A dual-frequency system is needed to better understand natural processes that constitute the environment and seasonal cycles of the Earth. A system working at two different wavelengths acquiring data simultaneously will give a valuable dataset since the conditions on the ground will be exactly the same. Hence, elements such as wind, soil moisture or any other changes on the ground will not interfere in the measurements.
This thesis explains how an S-band radar was built and tested. Moreover, the experiments done with a Ka-band radar used as a scatterometer are explained as well as the data processing and analysis. Finally, the two systems are used to get dual-frequency measurements from an airborne platform. The dual-frequency data is explored, showing the differences in normalized radar cross-section between frequencies and discussing the interferometric measurements.
Traditional pulsed radar transmits a pulse and waits for the echoes, or backscattered pulses. The echoes are downconverted to baseband and sampled. Then, a matched filter is applied to range compress the pulses. The pulsed radar system requires high sampling rates that generate big amounts of data. A different approach is used in Frequency Modulated Continuous Wave (FMCW) radar, where the system transmits and receives simultaneously.
FMCW Radar Basics:
As mentioned previously, FMCW radar uses a linear chirp that changes its frequency linearly with time. The time-domain expression for the pulse is given by
xt(t) = cos(2πf0t + πβt2 + φ0)
Synthetic Aperture Radar:
Synthetic Aperture Radar is a radar system that uses a processing technique that improves the azimuth radar resolution. The principle of the technique is to take advantage of the platform’s motion where the radar is mounted to synthesize an antenna array. Theoretically, the azimuth resolution achieved with SAR processing is half of the antenna length:
∆azSAR = L/2
Interferometry is a technique used to resolve a target’s third dimension: height. As explained in previous sections, by using one antenna, a radar can resolve a target in two dimensions: range and azimuth. Observing the same target from slightly different spatial positions and by using trigonometry, the remaining third dimension of a target can be inferred.
The S-band radar system is composed of the radar transceiver, with a transmit antenna and two receivers; a signal generator, the power supply, that includes a GPS receiver and a 10 MHz local oscillator and the acquisition system.
The signal generator generates a chirp pulse with a pulse repetition frequency (PRF) of 999 Hz and a trigger interval of 1.001 ms. Every time a new pulse is generated, the TTL rising pulse (trigger) is sent to the Analog to Digital Converter (ADC).
Before taking measurements from the airplane, the system was tested from the ground. Since at this stage, the radar was on its initial design, several measurements were done to characterize the system, to decide the appropriate chirp parameters, the position of the antennas and the acquisition configuration. The initial measurements were taken from the roof of the Lederle Graduate Research Center (LGRC) with a height of 60 m.
Once the results from the ground measurements were shown to be repeatable, the next step was to bring the system to the airplane. This section presents how the system is installed in an aircraft and the results obtained from the first airborne measurements.
The Ka-band radar described here is an FMCW 35 GHz radar with one transmit and two receive channels. It has been mounted on an airborne platform to be used as a side-looking interferometer as well as a nadir-looking scatterometer. A detailed description of the side-looking system can be found. This chapter describes the work done with the nadir-looking scatterometer during fall 2014 and spring 2015 and the systems transition to a side-looking interferometer that is combined with the S-band system presented in the previous chapter. It is anticipated that observations from the scatterometer will prove useful on their own and will be relevant to the side-looking interferometric configuration. The results might be useful in future processing of the Ka-band measurements as well as interpreting the results and errors associated with the interferometric measurements.
KASI: KA-BAND AND S-BAND CROSS-TRACK INTERFEROMETER
A dual-frequency system using both the S-band and Ka-band radars is of interest for a variety of reasons. Two systems working at different wavelengths acquiring data simultaneously will give a valuable dataset since the conditions on the ground will be exactly the same.
The dual-frequency system described here combines the existing S-band and Ka-band transceivers and the supporting electronics to build a system that can be mounted on an airborne platform to acquire data at the two frequencies simultaneously. Each of the transceivers is kept separate and the efforts to reduce system weight and volume focus on combining the supporting electronics the block diagram for the dual-frequency system.
The antennas used in the dual-frequency system are the respective antennas for the side-looking configuration of each system. Each of the two transceivers have two receive channels and hence, three antennas are needed: one transmitter and two receivers. A detailed description of the S-band antennas can be found The Ka-band antennas are slotted waveguide antennas with an azimuth beamwidth of 1 ◦and 45◦in elevation.
The work presented in this thesis has proven to be useful in measuring normalized radar-cross section of water and vegetation at Ka-band in a nadir-looking geometry. The scatterometer data match the observations found in the literature which opens possibilities for future data collections that will expand our knowledge on the radar Ka-band response. Moreover,these measurements are currently being used at the Jet Propulsion Laboratory and could potentially be used to design algorithms for the SWOT mission.
The working dual-frequency system is a unique instrument capable of producing valuable datasets. Although the data still needs to be explored more in depth, the first results have shown differences in both frequencies that can be exploited to better classify natural targets. Finally, even though more effort needs to be put in improving the interferograms, the work done is the first step towards the goal of estimating biomass and snow depth.
This thesis covers three different systems. Each one of them has its own problems, challenges and applications. Future work can be done in any direction to improve any of the radars or the dual-frequency system. Therefore, the future work is here divided in the three systems:
- S-band radar
- Ka-band scatterometer
- Dual-frequency system
Source: University of Massachusetts
Author: Gerard Ruiz Carregal