JoBea Way

Imaging radar for ecosystem studies

Recently a number of satellites have been launched with radar sensors, thus expanding opportunities for global assessment. In this article we focus on the applications of imaging radar, which is a type of sensor that actively generates pulses of microwaves and, in the interval between sending pulses, records the returning signals reflected back to an antenna.

The effect of changing environmental conditions on microwave signatures of forest ecosystems: preliminary results of the March 1988 Alaskan aircraft SAR experiment

Abstract In preparation for the first European Space Agency (ESA) Remote Sensing(ERS-I) mission,a series of multitemporal, multifrequency, multipolarization aircraft synthetic aperture radar (SAR) data sets were acquired over the Bonanza Creek Experimental Forest near Fairbanks, Alaska in March, 1988. P-, L- and C-band data were acquired with the NASA/JPL Airborne SAR on five differentdays over a period of two weeks. The airborne data were augmented with intensiveground calibration data as well as detailed, simultaneous in situ measurements of the geometric, dielectric and moisture properties of the snow and forest canopy. During the time period over which the SAR data were collected, the environmental conditions changed significantly; temperatures ranged from unseasonably warm (I to 9°C) to well below freezing (-8 to - 15°C), and the moisture content of the snow and trees changed from a liquid to a frozenstate. The SAR data clearly indicate the radar return is sensitive to these changing environmental fa...

The evolution of synthetic aperture radar systems and their progression to the EOS SAR

The authors describe the evolution of the spaceborne imaging radar starting with the Seasat SAR, through the SIR-A, SIR-B, and SIR-C/X-SAR missions, to the Earth Observing System (Eos) SAR which is scheduled for launch as part of Eos in the late 1990s. A summary of the planned international missions, which may produce a permanent active microwave capability in space starting as early as 1991, is also presented, along with a description of the airborne systems which will be essential to the algorithm development and long-term calibration of the spaceborne data. A brief summary of the planetary missions utilizing SAR and a comparison of their imaging capabilities with those available on Earth are presented. >

Opportunities for using the EOS Imaging Spectrometers and synthetic aperture radar in ecological models

Several promising approaches to assessing biochemical and architectural properties of landscapes are outlined. Strategies for using new EOS sensors in ecological models are examined. Ways in which ecological and remote sensing models can utilize information provided by the new sensors to characterize ecological properties at coarse scales and to estimate within-ecosystem properties are addressed.

Evaluating the type and state of Alaska taiga forests with imaging radar for use in ecosystem models

Changes in the seasonal CO/sub 2/ flux of the boreal forests may result from increased atmospheric CO/sub 2/ concentrations and associated global warming patterns. To monitor this potential change, a combination of information derived from remote sensing data, including forest type and growing season length, and ecophysiological models which predict the CO/sub 2/ flux and its seasonal amplitude based on meteorological data, are required. The authors address the use of synthetic aperture radar (SAR) to map forest type and monitor canopy and soil freeze/thaw, which define the growing season for conifers, and leaf on/off, which defines the growing season for deciduous species. Aircraft SAR (AIRSAR) data collected in March 1988 during a freeze/thaw event are used to generate species maps and to determine the sensitivity of SAR to canopy freeze/thaw transitions. These data are also used to validate a microwave scattering model which is then used to determine the sensitivity of SAR to leaf on/off transitions and soil freeze/thaw. Finally, a CO/sub 2/ flux algorithm is presented which utilizes SAR data and an ecophysiological model to estimate CO/sub 2/ flux. CO/sub 2/ flux maps are generated, from which areal estimates of CO/sub 2/ flux are derived. >

Winter and spring thaw as observed with imaging radar at BOREAS

Measurements of the length of the growing season in the boreal regions, during which significant carbon exchange due to metabolic activity occurs, may improve current estimates of annual CO2 fluxes at high northern latitudes. For coniferous, evergreen forest species, the summer frost free period bounds the growing season length and period of net carbon uptake. Spring soil thaw bounds the period of soil respiration and decomposition and thus carbon release. The balance of these two exchanges determines whether the boreal region is a net carbon source or sink. Imaging radar data can potentially be used to monitor these periods of soil and canopy thaw due to the sensitivity of radar to surface freeze/thaw state. In considering the use of imaging radar, two issues must be addressed. First, the temporal relationship between the time of freezing and thawing of the forest canopy and soil and the periods of photosynthetic and respiration activity must be ascertained. Second, the sensitivity of imaging radar to freeze/thaw processes in each of the forest components must be assessed. Of particular interest is the extent to which radar is selectively sensitive to tree and soil thawing. In 1994, in situ soil, stem and root temperatures, and stem xylem flux were measured over a complete annual cycle at the Boreal Ecosystem-Atmosphere Study (BOREAS) test sites in Canada. Imaging radar data from the European Space Agency Remote Sensing (ERS-1) satellite were also acquired throughout 1994. The in situ temperature data show clear transitions in soil and stem thawing related to the start of soil respiration and canopy photosynthesis, respectively. The imaging radar data show clear shifts in backscatter related directly to soil thaw, and possibly to canopy thaw, as two independent transitions. These results are compared to seasonal ecosystem model results for carbon exchange.

