Lunar Prospector Level 0 Volume Software Interface Specification

1.1. Overview

This Software Interface Specification (SIS) document is a description of the CD archive volume for the Lunar Prospector (LP) Level 0 data sets. These Level 0 products are basically raw data. Derived products will be included in separate archives. Lunar Prospector is a simple, spin-stabilized spacecraft operating in a circular, polar orbit around the Moon. During the LP primary mission, the orbit has a 118 minute period and average altitude of about 100 km. The science goals of LP are to map the Moon's surface composition and its magnetic and gravity fields, to determine the frequency and location of gas release events, and to search for polar ice deposits. To meet these objectives, LP has five science instruments, housed on three booms: a gamma ray spectrometer, a neutron spectrometer, an alpha particle spectrometer, a magnetometer, and an electron reflectometer. In addition, a gravity experiment uses Doppler tracking data to derive gravity measurements. This archive includes data collected by the three spectrometers, the magnetometer and the electron reflectometer. The gravity data sets are processed and archived separately.

The primary level 0 data set in this archive is the merged telemetry data set, which contains raw data acquired by the five science instruments and spacecraft engineering data. The telemetry records in these data files are ordered by time. The merged telemetry data do not contain information on spacecraft location or attitude. Thus, this archive includes several ancillary data sets needed for analysis of the science data. The ephemeris and spacecraft position data sets provide information on the spacecraft location as a function of time. The sun pulse and spacecraft attitude data sets provide information on the spacecraft spin rate and attitude. In addition, this archive includes a record of commands sent to the LP spacecraft.

1.2. Scope

The format and content specifications in this SIS apply to the Lunar Prospector Level 0 Data Archive produced by the Planetary Data System's Geosciences Node at Washington University in conjunction with the LP project. This archive is stored on CD write-once (CD-WO) volumes.

This document is a companion to the Lunar Prospector Science Data Interface Specification [6] document, which describes the detailed structure of the data products within the Level 0 data sets. This archive volume SIS describes the archive structure and how it was produced. It also documents how file formats or names are different from the descriptions in the LP Science Data Interface Specification document. This archive volume SIS also includes discussion of time and coordinate system conventions used in the archive.

1.3. Applicable Documents

International Standards Organization (ISO) document:

1. ISO 9660-1988, Information Processing - Volume and File Structure of CD-ROM for Information Exchange, April 15, 1988.

   Jet Propulsion Laboratory (JPL) documents:

2. JPL D-7669, PDS Data Preparation Workbook, Version 3.1, February 17, 1995.
3. JPL D-7669, PDS Standards Reference, Version 3.2, July 24, 1995.
4. JPL D-7116, Planetary Science Data Dictionary Document, Revision D, July 15, 1996.

   Lunar Prospector Mission documents:

5. Lunar Prospector Mission Data Management Plan, September 22, 1995.
6. Lunar Prospector Science Data Interface Specification.
7. Lunar Prospector Ground Software User's Guide and Programmer's Guide, February, 1, 1998.
8. Lunar Prospector Spacecraft Handbook.
9. Interface Control Document between the Lunar Prospector Project and the Flight Dynamics Analysis Branch, NASA Goddard Space Flight Center, 1997.

1.4. System Compatibility

The CD-WO volumes in this archive are readable on UNIX, PC, and MacIntosh systems.

2.1. ISO 9660 Compatibility

Each volume is an ISO-9660 compliant CD and may be read on any standard CD-ROM player. The volumes were created according to the ISO-9660 standards [1]. However, the volume does not contain any Extended Attribute Records (XAR). As a result some VAX/VMS users may have problems accessing files on this system.

2.2. PDS Compatibility

The archive complies with the PDS procedures and standards outlined in references [2], [3], and [4].

Each Lunar Prospector data product has a detached PDS label that describes the format and content of the product. Ancillary data files (e.g., index files) also have detached PDS labels. Each PDS label file is stored as an ASCII text file in stream format. A stream format file can have records of varying length. The end of a record is marked by carriage return <CR> and line feed <LF> characters to accommodate various operating systems.

2.3. Data Sources

After Lunar Prospector data are transmitted to Earth, they are subjected to programs that organize the data and merge multiple playbacks to minimize errors from data transmission. The merged data are distributed electronically (via ftp) to the LP science team members and the PDS Geosciences Node at Washington University.

