TRMM (Tropical Rainfall Measuring Mission)
TRMM is a joint NASA/JAXA (formerly NASDA) mission within NASA's ESE (Earth Science Enterprise) program with a low-inclination (equatorial) orbit. NASA/GSFC provides the satellite, four passive sensors, and mission operations, NASDA the launch vehicle (H-II rocket) and the precipitation radar instrument. Each agency processes the data from its own instruments. TRMM is the first mission dedicated to observing and understanding tropical and subtropical rainfall, one of the most important, but least understood parameters in global change. 1) 2) 3) 4) 5) 6) 7)
Objectives: Global change studies, especially in developing an interdisciplinary understanding of atmospheric circulation, ocean-atmospheric coupling, and tropical biology. General circulation models require detailed data on the latent heating of equatorial air masses, and the forcing and propagation speed of waves involved in the 30-60 day tropical oscillations.
• Measure diurnal variation of precipitation and evaporation in the tropics to provide increased understanding of how substantial rainfall affects global climate patterns.
• Obtain a minimum of three years of climatologically significant observations of rainfall in the tropics.
• In tandem with cloud models, provide accurate estimates of the vertical distributions of latent heating in the atmosphere.
• Provide TRMM rain products to weather organizations/researchers as close to real-time as possible to facilitate research into their applicability for forecast improvements and broader weather research.
The idea of measuring rainfall from space using a combined instrument complement of passive and active microwave (radar) instruments was generated in Japan in the late 1970s and in the USA in the early 1980s. 8) 9)
- Already in 1978 CRL (Communication Research Laboratory) of Tokyo, Japan started with the design of a so-called precipitation radar instrument.
- In Sept. 1984, a proposal titled, ‘‘Tropical Rain Measuring Mission,’’ was submitted to NASA/HQ by a team of GSFC investigators (G. North, T. T. Wilheit, and O. Thiele). The CRL of Japan (N. Fugono) joined in the activities soon thereafter.
- In 1985, a joint satellite project was proposed by NASA. The Japanese Space Commission accepted the NASA invitation to study jointly the feasibility of a TRMM mission. In November 1985, the first major workshop was convened near Goddard to develop the proposed TRMM further (NASA/GSFC and CRL). The group released a report establishing the science priorities for the mission in 1986. These goals are given in Table 1.
Although tropical precipitation is organized on the mesoscale, it is noteworthy that the primary objectives of the mission were to improve climate models and to aid them in climate prediction. It was proposed to have a dual-frequency radar, a multichannel dual-polarized, conically scanning passive microwave instrument similar to SSM/I, a single-frequency cross-track scanning radiometer to sample along with the radars, and a VIRS (Visible/Infrared Scanner) similar to the AVHRR (Advanced Very High Resolution Radiometer). The purpose of the visible/infrared instrument was to enable TRMM to establish the connection between TRMM and operational geostationary platforms and thus to serve as a ‘‘flying rain gauge.’’
Table 1: Goals of TRMM established by the Science Steering Group in 1986
- Agreements between the United States and Japan were formalized in 1988, leading to a new start for a joint U.S.–Japan mission at that time. The Japanese side (NASDA) agreed to provide the precipitation radar and a launch of the TRMM spacecraft by their new HII rocket. NASA would provide the spacecraft and the other rain-sensing instruments. The U.S. Congress passed support for TRMM for a new start in 1991, and the project got under way formally.
- In 1988, Japan (CRL) started with a breadboard model of the PR (Precipitation Radar) which was completed in 1993.
- Joint aircraft flights with an experimental radar suggested that instrument accuracy was promising. The low Earth orbit needed to realize such measurements from a spaceborne platform, however, immediately raised concerns regarding the sampling adequacy of such a satellite.
The radar data from the four Global Atmospheric Research Program Atlantic Tropical Experiment ships stationed in the ITCZ (Inter-Tropical Convergence Zone) off Africa in 1974 were used for a series of sampling studies. Several orbits and altitudes were considered. An inclined orbit extending between 35º N and 35º S at 350 km altitude was found to be the most suitable. The inclined orbit precessed such that the satellite would overfly a given location at a different time every day with an approximate 42 day cycle. This orbit would allow the documentation of the large diurnal variation of tropical rainfall.
