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Chapter 11. Onboard Software: LASCO Observing Sequences
The LASCO observing program has been constructed using a building block approach. The fundamental operational modes of the instrument are pre-programmed, with the variables of exposure times, filter settings, compression techniques, etc., stored in parameter tables. Layers of observing sequence software are built from the fundamental blocks. The observing program will then consist of high level commands building a sequence of observing programs that begin at a specific time of day.
Patches to the software stored in ROM and modifications to the table parameters, such as exposure time for each filter, will initially be stored in electrically eraseable programmable read- only-memory (EEPROM). This technology is not radiation-hard, and will degrade over some time period after launch. However, it will be acceptable during the initial post-launch checkout, eliminating the need to upload large numbers of commands after any power outage. The length of time until the EEPROM fails is not exactly known, since the failure mechanism depends on the amount of time the EEPROM is powered, which in this case is very short since it is only powered when the code is being transferred to RAM. After the EEPROM fails, the software will be loaded from ROM, and patches will be uploaded from the ground.
11.1 LASCO Electronics Box (LEB) Programs
A LEB Program (LP) is a software program that is part of the Observation Executive (OBE) in the LEB. It performs a single observation or set of observations. The LP controls the entire procedure of collecting and processing an image. First it configures the telescope mechanisms, Fabry-Perot interferometer, and cameras. Then it commands an exposure■readout cycle, including commanding the shutter to open and close as appropriate. The camera data is then transferred to the LEB, where it is compressed and passed to the telemetry system.
An LP can be scheduled to run immediately, or at some future time. As part of the scheduling process, the LP duration must be specified, as a number of iterations, as a delta time measured from the beginning of the observing program, or as an absolute time.
Various instrument parameters such as exposure time, readout coordinates, etc., are specified through tables stored in RAM. The contents of the tables can be modified by ground command. These table content commands can also be scheduled much like an LP. This permits an observation to be made with one set of table entries, and then later the same LP can be performed with new table entries.
There are three tables for exposure times: primary, alternate, and calibration. This distinction between the tables is meant for clarification only, and each table is functionally the same. The primary and alternate tables provide the optimum exposure times for full field and reduced field readout, respectively. The calibration table provides the exposure times for calibrations using the internal lamps.
Three tables, primary, alternate and calibration, provide the parameters for all of the camera modes, including readout port, dump speed, preamplifier power status, etc. Again the designation of primary, alternate, or calibration is for clarification only.
The LPs have been classified into four categories, depending upon the number of images that the LP generates in a single iteration, and how many telescopes participate in the LP (as discussed in previous chapters, the telescopes included in the LP complement include the EIT). These categories are:
11.2 Single Image from a Single Telescope
The LPs that take a single image from a single telescope are the following. In all cases the tables defining the camera parameters, the exposure times, the method of compressing the image, and, in the case of C1, the Fabry-Perot wavelengths, must be specified.
11.3 Multiple Images from a Single Telescope
The LPs that take multiple images from a single telescope are the following. In all cases the tables defining the camera parameters, the exposure times, the method of compressing the image, and, in the case of C1, the Fabry-Perot wavelengths, must be specified.
11.4 Images from More than One Telescope
The observing programs that use more that one telescope are the following. In all cases the tables defining the camera parameters, the exposure times, the method of compressing the image, and, in the case of C1, the Fabry-Perot wavelengths, must be specified.
An observing program that does not fit into one of the above categories is the following:
11.6 General LASCO Operations During the Mission
The SOHO satellite will take about four months after launch to reach, and attain station at, the L1 point. During this phase the SOHO instruments, including LASCO, will undergo engineering checkout. Toward the end of this phase some scientific observations will probably be made by LASCO, and it should be ready for full scientific operations once on station.
Previous sections described a number of available LEB Programs. The LPs of actual interest for science operations are basically NORMAL for single images from C1 (and the Fabry-Perot if no configuring is required), C2, or C3; FP SCAN LINE for images from the Fabry-Perot alone; POLARIZATION SEQUENCE for images from one telescope; SEQUENCE for general multi-telescope operations; and CONCURRENT for multi-telescope exposures taken together at nearest the same sarting time. From the basic LPs, using individualized parameter tables, higher level observing programs can be constructed.
The reader should realize that LASCO is basically telemetry limited in its operations. After taking and processing an image, controlled by the LEB, the data is passed to a 2 Mbyte telemetry buffer. Its storage capacity is approximately 10 compressed images, and with the low available telemetry rate, 4.2 Kbps, it would take approximately one hour to empty the buffer by telemetry to ground. We will call this the "One Hour" buffer. A goal of daily planning is to keep this One Hour buffer full but not overloaded, allowing approximately 200 images a day to be downlinked.
A small group of synoptic programs will probably occupy most of the observing day. The synoptic programs will obtain observations of the white light corona at least hourly in each telescope, and also obtain a complete description of the emission line corona several times a day, to generate synoptic maps of turbulance, velocity, temperature, etc. These observations will be designed to thoroughly use the capabilities of all three coronagraphs, and will collect a data set which can satisfy a wide range of scientific objectives.
The most direct measure of the observational resources required by an observing program is the telemetry necessary for it. The synoptic programs will utilize something like 85% of the available daily telemetry. Within the synoptic programs, approximately 50% of the telemetry will be devoted to C1 programs, with thorough exercize of the Fabry-Perot capabilities, and 30% to C1 and C2 programs. The EIT telescope, which is controlled by the LEB, receives an allocation of about 20% of available resources (including telemetry) for all of its programs.
The C1 spectral line synoptic program will take a complete spectral line description of the Fe XIV and Fe X coronae, 2-3 times per day. The Ca XV corona will be imaged once per day. The polarization of the Fe XIV corona will be determined 1-2 times per day.
The white light synoptic program will obtain an image from C1 and C2 every 30 minutes, while C3 will take an image every hour. In addition, a polarization sequence using C2 and C3 will be taken 1-2 times per day. A color sequence will be taken with C2 and C3 once per day. The polarization and color sequences are used to help separate the F corona from the K corona.
The remaining available telemetry can be devoted to special observations. These will consist of both special observing programs which will optimize some particular instrumental capability, such as increased timeporal resolution, and programs for special targets, such as individual CMEs or streamers.
11.7 Exposure Times
We give here a preliminary table of nominal exposures times for various filter-polarizer combinations for the three coronagraphs. The actual optimal exposures times will be determined during the early instrument operations. We also refer the reader to the previous Tables 4-1, 5-1, and 6-1.