TOVS/ATOVS
Introduction

The Advanced TIROS Operational Vertical Sounder (ATOVS) system was deployed when NOAA-15 was launched, replacing the aging TOVS suite of instruments that were flown on NOAA-6 through NOAA-14 Polar-orbiting Operational Environmental Satellites. The ATOVS consists of the Advanced Microwave Sounding Unit-A (AMSU-A), the Advanced Microwave Sounding Unit-B (AMSU-B)  and the High Resolution Infrared Radiation Sounder Version 3 (HIRS/3).  These instruments replaced the Stratospheric Sounding Unit (SSU), the Microwave Sounding Unit (MSU) and the HIRS/2 in that same order.  Beginning with NOAA-18 the Microwave Humidity Sounder replaced the AMSU-B and the HIRS/3 was upgraded to HIRS/4.  The European MetOp satellites also carry the same suite of instruments.  Each of these instruments vary in characteristics, resolutions, scan properties, etc., which are described below.

ATOVS

Detailed information on the ATOVS system can be found in the NOAA KLM User's Guide located at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/index.htm

HIRS (ver3 and ver4)

HIRS Instrument Description

The HIRS is a discrete stepping, line-scan instrument designed to measure scene radiance in 20 spectral bands to permit the calculation of the vertical temperature profile from the Earth's surface to about 40 km. Multispectral data from one visible channel (0.69 micrometers), seven shortwave channels (3.7 to 4.6 micrometers) and twelve longwave channels (6.5 to 15 micrometers) are obtained from a single telescope and a rotating filter wheel containing twenty individual filters.

An elliptical scan mirror provides cross-track scanning of 56 increments of 1.8 degrees. The mirror steps rapidly (<35 msec), then holds at each position while the 20 filter segments are sampled. This action takes place each 100 msec. The instantaneous FOV for each channel is approximately 1.4 degrees in the visible and shortwave IR and 1.3 degrees in the longwave IR band which, from an altitude of 833 kilometers, encompasses an area of 20.3 kilometers and 18.9 kilometers in diameter, respectively, at nadir on the Earth.

HIRS data can be ordered from CLASS.  Users would receive entire orbital files regardless if a subset was entered on the Search page. .

Summary of Parameters

Parameter

Value

IR Calibration

Warm target and space background

Channels

20

Cross-track scan angle

+/- 49.5 degrees

Scan time

6.4 seconds

Number of steps

56

Step angle

1.8 degrees

Step time

100 milliseconds

Data precision

13 bits

Time between start of each scan

6.4 seconds

Angular FOV (visible, shortwave IR)

1.40 degrees

Angular FOV (longwave IR)

1.30 degrees

At an altitude of 833 km:

Parameter

Value

Nadir Ground IFOV (visible, shortwave IR)

20.3 km diameter

Nadir Ground IFOV (longwave IR)

18.9 km diameter

Scan-end Ground IFOV (visible, shortwave)

68.3 km cross-track by 34.8 km along-track

Swath width (visible, shortwave IR)

2,248.8 km to far edge of outer FOV

HIRS Applications

In addition to profiles of temperature and moisture, the HIRS data are used for a diverse range of applications including the derivation of surface skin temperature, sea surface temperature, outgoing longwave radiation, cloud fraction, cloud-top height, total integrated column ozone, precipitation estimates, and wind speed and direction.

The HIRS/3 instrument is used as part of the NOAA sounding instrument suite which was changed significantly starting with the NOAA-15 satellite. The four-channel MSU instrument was replaced by the 20-channel AMSU instrument suite, made up of AMSU-A for temperature and moisture and AMSU-B for moisture. In addition, AMSU-A essentially replaced the function of the SSU instrument. ATOVS uses the HIRS and AMSU-A to generate the retrieved profiles. The AVHRR instrument is used for cloud detection along with the HIRS and AMSU-A.

HIRS Data Acquisition and Description

The NOAA ATOVS processing flow begins with sensor data receipt by the Command and Data Acquisition (CDA) stations where the data are re-broadcast via communications satellites to NOAA/NESDIS in Suitland, MD where they are processed. Earth location and calibration data are appended, but not applied, to the data as part of the Level 1b processing. The earth locations are computed for specific pixels using the data time corrected for clock drift, if any. Shortly after arriving at NOAA/NESDIS, the data are ingested into CLASS.

