GPS Based Positioning System for Geophysical Surveys in Heavy Foliage Areas
David Hodo | Integrated Solutions for Systems
Geophysical mapping is a key step in the process of remediating sites contaminated with unexploded ordnance (UXO). In order to accurately locate UXO, accurate measurements of the locations of the geophysical sensors used are needed. The need for accurate, real-time pose (position and attitude) estimates has increased in recent years as more advanced sensors are developed that are capable of not only detecting anomalies, but also discriminating items of interest from cultural debris. A combination of a global positioning system (GPS) receiver and an inertial measurement unit (IMU) is a common method of determining the pose of these sensors. GPS signals are extremely low power, however, and the performance of GPS receivers degrades rapidly when these signals are attenuated or blocked by nearby buildings or foliage.
This work will attempt to improve the accuracy and availability of position measurements for geophysical surveys. The resulting system will provide GPS-based positions in environments where standard GPS receivers provide a poor solution or fail to provide a solution at all. A GPS software defined receiver (SDR) will be developed that is capable of providing a position solution in degraded GPS environments such as under heavy foliage by making use of vector tracking techniques. The GPS position will be blended with the output of a commercial IMU to provide attitude measurements and allow brief periods of dead reckoning during complete GPS outages. A chip scale atomic clock (CSAC) will be used as the receiver clock to further improve reacquisition time and the GPS receiver’s ability to operate with a limited number of satellites.
In this work, the scalar tracking loops used in traditional receivers are replaced with a vector tracking architecture. Vector tracking loops are an advanced receiver architecture that combine signal tracking and position, velocity, and time (PVT) estimation into a single process. The vector tracking algorithm used is based on a vector delay/frequency locked loop (VDFLL). Standard GPS receivers use scalar tracking loops to process the received satellite signals. The tracking loops in different channels of the receiver operate independently of each other. No information is shared between channels and consequently, there is no feedback from the navigation processor to the tracking loops. Vector tracking exploits the correlations between the received satellite signals and receiver position. Vector tracking loops allow the different channels of the receiver to share information which allows strong signals to assist in the tracking of weaker signals. This also allows for instant reacquisition after brief outages. IMU measurements are combined with the GPS measurements in a deeply integrated (DI) architecture, which allows the tracking loops to also be aided by the IMU. The receiver performance will be improved further through the integration of a CSAC. A CSAC’s stability is several orders of magnitude better than the oscillators used in traditional GPS receivers, resulting in the ability to navigate for moderate periods with less than four visible satellites and provides longer time holdover periods during a GPS outage.
The primary benefit to DoD is the reduced cost, increased production rates, and increased data quality made possible by using standard GPS surveying techniques on a larger percentage of surveyed sites. The technology is applicable on any formerly used defense site (FUDS) or active range site where vegetation or man-made structures are present that block or attenuate GPS signals. (Anticipated Project Completion - 2015)