PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT PUBLICATION_DATE = 2008-04-25 NOTE = "GEO_INDEX_DESC.ASC describes how data maps to geometry indexes." END_OBJECT = TEXT END This file describes how the Active Ionospheric Sounder (AIS) data files map to the geometry index (INDEX/GEO_MARS.TAB) entries. The AIS data files (DATA/ACTIVE_IONOSPHERIC_SOUNDER) are organized by orbits; all data collected from an orbit is grouped into a single file FRM_AIS_RDR_nnnn.DAT, where nnnn is the orbit number. The AIS instrument rarely collects data continuously during the orbit pass. Typical data collection is limited to portions at the beginning and end of the orbit pass. \ MARS / x _ x x (_) x \_____/ x = AIS data collection during orbit pass The base unit of data collection is called a frame. A frame consist of 160 echos with frequencies ranging from 100KHz to 5.5MHz. When this frame is displayed graphically, it's called an ionogram. For each frame collected, there is one and only one entry in the geometry index (GEO_MARS.TAB). The AIS data files (FRM_AIS_RDR_nnnn.DAT) contain all of the frames for a given orbit pass in time order. Remember a frame consists of 160 echos, so within each frame the echos are stored in time order. The data files looks something like the drawing below, where each line is a record. AIS DATA FILE STRUCTURE ----------------------- Frequency 1 (beginning of frame 1) Frequency 2 ... Frequency 160 (end of frame 1) Frequency 1 (beginning of frame 2) Frequency 2 ... Frequency 160 (end of frame 2) ... For example, orbit 1847 consists of 33 frames so there are 33 entries in the geometry index representing the start of each frame. The data file contains 33 frames * 160 echos = 5280 records. This first two numbers in the geometry index (INDEX/GEO_MARS.TAB) are used to implement PSA's "Line Concept", see SOP-RSSD-TN-010 Issue 3 Revision 5 4 May 2005. Per PSA's request, the line ends when there are gaps in the data coverage. For example, orbit 1847 consists of data collection before and after the spacecraft's nearest approach to mars. Note there is about 2 minutes of data collection before and after the nearest approach to mars, separated by approximately 30 minutes of no data collection. The orbit pass is illustrated below. ORBIT 1847 AIS data AIS data collection collection |<---->| |<--->| __xxxxxxxx____________________________________________________xxxxxxx__ | | | | | | | 22:05 22:10 22:15 22:20 22:25 22:30 22:35 The entries in the geometry index reflect the gaps in data coverage. The first 18 entries correspond to data collection at the beginning of the pass and the following 15 correspond to data collection at the end of the pass. The geometry index is depicted below. GEO_MARS.TAB ----------------------------------------------------- ... 18, 1 geometry data for frame No. 1 of orbit 1847 18, 2 geometry data for frame No. 2 of orbit 1847 ... 18,17 geometry data for frame No. 17 of orbit 1847 18,18 geometry data for frame No. 18 of orbit 1847 15, 1 geometry data for frame No. 19 of orbit 1847 15, 2 geometry data for frame No. 20 of orbit 1847 ... 15,14 geometry data for frame No. 32 of orbit 1847 15,15 geometry data for frame No. 33 of orbit 1847 ... ---------------------------------------------------- HOW TO MAP GEOMETRY INFORMATION BACK TO THE AIS DATA ----------------------------------------------------- Method 1 - using orbit number and frame number The orbit number can be obtained from the name of the data file you are reading. If the data file is named FRM_AIS_RDR_1847.DAT, then the orbit is 1847. The frame number is known by which record in the file you are reading, see data file illustration above. The first 160 records belong to frame number 1; the second 160 records belong to frame number 2; and so forth. By knowing the orbit number and the frame number, the geometry information can be extracted from the geometry index. Simply go to the first line in the geometry file containing the orbit number. This line will contain the geometry information for frame number one; the next line will be frame number two; and so forth. Continue counting down down lines until the correct frame number is reached in the geometry file. Hint: This process can be automated with a script in a few lines. Method 2 - using time Time may be used to reference geometry data in the geometry index file, since all frames have unique time stamps. From the AIS data file, choose one of the many available time formats. In this example, Spacecraft clock or SCLK is used. Extract the SCLK from the data file. Next use the SPICE functions provided by NAIF to output the SCLK into a human readable format (ASCII). Compare the time from the data file with the time listed in the geometry file. The line in the geometry index where both of these times match is the correct line for the geometry information. In this example, the human readable ASCII time from the AIS data file is used. From the AIS data file, extract or write down the time for the frame of interest. Next, scroll through the geometry index until the time from the AIS data file matches the time in the geometry index. When the two times match, the correct geometry information has been reached. Please Note: NAIF's pursuit of excellence will cause their SPICE kernels to be improved over time. NAIF's SPICE kernels are used to calculate geometry information and correlate spacecraft clock with universal coordinated time UTC. Geometry and time information submitted to PSA and PDS is NAIFs best information at that point in time. Therefore all data set users are encourage to compute the geometry information for themselves. Remember, when NAIF improves SPICE kernels, geometry information will change. The information archived PSA and PDS is the best information at that point in time.