HIRS Spatial Coverage

Global coverage is available for the HIRS data which have an Instantaneous Field of View (IFOV) of approximately 1.4 degrees in the visible and shortwave IR and 1.3 degrees in the longwave IR band. From an altitude of 833 kilometers, these translate into a ground IFOV encompassing an area of 20.3 kilometers and 18.9 kilometers in diameter, respectively, at nadir on the Earth. At scan end, a 68.3 km cross-track by 34.8 km along-track footprint is registered for the visible and shortwave IR channels, again assuming a nominal altitude of 833 km. The HIRS instrument provides a global (pole-to-pole) on-board collection of data from all 20 spectral channels. At the equator, the 99 degree scan equates to a swath width of 2,248.8 km to the far edge of the outer FOV centered on the subsatellite track. As a result of the scan geometry, there is variable underlap between steps and scans.

HIRS Temporal Coverage

The overall coverage of the archived HIRS/3 and HIRS/4 Level 1b data base is shown in the following table. However, associated with equipment malfunctions, there may be short gaps in the time ranges.

 Instrument

Satellite

Start Date (mm/dd/yyyy)

End Date

HIRS/3

NOAA-15

10/26/1998

Present

HIRS/3

NOAA-16

02/26/2001

Present

HIRS/3
NOAA-17 08/24/2002 Present
HIRS/4
NOAA-18
06/05/2005
Present
HIRS/4
NOAA-19
04/14/2009
Present
HIRS/4
MetOp-A
05/21/2007
Present

HIRS Calibration

Information on the calibration of the HIRS may be found at http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c7/sec7-2.htm     

HIRS Comprehensive Information

Additional information on the HIRS sensor can be found in the NOAA KLM User's Guide at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c3/sec3-2.htm

Details of the HIRS data sets can be found in the NOAA KLM User's Guide at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c8/sec831-5.htm

If you need information on the HIRS/2 Instrument, go to HIRS/2.

AMSU-A

AMSU-A Introduction

The AMSU-A is a multi-channel microwave radiometer used for measuring global atmospheric temperature profiles and for gathering information on atmospheric water in all of its forms, save small ice particles which are transparent at microwave frequencies.

AMSU-A is a cross-track, line-scanned instrument designed to measure scene radiances in 15 discrete frequency channels. These measurements permit the calculation of the vertical temperature profile from about 3 millibars (~45 km) to the Earth's surface. At each channel frequency, the antenna beamwidth is a constant 3.3 degrees (at the half power point). Thirty contiguous scene resolution cells are sampled in a stepped-scan fashion every eight seconds with each scan covering 50 degrees on each side of the subsatellite path. The scan pattern and geometric resolution translate to a 50 km diameter cell at nadir and a 2,343 km swath width from the 833 km nominal orbital altitude.

The AMSU-A system is implemented in two separate modules: the AMSU-A1 and AMSU-A2. AMSU-A1 consists of 12 V-band channels (3 through 14) and one W-band channel (15). This module provides a complete and accurate vertical temperature profile of the atmosphere from the Earth's surface to a height of approximately 45 km.

AMSU-A2 contains the two lower frequencies (K-band channel 1 and Ka-band channel 2). This module is used to study atmospheric water in all of its forms with the exception of small ice particles.

Summary of Parameters:

Parameter

Value

Calibration

Internal target and space background

Channels

15

Cross-track scan angle

+/- 48.33 degrees

Scan time

8.0 seconds

Number of steps

30

Step angle

3.33 degrees

Step time

202.5 milliseconds

Data precision

16 bits

Time between start of each scan

8.0 seconds

Angular FOV

3.33 degrees

At an altitude of 833 km:

Parameter

Value

Ground IFOV at Nadir

48.05 km diameter

Ground IFOV at Center of Outer FOV

149.1 km cross-track by 79.4 km along-track

Swath width

2,226.8 km to far edge of outer FOV

AMSU-A Applications

The AMSU-A instrument is part of the NOAA sounding instrument suite which was changed significantly starting with the NOAA-15 satellite. The four-channel MSU instrument was replaced by the 20-channel AMSU instrument suite, made up of AMSU-A for temperature and moisture and AMSU-B for moisture. In addition, AMSU-A essentially replaced the function of the SSU instrument. ATOVS uses the HIRS/3/4 and AMSU-A to generate the retrieved profiles. The AVHRR/3 instrument is used for cloud detection along with the HIRS/3/4 and AMSU-A. Primarily because of resource limitations, the AMSU-B instrument is currently not part of the ATOVS system. A slightly modified version of the software developed and used to process the SSM/T-2 on the DMSP satellites is used to process the AMSU-B. The AMSU-B system became operational about one year after ATOVS because of satellite antenna interference in the AMSU-B data.

