The concept of Relative Receiver Autonomous Integrity Monitoring (RRAIM) is brought up in the context of Modernized GNSS by GNSS Evolutionary Architecture Study (GEAS) Panel (GEAS 2008) to provide LPV-200 performance worldwide in mid-term future. Three new integrity architectures were provided in the first report (GEAS 2008) where RRAIM is one of them. The idea of RRAIM is to use carrier phase smoothed code measurement for positioning at the initial time with integrity validated by GNSS Integrity Channel (GIC) and this position is then projected to the current position using relative carrier phase measurements. Integrity burden is shared between ground augmentation system and GNSS receiver onboard the aircraft. Consequently, the latency of GlC integrity is greatly relieved. Also another advantage of the RRAIM model is that by using relative carrier phase measurements, high precision can be achieved with no need to fix integer ambiguities. In this paper, weak geometry as an important issue to improve service availability in terms of integrity is discussed. In the basic model for calculation of VPL, RRAIM positioning and Fault Detection methods based on position domain are used. Optimization with dynamic coasting time and predicted information to improve service unavailability is explored. And Vertical Protection Level (VPL) as the criteria to evaluate integrity is provided in simulation results.