Ancillary data, such as spacecraft attitude and sun pulse data, are generated at the Ames Research Center. Spacecraft ephemeris and position data files are generated by the Flight Dynamics Division of the NASA Goddard Space Flight Center and are distributed to the LP Mission Operations Center [9]. All of these data files are electronically sent to the PDS Geosciences Node for inclusion in the Lunar Prospector Level 0 Data Archive. The PDS labels in this archive are created by the PDS Geosciences Node at Washington University.

2.4. Data Validation

The merged telemetry data within the Lunar Prospector Level 0 Data Archive has been subjected to a variety of data validation procedures. The LP mission operations group at the Ames Research Center check the merged telemetry files for data gaps by searching for missing spacecraft clock counts. Data are also flagged as noisy if: A) the merge program cannot identify records due to corrupted spacecraft clock counts, B) the LP operations team detects noise in the real-time telemetry displays, or C) there are low signal-to-noise ratios. Data gaps and noisy data are documented in a file stored in the INDEX directory of each Lunar Prospector CD volume.

The Lunar Prospector Level 0 archive volume has also gone through a standard PDS peer review.

2.5. Volume Content and Generation Frequency

Each volume in the Lunar Prospector Level 0 Data Archive stores one calendar month of data collected by the spacecraft. Each volume also has associated ancillary data for that month (e.g., ephemeris, uplink commands, sun pulse, and attitude data), along with a complete set of documentation. Each volume contains a series of index files that lists all the data products on that volume and a series of cumulative index files that lists all data products on all of the volumes released so far.

Archive volumes are produced and released approximately 2 to 3 months after a complete month of data has been received on Earth.

3.1. Volume Labels

Volumes within the Lunar Prospector Level 0 Data Archive are labeled and uniquely identified using the PDS volume object, which is stored in the VOLDESC.CAT file within the top-level directory of each volume. A combination of the volume_id and volume_version_id keywords can be used to uniquely identify a volume. The Volume_id value has the form LP_0xxx, where LP indicates Lunar Prospector, the digit 0 indicates level 0 volumes, and xxx is a volume number starting with 001 for the first volume. The volume_version_id value is "Version 1" for the first release of a volume. If a volume is ever revised and remade, then the volume_id will remain the same as the original version and the volume_version_id will be incremented to indicate the revised volume.

3.2. Directory Structure

Each Lunar Prospector Level 0 Data Archive volume is an NFS-mountable volume containing a number of directories. The directories: COMMAND, GEOMETRY, MERGED, and SUNPULSE contain data product files and PDS labels. The directories: CATALOG, DOCUMENT, and INDEX store ancillary data and documentation.

In addition to subdirectories, the top-level directory contains the following files:

AAREADME.TXT	-	Brief description of volume contents.
AAREADME.HTM	-	HTML version of the AAREADME.TXT file.
ERRATA.TXT	-	Cumulative set of comments and errors about the archive.
VOLDESC.CAT	-	PDS volume object definition.

The COMMAND directory has a series of ASCII text files that contain uplink commands sent to the Lunar Prospector spacecraft for a given month. Each command file has an associated PDS detached label file.

The GEOMETRY directory contains files for the spacecraft attitude, ephemeris, and latitude/longitude position data sets. The spacecraft attitude file is cumulative over the entire mission. Ephemeris and spacecraft latitude/longitude position files cover the same time range as the spacecraft telemetry data for a given volume. Each data file in the GEOMETRY directory has an associated PDS detached label file.

The MERGED directory has the set of merged telemetry files received from the spacecraft during a given month. The data are in binary format. Because several merged telemetry files are generated each day (each covering a period of a few hours), these data files are grouped into subdirectories based on the day of year that the data were received. Each subdirectory under the MERGED directory contains data files for a block of 10 days. The subdirectories are named as follows: DddX, where the dd is replaced with the 2 most significant digits in the day of year. Each merged telemetry file has an associated PDS detached label file.

The SUNPULSE directory contains the set of sun pulse data files associated with the merged telemetry files for a given month. A sun pulse is the time that the sun sensor detects the Sun. The sun pulse data are stored in binary format. Generally, there is a sun pulse file that covers the same time period as a corresponding merged telemetry data file. Thus, sun pulse files are divided into subdirectories in the same manner as described above for the merged telemetry files and with the same convention for naming the subdirectories. Each sun pulse file has an associated PDS detached label file.

