SP-399 SKYLAB EREP Investigations Summary

 

APPENDIX A

EREP Sensor Systems a

ROY L. EASON b

 


a The primary source of information for this appendix was the Skylab EREP investigator's Data Book.
b NASA Lyndon B. Johnson Space Center.

 

[343] The rationale for selecting the Earth Resources Experiment Package (EREP) sensors was based on the desire to explore various portions of the electromagnetic spectrum; on the need for correlating data among the various sensors; on the status of sensor development; and on the adaptability of the sensors, as a package, to the mission requirements. The sensor selection, as first considered in 1969, resulted in the proposal of four sensors.

For investigations in the visible portions of the spectrum, the prime candidate was a camera system. A multiband, high-resolution camera system was proposed to provide data correlation with other remote-sensor systems. The potential of the multi-band camera system was demonstrated during the Apollo 9 mission. The spectral regions and film/filter-combination proposals were based on experience gained during the NASA Earth Resources Aircraft Program, the Apollo Program, and other multispectral photographic studies.

The second sensor proposed was a wide-range imager that would extend observations from the visible through the near-infrared into the far-infrared portion of the spectrum (0.5 to 2.4 µm and 10.5 to 12.5 µm). The shorter wavelengths were proposed to overlap the multiband camera system. Other bands were proposed to extend into the near infrared with the longer wavelengths extending into the thermal infrared. The longer wavelengths were to permit monitoring of nighttime surface emissions.

An infrared spectrometer was proposed to extend measurements from 3.2 to beyond 14 µm. This wavelength range would provide correlation with the wide-range imager at 10.5 to 12.5 µm.

A microwave system that was a combination radar scatterometer and passive microwave radiometer operating at approximately 10 GHz (3 cm) was proposed. The advantages of this system were that it could operate day or night and that it was not generally affected by clouds and weather. The objectives of this system primarily concerned measurements of the winds over the oceans, the capability for snow mapping, and measurement of rainfall. Portions of this type of system had been used in the NASA aircraft program and in the Nimbus satellite program.

Later in the Skylab Program, three additional sensors were proposed for the EREP system.

 

1. The L-band 1.4-GHz passive radiometer for measurement of soil moisture and oceanographic data
 
2. A radar altimeter applicable to both geodesy and oceanography (This system was to be capable of measuring the distance from the spacecraft to the surface of the oceans within an accuracy of I to 2 m.)
 
3. An Earth terrain camera system to provide higher resolution data to serve as a truth source for the other sensors and to assist those investigators interested in mapmaking (This camera system was similar to the high-resolution camera flown on the Apollo 14 mission.)

 

The following five systems were selected for EREP. (See fig. A-l.)

 

1. The Multispectral Photographic Facility (S190) consisting of the Multispectral Photographic Cameras (S19OA) and the Earth Terrain Camera (S19OB)
 
2. The infrared Spectrometer (S191)
 
3. The Multispectral Scanner (S192)

 


[
344]

FIGURE A-1.

FIGURE A-1.-Skylab spacecraft, showing location of EREP sensors (SL3-114-1659). [For a larger picture, click here]

 

[345] 4. The Microwave Radiometer/Scatterometer an Altimeter (S193)
 
5. The L-Band Radiometer (S194)

 

Figure A-2 shows the wavelength coverage of the Earth-viewing EREP Skylab sensors. The EREP ground coverage is shown in figure A-3.

 

Photographic Systems

Cameras provided the primary source of information for most of the Skylab investigators. The carefully designed cameras and films used for the EREP were not fundamentally different from conventional cameras and films.

Images are formed on films by different wavelengths or colors of light. Each film has a different sensitivity to the various wavelengths of light. Figure A-4 shows the wavelength sensitivity of one of the Skylab films (Eastman Kodak (EK) 3414). This film is similar to the commercially available Plus-X film; however, it is capable of achieving higher resolution and is coated on a thinner base. The thin base permits a large volume of film to be packed into a small space.

The Skylab cameras used the wavelength sensitivity of the films to photograph the Earth in well-defined.....

