The GNSS (Global Navigation Satellite Systems) are a valid aid in support of the aeronautic science. GNSS technology has been successfully implemented in aircraft design, in order to provide accurate position, velocity and heading estimations. Although it does not yet comply with aviation integrity requirements, GNSS-based aircraft navigation is one of the alternative means to traditional dead-reckoning systems. It can provide fast, accurate, and driftless positioning solutions. Additionally, ground-based GNSS receivers may be employed to aid navigation in critical applications, such as precision approaches and landings. One of the main issues in airborne navigation is the determination of the aircraft attitude, i.e., the orientation of the aircraft with respect to a defined reference system. Many sensors and technologies are available to estimate the attitude of a aircraft, but there is a growing interest in GNSS-based attitude determination (AD), often integrated at various levels of tightness to other types of sensors, typically Inertial Measurements Units (IMU). Although the accuracy of a stand-alone GNSS attitude system might not be comparable with the one obtainable with other modern attitude sensors, a GNSS-based system presents several advantages. It is inherently driftless, a GNSS receiver has low power consumption, it requires minor maintenance, and it is not as expensive as other high-precision systems, such as laser gyroscopes. GNSS-based AD employs a number of antennas rigidly mounted on the aircraft’s structure, as depicted in Figure 1. The orientation of each of the baselines formed between the antennas is determined by computing their relative positions. The use of GNSS carrier phase signals enables very precise range measurements, which can then be related to angular estimations. However, carrier phase measurements are affected by unknown integer ambiguities, since only their fractional part is measured by the receiver. The process of reconstructing the number of whole cycles from a set of measurements affected by errors goes under the name of ambiguity resolution (AR). Only after these ambiguities are correctly resolved to their correct integer values, will reliable baseline measurements and attitude estimations become available. This chapter focuses on novel AR and AD methods. Recent advances in GNSS-based attitude 2 Will-be-set-by-IN-TECH determination have demonstrated that the two problems can be formulated in an integrated manner, i.e., aircraft attitude and the phase ambiguities can be considered as the unknown parameters of a common ambiguity-attitude estimation method. In this integrated approach, the AR and AD problems are solved together by means of the theory of Constrained Integer Least-Squares (C-ILS). This theory extends the well-known least-squares theory (LS), by having geometrical constraints as well as integer constraints imposed on parameter subsets. The novel AR-AD estimation problem is discussed and its various properties are analyzed. The method’s complexity is addressed by presenting new numerical algorithms that largely reduce the required processing load. The main objective of this chapter is to provide evidence that: • GNSS carrier-phase based attitude determination is a viable alternative to existing attitude sensors • Employing the new ambiguity-attitude estimation method enhances ambiguity resolution…