The CATALOG directory contains a series of files with summary descriptions of the Lunar Prospector mission, spacecraft, instruments, and data sets in the form of PDS catalog objects.

The DOCUMENT directory contains the detailed documentation files for the LP Level 0 Data Archive, including the file you are reading.

The INDEX directory contains a PDS index table and cumulative index table for each data set. An index table has one record for each data product on a given volume. A cumulative index table has records for data products on all volumes released thus far. Records in an index table contain metadata about a given data product including the directory path and file name for the product. The INDEX directory also has an index table that is a cumulative list of times of data gaps and noisy data for the merged telemetry data files. All tables are formatted so that they can be read directly into many data management systems or spreadsheet programs.

The only time tag in the merged telemetry data set (see section 6.6.1 for a description of the merged telemetry data set) is the Earth Received Time (ERT) in UTC of the first bit in a record (i.e., frame). To determine the time that data were collected on the spacecraft, this ERT has to be converted to a corresponding spacecraft event time. The spacecraft event time for a measurement is needed to determine where the spacecraft was from the ephemeris or spacecraft position data sets and what its attitude was from the attitude data set because these data sets tag time as spacecraft event times in UTC.

The LP telemetry data stream includes data transmitted in real-time and data recorded about 50 minutes earlier (the delayed stream). Thus, data are transmitted twice in order to recover data during transmission outages. The merged telemetry data set is constructed by selecting and storing one of these two transmissions. For merged telemetry records where the real-time stream is stored, the ERT is time of receipt of the first bit of the record. For records where the delayed data stream is selected, the ERT in the record is the time when the real-time data would have been received. The ERT in each merged telemetry record should increment by about 2 seconds. Some variation may occur due to changes in the one-way light time between the Earth and Moon. The LP team uses a constant one-way light time, even though it changes due to the slight eccentricity in the Moon's orbit. The average one-way light time is 1.28 seconds.

The data in each merged telemetry record are 2 seconds old before transmission, due to buffering in the spacecraft Command and Data Handling (C&DH) unit. Thus, to determine the spacecraft time when a given engineering data record was collected, 3.28 seconds for the buffering time and the one-way light time needs to be subtracted from the ERT. Determining the spacecraft time for science measurements is more complex, since the instruments take another 2 seconds to transmit data to the C&DH. They also integrate for 32 seconds for many of their measurements and transmit the data in 16 separate records (i.e., frames).

The merged telemetry data files do not contain any embedded information on the spacecraft location at the time data were acquired. Spacecraft location information is stored in the ephemeris and spacecraft position data sets. To determine the spacecraft location associated with a given science measurement in the telemetry file, the earth received time from the telemetry file must first be converted to a spacecraft event time (see Section 4 of this document). The ephemeris or spacecraft position data set can then be used to determine the spacecraft position for that spacecraft event time.

Spacecraft state vectors (i.e., position and velocity) stored in the ephemeris data set are given in Moon-centered, Earth equator, Mean of J2000 coordinates. Note that this coordinate system is an inertial system that does not rotate with the Moon. Thus, in order to convert the spacecraft position from these state vectors into selenocentric or selenographic coordinates, the relationship between the inertial system and the rotating lunar system must be known for the time of interest. The spacecraft attitude data set also uses the J2000, Earth equator inertial coordinate system, with the spacecraft spin axis defined in right ascension and declination.

The spacecraft position data set uses a rotating selenographic coordinate system for the spacecraft latitude and longitude position. For the Moon, longitudes are positive in the east direction. Because the flattening of the Moon is close to zero (1x10^-30), these selenographic latitudes are essentially the same as selenocentric latitudes. The radius of the Moon used to compute the height of the spacecraft above the surface is 1738 km. A spherical planet (i.e., no topography) is also assumed in determining the spacecraft height.

6.1. Spacecraft Attitude Data Set

6.1.1. Data Set Description

The Lunar Prospector attitude data set consists of values for the spacecraft spin rate and spin axis orientation (attitude) as a function of time. These values are determined from spacecraft attitude data (sun and limb crossing sensors) and ephemeris data. Details on the method used to determine attitude and spin rate are given in the Project's Ground Software User's Guide and Programmer's Guide [7]. The attitude data set consists of a single, cumulative file for the entire mission.