 


FIGURE A-2.

FIGURE A-2.-Wavelength sensitivity of Earth-viewing Skylab sensors.

 


FIGURE A-3.

FIGURE A-3.-The ground area coverage provided by EREP sensors (S-73-005-5). [For a larger picture, click here]

 

....color, or wavelength, regions by placing a filter over the camera lens. The effect of placing a filter that transmits only red light over EK-3414 film is shown in figure A-5. Use of the filter results in the film recording information in the red wavelength region only. Color films do not need such filtration to record an image in spectral bands. An ordinary color film has three light-sensitive layers, each of which is sensitive to a different group of wavelengths. One film layer records only blue light, a second layer only green light, and a third layer only red light (fig. A-6). The information recorded on each of these film layers can be separated by means of several analysis techniques that are described in section 6 of this report.

Whereas standard color film has the normal blue-, green-, and red-sensitive layers (fig. A-7(a)), color-infrared film does not have a blue-sensitive layer but instead has a layer sensitive to infrared wavelengths. Because the human eye is not sensitive to infrared wavelengths, the information on this film layer is made....

 


[
346]

FIGURE A-4.

FIGURE A-4.-Wavelength sensitivity curve for EK-3414 film.

 

.....visible after development by having the infrared layer appear red. The red-sensitive layer is made to appear green, and the green-sensitive layer appears blue. This rather complicated situation in which red is not red and green is not green is depicted schematically in figure A-7(c). The apparent confusion is more than justified, however, by the amount of information obtained by making infrared wavelengths visible in a photograph. Color-infrared film is particularly valuable in vegetation studies. Figure A-7(b) contains a color-film image of the same scene shown in figure A-7(d).

The photographic image is determined not only by the film used but also by the camera. Three camera characteristics are most important: focal length, aperture, and shutter speed. At a fixed distance, the focal length of a camera determines the size of an object relative to the size of a photograph on which it appears. Thus, a telephoto lens with long focal length will make a distant object appear larger on a photograph than it would appear if a short-focal-length lens were used. A long-focal-length lens will also provide better reproduction or resolution than a short-focal-length lens of equal quality. The detail reproduction in a photograph can be limited not only by lens distortions but also by film....

 


FIGURE A-5.-Wavelength sensitivity curve for EK-3414 film showing filtering to transmit only red light.


FIGURE A-6.

FIGURE A-6.-Cross section of a typical color emulsion layer.

 

....grain. A view of exposed film under high magnification will reveal a nonuniform pebbly, or grainy, appearance called graininess. A long-focal-length lens will map a smaller object onto the limiting graininess to achieve better resolution because of the small blur circle. The focal length cannot be increased arbitrarily. It is more difficult and expensive to make a high-quality lens of longer focal length. A long-focal-length lens also covers less area. The field of view (FOV) is decreased with a long-focal-length lens, and a small FOV can be a disadvantage in Earth resources studies.

The aperture and shutter parameters of a camera affect the amount of energy reaching the film during the exposure. The camera aperture (f-stop) represents the ratio of the focal length to the diameter of the lens aperture. The amount of energy reaching the film is proportional to the square of the ratio of aperture diameter to focal length.

Multispectral Photographic Cameras (S190A). -Six high-precision cameras with matched optical systems were mounted and boresighted to form the camera assembly shown in figure A-8. Each camera had an f/2.8 lens with an aperture variable to f/16 in 0.5-stop increments and a focal length of 15.2 cm. At a nominal.....

 


[
347]

FIGURE A-7.

FIGURE A-7.-Color as opposed to color-infrared spectral layer. (a) Sensitivity curve for color film. (b) Color photograph of the Yuma, Arizona, area taken on January 26, 1974 (SL4-92-356). (c) Sensitivity curve for color-infrared film. (d) Color-infrared photograph of the Yuma, Arizona, area taken on January 14, 1974 (SL4-93-057). [For a larger picture, click here]

 


[
348]

FIGURE A-8.

