SubSystems

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ISTSAT-1 is a 1U (10cm cube) CubeSat composed of different subsystems that are fitted together into a flight module plus a payload module comprising a subsystem capable of carrying out the flight mission.

The flight module will comprise the following functional subsystems: sensors & actuators for attitude determination and control - ADCS; main controller - OBDH; power supply - EPS; communications processor - COM; modem, transceiver and antennas – RF. In the case of the ISTSAT-1 the ADCS and OBDH modules are integrated in a single physical subsystem (On-Board Computer – OBC). Also de COM and RF subsystems are joined as the TT&C subsystem. A Transponder and a GNSS Receiver are optional sub-sys-systems.

The following image displays a block diagram of the ISTSAT-1’s System Architecture.

i) Structure


The structure needs to provide a proper vessel for the different subsystems and the payload while being both durable and light. The main structure is composed of six rectangular isogrid panels and the aluminium used in the structure is the AA5052 H32.



ii) ADCS/OBDH


The purpose of the ADCS module is to constantly monitor and act on the satellite in order to remove external disturbances or to provide specific conditions for certain missions. Depending on the intended purpose it must initiate manoeuvres, detumbling and stabilization functions.

The ADS-B receiver payload is constituted by the receiver and corresponding antenna. It It is also responsible for the housekeeping task of the whole satellite. It is in charge of making periodical diagnostics of other modules and subsystems in addition of detecting and reporting failures when they occur. The system has permanent storage capability, using a flash memory, and is responsible for maintaining a time reference. It also gathers telemetry information on the status of all subsystems and supplies them to the beacon module.


iii) EPS


The electrical power subsystem uses five triple junction solar panels, two lithium rechargeable batteries and an in-house designed controller and supervisor system. In the first power processing stage the input voltage coming from the solar panels is converted to an appropriate intermediate voltage to charge the batteries and to provide a direct voltage to the S/C. The second stage involves the output voltage regulators and current limiting systems that will feed all the satellite subsystems.





iv) TT&C


ISTSAT-1 is equipped with a CW (OOK/ASK) tracking beacon, which periodically transmits a synopsis telemetry frame in Morse code. The telemetry data is sent periodically using the AX.25 protocol on the downlink VHF channel , and is a much more complete set of data (cubesat status) than the data conveyed by the beacon. The communications subsystem comprises the COM board, the multi-mode modem and the radio module. COM is responsible for processing all input/output digital data for the radio link. The modem is a multi-mode device capable of several digital modulation schemes such as FSK, BPSK, QPSK and GMSK. The radio module contains the beacon transmitter, a VHF transmitter (downlink) and a UHF receiver (uplink). The maximum RF output power is 0.5W.



v) ADS-B Payload


provides the ability to receive, decode and process the ADS-B messages transmitted by the aircraft. It includes an high sensitivity, low noise and low power dedicated receiver with and analog RF front-end and an FPGA for decoding the messages. The antenna, also being designed in-house, will be a circular polarization patch that will be placed in the nadir pointing face of the satellite. Being ADS-B a popular topic among amateur radio community and hobbyists, it is foreseen that the collected ADS-B messages can be made available to the community.


vi) Ground Segment




The objectives of the GS are to support: i) the retrieval of data from the satellite, both in the context of maintenance and mission; ii) the posterior analysis of this data by personnel on the ground; iii) the delivery of instructions or orders to the satellite.