- The ground validation program that followed included conduction of studies to improve rainfall measurement technology; establishment of ground validation sites consisting of radars, rain gauges, and disdrometers around the Tropics; development and expansion of techniques to measure rainfall in oceanic regions; improvement of ground-based rainfall estimation techniques; and development of radar processing and analysis software for producing and analyzing ground validation (GV) products.
- In 1993, the TRMM observatory passed its critical design review and moved into phase C/D of actual observatory construction.
- In 1994, the United States and Japan simultaneously selected new science teams that would be in place until the launch of TRMM in 1997. Although it was decided that the two teams should operate independently, a Joint TRMM Science Team made up of team leaders from both countries was established to coordinate the efforts of both teams. This joint team has worked effectively since then through the successful launch of TRMM from Tanegashima Island on November 27, 1997.
Figure 1: Artist's rendition of the deployed TRMM spacecraft over a tropical storm (image credit: NASA) 10)
The TRMM spacecraft is three-axis stabilized using zero momentum bias (designed and built at GSFC). The nominal Earth-pointing mission mode requires a rotation once per orbit about the spacecraft's y axis. The attitude determination hardware consists of ESA (Earth Sensor Assembly), DSS (Digital Sun Sensors), CSS (Coarse Sun Sensors), a TAM (Three-Axis Magnetometer), and gyroscopic rate sensors. The attitude control hardware includes three MTB (Magnetic Torquer Bars) which are used to provide magnetic momentum unloading capability, and a Reaction Wheel Assembly (RWA) which consists of four wheels in a pyramidal arrangement to maximize momentum storage capability along a preferred axis. Primary attitude determination is accomplished using the ESA and gyroscopes. The attitude knowledge is 0.18º per axis.
Two solar panels provide 1100 W of power (850 W average); the S/C mass = 3620 kg (including 890 kg of fuel); the S/C dimensions are approximately 5 m x 3.5 m; design life = 3 years. The SDS (Spacecraft Data System) features a dual-redundant command and data handling system which functions as a command decoding and distribution system, a telemetry/data handling system, and a data storage system. It provides onboard computational capability for processing attitude sensor data and generating commands for the attitude control actuators in a closed loop fashion. It also provides stored command processing and monitoring of the health and safety functions for the spacecraft and instrument subsystems. Use of MIL-STD-1773 protocol for data bus interfaces.
RF communications: TRMM employs two TT&C transponders permitting communications through four RF links via TDRS (Tracking and Data Relay Satellite), namely:
• LCP (Left Circular Polarization) forward or receiving
• LCP return or transmitting
• RCP (Right Circular Polarization) forward or receiving
• RCP return or transmitting
The forward (receiving) frequency is in S-band (2076.94 MHz). The return (transmitting) frequency is also in S-band (2255.5 MHz). Data rate = 170 kbit/s average and 2 Mbit/s on playback; TDRSS S-band communications (8.5 minutes/orbit playback time). The CCSDS protocols are being used for communications. TRMM science data are being received by NASA/GSFC and by JAXA/EORC (Earth Observation Research Center).
Figure 2: The TRMM satellite being assembled at Goddard Space Flight Center (image credit: NASA/GSFC)
Table 2: Major characteristics of the TRMM satellite
Figure 3: The deployed TRMM spacecraft (image credit: NASA)
Launch: The launch of TRMM [along with ETS-7 (Engineering Test Satellite-7) of NASDA, composed of two spacecraft: Chaser (Orihime) and Target (Ikoboshi)] took place on Nov. 27, 1997 (UTC) from the Tanegashima Space Center (TNSC), Japan. The NASDA H-II rocket served as the launch vehicle.
The mission goals required an extensive GVP (Global Validation Program) after launch, consisting of more than 10 ground validation sites throughout the tropics. This was complemented by airborne underflight campaigns like CAMEX-3 (Convection and Atmospheric Moisture Experiment-3).