FIGURE A-8.-Multispectral Photographic Cameras (S19OA). (a) Magazines (S-72-44415). (b) Lenses and filters (S-72-44416). [For a larger picture, click here]

 

 

.....spacecraft altitude of 435 km, the 21.2° FOV provided ground coverage of a square area approximately 163 km on each side (1:2 900 000 scale, approximately).

The film width was 70 mm, the shutter speeds were 2.5, 5, and 10 milliseconds, and the six shutter mechanisms were synchronized to within 0.4 millisecond. Programed camera rotation, variable from 10 to 30 mrad/sec, compensated for the forward motion of the spacecraft, and photographs could be taken singly or in automatic series in 2- to 20-second intervals. To provide for stereoscopic viewing, 60-percent overlaps were obtained using 10-second intervals.

Figure A-9 shows six sample images acquired by the S19OA cameras and includes information on film types and spectral ranges. The S19OA data were usually furnished to the Principal investigators in the form of contact positive and negative transparencies (70 mm) and enlarged transparencies (280 mm).

Earth Terrain Camera (S190B).-The Earth terrain single-lens camera assembly (fig. A-10(a)) had an f/4 lens and a focal length of 45.7 cm with a focal-plane shutter. Programed camera rotation, variable from 0 to 25 mrad/sec, compensated for the forward motion of the spacecraft. The 14.24° FOV provided ground coverage of a square area approximately 109 km on each side (1:950 000 scale, approximately).

The film width was 12.7 cm, and the shutter speeds were 1/100,1/140, and 1/200 second. Sequence photography intervals were possible from 0 to 25 frames/min.

To provide for stereoscopic viewing, 60-percent overlaps were obtained using a rate of 9.5 frames/min. Figure A-10(b) is an image of an area taken with the Earth Terrain Camera. Data were usually furnished to the Principal investigators in the form of positive and negative contact transparencies (140 mm) and enlarged transparencies (280 mm). Unless otherwise stated, "color film" should be assumed in S190B discussions throughout this report. The S190B film types used onboard the spacecraft were EK-3414 black-and-white high-definition aerial (0.5 to 0.7 µm), special order SO-242 high-resolution aerial color (0.4 to 0.7 µm), SO-131 high-resolution color-infrared aerial (0.5 to 0.88 µm), and EK-3443 color-infrared (0.5 to 0.88 µm).

 

Station

Wavelength µm

Film

Color

Type

.

1

0.7 to 0.8

Black-and-white (B &W)

EK-2424

2

0.8 to 0.9

B &W infrared

EK-2424

3

0.5 to 0.88

Color infrared

EK-2443

4

0.4 to 0.7

Color

Special order (SO) 356

5

0.6 to 0.7

B &W visible

SO-022

6

0.5 to 0.6

B &W visible

SO-022

 


[
349]

FIGURE A-9.-S190A sample data taken over Las Vegas, Nevada: Lake Mead; the Colorado River; and the Hoover Dam. Data on film parameters for each station are contained in the table on the facing page. [For a larger picture, click here]

 


[
350]

FIGURE A-10.

FIGURE A-10.-Earth Terrain Camera (S19OB). (a) Camera assembly. (b) Sample data taken over northwestern Florida (SL3-88-141). [For a larger picture, click here]

 

 

Spectroradiometric Sensors

Remote sensing depends on receiving energy from an object or a scene, viewing or recording it, and analyzing the received energy to deduce some of the characteristics of the scene. In recording the energy of an Earth scene with a space sensor, the many variables considered can generally be divided into two classes: those variations that affect or influence some characteristics of the scene, such as temperature and moisture, and those variations that, although they influence the radiation received at the sensor, do not represent scene characteristics. Examples are the intervening atmosphere and contamination around the recording instrument. The most influential agent within the wavelengths previously discussed is the atmosphere. The alterations of radiation passing through the atmosphere have serious ramifications for remote sensing. The S191 infrared Spectrometer was designed to investigate correction factors that might be applied. Although called an infrared spectrometer, the sensor operated in both the reflective (0.4 to 2.5 µm) and emissive (6.6 to 16 µm) wavelength intervals.

