Cicerone: Data — LAT Data Files - Column Descriptions

LAT File Types

There are two LAT data filetypes resulting from the Level 1 processing of the data downlinked from the Fermi spacecraft. This document describes the information contained in each column of those files. The Cicerone contains a detailed explanation of where this data originates. The two filetypes are:

Events Files - The two types of events files contain the same number of events
- Photon File - contains the science analysis data (time, energy, position, etc.)
- Extended File - same as the photon file, but with additional technical information for each event
Spacecraft File - contains spacecraft location/orientation information necessary for data analysis

Photon File

For each event the photon file contains the following information:

ENERGY (MeV) - Reconstructed energy of the event

RA (degrees) - Reconstructed direction of the event in Right Ascension

DEC (degrees) - Reconstructed direction of the event in Declination

L (degrees) - Reconstructed direction of the event in Galactic Longitude

B (degrees) - Reconstructed direction of the event in Galactic Latitude

THETA (degrees) - Reconstructed angle of incidence of the event with respect to the LAT boresight (+Z axis of the spacecraft - the line normal to the top surface of the LAT)

PHI (degrees) - Reconstructed angle of incidence of the event with respect to the +X axis (the line normal to the sun-facing side of the spacecraft)

ZENITH_ANGLE (degrees) - Angle between the reconstructed event direction and the zenith line (originates at the center of the Earth and passes through the center of mass of the spacecraft)

EARTH_AZIMUTH_ANGLE (degrees) - Angle of the reconstructed event direction with respect to North (line from spacecraft origin to north celestial pole) as projected onto a plane normal to the zenith. The angle is measured in degrees east of north, such that 90 degrees indicates that the event originated from the west.

TIME (seconds) - Mission elapsed time when the event was detected (MET is the total number of seconds since 00:00:00 on January 1, 2001 UTC)

EVENT_ID - Sequence number for the event in the LAT data acquisition period

RUN_ID - Unique identifier for each LAT data acquisition period

RECON_VERSION - Version of event reconstruction software in use at the time the event was detected

CALIB_VERSION (3-element array) - Version of the calibration tables for the ACD, CAL, and TKR (in that order) in use at the time the event was detected. (This column is currently unused)

EVENT_CLASS - For the current IRF, this indicates the fraction of residual background events, with higher values indicating lower background levels (i.e. higher confidence). A value of zero indicates the data was simulated. Note that the transient class is defined as CTBClassLevel >= 1, and so on, such that these classes are strictly nested.

CONVERSION_TYPE - Indicates whether the event induced pair production in the front (thin) layers or the back (thick) layers of the tracker (front=0, back=1)

LIVETIME (seconds) - A short-term measure of accumulated livetime of the LAT. This value can have gaps and it resets every few seconds. For large time intervals, the LIVETIME documented in the spacecraft file is correct. However, for short time intervals, this LIVETIME value can be compared between two events to gauge the fraction of dead time.

DIFRSP0 - Diffuse response for extragalactic component (only applicable for diffuse class events)

DIFRSP1 - Diffuse response for galactic component (only applicable for diffuse class events)

DIFRSP2 - Diffuse response for an additional component (currently unused)

DIFRSP3 - Diffuse response for an additional component (currently unused)

DIFRSP4 - Diffuse response for an additional component (currently unused)

Extended File

For each event the extended file contains the same columns as the photon file above. In addition it adds the following information:

CTBBestEnergyProb - The probability that the best energy chosen from the three energy estimators is correct (based on a Classification Tree analysis)

CTBBestEnergyRatio - The ratio of the most probable reconstructed energy with the raw energy estimate from the calorimeter subsystem, If the event missed the calorimeter, this is the reconstructed energy of the event.

CTBBestSXX - Analysis choice for the best XX slope element in the covariance matrix

CTBBestSXY - Analysis choice for the best XY slope element in the covariance matrix

CTBBestSYY - Analysis choice for the best YY slope element in the covariance matrix

CTBBestXDir - Analysis choice for the best X-direction cosine in the direction estimate

CTBBestYDir - Analysis choice for the best Y-direction cosine in the direction estimate

CTBBestZDir - Analysis choice for the best Z-direction cosine in the direction estimate

CTBCORE - The probability that the direction estimate is good (based on a Classification Tree analysis)

CTBClassLevel - This value is currently a copy of the information provided in EVENT_CLASS.

PtAlt - Spacecraft altitude at the time of the event

PtDecx - Declination of the spacecraft +X-axis at the time of the event

PtLat - Ground point latitude at the time of the event

PtLon - Ground point longitude at the time of the event

PtMagLat - Magnetic latitude at the time of the event

PtRax - Right Ascension of the spacecraft +X-axis at the time of the event

PtRaz - Right Ascension of the spacecraft +Z-axis at the time of the event

PtSCzenith - Angle of the spacecraft Z-axis from zenith (positive values indicate a rock toward the north)

Tkr1FirstLayer - First tracker layer to show evidence of a particle hit for the best track reconstruction (tracker layers are 0-17 where 0 is closest to the CAL).

Spacecraft File

The spacecraft file contains the following information for 30 second intervals (some intervals may be shorter):

START (seconds) - Mission Elapsed Time of start of interval

STOP (seconds) - Mission Elapsed Time of end of interval

SC_POSITION (meters) - Three element array giving the position (x, y, z) of spacecraft in inertial (ECI) coordinates at START

LAT_GEO (degrees) - Ground point latitude at START

LON_GEO (degrees) - Ground point longitude at START

RAD_GEO (meters) - Spacecraft altitude at START

RA_ZENITH (degrees) - RA of zenith at START

DEC_ZENITH (degrees) - DEC of zenith at START

B_MCILWAIN (Gauss) - McIlwain B parameter, magnitude of the magnetic field at START

L_MCILWAIN (Earth radii) - McIlwain L parameter, distance/shell value at START
- More information on the McIlwain parameters

GEOMAG_LAT (degrees) - Geomagnetic latitude at START

IN_SAA (T/F) - Whether the spacecraft was in the SAA at START

RA_SCZ (degrees) - viewing direction at START (RA of S/C +z axis)

DEC_SCZ (degrees) - viewing direction at START (Dec of S/C +z axis)

RA_SCX (degrees) - viewing direction at START (RA of S/C +x axis)

DEC_SCX (degrees) - viewing direction at START (Dec of S/C +x axis)

RA_NPOLE (degrees) - RA of north orbital pole at START

DEC_NPOLE (degrees) - Dec of north orbital pole at START

ROCK_ANGLE (degrees) - Angle of the spacecraft Z-axis from zenith (positive values indicate a rock toward the north)

LAT_MODE - Spacecraft GNC mode, where the 3 nominal modes are 3(inertial point), 4 (Maneuver) and 5 (zenithpoint/survey). Other modes include 1 and 2 (capture and sunpoint - rarely used) and 6 and 7 (reentry modes).

LAT_CONFIG - Flag for the configuration of the LAT (1 = nominal science configuration, 0 = not recommended for analysis)

DATA_QUAL - Flag indicating the quality of the LAT data, where 1 = ok, 2 = waiting review, 3 = good with bad parts, 0 = bad

LIVETIME (seconds) - Accumulated livetime of the LAT during the interval from START to STOP

QSJ_1 - First component of SC attitude quaternion

QSJ_2 - Second component of SC attitude quaternion

QSJ_3 - Third component of SC attitude quaternion

QSJ_4 - Fourth component of SC attitude quaternion