(ATBD) SMAP Level 1 Radar Data Products - NASA

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hang out by 500 m thus broadening resolution by 50%. In level 2 processing, the 1
km radar data will be further averaged down to at least 3 km posting which is the
resolution that requirements are specified for.
6 Point Target Response and Time Range Definitions
Before getting into the details of the processing algorithms, we need to understand
four key time ranges that come out of the way SMAP radar data are collected. This
section, will examine the radar point target response which is of central importance
in SAR processing, and define the associated time ranges.
The radar operates by continuously transmitting pulses and recording echoes
during receive windows interleaved between the transmit times. High resolution
data consist of the basebanded complex (I,Q) data samples spaced at the sample
rate (1.25 MHz) from each consecutive receive window spaced at the pulse repetition
interval (PRI). Round trip travel times at light speed from the spacecraft to the
Earth’s surface will lead to about 18 pulses in flight. Thus, the received echo from
one particular transmit event would be recorded 18 PRI later in the received data.
The precise position of the echo in the receive window depends on the range to the
surface which will include some variability due to topography. The receive windows
are long enough to accomodate most of this topographic variability, although
there may be some data loss in very mountainous areas such as the Himalayas. If
a single idealized point target on the surface is considered with no reflectivity anywhere
else, then the echo recorded is called a point target response, or an impulse
response. The point target response is a time and frequency shifted replica of the
transmitted waveform. The next section describes the transmitted waveform.
6.1 Radar RF Signal Generation
Figure 6 shows a schematic view of the SMAP radar RF front end as currently
designed. This figure is still a preliminary version and the final design will likely
differ in the details. The following frequency and time definitions consistent with
Fig. 6 are established:
f stalo = 10 MHz (3)
f ctu = f stalo /8 = 1.25 MHz (4)
f cg(n,h,v) = m cg(n,h,v) f ctu (5)
f u = m u f ctu (6)
f d = m d f ctu (7)