# A Reliability Engineering Approach for Managing Risks in CubeSats
## 3 Materials and Methods
### Step 3. Perform the Criticality Analysis
#### 3.3.3 Defining the Detectability Index  
Detectability (D) represents the capability of identifying a failure mode. In a CubeSat, this capability acquires a crucial role to manage the possibility of detecting and then recovering from failure modes. The index ranges in a scale from 1 (easy detection of the failure mode) to 5 (impossibility to detect the failure mode), as shown in Table 5. \begin{table}
\begin{tabular}{c l} \hline \hline
**Score** & **SEVERITY: Impact on the System of the Potential Failure Cause** \\ \hline
1 & No vital subsystem is affected by any failure. \\ \hline
2 & One (or more) component(s) are affected by any failure, but the system can overcome the failure through redundancies or other subsystems. \\ \hline
3 & One subsystem is completely inefficient, but mission is still active by means of other subsystems, eventually cutting off operations not vital for mission purposes. \\ \hline
4 & One (or more) subsystem(s) are affected and mission is partially compromised in terms of efficiency, limited operations or lifetime. \\ \hline
5 & Mission is totally compromised. \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Severity index. Figure 1: Logical scheme if inference rules for the FMECA index Occurrence (O). \begin{table}
\begin{tabular}{c l} \hline \hline
**Score** & **DETECTABILITY: Capability to Understand the Subsystem/Component/Unit/Operation** \\  & **Affected by the Failure** \\ \hline \multirow{2}{*}{1} & Just one source of failure in one subsystem might produce this effect; the cause of the failure is detectable. \\ \hline
2 & Subsystems affected by failure are less than 2 different, independent ones \\ \hline
3 & Subsystems affected by failure are less than 3 different, independent ones \\ \hline
4 & Sources of the failure or subsystem affected might be more than 3 different, independent ones \\ \hline
5 & No chances to understand either the source of the failure and the subsystem affected as well \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Detectability index. Rather than relying on qualitative judgments, detectability has to be calculated combining two different indexes, i.e., the falsifiability and the observability. This choice confirms the purpose of generating systematic assessments for the indexes included in the FMECA calculations. The falsifiability refers to the possibility of testing or observing how one or more sub-systems may lead to the same effect. More specifically, falsifiability aims to identify how many components may fail in different ways or for different causes, but anyway, producing the same observable effects. The falsifiability also includes an assessment of how many components, operations or units may generate useful information to discriminate failures, and then identify them properly (see Table 6). In addition, another variable, i.e., observability, has to be considered to take into account the number of independent ways to observe the failure mode through specific data. The assessment of the observability follows the scale presented in Table 7.  
At this step, as for the occurrence index, specific IF-THEN FAM inference rules have to be adopted to integrate the two coefficients (falsifiability and observability) and gather a systematic assessment of the detectability index (see Figure 2).