IEEE Geoscience and Remote Sensing Magazine - September 2017 - 39

A surface coverage simulator uses the ISS orbital information with the RapidScat antenna beamwidths, look angles, and rotation rate; it fills in a map with all areas above
a certain illumination threshold, allowing gaps in coverage
to be identified. A timing tool then calculates the timing of
the nadir return (through antenna sidelobes) and the returns for both the inner and outer beams. The inputs are
the radar timing parameters, the radar look angles, and the
ISS altitude. This tool allows verification that the chosen
timing parameters result in received signals at the desired
times, with no clipping or nadir-return contamination. A
variant of that tool allows the calculation of the effect of
the ISS attitude variation from nominal values. To compare
measurements with expected performance, we developed a
software package for analyzing test and on-orbit data.
TIMING PARAMETER SELECTION
We used the timing calculator tool to derive new timing for
RapidScat, because its altitude is roughly half that of previous SeaWinds missions (400 versus 800 km). At the 400-km
altitude of the ISS, the PRI must be reduced to fewer than
3 ms to transmit on one beam and then receive on the other,
as described in the "SeaWinds Overview" section. The legacy SES hardware does not allow such small PRIs. Hence,
the RapidScat team disabled the interleaved operation by
performing a modification to the SES that causes the beam
switching to occur after the receive window rather than prior
to it. This allows RapidScat to operate with timing similar to
an airborne system's; it transmits a pulse and receives that
same pulse (on the same antenna beam) prior to the next
transmit event. The next transmit event and pulse reception
occur on the other beam so that the pulsing alternates between the inner and outer beams.
The system timing is constrained by the commandability of the legacy hardware. Within this constraint, we
should choose timing so that 1) the nadir return completes
prior to the start of the inner beam and 2) the outer beam
return completes prior to the next transmit pulse. This is
complicated by the desire to have a large swath and the
need for robustness relative to changes in the ISS attitude
(expected to be a more significant issue than on previous
SeaWinds missions). To avoid chopping the end of the received data on the outer beam (especially at higher ISS altitudes and nonzero attitudes), the PRI for RapidScat is set to
the maximum allowable, i.e., 6 ms. To further increase the
timing margin, we reduced the pulsewidth from the 1.5 ms
used on previous SeaWinds missions to 1 ms. The receive
window length is set to 1.4 ms, and the 0.4-ms difference
relative to the pulsewidth maximizes the range coverage
of the slices (see the "SeaWinds Overview" section) and
also maximizes the likelihood of getting the surface return
within the 12 slices when the ISS attitude varies.
While including the ability to adjust instrument pointing based on the ISS attitude would have been ideal, this
was not within the project's scope (relative to schedule
and/or cost). Hence, the instrument was mounted on the
SEPTEMBER 2017

IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE

1
0.8
0.6 T
0.4

T
R

0.2
0
0

R

N
2

N
4
6
8
Elapsed Time (ms)

10

FIGURE 2. A timing diagram showing transmit pulses and receive
signals over two PRIs: the inner beam and then the outer beam.
The altitude is 400 km, and the attitude is zero (with the antenna
spin axis at nadir). The vertical axis is a qualitative illustration of
possible signal levels. T: transmit pulse; N: nadir return; R: received
main beam return.

ISS with a pitch offset to compensate for the mean pitch
predicted for the ISS over the RapidScat lifetime (with the
prediction provided by the ISS program). The ISS normally
flies with a slightly downward pitch. We selected the compensating RapidScat pitch offset well before launch, and
the choice worked well, resulting in a very small attitude
bias of the RapidScat instrument during its operation on
the ISS. The remaining inputs for the timing calculation
are the antenna look angles; Figure 2 shows the output of
the timing calculation tool, using the look angles and other
characteristics listed in Table 1.
NEW ANTENNA REQUIREMENTS
While an engineering model SAS was inherited (a spin mechanism and its controller rotary joint), a new antenna reflector was developed for the SAS because no suitable spare

TABLE 1. RAPIDSCAT CHARACTERISTICS.
PARAMETER

VALUE

UNIT

Orbital attitude

405

km

Antenna diameter

0.75

m

3-dB beamwidth-elevation (inner, outer)

2.4, 2.2

°

3-dB beamwidth-azimuth

2.0

°

Gain (inner, outer)

36, 37

dBi

Antenna rotation rate

18

r/min

Operating frequency

13.4

GHz

Chirp rate

250

kHz/ms

Pulsewidth

1.0

ms

PRI

6.0

ms

Peak power

90

W

Incidence angle (inner, outer)

49, 55.5

°

Look angle (inner, outer)

45, 50.5

°

Ground-range resolution (inner, outer)

22, 20

km

Azimuth resolution (inner, outer)

16, 18

km

Ground swath (inner, outer)

900; 1,100

km

Data window length

1.4

ms

Noise-equivalent v 0

-32, -31

dB

39



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