The basic principle concerned viewing a single homogeneous scene long enough to record the radiance values over the entire wavelength range and thereby obtaining a plot of energy level as a function of wavelength for the target scene. If the energy levels departing from ground level are known (either from direct measurements or by inference) and if they are compared to the levels received at the sensor, the major differences can be attributed to the intervening medium. Even if there are no differences, however, one cannot assume the medium has no effect. For example, at some wavelengths, the medium may completely absorb the emissions of the ground scene and replace the absorbed radiation with its own emissions. The experimenter relies on the spectral details of the comparison to unravel the confounding effects of the atmosphere.

Infrared Spectrometer (S191).-The S191 infrared Spectrometer was unlike the other visible and infrared recording sensors (cameras and multispectral scanner) and also unlike many other infrared systems in that no image was acquired or derived.

The S191 sensor (fig. A-11(a)) was composed of a filter-wheel spectrometer that spectrally scanned the radiation entering its aperture and a tracking telescope alined along the spectrometer line of sight that enabled the crewman to acquire and track the test site and take.....

 


[
351]

FIGURE A-11.

FIGURE A-11.-Infrared Spectrometer (S191). (a) Instrument. (b) Sample data. [For a larger picture, click here]

 

....16-mm photographs of the scene. Incoming radiation, recorded at I spectral scan/sec from a Cassegrainian collecting telescope, was split into short-wavelength (0.4 to 2.5 µm) and long-wavelength (6.6 to 16.0,u m) bands by a dichroic beamsplitter. The detector alternately sensed radiation from the external target and from the internal reference sources.

Visible and near-infrared energy was detected by a silicon and lead sulfide sandwich detector; thermal energy was detected by a mercury-cadmium-tellurium detector that was cooled to 90 K by a miniaturized closed-cycle engine. In-flight calibration spectra recorded before and after each data-gathering pass enabled conversion of the spectral voltage signals to radiance values. The data from this experiment sensor were furnished to the Principal investigators in the form of computer tapes and 16-mm film.

Typical spectral data obtained with the spectrometer are shown in figure A-11(b). The lower spectrum was obtained when the Skylab spacecraft was over White Sands, New Mexico, on a foggy morning. Only the thermal region of the spectrum is shown. For comparison, the line at the top of the figure shows the spectrum measured by a similar spectrometer mounted in a helicopter. The difference between the two is due to energy being absorbed in the atmosphere by carbon dioxide, ozone, and water vapor. The differences at the various wavelengths illustrate atmospheric effects on data recorded above the atmosphere.

Multispectral Scanner (S192).-Another method of forming an image is through a point-by-point reconstruction of an area that has been scanned by an optical mechanical scanner. Each point actually represents the integrated energy from a small area called a picture element (pixel) or resolution cell. The size of the picture element on the ground is governed by the optical design parameters of the sensor and the height of the satellite. The energy from each pixel is collected by an optical assembly and focused onto a detector. The detector converts the energy received at each instant into an analog electrical signal that can be amplified and recorded. The electrical signal varies in direct proportion to the changes in the amount of energy received at the detector and thus carries information about changes in the reflection or emission of radiation from the objects scanned.

The mechanical movement (usually rotation) of a mirror in the optical assembly produces the scan lines perpendicular to the satellite track. As the spacecraft [352] moves, each line is scanned so that successive lines will fall exactly adjacent to each other and a continuous swath of the Earth can be mapped. The scan lines can be curved or straight depending on the method chosen to generate the scan motion. The quality of the data depends on the swath width, defined by the unobstructed angle through which the scanning mirror is designed to rotate, and is limited by changes in the atmospheric path lengths and in the size of the pixel for off-nadir angles and by the available electrical bandwidths for tape recording or telemetry of the image data.

