SECTION 5

5.  ENVIRONMENTAL SATELLITES

This section describes the environmental satellites currently aloft, their sensors and their potential use for oceanographic studies. The sensors for some of these satellites are actually out of commission or, in the case of the manned satellites, uninhabitated. Nevertheless, due to the potential interest in the data they have acquired during their activity period, they are included in this section.

The major environmental satellites aloft today are:

1. LANDSAT series - U.S.A.
2. TIROS series - U.S.A.
3. NOAA series - U.S.A.
4. HEAT CAPACITY MAPPING MISSION (HCMM) - U.S.A.
5. NIMBUS series - U.S.A.
6. SEASAT-A - U.S.A. (now defunct)
7. GOES/METEOSAT - U.S.A./E.S.A.
8. SPOT - FRANCE
9. BHASKARA series - INDIA
10. MOS-1 - JAPAN

Environmental remote sensing programs scheduled or proposed include:

5.1  The LANDSAT Series

The LANDSAT series of satellites was formerly known as the Earth Resources Technology Satellites (ERTS). The name LANDSAT is a misnomer since the LANDSAT satellites have significant applicability to ocean and coastal studies. This series can be divided into two generations:

5.1.1 The first generation: LANDSAT-1, 2, 3

LANDSAT-1, 2 and 3 were launched respectively in 1972, 1975 and 1978. LANDSAT-1 was decommissioned in 1978 after a sensor malfunction and LANDSAT-2 and 3 were decommissioned in 1983. These satellites have basically the same orbital parameters and carry the same sensors. Both spacecraft are in near-polar (inclination angle of about 99°) sun- synchronous orbits with a period of 103 minutes. The satellites make 14 revolutions per day with inter-track distances of 2875 km. Each satellite provides coverage of most of the earth every 18 days, i.e. overlays the same track every 18 days (refer to Figure 5. 1a, b and c).

LANDSAT-2 and 3 carry two types of imaging sensors:

i) the MultiSpectral Scanner (MSS);
ii) the Return Beam Vidicon (RBV) camera.

The RBV has been used rarely for environmental studies. It is described briefly in Section 4.1.2.

5.1.1.1  MultiSpectral Scanner (MSS):

The MSS is an airborne or spaceborne line scanning device which produces a specific number of synchronous images each at a different wave band. The individual scene of an MSS image covers approximately 185 × 185 km and overlaps its neighbour by about 10% along the spacecraft ground track. At the ground station the images are usually converted from electronic signals to black and white image positives on 7 mm film by an electron beam recorder. The original images have a scale of approximately 1 : 3,369,000.

Figure 5.1a LANDSAT configuration.

Figure 5.1b Typical daytime LANDSAT orbit paths for a single day. Each day the paths shift westward 160 km at the eqyuator so that every 18 days the paths are repeated. Image are acquired between 9:30 and 10:00 a.m. local sun time, except at high latitudes. Note location and ranges of receiving stations in the United States.

Figure 5.1c LANDSAT orbits over the United States on successive days. Note the 65 km sidelap of successive image swaths at 40°N latitude. The sidelap between adjacent orbits ranges from 14% at the equator to 85% at 80° parallels of latitude.

The LANDSAT-2 MSS operates in four different wave bands. The characteristics of the sensor are given in Table 5.1.

TABLE 5.1

The payload for LANDSAT-3 includes a five band MSS and a two camera RBV. The MSS has four bands identical to those on LANDSAT-2 and a fifth band (thermal IR) which was designed to operate at any time during orbit, including night operations, and at all solar elevation angles but did not become operational. The first four bands are operated only when the solar elevation is greater than 10 degrees.

There are six detectors for each of the four MSS spectral bands and two detectors for the thermal IR band. For this reason, the fifth band has one third the resolution of the other four bands.

5.1.2  The second generation: LANDSAT-4, 5

In the U.S.A., the National Oceanic Atmospheric Administration (NOAA) has been assigned the task of establishing an operational remote sensing system. LANDSAT-4 and 5 are the result of NOAA's mandate. Systems development per se has been the responsibility of NASA.

LANDSAT-4 and 5 were launched respectively in 1982 and 1984. They have an inclination angle of 98.3° and a period of 98.5 minutes. The satellites make 14 to 15 revolutions per day with inter-track distances of 2752 km. They overlay the same track every 16 days.

The major difference between LANDSAT-4 and 5 and the previous LANDSATs is that the RBVs have been removed and have been replaced with a new generation of MSS called the Thematic Mapper (TM). This sensor provides more spectral bands and offers an improved ground resolution.

