Satellite Passive Systems

A passive microwave instrument of particular note is the Special Satellite Microwave/Imager (SSM/I) onboard the Defense Meteorological Satellite Platforms. These polar orbiting satellites have been in operation since 1987 and provide high frequencies and two polarizations (table 7.1) except for 22 GHz (V only). Spatial resolution of the SSM/I is very coarse, as shown in table 7.1.

The SSM/I utilizes conical scanning, which provides measurements at the same viewing angle at all beam positions on a swath of 1200 km. This makes data interpretation more straightforward and simplifies image comparisons. There have been as many as four SSM/I satellites in operation at any given period. Therefore, frequent and even multiple daily passes are typical for most regions of the earth. Data from the SSM/I are publicly available (http://www.saa.noaa.gov/).

Interpreting data from the SSM/I to extract surface information requires accounting for atmospheric effects on the measurement. When one considers the atmospheric correction, the significance of vegetation attenuation, and the shallow contributing depth of soil for these high frequencies, it becomes apparent that the data are of limited value for estimating soil water content. However, data from the SSM/I can be used under some circumstances, such as in arid and semiarid areas with low amounts of vegetation. Figure 7.1 includes the results obtained over the southern Great Plains of the United States, from Jackson (1997), using SSM/I data. Value-added products from the SSM/I sensors include a wide range of atmospheric and oceanic variables. However, for reasons noted above, there have been few attempts to generate standard land surface products.

Another current satellite option is the Tropical Rainfall Measurement Mission (TRMM) Microwave Imager (TMI). It is a five-channel, dual-polarized passive microwave radiometer with a constant viewing angle. The lowest TMI frequency is 10 GHz (table 7.1), about half that of the SSM/I. The TMI has higher spatial resolution as compared to the SSM/I. TRMM is not a polar orbiting satellite and only provides coverage of the tropics, which includes latitudes between 38°N and 38°S for the TMI instrument. However, a unique capability of the TMI is its ability to collect data daily, and in many cases more often, within certain latitude ranges. This could facilitate multitemporal and diurnal analyses. These data are publicly available (http://trmm.gsfc.nasa.gov/data_dir/data.html). Jackson and Hsu (2001) have retrieved soil moisture from TMI observations. These studies showed the potential of the improved spatial resolution, higher temporal repeat coverage, and lower frequency as compared to the SSM/I.

Although no longer in operation, another satellite instrument of interest is the scanning multifrequency microwave radiometer (SMMR). This

Table 7.1 Characteristics of passive microwave satellites

Horizontal

Equatorial

Frequency

resolution

crossing time

Satellite

(GHz)

Polarization3

(km x

km)

(local)

SSM/I

19.4

H and V

69 x

43

0543 h

22.2

V

60 x

40

0922 h

37.0

H and V

37 x

28

1000 h

85.5

H and V

15 x

13

TMI

10.7

V, H

59 x

36

19.4

V, H

31 x

18

21.3

H

27 x

17

Changes

37.0

V, H

16 x

10

85.5

V, H

7x

4

AMSR (NASA)

6.9

H and V

75 x

43

10.7

H and V

48 x

27

18.7

H and V

27 x

16

23.8

H and V

31 x

18

1330 h

36.5

H and V

16 x

9

89.0

H and V

7x

4

AMSR (Japan)

6.9

H and V

71 x

41

10.7

H and V

46 x

26

18.7

H and V

25 x

15

23.8

H and V

23 x

14

1030 h

36.5

H and V

14 x

8

89.0

H and V

6x

4

aH = horizontal; V = vertical.

aH = horizontal; V = vertical.

instrument operated on the Nimbus-7 satellite between 1978 and 1987. It was a polar orbiting satellite and SMMR had a constant view angle of 50.3° and a swath of 780 km. Dual polarization TB was measured at frequencies of 6.6, 10.7, 18, 21, and 37 GHz. Spatial resolution for C band was very coarse (~ 150 km), and peculiarities of operation resulted in a long repeat cycle. SMMR data are also available (http://nsidc.org/data/nsidc-0036.html).

Recently several multifrequency passive microwave satellite systems have been launched, and more systems are planned. These systems offer lower frequency channel operating at C or L band, which should provide a more robust soil moisture measurement, and better spatial resolution. These satellites include, for example, the National Aeronautics and Space Administration (NASA) Aqua, the Japanese Advanced Earth Observation Satellite (ADEOS-II), the Naval Research Lab Windsat, the European Space Agency Soil Moisture Ocean Salinity Mission (SMOS), and the NASA Hydrosphere States Mission (Hydros).

Aqua was launched in May 2002 (http://aqua.nasa.gov/AMSRE3.html), and ADEOS-II was launched in the same year. Each includes an instrument called the advanced microwave scanning radiometer (AMSR). As shown in table 7.1, these are multifrequency systems that include a 6.9-GHz channel with 60-km spatial resolution. AMSR holds great promise for estimating soil water content in sparsely vegetated regions and is the best possibility in the near term for mapping soil water. Based on published results and supporting theory (Owe et al., 1992; Njoku and Li, 1999), this instrument should be able to provide information about soil water content in regions of low vegetation cover, with less than 1 kg/m2 vegetation water content. Aqua and ADEOS-II can provide observations with nominal equatorial crossing times of 1330 and 1030, respectively.

As opposed to previous passive microwave satellite missions, Aqua and ADEOS-II include soil moisture as a product. On Aqua it is a standard product, and on ADEOS-II it is a research product. The algorithm planned for use with Aqua is a variation of the multichannel approach described in Njoku and Li (1999). Several types of soil moisture products are to be produced. These include a daily swath product and a global composite. The swath products include a retrieval of soil moisture for each pixel observed. Results will be composited to a standard grid to generate a global map of surface soil moisture with a nominal spatial resolution of 25 km. Following a period of calibration/validation, the soil moisture products should be available on a daily basis. Examples of the types of products can be found on the Web (http://sharaku.eorc.nasda.go.jp/AMSR/index_e.htm).

Windsat was launched in 2003 and includes a multifrequency passive microwave radiometer system with a C-band channel. This system includes the AMSR and other frequencies and offers additional polarization options. The equatorial crossing time is 0630 h. It is a prototype of one component of the next generation of operational polar orbiting satellites that the United States will be implementing by 2010. Experience gained by using these science missions will provide the basis for future operational products. Research programs are underway to develop and implement space-based systems with a 1.4-GHz channel that would provide improved global soil moisture information. The challenge with low-frequency passive microwave remote sensing from satellite platforms has been to achieve a useful spatial resolution subject to the constraints of antenna size. Toward that goal, the European Space Agency is developing a sensor system called the Soil Moisture Ocean Salinity (SMOS) mission (Wigneron et al., 2000). SMOS will use synthetic aperture radiometry techniques to overcome the resolution-antenna-size problem. It is scheduled for launch after 2007.

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