Specifically, the AMSU-A is used for measuring global atmospheric temperature profiles and for gathering information on atmospheric water in all of its forms, save small ice particles which are transparent at microwave frequencies.

AMSU-A Data Acquisition and Description

The NOAA ATOVS processing flow begins with sensor data receipt by the Command and Data Acquisition (CDA) stations where the data are re-broadcast via communications satellites to NOAA/NESDIS in Suitland, MD where they are processed. Earth location and calibration data are appended, but not applied, to the data as part of the Level 1b processing. The earth locations are computed for specific pixels using the data time corrected for clock drift, if any. Shortly after arriving at NOAA/NESDIS, the data are made available through the CLASS system.

AMSU-A Spatial Coverage

Global coverage is available for the AMSU-A data which have an Instantaneous Field of View (IFOV) of approximately 3.3 degrees. From an altitude of 833 kilometers, this translates into a ground IFOV encompassing an area of 48.05 kilometers in diameter at nadir on the Earth. At scan end, a 149.1 km cross-track by 79.4 km along-track footprint is registered, again assuming a nominal altitude of 833 km. At the equator, the 96.66 degree scan equates to a swath width of 2,226.8 km to the far edge of the outer FOV centered on the subsatellite track.

AMSU-A Temporal Coverage

The overall coverage of the archived AMSU-A Level 1b data base is shown in the following table. However, associated with equipment malfunctions, there may be short gaps in the time ranges.

Satellite

Start Date (mm/dd/yyyy)

End Date

NOAA-15

10/26/1998

Present

NOAA-16

01/24/2001

Present

NOAA-17 08/24/2002 Present
NOAA-18
05/24/2005
Present
NOAA-19
04/14/2009
Present
MetOp-A
05/21/2007
Present

AMSU-A Calibration

The instrument is automatically calibrated each data cycle by measuring both warm and cold calibration targets. Once every 8 seconds, the AMSU-A measures 30 Earth views, the space view twice and the internal blackbody target twice.

Additional information on the calibration of the AMSU-A may be found at:  http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c7/sec7-3.htm

AMSU-A Comprehensive Information

Additional information on the AMSU-A sensor can be found in the NOAA KLM User's Guide at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c3/sec3-3.htm

Details of the AMSU-A data sets can be found in the NOAA KLM User's Guide at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c7/sec7-3.htm

AMSU-B

AMSU-B Introduction

The Advanced Microwave Sounding Unit-B (AMSU-B) is a 5 channel microwave radiometer. The purpose of the instrument is to receive and measure radiation from a number of different layers of the atmosphere in order to obtain global data on humidity profiles. It works in conjunction with the AMSU-A instruments to provide a 20 channel microwave radiometer. AMSU-B covers channels 16 through 20. Channels 18, 19 and 20 span the strongly opaque water vapor absorption line at 183 GHz to provide data on the atmosphere's humidity level. Channels 16 and 17, at 89 GHz and 150 GHz, respectively, penetrate through the atmosphere to the Earth's surface.  Beginning with NOAA-18 AMSU-B was replaced by the Microwave Humidity Sounder.  See next section for details.

AMSU-B is a cross-track, line scanned instrument designed to measure scene radiances in 5 channels. At each channel frequency, the antenna beamwidth is a constant 1.1 degrees (at the half power point). Ninety contiguous scene resolution cells are sampled in a continuous fashion. Each scan covers roughly 50 degrees on each side of the subsatellite path. The scan pattern and geometric resolution translate to a 16.0 km diameter cell at nadir at a nominal altitude of 833 km.

Summary of Parameters:

Parameter

Value

Calibration

Internal target and space background

Channels

5

Cross-track scan angle

+/- 48.95 degrees

Scan time

2.67 seconds

Number of steps

90

Step angle

1.1 degrees

Step time

19 milliseconds

Data precision

16 bits

Time between start of each scan

2.67 seconds

Angular FOV

1.1 degrees

At an altitude of 833 km:

Parameter

Value

Ground IFOV at Nadir

16.0 km diameter

Ground IFOV at Center of Outer FOV

51.6 km cross-track by 26.9 km along-track

Swath width

2,178.8 km to far edge of outer FOV

AMSU-B Applications

The AMSU-B instrument is part of the NOAA sounding instrument suite which was changed significantly starting with the NOAA-15 satellite. The four-channel MSU instrument was replaced by the 20-channel AMSU instrument suite, made up of AMSU-A for temperature and moisture and AMSU-B for moisture. In addition, AMSU-A essentially replaced the function of the SSU instrument. ATOVS uses the HIRS/3 and AMSU-A to generate the retrieved profiles. The AVHRR instrument is used for cloud detection along with the HIRS/3 and AMSU-A. Primarily because of resource limitations, the AMSU-B instrument is currently not part of the ATOVS system. A slightly modified version of the software developed and used to process the SSM/T-2 on the DMSP satellites is used to process the AMSU-B. The AMSU-B system became operational about one year after ATOVS because of satellite antenna interference in the AMSU-B data.

