2 Active Sensing Payloads for Satellites

An SAR is a common type of active sensing payload found in satellites. Like any radar system, SARs emit a pulse or frequency chirp of radio waves from their antenna, which propagates to the target and then is reflected back for receiving by the same antenna. Measuring the time it takes for this journey to occur makes it possible to determine the distance to the target. However, since a satellite flies at an angle to the target on the ground (see Figure 2-1), the amount of reflected energy is determined by the smoothness and angle of the target.

Figure 2-1 Radio Waves Reflecting off a Smooth Surface Target

For example, smooth water reflects all of the radio waves used for radar sensing away from the receiver (i.e. the antenna). The amount of reflected energy is displayed as a gray scale image where white areas indicate high reflection and dark areas indicate low reflection.

Radio waves can also reflect off of multiple objects. For example, first water and then a tree, which is called a double bounce. A double bounce can make a smooth water surface appear bright in an SAR image instead of dark, as the radio waves first reflect off the water’s surface and then the tree, as you can see in Figure 2-2.

Figure 2-2 Double Bounce of Radio Waves

The frequency of the radar is an important property that significantly affects what it can observe. Radio waves only reflect off of objects that are larger than the wave’s wavelength.

The property of radio wave reflection enables a radar to see through clouds and even vegetation on the ground. Very low frequency radars can even penetrate soil to determine moisture levels or features undetectable by optical imaging.

Another feature of the radar is its aperture size. The effective size of the radar is inversely proportional to the spot size of the radar beam as it scans the ground. The spot size is referred to as the azimuth resolution of the radar, which is the ability of a satellite to distinguish between two objects that are close to each other. A bigger radar creates a smaller spot size on the ground and has better azimuth resolution. Unfortunately, it isn’t practical to launch very large radars into space given their size and weight. System designers, however, are able to use computer processing to make the radar appear larger than its physical size. This method takes advantage of the fact that the satellite is moving with respect to its target and synthesizes an aperture by using the reflection of multiple overlapping pulses of the radar spot size.

Implementing an SAR instrument in a satellite requires very specialized radio frequency (RF) components. Figure 3 shows a typical block diagram of a radar imaging payload for implementing SAR in a satellite.

Figure 2-3 Radar Imaging Payload Block Diagram

High-speed data converters in the radar imaging payload can help determine the performance and architecture of the radar. For example, an RF-sampling data converter can directly convert the radar frequency band into digital information for processing. The most important requirements for these data converters are:

• An analog input bandwidth greater than the maximum input frequency.
• A sampling rate greater than twice the instantaneous bandwidth of the radar signal.
• High signal-to-noise ratio and spurious-free dynamic range to meet the system performance needs at the frequency of interest.
• Radiation tolerance to meet the mission needs.

For example, the AFE7950-SP RF-sampling transceiver provides these features:

• 10.6GHz, –3dB analog input bandwidth to support RF sampling from the L-band to X-band.
• Six 3GSPS analog-to-digital converters (ADCs) and four 12GSPS digital-to-analog converters (DACs). A maximum instantaneous bandwidth of 1.2GHz provides for better range resolution and implementation of anti-jamming techniques.
• Noise spectral density better than –155dBc/Hz and third-order intermodulation distortion (IMD3) >76dBc through 5GHz input frequency enable high receiver sensitivity.
• Pin-compatible 100krad/75MeV enable use in low Earth orbit (LEO) to geostationary orbit (LEO).

Another type of active sensor uses a laser as the illumination source, rather than the electromagnetic waves from the radar, but the same principle of using time to measure distance still applies. However, lasers operate at a very high frequency and with a short wavelength, which is unable to penetrate clouds or other objects on the ground and therefore requires clear conditions. Instead of an antenna, a laser system uses a photo diode to receive and measure laser light reflected from the target. Figure 2-4 shows a block diagram of this type of system.

Figure 2-4 Laser Imaging Block Diagram

As Figure 2-4 illustrates, the photodiode sensor array is followed by a transimpedance amplifier to convert its current output to a voltage that the data converter can sample. In this case, the data converter needs to be fast enough to sample the rising edge and wave shape of the reflected pulse of light, which depends on the laser rise time and pulse repetition rate.