Introduction to the Special Sensor Microwave Atmospheric Temperature Sounder Sensor Counts Data Set TABLE OF CONTENTS 1.0 Introduction 2.0 Special Sensor Microwave Atmospheric Temperature Sounder 3.0 SSM/T-1 Data Set Summary 3.1 File Naming Conventions 3.2 SSM/T-1 Header Record Format 3.3 SSM/T-1 Data Record Format 4.0 References 1.0 Introduction This README file contains information on the Special Sensor Microwave Atmospheric Temperature Sounder (SSM/T-1) sensor count data sets. Brief descriptions of the Defense Meteorological Satellite Program (DMSP) satellites, the SSM/T-1 instrument, the computation of antenna temperatures and the format of the data sets are included. Pertinent scientific references are also included. The Defense Meteorological Satellite Program (DMSP) is a Department of Defense program which is responsible for designing, building, launching and operating polar orbiting meteorological satellites. The satellites can broadcast visual, infrared and microwave imagery directly to transportable tactical sites around the world. The data are also stored for transmission to the Navy's Fleet Numerical Meteorology and Oceanography Center (FNMOC) and to the Air Force Global Weather Central (AFGWC). Each of the DMSP satellites flies in a sun-synchronous, near-polar orbit. For a satellite in sun synchronous orbit, the ascending equatorial crossing time remains relatively constant with respect to the local time throughout the lifetime of the satellite. The SSM/T-1 sensor is flown aboard the DMSP F-11, F-12 and F-13 satellites. Table 1 summarizes the orbits for each spacecraft (valid for the orbit shown). Table 1. Spacecraft F-11 F-12 F-13 Launch Date Nov 1991 Sep 1994 Mar 1995 Ascending Equator Crossing Time (Local) 18:11 21:22 17:42 Inclination (Degrees) 98.8 deg 98.9 deg 98.8 deg Degrees Period (Min) 101.9 102.0 101.9 min Maximum Altitude (Km) 878 851 856 Km Minimum Altitude (Km) 841 844 844 Km 2.0 The Special Sensor Microwave Atmospheric Temperature Sounder The information in this section was taken from the DMSP SSM/T-1 Physical Retrieval Critical Design Document (1994). The SSM/T-1 sounder consists of seven microwave channels with atmospheric sensitivity in the oxygen absorption band, at frequencies ranging from 50.5 to 59.4 GHz, with one channel acting as a surface window channel. Channel parameters and regions of peak sensitivity in the atmosphere are given in Table 2. Frequency Bandwidth Approximate Peak Sensivitity Channel (GHz) (MHz) Altitude Pressure 1 50.50 400 Surface - 2 53.20 400 1 800 3 54.35 400 6 400 4 54.90 400 10 280 5 58.40 115 30 10 6 58.825 400 18 70 7 59.40 250 23 40 The radiometer antenna completes one scan of seven earth view measurements and two calibration measurements every 32 seconds. In order to obtain the seven earth view measurements, the antenna is rotated in seven equal angular increments, resulting in a center nadir view and six limb views. At nadir, the antenna observes a circular area (footprint) of 175 km in diameter at the earth's surface. As the antenna scans away from the nadir on either side, the footprint becomes elliptical, increasing to 230x305 km at the largest viewing angle. At each viewing angle, the instrument collects measurements for each of the seven channels. Each scan of the instrument covers a swath of 1600 km perpendicular to the orbital tract with 195 swaths per orbit. The satellite completes a single orbit in approximately 104 minutes, resulting in approximately 14 orbits per day. The SSM/T-1 sounding products are disseminated in near real time by a variety of methods for use in local analyses and numerical weather prediction forecast models. The data sets are then placed into the SPN format. The resulting sounds products are mean lay virtual temperatures at 30 atmospheric levels (lowest TOVS level) from the surface to 10 mb. 3.0 SSM/T-1 Data Set Summary 3.1 File Naming Conventions The SSM/T-1 orbit files are stored as one logical daily file by employing the Unix tape archive (tar) procedure. Each daily file is then compressed using the standard Unix compression routine. The SSM/T-1 daily files are named using the convention: scxxt1yy.ddd_nesdis_swath.tar.Z where sc = sensor counts xx = satellite number (F11, F12 or F13) yy = year of the century ddd = julian day tar = Unix tar format Z = Unix compress format When an SSM/T-1 data file is uncompressed and untarred, the results will be one day's worth of data in individual files. These files will follow the naming convention: NSS.SSMT.Sn.Dyyjjj.Shhmm.Ehhmm.Axxxxxxx.NS where the upper case characters remain fixed and the lower case characters vary from spacecraft to spacecraft and from orbit to orbit. The lower case characters correspond to the following variables: n = Spacecraft identification (5 for F11, 6 for F12 and 7 for F13) yy = Year of the century jjj = day of the year hhmm = Time in hours and Minutes xxxxxxx = Orbit Number The upper case characters represent the following: NSS.SSMT = NESDIS SSM/T-1 D = Day S = Start Time E = End Time 3.2 SSM/T-1 Header Record Format Each SSM/T-1 level 1b data set contains a header record. A header record contains the following data groups: Idenfication Block 2 Bytes Data Type 2 Bytes Start Time 8 Bytes Number of Scans 4 Bytes End Time 8 Bytes Number of Data Gaps 4 Bytes Processing Block ID 8 Bytes Preflight Calibration Data 72 Bytes Scan Bias Correction 98 Bytes Antenna Pattern Correction Data 98 Bytes Manual Coefficients 14 Bytes Raw Counts Limits 14 Bytes Spare 168 Bytes 3.3 SSM/T-1 Data Record Format Each data record contains one scan of SSM/T-1 data. The data record contains the following data groups: Scan Information 2 Bytes Orientation Vectors 6 Bytes Satellite Altitude 2 Bytes Satellite Angle 2 Bytes Time Code 8 Bytes Orbit Number 4 Bytes Earth Location Data 28 Bytes SSM/T Data FOV & Telemetry 324 Bytes Scene Data 336 Bytes Calibration Coefficients 44 Bytes Quality Control Data 25 Bytes Spare 55 Bytes. The SSM/T-1 data files have an approximate size of 4.7Mb per satellite per file. 4.0 References Donahue, David, M. Pettey, B. Whistler, G. Kratz: DMSP SSM/T-1 Physical Retrieval System Critical Design, May 1994.