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Session F3: Navigating in Challenged Environments (e.g., Urban, Indoor and Subsurface Navigation)

RF Trilateration Positioning in Vegetated and Urban Settings Leveraging One World Terrain
Simone B. Bortolami, Nicholas Saluzzi, and David Redington, Navigation Research and Development Center, Applied Research Laboratory, Pennsylvania State University
Location: Room 6-8
Alternate Number 1

BACKGROUND & PROBLEM STATEMENT
Accurately tracking soldiers’ positions is a challenge when GPS is not accessible. Available radio-frequency (RF) ranging technologies fall short in the battlefield where soldiers operate or do not provide desired accuracies. For example, GPS and (GPS/RTK) fail when a soldier is surrounded by vegetation due to the attenuating effects on low power signals. Technologies such as Bluetooth, Wi-Fi, and Ultra-Wideband systems fail because of low accuracy or susceptibility to multi-path interference. We describe an innovative RF ranging system that overcomes the limitations of other systems with simulations showing good positioning accuracy in environments where soldiers operate. This technology originates from the U.S. Army research in training technologies, but its impact is broader.
THE TECHNOLOGY
The Resilient RF Ranging Positioning System, we dubbed R3PS, uses mobile or fixed reference towers (RT) to broadcast RF ranging signals to the R3PS user group. The light COTS user equipment performs range-measurements to the RTs to trilaterate user’s position with the assist of the One World Terrain (OWT). This approach functions similarly to GPS, but with innovations that allow for the system to operate in locations were GPS and in particular GPS-RTK fail. Instead of using satellites, R3PS uses RTs that could be fixed or mobile, e.g. carried by soldiers, ground vehicles, or UAVs. R3PS operates in environments where significant signal attenuation occurs due to foliage and where multi-path interference is high.
THE INNOVATION
R3PS uses terrain and surface elevation map data from the OWT for design analyses and running-time operation. RF channels are modeled to include terrain-induced line-of-sight (LOS) obstructions, vegetation-induced path loss, free-space path loss, and multi-path interference due to foliage and structures. Our simulations produced RF channel models for the optimal selection of the RF parameters and structures of the ranging-signals. The robust positioning system was then designed after the worst-case scenarios were properly understood.
The RTs are required to know their geodetic position with centimeter level accuracy. This is achieved in several ways using technology that is currently available. If the RT is in an uncontested GPS environment and has a relatively clear view of the sky, the ground- or UAV-based RTs use GPS/RTK to calculate their position in real-time. In contested GPS environment RTs can use anti-jamming (AJ) antennas, which are commercially available. If AJ antennas are unavailable or undesirable, stationary RT positions are calculated using traditional surveying techniques. RTs can be rapidly deployed in areas with no existing infrastructure. For clarity, the user side of R3PS never requires GPS or other GNSS capabilities and is designed to function in areas where GPS/GNSS/RTK with or without AJ antennas typically fail.
Since, the attenuation effects of foliage on received RF power levels is a function the carrier frequency, we simulated hundreds of thousands of RF channel models between virtual RTs and users using real elevation map data (OWT) of several U.S. Army training locations. These locations have a variety of terrains including mountains, plains, and woodlands. The analyses indicated that frequencies in the neighborhood of 300MHz provide adequate penetration through foliage while minimizing antenna size and transmitted power-levels.
To overcome multi-path interference, we used the elevation data from the OWT to model volumes of foliage as clusters of RF scatter points. We have studied the collection of RF signal power-levels and delays seen by each user based on how the signals were reflected in the simulation space. The simulated multi-path results ranged from benign in areas with wide-open spaces to stronger multi-path effects in urban-like areas. We defined a multi-path mitigation approach which handles the worst cases of multi-path interference seen in the simulations. The mitigation approach is based on simultaneously transmitting data and training sequencies using orthogonal spread spectrum codes to modulate a common carrier frequency. The training sequency is used to measure the multi-path effects in a RF channel. In turn, the measured effects are used to control a multi-path cancelation filter that cleans up the data message and remove the carrier phase error induced by the multi-path.
The RTs are equipped with local clocks that must be synchronized and their positions must be known at the centimeter level. Methods for finding the RTs positions were described previously. Time synchronization is achieved performing two-time transfers between RTs. At power-up the R3PS user equipment local clock is not synchronized to the RT clocks. To synchronize the user equipment clock to the RTs’ clocks the users initial clock error must be measured. Time synchronization is necessary because the range measurements are based on signals time-of-flight between RTs and users. The user equipment solves for the three-dimensional position and local clock error simultaneously using the relative delays between each of the ranging signals received and the assistance of the OWT for the vertical direction.
Each tower transmits data and a training sequency on two carrier frequencies which are both unique to an individual tower. These two carriers are separated by a particular number of Hertz so that the two carrier phases are equal at a chosen frequency. This frequency is referred to as the delta-frequency. To achieve a precise position solution, the user equipment makes a series of three increasingly precise range measurements. The coarsest measurements are made using relative delays between the spreading sequencies send sent by the RTs. These range measurements are refined by measuring the phase of the delta-frequency. The final, most precise measurement is made by directly measuring the phase of the carrier frequencies.
R3PS trilateration yields centimeter-level accuracy in the horizontal plane, while the vertical direction is assisted by the OWT. In its native state OWT data are derived mostly from satellite ranging that carries reasonable accuracy in the horizontal plane, but very cursory on the vertical dimension. The use of RTs sorts out this limitation without the use of additional surveys of optical scans, since the very moment fixed or mobile RTs touch the ground – relative to an uncorrected OWT dataset or point cloud – the vertical positions of the terrain under the RTs are known with respect to the WGS84 with great certainty, and the horizontal position as well. Even in tactical situations this allows for the correct geo-registration/rectification of the raw OWT data to the WGS84 in real time.
Our presentation will describe our R3PS analyses and design in detail, show predicated performance figures, and COTS hardware for the rapid development and deployment of R3PS.
This work was supported by U.S. Army Combat Capabilities Development Command Soldier Center (CCDC SC).

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