4 -- SOME PRESENT AND FUTURE SATELLITE/SENSOR SYSTEMS FOR MONITORING VEGETATION

Over 100 remote sensing satellites have been launched and a number are planned for launch in the near future. Two satellite programs that have a long record of collecting land cover data are the Landsat and SPOT (Systèmé Probatoire d'Observation de la Terre) series. The NOAA (National Oceanic and Atmospheric Administration) satellite series with the AVHRR (Advanced Very High Resolution Radiometer) onboard have provided long-term data very useful in global vegetation monitoring. These systems along with NASA's planned Earth Observing System (EOS) are discussed below.

4.1 Landsat

The Landsat series of satellites has been providing continued satellite imagery of Earth from July 1972 to the present. Landsat was the first satellite launched with the primary intent of providing long-term collection and continuity of Earth resources imagery. The program was initially managed by the government of the United States, privatized from 1985 through 1991, and returned to federal control in 1992. To date, six satellites have been launched in this series although the sixth never achieved orbit. The seventh Landsat satellite is planned to be launched in early 1999.

All of the Landsat satellites were placed in near polar, Sun synchronous orbits. The first three flew at altitudes of approximately 900 km with repeat coverage over the same area every 18 days; for Landsat-4 and -5 the orbital altitude was lowered to 705 km and the repeat coverage was every 16 days. Landsat-5 was placed 8 days out of phase with Landsat-4 to effectively produce 8-day repeat coverage. Frequent repeat coverage is a useful feature for monitoring vegetation through time. The ground swath width (total area covered by a scan line) acquired by Landsat as it flies is 185 km.

The Landsat satellites have always carried passive sensors capable of recording electromagnetic energy in the visible and infrared wavelengths. Launched in 1972, Landsat-1 was originally called ERTS (Earth Resources Technology Satellite) and it carried two sensors, the Return Beam Vidicon (RBV) and the Multispectral Scanner (MSS). Although the RBV was intended to be the primary sensor, over the lifetime of the satellite the performance and utility of the MSS proved to be superior.

The MSS is a 4-channel scanner able to measure reflected energy in wavelengths ranging from 0.4 through 1.1 micrometers with a spatial resolution of 80 meters. The 4 channels included a green, red and 2 near-infrared bands. The MSS bands have historically been referenced as bands 4-7, since the 3 channels of the RBV were labeled bands 1-3. In 1975, the ERTS program was renamed Landsat, and Landsat-2 was launched. Landsat-1 and -2 were identical missions, carrying duplicate RBV and MSS sensors. Landsat-3 (launched in 1978) contained a modified RBV, and an MSS with an additional thermal infrared channel that unfortunately failed.

On Landsat-4 and -5 (launched in 1982 and 1984 respectively), the MSS and a new sensor, the Thematic Mapper (TM), were flown. The TM scanner has seven spectral bands ranging from about 0.4 - 2.4 µm (see Table 1 for wavelengths, spatial resolutions, and applications). Channels 1-5 and 7 have spatial resolutions of 30 m while channel 6 differs with a spatial resolution of 120 m. Landsat-4 and -5 are still in operation, however Landsat-4 is no longer transmitting TM data. Landsat-7 will have an Enhanced Thematic Mapper (ETM+) sensor, which will include an additional channel with 15-meter spatial resolution. Table 2 lists the launch date, satellite altitude, and sensors of the Landsat satellite series.

Another new satellite will be launched to fly several minutes ahead of Landsat-7. This satellite will carry a new generation remote sensing instrument called the Advanced Land Imager (ALI). ALI image quality will be evaluated using ETM+ imagery.