5.1.2.1  Thematic Mapper (TM):

The TM has 3 visible, 1 near and 2 middle IR bands with a 30 m ground resolution cell size and a thermal IR band with a 120 m ground resolution cell size. The characteristics of the sensor are given in Table 5.2.

TABLE 5.2

LANDSAT TM CHARACTERISTICS

The TM has the capacity to monitor a wide range of spectral bands (blue to infrared) and, hence, has a wide range of applications, e.g.:

1. Coastal water mapping (Band 1);
2. Identification of healthy vegetation by green reflectance (Band 2);
3. Plant differentiation by chlorophyll mapping (Band 3);
4. Water body delineation (Band 4);
5. Snow/Cloud measurement (Band 5);
6. Thermal mapping (Band 6);
7. Hydrothermal mapping (Band 7).

The TM provides an overall resolution of 30 m. This high resolution is achieved by sensitive detectors and by an 8 bit quantization in the analog-to-digital conversion process (256 grey levels). In contrast the MSS has only 6 bit quantization (64 grey levels). This means that TM scenes contain larger numbers of pixels with greater radiometric range. This also results in a very high data bit rate of 84.9 megabytes per second.

5.2  The NOAA Series

With the experience gained from the NIMBUS, TIROS and TOS (TIROS Operational Satellite) satellite series, the operational NOAA satellite series was commenced. The U.S. National Oceanic and Atmospheric Administration (NOAA) funded the series, hence the name designation.

The satellites of the NOAA series are dedicated to meteorological observations and sea surface temperature studies. This series can be divided into two generations:

5.2.1  The first generation: NOAA-2 to 5

NOAA-2 to 5 were launched respectively in 1972, 1973, 1974 and 1976. The NOAA satellites were launched into circular, near-polar (inclination angle of 102°) sun-synchronous orbits, arranged to pass over any local receiving station twice a day. These satellites have a period of 115 minutes and they make 12 to 13 revolutions per day with inter-track distances of 3200 km.

They carry the VHRR (Very High Resolution Radiometer), which has been used extensively for oceanographic studies. This sensor will not be detailed in this manual; it became inactive in 1979 and was subsequently replaced in later NOAA satellites by the Advanced Very High Resolution Radiometer (AVHRR), an improved version of the VHRR.

5.2.2  The second generation: TIROS-N, NOAA-6 to 9

These satellites were launched respectively in 1978, 1979, 1981, 1983 and 1984. They have an inclination angle of 102° as do the first generation of NOAA satellites. In contrast, they have a period of 99 minutes and they make 14 to 15 revolutions per day with inter-track distances of 2760 km.

TIROS-N (Television and Infrared Observational Satellite) ended its mission in 1981. The main purpose of the TIROS series was weather forecasting and cloud cover observation. TIROS-N was used to test instruments for the NOAA satellite series.

The sensor used on the second generation of NOAA satellites for oceanographic studies, mainly sea surface temperature determination, is the AVHRR (Advanced Very High Resolution Radiometer.

NOAA-8 carries another sensor of interest for fisheries, SARSAT (Search and Rescue Satellite-Aid Tracking), which detects the distress signal emitted by vessels in difficulty.

5.2.2.1  Advanced Very High Resolution Radiometer (AVHRR):

The AVHRR is a scanning radiometer of four or five channels (depending on the version) operating in the visible band and the near and thermal infrared bands. The characteristics of the sensor are given in Table 5.3.

TABLE 5.3

NOAA-7 - AVHRR CHARACTERISTICS

5.3  Heat Capacity Mapping Mission (HCMM)

The Heat Capacity Mapping Mission (HCMM) was launched in 1978. It has an orbit designed to permit the measurement of the earth's temperature at 12-hour time intervals, when the temperature variation is at a maximum. This day/night temperature difference can be used to determine thermal inertia. The circular and sun-synchronous orbit has an inclination angle of 98°. The satellite makes 14 to 15 revolutions per day with an inter- track distance of 2712 km. It overlays the same track every 16 days at an altitude of 620 km.

The sensor onboard HCMM is the HCMR (Heat Capacity Mapping Radiometer) which was functional from 1978 to 1980.

5.3.1  Heat Capacity Mapping Radiometer (HCMR)

The HCMR is a two channel scanning radiometer and its characteristics are described in Table 5.4.

TABLE 5.4

HCMM - HCMR CHARACTERISTICS

The two channels provide measurements of reflected solar and emitted thermal energy respectively. In addition to terrestrial studies, the HCMR was designed for research in the following areas:

i) Coastal zone thermal gradient determination. This includes mapping of thermal gradients in coastal zones and diurnal heat exchanges of the oceanic surface layer;

ii) Monitoring of large-scale marine pollution effects, mainly oil;

iii) Study of the life span, formation, transportation rate, decay and aggregation of mesoscale ocean eddies.

