(ATBD) SMAP Level 1 Radar Data Products - NASA

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positions in a half-orbit), and suffers from much less distortion. Furthermore, many
important performance characteristics of the radar data such as the number of looks,
the effective resolution, and the random error are functions of cross-track position
but not of along-track position. Using a swath grid will then make validation of the
data an easier task.
The L1C product contains three sections of data. The first is the same meta-data
present in the L1B file. The second is the spacecraft data group which contains
time-ordered geometry data (spacecraft position, velocity, and attitude) at the times
when the spacecraft nadir point crosses the midpoints of each swath row. The third
is the σ 0 data group organized as 2-D swath grid arrays. The primary field again
is the σ 0 field. The L1C SAR processing algorithms will geolocate σ 0 results with
respect to the 3-D ellipsoidal surface of the Earth as defined by the WGS84 Earth
model. These data are then projected onto the 2-D swath grid. The same basic data
fields listed earlier for the L1B file are also present in L1C files, but they will be
2-D spatially organized arrays rather than 1-D time-ordered vectors.
4.3 Bit Flags in L1B and L1C products
Numerous bit flags are included in the L1B and L1C products to convey useful
information about the associated data fields. As a general rule, bit flags are defined
so that the value of one (true) indicates the presence of some adverse condition
such as an error above a requirement threshold, or the absence of data needed to
computed a result. Conversely, the value of zero (false) means the associated data
are “good”. Related bit flags are grouped together in 16-bit integers with array
dimensions matching the associated data. The L1B product has time-ordered bit
flag fields with shapes corresponding to both σ 0 arrays and σ 0 slice arrays. The L1C
product has spatially ordered bit flag fields with shapes corresponding to the swath
grid. The 16-bit integer fields containing bit flags are summarized in table 4 for the
L1B product, and in table 5 for the L1C product. The individual bit flags are then
described in tables 6 - 11. Table 8 represents eight tables; four for whole footprint
flags (HH,VV,VH,HV), and four for range slice flags (HH,VV,VH,HV). The range
slice flag fields are 2-D with the 2nd dimension indexing the specific range slice.
Table 11 represents three tables for the three polarization channels present in highres
data. These three channels are the two co-pol channels (HH,VV) and one of the
cross-pol channels indicated by XY ∈ {HV,VH}. The specific cross-link channel
used is selected by an uplinked command to the radar.