# InnCube--A Wireless Satellite Platform to Demonstrate Innovative Technologies
## 2 Mission
### Scientific and Technical Goals and Payloads
#### 2.4.6 Experiment for Precise Orbit Determination (EPISODE)  
Experiment for Precise Orbit Determination (EPISODE) is a payload consisting of a GNSS receiver and miniaturized laser retro-reflector (LRR). The development is done through coursework and student work. The corresponding hardware for the GNSS receiver and the retro-reflectors is custom developed with COTS components. EPISODEs insights will support Space Debris research at the Institute of Space Systems. It further enables the provision of precise position data from the InnoCube, which contributes to a safer space environment by allowing collision warnings to be more accurately evaluated and assessed. The GNSS system consists of a front-end receiver with antennas for reception of GNSS satellite signals and a Field Programmable Gate Array (FPGA) based processing unit for generating the pseudo-random code, within the GPS system called the C/A (coarse acquisition) code, and correlation with the input signals. EPISODE will be composed of low-cost commercial off the shelf components and shall provide tracking solutions and position measurement data to the ground station at regular intervals. The main objective of the payload design is to provide a compact and low power solution for small spacecraft. For compactness, better power routing and less signal loss, a 4-layer circuit board design was chosen. As the antenna is one of the biggest parts of the receiving front-end, it will be placed on the outer structure of the spacecraft, instead of being integrated on the board. This leads to a smaller board and greater signal strength. The front-end utilizes a GNSS chip, designed for both industrial purposes with high requirements as well as for broad end-users, with an entire receiver chain on the chip. Its frequency range covers the L1/E1, B1 and G1 bands for GPS, Galileo, BeiDou and GLONASS. Currently, the combined system of front-end and processing unit is designed. For initial code development and testing, an Artix 7 FPGA with a Nexys 4 development board is used. The front-end receives a GNSS signal as C/A- code, where each transmitter has a unique code. An identical code must be generated and overlaid with the received code to correlate the relating satellite and determine its pseudo-range. To enable a position determination, four signals need to be correlated. Figure 6 shows the necessary steps to determine the position from the acquired signal. To determine the position of the receiver, a triangulation problem has to be solved by the processing unit. The generation of the C/A code for GPS, using the developer FPGA, and matching to the received message was accomplished lately. Further developing steps include decoding of the navigation message and calculating the position data. The signal acquisition from Galileo and GLONASS satellites is implemented to compare different navigation systems. As an alternative, the highly accurate, independent and supportive method of laser ranging (SLR) will be used for orbit determination. Its main limitation is poor spatial and temporal coverage, but SLR measurements are unambiguous and directly related to the terrestrial reference system. This knowledge allows independent calibration and validation of the on-board GNSS hardware. A fundamental requirement for a laser ranging reflector is to provide observation through the global SLR station network with both high accuracy and sufficient signal strength. Improvements in laser ranging measurement technology make it appear promising to miniaturize the laser retro reflector of a suitable size for a CubeSat, enabling centimeter level measurements. The smaller the number of individual prisms within such a reflector, the higher the achievable accuracy. An ideal reflector would, therefore, consist of only one individual prism. However, due to the limited field of view, this solution is not possible for low-flying satellites.