This is a description of the contents of the GENESIS Level 1A "txt" format files. ============================================================ SUMMARY ============================================================ The Genesis GPS Occultation Observations Level 1A data set consists of LEO-GPS (low-earth-orbiter satellites and Global Positioning System satellites) radio occultation (limb sounding) atmospheric phase delay and amplitude data for use in computing atmospheric refractivity, temperature, pressure, and water vapor profiles. LEOs collecting occultation data include GFZ's CHAMP (CHallenging Minisatellite Payload satellite) and CONAE's SAC-C. Data are available at the two GPS frequencies, 1.2 and 1.6 GHz, the latter at a time resolution of 0.01 or 0.02 seconds. These are the Level 1A products of this radio occultation data set. The full set of products available include * Level 0: Raw GPS data * GPS data for orbit determination in RINEX format * Orbit products * Level 1A: Atmospheric phase delay and signal amplitude * Level 1B: Atmospheric doppler shift and bending * Level 2: Atmospheric refractivity, temperature, pressure, water vapor pressure profiles, and comparisons to weather analysis and radiosondes. CHAMP and SAC-C each carry a JPL Blackjack GPS receiver and a rearward-facing antenna to record the signals of GPS satellites setting behind the Earth's limb as the signal passes through the atmosphere. (SAC-C also carries a forward-facing antenna, currently inactive.) Typically over 200 of these "occultations" occur per day per LEO with fairly uniform global distribution. By measuring the precise phase delay experienced by the GPS signals, and using precise clock-offset and orbit information, the atmospheric component of the phase delay can be extracted. Assuming local spherical symmetry about the tangent point, inversion of the atmospheric phase delay phase measurements during an occultation yields atmospheric refractivity profiles, which can be converted to temperature and pressure profiles between 60 km and the middle troposphere, and, with independent knowledge of temperature, into water vapor density in the middle and lower troposphere. Valuable properties of radio occultation measurements of atmospheric profiles include: * Self-calibrating, making them ideal for climate detection. * Sub-kilometer vertical resolution. * Sub-Kelvin temperature accuracy below 45 km. * All-weather operation * Independent height and pressure data allowing computation of geopotential heights and derived wind fields * Concurrent global coverage with a small constellation Comparison of CHAMP and SAC-C to National Center for Environmental Prediction (NCEP) and the European Center for Medium-range Weather Forecast (ECMWF) analyses show that GPS occultation temperature profiles are consistent with the models to better than 0.5 K in the mean, and better than 1.5 K in standard deviation. Geometric optics inversion techniques primarily use the phase delay, but physical optics inversion can also make precise use of signal amplitude variation to account for diffraction effects. Each Level 1A data file contains atmospheric phase delay and amplitude data from a single occultation at a rate of 50 or 100 samples/sec at the 1.6 GHz frequency. 1.2 GHz data is also included, but may be noisier and at a lower sample rate. ============================================================ See README_genesis.txt for general description of GENESIS txt format. ============================================================ Following is the metadata header of an L1A txt file interleaved with commentary. ============================================================ ProductCreationTime = 2002-01-12T00:10:21 # Time of file creation (YYYY-MM-DDThh:mm:ss) ShortName = GPS-OCC-L1A LongName = GPS-Occultation-Level-1A # Short and Long names of data file type DataSetID = 20011029_0603sac_g46_1p0 # Data Set ID: Time of occultation (YYYYMMDD_hhmm), occulting link satellites (sac = SAC-C, g46 = GPS SVN 46), version (1p0 = 1.0) ParameterName = {"Atmosphere", "PhaseDelay", "PhaseSNR"} # Indicate general information content of file PlatformShortName = SACC SensorShortName = SACC-BlackJack # The sensor is the BlackJack GPS receiver on-board the SAC-C satellite VersionID = 1.0 # Product version is 1.0 ProcessingLevel = 1A # Product type (Level 1A) GranulePointer = 20011029_0603sac_g46_1p0.L1a.txt # Name of the "txt" format file TextFilePointer = ftp://sideshow.jpl.nasa.gov/pub/genesis/glevels/sacc/1p0/y2001/2001-10-29/L1a/txt/20011029_0603sac_g46_1p0.L1a.txt # Location of txt format file (.gz has been elided) XmlFilePointer = ftp://sideshow.jpl.nasa.gov/pub/genesis/glevels/sacc/1p0/y2001/2001-10-29/L1a/xml/20011029_0603sac_g46_1p0.L1a.xml # Location of xml format file (if available. .gz has been elided) HdfFilePointer = ftp://sideshow.jpl.nasa.gov/pub/genesis/glevels/sacc/1p0/y2001/2001-10-29/L1a/hdf/20011029_0603sac_g46_1p0.L1a.hdf # Location of hdf format file (if available. .gz may be elided) WestBoundingCoordinate = 291.11 NorthBoundingCoordinate = -63.06 EastBoundingCoordinate = 291.12 SouthBoundingCoordinate = -63.07 # Bounding coordinates of location of tangent point of occulting link (degrees latitude or east longitude) # The values in the L1a and L1b files are based on a straight-line approximation at some random point in the occultation; # the coordinate window is arbitrarily set to 0.01 degrees in latitude and longitude. # The latitude/longitude profile is not computed until the end of processing, # and the correct bounding coordinates are in the L2 file. RangeBeginningDate = 2001-10-29 RangeBeginningTime = 06:03:27.500 RangeEndingDate = 2001-10-29 RangeEndingTime = 06:04:41.480 # Data begin and end date/time Transmitter = gps46 # Occulting link transmitter (gps46 = GPS SVN 46) Receiver = sacc # Occulting link receiver (sacc = SAC-C) ReferenceTransmitter = gps31 # Reference link transmitter (gps31 = GPS SVN 31) ReferenceReceiver = sutm # Reference links receiver (sutm = SUTM GPS ground receiver) LowestAltitude = 0 # Lowest altitude of tangent point (km). See Level 2 file for final value. LocalTime = 01:28:00 # Local time of occultation at tangent point LinkOrientation = -23.40 # Orientation of occulting link from receiver to transmitter (degrees, counterclockwise from east) AS = ON # GPS satellite Anti-Spoofing status (AS = ON indicates Anti-Spoofing is on) CalibrationFileReference = /genesis/occult/calibration/sacc/g150c5_q/y2001/2001-10-29/2001-10-29-06:03sacc_gps46 # JPL internal file reference AngleToVelocity = 156.3 # Angle between receiver velocity vector, and vector from receiver to transmitter, both vectors projected to receiver's horizontal plane (deg) ErrorFlags = 1111111 # 7 1-character flags indicating processing error status at 7 steps: 1 = nominal completion # 1. Extracting atmospheric delay # 2. Retrieving profiles # 3. Quality control # 4. Level 1a format conversion # 5. Level 1b format conversion # 6. Level 2 format conversion # 7. Publish under http://genesis DayOfYear = 302 # Day of year (1...366) ReceiverSoftwareVersion = TBD DataTypeName = { "Ca_L1a", "L2_L1a" } DataTypeID = { 1, 2 } Fields(1) = { "Time", "Phase", "SNR", "TransTime"} Fields(2) = { "Time", "Phase", "SNR", "TransTime"} # Definition of data in quasi-tabular numerical data set below: # # Two data types: # 1 = Ca_L1a , GPS CA-code phase delay data at the L1 (1575.42 MHz) frequency (L1A level product). # 2 = L2_L1a , GPS P-code phase delay data at the L2 (1227.60 MHz) frequency (L1A level product). # # Fields for both data types are # Time = Time phase received at receiver (seconds past StartTimeInSecondsFromJ2000) # Phase = Atmospheric excess phase delay (km) (biased) # SNR = Voltage SNR of carrier phase (Signal amplitude / RMS background noise) # TransTime = Time signal was transmitted by transmitter (seconds past StartTimeInSecondsFromJ2000) StartTimeInSecondsFromJ2000 = 0.5760740750E+08 # Reference time for data in tabular data in seconds past J2000 GPStime. # J2000 = 1 Jan 2000 12:00 GPS time CA_StartPhase = 0.27790739E-01 CA_EndPhase = 0.22948012E+01 CA_MinimumPhase = 0.27789204E-01 CA_MaximumPhase = 0.22948012E+01 CA_StartSNR = 0.67462666E+03 CA_EndSNR = 0.11722047E+02 CA_MinimumSNR = 0.83728909E+00 CA_MaximumSNR = 0.74218067E+03 First, last, minimum, and maximum phase delay and SNR values in the numerical data for CA-code L1 frequency Phase delays are atmospheric excess phase delay in km, with an overall bias. SNRs are carrier phase voltage SNR: (signal amplitude)/(RMS noise). P2_StartPhase = 0.46402915E-01 P2_EndPhase = 0.21985969E+01 P2_MinimumPhase = 0.46402915E-01 P2_MaximumPhase = 0.21985969E+01 P2_StartSNR = 0.96000000E+02 P2_EndSNR = 0.00000000E+00 P2_MinimumSNR = 0.00000000E+00 P2_MaximumSNR = 0.11600000E+03 First, last, minimum, and maximum phase delay and SNR values in the numerical data for P-code L2 frequency