Due to calibration problems and the short life of the sensor, its oceanographic applications were not fully explored.

5.4  NIMBUS Series

The NIMBUS satellite program was initiated by NASA in 1960 to study the atmosphere and the earth's surface. The latest in this series of satellites, NIMBUS - 7, was launched in November 1978. This satellite is one of the few dedicated to oceanographic studies. The sun-synchronous near-polar orbit of NIMBUS - 7 has an inclination angle of 99°. The satellite has a period of 104 minutes and makes 13 to 14 revolutions per day with an inter-track distance of 2904 km. It overlays the same track every 6 days and carries nine sensors, two of which are important to oceanography: the Coastal Zone Colour Scanner (CZCS) and the Scanning Multichannel Microwave Radiometer (SMMR).

5.4.1  Coastal Zone Colour Scanner (CZCS)

CZCS is a 6 channel radiometer designed specifically for ocean colour mapping. The characteristics of the NIMBUS-7 CZCS are included in Table 5.5.

TABLE 5.5

NIMBUS-7 CZCS CHARACTERISTICS

Five of these bands can sense solar energy and, therefore, water colour as affected by absorption and scattering due to chlorophyll, sediment and gelbstoffe. Chlorophyll strongly absorbs energy in the wavelength bands centred at 0.44 and 0.52 micrometres (Bands 1 and 2) respectively. The wavelength of 0.50 micrometres (Band 3) is the hinge point, the wavelength of minimum absorption. The ratios of measured energies in these bands have been shown to closely parallel the surface chlorophyll concentrations. The data from the CZCS is processed to produce maps of the above measured materils. The measurement of pigment concentration is made possible by unique characteristics of the CZCS, i.e. the fine spectral resolution centered on specific wavelengths in the blue and green spectral regions and the extreme sensitivity suitable to discriminate the subtle variations of the oceanic signal caused by various water constituents. These characteristics are not shared by other sensors observing visible light, such as the MSS, TM or HRV. The temperature of coastal waters and the open ocean is measured in a spectral band centred at 11.5 micrometres (Band 6).

5.5  SEASAT-A

This is the first of a series of satellites specifically designed for oceanographic research (refer to Figure 5.2). SEASAT-A was launched by NASA in 1978 into a near-polar orbit with an inclination angle of 108°, a period of 100.7 minutes and an altitude of an 800 km. It ceased to function after a few months, apparently because of a power failure.

SEASAT-A carries five high resolution sensors: Scanning Multichannel Microwave Radiometer (SMMR); Radar Altimeter (Alt); Radar Scatterometer (SASS - SEASAT-A Satellite Scatterometer); Synthetic Aperture Radar (SAR); and Visible and Infrared Radiometer (VIRR).

5.5.1  Scanning Multichannel Microwave Radiometer (SMMR)

This type of microwave radiometer also was installed onboard NIMBUS-7. It provides information on numerous parameters, particularly sea surface temperature and sea ice cover. The characteristics of the NIMBUS-7 and SEASAT-A SMMR are included in Table 5.6

Figure 5.2 SEASAT-A configuration.

TABLE 5.6

NIMBUS-7 / SEASAT - SMMR CHARACTERISTICS

5.5.2  Radar Altimeter (Alt)

The Radar Altimeter illuminates a target from 1.6 to 12 km in diameter at the nadir. The main purpose of this sensor is to measure the sea level (ocean geoid) with an accuracy of 0.1 metre and to determine the average wave height with an accuracy of 0.5 m.

The characteristics of the Radar ALtimeter are included in Table 5.7.

TABLE 5.7

SEASAT - RADAR ALTIMETER CHARACTERISTICS

5.5.3  SEASAT-A Satellite Scatterometer (SASS)

The main purpose of this radar sensor is to measure near surface wind speed over the oceans in all weather conditions. The SASS was designed to record wind speed and wind direction with errors not exceeding 10%.

The characteristics of the Radar Scatterometer are included in Table 5.8.

Table 5.8

SEASAT - RADAR SCATTEROMETER CHARACTERISTICS

5.5.4  Synthetic Aperture Radar (SAR)

The Synthetic Aperture Radar of SEASAT-A was the first imaging SAR system used as a scientific sensor from earth orbit. This sensor is used for wave characterization (wave height, wave length, etc.). It provided a resolution of 25 m and covered a 100 km swath on the north side of the satellite ground track. The SAR was operated on a real time basis only, when near a receiving station and at the latters command. This was mainly due to the very high rate (110 megabytes per second) at which the data was recorded and to the problems associated with the storage of such a large volume of data onboard the satellite.