Variations in spacecraft height or velocity or in mirror scan rotation can cause an underlap or overlap of adjacent lines that must be corrected during image reconstruction at a ground facility. If the height increases, as when viewing the Earth obliquely, the width of the area scanned will increase; and, if no change is made in the rotational speed of the mirror or in the forward velocity of the spacecraft, overlapping lines will be sensed. For contiguous imagery, the width of each line on the ground must equal the distance the spacecraft moves while scanning each line.

By using the proper optical design and an array of detectors, a set of coincident spectral bands can be obtained for each line. In storing the signals on a multiband recorder, a multispectral set of data is made available to the analyst for interpretation. One of the advantages of an optical mechanical scanner is that it can collect radiation in spectral regions outside as well as coincident with those viewed by a camera, particularly infrared wavelengths beyond 1 µm. The particular design for an optical mechanical scanner used for the EREP was called the Multispectral Scanner (S192) (fig. A-12).

This optical electromechanical scanner collected incoming radiant energy using a rotating mirror in the image plane to scan the scene conically. A spherical mirror was the major element of a folded reflecting telescope that had a 43.2-cm entrance pupil. The energy scanned in the image plane passed through a reflective Schmidt corrector mirror and through a field stop that was the entrance slit of a prism spectrometer. A dichroic mirror then separated the short wavelengths (0.41 to 2.35 µm) from the long thermal wavelength band (10.2 to 12.5 µm). The spectrally dispersed electromagnetic energy received from the scene simultaneously irradiated 13 detectors. Each detector responded to a specific wavelength band as given in table A-l. The multispectral scanner had 22 scientific data outputs. One scientific data output (SDO) was assigned to each....

 


FIGURE A-12.

FIGURE A-12.-Multispectral Scanner (S192). (a) Cutaway diagram. (b) Scanner Optics. (c) Lens system. [For a larger picture, click here]

 

[353] TABLE A-1.-Detectors and Corresponding Wavelength Bands for the Multispectral Scanner (S192).

 

Detector no.

Band

SDO (or channel)

Color

Wavelength, µm

.

.

1

Violet

0.41 to 0.46

22

2

Violet blue

0.46 to 0.51

18

3

Blue-green

0.52 to 0.56

1, 2

4

Green yellow

0.56 to 0.61

3, 4

5

Orange-red

0.62 to 0.67

5, 6

6

Deep red and infrared

0.68 to 0.76

7, 8

7

Near infrared

0.78 to 0.88

9,10

8

Near infrared

0.98 to 1.08

19

9

Near infrared

1.39 to 1.19

20

10

Near infrared

1.20 to 1.30

17

11

Middle infrared

1.55 to 1.75

11,12

12

Middle infrared

2.10 to 2.35

13,14

13

Thermal infrared

10.20 to 12.50

15, 16, 21

 

....detector sampled at 1240 times/scan (bands 1, 2, 8, 9, and 10). Two SDO's were assigned to the detectors sampled at 2480 times/scan (bands 3, 4, 5, 6, 7, 11, 12, and 13). Band 13 was assigned an additional redundant SDO.

Each detector produced an electronic signal that corresponded to the average value of the radiance received in its spectral band from the area on the Earth's surface in the instantaneous FOV of the instrument. The detector outputs were amplified, converted to digital values, multiplexed, buffered, and recorded on magnetic tape.

The 0.182-mrad FOV measured by each detector provided an instantaneous ground coverage of a square area 79 m on each side. Although the scan assembly rotated a full 360°, only the forward 110° were used to obtain surface data with the calibration data taken on the remainder of the scan. The corresponding sweep angle viewed from the sensor was 10.4°, which provided a groundswath width of 74 km.

Because the original thermal detector (Y-3) had less than specified sensitivity, a more sensitive detector (X-5) was installed in January 1974 during the Skylab 4 mission. Checkout of this instrument was accomplished January 15 to 17, 1974.

An example of the multispectral scanner imagery is shown in figure A-13. The data from this experiment were furnished to the Principal investigators in the form of imagery (from one SDO per detector) and computer tapes.

 

Active and Passive Microwave Sensors

Microwave sensors operate in the millimeter to meter region of the electromagnetic spectrum. Because the longer wavelengths require larger antennas for a given angular resolution, higher resolution microwave space sensors usually operate at the shorter wavelengths, with the exception of special applications.