Table 1: Landsat TM Channels and Related Applications
Channel #	Wavelength	Spatial Resolution	Some Applications
1	0.42-0.52µm	30 meters	Water depth and quality, soil and vegetation differentiation, deciduous and coniferous differentiation
2	0.52-0.60µm	30 meters	Green reflectance by healthy vegetation
3	0.63-0.69µm	30 meters	Chlorophyll absorption for plant species differentiation
4	0.76-0.90µm	30 meters	Biomass surveys, water body delineation
5	1.55-1.75µm	30 meters	Vegetation moisture measurement, snow and cloud differentiation
6	10.4-12.5µm	120 meters	Plant heat stress measurement, thermal mapping
7	2.08-2.35µm	30 meters	Hydrothermal mapping

Table 2: Landsat 1 - 7 Summary
Satellite	Launch Date	Altitude	Sensors
Landsat-1	July 23, 1972	907km	Multispectral Scanner (MSS), Return Beam Vidicon (RBV)
Landsat-2	January 22, 1975	908km	MSS,RBV
Landsat-3	March 5, 1978	915km	MSS,RBV
Landsat-4	July 16, 1982	705km	Thematic Mapper (TM), MSS
Landsat-5	March 1, 1984	705km	TM, MSS
Landsat-6	October 5, 1993	n/a	this satellite never achieved orbit
Landsat-7	1999	705km	Enhanced TM (ETM+)

4.2 SPOT

The SPOT (Systèmé Probatoire d'Observation de la Terre) earth observation satellite program was conceived and designed by the French CNES (Centre National d'Etudes Spatiales), with Belgian and Swedish participation added shortly after its origination. Like the Landsat program, the SPOT series aims to provide long-term data continuity. SPOT-1, -2 and -3 were launched in 1986, 1990 and 1993 respectively; these first three satellites had the same orbits and sensors. They were placed in Sun synchronous, near polar orbits 832 km above Earth. Repeat coverage (passing over the same place) occurs every 26 days.

The sensor payload consisted of two identical HRV (High Resolution Visible) pushbroom imaging systems. Each HRV was designed to operate in either of two modes -- (1) a panchromatic mode with 10-m spatial resolution, or (2) a multispectral mode with 20-m spatial resolution (see Table 3 for the wavelengths detected by each mode).

An interesting feature of the HRV is that it can be pointed to look off to the side (called off-nadir viewing). This provides the capability to acquire imagery of areas not directly below the satellite (effectively increasing the repeat coverage if desired) and to obtain stereoscopic images, useful in terrain mapping and other applications. At a nadir viewing angle, the ground swath width (total ground width acquired by the detector array) for each individual HRV is 60 km. When pointing off-nadir, the swath increases. The two HRVs together can be pointed to acquire adjacent image fields at nadir, with a combined total swath of 117 km and 3 km overlap.

Table 3: SPOT HRV (called HRVIR on SPOT-4) Mode Information
Sensor and Mode	Spatial Resolution	Wavelength Range
HRV Panchromatic	10 meters	0.51-0.73&m; 0.61-0.68&m for SPOT-4
HRV Multispectral	20 meters	0.50-0.59&m, 0.61-0.68&m, 0.79-0.89&m; 1.58-1.75&m added for SPOT-4

 

SPOT-4 was launched March 24, 1998 and is now fully operational. The 2 HRVIR (High Resolution Visible and InfraRed, formerly HRV) sensors will have an additional spectral channel in the middle-infrared (1.58-1.75 µm), useful for improved vegetation monitoring and mineral discrimination. SPOT-4 carries a new instrument with a wide field of view designed specifically for monitoring vegetation. The VMI (Vegetation Monitoring Instrument) has a spatial resolution of 1 km and a swath width of 2000 km, providing global coverage every day. It uses the same spectral bands as the HRVIR with an additional spectral band (0.43-0.47 µm) for oceanographic applications. The SPOT-5 satellite is planned to have a sensor (corresponding to the HRVIR) with improved spatial resolution (5 m) and launch is expected early in 2001.