The characteristics of SAR are included in Table 5.9.

TABLE 5.9

SEASAT -SAR CHARACTERISTICS

5.5.5  Visible and Infrared Radiometer (VIRR)

The purpose of this sensor is to provide IR and visible low resolution images of the ocean surface, cloud positions and clear air sea temperature.

5.6  GOES/METEOSAT Series

These high altitude (36,000 km) geostationary satellites are used for global meteorological observations and communications (refer to Figures 5.3 and 5.4 respectively). The aspects of meteorology that are the focus of these satellites include cloud mapping and infrared and visible imaging of the earth's surface to detect large-scale changes in ocean parameters.

Figure 5.3 Positions of the five geo-synchronous meteorological satellites that provide global weather watch capabilities. (After J.A. Richards, 1986)

Figure 5.4 METEOSAT data collecting system.

The countries and agencies which are participating in the WMO (World Meteorological Organization) program are the European Space Agency (ESA), the U.S.A., Japan and the U.S.S.R. Five satellites, GOES West (U.S.A.), GOES-East (U.S.A.), METEOSAT (ESA), INSAT (India) and GMS (Japan), collectively allow the earth to be fully imaged each 30 minutes.

The first satellite of the GOES (Geostationary Operational Environmental Satellite) series was launched in 1974. The latest in the series, GOES-4 (West) and 5 (East), were launched in 1980 and 1981 respectively. GOES-4 and 5 carry a special scanner, a spin scan radiometer, which has six infrared detectors.

The characteristics of the scanning radiometer onboard METEOSAT are described in Table 5.10. Note that the given ground resolution cell size corresponds to the nadir view of the sensor.

TABLE 5.10

METEOSAT - SCANNING RADIOMETER CHARACTERISTICS

5.7  SPOT

This satellite was launched in 1986 and is dedicated to land observation, although it has oceanographic applications as does LANDSAT.

It has a sun-synchronous near-polar orbit with an inclination angle of 99° and a period of 101 minutes. It makes 14 to 15 revolutions per day and overlays the same track every 26 days with an inter-track distance of 2818 km. The altitude of this satellite is 820 to 840 km. The sensor onboard is the push-broom radiometer, HRV (Haute Résolution Visible).

5.7.1 Haute Résolution Visible (HRV) radiometer

The special feature of this scanner is that it has a multispectral (3 bands) ground resolution of 20 metres. The scanners are also pointable by ground control to allow imaging of the same scene from different orbital tracks with an incidence angle from 0° (nadir) to a maximum of 27°. The images are obtained using two identical “push-broom” HRV scanning radiometers which permit stereoscopic imaging (refer to Figure 5.5). The characteristics of the HRV are described in Table 5.11.

Figure 5.5 Attributes of the SPOT system. (After D.J. Carter, 1986)

TABLE 5.11

SPOT - HRV CHARACTERISTICS

5.8  Space Shuttle and Space Stations

The space shuttle is not the first manned satellite to be used as a remote sensing platform. The U.S.A. launched an orbital station, SKYLAB, in 1973 which received astronauts in 1973 and 1974. The remote sensors onboard included two photographic cameras, which acquired 35,000 photographs, and a multispectral (13-channel) radiometer, which recorded 800 km of magnetic tape. Three years later the U.S.S.R. launched the SOYUZ station which carried a photographic camera similar to the main SKYLAB equipment. Numerous sensors have been used in the space shuttle of which three have oceanographic applications:

i) SIR-A (Shuttle Imaging Radar), an improved version of the SEASAT SAR;

ii) SMIRR (Shuttle Multispectral Infrared Radiometer), a 10-channel infrared radiometer with a spectral range of 5 to 25 micrometres;

iii) OCE (Ocean Colour Experiment), an 8-channel scanning radiometer with a spectral range of 0.49 to 0.79 micrometres.

5.9  Bhaskara Series

The Bhaskara-1 and 2 satellites were launched by India in 1979 and 1981 respectively with the assistance of the U.S.S.R. A radiometer operating at frequencies of 19 and 22 GHz is carried onboard both satellites for ocean survey purposes. The Bhaskara satellites are no longer operational.

5.10  MOS-1

MOS-1 is the first Japanese earth observation satellite and was launched successfully in February 1987. Data collected by this satellite will be available in early 1988. This is an experimental satellite designed to collect information on sea colour and sea surface temperature in addition to terrestrial parameters.