An active microwave sensor transmits repetitive pulsed bursts of energy that are directed in a given direction by the antenna beam. A discontinuity, such as the atmosphere/lithosphere interface, will reflect or scatter a part of the energy back to the transmitting antenna, where it is accepted by the receiver between the transmitted bursts. After the receiver, which is designed to match the transmitted signal characteristics for optimum detection, converts the radiofrequency energy to video frequencies, signal processing is performed. In remote-sensing applications, in which the effect of either the Earth's surface or the atmosphere on the transmitted radiation is measured, the geometric configuration of the observations will depend on both the target characteristics and the radar system parameters. Generally, when the spatial resolution depends only on the antenna size, the operation is termed beamwidth limiting and is used to measure the amount of backscatter from a given area within the antenna beam (scatterometer). To derive a radar signature for each measurement, the measured backscatter is normalized relative to the beamwidth-limited area. The resultant radar cross section per unit area, or backscatter coefficient, will depend only on the surface characteristics, the incidence angle, and the polarization for which the values were determined.

The backscatter coefficients measured by a scatterometer are estimates of the mean return of a noiselike signal. To reduce fluctuations as well as increase the signal-to-noise ratio, a long pulse is transmitted. For a monostatic radar that uses the same transmitting and receiving antenna, the transmitted pulsewidth cannot exceed the expected round-trip traveltime (2R/c, where R is the range and c is the electromagnetic wave propagation speed), so that the received and transmitted energy will not interfere. The pulsewidth used for the Skylab scatterometer was approximately 4 milliseconds for higher incident angles.

The active sensor can also be used as an altimeter to measure precisely the spacecraft height relative to the subsatellite groundtrack. Narrow transmitted pulses are....

 


[
354]

FIGURE A-13.

FIGURE A-13.-Imagery from the 13 S192 detectors taken over Las Vegas, Nevada; Lake Mead; the Colorado River; and the Hoover Dam. (a) Bands 1 to 8. (b) Bands 9 to 13. [For a larger picture, click here]


[
355]

FIGURE A-13.

FIGURE A-13.- Continued. [For a larger picture, click here]

 

....used for this application, and the arrival time of each pulse relative to the transmitted time is measured. Time precision (1 nanosecond = 15 cm) is easily obtained, and a profile of the subsatellite groundtrack can be obtained by plotting a time history of the altimeter measurements. At nadir, the spatial resolution is pulse-width limited rather than antenna beamwidth limited (fig. A-14), if sufficiently narrow transmitted pulses are used.

The operation of both the scatterometer and the altimeter is based on the reflective properties of a rough surface. However, when smooth areas are encountered, the scatterometer will become inoperative at larger incidence angles, whereas the altimeter, operating at nadir, will be activated by mirrorlike returns that preserve the transmitted pulse shape. In this case, the effective area of reflection is reduced to the first Fresnel zone, which may be only a fraction of the pulsewidth- or beamwidth-limited area (fig A-14). The size of the Fresnel zone depends only on the platform height and the radar wavelength and thus is independent of both the pulsewidth and the antenna size.

 

 


[
356]

FIGURE A-14.

FIGURE A-14.-Antenna beamwidth, pulsewidth, and Fresnel region, where c is electromagnetic wave propagation speed, T is time, h is height, and Greek letter alpha / 4is quarter wavelength. [For a larger picture, click here]

 

 

Passive microwave receivers have many components similar to radars. An antenna is focused on those targets that are emitting the wavelengths of electromagnetic radiation. Generally, the passive receiver bandwidth is much broader than radar bandwidths because the naturally generated radiation is much weaker. Passive microwave receivers therefore require very sophisticated calibration techniques involving internal radiation sources arid a consideration of the internally generated receiver noise.

The measurements made by passive microwave receivers are converted to passive microwave temperatures, which are interpreted in terms of the temperature of the target and its emissivity. These physical properties change for each target.

