Remote sensing satellites must handle large amounts of data and thus transmit at high data rates. Because of this, the antenna design is critical for these satellites. Spherical phasedarray antennas can provide a workable solution for this application, with consistently high data-rate performance across the full distance of the satellite’s coverage area. The effectiveness of the approach will be shown with an X-band design.

Spectrally efficient modulation is critical to a satellite- based remote sensing system,^1-3 and quadraturephase- shift-keying (QPSK) modulation has generally been applied for high-datarate transmissions at Xband frequencies. To make optimum use of available power from the satellite, special antennas are required. For example, when a satellite is visible to the ground station, the path loss depends on the elevation angle of the ground station. Typically, isoflux antennas, which ensure near-constant signal strength at the receiving station when the satellite is traveling through the visible portion of its orbit, are often used on remote sensing satellites.^4,5

The range of a satellite in a circular orbit (Fig. 1) can be computed from Eq. 1.

where
r = the earth’s radius (6380 km),
h = the satellite’s orbital height,
θ = ground antenna elevation, and
α = the angle between the axis of the antenna and the direction of the earth station.

The visibility or “look angle” is computed as 66.10 deg. from triangle OAS shown in Fig. 1 where AGO is the axis of the antenna looking at the earth, OG is the earth’s radius, and GA is the satellite’s orbital height (600 km).

The range of a satellite in an orbit from an antenna at ground station S varies from 2831 km (SA or SB in Fig. 1) when satellite is at point A and B corresponding to 0 and 180 deg. ground antenna elevation to 600 km when the satellite is overhead at point C. From this, the path loss can be computed according to (4r/λ)² at a carrier frequency of 8300 MHz The path loss was found to vary in level from 179.9 to 166.4 dB (Fig. 2). The signal received at the ground station, when the satellite transmits with a constant-gain antenna, increases by about 13.5 dB for ground-station elevation angles from 0 to 90 deg.

For isoflux antennas for onboard satellite use, the antenna radiation (gain) pattern is shaped to compensate for path-loss variations (Fig. 2). The gain at a ±66-deg. look angle for the onboard antenna in an X-band remotesensing system can be increased by decreasing the gain at a 0-deg. angle so that the gain pattern matches the path-loss pattern over the range of elevation angles. The gain at ±66 deg. was measured as +7 dBi, with a gain pattern nearly equivalent to the system’s path-loss pattern, which is typical for a shaped-beam antenna.

With this antenna, the received signal strength at the ground station is nearly constant. But due to low gain of only +7 dBi at the maximum range angle, to maintain the required effective isotropic radiated power (EIRP), high onboard RF power must be available to transmit at high data rates. Typically about 40 W RF power is needed onboard the remote sensing satellite for a data rate of 105 MB/s. Such an isoflux antenna is unsuitable for frequency reuse due to poor cross polarization isolation and, with its high power requirements, is not suitable for high-data-rate transmissions. Also, due to its wide beamwidth, this type of antenna can spread a signal over a wide ground area, subjecting other receivers in the ground receiving area to unintended transmissions.

Typically, remote sensing satellites are tilted while imaging to increase the imaging range. Also, a “step-andstare” technique is often used to improve imaging resolution. To comply with these requirements, imaging satellites are designed for agility, with a tilt angle range of more than ±25 deg. This agility requires antennas capable of radiating signals over a more than ±90-deg range, and isoflux antennas cannot support this with adequate gain. But high-gain, narrow-beam antennas can provide a practical solution while reducing requirements for satellite RF and DC power.

Because of its narrow beamwidth, a high-gain antenna must be steered to direct the beam to a desired ground station. This can be accomplished by either mechanical or electrical steering by controlling the phase of the signals fed to the radiating elements of an array antenna. For high-resolution imaging satellites, electrical steering is preferred since mechanical steering may cause jitter due to rotating motors. Phased-array antennas offer a practical solution to systems requiring electrical beam steering.

In a planar phased-array antenna, the beam can usually not be steered more than 60 to 70 deg. from normal without suffering a reduction in antenna gain. The variation is approximately given by (cos θs)α, where θs is the scan angle from normal to the surface and α is a parameter used to account for scan loss. A value of α = 1 corresponds to an ideal case with no mismatch. A value greater than 1.0, to as high as 1.3, accounts for increasing mismatch loss as the beam is scanned away from normal.6 The reduction varies roughly as the projection of the antenna surface in the scan direction (Fig. 4, left).

In principle, several planar arrays, each pointing in different directions, can scan a wide coverage area. In a multisurface array having few large planar surfaces with several radiators per surface, one surface is used at a time. The beam from one planar array is phase steered to a maximum value and at the edge of scan, the adjacent plane surface takes over for wide angular coverage. A more effective approach, however, is a conformal array antenna with radiating elements evenly distributed on a smoothly curved surface. The surface can be made of small planar facets with one or more radiators per facet.

Continue on Page 2