The instruments onboard this satellite are: Multispectral Electronic Self Scanning Radiometer (MESSR); Visible and Thermal IR Radiometer (VTIR), to measure sea surface temperature (SST); and Microwave Scanning Radiometer (MSR), to measure atmospheric water vapour. The Japanese also plan to launch MOS-2 and 3 which will carry active microwave sensors.

5.11  Satellites of the Future

Numerous satellites are planned for launching in the late 1980's and in the 1990's. The following will be dedicated to oceanographic studies:

5.11.1  IRS-1

IRS-1 (Indian Remote Sensing Satellite 1) of India is scheduled for launching in the late 1980's from the U.S.S.R. It will carry two push- broom sensors, LISS 1 and 2 (Linear Imaging Sensor System), whose bands and resolution (both radiometric and spatial) are comparable to MSS.

5.11.2  ERS-1

ERS-1 (ESA Remote Sensing Satellite) will be the first ESA (European Space Agency) satellite and is proposed for launching in 1989. The objectives of this satellite may be compared to those of the defunct SEASAT satellite. Its primary task will be the measurement of wind and waves over the ocean but other objectives include coastal ocean monitoring, ice movement studies and global weather studies.

The sensors that it will carry include: Synthetic Aperture Radar (SAR) with 30 metre resolution; Imaging Microwave Radiometer (IMR); a scatterometer for wind direction and velocity; and a radar altimeter for sea state determinati7on (refer to Figure 5.6).

Figure 5.6 ERS-1 configuration.

5.11.3  TOPEX

TOPEX (Topographic Experiment) is a satellite designed exclusively for ocean scientific research. This satellite, designed by the Jet Propulsion Laboratory, Pasadena, U.S.A., the organization which designed and managed SEASAT-A, is proposed for launching in the late 1980's. The main objective of TOPEX is microwave sounding of the atmosphere to establish water vapour corrections.

5.11.4  NROSS

The U.S. Navy Remote Ocean Sensing System (NROSS) is scheduled for a three year mission beginning in 1989. The major sensor onboard will be the NASA scatterometer (NSCAT) designed to measure wind vectors in a 600 km swath on either side of the spacecraft.

5.11.5  OCI

The primary goal of the Ocean Colour Imager (OCI) mission, proposed for launching in 1990 by the U.S.A., is to measure visible and near- infrared ocean radiance to derive chlorophyll concentration as an estimate of ocean productivity and to provide flow visualization for ocean currents. The OCI is a development of the present Coastal Zone Colour Scanner (CZCS) carried onboard NIMBUS-7.

5.11.6  RADARSAT

RADARSAT, a Canadian satellite, is proposed for launching in the early 1990's. Its major sensor will be an advanced Synthetic Aperture Radar (SAR) for the monitoring of ice, the measurement of ocean waves and winds, and the detection of ships, oil slicks, etc.

5.11.7  The Sea Wide Field Sensor (Sea-WIFS) Program (NASA/EOSAT)

The Sea-WIFS program, which has been planned jointly by the National Aeronautics and Space Administration (NASA) and the Earth Observation Satellite Company (EOSAT), is designed to meet the needs of ocean science and industry communities for space-acquired ocean colour and sea surface temperature information in the 1990's. The Sea-WIFS sensor will be flown on the LANDSAT-6 satellite scheduled for launching in early 1991. The Sea-WIFS will build on the experience gained from the Coastal Zone Colour Scanner (CZCS) mission, flown by NASA on the NIMBUS-7 satellite from 1978–1986.

5.11.8  Earth Observing System (EOS)/NASA

Important global-scale research in oceanography, geology, limnology, hydrology, glaciology and terrestrial ecology is just beginning to be addressed with the most advanced kinds of sensors now available on satellites. A new generation of optical sensors, imaging spectrometers, is currently flying on aircraft and is scheduled to fly aboard the polar- orbiting space platform in the mid-1990's as part of the Earth Observing System (EOS). These new generation sensors include:

- Airborne Visible and Infrared Imaging Spectrometer (AVIRIS)
- High-Resolution Imaging Spectrometer (HIRIS)
- Moderate-Resolution Imaging Spectrometer (MODIS)

The EOS concept envisions the synergistic use of a number of remote sensing instruments together with an advanced data management system to provide global data sets to further our understanding of geological and glaciological processes, the hydrologic cycle, oceans and inland waters, and biogeochemical cycles. Only with such an integrated program can a rational sampling strategy be planned to obtain measurements needed to produce and verify models of Earth-System processes.