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GO JUP SSD DERIVED ELECTRON FLUX V1.0 (PDS)

NSSDCA ID: PSFP-00306

Availability: Archived at NSSDC, accessible from elsewhere

Description

This description was generated automatically using input from the Planetary Data System. Data Set Overview ================= The Galileo star scanner is providing data on the instantaneous flux of 1.5 to 30 MeV electrons in the Jovian environment. It is able to measure fluxes of ~1 x 105 electrons cm-2 sec-1 or greater which generally means the data are usable inside of about 12 and rarely as far out as 16.5 Jovian Radii (RJ). Typically, the data points are spaced about 400 or 80 seconds apart with infrequent periods of more rapid data depending upon the operational mode. It should be noted that data are generally accurate in time to only within 20 seconds without special processing. Pitch angle information generally cannot be extracted from this data set. Separate files exist for the Jovian insertion event (J0) and all subsequent perijove passes except J5 which occurred during a period of solar conjunction. Data ==== Time: GMT-UTC time of the measurement in PDS format: yyyy-mm-ddThh:mm:ss.sss. (Note: This accurately reflects when the data was telemetered, not the actual time the data was taken which could be up to 20 seconds earlier. All data is referenced to the time it occurred on the spacecraft time and not ground receipt time. Occasional periods of much higher rate telemetry do occur (e.g. parts of orbit C21) where data is telemetered every 2/3 second.) Sclk: A spacecraft counter from which time information is derived. The left hand integers are termed RIMs and increment 1 unit per 60.66666 seconds. The center two integers are termed 'minor frames' and increment from 0 to 90 at a rate of 1 unit per 2/3 second. When this cycle is complete one digit is added to the RIM count. The final integer is termed an 'RTI' and increment from 0 to 9 at a rate of 1 unit per 0.0666666 seconds. When this cycle is complete, one unit is added to the minor frame count. Star Code: This is a hexadecimal representation of the star scanner health and status. It is collected at the same instant as Raw Star Intensity and the reading of Raw Background count. Position 1 - always should be '0'. Any other number indicates data is suspect. Position 2 - always should be 'x'. Any other letter is a flag for suspect data. Position 3 - 'f' indicates normal operation '1', '2', '5', '6' or '7' indicates anomalous star scanner operation. Suspect data. '3' or '4' star scanner is having difficulty recognizing stars. All data is fine unless otherwise noted in notes column. Position 4 and 5 - Two letters that convert to an 8 bit binary word (hexadecimal). For example C7 = 11000111. First two bits should always be 11. Other numbers mean that the star scanner is re-starting and data is suspect. The final six numbers are a binary representation of recognized stars. In the C7 example, three stars are recognized, most likely because only three stars have been loaded into the star scanner memory. Day of Year (DOY): A fractional representation of the time at which the data was telemetered in GMT-UTC. All data is in spacecraft time and not ground receipt time. January 1 is DOY 001. For example, 6 pm on March 1, 1996 (a leap year) is DOY 61.7500000. Twist: Spacecraft rotor twist angle in degrees. It is an inertially based angle using the EME-50 coordinate system. Twist is the angle between the projection of the 1950 North Celestial Pole onto the spacecraft X-Y plane and the -X axis of the rotor (spinning portion of the spacecraft). This represents the angle at which the raw background and star intensity data was telemetered, but not necessarily the angle at which the data was taken. It is used to subtract out the effects of 'star corruption' (see Fieseler [2000]) in the creation of the filtered and compensated data sets. It is a real number which varies from 0 to 360 degrees. Raw Star Intensity: The intensity of the most recently recognized star in counts. An integer which can scale from 0 to 16,383. Sirius is about 4200 counts and the star scanner is linear for spectrally similar stars. The instrument, however, has a sharp spectral response in the blue and so reddish stars are much dimmer than might otherwise be expected. Star intensity is put into a buffer for downlink only when a star is recognized. After this, the stale data remains in the buffer until the next star is seen. This buffer is generally only checked and relayed to the ground once per several minutes in most telemetry modes. Star intensity information is primarily used to subtract out the effects of 'star corruption' [Fieseler, 2000] in the creation of the compensated data set. Raw Background: Far from Jupiter, this is just average brightness of the sky, including stars, nebulae, zodiacal light taken over the previous 25.6 milliseconds. This is termed 'star corruption' Near Jupiter, electron bombardment adds signal and eventually overwhelms the light from background sources. An integer which can scale from 1 to 16,383. It is placed into at the same time as star intensity. This buffer is generally only checked and read-out once per several minutes in most telemetry modes. Filtered: This is the filtered data which is derived from the Raw background, Star Intensity and Twist data. It represents the measured background light with the star corruption (deterministic effects of background light) subtracted out. A value of -50 means that this data had to be thrown out of the processing because the star light could not be subtracted out in this instance. Although this data frequently appears to be an integer, it is actually real as fractional values are do occur in some orbits. Compensated: This is the compensated data set which is derived from the filtered data. It linearizes the star scanner data (detector saturation becomes a factor in some cases) and corrects for gain changes in the photomultiplier tube. A real number. A value of -50 means that this data had to be thrown out of the processing because the star light could not be subtracted out of the filtered data in this instance. Error low: This is the estimated error in the compensated data applied on the low side. It is comprised of an RSS'ed series of independent or nearly independent worst case instrument errors, biases and computational errors. It accounts for aging and radiation damage to the star scanner equipment, noise in the background radiation data after being averaged, noise in the