Constellation availability is an important assessment criteria of a constellation usually modeled with the assumption of the Markov property. We show this assumption does not hold for the time to long-term outage distribution of GPS satellites. In this paper, we propose an alternate method to calculate slot availability by modelling the outage process as an alternating renewal process. This proposed method avoids the assumption of the Markov property. The method is then validated against the slot availability data from the GPS 24 constellation, where both long-term and short-term outages are shown to approximate independent alternate renewal processes. In addition, the similarities of various constellation state probabilities with the binomial distribution based on average slot availability are shown. Using the model, future constellation availability performance can be estimated directly from historical data and also provide accurate estimates of the expected number of satellite replacements.

星座可用性是一个重要的星座评估标准，通常以马尔可夫特性的假设为模型。我们表明这一假设不适用于 GPS 卫星长期中断分布的时间。在本文中，我们提出了一种替代方法，通过将中断过程建模为交替更新过程来计算插槽可用性。这种提出的方​​法避免了马尔可夫性质的假设。然后根据来自 GPS 24 星座的时隙可用性数据验证该方法，其中长期和短期中断都显示为近似独立的替代更新过程。此外，还显示了各种星座状态概率与基于平均时隙可用性的二项式分布的相似性。使用模型，