Engineering topics

Brunfeldt, D. R.

Publications and source records attributed to Brunfeldt, D. R..

8 records

NASA NTRS1986

External calibration of SIR-B Imagery with area-extended and point targets

Data collected by the Shuttle Imaging Radar-B (SIR-B) on two ascending orbits over a site in west-central Illinois were used to establish end-to-end transfer functions for conversion of the digital numbers on the eight-bit image to values of the radar backscattering coefficient, sigma(0). The transfer function for each data take was defined by the SIR response to an array of six calibrated point targets of known radar cross section and to a large number of area-extended targets, also with a known cross section, as measured by externally calibrated, truck-mounted scatterometers. In spite of variance in the SIR transmit power, end-to-end transfer functions generated from either point targets or area-extended targets were found to yield close agreement for a given pass, indicating that targets of known radar cross section can be used to provide transfer functions for orbital SAR. Although the use of area-extended targets yielded the most satisfactory results, the use of point targets is far simpler and may be the preferred alternative for many situations.

Dobson, M. C.↗

NASA NTRS1985

External calibration of SIR-B imagery with area-extended and point targets

Data-takes on two ascending orbits of the Shuttle Imaging Radar-B (SIR-B) over an agricultural test site in west-central Illinois were used to establish end-to-end transfer functions for conversion of the digital numbers on the 8-bit image to values of the radar backscattering coefficient sigma sup 0 (sq m/sq. m) in dB. The transfer function for each data-take was defined by the SIR-B response to an array of six calibrated point targets of known radar cross-section (transponders) and to a large number of area-extended targets also with known radar cross-section as measured by externally calibrated, truck-mounted scatterometers. The radar cross-section of each transponder at the SIR-B center frequency was measured on an antenna range as a function of local angle of incidence. Two truck-mounted scatterometers observed 20 to 80 agricultural fields daily at 1.6 GHz with HH polarization and at azimuth viewing angles and incidence angles equivalent to those of the SIR-B. The form of the transfer function is completely defined by the SIR-B receiver and the incoherent averaging procedure incorporated into production of the standard SIR-B image product.

Dobson, M. C.↗

NASA NTRS1984

Measured microwave emission and scattering in vegetation canopies

Reflecting metal screens were placed beneath vegetation of three types to allow measurement of the brightness temperature of the vegetation cover alone. A two-frequency radiometer (wavelengths: 11.1 and 5.88 cm) was used to measure the emission from the vegetation, and a theoretical model involving a loss term and a scattering albedo was fitted to the measured temperatures. Passive and active microwave measurements were also made on fields immediately adjacent to the 'screened' fields. Emission and backscattering were measured for these fields under two conditions: with full vegetation cover, and with vegetation removed. Using the same theoretical model and the calculated values for absorption and scattering derived from the screened fields, the effect of the vegetation cover on the soil emission was predicted. The predicted full-canopy temperatures were then compared to those measured. The values for loss and scattering in the vegetation canopy were used in another model to predict the effect of vegetation on backscattering from the soil surface. Again, the predicted backscattering from the vegetation canopy was compared with the measured values of the backscattering coefficient.

Brunfeldt, D. R.↗

NASA NTRS1984

The effect of row direction on the microwave emission from vegetation canopies

Experiments were conducted to determine the microwave brighthness temperature of corn and soybean canopies as a function of polarization and azimuth look direction. A strong dependence on row direction and polarization is observed for soybeans. The corn canopy shows a similar dependence, although to a lesser extent. The variation in brightness temperature is shown to be related to the canopy alone, since the underlying soil surface was blocked by reflecting metal screens.

Brunfeldt, D. R.↗

NASA NTRS1983

Measured microwave emission and scattering in vegetation canopies

Radiometric measurements of vegetation have been conducted by placing reflecting metal screens under three types of vegetation, and using a radiometer operating in the 11.1 and 5.88 cm wavelengths. A theoretical model involving a loss term and a scattering albedo value was fitted to the measured temperatures. Canopy absorption and scattering must be determined to assess the vegetation's effect on microwave sensor sensitivity to the dielectric properties of the underlying soil. Using the theoretical model and calculated absorption and scattering values derived from the screened fields, the effect of vegetation cover on soil emission was predicted. A comparison of predicted full canopy temperatures with those measured was then conducted.

Brunfeldt, D. R.↗

NASA NTRS1982

The effects of vegetation cover on the radar and radiometric sensitivity to soil moisture

The measured effects of vegetation canopies on radar and radiometric sensitivity to soil moisture are compared to emission and scattering models. The models are found to predict accurately the measured emission and backscattering for various crop canopies at frequencies between 1.4 and 5.0 GHz, especially at theta equal to or less than 30 deg. Vegetation loss factors, L(theta), increase with frequency and are found to be dependent upon canopy type and water content. In addition, the radiometric power absorption coefficient of a mature corn canopy is 1.75 times that calculated for the radar. Comparison of an L-band radiometer with a C-band radar shows the two systems to be complementary in terms of accurate soil moisture sensing over the extreme range of naturally occurring soil moisture conditions.

Ulaby, F. T.↗

NASA NTRS1982

An active radar calibration target

An active radar calibrator (ARC), consisting of a receive antenna and a transmit antenna with an RF amplifier in between, is proposed as a tool for conducting high-precision calibration measurements of radar systems. The ARC can be designed to have a large radar cross-section with a broad pattern. Its major advantages over passive reflectors are its small physical size and its suitability for calibrating radars operating in a cross-polarized antenna configuration.

Brunfeldt, D. R.↗

NASA NTRS1981

AgRISTARS. Supporting research: MARS x-band scatterometer

The design, construction, and data collection procedures of the mobile agricultural radar sensor (MARS) x band scatterometer are described. This system is an inexpensive, highly mobile, truck mounted FM-CW radar operating at a center frequency of 10.2 GHz. The antennas, which allow for VV and VH polarizations, are configured in a side looking mode that allows for drive by data collection. This configuration shortens fieldwork time considerably while increasing statistical confidence in the data. Both internal calibration, via a delay line, and external calibration with a Luneberg lens are used to calibrate the instrument in terms of sigma(o). The radar scattering cross section per unit area, sigma(o), is found using the radar equation.

Ulaby, F. T.↗