**1. Introduction**

The Earth has two van Allen radiation belts, i.e., the inner and outer, which consist of trapped high-energy charged particles. Most of the particles within the radiation belts originate from solar winds and galactic cosmic rays [1].

The approximately dipolar magnetic field at low altitudes generated by Earth's core is tilted and shifted from the geographic pole. This creates a region with reduced magnetic field intensity, namely the South Atlantic Anomaly (SAA), which is located approximately on the eastern coast of Brazil. The SAA is a place where the radiation of the inner van Allen belt approaches the closest to Earth [2,3]. One of the manifestations of the SAA is the enhanced count rate of protons and electrons coming from the inner radiation belt [4].

The SAA region presents a threat to low Earth-orbiting satellites due to the high probability of single event upsets, failure of microelectronics and premature ageing, as well as to astronauts and their health [5–7]. Therefore, the region has been thoroughly investigated since its discovery. Many of the published papers have focused on the dynamics of the SAA, mainly the variation in its intensity over time and its drift, to predict its future movement. It has been observed that the SAA moves steadily in a northwest direction. This movement has been registered by measurement of the magnetic field [8,9] and by radiation [2,3,10–21]. Table 1 summarizes the publications that have focused on examining the drift of the radiation center of the SAA. As shown in previous studies, a large spread of drift velocities has been reported. Such large discrepancies could be explained by the dependence of the particle flux on the altitude and on particle energy, as described in [2], as many measurements

**Citation:** Kováˇr, P.; Sommer, M. CubeSat Observation of the Radiation Field of the South Atlantic Anomaly. *Remote Sens.* **2021**, *13*, 1274. https://doi.org/10.3390/rs13071274

Academic Editor: Serdjo Kos

Received: 8 February 2021 Accepted: 24 March 2021 Published: 26 March 2021

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were performed on different orbits with different instrumentation. Other phenomena that influence the drift of the SAA and its velocity are sun modulation and "geomagnetic jerks". The effect of the solar cycle on drift velocity was described in [3,20,21]. The authors of [19] theorized that the rapid short-term change in the drift of the SAA was caused by a "geomagnetic jerk", which was reported in 2003 by [22]. Geomagnetic jerks occur when the secular acceleration of the magnetic field rapidly changes.


**Table 1.** Summarized results of measured South Atlantic Anomaly (SAA) drift rates.

Another systematic error in the evaluation of SAA drift can be caused by different methods of data processing. One of the most deployed methods for evaluation of the SAA position is Gaussian or Weibull fitting of measured maxima data (flux, dose, and dose rates) over a period of time and calculation of the SAA position based on the fit maximum [3,14–17,21]. In recent years, a new interpolation method based on the calculation of the SAA centroid has been presented [12,13]. The shortcoming of the centroid method used in [12,13] is that it does not consider cosine-latitude effects.

CubeSats have been used for many scientific studies in recent years. Although CubeSat missions cannot fully replace mainstream space missions, they have proven to be a useful and inexpensive platform for small payloads [23,24]. CubeSats have been used in scientific fields of Earth science [25–27], space weather [28–30], and astrophysics [27,31].

Our 1U CubeSat Lucky 7 was launched with the primary objective of testing communication systems and global positioning systems (GPSs). The secondary objective was to study the ionizing radiation field with two devices, i.e., a piDOSE radiation detector and a silicon diode spectrometer. Due to the failure of the spectrometer, only data from piDOSE were obtained.

The objective of this study is to prove that 1U CubeSats can be exploited for the longterm monitoring of the SAA movement, as all measurements introduced in Table 1 were done on professional large satellites. Moreover, this study focuses on the improvement of the centroid method used for the localization of the SAA center.

In our study, we present a modest dataset of processed SAA location data gathered by the radiation detector piDOSE flown on the Lucky 7 satellite. The SAA location was derived from the radiation data by the centroid method normalized for cosine-latitude effects. We show that the centroid method calculates the SAA location with much higher accuracy than the maxima fitting method. Hence, fewer data are needed to evaluate the SAA location when the centroid method is used. Such features might be crucial in the case of 1U CubeSat, which has very limited power, and broadcasting resources and data transfer to Earth might be problematic. We believe that lightweight, cost-effective CubeSats equipped with radiation detectors such as piDOSE can be utilized for the continuous monitoring of SAA drift in the future.