Spacecraft orientations are given as right ascension and declination of the spacecraft spin axis in the Mean-of-J2000 Ecliptic reference frame. Records are added to this file following each significant change in either the spin rate or spin axis orientation (e.g., before and after a spacecraft maneuver). The spin rate and attitude given in the attitude file are orbit averages. The instantaneous spin rate can be obtained from the sun pulse data set. During eclipse, the instantaneous spin rate may differ from the mean spin rate by up to 0.01 revolutions per minute as the spacecraft passes into and out of the Moon's shadow.

The uncertainty in spacecraft attitude (right ascension and declination) is estimated to be about one degree of arc. The spacecraft attitude generally varies by less than one degree per day, except during maneuvers. Users can approximate the instantaneous attitude by linearly interpolating between consecutive days. No attempt is made to determine the instantaneous attitude throughout a maneuver. Following a maneuver the spacecraft can nutate for 6 to 8 hours.

6.1.2. File Name Convention

There is only one attitude file on a given Level 0 archive volume. The name of the file is ATTITUDE.TAB and the PDS label file is named ATTITUDE.LBL. Each version of the attitude file has a unique product_id, with a the format of LP_ATTITUDE_yymmdd, where yy is the 2-digit year, mm is the month, and dd is the day that a given version of the file was created.

6.1.3. File Format

The format of the spacecraft attitude file stored in this archive is different from the original Lunar Prospector Project format. The version in this archive has been reformatted in order to comply with PDS standards. Records within the file are in a fixed-length ASCII text format. All records are terminated with a carriage return <CR> and line feed <LF> character pair to accommodate various computer operating systems. All fields are fixed-length and are delimited by commas. Time values are given in UTC in the ISO standard format of yyyy-mm-ddThh:mm, where yyyy is the year, mm is the month, dd is the day, hh is the hour, and mm is the minute [3]. Numeric fields are right justified.

6.2. Uplink Command Data Set

6.2.1. Data Set Description

The command file data set contains a record of all commands sent to the Lunar Prospector spacecraft. These files document commands sent to the spacecraft to change operating modes to perform spacecraft maneuvers. Commands sent to the science instruments are also documented in these files. Command files are ASCII text records of the commands and associated parameters. A complete list of spacecraft commands can be found in Appendix B of the Level 0 Data Product SIS [6] or Appendix A of the Lunar Prospector Ground Software User's Guide and Programmer's Guide [7], which are both included in the Lunar Prospector Level 0 Data Archive.

Each file in the command data set contains uplink commands for a given commanding session. Such uplink sessions typically occur every few days.

The uplink command files contain a field to indicate the status of sending each command to the spacecraft. Status values are CONFIRM, ABORT, and SKIP. Confirm indicates that the entire command was successfully transmitted. Abort indicates that the transmission of the command began, but not all command bits were sent. Skip means that the command was never sent.

6.2.2. File Name Convention

File names for the uplink command files have the form of Cyydddss.TAB, where yy is the 2-digit year, ddd is the day of year, and ss is the number of the DSN station that sent the commands. The PDS label file name has the same form, but with an extension of .LBL. This file name convention is different from that used by the Lunar Prospector Project. File names have been changed conform to the "8.3" standard used by PDS for compatibility with ISO-9660 CD-ROM standards. The original Lunar Prospector file name is used as the product_id in the PDS label as a cross reference with the original LP file.

6.2.3. File Format

The format of the uplink command files stored in this archive is different from the original Lunar Prospector Project format. Command files in this archive have been reformatted in order to comply with PDS standards. Records within a command file have a fixed-length ASCII text format. All records are terminated with a carriage return <CR> and line feed <LF> character pair to accommodate various computer operating systems. All fields are fixed-length and are delimited by commas. Time values are given in UTC in the ISO standard format of yyyy-mm-ddThh:mm:ss.sss, where yyyy is the year, mm is the month, dd is the day, hh is the hour, mm is the minute, and ss.sss is the second [3]. Numeric fields are right justified. Text fields are left justified and are enclosed in double quotes. Text fields are also padded with spaces to maintain a fixed width for the field.

6.3. Sun Pulse Data Set

6.3.1. Data Set Description

The sun pulse data set is a record of times that the LP sun sensor detects the Sun. Sun pulse times are used in determining the spacecraft spin rate and orientation as a function of time. The sun pulse data set is also used to replace the sun pulse times in the merged telemetry files. Because the Lunar Prospector spacecraft is sometimes in eclipse with the Sun, the sun pulse data set is generated on the ground to include both actual sun pulse times based on the sun sensor measurements and estimated sun pulse times based on the history of spin rate change during prior eclipses. As such, the sun pulse data set is a more complete and accurate record of times of Sun detection than the values in the merged telemetry files.

