# Combined Navigation and Tracking with Applications to Low Earth Orbit Satellites +
## 1 Introduction  
Xona's PULSAR(tm) constellation is a modern Low Earth Orbit (LEO) navigation system, which can provide several enhancements to legacy GNSS. Being closer to Earth, LEO satellites can broadcast stronger signals with better immunity to interference. The fast motion across the sky typical of LEO orbits (quick geometry change) gives much in the way of rapid convergence for Precise Point Positioning (PPP) as well favorable characteristics for multipath because signals change over shorter timescales. Another important aspect is related to security. New LEO signals are not bound by legacy, so there can be the inclusion of native encryption and data authentication [1, 2, 3, 4, 5]. When completed, the Xona LEO space segment will include about 260 satellites, with an average of 10-20 satellites in view, up to 170\(\times\) stronger signals, and cm level target positioning accuracy. Finally, a commercial LEO layer can add capability and evolve quickly--on a 5-year timescale--to keep up with the pace of commercial demand. All these aspects are illustrated in Figure 1.  
Combined navigation and tracking refers to a class of algorithms, where signal tracking and the navigation solution are jointly estimated--usually by a single Kalman filter [6]. When the dumped values (Is and Qs) and the Numerically Controlled Oscillator (NCO) commands are directly accessed by the navigation filter, the resulting structure is known as vector tracking and offers advantages in weak-signal, high-dynamics, and jamming scenarios [7]. Intermediate solutions between scalar and vector tracking retain all or partof the original control loops and inject deep aiding information such that outer feedback is created [8]. These structures present a cascaded configuration of control loops and Kalman filter, and their performances are expected to be sub-optimal with respect to a fully vectorized implementation [9]. Modular design and simplicity are the primary advantages of this kind of loop aiding--especially when a legacy scalar implementation is already available [10]. Inertial Navigation Systems (INSs) can be integrated with essentially any form of vector tracking or loop aiding [7]. This paper proposes a loosely combined navigation and tracking architecture to mitigate the effects of LEO orbits, which are perceived by the receiver tracking loops as high-dynamic stress. The legacy code and carrier control loops are retained, and deep aiding is realized through code-phase and frequency predictions as produced by the navigation engine. Observable predictions are derived from the receiver-to-satellite relative kinematics, and LEO orbits must be accurately estimated before loop aiding can start. To solve this \"chicken-and-egg\" situation, LEO broadcast ephemeris should be either uploaded to the receiver at startup or downloaded from the navigation message while temporarily tracking and decoding in unaided mode. Both these solutions are valid and applicable. A Xona PULSAR(tm) signal in the L-Band was used to simulate the LEO constellation. Special firmware was then developed to support the reception of this signal on the STMicroelectronics TeseoV multi-band GNSS chipset. Regular GPS L1 C/A signals were also simulated and synchronized with LEO. As part of the platform validation process, single-point solutions for Xona stand-alone and Xona + GPS have been generated. Other practical aspects of the Xona PULSAR(tm) receiver are also discussed, including assistance during acquisition, handling of navigation messages for large LEO constellations, and the usage of the LEO carrier phase with classic PPP algorithms. The rest of the paper is organized as follows. Section 2 reviews the proposed deep aiding architecture--originally introduced in [10]--and adapts it for LEO combined navigation and tracking. Section 3 illustrates the Xona PULSAR(tm) demonstration platform, including aspects that showed up during software development.