Orbit: Non-sun-synchronous near-circular (low inclination) orbit, altitude = 350 km. (in Aug. 2001 TRMM was boosted to an altitude of 402.5 km to extend its life by 2 years), inclination = 35º, period = 96 min. An observation coverage of the tropics was chosen since the volume of rainfall in the tropics accounts for about two-thirds of the total rainfall on the earth. The low orbital altitude was selected to ensure sufficient detection capability of the weak radar echoes of the PR instrument, the prime sensor on TRMM.
Note: In August 2001, NASA boosted the TRMM orbit to an average altitude of 402.5 km to extend the mission life of TRMM. The choice of 402.5 km was determined by the next higher altitude at which the PR would work, given the designed pulse repetition rate. 11)
After the orbit boost, a 6-state Kalman filter used 3-axis gyro data with sun sensor and magnetometer data to estimate onboard attitude and gyro rate biases. Originally, the backup Kalman filter control mode was intended to meet a degraded attitude accuracy of 0.7º; however, after improving the onboard ephemeris accuracy and adjusting an onboard magnetometer calibration, it proved possible to meet the original 0.2º accuracy requirement.
Figure 4: Artist's view of the deployed TRMM spacecraft (image credit: NASA)
Table 3: Overview of TRMM orbit parameters
Operational status of the TRMM mission:
• The TRMM spacecraft and its payload (with the exception of CERES) are operating nominally in early 2014 (Ref. 4).
- Jan. 17, 2014 - Deadly Philippine Flooding And Landslides: People in the southern Philippines are used to heavy rainfall this time of the year but rainfall totals have recently been exceptionally high. A tropical low northeast of Mindanao has been an almost permanent feature on weather maps for the past week. It has caused nearly continuous rain in the area of northeastern Mindanao triggering floods and landslides that have caused the reported deaths of 34 people. 12)
The TMPA (TRMM Multi-Satellite Precipitation Analysis) service, produced at GSFC (Goddard Space Flight Center), combines the rainfall estimates generated by TRMM and other satellites. The analysis of Figure 5 shows a near-real time TMPA for the period from January 10-17, 2014. Extremely high rainfall totals of over 1168 mm for the past week were found near northeastern Mindanao. This past Monday (Jan. 13, 2014) a deadly landslide in this area caused the deaths of six people on Dinagat Island.
Figure 5: TMPA of the heavy rainfalls of the Mindanao region (Philippines), mostly from TRMM data, in the period Jan. 10-17, 2014 (image credit: NASA/GSFC)
• July 2013: TRMM lifetime predictions, issued July 30, 2013. 13)
The big challenge faced by TRMM is the uncertainty associated with the end of fuel date. GPM is planned to be launched by February 2014. Assuming the worst case scenario for fuel consumption, there should be 2+ years between end-of-fuel and the end-of-life date, during which TMI data will always be available, including about 1 year during which the PR will also be available as the satellite drifts to the 350 km elevation range (Ref. 14).
Figure 6: TRMM Lifetime - January 2013 Schatten Update utilizing PVT Method (image credit: NASA)
• June 2013: The 2013 Senior Review evaluated 13 NASA satellite missions in extended operations: ACRIMSAT, Aqua, Aura, CALIPSO, CloudSat, EO-1, GRACE, Jason-1, OSTM, QuikSCAT, SORCE, Terra, and TRMM. The Senior Review was tasked with reviewing proposals submitted by each mission team for extended operations and funding for FY14-FY15, and FY16-FY17. 14)
- TRMM has now passed the 15-year benchmark, which is a remarkable accomplishment considering that the initial life of the mission was to be three years. Because of this long record, and the broad constituency of data users for myriad applications, the continuity of the TRMM mission alone is justified in order to preserve the continuity and robustness of the longest global scale precipitation data set and meet operational demands. In addition, the record is currently enabling studies of global scale processes from annual to 3-5 years variability such as the Asian and Nth and South American Monsoons and ENSO. TRMM is the precursor to the upcoming GPM mission to be launched in 2014, which will benefit from TRMM sensor technology, ground validation and algorithm legacies, and also from cross-calibration of radar and radiometer instruments. TRMM and GPM are commended for having devised a well-planned transition process that preserves TRMM strengths while building day-1 capacity for GPM algorithms and products.