The overall purpose of AMSU-B is to receive and measure radiation from a number of different layers within the atmosphere in order to obtain global data on humidity profiles.

AMSU-B Data Acquisition and Description

The NOAA ATOVS processing flow begins with sensor data receipt by the Command and Data Acquisition (CDA) stations where the data are re-broadcast via communications satellites to NOAA/NESDIS in Suitland, MD where they are processed. Earth location and calibration data are appended, but not applied, to the data as part of the Level 1b processing. The earth locations are computed for specific pixels using the data time corrected for clock drift, if any. Shortly after arriving at NOAA/NESDIS, the data are made available at the Comprehensive Large Array-data Stewardship System (CLASS).

AMSU-B Spatial Coverage

Global coverage is available for the AMSU-B data which have an Instantaneous Field of View (IFOV) of approximately 1.1 degrees. From an altitude of 833 kilometers, this translates into a ground IFOV encompassing an area of 16.0 kilometers in diameter at nadir on the Earth. At scan end, a 51.6 km cross-track by 26.9 km along-track footprint is registered, again assuming a nominal altitude of 833 km. At the equator, the 97.9 degree scan equates to a swath width of 2,178.8 km to the far edge of the outer FOV centered on the subsatellite track.

AMSU-B Temporal Coverage

The overall coverage of the archived AMSU-B Level 1b data base is shown in the following table. However, associated with equipment malfunctions, there may be short gaps in the time ranges.

Satellite

Start Date

End Date

NOAA-15

10/26/1998

Present

NOAA-16

01/24/2001

Present

NOAA-17 08/24/2002 Present

AMSU-B Calibration

The instrument is automatically calibrated each data cycle by measuring both warm and cold calibration targets. Once every 8/3 seconds, the AMSU-B measures 90 Earth views, four space views and four internal blackbody target views.

Information on the calibration of the AMSU-B may be found at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c7/sec7-3.htm

AMSU-B Comprehensive Information

Additional information on the AMSU-B sensor can be found in the NOAA KLM User's Guide at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c7/sec7-3.htm

Details of the AMSU-B data sets can be found in the NOAA KLM User's Guide at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c3/sec3-4.htm

MHS

MHS Introduction

The Microwave Humidity Sounder (MHS) is a self-calibrating microwave radiometer, observing the Earth with a field of view of ±50 degrees across nadir, in five frequency channels of the millimeter-wave band (89-190 GHz). MHS, together with the complementary AMSU-A instruments, provides the operational microwave sounding capability for the NOAA-18, -19 and MetOp-A polar-orbiting satellites. 

Channels at 157 GHz and around the 183 GHz water vapor absorption line provide a humidity profile sounding capability, while the 89 GHz channel provides information on surface temperature and emissivity (in conjunction with AMSU-A channels) and detects cloud and precipitation contaminated pixels. The MHS instrument represents an improvement to the AMSU-B radiometer on board previous NOAA KLM series satellites, while providing continuity to its data.

Summary of Parameters:

Parameter

Value

Calibration

Internal blackbody target and space background

Channels

5

Cross-track scan angle

+/- 49.44 degrees

Scan time

2.67 seconds

Number of steps                                                            

90

Step angle

N/A

Step time

N/A

Data precision

16 bits

Time between start of each scan

1.11 seconds

Angular FOV

1.1 degrees


At an altitude of 870 km:

Parameter

Value

Ground IFOV at Nadir

17 km diameter

Ground IFOV at Center of Outer FOV

51.6 km cross-track by 26.9 km along-track

Swath width

2348 km to far edge of outer FOV


MHS Applications

The MHS instrument is part of the NOAA sounding instrument suite replacing AMSU-B beginning with NOAA-18 and MetOp-A.  The sounding data is used extensively in weather prediction. Brightness temperatures are processed as quickly as possible and sent to numerical weather prediction  (NWP) centers around the world. This data helps keep the assessment of the current state of the atmosphere correct, which in turn helps make predictions more accurate. Long-term records are also used in studies of climate. 

