Locata Correlator-Based Beam Forming Antenna Technology for Precise Indoor Positioning and Attitude (2011)

Multipath is the largest unmodeled error source for most radio frequency (RF) positioning applications, and is especially prevalent in indoor, industrial and urban environments. Although there are many ways to mitigate multipath in the receiver (such as filtering and correlation techniques) it is well understood that pointing a narrow beam antenna in the desired signal direction inhibits the reception of signal from all but the desired direction – and so reduces the effects of multipath corruption of the positioning signal. Beam forming antennas are well known to the positioning industry, and are commonly used for demanding applications such as GPS anti-jamming. Almost all beam forming antennas used professionally have multiple RF front-ends to form beams because engineers have long known that the more RF front-ends used, the tighter the beams that can be formed. As a result, the many benefits afforded by tight beams for multipath mitigation are only gained at the cost of increasingly complex and expensive antenna and receiver hardware. Further, with multiple RF front-ends, precise phase-based positioning (which can provide the centimeter-level accuracies essential for applications such as autonomous machine control in warehouses) is problematic because each RF front-end will have different filter delays. Locata Corporation has developed and commenced demonstrating a unique beam-forming antenna capable of centimeter-level positioning, using standard Locata positioning signals, in extremely high multipath environments such as indoor warehousing and container-terminal applications. This multipath mitigation capability is enabled by a new beam forming method called Correlator Beam Forming™. This method creates beams by sequentially switching through each element of an antenna array and forming the beam with phase and gain corrections in thereceiver’s individual channel correlators, using only one RF front-end. This elegant approach is the antithesis of traditional beam forming techniques which utilize a combination of multiple parallel RF streams from multiple RF front-ends, resulting in a very complex baseband arrangement. The physical antenna design has been trademarked as a “Small TimeTenna”, styled after the inventor’s name and to highlight the novel time-based methodology it employs. Correlator Beam Forming gives each receiver channel the capability to independently “point” beams. The physical TimeTenna is therefore capable of pointing multiple beams simultaneously in different directions, using a standard Locata positioning receiver. It should be noted that Correlator Beam Forming is not constrained solely to a LocataNet positioning system where a TDMA transmission scheme is a core part of the network architecture. In fact the method is equally applicable to the typical CDMA schemes used by GPS and other satellite-based navigation systems. Correlator Beam Forming technology can therefore be adapted for effective new GPS anti-jam applications through relatively minor changes to a conventional GPS correlator design. In order to point beams in the desired direction, accurate knowledge of the antenna’s orientation is essential. The Correlator Beam Forming method’s capacity to create multiple beams which independently search for signals endows a suitably-equipped receiver with the unique ability to independently resolve 2-D or 3-D attitude using only a single antenna. Locata’s development effort has shown that the attitude information delivered by a TimeTenna is extremely accurate and stable, and does not exhibit the drift intrinsic to gyro-based devices or the accumulated errors encountered with dead-reckoning IMU systems. This technical paper presents real-world test results from a high-multipath all-metal indoor facility which approximates a distribution warehouse environment. These tests utilized a TimeTenna model made of 64 elements which are distributed around a cylindrical frame. This combination of elements is shown to generate beams with a width of about 23 degrees. The elements are sequentially switched under precise time control from the receiver such that only a single element is “hot” at any time and all 64 elements are sequenced completely during a signal integration interval. During each switch interval, the gain and phase of the incoming signal is adjusted within the correlator by modifying the phase and amplitude of the carrier DCO to form the desired beam. The beam direction can be changed at each integration interval independently within each channel. This paper provides an overview of the Correlator Beam Forming architecture and the design of the TimeTenna which will detail how this new correlator-based multi-beam capability is achieved. A Leica total station laser-based solution is used as a truth during the tests to corroborate positioning and orientation accuracy. The TimeTenna positioning performance is shown to be on the order of a few centimeters from truth. Additionally, orientation solutions are shown to be accurate to within a few degrees. This paper will be the first to publicly present the TimeTenna’s unique “accurate orientation with a single antenna” capability, along with examples of the precise indoor positioning results. Locata Corporation will present live demonstrations of the TimeTenna indoor positioning capabilities as part of it’s involvement as an industry participant at the ION 2011 Conference.