• On Nov. 27, 2012, the TRMM spacecraft was 15 years on orbit. TRMM has become the world’s foremost satellite for the study of precipitation and associated storms and climate processes in the tropics. While TRMM continues to collect valuable science data, its fuel will eventually run out for operations. Its successor, the GPM (Global Precipitation Measurement) mission, is gearing up for launch in February 2014. 16) 17)
Figure 7: This 3-D image of Hurricane Sandy's rainfall was created using TRMM Precipitation Radar data. It shows the storm as it appeared on Oct. 28, 2012. The red areas indicate rainfall of 50 mm per hour. (image credit: NASA/SSAI)
Figure 8: Illustration of TRMM observed average daily rainfall for the month of November in the period 1998 to 2010 (image credit: NASA) 18)
• Gridded climatologies of total lightning flash rates observed by the spaceborne OTD (Optical Transient Detector) and LIS (Lightning Imaging Sensor) instruments have been updated. OTD collected data from May 1995 to March 2000. LIS data (equatorward of about 38º) adds the years 1998–2010. Flash counts from each instrument are scaled by the best available estimates of detection efficiency. The long LIS record makes the merged climatology most robust in the tropics and subtropics, while the high latitude data is entirely from OTD. The gridded climatologies include annual mean flash rate on a 0.5º grid, mean diurnal cycle of flash rate on a 2.5º grid with 24 hour resolution, mean annual cycle of flash rate on a 0.5º or 2.5º grid with daily, monthly, or seasonal resolution, mean annual cycle of the diurnal cycle on a 2.5º grid with two hour resolution for each day, and time series of flash rate over the sixteen year record with roughly three-month smoothing. 19)
The mean global flash rate from the merged climatology is 46 flashes s-1. This varies from around 35 flashes s-1 in February (austral summer) to 60 flashes s-1 in August (boreal summer). The peak annual flash rate at 0.5º scale is 160 fl km-2 yr-1 in eastern Congo. The peak monthly average flash rate at 2.5º scale is 18 fl km-2 mo-1 from early April to early May in the Brahmaputra Valley of far eastern India. Lightning decreases in this region during the monsoon season, but increases further north and west. An August peak in northern Pakistan also exceeds any monthly averages from Africa, despite central Africa having the greatest yearly average.
Figure 9: a) HRFC mean annual global flash rate from combined LIS and OTD observations on a 0.5º grid; b) LRFC mean annual flash rate from combined LIS and OTD, 2.5º grid (image credit: NASA, Ref. 19)
• The TRMM spacecraft and its payload (with the exception of CERES) are operating nominally in 2012. In June 2011, the NASA Earth Science Senior Review recommended an extension of the TRMM mission to 2015. 20)
Executive summary: The TRMM satellite and its instruments are in excellent shape and there is sufficient station-keeping fuel on board to potentially maintain science operations until 2014 or later. TRMM flight operations and data processing costs have been significantly reduced for the extension period. TRMM data processing has shifted to the PPS (Precipitation Processing System), being developed as part of NASA’s Precipitation Program to process TRMM, GPM and other relevant satellite precipitation data. The basic mission extension will continue production of validation products that continue to contribute toward algorithm validation and improvement. A multi-year extension of TRMM has a very high payoff for science and applications, but at a low additional cost to NASA. 21)
Figure 10: Sample image of 7 day accumulated global reainfall data (image credit: NASA) 22)
• The TRMM spacecraft and its payload (with the exception of CERES) are operating nominally in 2011. NASA extended the mission trying to achieve an overlap with the GPM (Global Precipitation Measurement) mission, due for launch in 2013.