Microwave Radiometer/Scatterometer and Altimeter (S193).-The active microwave scatterometer, the passive microwave radiometer, and the altimeter shared one antenna (fig. A-15). The radiometer and scatterometer functions of S193 were combined into a single instrument. Features of microwave radiometry were incorporated in the radar scatterometer design to improve the accuracy of the backscatter measurements.

From radiometer measurements, the brightness temperature of the Earth's surface within the 1.6° half-power point of the antenna pattern was determined as a function of incidence angle from 0° (vertical) to 48° with a bandwidth of 0.200 GHz centered at a frequency of 13.9 GHz for two polarizations. The mean value of the Earth's thermal noise signal was determined by sufficiently long integration of the received signal. The....

 


FIGURE A-15.-Microwave Radiometer/Scatterometer and Altimeter (S193). [For a larger picture, click here]


[
357]

FIGURE A-16.

FIGURE A-16.-Radiometer and scatterometer data. (a) Intrack noncontiguous scanning mode with varied pitch angle Greek letter theta ; mathematical symbol is the local vertical vector. (b) Crosstrack noncontiguous scan mode. (c) Intrack contiguous scan mode. (d) Crosstrack contignous scan mode. [For a larger picture, click here]

 

....measured energy, converted to brightness temperature, was compared to the mean noise energy from two known internal temperature sources for calibration to yield an accurate proportional measurement of the microwave emission of the Earth within the antenna half-power points.

The scatterometer measured the radiation backscatter from the Earth at a center frequency of 13.9 GHz as a function of incidence angles from 0° (vertical) to 48° for different polarization combinations and scanning modes as shown in figure A-16. The calculated scattering coefficient was related to the roughness and the dielectric properties of the surface reflections. Several measurements of the scattered return signal (which resembles thermal noise) and receiver noise were taken and integrated to obtain an accurate measurement of average return power, from which the backscattering coefficient was calculated. Concurrent operation of the radiometer and the scatterometer enabled collections of values of the backscattering coefficient and apparent black-body temperatures for each surface area. This method resulted in the ability to study emissivity effects from reflectivity effects in the same area.

An example of radiometer/scatterometer data (in two polarizations) collected from Hurricane Ava off the coast of Mexico is shown in figure A-17. The winds at the closest approach of the S193 sensors to the center of the hurricane had speeds of approximately 90 km/hr, with 10-m wave heights. The changes in the backscatter as this spacecraft passed by the storm were caused by changes in surface roughness, as slightly attenuated by the clouds. As the windspeeds increased near the storm.....

 


[
358]

FIGURE A-17.

FIGURE A-17.-Radiometer and scatterometer data from Hurricane Ava. (V = vertical transmit; H = horizontal transmit; VV = vertical transmit, vertical receive; HH = horizontal transmit, horizontal receive.) [For a larger picture, click here]

 

.....center, the scattered signal intensity increased because of increased surface roughness. The microwave temperature increased because of both thickening clouds and increased roughness. These data were analyzed to determine the feasibility of using the passive microwave data to correct the radar data for attenuation and then determining the windspeed.

The S193 altimeter was designed to operate in a preprogramed sequence of approximately 3 minutes. Each sequence consisted of a data acquisition and system calibration subsequence. To increase the flexibility of the system and to study the various effects of different system parameters, five different combinations of pulsewidth, receiver bandwidth, and off-nadir....

 

[359] TABLE A -II.-Basic Sequence of Altimeter Operation.

 

Transmitter

IF b filter bandwith (receiver), MHz

Antenna pointing angle deg

No of data frames

(c)

Step

Submode (a)

Pulsewidth, nsec

.

Mode 1 (pulse shape)

0

DAS-1

100

10

Subsatellite

50

1

DAS-2

100

100

Subsatellite

61

2

DAS-3

100

100

0.431 pitch

61

3

CDS-1

100

100

NAd

7

4

CDS-2

100

10

NA

5

5

CDS-3

100

10

NA

5

.

Mode 5 (pulse compression)

.

