(ATBD) SMAP Level 1 Radar Data Products - NASA

19
the spacecraft velocity vector with the Earth’s surface. These iso-doppler curves
are shown on the figure as straight to hyperbolic thin lines. The SMAP concial scan
maintains an incidence angle of 40 degrees, and the actively imaged area is thus an
annulus covered by the beam footprint. The swath is filled in by repeated conical
scans which slightly overlap along the nadir track as shown in Fig 4.
SAR processing yields single look resolution cells bounded by iso-range and
iso-doppler contours within the beam footprint. For conically scanned systems, it
is convenient to think in terms of range or elevation resolution directed radially
away from the sub-spacecraft nadir point and azimuth resolution directed along the
scan motion of the beam footprint. Assuming a spherical Earth, projected range
resolution and azimuth resolution are given by,
δR g =
c
2B r sin θ i
, (1)
δx = λR
2τ d v f(θ az), (2)
where δR g is the projected range resolution on the surface, c is the speed of light,
B r is the transmitted chirp bandwidth, θ i is the incidence angle, δx is the azimuth
resolution on the surface, λ is the transmitted wavelength, R is the slant range, τ d is
the beam dwell time or synthetic aperture time, v is the magnitude of the spacecraft
velocity relative to the target body, and f(θ az ) is the squint elongation varying as a
function of the scan angle position θ az as decribed in [6]. Projected range resolution
is constant around the scan at 230 m. Azimuth resolution degrades as the squint
angle increases due to varying doppler spread and elongation of the resolution cells
as iso-range and iso-doppler lines become more parallel to each other. Figure 5
shows the variation of single look resolution as a function of cross track distance.
Although Fig. 5 shows highly variable azimuth resolution across the swath, and
fairly fine range resolution, the L1C product will not provide access to the single
look resolution cells. Instead, multiple resolution cells are averaged together using
area weighting into 1 km grid cells in the L1C product file. Two kinds of multilooking
occur in this step. First, resolution will be degraded to the 1 km posting of
the L1C swath grid. This tradeoff is accepted in return for reduced statistical variation
of the measurements and reduced data volume in the product. Second, multiple
scans that cover the same 1 km cell will have their measurements averaged together
for further reduction of statistical variation. However, forward and aft directed look
directions will be kept separate so that azimuthal assymetry of the target scene can
still be observed. The resolution of the L1C product is then slighty less than 1 km
with slight variation crosstrack due to the area weighting of resolution cells into the
1 km cells. Near the outer edge of the swath, resolution cells that stick outside of the
1 km cell by half their size (200 m) will broaden resolution by 20%. Near the inner
edge of the usable swath (150 km crosstrack) resolution cells are elongated and can