- The TRMM-based, near-real time TMPA (Multi-satellite Precipitation Analysis) at the NASA Goddard Space Flight Center (GSFC) is used to monitor rainfall over the global Tropics. In January/February 2011, TRMM covered the Cyclone Yasi, a massive storm, made landfall along the northeast coast of Queensland, Australia.23) 24)
- As the TRMM mission has now (2011) continued into its 14th year, the science objectives have extended beyond just determining the mean precipitation distribution but have evolved toward determining the time and space varying characteristics of tropical rainfall, convective systems, and storms and how these characteristics are related to variations in the global water and energy cycles. Significant scientific accomplishments have already come from TRMM data, including reducing the uncertainty of mean tropical oceanic rainfall; a documentation of regional, diurnal, and inter-annual variations in precipitation characteristics; the first estimated profiles of latent heating from satellite data; improved climate simulations; increased knowledge of characteristics of convective systems and tropical cyclones; and new insight into the impact of humans on rainfall distributions. The availability of real-time TRMM data has led to significant applications and fulfillment of national operational objectives through use of TRMM data, primarily in the monitoring of tropical cyclones, in hydrological applications, and in assimilation of precipitation information into forecast models. The TRMM satellite and its instruments are in excellent shape and there is sufficient station-keeping fuel onboard to maintain science operations potentially until 2014 or later.25)
- The successful TRMM partnership between NASA and JAXA has allowed for a better understanding of tropical rainfall and its relationship to the global climate, and has paved the way for the next JAXA-NASA partnership in the form of GPM. As with TRMM, the success of GPM will be tied closely to the ability of all partners’ data systems to closely coordinate efforts in the areas of science data processing and distribution.
• The TRMM spacecraft and its payload (with the exception of CERES) are operating nominally in 2010 after more than 12 years in orbit. The combined use of the PR and the TMI instrument data has greatly improved the estimation of rainfall amount. It has also revealed the three-dimensional structure of tropical cyclones over the ocean, which was rarely observed before the TRMM satellite.
Legend to Figure 11: The instruments PR (Precipitation Radar) and TMI (TRMM Microwave Imager) revealed that Magda had developed an eye before coming ashore with hurricane force winds and powerful thunderstorms were dropping rainfall at a rate greater than 50 mm per hour in an area west of the eye. TRMM’s 3-D perspective of Magda showed that some of the intense thunderstorms near its eye reached heights above ~16 km.
• In 2008, the TRMM mission life was extended with current estimates that mission operations will continue through the 2012-2013 timeframe. 27)
• TRMM is operating nominally as of 2008 after more than 10 years in orbit (10th anniversary of spacecraft launch on Nov. 27, 2007). TRMM has yielded significant scientific research data accumulated over the past years to users around the globe, well beyond its original design life of 3 years. TRMM started as an experimental mission, but has become the primary satellite in a global set of satellites to observe and study precipitation characteristics and variations. 28) 29) 30)
- All spacecraft systems are in excellent shape for continuation; one of two solar arrays is parked in horizontal position to avoid a failure of SADA (Solar Array Drive Assembly).
- The TRMM instruments (PR, TMI, VIRS, LIS) and spacecraft remain in excellent operating shape with some minor degradations
- Based on current fuel consumption expectations (in late 2007), TRMM data could be available into 2012-2013, providing the potential for overlap with GPM.
• Continuation of the TRMM mission: As of October 2005, NASA management decided to continue the TRMM mission until at least 2009 and possibly until 2012. Earlier agency plans had called for discontinuing TRMM this year (2005) while the spacecraft still had enough fuel for a controlled reentry. Although initially intended as a purely research-oriented mission, TRMM data are now being used in operational applications such as hurricane forecasting, because the data from its complementary sensor suite are unique and available in near real-time. The TRMM data are being used by several US weather forecasting centers as well as by JAXA, the ECMWF (European Center for Medium-Range Weather Forecasts), and by the tropical cyclone warning centers of WMO (World Meteorological Organization). - NASA's extension of the TRMM mission followed an internal NASA review that determined that the benefits to increased public safety from continued TRMM data service, in particular in such programs as hurricane modeling, outweighed by far any potential dangers from an uncontrolled reentry. 31)
• On Jan. 4, 2005, NASA announced that it will continue to operate the TRMM spacecraft through spring 2005.
• As of summer 2005, NASA policy is to decommission the TRMM mission in the later part of 2005. 32)
• Although the spacecraft could be operated for several more years, NASA informed JAXA in early 2004 that it intends to decommission and to de-orbit TRMM. However, this intended policy action (due to budgetary constraints) caused a heated debate by the data user community - a number of weather forecasting agencies are using TRMM data to improve hurricane and typhoon tracking. As of July 2004, NASA and JAXA were discussing the future of the joint mission. The outcome is a mutual agreement to deorbit TRMM within the next year. The science operations on TRMM are due to retire by the end of 2004.
