Sensors of Polar-Orbiting Satellites

Sensors of L-Band and S-Band Satellites:  
NOAA  |  DMSP  |  OrbView-2 (SeaWiFS)  |  FY-1C

Sensors of the NOAA Satellites

The NOAA polar-orbiting satellites carry a suite of instruments capable of remotely sensing conditions of the Earth's atmosphere, its surface, and cloud cover:

The AVHRR

The primary sensor of the NOAA polar-orbiting satellites is an imager known as the Advanced Very High Resolution Radiometer (AVHRR). This instrument makes calibrated measurements of visible, near-infrared, and infrared radiation from the Earth and its atmosphere.

The AVHRR/2 of NOAA-12 and NOAA-14 has five channels: avhrr_ch1, avhrr_ch2, avhrr_ch3, avhrr_ch4, and avhrr_ch5. Each channel records radiation over a narrow band of wavelengths (other wavelengths are filtered out).

The AVHRR/3 of NOAA-15, NOAA-16, and NOAA-17 has six channels: avhrr_ch1, avhrr_ch2, avhrr_ch4, and avhrr_ch5 are the same as for AVHRR/2., but channel 3 has been split into 3A and 3B. Channel 3B is the same as the original channel 3 and operates during the nighttime portion of the orbit. avhrr_ch3A has a different wavelength range and operates during the daylight portion of the orbit. Note: For consistency with data processed from NOAA-12 and NOAA-14, channel 3B is called avhrr_ch3.

The spectral bands of each of the AVHRR channels are shown in the following table: 

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The TOVS Instruments

The NOAA satellites also carry the TIROS Operational Vertical Sounder (TOVS) package. On the NOAA-12 and NOAA-14 satellites, this includes the MSU (Microwave Sounder Unit) and the HIRS (High Resolution Infrared Sounder), instruments that make measurements at stepped levels of the atmosphere, providing a vertical profile of atmospheric conditions.

The MSU measures temperature only but can see through clouds. Its primary purpose is to measure temperature on cloudy days. The HIRS measures both temperature and humidity but can’t see through clouds. The following table lists the parameters measured at different levels of the atmosphere by the TOVS instruments.

Level (mb)	Temp	Dew Pt	GP Height	Wind Speed	Wind Direction	Pressure
Surface	°K	°K	-	-	-	mb
1000	-	-	meters	-	-	-
850	°K	°K	meters	m/sec	°	-
700	°K	°K	meters	m/sec	°	-
500	°K	°K	meters	m/sec	°	-
400	°K	°K	meters	m/sec	°	-
300	°K	°K	meters	m/sec	°	-
250	°K	-	meters	m/sec	°	-
200	°K	-	meters	m/sec	°	-
150	°K	-	meters	m/sec	°	-
100	°K	-	meters	m/sec	°	-
70	°K	-	meters	-	-	-
50	°K	-	meters	-	-	-
30	°K	-	meters	-	-	-
20	°K	-	meters	-	-	-
10	°K	-	meters	-	-	-

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The ATOVS Instruments--AMSU-A and AMSU-B

On NOAA-15, -16, and -17, the AMSU-A and AMSU-B (Advanced Microwave Sounder Units) have replaced the MSU.

AMSU-A is a 15-channel microwave radiometer made up of two separate units: AMSU-A1 and AMSU-A2. Together, they provide improved temperature soundings (channels 1, 2, and 15 provide information on precipitation, sea ice, and snow coverage).

AMSU-B is a 5-channel microwave radiometer, with three channels centered at the 183.31 GHz water vapor line, one channel at 89 GHz, and one channel at 150 GHz. When combined with AMSU-A, this instrument permits retrievals of vertical profiles of atmospheric humidity.

The following table lists the parameters measured at different levels of the atmosphere (and the layers between the levels) by AMSU-A and AMSU-B.

NOTE:  These parameters are measured for the layers between the levels listed.

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The DCS

The NOAA polar-orbiting satellites can also receive, process, and re-transmit data from small specialized radio transmitters—automatic data-collection platforms located on land, on ocean buoys, or aboard free-floating balloons. This data is handled by the ARGOS Data Collection System (DCS).

