IEEE Geoscience and Remote Sensing Magazine - September 2020 - 22

Since the microwave SM product corresponding to the
NSO experimental territory has a large pixel, there are few
areas in Tibet that can meet the conditions of good vegetation and flat terrain. As for the A'li area in the western TP,
the soil conditions around the wetlands are better, but they
are relatively scattered. Hence, large-scale test sites are rare.
Therefore, the two test areas (NSO and ASO) are easily determined by geostatistical methods from topographic, vegetation, and land use maps.

surface pattern. The three patterns are integrated through
the organic combination of observational elements.
Since microwave SM algorithms are greatly influenced
by the optical thickness of vegetation and the dielectric
constant of the soil, the NSO experimental area was selected and established in the typical seasonal frozen soil
regions. Throughout Tibet, the vegetation cover is relatively uniform, and the terrain is relatively flat. The NSO
region was chosen not only for the validation of multiple
products but also, importantly, for the improvement of
the product algorithm in evaluating the influence of the
vegetation optical thickness and frozen soil.
To ensure that each site falls into 2 # 2 pure pixels, the
SM product is first geocorrected, and four corner latitudes
and longitudes of each pixel in the 25-km # 25-km scale are
directly determined on the image. Then, the sites on other
scales (1 km, 5 km, 15 km, and 75 km) are determined one
by one, according to their distance from the four-corner site. A
nested and satellite pixel-oriented site layout was established
to use fewer sites to represent the actual situation of larger areas and scale down the SM products step by step. Hence, these
characteristics are different from past experimental protocols.
The ASO experimental area is a mixed-pixel zone, with
wetland and desertification grassland. To ensure the representativeness of the sites, they were established from wetland to desert grassland, centered on the wetland. And the
density of the site is changed from dense to sparse.

OBSERVED VARIABLES
Three categories of variables/parameters are observed: atmospheric, soil, and vegetation. The atmospheric state variables include air temperature and humidity at 2 m, wind
direction and speed at 2 m, and precipitation at 3.5 m. The
soil state variables include skin surface infrared radiation
temperature and emissivity, 0-cm skin surface temperature, soil temperature and moisture at different depths, soil
dielectric constant, soil texture, and soil organic carbon
(SOC). As an important factor in passive microwave SM inversion in vegetated areas [11], SOC was considered in the
design of this experiment. The vegetation state variables include vegetation type, vegetation coverage, surface infrared
temperature, and emissivity. All of the observed variables
are closely related to the inversion and inversion analysis
of the SM. The sensors associated with these observed variables are listed in Table 1.

TABLE 1. THE OBSERVED VARIABLES AND SENSORS IN THE TP-IMSO.
OBSERVED VARIABLES
Soil temperature and
moisture (depth: 0-3 cm,
5 cm, 10 cm, 20 cm,
30 cm)

PHOTO

INSTRUMENT
Soil Temperature
and Moisture
Sensor Decagon
ECH2O 5TM

MAJOR INSTRUMENT CHARACTERISTICS
Accuracy: apparent dielectric permittivity (fa): ±1 fa from
1 to 40 (soil range); ±15% from 40 to 80 (soil)
Volumetric water content (VWC): using the Topps equation:
- lectrical
±0.03 cm3 · cm -3, typical in mineral soils that have an e
conductivity <10 dS/m; using allocation-specific calibration:
±0.02 cm3 · cm -3
Temperature: ±1 °C
Resolution: VWC: 0.0008 cm3 · cm -3 (0.08% VWC) from
0 to 50% VWC
Temperature: 0.1 °C
Range: VWC: 0 to 1 cm3 · cm -3; temperature: -40-50 °C
Measurement time: 150 ms

0-cm surface temperature

Campbell Scientific
109 temperature
probe

Sensor: BetaTherm 10K3A1 thermistor
Measurement range: -50°-70 °C
Accuracy: thermistor interchangeability error: <±0.36 °C over
-25-50 °C, <±0.6 °C over -50-70 °C
Survival range: −55-100 °C
Linearization error: Steinhart and Hart equation used to calculate temperature; maximum error: 0.03 °C over -50-70 °C

Infrared radiation
temperature (band:
8-14 μm)

Apogee infrared
radiometer (SI-111)

Mean of differences from target: 0.002 °C
Target temperature uncertainty (95% confidence) from
-30-65 °C: 0.122 °C
Maximum differences from target: 0.14 °C
Minimum differences from target: -0.106 °C
(Continued)

IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE

SEPTEMBER 2020

IEEE Geoscience and Remote Sensing Magazine - September 2020

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