What are vector tracking loops , and what are their benefits and drawbacks ?

GNSS Solutions: V ector tracking loops are a type of receiver architecture. The difference between traditional receivers and those that use vector tracking algorithms is the manner in which they process the received GNSS satellite signals, and how they determine the receiver's position and velocity. Vector-based tracking loops combine the two tasks of signal tracking and position/velocity estimation into one algorithm. In contrast, traditional — or scalar — tracking methods track each satellite's signal(s) independently; both of each other and of the position/ velocity solution. Vector tracking has many advantages over scalar tracking loops. The most commonly cited advantage is increased immunity to interference and jamming. The minimum carrier-to-noise power density ratio (C/N 0) at which the receiver can operate is lowered by processing the signals in aggregate instead of separately. Vector tracking algorithms also have the ability to bridge signal outages and immediately reacquire blocked signals. Moreover, vector tracking loops have a greater immunity to receiver dynamics than scalar tracking loops. A final advantage: The vector tracking architecture allows the receiver's motion to be constrained in different dimensions, which can be exploited by receivers whose motion occurs primarily in one or two directions, such as ships or automobiles, for example. The primary drawbacks of vector tracking loops relative to traditional approaches are their processing load and complexity. The Kalman filter used by the vector tracking architecture (more details to follow) must be iterated on a time scale commensurate with the integrate-and-dump period used by the algorithm (~ 50 Hz). The numerically controlled oscillators (NCOs) in each channel also must be controlled directly by the central Kalman filter. Another drawback of vector tracking is that the presence of a fault in one channel will affect all the other channels , possibly leading to receiver instability or loss of lock on all satellites. Before discussing how vector tracking loops operate, let's first review how a traditional receiver operates. Figure 1 shows a block diagram of a typical GPS receiver. In the traditional GNSS receiver, scalar tracking loops are used to estimate the pseudoranges and pseudor-ange-rates for the available satellites. A delay lock loop (DLL) is generally used for estimating the pseudoranges, and either a Costas loop or frequency lock loop (FLL) is used to estimate the pseu-dorange-rates or carrier Doppler. (A phase lock loop can also be implemented , although it is not strictly required for signal tracking). The pseudoranges and …