MHS Data Acquisition and Description

The NOAA ATOVS processing flow begins with sensor data receipt by the Command and Data Acquisition (CDA) stations where the data are re-broadcast via communications satellites to NOAA/NESDIS in Suitland, MD where they are processed. Earth location and calibration data are appended, but not applied, to the data as part of the Level 1b processing. The earth locations are computed for specific pixels using the data time corrected for clock drift, if any. Shortly after arriving at NOAA/NESDIS, the data are made available at CLASS.

MHS Spatial Coverage

Global coverage is available for the MHS data which have an Instantaneous Field of View (IFOV) of approximately 1.1 degrees. From an altitude of 870 kilometers, this translates into a ground IFOV encompassing an area of 17.0 kilometers in diameter at nadir on the Earth. At scan end, a 51.6 km cross-track by 26.9 km along-track footprint is registered, again assuming a nominal altitude of 870 km. At the equator, the 98.8 degree scan equates to a swath width of 2348 km to the far edge of the outer FOV centered on the subsatellite track.

MHS Temporal Coverage

The overall coverage of the archived MHS Level 1b data base is shown in the following table. However, associated with equipment malfunctions, there may be short gaps in the time ranges.

Satellite

Start Date

End Date

NOAA-18
05/25/2005
Present
NOAA-19
04/14/2009
Present
MetOp-A
05/21/2007
Present

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TOVS (pre-ATOVS)

Detailed information on the TOVS system can be found in the NOAA POD Guide located at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/podug/index.htm

HIRS/2

HIRS/2 Introduction

The High Resolution Infrared Radiation Sounder Version 2 (HIRS/2) is part of the TOVS instrument suite flown onboard the TIROS-N and NOAA 6 through NOAA 14 satellites.

The HIRS/2 is a step-scanned multi-channel spectrometer with 20 channels, 19 of which are in the infrared region of the spectrum with the remaining channel in the visible region. The HIRS/2 Level 1b data include 13 bit precision data values from these 20 spectral channels. The visible data values (Channel 20) may be converted into albedos and the IR data (Channels 1-19) may be converted into brightness temperatures using the calibration information which is appended but not applied to each scan line. Latitudes and longitudes for each of the 56 Earth FOVs in each scan are included as are time tags.

HIRS/2 data from CLASS is provided as whole orbital files, whether or not a subset is desired.

Summary of Parameters:

Parameters

Values

Calibration

Stable blackbodies (2) and space background

Channels

20

Cross-track scan angle

+/- 49.5 degrees

Scan time

6.4 seconds

Number of steps

56

Step angle

1.8 degrees

Step time

100 milliseconds

Data rate

2,880 bits/second

Data precision

13 bits

Time between start of each scan

6.4 seconds

Angular FOV

1.25 degrees

Parameter

Value

Ground IFOV (nadir)

17.7 km diameter

Ground IFOV (end of scan)

59.5 km cross-track by 30.4 km along-track

Distance between IFOV centers

42.0 km along-track

Swath width (to far edge of outer FOV)

2,239.6 km

HIRS/2 Applications

In addition to profiles of temperature and moisture, the HIRS/2 data are used for a diverse range of applications including the derivation of surface skin temperature, sea surface temperature, outgoing longwave radiation, cloud fraction, cloud-top height, total integrated column ozone, precipitation estimates, and wind speed and direction.

TOVS was designed so that the data from the HIRS/2, SSU and MSU instruments could be combined to compute: 1) atmospheric temperature profiles from the surface to 10 millibars (mb), 2) water vapor content at three levels of the atmosphere, and 3) total ozone content. Improved accuracy of retrieval profiles and a better definition of the water vapor profile, even in the presence of clouds, are the objectives of this system.

HIRS/2 Data Acquisition and Description

The NOAA TOVS processing flow begins with sensor data receipt by the Command and Data Acquisition (CDA) stations where the data are re-broadcast via communications satellites to NOAA/NESDIS in Suitland, MD. The ephemeris data (orbital reference information) are funneled through the Advanced Earth Location Data System (AELDS) software. Earth location and calibration data are appended, but not applied, to the data as part of the Level 1b processing. The earth locations are computed for specific pixels using the data time corrected for clock drift, if any. Shortly after arriving at NOAA/NESDIS, the data are made available at the Comprehensive Large Array-data Stewardship System (CLASS).