Using the space-borne NASA scatterometer (NSCAT) to determine the frozen and thawed seasons

We hypothesize that the strong sensitivity of radar backscatter to surface dielectric properties, and hence to the phase (solid or liquid) of any water near the surface, should make space-borne radar observations a powerful tool for large-scale spatial monitoring of the freeze/thaw state of the land surface, and thus ecosystem growing season length.

Radar remote sensing proposed for monitoring freeze‐thaw transitions in boreal regions

New research is finding that satellite-based radar remote sensing techniques are particularly well-suited for quantifying the transition of remote boreal regions from a frozen to a thawed condition. The implications for studying global warming are far reaching. If the timing or areal extent of this freeze/thaw transition were to change significantly, measurable changes in boreal climate, hydrology, and biogeochemistry would result. Abrupt transition from frozen to thawed conditions occurs each year over roughly 50 million km2of the Earths remote terrestrial surface at latitudes above 40°N. Radar remote sensing works well to capture this transition because of the way electromagnetic radiation at radar wavelengths interacts with polar water molecules in solid and liquid states. Also, radar has the substantial advantages at high latitudes of both penetrating through clouds and not requiring solar illumination of the land surface.

Diurnal change in trees as observed by optical and microwave sensors: the EOS synergism study

The EOS Synergism Study examined the temporal variability of the optical reflectance and microwave backscatter due to diurnal change in canopy properties of interest to eco- system modelers. The experiment was designed specifically to address diurnal changes in canopy water status (including water potential and content) that relate to transpiration. Multispectral optical and multifrequency, multipolarization microwave measurements were acquired using boom-truck- based systems over a 2-week period in August 1987. Sensor and canopy properties were collected around the clock. The canopy under study was a walnut orchard in the San Joaquin Valley of California. The results demonstrate large diurnal variations in the di- electric properties of the tree (the stem, or bole, specifically) that in turn produce significant diurnal changes in the micro- wave backscatter. The change in dielectric constant is related to the diural change in water potential, although the specific physical or physiological relationship has not yet been estab- lished. A diurnal change in optical reflectance could be attrib- uted primarily to sun angle-view angle change. Although some changes in canopy spectral properties were also observed, these could not be distinguished from edge effects of the canopy. The results suggest that permanently orbiting spaceborne sensors such as those on EOS should be placed in orbits that are optimized for the individual sensor and need not be tied together by a tight simultaneity requirement on the order of minutes to hours for the purpose of monitoring ecosystem properties. Microwave sensors, although able to image day or night, are sensitive to the diurnal pattern of change in vegeta- tion canopies and, therefore, should be in sun synchronous or- bits with a node crossing time selected to optimize sensitivity to the diurnal patterns (4-6 p.m. for this canopy and season). Furthermore, comparison of the long-term spaceborne syn- thetic aperture radar (SAR) data sets collected with SAR in different equator crossing time orbits must consider potential diurnal variation in the surface properties and the effects on the backscatter. Optical reflectance is affected more by sun-an- glehiew angle variations than a diurnal change in canopy water properties. Therefore, based on the results of this study, opti- cal sensors should be placed in orbits which minimize the influ- ence of clouds on terrestrial data acquisition.

Polarization signatures of frozen and thawed forests of varying environmental state

During the two different overflights of the Bonanza Creek Experimental Forest (near Fairbanks, Alaska) by the NASA/JPL radar polarimeter in March 1988, the environmental conditions over the region changed significantly with temperatures ranging from unseasonably warm (1 to 9/spl deg/C) during one day to well below freezing (-8 to -15/spl deg/C) during the other. The moisture content of the snow and trees changed from a liquid to frozen state causing significant changes in the radiometric and polarimetric responses of the forest to the radar wave. The L-band polarimetric observations are summarized in this paper. Up to a 6 dB change in the backscatter was observed in certain forest stands at L-band. Features extracted from the Stokes matrices of the same stands from the thawed and frozen days suggest the changes in the relative contribution of the different scattering mechanisms to the radar return. Comparison of the polarimetric signatures indicate relatively higher contribution from diffuse scatterers on the thawed day than on the frozen day. The sensitivity of the polarimetric signatures to changing environmental conditions is clearly demonstrated. >