• Once operations of TRMM cease, NASA plans to use the naturally decaying orbit (without any orbit-raising maneuvers) for about a year in preparation for a thruster firing, designed to drop the spacecraft eventually into the Pacific Ocean (this maneuver requires fuel). 33) 34)
The follow-on mission to TRMM is GPM (Global Precipitation Mission), a joint project of NASA and JAXA, with a planned launch for Dec. 2010. However, as of Nov. 2005 this date is in jeopardy because GPM is still in its definition phase at NASA and at JAXA.
• The CERES instrument suffered a voltage converter anomaly in August 1998; hence, it acquired only 9 months of useful science data.
• Data from all the instruments first became available approximately 30 days after the launch. Since then, much progress has been made in the calibration of the sensors, the improvement of the rainfall algorithms, and applications of these results to areas such as data assimilation and model initialization (Ref. 8).
Figure 12: Alternate view of the TRMM spacecraft and its payload (image credit: UCB)
Sensor complement: (PR, VIRS, TMI, CERES, LIS)
The first three instruments (PR, VIRS and TMI) are the primary sensors on TRMM forming what is called the 'rain package'. 35)
Table 4: Characteristics of primary TRMM instruments
Table 5: Overview of TRMM sensor complement and objectives 36)
Figure 13: Schematic view of rainfall measurement with the TRMM sensor complement (image credit: JAXA)
Figure 14: Rainfall measurement with the TRMM sensor complement after orbit boost (image credit: NASA)
PR (Precipitation Radar):
PR is a JAXA instrument in cooperation with NICT (National Institute of Information and Communications Technology) of Tokyo, formerly CRL (Communications Research Laboratory), Japan. Active phased array microwave radar operating with horizontal polarization (orbit permits monthly sampling over the complete diurnal cycle), minimum measurable rain rate of 0.5 mm/h. Objective: 3-D rainfall distribution over land and oceans (combined with TMI sensor). Periodically the PR performs an external calibration with ARC (Active Radar Calibrator) and an internal loop calibration to measure the transfer function of the PR receiver. 37) 38) 39) 40) 41)
The radar echo of the instrument consists of the following three components: rain echo, surface echo, and mirror image echo. The surface echo is measured for estimating the total path attenuation and for providing the range of the surface along the radar beam. The mirror image, which is the rain echo received through the double reflection at the surface, may be useful to estimate rain rate retrieval; it is measured at nadir incidence.
Table 6: PR radar echo collection parameters
Figure 15: Functional block diagram of the PR instrument (image credit: JAXA)
Figure 16: Illustration of the PR instrument (image credit: JAXA)
The PR is a 128-element active phased array system. The transmitter/receiver consists of 128 Solid-State Power Amplifiers (SSPA) LNAs and PIN-diode phase shifters. Each transmitter/receiver element is connected to a 2 m slotted waveguide (WG) antenna (2 m x 2 m planar array). The PR uses a frequency-agility technique to obtain 64 independent samples (NS) with a single PRF of 2776 Hz, in which a pair of 1.6 µs pulses - having two different frequencies at 6 MHz apart from each other - are transmitted. There are 49 angle bins with the angle-bin interval of 0.71º over the swath width; 32 pairs of pulses are transmitted for each angle bin. 42)
Table 7: Instrument parameters of PR
Table 8: Summary of PR operational modes
VIRS (Visible Infrared Scanner):
VIRS is a NASA/GSFC instrument, built by Hughes SBRC, a passive cross-track scanning radiometer which measures scene radiance in five spectral bands: 0.63 µm (±0.05), 1.6 µm (±0.03), 3.75 µm (±0.05), 10.8 µm (±0.05), and 12 µm (±0.05). The horizontal resolution is 2 km at nadir. The telescope is a two-mirror Cassegrain-type focusing the image onto five collocated detectors, each with its own bandpass interference filter. Spectral separation: discrete filters mounted on a cooled focal plane. Swath width = 720 km (FOV=±45º).
