but it seems highly propable that data will be available on a commercial basis too (at least for selected instruments).
Allthough the scientific program for the PRIRODA mission is in principle allready closed, the Scientific Council, however, is still ready to take into account additional experiment proposals, depending on the data acquisition ressources and the general scientific interest for the proposed experiment.
German Payload Contribution to PRIRODA
Germany contributes with to intruments to the entire remote sensing complex: the imaging spectrometer MOS, which is an imaging VIS/NIR spectrometer provided by the German Aerospace Research Establishment DLR, Institute for Space Sensor Technology in Berlin and the Modular Optoelectronic Multispectral Scanner MOMS-02, which is an instrument provided by the German Aerospace (DASA). Both instruments and the corresponding scientific programs are sponsored by the German Space Agency DARA whithin the frames of the German Space Research Program.
Modular Optoelectronic Scanner MOS
The MOS spectrometer follows a concept of a specialized instrument for remote sensing of the atmosphere-ocean system that has been tested with non-imaging spectrometers built by the Institute for Space Research in Berlin for several missions (MKS, MKS-M on Intercosmos-21, space stations Salyut-7 and MIR). It consists of two seperate spectrometer blocks with medium spatial resolution and appropriate chosen spectral and high radiometrical resolution. The atmospheric spectrometer MOS-A provides 4 narrow channels in the O 2A-absorption band at ~760 nm to allow measurements that can be used to estimate the aerosoloptical thickness and the aerosol content of the atmosphere (Fig. 2). It measures simultaneously with the bio-spectrometer MOS-B that has 13 channels of 10 nm width in the range from 408 to 1010 nm. For the main parameters of the two spectrometers see table 1. Using the MOS-A measurements and the NIR-channels of MOS-B it is possible to remove the atmospheric influence from the MOS-B data and one can compute the water leaving radiance (reflectance) on the surface level. This atmospheric correction is essential for the retrieval of water constituents from the measurements in the VIS. The advantage of the O 2A-method is the use of "direct" measurements to estimate the aerosoloptical thickness.
Fig. 2: O 2A absorption band and position of the MOS-A channels
The center wavelengths of the MOS-B channels are chosen in accordance with the spectral characteristics of the ocean and appropriate to construct retrieval algorithms of water constituents. They also give the opportunity of vegetation signature determination (red edge) and H 2O vapour content in the atmosphere from the NIR-measurements. For land applications the suitability of the O 2A-method of atmospheric correction is actually under investigation.
no. of channels
center wavelengths [nm]
757.0, 760.5, 763.5,
408, 443, 485, 520,
570, 615, 650, 685,
750, 815, 870, 945,
2.8x2.8 km 2
0.7x0.7 km 2
radiance L min
DL/L at L min
Table 1: Main parameters of the imaging spectrometer MOS
Optical Concept of MOS
Both the MOS-A and -B are imaging pushbroom spectrometers, realized in a low-cost design using commercial available components as far as possible. In the optics are used modified refractive photo-objectives and reflective gratings. To fit the spectral and radiometrical requirements there are used specially manufactured CCD-lines in the focal plane with 512 pixel elements having a size of 480x23 mm 2. The incoming radiation in the spectrometer is focussed by the entrance optics onto the entrance slit which is imaged via the collimator objective and the plane grating into the focal plane. For each wavelength channel the input slit is imaged on one CCD-line. Figure 3 shows the principle for MOS-A. The pixel size in the across track direction is defined by the size of the CCD-line pitches, the along track dimension of the imaged swath is determined by the width of the entrance slit. Allthough both spectrometers have the same principle design in accordance with the expected spatial structure of the phenomena to be investigated and the different needs in spectral resolution the designed parameters are different: MOS-A has a halfwidth of 1.4 nm for each channel according to the shape of the O 2A absorption band, the swath consists of 28 pixels of 2.8x2.8 km 2. MOS-B is designed for channel halfwidths of 10 nm and a ground resolution of 0.7x0.7km 2 and has 128 pixels per swath. The pixels are formed by superimposing the analogue read-out signals of 15 CCD elements in MOS-A and 3 CCD-elements in MOS-B. The signals of each pixel is on-board corrected for dark value and groups of channels are attenuated according to the expected spectral radiance to be measured: channels with high sensitivity and those with high input radiance are attenuated to get a nearly constant output level for all spectral channels to optimize radiometric sensitivity and S/N ratio. The attenuation is performed in two ways: different amplification coefficients for the channels and a special designed feature of the CCD-lines, the dynamically controlled antiblooming. Two different operational modes allow it to adapt the spectral sensitivity to ocean and land scenes. The analogue to digital conversion is performed with a 12 bit ADC to cover the dynamic range of the incoming radiance and to assure high measuremtnt resolution and accuracy. The focal plane CCD-arrays, read-out electronics as well as the analogue and digital data channels are controlled by an 80186 microprocessor, which also realizes data formatting, telemetry output, command control and overall system check and watch functions.
Fig. 3: Principle of the imaging spectrometer MOS-A
Since both spectrometers work simultaneously and shall provide data sets for joint interpretation of both devices there is a definite synchronization of the read-out that leeds to a special "joint" scene geometry for the entire MOS data: each pixel of MOS-A covers 4x4 pixels of MOS-B thus giving 4 total swaths of MOS-B (4x128 pixels in 13 channels) per swath of MOS-A (28 pixels in 4 channels). This subset of data is called a "subscene". The overlapping between subsequent swaths thereby is, determined by the timing of CCD read-out, chosen to be 10%. A MOS-scene corresponds to a quadratic image on the earth's surface and contains of 128x128 pixels of MOS-B and 28x29 pixels of MOS-A.