HIRS/2 Spatial Coverage

Global coverage is available for the HIRS/2 data which have an Instantaneous Field of View (IFOV) with a 17.7 km diameter at nadir and a 59.5 km cross track by 30.4 km along-track footprint at the end of the scan, assuming a nominal altitude of 833 km. The HIRS/2 instrument provides a global (pole-to-pole) on-board collection of data from all 20 spectral channels. The 99 degree scan equates to a swath width of 2239.6 km (at the Equator) centered on the subsatellite track. As a result of the scan geometry, there is variable underlap between steps and scans.

HIRS/2 Temporal Coverage

The overall temporal coverage of the archived HIRS/2 Level 1b data base is shown in the following table. Some gaps could be present due to a number of reasons. Any notable gaps are indicated.

Satellite

Start Date

End Date

TIROS-N

10/21/1978

02/27/1981

NOAA-6

06/30/1979
04/08/1985
10/15/1985

04/17/1983
06/01/1985
11/17/1986

NOAA-7

06/24/1981

02/18/1985

NOAA-8

04/25/1983
07/01/1985

06/20/1984
10/14/1985

NOAA-9

12/13/1984

11/07/1988

NOAA-10

11/25/1986

09/16/1991

NOAA-11

09/24/1988
07/15/1997

04/10/1995
04/26/2000

NOAA-12

05/14/1991

12/14/1998

NOAA-14

12/30/1994

10/10/2006

Each scan of the HIRS/2 views the Earth for a period of 6.4 seconds. The analog data output from the sensor is digitized on-board the satellite at a rate of 2,880 bits per second. At this rate, there are 288 bits per step (step time = 100 milliseconds). Each step corresponds to an angle of scanner rotation of 1.8 degrees and there are 56 steps per scan.

Because the satellite is sun-synchronous, visible data revisit time is daily. Infrared imaging is accomplished twice daily with the second visit occurring during the pass over the dark side of the Earth. Instrument operation is continuous.

HIRS/2 Calibration

The HIRS/2 sensor can be commanded to automatically enter a calibration mode every 256 seconds. When the instrument is in the calibration mode, the mirror (starting from the beginning of a scan line) rapidly slews to a space view and samples all channels for the equivalent time of one complete scan line of 56 scan steps. Next, the mirror is moved to a position where it views a cold calibration target and data are taken for the equivalent of 56 scan steps. The mirror is then stepped to view an internal warm target for another 56 scan steps. Upon completion of the HIRS/2 calibration mode, the mirror continues its motion to the home position where it begins normal Earth scan. The total calibration sequence is equivalent to three scan lines (no Earth location data are obtained during this period).

Additional calibration information may be found at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/podug/html/c4/sec4-5.htm

HIRS/2 Comprehensive Information

Additional information on the HIRS/2 can be found in the NOAA Polar Orbiter Data User's Guide at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/podug/html/c4/sec4-1.htm

MSU

MSU Introduction

The Microwave Sounding Unit (MSU) is part of the TOVS instrument suite flown onboard the TIROS_N and NOAA 6 through NOAA 14 satellites.

The MSU is a passive scanning microwave spectrometer with four channels in the 5.5 micron oxygen region. The MSU consists of two four-inch diameter antennas, each having an angular Instantaneous Field of View (IFOV) of 7.5 degrees. Assuming a nominal altitude of 833 km, ground resolution is 124 km at the subpoint, while the distance between adjacent scan lines is 168.1 km at nadir. Data from the four channels are at 12 bit precision and may be converted into brightness temperatures using the calibration information which is appended but not applied. Latitudes and longitudes for each of the Earth FOVs in each scan are included as are time tags.

MSU data from CLASS is delivered as complete orbital files whether or not a subset was specified on the search page.

Summary of Parameters:

Parameter

Value

Calibration

Hot reference body and space background each scan cycle

Channels

4

Cross-track scan angle

+/- 47.35 degrees

Scan time

25.6 seconds

Number of steps

11

Step angle

9.47 degrees

Step time

1.84 seconds

Data rate

320 bits/second

Data precision

12 bits

Time between start of each scan

25.6 seconds

Angular field of view

7.5 degrees (3 dB)

At an altitude of 833 km:

Parameter

Value

Ground IFOV (nadir)

109.3 km diameter

Ground IFOV (end of scan)

323.1 km cross-track by 178.8 km along-track

Distance between IFOV centers

168.1 km along-track

Swath width

2,348 km

MSU Applications

Because the longer wavelengths of microwaves can penetrate most cloud systems, observations of thermal emission in the microwave region can provide temperatures below clouds. Interpretation of microwave observations is hampered by surface emissivity effects and sensitivity to precipitation. But these mechanisms also open possibilities for sensing precipitation as well as surface properties such as snow, sea ice, and soil moisture.