0

DAS-1

100

10

Subsatellite

16

1

DAS-2

130

100

Subsatellite

99

2

DAS-3

20

100

Subsatellite

51

3

CDS-1

20

100

NA

7

4

CDS-2

130

100

NA

7

5

CDS-3

100

10

NA

5

a DAS = data acquisition step: CDS = calibration data step
b intermediate frequency
c One frame corresponds to approximately 1 second of data
d Nor applicable

 

 

 

....angle were available. The basic sequence of operation for modes 1 and 5, which were most frequently used in the altimeter operation, is shown in table A-ll.

An example of the altimeter data over an anomaly in the gravitational field of the Earth is shown in figures A-18 and A-19. Because of this anomaly, the mean sea level deviates considerably from the spherical Earth model ellipsoid representing the shape of the Earth. These illustrations show a 20-m depression of mean sea level as obtained directly from the altimeter sensor. The altimeter measurements correlate with independent measurements of sea level in this region.

L-Band Radiometer (S194).-The objective of the L-Band Radiometer was to evaluate the applicability of a passive microwave radiometer to the study of the Earth from orbital altitudes. The radiometer measured the brightness temperature of the terrestrial surface along the spacecraft groundtrack to a high degree of accuracy.

 


FIGURE A-18.

FIGURE A-18.-Skylab 2 groundtrack for altimeter data. [For a larger picture, click here]

 

The S194 sensor (fig. A-20) had a fixed antenna with a 3-dB half power) beamwidth of 15.0°. The energy received by the antenna was integrated at a rate that ensured a minimum of 80-percent ground coverage overlap. The receiver provided a digital representation of the 0- to 350-K input radiometric temperature range. The system had an internal calibration network referenced to a fixed hot- or cold-load input.

The radiometric brightness temperature was measured with a resolution of ±1.0 K at a wavelength of approximately 21 cm. The system operated at a center frequency of 1.4125 GHz with a bandwidth of 27 MHz. Operating at this frequency, the sensor provided measurements that were minimally affected by meteorological conditions.

The 3-dB beamwidth implies that 50 percent of the energy received by the antenna was received in the 15°...

 


[
360]

FIGURE A-19.

FIGURE A-19.-Altimeter range measurements.

 


FIGURE A-20.

FIGURE A-20.-L-Band Radiometer (S194).

 

....by 13.9° solid pyramid centered about the vertical axis. The antenna received more than 90 percent of the energy available in a first-nulls beamwidth (primary lobe) that encompassed a swath width of approximately 282 km at the 435-km orbital altitude, and the signature was influenced by the entire view area. However, the signature recorded by the facility was influenced to a much larger degree by the brightness of the material contained within the 3-dB beamwidth: a 111-km swath centered about the nadir point Data output was eighteen 10-bit words/sec. Sensor radiometric calibration was acquired by viewing the Moon and deep space.

Examples of the data produced by the S194 sensor are shown in figures A-21 and A-22. The spacecraft moved from Baja California across the Gulf of California and on into Mexico along the groundtrack shown in the map at the top of figure A-21. The antenna footprint is shown as circles on the flightpath. The solid circle represents the half-power point on the antenna pattern and corresponds to a circular area 124 km in diameter. The first null in the antenna pattern is shown by the dashed circle representing 285 km in diameter. The plot in the lower portion of figure A-21 shows the radiometer response along this groundtrack.

 

 


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FIGURE A-21.

FIGURE A-21.-L-Band Radiometer data from Baja California. [For a larger picture, click here]

 


FIGURE A-22.

FIGURE A-22.-Brightness temperature plots of data from the L-Band Radiometer. [For a larger picture, click here]

 

Radiometer response is shown in terms of microwave brightness temperature. The low temperature of the sea is caused by its low emissivity. Land surfaces have emissivities approaching that of a black body, so that the microwave temperatures for land surfaces are close to actual temperatures.

Correlations were obtained between moisture content of the soil and radiometric data from S193 and S194. The correlations indicate that microwave sensors may be quite useful for such measurements in the future.


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