To assure the required high radiometric accuracy there is realized an extensive calibration concept including both laboratory and in-flight calibration of the spectrometers. The absolute calibration in laboratory is based on spectral irradiance standards provided by the State Office of Standards (Physikalisch-Technische Bundesanstalt) which are converted to spectral radiances via a white reflecting disc having known spectral radiance coefficients. The achieved accuracy is of about 3.5%.
Fig. 4: Principle of the suncalibration
Extensive measures are taken to realize an absolute in-flight calibration of the spectrometers to the sun radiation and to assure a frequent control of spectral and sensitivity parameters during the mission time. There are two ways of sun calibration: a direct and an indirect one (see fig.4). During sun calibration a transmitting diffusor disk with a known transmissivity is put in front of the entrance optics to attenuate the direct sunlight and to convert the sun irradiance into radiance measured by the spectrometers. The space station will be oriented by cosmonaute control to catch the sunlight perpendicular to the MOS window and entrance optics. Allthough this direct sun calibration is rather extensive in fuel consumption (the entire space station will be turned) and exhaustive in control it is of great importance since only by this kind of calibration there can be achieved high accuracy absolute calibration of the spectrometers and accounting the influence of the (changing in time) spectral transmissivity of the spacecraft's window. It is planned to perform this direct sun calibration twice a year.
The second opportunity of calibration to the sun is the indirect one via a titanum mirror that is mounted inside the movable window cover outside the hermetic section of the module. To achieve perpendicular input angle of the sun radiation in this case at definite points of the orbit it is neccessary to rotate the space station around one axis (instead of three for direct calibration), what results in simplier controlling operation for the cosmonaut and less fuel consumtion. This kind of calibration will be performed every quarter of a year.
To watch the main parameters of the spectrometers during normal operation there will be performed an internal control cycle using special high-stability calibration lamps inside of each optics block. The two mini-bulbs are mounted close to the entrance slit (see fig. 3) and illuminate through special color filters the spectrometer's focal planes producing different spectra for each lamp. The response function for each of the lamps gives two different output values for every detector element for a definite lamp voltage. These control values are used to detect changes in the sensitivity of the CCD-lines as well as in spectral alignment of the focal planes.
The internal control cycle is performed after each powering on of the spectrometers so that for every scene there will exist actual calibration information. In addition to the automatically "dayly repeated" short internal control cycle (using 4 different illumination levels) there is the possibility to select via telecommand a long control cycle. The long cycle provides 16 different illumination levels (using both lamps in each spectrometer simultaneously) and gives the opportunity to check the linearity and the dynamic range for each focal plane array.
Scientific Goals of the MOS Experiment
As mentioned above the MOS spectrometer was designed especially for ocean remote sensing. Accordingly the goals of the MOS-related experiments are mainly oriented on the developement and validation of methods for:
The scientific and interpretational work will be supported by dedicated ground-truth measurement campaigns that are carried out by the DLR Institute for Space Sensor Technology together with other institutions. The main activity in this field will be a ship campaign in the Meditteranean and the East Atlantic under participation of specialists from the Ukraine, the USA and Germany, supporting both the MOS-PRIRODA and the SeaWiFS experiments. This campaign will take place in spring 1995.
Taking into account the similar mission goals of the MOS and the SeaWiFS instruments it is also planned to perform intercalibration and joint interpretation of both experiments.
Entire German Scientific Program
Due to the contribution to the PRIRODA payload and the realization of a ground receiving and processing facility scientific institutions in Germany will have free access to all available PRIRODA data. Based on this opportunity the German Space Agency DARA and the DLR initiated a broad scientific program with contributions from 16 institutes and universities. The entire program is coordinated by the DLR Institute for Space Sensor Technology in Berlin, which provides the staff to work within the International PRIRODA Science Council. The program devides into three major groups with the following subtasks:
In addition to this general scientific program on PRIRODA there will be established a dedicated program on the second German payload, the MOMS-02 stereo scanner. This program is yet under definition.
To ensure data acquisition, preprocessing and primary processing of the different intruments data as well as data exchange there is established the German PRIRODA User Data Centre at the Remote Sensing Ground Station of the DLR in Neustrelitz near Berlin. This User Data Centre will provide data links to the PRIRODA ground control centre and the processing centre of the scientific company NPO ENERGIA near Moscow, who is the organization operating and controlling the entire space station and the PRIRODA-module. Together with the Institute of Radioelectronics and Engineering of the Russian Academy of Sciences in Moscow there will be established the main (international) data base for all PRIRODA experiments and related ground truth campaigns of all participating organizations. In Germany the PRIRODA user Centre provides a data base management system giving access to the experiment data according the German scientific program, ground truth data for the experiments (as far as available) and information on entire data sets from the PRIRODA intruments which are available in the main data base. Within the frames of dedicated experiments of the scientific program all data will be available as raw data and on processing levels 0 and 1 for the scientific users. As mentioned above the German User Data Centre issues a monthly informational bulletin providing general information for the international user community. Up to now it is planned to ditribute this bulletin via e-mail.
The PRIRODA mission tries a complex approach to earth's remote sensing, using for the first time a powerfull and broad equiped platform. The Modular Optoelectronic Scanner MOS will give essential measurements to solve different scientific tasks, especially for remote sensing of the ocean-atmosphere system. The just now developing scientific and experimental interaction with other remote sensing missions, as for example the SeaWiFS and the ERS missions, will give interesting opportunities for the developement of parameter retrieval algorithms. And, as a pre-operational scientific experiment, MOS will be the first imaging spectrometer in space, giving experience for future missions and instrument design.