This paper describes the prototyping of an offline ground monitor for advanced receiver autonomous integrity monitoring (ARAIM). The ARAIM user algorithm, which includes fault detection and exclusion (FDE), is autonomously executed at the airborne receiver. To achieve specific integrity and continuity requirements, the real-time FDE process requires assertions on the signal-in-space (SIS) performance, in particular on satellite clock and orbit ephemeris error characteristics. This information is broadcast in the integrity support message (ISM). To validate the ISM, the offline ground monitor estimates precise GNSS satellite orbits and clocks which are utilized to validate the ISM. There are many sophisticated orbit determination processes such as the one used by the international GNSS service (IGS), whose performance is specified in terms of accuracy. In contrast, the proposed offline monitor (OFM) architecture is mainly intended for safety-critical aviation applications, in which integrity is of primary concern. This monitor employs a straightforward approach to estimate satellite orbit/clock using the existing satellite based augmentation system (SBAS) ground infrastructure. For prototyping purpose, twenty sparsely-distributed reference stations (RS) are selected from a worldwide network of IGS stations, and their publicly-available observation data is utilized. A parametric satellite orbital model is employed in the estimator, whose implementation is described step by step in the paper. Receiver noise and multipath (RNM) error models are analyzed and implemented for all twenty RS and the sensitivity of the monitor’s performance to the RNM model is evaluated. The prototype uses the GPS legacy orbit model and does not assume a reference station clock model. Previous covariance analyses showed that the standard deviation of the monitor’s orbit/clock estimation error was expected to be on the order of 30 centimeters. In this preliminarily evaluation, satellite orbits and clock errors are on the order of meters.