# A Femto-Satellite Localization Method Based on TDOA and AOA Using Two CubeSats
## 1 Introduction
In the Space and Planetary Exploration Laboratory (SPEL) at the University of Chile, we are working on developing such types of missions, with a preliminary technology demonstration that will be carried in the following missions of the laboratory [9]. Preliminarily, we plan to deploy a femto-satellite version that contains a GPS to verify the position. This version will carry a magnetometer and a miniaturized particle counter that was tested in the SUCHAI-1 mission [10]. We will deploy the femto-satellite opportunistically to increase the sensing points in the ionosphere/magnetosphere. This increase in the sensing points allows us to study the dynamic evolution during a geomagnetic storm. An ionospheric/magnetospheric anomalous activity usually lasts for a couple of days [11]. This duration requires the femto-satellite to operate for a few days. The experiment will be with two 3U CubeSats, the SUCHAI-2 and SUCHAI-3 (Figure 1). We expect to launch them together in a similar orbit. SUCHAI-3 will serve as the mother-spacecraft of the femto-satellite, carrying and deploying it in some moment once in orbit. The femto-satellite will send the gathered data with a radio communication link to both CubeSats. In this research, we simulate a similar scenario to study the feasibility of using the radio links to estimate the location of a femto-satellite with only two CubeSats. This location estimation combines the Time Difference Of Arrival (TDOA) with the Angle Of Arrival (AOA) of the received signals. The TDOA method has been used for drone detection and localization [12], in store-house environments [13], IoT [14] and by mobile users [15]. This method has also been used on larger satellites, using it to localize a satellite from the ground [16], as well as from other satellites in space [17]. The AOA method uses an antenna array to know the yaw (azimuth) and pitch (elevation) of the received signal. Placing the antennas in a square array [18] allows estimating both yaw and pitch instead of just one angle. A target can send a signal to multiple receivers with antenna arrays. The angles measured at the receivers with known locations provide the target position. This method has been used mostly on the ground for wireless sensor networks [19]. There is also research for the 5G cellular system, which uses antenna arrays in the stations, suggesting the possibility of locating a phone using the AOA with multiple transmission-reception points [20]. A work even suggested using two satellites to passively locate an object on the surface of the Earth using TDOA, Frequency Difference Of Arrival (FDOA), and AOA measurements [21]. Figure 2 shows our implementation of an antenna array. Figure 1: Images of the SUCHAI-2 and -3 CubeSats during the integration process together with part of the development team at the Space and Planetary Exploration Laboratory (SPEL). The launch of these CubeSats is scheduled for 2022. They will carry magnetometers and a communication system to receive the data from two femto-satellites. Each of these femto-satellites will also carry a magnetometer. The femto-satellites will be carried and deployed by the SUCHAI-3 CubeSat. We are studying the feasibility of estimating the location of a radio source (femto-satellite) in a concrete scenario, in low earth orbit where there are only two CubeSats available to estimate the location of two femto-satellites in a future mission. Figure 2: We are currently developing an antenna array able to fit in one face of a CubeSat. This figure shows a four-antenna array located at the bottom of the satellite, near the deployment switches. Another antenna array will be on the side of the CubeSat. We will deploy the femto-satellites through the hole of one of the antenna arrays. We based our work on [PERSON] et al. [22] localization method. [PERSON] et al. estimated the location of a source using two stations in fixed and well-known positions. Our work studies the accuracy of this method when the stations are moving and have uncertainty in their positions.