# Asymmetric Dual-Band Tracking Technique for Optimal Joint Processing of B1I and B1C Signals
## 1 Introduction  
It is well known that the widespread application of the Global Navigation Satellite System (GNSS) has promoted the construction, improvement, and modernization of satellite navigation systems such as GPS and Galileo in major countries and regions of the world [1; 2]. Among them, China's BeiDou system has seen rapid development in recent years. The BeiDou regional system began its service to the Asia Pacific region in 2012, while the BeiDou global system has entered the deployment phase in 2017 and will complete global networking in 2020 [3; 4]. On the premise of guaranteed forward compatibility of open service to legacy users, the BeiDou global system will broadcast several new navigation signals in the B1, B2, and B3 frequency bands to achieve better performance in terms of accuracy, sensitivity, and robustness [5]. Of particular note is that two civilian navigation signals--B1I and B1C--modulated at different center frequencies in the B1 frequency band will be provided in the BeiDou global system. While the B1I signal [6], with a center frequency of 1561.098 MHz, is retained from the regional system to offer service to legacy users, the new B1C signal, with a center frequency of 1575.42 MHz and which is implemented by the state-of-the-art multiplexed binary offset carrier (MBOC) modulation scheme, will be broadcasted mainly to offer interoperability with GPS L1C and Galileo E1C signals [7]. Traditionally, each satellite in the GNSS constellation broadcasts multiple navigation signals in the same or different frequency bands. For example, GPS broadcasts both a L1C/A signal and a L1P (Y) signal in the L1 band while broadcasting a L2P (Y) signal in the L2 band; in the modernization of GPS, the newly added L5 signal contains both data and pilot channels, which can also be viewed as a special case of broadcasting two signals at the same frequency [8]. Because two or more navigation signals broadcast by one satellite are generated based on the same onboard reference clock and their propagation paths to the receiver antenna are nearly the same, the time delay and Doppler-shift of these signals at the receiver are strongly correlated. This is the basis for the joint processing of multiple signals to achieve better measurement performance. Various techniques have been developed for the joint processing of multiple navigation signals, and they can be grouped into three types. The first type of joint processing is based on a tracking loop. The time delay and Doppler-shift parameters obtained by tracking a strong signal are used to assist in tracking the weak signal. One typical example is the GPS receiver with dual-frequency positioning, which utilizes the L1 signal to assist semi-codeless tracking of the L2P(Y) signal [8; 9; 10]. This type of joint processing handles two signals separately in essence, and can improve tracking sensitivity for the weak signal at the parameter level. The second type of joint processing is based on code and/or phase discriminators. For instance, when receiving the L5 signal, the outputs from code and phase discriminators of the data and pilot channels can be coherently or non-coherently combined to improve the accuracy and sensitivity of tracking [11; 12]. The third type is based on the auto-correlation function (ACF). By quasi-coherently combining correlator outputs of L1C and L1C/A signal components, the proposed algorithm in [13] successfully constructs a synthesis correlation function with no side-peaks. In addition, it combines the power of two signals to improve the tracking accuracy. These three types of joint processing implement different ways of jointly utilizing the power of multiple signals. We hence refer to this kind of performance improvement as power gain. The unique features of the B1I and B1C signals in the BeiDou global system, however, offer more potential than can be exploited by the three schemes described above.