When Lunar Prospector is in sunlight, the sun pulse time is set equal to the sun sensor time extracted from the telemetry files. When the spacecraft is in eclipse, the sun pulse is estimated using the pre-eclipse measured spin rate, the calibrated spin rate change during eclipse, and Earth-Moon limb sensor measurements. The uncertainty in an estimated sun pulse time is also determined and stored in the data set. Each sun pulse record contains a flag indicating whether the value is an actual or estimated value.

The sun pulse data set is organized into a set of files, each of which cover about 4-6 hours of time. In general, each sun pulse file matches the time range of a corresponding merged telemetry file. There is usually one sun pulse record for every other minor frame (i.e., record) of the merged telemetry file.

6.3.2. File Name Convention

File names for the sun pulse files have the form of Syydddhh.B, where yy is the 2-digit year, ddd is the day of year, and hh is the hour for the start of the file. The PDS label file name has the same form, but with an extension of .LBL. This file name convention is different from that used by the Lunar Prospector Project. File names have been changed conform to the "8.3" standard used by PDS for compatibility with ISO-9660 CD-ROM standards. The original Lunar Prospector file name is used as the product_id in the PDS label as a cross reference with the original LP file.

6.3.3. File Format

Sun pulse files in this archive have the same format as the original Lunar Prospector Project files. These files are stored in binary format and have fixed-length records. Records do not contain embedded information to indicate the beginning or end of a record. The starting byte of any record can be calculated using the record number and the record length in bytes. Most binary fields within a record are multiples of 8-bit bytes. In one case, two fields are packed into one byte. There are no delimiters between fields. The detailed structure of a sun pulse data file is given in the Lunar Prospector Science Data Interface Specification [6].

6.4. Spacecraft Ephemeris Data Set

6.4.1. Data Set Description

The Lunar Prospector ephemeris data set consists of spacecraft position and velocity determinations as a function of time in Moon-centered, Earth equator, mean of J2000 coordinates. Tracking data collected by ground stations are used by the Goddard Flight Dynamics Analysis Branch (FDAB) to compute the Lunar Prospector orbit ephemeris. The Level 0 Data Archive contains the ephemeris type known as the "definitive ephemeris", which is based on an updated lunar gravity model [9]. Each ephemeris file contains orbit determinations for approximately 24 hours. In some cases, a single file may contain two separate orbit solutions within one 24 hour period.

During the mapping phase of the mission the spacecraft position is given to the center of the Moon as indicated by the "CENTRAL BODY INDICATOR = 2" record in the header section of the ephemeris file. The orientation of the x, y, z axes is in Earth equator, mean of J2000 coordinates.

6.4.2. File Name Convention

File names for the ephemeris files have the form of Eyyddd.DAT, where yy is the 2-digit year, and ddd is the day of year for the file. The PDS label file name has the same form, but with an extension of .LBL. This file name convention is different from that used by the Lunar Prospector Project. File names have been changed conform to the "8.3" standard used by PDS for compatibility with ISO-9660 CD-ROM standards. The original Lunar Prospector file name is used as the product_id in the PDS label as a cross reference with the original LP file.

6.4.3. File Format

Ephemeris data files in this archive are in the original format as delivered to the Lunar Prospector Project by the Goddard Flight Dynamics Analysis Branch. The original format has been maintained to be compatible with tools used by the LP science teams. The files are ASCII text files. For each orbital solution there is a header section that has a "keyword = value" format and contains parameters used in the solution. The second section of the file contains records with time, spacecraft position, and spacecraft velocity. Note that some ephemeris files contain two solutions, each of which has a header and data section.

The format of the ephemeris data file is not compatible with PDS objects such as the table object. Instead the simple file object is used in the PDS labels of the ephemeris files. The detailed structure and format of the ephemeris file, along with the coordinate system, are documented in references [6] and [9].

