It is apprized that GNSS satellites transmit free source of coherent radio waves illuminating the Earth’s surface 24 hours a day, these signals are also reflected from different objects (known as GNSS-R signals) and can be utilized in various remote-sensing applications as they contain valuable information regarding the reflecting surface.
The Global Navigation Satellite System (GNSS) signals are always available globally and the signal structures are well known. They have some distinctive characteristics, including the use of L-band frequencies, which are particularly suited for remote sensing purposes. The advantage of GNSS remote sensing is that a large number of GNSS satellites are available including US NAVSTAR GPS (24 to 32 satellites), the Russian GLONASS (24 satellites), the European GALILEO (30 satellites) and the Chinese BEIDOU or COMPASS (35 satellites). These numerous GNSS sources are continuous, all-weather and near-real-time microwave (L-band) signals are available through the Earth’s atmosphere.
Applications of GNSS-R Signals
GNSS based remote sensing has the attraction that user can take advantage of the expensive GNSS infrastructure maintained for navigation purposes and no dedicated transmitter is required. The reflected GNSS signals have the potential to be applied in various remote sensing applications and the data can be processed to extract: –
- Over land to measure soil moisture content, biomass, wetland, forest change detection and bi-static imaging.
- Over the ocean to calculate mean sea height, wind speed and direction and significant wave height.
- Over ice to ascertain ice age, thickness and surface ice density.
Other applications include cyclone monitoring, passive altimetry, ship detection and GNSS based passive radar.
Hardware used for GNSS-R Signals
Typically, the GNSS remote sensing hardware consists of a LHCP (left hand circularly polarized ) high gain and directional antenna to acquire the reflected signals and an electronic circuit that consists of a front end to receive the RF data, which is later on converted to IF (Intermediate Frequency). At least two RF front ends are required, one for direct signals and other for reflected signals. It is apprized that extraction of various information for remote sensing purposes is only possible with the help of customized algorithms / software modules. The GNSS signals are received, amplified, down-converted and digitized into near base band samples, which are than processed using software routines.
Challenges in Receiving GNSS-R Signals
The major challenge is the reception of weak GNSS-R signals, which are not expected to have enough SNR (Signal to Noise Ratio) to permit successful signal acquisition. Therefore, the direct signal from a specific satellite can be selected, locked and used as a reference for the reflected signal. Recent advances in the array signal processing, beam forming and adaptive antennas have made it possible to design a system which includes a number of antenna elements, whose gain and phase can be adjusted in an adaptive manner. Thus, a desired beam pattern can be achieved and steered in the direction of the reflected signal or specular point (a point that gives maximum return).
Products at ZATNav
The GNSS team at ZATNav has developed a four (4) channel SDR (Software Defined Radio) device for acquisition of GNSS signals that can also perform onboard signal processing.
The device can be used for the simultaneous acquisition of direct and reflected signals (GPS L1 at the moment), it can also store the signals and perform digital signal processing related to GNSS remote sensing. It consist of two PCBs, the motherboard is based on Xilinx ZYNQ7020 and RF front end on Maxim MAX2771 IC. The design and manufacturing work is complete and initial lab prototype is being tested and verified.