# A Sensitivity Study of POD Using Dual-Frequency GPS for CubeSats Data Limitation and Resources
## 1 Introduction
Due to these constraints and the potential limit on computational load in case of the real-time onboard processing, a relatively low sampling rate might be preferable for CubeSat POD, or low in general but only high in specific areas [24]. Furthermore, one may need to prepare for a low number of tracked satellites in case of assigning channels for the purpose of initial signal acquisitions [20] and in case of improper data tracking due to possible antenna control problems, which will be discussed later in this section. In addition to the data continuity and availability, other information required for the POD may also be limited for CubeSat missions. Due to the limitation of the CubeSats in terms of their size, power, and capacity to install extra sensors, the antenna attitude information may not be well measured, estimated, documented, and transferred back to the GPC with sufficient details. For example, due to power management needs, the IMU under its own high-accuracy experiment [25] onboard the CubeSat Simulation To Flight-1 (STF-1), investigated by NASA's Katherine Johnson Independent Verification and Validation (IV&V) Facility and West Virginia University [21; 26], was unable to be activated during the GNSS data collection. Depending on the latency requirements of the CubeSat orbits, the high-accuracy GNSS satellite orbits and clocks might not be available for the processing, e.g., the international GNSS service (IGS) final products [27; 28], which has a latency of 12-18 days. As explained in [11], this could lead to dramatic accuracy differences already in case of good data continuity and availability, mainly due to the decreased sampling rates of the satellite clocks. Moreover, instead of processing orbits with long arcs of 24 h or 12 h (which accounts for about 16 and 8 orbital cycles for an orbital period of 1.5 h), rapid processing may also be used to compute the orbit with shorter arcs of several hours. While some conditions mentioned above may not significantly influence the orbital accuracy when individually being considered, large degradation of different levels could happen when several conditions happen together. Therefore, at the planning phase of a CubeSat mission, it is important to assess the orbital accuracy under different scenarios, so that one can properly compromise between the limits on diverse resources and the expected orbital accuracy. The GNSS-based orbit determination of small satellites is often performed with single-frequency signals [23; 24; 29; 30], which leads to an accuracy of decimeters to meters depending on the data availability and the dynamic models used. In recent years, to enable a higher POD accuracy, the usage of dual-frequency code and phase GPS data has been attempted and planned in CubeSat missions, even for a multi-constellation scenario [31]. Having dual-frequency phase and code observations at hand, the first-order ionospheric delays, which amounts to about 99% of the total ionospheric delays [32], can be eliminated by forming the ionosphere-free (IF) combination instead of the group and phase ionospheric correction (GRAPHIC) combination, which is often formed in the single-frequency case involving both the code and phase signals. While having the second frequency should theoretically enables better POD accuracy, a higher challenge for data tracking, storage, and transfer is also associated with the use of the dual-frequency GNSS in CubeSat missions. In addition to the planned constraints on diverse resources, the actual data collection of the dual-frequency CubeSat missions may not fulfil the expectation due to the lower robustness compared to the large LEO satellites. As an example, Figure 1 shows the real data condition of the 3U CubeSat STF-1 experimented by NASA's Katherine Johnson IV&V Facility and West Virginia University, which was expected to collect dual-frequency GPS data with a duty-cycle of 100% during a specific 12 h period. The actual data condition, however, did not meet the expectation, possibly due to the lack of the antenna attitude control.