Applications: Data will be used in conjunction with data from CERES to determine cloud radiation. VIRS will enable “calibration” of precipitation indexes derived from data of other sources (rain estimation from brightness temperature). Data rate = 50 kbit/s (day) = 28.8 kbit/s (night), instrument mass = 34.5 kg, power = 40 W. In-flight radiometric calibration is provided by an on-board blackbody, a solar diffuser, and a space view for a zero radiance calibration reference. Radiometric accuracy of at least 5% in the thermal bands and 10% in the visible region. 43) 44) 45) 46)
Figure 17: Detailed view of Visible Infrared Scanner (image credit: NASA)
Figure 18: Schematic view of the VIRS instrument (image credit: NASA)
Figure 19: Illustration of the VIRS instrument (image credit: NASA)
TMI (TRMM Microwave Imager):
TMI is a NASA instrument built by BSS (Boeing Satellite Systems). TMI is a passive multichannel/dual-polarized microwave radiometer (heritage of SSM/I on DMSP series) with frequencies in five discrete channels at 10.7, 19.4, 21.3, 37.0, and 85.5 GHz. Resolution (IFOV): 7 km x 5 km to 63 km x 37 km depending on the frequency used. TMI is a conically scanning radiometer, maintaining a nearly constant Earth incidence angle throughout the scan. Swath width = 780 km. 47)
TMI detects microwave energy in the form of of brightness temperatures from Earth's surface and atmosphere. Applications: The 9 channels on TMI allow for simultaneous retrieval of SST (Sea Surface Temperature), wind speed, columnar water vapor, cloud liquid water, and rain rate. TMI is crucial for determining the diurnal dependence of the bulk-skin SST difference. The inclusion of the new 10.7 GHz channel on TMI provides the additional capability to accurately measure SST through clouds. Data is related to rainfall rates over oceans (vertically integrated rainfall distribution). Data rate = 8.8 kbit/s, instrument mass =65 kg, power = 50W. 48)
Table 9: Performance parameters of TMI
TMI consists of nine separate total-power radiometers, each simultaneously measuring the microwave emission coming from the Earth's surface with the intervening atmosphere. The channels are defined in Table 9. TMI employs an offset parabolic reflector (antenna aperture size of 61 cm) to collect the microwave radiation. The reflector focuses the radiation into two feedhorns (at 10.7 the other for the 19-85 GHz). The reflector and feedhorns spin as a unit about an axis parallel to the S/X nadir direction. The rotation period is 1.9 s. A cold-space mirror and a warm reference load are attached to the spin axis and do not rotate. The rotating feedhorns observe the fixed cold mirror and warm load once each scan for calibration purposes. 49) 50)
Earth observations are taken during a 130º segment of the rotation. The 130º arc is centered on the S/C sub-track and maps out a 760 km swath on the Earth's surface. During each scan, the 10.7-37 GHz observations are sampled 104 times over the 130º arc. The 85 GHz observations are at a higher spatial resolution and are sampled at 208 observations/scan.
The antenna beam views the earth surface with a ‘‘nadir’’ angle of 49º, which results in an incident angle of 52.8º at the Earth’s surface. The TMI antenna rotates about a nadir axis at a constant speed of 31.6 rpm. The rotation draws a ‘‘circle’’ on the earth’s surface. Only 130º of the forward sector of the complete circle is used for taking data. The rest is used for calibrations and other instrument housekeeping purposes. From the TRMM orbit, the 130º scanned sector yields a swath width of 758.5 km.
During each complete revolution (i.e., a scan period of about 1.9 s), the subsatellite point advances a distance d of 13.9 km. Since the smallest footprint (85.5 GHz channels) size is only 6.9 km (down-track direction) by 4.6 km (cross-track direction), there is a ‘‘gap’’ of 7.0 km between successive scans. However, this is the only frequency where there is a small gap. For all higher-frequency channels, footprints from successive scans overlap the previous scans. The ‘‘footprint’’ size here is the EFOV (Effective Field of View).
Figure 20: The footprint sizes of the various TMI channels (image credit: NASA)
Figure 21: Line drawing of the TMI instrument (image credit: NASA)
CERES (Clouds and the Earth's Radiant Energy System):
CERES is an EOS program NASA funded instrument provided by NASA/LaRC [instrument heritage of ERBE (Earth Radiation Budget Experiment)]. The objective of CERES is to study the energy exchanged between the sun, the Earth's atmosphere, surface and clouds, and space. CERES measures the energy at the top of the atmosphere, it also permits energy level estimates within the atmosphere and at the Earth's surface. Using information from very high resolution cloud imaging instruments on the same spacecraft, CERES also determines cloud properties, including cloud-amount, altitude, thickness, and the size of the cloud particles. All of these measurements are critical for advancing our understanding of the Earth's total climate system and further improving climate prediction models.