Sensors of the DMSP Satellites

The DMSP satellites carry four meteorological sensors - a visible and infrared imager called the Operational Linescan System (OLS), a microwave imager (SSM/I), and two microwave sounders (SSM/T1 and SSM/T2). Data from all of these instruments is transmitted to Earth via a telemetry known as Real-Time Data (RTD).

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The OLS

The Operational Linescan System (OLS) is the primary sensor on the DMSP satellites. It is a two-channel radiometer with visible (0.4 - 1.1µm) and infrared (10 - 12µm) channels. Each has a nearly constant spatial resolution of 0.55 km across the scan line. This produces about 12 lines of data per second with about 7300 samples per line, per channel. The spectral properties of the OLS channels are given in the table below.

The visible sensor on the OLS has a much broader bandwidth than the AVHRR channels 1 and 2 combined. This greater bandwidth means that more radiation is sensed, giving the instrument greater sensitivity. OLS visible data can also be collected at night using reflected moonlight. City lights are also detectable on OLS images.

Channel	Wavelength	Spectrum	Unit of Measure	Ground Resolution
OLS-vis-fine	0.4 - 1.1 µm	visible	albedo (%)	0.55 km
OLS-ir-fine	10-12 µm	thermal ir	temperature (°)	0.55 km
OLS-vis-smooth	0.4 - 1.1 µm	visible	albedo (%)	2.5 km
OLS-ir-smooth	10-12 µm	thermal ir	temperature (°)	2.5 km

Special Sensors - SSM/I, SSM/T1, SSM/T2

There are three instruments called special sensors that are flown on various DMSP satellites. These are:

SSM/I (Special Sensor Microwave Imager), a conical scanning microwave radiometer with 7 channels—4 vertical and 3 horizontal.

SSM/T1 (Special Sensor Microwave Temperature Sounder), a cross-track scanning microwave radiometer with 7 channels.

SSM/T2 (Special Sensor Microwave Water Vapor Sounder), a cross-track scanning microwave radiometer with 5 channels.

To process the data from the SSM/T2 sensor, you must also have the data from SSM/T1.

Sensor Status

The following table shows the eight most recent DMSP satellites and the status of the sensors they have on board. A bullet (·) indicates the sensor is on board; an × indicates that the sensor is not operational.

Satellite	Launch Date	OLS	SSM/I	SSM/T1	SSM/T2
F-8	June, 1987	·	·	·
F-9	February, 1988	·		·
F-10	December, 1990	·	·	·
F-11	November, 1991	·×	·	· (not stable)	· (ch 5 bad)
F-12	August, 1994	·	·×	·×	·
F-13	March, 1995	·	·	·
F-14	April, 1997	·	·	·×
F-15	December, 1999	·	·	·	·

Click here for specific DMSP sensor details - such as geophysical parameters, 2-D and 3-D data variables, pressure levels/layers.

Note: The f-15 is the last DMSP that will carry SSMI, SSM/T1, and SSM/T2.  Beginning with f-16 (anticipated launch date is May 2001), these three instruments will be replaced with a single instrument - the SSMIS (Special Sensor Microwave Imager Sounder).

The SeaWiFS Sensor of the OrbView-2 Satellite

The SeaWiFS (Sea-viewing Wide Field-of-view Sensor) instrument of OrbView-2 is an imager with eight channels, six channels in the visible range and two in the near-IR range. Spectral properties of these channels are given in the following table:

The SeaWiFS imager provides ocean color images. These images can be used to

• Determine the spatial and temporal distribution of phytoplankton.
• Quantify the ocean’s role in the global carbon cycle.
• Study optical properties in the coastal and open ocean.
• Better understand the processes associated with mixing along the edge of eddies and boundary currents.

The MVISR Sensor of the FY-1C/D Satellite

The MVISR (Multichannel Visible and IR Scan Radiometer) instrument of FY-1C/D is a scanning imaging radiometer similar to AVHRR on the NOAA satellites, but with 10 channels—4 channels in the visible range, 4 in the near-IR range, and 2 in the IR range.

Spectral properties of these channels are given in the following table:

Last Update: $Date: 2003/04/29 20:00:11 $