# A Sensitivity Study of POD Using Dual-Frequency GPS for CubeSats Data Limitation and Resources
## 3 Orbit Determination under Different Scenarios
### Latency Applying Different GPS Products  
In Sections 3.1 and 3.2, the IGS final products for GPS satellite orbits and clocks were used for the estimation of the reduced-dynamic orbits. The IGS final products are produced weekly and have a latency of 12 to 18 days. In case that the CubeSat orbits are required within a shorter time, the IGS (or one of its analysis centres) rapid products or products from the IGS real-time service (RTS) [45; 46] can be used instead. Table 4 gives the latencies of different IGS products. In this study, the decoded IGS real-time stream IGC01, which is denoted as the IGC products and available at [47], is used for simulation of the scenario retrieving the GPS products in (near)-real-time. Products of the single IGS analysis centers, e.g., the real-time products from the National Centre for Space Studies (CNES), are also of high quality [48]. In this study, for consistency purpose, we used the IGS products of different latencies and satellite clock sampling rates. Despite the accuracy differences [46; 49], from Table 4 it can also be seen that the sampling rate of the IGR satellite clocks is lower than the other two products. In [11], the sampling interval of GPS satellite clock-offset corrections was identified as a limiting factor for the LEO orbital accuracy. This possibly explains the fact that the 3D orbital RMSE using the IGR products are not better than those using the IGC products. Figure 7 shows the differences of the between-satellite clock corrections for G02 and G32 between the IGS final and rapid clocks. The between-satellite clock differences are shown to remove possible inconsistencies between the reference time scales of different products. The blue dots indicate the differences at the given sampling interval of the rapid clocks, i.e., 300 s. The red dots illustrate the differences at a sampling interval of 30 s, where the rapid clocks were linearly interpolated using the nearest epochs to this higher sampling rate. From Figure 7 it can be observed that although the differences are generally at centimeters for the blue dots with the given sampling rate of the rapid clocks, after interpolation, the differences are increased to dm-level (see the red dots). For the test day, the average RMS of the clock differences for all satellites between the IGS and IGR final products, divided by \(\sqrt{2}\) for the between-satellite differencing, amount to about 2 cm. Note that the large pattern of the red dots is caused by the stochastic behaviors of the clocks, and an interpolation over a longer period with a higher-order polynomial does not help to reduce the biases. As an example, interpolation with a quadratic polynomial within each half hour increases the average RMS by several millimeters. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Orbits/Clocks** & **Identifier** & **Latency** & \begin{tabular}{c} **Accuracy [cm]** \\ **Orbit/Clock** \\ \end{tabular} & \
\begin{tabular}{c} **Satellite Clock** \\ **Sampling Interval [s]** \\ \end{tabular} &
\begin{tabular}{c} **3D RMSE [cm]** \\ \end{tabular} \\ \hline IGS Final & IGS & 12–18 days & 2.5/2.25 & 30 & – \\ \hline IGS rapid & IGR & 17–41 h & 2.5/2.25 & 300 & 4.8 \\ \hline IGS RTS & IGC & (Near)-real-time & 2-5/3–5 & 30 & 3.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Information on different IGS products and the 3D orbital RMSE using 10 s observation data. Assuming a duty-cycle of 100%, an observation sampling interval of 10 s, and a mean satellite number of 9, the last column of Table 4 shows the 3D RMSE of the final LEO orbits determined using different IGS products compared with the best-case scenario, i.e., using the IGS final products. A 3D RMSE of several centimeters can be achieved for the IGR and IGC products. This is further illustrated by Figure 8, which shows the time series of the 3D orbital errors applying the IGR and IGC products.