CERES provides geolocated broadband shortwave and total (shortwave and longwave) filtered radiances as well as narrowband filtered radiances in the 8 to 12 µm water vapor window region. The CERES detectors (thermistor bolometers) sense radiances in the broadband shortwave and total spectral regions. Daytime longwave radiances are derived from the differences of the total and shortwave bolometer measurements while the nighttime longwave radiances are derived from the total bolometer measurements, with the nighttime shortwave radiances equal to zero.
Table 10: CERES instrument accuracy requirements (1σ)
CERES on TRMM has a 46-day repeat cycle (precessing orbit of 350 km altitude and an inclination of 35º), cycle, so that a full range of solar zenith angles over a region are acquired every 46 days. On TRMM, CERES has a spatial resolution of approximately 10 km (equivalent diameter) and operates in three scan modes: cross-track, along-track, and rotating azimuth plane (RAP) mode. In RAP mode, the instrument scans in elevation as it rotates in azimuth, thus acquiring radiance measurements from a wide range of viewing configurations. The CERES instrument on TRMM was shown to provide an unprecedented level of calibration stability (0.25%) between in-orbit and ground calibration]. Unfortunately, the CERES/TRMM instrument suffered a voltage converter anomaly and only acquired 9 months of science data. 51) 52)
CERES calibrations. The instrument can perform flight calibrations while operating in the fixed azimuth (cross track) or rotating azimuth plane scan mode. During flight calibrations, the internal calibration sources are cycled on and off via a programmed sequence of commands while the instrument continues to perform a normal Earth scan profile. Earth measurement data taken during internal calibrations are also included in the archival science data.
Figure 22: Illustration of the CERES instrument (image credit: NASA)
LIS (Lightning Imaging Sensor):
LIS is a NASA/MSFC instrument, PI: H. J. Christian. LIS is an EOS-funded instrument. Objective: Measurement of lightning distribution and variability over the Earth, its correlation with rainfall, and its relationship with the global electric circuit. Measurement approach: LIS is an optical staring telescope/filter imaging system that detects the rate, position, and radiant energy of lightning flashes. LIS detects intracloud and cloud-to-ground lighting with storm-scale resolution. Applications: study of mesoscale phenomena such as storm convection, dynamics, and microphysics. These will be related to global rates, amounts, and distributions of convective precipitation, as well as to the release and transport of latent heat, which are all influenced by global-scale processes. Further applications are: cloud characterization, hydrologic cycle studies, storm convection, microphysics and dynamics, seasonal and interannual variability of thunderstorms. 53) 54) 55) 56) 57) 58)
The LIS instrument consists of two main elements: a) a telescope with a CCD detector matrix, and b) the real-time data processing unit. LIS uses an expanded optics wide-FOV lens, combined with a narrow-band interference filter that focuses the image on a small, high-speed CCD focal plane.
- special filter to image at 777.4 nm (OI line) onto a 128 x 128 high-speed CCD array detector
- event processor to subtract out the bright background during daylight (sensor records data during day and night)
- location coverage of lightning flashes within 5 km over a FOV of 600 km x 600 km
- spatial resolution: 5 - 10 km
- temporal resolution: 2 ms
Figure 23: Illustration of the LIS instrument (image credit: NASA)
Primary products of TRMM mission data:
• Average monthly rainfall over the tropics and subtropics for at least 3 years
• Cloud cover (VIRS, CERES)
• Rain rates (TMI)
• Rain rate vertical profile (PR)
• Path-averaged rain rate and liquid water content (PR)
• Lightning distribution and variability (LIS)
Data Validation Program:
• Rain rate spatial distribution (surface radars)
• Rain rate point measurements (in situ measurements)
Table 11: TRMM scientific accuracy requirements
Table 12: Uses for the data collected by the TRMM instruments
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.