6.5. Spacecraft Position Data Set

6.5.1. Data Set Description

The Lunar Prospector spacecraft position data set consists of spacecraft position in selenographic latitude, longitude, and altitude coordinates in one minute increments. Each file in the spacecraft position data set covers 24 hours of time. The data set also contains spacecraft velocity and distance from the center of the Moon as a function of time. Velocity values are the magnitude of the spacecraft velocity vector with respect to the moon's center of gravity. The data set is generated by the Goddard Flight Dynamics Analysis Branch.

A negligible amount of flattening (1x10^-30) is used for the Moon. Thus, latitude can be considered selenographic or selenocentric with almost no error. Longitudes are east longitude positive. The mean radius and GM of the Moon used to generate this data set are 1738 km and 4902.8014 km^3/s^2, respectively.

6.5.2. File Name Convention

File names for the spacecraft position files have the form of Pyyddd.DAT, where yy is the 2-digit year, and ddd is the day of year for the file. The PDS label file name has the same form, but with an extension of .LBL. This file name convention is different from that used by the Lunar Prospector Project. File names have been changed conform to the "8.3" standard used by PDS for compatibility with ISO-9660 CD-ROM standards. The original Lunar Prospector file name is used as the product_id in the PDS label as a cross reference with the original LP file.

6.5.3. File Format

Spacecraft position data files in this archive are in the original format as delivered to the Lunar Prospector Project by the Goddard Flight Dynamics Analysis Branch. The original format has been maintained to be compatible with tools used by the LP science teams. The files are ASCII text files. The first line of each file contains the date covered by the file. The remainder of the file contains a series of spacecraft position determinations at one minute intervals. Each position determination occupies 2 lines in the file. Each spacecraft position provides the following information: date and time in UTC, Julian date, the distance of the spacecraft from the center of the planet in km (labeled R in file), spacecraft velocity in km/sec (V), latitude in degrees (LAT), longitude in degrees (LON< east is positive), and spacecraft altitude above the surface in km assuming a spherical planet of radius 1738 km (HGT).

The format of a spacecraft position data file is not compatible with PDS objects such as the table object. Instead the simple file object is used in the PDS labels of the spacecraft position files.

6.6. Merged Telemetry Data Set

6.6.1. Data Set Description

The Lunar Prospector merged telemetry data set is a result of comparing the two Lunar Prospector telemetry data streams and selecting one of them. The Lunar Prospector raw telemetry data downlink stream contains data transmitted in real-time and data that was collected about 50 minutes prior and stored on the spacecraft. The purpose of the delayed stream is to capture data acquired during communications blackouts.

The merged telemetry data set contains raw science and instrument engineering data acquired by all of the science instruments (Gamma Ray, Neutron, and Alpha Particle Spectrometers, and the Magnetometer and Electron Reflectometer), along with spacecraft engineering data. The data set is organized into binary files with each file containing about 4-6 hours of data. Each record in a given file is a main (minor) frame containing 2 seconds of downlinked data. A major frame consists of 16 minor frames.

The merging process compares each minor frame of the real-time and delayed data streams. When mismatches occur between the two data streams, the data stream with the highest signal-to-noise ratio is chosen for storage in the merged file. When there are no mismatches, the real-time data stream is chosen. When mismatches occurs between the two streams, and the signal-to-noise ratios are the same, the real-time stream is selected. Where there is only one data stream for a given time, that stream is stored in the merged file. Each minor frame (i.e., each record in a merged telemetry file) has a flag to indicate which stream was selected and which criteria was used for the selection.

6.6.2. File Name Convention

File names for the merged telemetry data files have the form of Myydddhh.B, where yy is the 2-digit year, ddd is the day of year, and hh is the hour for the start of the file. The PDS label file name has the same form, but with an extension of .LBL. This file name convention is different from that used by the Lunar Prospector Project. File names have been changed conform to the "8.3" standard used by PDS for compatibility with ISO-9660 CD-ROM standards. The original Lunar Prospector file name is used as the product_id in the PDS label as a cross reference with the original LP file.

6.6.3. File Format

Merged telemetry data files in this archive have the same format as the original Lunar Prospector Project files. These files are stored in binary format and have fixed-length records. Records do not contain embedded information to indicate the beginning or end of a record. The starting byte of any record can be calculated using the record number and the record length in bytes. Most binary fields within a record are multiples of 8-bit bytes. In some cases more than one field is packed into one byte. There are no delimiters between fields. The detailed structure of a merged telemetry data file is given in the Lunar Prospector Science Data Interface Specification [6].