Under the hood of the EOStation
Let us to introduce in brief the internal design of our system that works hard under the hood of the EOStation web-interface and data services.
This system was designed, assembled and is operated by R&D center ScanEx which is one of the leading Russian companies offering integrated remote sensing satellite data acquisition and processing solutions.
The E(arth)O(bservation)Station is a set of interconnected state of the art hardware and software technologies that were combined together in order to bring this service to operation. The primary goal of the system design were creation of the comprehensive, flexible and yet low cost and easy to operate system for satellite data acquisition and dissemination. Such a system is intended to be operated by a wide range of customers that need highly operative access to satellite information. Data acquisition and processing procedures as well as system management are automated as possible and do not require dedicated staff, user's experience nor much attention. Access to the processed data is based on the well established internet technologies that lets operating organization to use the system in own intranet as well as provide public service to customers worldwide via Internet.
EOScan Receiving station
There an EOScan receiving station is the core component the system. This station was designed and manufactured by R&D center ScanEx and represents one of the offered ground station families . The EOScan series stations were specially designed for reception X-band Direct Broadcast downlink from the EOS-series satellites (Terra, Aqua etc).
The station lets to acquire EOS direct broadcast data stream that carries information from the MODIS sensor. The station consist of the roof-mounted antenna system with 3m parabolic dish, an in-door block assembling antenna control circuits, demodulator and decoding modules, and a PCI-bus interface card. Receiving equipment provides reliable acquisition of the downlink starting at elevation near 3-5deg . Due to the using 2-axis tracking scheme and Terra's DB transmitter design the system has a "zenith hole" that may lead to temporal data loss at the center of extremely high satellite passes.
The computing power of the system is formed by two computers each of them is playing a dedicated role in frame of the whole system design.
The receiver control PC includes interface to the other EOScan hardware components and supports data stream acquisition and recording it to the hard drive as well as all the tasks on satellite passes scheduling, tracking and receiving equipment control. This computer is also connected to the GPS-receiver providing autonomous accurate time keeping.
The data processing server (DPS) is a primary working horse. It performs the most computational work on data processing and contains primary data storage. Its storage capacity determines amount (and age) of data that is maintained accessible on-line. Modern storage hardware lets to assemble quite large storage at reasonably low costs. Our server uses cheap IDE drives gathered into the stripped RAID array. At this time total storage volume is formed by 4 disks by 40 GB each. Such a capacity lets keep and maintain full operational MODIS data during about a weeks. The server is operated in the 24x7 mode.
The most of the in-door equipment are mounted inside of a 19" rack. Such an assembly lets to keep the station on a limited area in quite maintainable and accurate looking manner, as well as to control access to core system components. Besides computer processor modules the cabinet includes some other supporting hardware like uninterruptible power supply (UPS) unit, console switch and network equipment. All three computers are operated with help of one console set ( monitor&mouse&keyboard) placed beside and commutated to one of the processors at time.
Software components and Operation overview
From space to hard drive
The operational data reception procedures require a minimal operator's intervention and may be mainly accomplished in unattended mode. Preparation steps are executed automatically: precise time synchronization is maintained in the background with help of the GPS unit, recent orbital elements (NORAD TLE files) are routinely fetched from the Internet, station hardware are initialized and tested during startup. Then station control software automatically switch the station from idle to working mode (and back) in accordance with the precalculated satellites passes schedule. As a result of the acquisition session the raw (or Level-0) data stream is recorder to a file on the hard drive. Just as the recorded file is freed by the receiving software it is caught and transferred to the processing server.
To higher levels.
Level-0 MODIS file contains packetized data stream that is quite complex and nearly unusable in such a format. In order to bring the data to the scientist's desk received data should be passed through the processing chain. We use the IMAPP for this task. This package has been derived at Wisconsin Un. SSEC from the EOSDIS's operational processing software and is widely used by the EOS DB community to work with the low level MODIS data. One of the advantages of using this well-known software is that we deal with and can offer to users standard (and nearly identical to those distributed by EOSDIS) data products up to Level-1B. IMAPP let to do data decommutation, geolocation and calibration. These three operations are accomplished consequently for each acquired satellite pass.
As a result of first-step processing the all MOD01,MOD02 and MOD03 product files as well as browse products (reduced resolution and JPEG or ECW compressed images) are archived at the processing server.
After obtaining calibrated and geolocated MODIS Level-1B files the processing system in sequence performs generation of configured custom products as well as several kinds of higher level processing. The processing system may be easily extended with specialized software components to accomplish nearly any tasks on the user's demand. All the processing steps are executed automatically by a batch job control system.
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