4.3 NOAA

The NOAA (National Oceanic and Atmospheric Administration) series satellites are meteorological platforms, designed to assist in weather prediction and monitoring. Sensors onboard meteorological satellites generally have very low spatial resolution compared to sensors that specifically study land features, however, their temporal resolution is high and areal coverage large, obtaining global information very frequently.

In the series from NOAA-6 (launched 1979) through NOAA-14 (launched 1994), the orbits were near polar Sun synchronous, with even numbered missions passing southbound over the equator in the early morning and northbound over the equator in early evening. The odd numbered missions have southbound passes over the equator during the middle of the night, and northbound equatorial crossovers in midafternoon.

These satellites carried the AVHRR (Advanced Very High Resolution Radiometer), a wide-angle-view sensor with 5 spectral channels [0.58-0.68 µm, 0.72-1.10 µm, 3.55-3.93 µm, 10.3-11.5 µm, and in NOAA- 7, -9, -11, and -13, 11.5-12.5 µm]. The spatial resolution at nadir is 1.1 km, and NOAA archives AVHRR data in two forms: LAC (Local Area Coverage) data retain full resolution (1 .1 km) and GAC (Global Area Coverage) data are resampled to 4 km spatial resolution.

A new series of NOAA satellites, called POES (Polar-orbiting Operational Environmental Satellite) began with the launch of NOAA-K/NOAA-15 (NOAA satellites in the prelaunch stage carry a letter designation; after launch and transfer of operation from NASA to NOAA, naming converts to a number designation) in May 1998. This satellite series has improved imaging capabilities and will operate over the next 12 years.

Annual sequences of NDVI (Normalized Difference Vegetation Index, see Chapter 4 Section 5.2) over the globe have been calculated using AVHRR spectral channels 1 and 2. The NDVI is useful for monitoring vegetation dynamics in time and space.

4.4 Earth Observing System (EOS)

The Earth Observing System (EOS) serves as the foundation of NASA's Earth Science Enterprise (ESE). EOS involves a number of instruments and platforms, a community of funded scientists, and the infrastructure to consolidate data and information from surface campaigns and remote sensing satellites. The goal of the EOS program is to determine the extent, causes, and regional consequences of global climate change. To this end, seven science priorities have been identified for EOS objectives:

1. Water and Energy Cycles -- Cloud formation, dissipation, and radiative properties, which influence response of the atmosphere to greenhouse forcing; large-scale hydrology and moisture processes, including precipitation and evaporation.
2. Oceans -- Exchange of energy, water, and chemicals between the ocean and atmosphere, and between the upper layers of the ocean and deep ocean (includes sea ice and formation of bottom water).
3. Chemistry of Troposphere and Lower Stratosphere -- Links to the hydrologic cycle and ecosystems, transformations of greenhouse gases in the atmosphere, and interactions inducing climate change.
4. Land Surface Hydrology and Ecosystem Processes -- Improved estimates of runoff over the land surface and into the oceans; sources and sinks of greenhouse gases; exchange of moisture and energy between the land surface and atmosphere.
5. Glaciers and Polar Ice Sheets -- Predictions of sea level and global water balance.
6. Chemistry of the Middle and Upper Stratosphere -- Chemical reactions, solar-atmosphere relations, and sources and sinks of radiatively important gases.
7. Solid Earth -- Volcanoes and their role in climatic change.

To meet these ambitious objectives, a series of satellites are scheduled to be launched (see Table 4) which will collect data from specially designed instruments. Satellite launches will begin in the next several years and continue throughout the next several decades. Specific launch dates are not listed in Table 4 since launch dates are subject to modification. Check the EOS website (URL is given in reference section) for updates.

The EOS Data and Information System (EOSDIS) is a comprehensive data and information system designed to perform a wide variety of functions in support of a heterogeneous national and international user community, committed to make Earth science data easily available to a wide community of users. Functions include user support, data archive management and distribution, information management, product generation, spacecraft command and control, and data capture and telemetry processing.