ACE Data Processing Levels




Contents

Data Telemetry and Level 0 Data Processing

 The ACE spacecraft Command and Data Handling (C&DH) system gathers data from the instruments and formats the data into minor and major frames. One minor frame (996 bytes, including sync codes, headers and R-S check symbols) is read into the C&DH system each second and there are 16 minor frames per major frame. The C&DH system also gathers data from various analog sensors and digital telltales, from the sun sensors and star sensor, and from the command system, etc. Most of the time the spacecraft is not in touch with the ground facilities and these data are stored in an onboard Solid State Data Recorder (SSDR). Typically one contact per day is initiated by ground facilities and lasts roughly two to four hours. The SSDR is large enough to allow contacts to be spaced by more than 50 hours when necessary. The SSDR contents are read out to the ground at a rate exceeding 10 minor frames per second while current data are being simultaneously telemetered to the ground and stored in the SSDR for the next contact. The telemetry data are formatted into two virtual channels (real-time and playback) and received by the Caltech Jet Propulsion Laboratory Deep Space Network (DSN). The telemetry data are then forwarded in near real-time via the Internet to the ACE Integrated Mission Operations Center (IMOC) at the Goddard Space Flight Center (GSFC). There the data are reviewed in near real time for purposes of monitoring spacecraft and instrument status. At the end of each spacecraft contact, the DSN forwards the telemetry data to the ACE Science Center via ftp.

The data then undergo level zero processing (per NASA's standard terminology) as soon as all the data contained within the current 24 hour time frame have been received. In level zero processing, duplicate data are removed from the data stream, data are time ordered, and data quality and accounting summaries are appended. The data are formatted into a 24-hour Science Routine Data Set File (Level 0 data file). Production of Level 0 data is now the responsibility of the ACE Science Center at Caltech.

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Level 1 Data Processing

 At the ACE Science Center, the data undergo level one processing, usually within a few days of receipt. In level one processing, the data are separated out by instrument and each instrument data set is formatted (using the NCSA HDF standard) in a fashion which is both consistent with the other instruments and customized to meet the special requirements of that data set and team. At this point in the processing, i.e., in level one, the data are supplemented with ancillary data including position, attitude, and spin phase of the spacecraft; command history and comments; calibration of the spacecraft clock; and documentation of the data items. Excepting the documentation, these ancillary data are all received by the Science Center from the IMOC. The level one data are archived at the Science Center and a copy is transmitted to the National Space Science Data Center (the NSSDC) for long term archiving. Each instrument team receives a copy of all the level one data, including, of course, that from their own instrument.

In addition to formatting, level one processing includes those data processing steps which are judged to be of sufficient simplicity that they can be understood, defined, and coded before launch, and do not require iterated processing with increasing experience. Examples of such steps include decompression of compressed rate scaler data and proper time labeling of data which are buffered for a number of minor frames within the instrument before readout. A counter-example (a process which clearly does not belong in level one) is application of calibration data to convert digital pulse heights from detector signals to engineering units. Experience indicates that calibrations are often adjusted repeatedly to improve resolution based on extended iterative study of the instrument response.

In parallel with the level one processing, the level zero data is processed to yield Browse Parameters. Browse parameters are a subset of ACE measurements which allow monitoring of the solar wind and large-scale particle and magnetic field behavior. They also allow the selection of time intervals of particular interest for more intensive study. Since it is considered important to distribute first-order ACE results as soon as possible, the browse parameters are delivered to the public domain immediately, at the expense of full verification.

Click here for more detailed level 1 data documentation.

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Level 2 Data Processing

Data processing beyond Level 1 is the responsibility of the individual instrument teams. Level 2 processing includes such operations as application of calibration data and detector response maps, organization of data into appropriate energy and time bins, and application of ancillary data (for example, conversion of magnetic field vectors to useful coordinate systems using the spacecraft attitude data). The Science Center attempts to facilitate these efforts within its resources, especially when high-level processing involves multiple instrument teams. For example, much of the anisotropy/flow data for the particle instruments, in particular for the Electron, Proton, and Alpha-particle Monitor (EPAM), will be computed in terms of the direction of the magnetic field. Thus the EPAM team will need high level results from the MAG team to do high level EPAM analysis. The Science Center can facilitate data sharing and communications with its substantial data storage capabilities and its data formatting experience. Another example is the high level processing for the Cosmic Ray Isotope Spectrometer, CRIS. Four institutions are involved in this processing, each contributing expertise and experience in a different sub-assembly of this very complex instrument. Communications and iteration of the data processing are being facilitated by the Science Center for this team.

Each instrument team is required to deliver level two data back to the Science Center, which will then make the data available to the other instrument teams, the space science community (as required by NASA), and the NSSDC for long term archiving. Delivery of level two data back to the Science Center is expected to begin about three months after the spacecraft enters orbit about the L1 Lagrangian point. Thereafter, roughly a two month lag time is expected between receipt of level one data by the instrument teams and delivery of level two data back to the Science Center. However, these delivery schedules may require revision if instrument checkout and debugging take longer than expected. In addition, the level two dataset is expected to be evolutionary, in the sense that an instrument team may enhance their level two data with additional products in the future, as the sophistication of their analysis increases.

Access ACE Level 2 data.

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Ancillary Data

Ancillary data is data provided by various sources in addition to what is telemetered from the spacecraft. This includes attitude and position solutions from Flight Dynamics and onboard clock calibration data from the Flight Operations team. These data are either folded into the Level 1 data at the Science Center, and/or provided to the instrument teams in addition to the Level 1 data.

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Last Updated: Sep 24, 1998

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