Cooperative position location with signals of opportunity

When the reception of GPS signals becomes unreliable, an alternative is to explore signals of opportunity (SOOP) for positioning. Broadcast digital radio transmissions (e.g., digital TV signals) contain field and segment sync codes, which can be used for ranging even though it was not originally designed for so. Another example is the wireless local area network (WLAN) signals. However, there are two major difficulties. Although the location of SOOP sources is known, the number of independent SOOP sources and their geometric distribution may not be favorable for precise positioning. Besides, the clocks of SOOP transmitters are not synchronized, each subject to a different bias and drift. To respond to the 2009 NAECON Grand Challenge, we set forth a cooperative position location approach. The proposed concept makes use of differential ranges between cooperative devices to a common SOOP source, the relative ranges between the cooperative devices, and displacement measurements by the cooperative devices. The cooperation among networked location devices not only allows them to choose the most appropriate positioning mechanism but also provides them with additional measurements to reduce the number of SOOP otherwise required. In addition to data exchange, the radio link between two cooperative devices also allows for estimation of their clock offset. This leads to a joint position location solution via fixed-point smoothing. In this paper, we present the proposed system concept, its subsystems, and their operations and also analyze preliminary simulation results.