GNSS-R
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 and can be utilized in various remote-sensing applications as they contain valuable information regarding the reflecting surface. 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. 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 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.
- Over land to measure soil moisture content, biomass, wetland, forest change detection and bi-static imaging.
Other applications include cyclone monitoring, passive altimetry, ship detection and GNSS based passive radar.
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. 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.
The major challenge is the reception of weak reflected GNSS 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).
We have expertise related to GNSS signal processing and have developed a Four channel GNSS SDR device that can acquire GNSS signals and perform signal processing. Our hardware is versatile and can be used for GNSS-based remote sensing, anti-jamming, and array antenna beamforming. Additionally, we are working on software algorithms for GNSS-R, as well as anti-jamming and spoofing GNSS receivers. If you require further information about our services, please do not hesitate to contact us info@zatnav.com.