TOVS was designed so that the data from the HIRS/2, SSU and MSU instruments could be combined to compute: 1) atmospheric temperature profiles from the surface to 10 millibars (mb), 2) water vapor content at three levels of the atmosphere, and 3) total ozone content. Improved accuracy of retrieval profiles and a better definition of the water vapor profile, even in the presence of clouds, are the objectives of this system.

MSU Data Acquisition and Description

The NOAA TOVS processing flow begins with sensor data receipt by the Command and Data Acquisition (CDA) stations where the data are re-broadcast via communications satellites to NOAA/NESDIS in Suitland, MD. The ephemeris data (orbital reference information) are funneled through the Advanced Earth Location Data System (AELDS) software. Earth location and calibration data are appended, but not applied, to the data as part of the Level 1b processing. The earth locations are computed for specific pixels using the data time corrected for clock drift, if any. Shortly after arriving at NOAA/NESDIS, the data are made available at the Comprehensive Large Array-data Stewardship System (CLASS).

MSU Spatial Coverage

The MSU provides a global (pole-to-pole) on-board collection of data from all four spectral channels. The two four-inch diameter MSU antennas scan 47.35 degrees on either side of nadir in 11 steps. Assuming a nominal altitude of 833 km, the 7.5 degree antenna beamwidth results in a ground resolution at the subpoint of 109.3 km, while the Instantaneous Field of View at the end of the scan is 323.1 km cross-track by 178.8 km along-track. The swath width is 2,348 km, while the resolution at the subpoint creates an underlap of 115 km between adjacent scan lines.

MSU Temporal Coverage

The overall coverage of the archived MSU Level 1b data base is shown in the following table. However, associated with equipment malfunctions, there may be short gaps in the time ranges.

Satellite

Start Date (mm/dd/yyyy)

End Date (mm/dd/yyyy)

TIROS-N

10/21/1978

01/30/1980

NOAA-6

06/30/1979
04/08/1985
10/15/1985

03/05/1983
07/01/1985
11/15/1986

NOAA-7

08/24/1981

02/01/1985

NOAA-8

05/03/1983
07/01/1985

06/20/1984
10/14/1985

NOAA-9

02/25/1985

11/07/1988

NOAA-10

11/25/1986

09/16/1991

NOAA-11

11/08/1988
07/15/1997

04/10/1995
02/27/1999

NOAA-12

09/16/1991

12/14/1998

NOAA-14

01/02/1995

10/10/1996

Each scan of the MSU views the Earth for a period of 25.6 seconds. The analog data output from the sensor is digitized on-board the satellite at a rate of 320 bits per second. At this rate, there are 579.2 bits per step (step time = 1.84 seconds). Each step corresponds to an angle of scanner rotation of 9.47 degrees. There are 11 steps per scan.

Because the satellite is sun-synchronous, imaging is accomplished twice daily with the second visit occurring during the pass over the dark side of the Earth. Instrument operation is continuous.

MSU Calibration

Unlike the HIRS/2 and SSU instruments, the MSU has no special calibration sequence that interrupts normal scanning. The calibration data are included in a scan line of data. From the last Earth view position, the reflector rapidly moves four steps to view space, 10 additional steps to view the housing, and then returns to the home position to begin another scan line. Since each scan line requires 25.6 seconds, synchronization of MSU within the other two TOVS instruments occurs every 128 seconds (5 scan lines).

Additional calibration information may be found at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/podug/html/c4/sec4-5.htm

MSU Comprehensive Information

Additional information on the MSU can be found in the NOAA Polar Orbiter Data User's Guide at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/podug/html/c4/sec4-3.htm

SSU

SSU Introduction

The Stratospheric Sounding Unit (SSU) is part of the TOVS instrument suite flown onboard the TIROS-N and NOAA 6 through NOAA 14 satellites.

The SSU is a step-scanned infrared spectrometer employing a selective absorption technique to make measurements at the top of the Earth's atmosphere in three channels in the 15 micron carbon dioxide absorption band. The three SSU channels have the same frequency but different cell pressures. The 10.0 degree angular Instantaneous Field of View (IFOV) gives a resolution of 147.3 km at nadir. The distance between adjacent scan lines is 62.3 km at nadir. Data are from three channels at 12 bit precision and may be converted into brightness temperatures using the calibration information which is appended but not applied. Latitudes and longitudes for each of the eight Earth FOVs in each scan are included as are time tags.

