Target Detection and Location by Fusing Delay-Doppler Maps

The use of delay-Doppler maps (DDMs) measured by Global Navigation Satellite System Reflectometry (GNSS-R) for target detection and location is a hot issue because of the potentially global coverage and short revisit periods of GNSS-R satellite missions. Existing researches have explored the detection of oil rigs, oil spills, sea ice, and other targets with spaceborne GNSS-R. However, these researches may not fully consider the advantages of multiple GNSS-R satellites, for example, Cyclone Global Navigation Satellite System (CYGNSS) which has eight satellites. Two problems of using multiple GNSS-R satellites for detection and location are how to estimate the sea clutter that is used to cancel the clutter in DDMs and how to remove location ambiguity caused by the transformation from the delay-Doppler (DD) domain to the spatial domain. In this article, an oil rig is taken as an example for detection and location with multiple GNSS-R satellites. To distinguish the oil rig from sea clutter, this article uses subspace projection to estimate the clutter and subtract the estimation from the DDMs. Moreover, after the clutter cancellation, this article projects the potential oil rig coordinates in the DD domain into a uniform coordinate system and removes the location ambiguity based on multisatellites fusion. The measured DDMs collected by CYGNSS are employed to conduct experiments to validate the feasibility of the proposed methods for detection and location with DDMs. By subtracting the estimated sea clutter from DDMs, the potential oil rig can be indicated in the DD domain. Two location experiments can calculate the unambiguous coordinate of the oil rig with a deviation less than 4 km.

I. Introduction

The global coverage and short revisit periods are the advantages of spaceborne Global Navigation Satellite System Reflectometry (GNSS-R), making it a potentially suitable technology for target detection. Recently, the use of spaceborne GNSS-R for sea targets detection, such as detecting oil rigs [1], oil slicks [2], [3], sea ice [4], [5], [6], [7], [8], red tides [9], and phytoplankton on the ocean surface [10], has been investigated apart from ocean wind and the target above the Earth’s surface detection [11]. Although researchers have provided contributions to the detection with spaceborne GNSS-R, two main problems have not been overcome. One problem is how to estimate sea clutter for clutter cancellation in delay-Doppler maps (DDMs). Given the different forms of sea clutter, sea targets are easily submerged in various noises and difficult to be accurately detected in the complex sea environment (calm sea or sea waves). Therefore, a key technology for the detection is to estimate sea clutter for clutter cancellation in DDMs. The other problem is how to locate the target when it is detected in the DDMs collected by spaceborne GNSS-R. When locating the target, e.g., oil rig, a location ambiguity is observed because the coordinate of the target in the delay-Doppler (DD) domain corresponds to two approximate symmetric coordinates around the specular point (SP) on the sea surface. The two problems hinder the further detection applications of the spaceborne GNSS-R technology.