1.2. Integrity monitoring on land

Both SBAS/GBAS and RAIM methods can in principle be adopted for IM in land applications, since the fundamental positioning problem is the same. However, some important differences in the applications may make the task not straightforward. GNSS positioning in aviation is generally restricted to Single Point Positioning (SPP), based on code observations on the civil frequencies, L1 (E1 for Galileo) and soon L5 (E5). With SPP, accuracy of few meters is attainable. However, most current and future land applications (such as ITS) require lane-level accuracy, i.e., sub-meter accuracy [7]. As such level of accuracy is considered unattainable with SPS, ITS applications are foreseen to be relying on Satellite Based Augmentation Systems (SBAS), RTK or Precise Point Positioning (PPP) techniques [7].

The different positioning methods and the corresponding higher precisions involved bring with them a new set of specific vulnerabilities. For instance, anomalies that would create positioning errors of too small magnitude in an SPP context, and could therefore been neglected, would now need to be taken into account. In case carrier phase observations are to be used, cycle-slip monitoring shall be included, as well as IM for ambiguity resolution. Another difference from the aviation case that shall be taken into account is the environment in which positioning is to take place. Land users are often located in urban environment, which is characterized by the presence of high-rise buildings: as a result, GNSS observations are highly more likely to be affected by multipath and Non-Line-of-Sight (NLOS). Furthermore, the urban environment brings extra vulnerabilities linked to the higher risk of interference.

As the SBAS integrity monitoring concept has not been defined yet for ITS applications, this chapter focus is on the RAIM concept. This is in fact the most versatile integrity monitoring approach, generally applicable to any estimation problem. The chapter is organized as follows: in Section 2 the integrity as a navigation performance parameter is introduced and the focus moves to the RAIM approach. The RAIM problem is defined and the most important performance parameters of RAIM algorithms (PL, Probability of HMI, etc.) are introduced. In Section 3, a number of possible approaches to deal with the RAIM problem are introduced, whereas in Section 4 the most popular RAIM methods developed in aviation are described. In Section 5 the challenges related to the adaptation of current aviation RAIM methods to land applications are illustrated, and in Section 6 an example of preliminary results of an IM prototype method in ITS is shown. Finally, in Section 7 conclusions on the state-of-art in IM and directions of present and future work are given.