Users wanting SSU data on-line may specify only whole data sets through the Comprehensive Large Array-data Stewardship System since, at this time, there is no select capability available for SSU data.

Summary of Parameters

Parameter

Value

Calibration

Stable blackbody and space

Channels

3

Cross-track scan angle

+/- 40.0 degrees from nadir

Scan time

32.0 seconds

Number of Earth views/line

8

Step angle

10.0 degrees

Step time

4.0 seconds

Data rate

480 bits per second

Data precision

12 bits

Time between start of each scan

32.0 seconds

Angular field of view

10.0 degrees

At an altitude of 833 km:

Parameter

Value

Ground IFOV (nadir)

147.3 km diameter

Ground IFOV (end of scan)

244.0 km cross-track by 186.1 km along-track

Distance between IFOV centers

62.3 km along-track

Swath width

1,474 km

SSU Applications

The primary objective of the SSU instrument is to obtain data from which stratospheric (~25-50 km) temperature profiles can be determined.

TOVS was designed so that the data from the HIRS/2, SSU and MSU instruments could be combined to compute: 1) atmospheric temperature profiles from the surface to 10 millibars (mb), 2) water vapor content at three levels of the atmosphere, and 3) total ozone content. Improved accuracy of retrieval profiles and a better definition of the water vapor profile, even in the presence of clouds, are the objectives of this system.

Satellite observations have been very valuable for establishing the climatology and phenomenology of large scale waves, especially the propagation of Rosby waves in the mid-latitudes. With satellite data, the mean zonal and temporal structures of stratospheric disturbances can be defined and their slow, large-scale variations can be resolved. The general circulation in the stratosphere can be defined largely in terms of radiative forcing and dynamical interaction between large-scale, slowly varying waves and the mean zonal flow.

SSU Data Acquisition and Description

The NOAA TOVS processing flow begins with sensor data receipt by the Command and Data Acquisition (CDA) stations where the data are re-broadcast via communications satellites to NOAA/NESDIS in Suitland, MD. The ephemeris data (orbital reference information) are funneled through the Advanced Earth Location Data System (AELDS) software. Earth location and calibration data are appended, but not applied, to the data as part of the Level 1b processing. The earth locations are computed for specific pixels using the data time corrected for clock drift, if any. Shortly after arriving at NOAA/NESDIS, the data are made available at the Comprehensive Large Array-data Stewardship System (CLASS).

SSU Spatial Coverage

The cross-track scan of the SSU instrument, combined with the satellite's motion in orbit, provides coverage of a major portion of the Earth's surface. The SSU instrument provides a nearly global (pole-to-pole) on-board collection of data from all three spectral channels. The SSU antennas scan 40.0 degrees on either side of nadir in eight steps. Assuming a nominal altitude of 833 km, the angular Field of View (FOV) of 10.0 degrees results in a ground resolution at the subpoint of 147.3 km diameter, while the IFOV at the end of the scan is 244.0 km cross-track by 186.1 km along-track. The swath width is 1,474 km (at the Equator) centered on the subsatellite track. At nadir, 62.3 km separate consecutive scan lines.

SSU Temporal Coverage

The overall coverage of the archived SSU Level 1b data base is shown in the following table. However, associated with equipment malfunctions, there may be short gaps in the time ranges.

Satellite

Start Date

End Date

TIROS-N

10/21/1978

01/30/1980

NOAA-7

08/24/1981

02/01/1985

NOAA-9

02/25/1985

11/07/1988

NOAA-11

11/08/1988

04/10/1995

NOAA-14

01/01/1995

10/10/2006

Each scan of the SSU views the Earth for a period of 32.0 seconds, including time for the mirror retrace. The analog data output from the sensor are digitized on-board the satellite at a rate of 480 bits per second. At this rate, there are 1920 bits per step (step time = 4.0 seconds). Each step corresponds to an angle of scanner rotation of 10.0 degrees and there are eight steps per scan.

Because the satellite is sun-synchronous, imaging is accomplished twice daily with the second visit occurring during the pass over the dark side of the Earth. Instrument operation is continuous.

SSU Calibration

A calibration sequence is initiated every 256 seconds (eight scans) during which the radiometer is, in turn, stepped to view unobstructed space and then to view an internal blackbody of a known temperature. This mode is synchronized with the HIRS/2 instrument.

Additional calibration information may be found at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/podug/html/c4/sec4-5.htm

SSU Comprehensive Information

Additional information on the SSU can be found in the NOAA Polar Orbiter Data User's Guide at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/podug/html/c4/sec4-2.htm