star light subtracted out of the filtered data, attitude errors, calculational errors and biases caused by magnetic and temperature effects. Error high: This is the estimated error in the compensated data applied on the high side. It is comprised of an RSS'ed series of independent or nearly independent worst case instrument errors, biases and computational errors. It accounts for aging and radiation damage to the star scanner equipment, noise in the background radiation data after being averaged, noise in the star light subtracted out of the filtered data, attitude errors, calculational errors and biases caused by magnetic and temperature effects. Flux: This is the omnidirectional flux of electrons in particles/cm-2 sec-1 between ~1.5 and 30 MeV external to the spacecraft. Due to the steep decrease of electron flux with energy, it can also be thought of as simply integral flux above 1.5 MeV. It is thought to be no more accurate than within a factor of five. It is an integer which would built from knowledge of the jovicentric distance and the Compensated data. The equation used was Flux = 1755 * Compensated Counts * RJ^1.1208 Where RJ is the most recently Estimated RJ rather than the actual RJ at the instant the data was taken. This does not cause an error of more than a few percent. The user is cautioned that this equation is preliminary - the star scanner data are self-consistent but the attempt to calculate a flux is less certain and was based primarily upon comparison with EPD data. This flux parameter was forced to a value of zero for all cases where the jovicentric distance was outside 20 RJ or when it, due to slightly negative values in the compensated counts, would have given a negative flux. Range: Distance from Jupiter center of mass in Jupiter Radii. Latitude: Planetographic latitude of the spacecraft in degrees. West Longtitude: Planetographic (West) longitude of the spacecraft in degrees. L: Distance at which a dipole field line crosses the dipole equator. L is given by L = R sin*2(Magnetic Latitude) where R is the distance from the center of Jupiter. Magnetic Latitude: Angle with respect to the dipole equator where the dipole axis tilts 9.6 degrees towards system III (1965.0) West Longitude. Magnetic Longtitude: Angle around the dipole axis with respect to the prime meridian. The prime meridian can be considered to intersect with 202 degrees west longitude of System III (1965.0). Notes: Text notes describing the transition to different star scanner operation modes, suspected problems with the data not captured under Star Code and other points of interest. The files of this data set are in the following structure: NAME TYPE Description ---------------------------------------------------------------------PDS Time char UTC time Spacecraft Clock char Spacecraft clock time Star Code char Star Scanner status code Day of Year float Decimal day of year Twist float Spacecraft rotor angle (degrees) Raw Star Intensity Int Star intensity (counts) Raw Background Int Background intensity (counts) Filtered Data float Raw intensity minus background Compensated Data float calibrated star intensity Error, Low float Bottom limit from error estimate Error, High float Top limit from error estimate Flux Int Electron flux R float Distance from Jupiter to spacecraft (Rj) Latitude float Planetographic latitude (degrees) West Longitude float Planetographic west longitude (degrees) L float Distance at which a dipole field line crosses the dipole equator Magnetic Latitude float angle to dipole equator (degrees) Magnetic Longitude float angle to dipole equator (degrees) Notes char note on changes in data Coordinate System ================= Satellite-Centered Planetographic (Right Handed) Coordinates (SPRH) ------------------------------------------------------------------SPRH is a spherical 'planetocentric' coordinate system, centered at the satellite. The magnetic field components are the standard right-handed spherical triad: R, Theta, and Phi. R is radial (along the line from the center of the satellite to the center of the spacecraft), and positive away from the satellite. Phi, the azimuthal component, is parallel to the satellite's planetographic equator (Omega x R) and positive in a right-handed sense. Theta, the 'southward' component, completes the right-handed set. Trajectory components are also right-handed. Since all of the satellites studied are nearly phase locked to Jupiter, the SPRH prime meridian is effectively the sub-Jupiter meridian. More precise definitions are provided by the IAU [1994] (see Table 4). Longitude is measured from the prime meridian and is positive in a right-handed sense (see figure below). R is the radial distance (satellite's center to spacecraft center). Latitude is planetocentric. System III [1965] Coordinates (SYS3) -----------------------------------System III [1965] (SYS3) magnetic field vector components form the standard right-handed spherical triad (R, Theta, Phi) for a Jupiter centered system. Namely, R is radial (along the line from the center of Jupiter to the center of the spacecraft), and positive away from Jupiter. Phi, the azimuthal component, is parallel to the Jovigraphic equator (Omega x R) and positive in the direction of corotation. Theta, the 'southward' component, completes the right-handed set. For SYS3 trajectory both east and west longitudes are provided. West longitudes are related to east longitudes by to the algorithm: west longitude = 360. - east longitude <degrees> West longitude is defined such that it appears to increase with time for a stationary observer [DESSLER1983]. Note, however, that R, latitude, and west longitude constitute a left handed set. The SYS3 1965 prime meridian is the sub-Earth longitude of 1965-01-01 00:00 UT. The spin rate (which was determined from the rotation rate of the magnetic field) is 9 hrs 55 min 29.719 sec. (See [DESSLER1983] for a discussion on Jovian longitude). R is the radial (Jupiter's center to spacecraft center) distance. Latitude is planetocentric.

Alternate Names

  • GO-J-SSD-5-DDR-STAR-SENSOR-V1.0

Discipline

  • Planetary Science: Fields and Particles

Additional Information

Spacecraft

Questions and comments about this data collection can be directed to: Dr. Edwin V. Bell, II

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Paul D. FieselerData ProviderNASA Jet Propulsion Laboratorypaul.d.fieseler@nasa.gov
Dr. Paul D. FieselerGeneral ContactNASA Jet Propulsion Laboratorypaul.d.fieseler@nasa.gov
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