FAQ

1. What is OBDH? What is it for?

The On-Board Data Handling (OBDH) is a subsystem of a satellite which carries and stores data. Its main tasks are to synchronize actions and peripheral readings, save and manage the data collected from the entire satellite and send this data to TTC. To perform these functions, OBDH needs some crucial features such as a relative large non-volatile memory to retain the data and a Real-Time Operating System(RTOS) to synchronize these periodic tasks.

2. Why are we using msp430?

This family of microcontrollers brings several advantages to the Floripasat team when it comes to building a nano satellite. Ultra-low-power design, a robust and flexible system (I²C interfaces, SPI, UART, internal flash memory, timers, ADC ...) and a step-by-step debugging interface through JTAG are some of these features that make the MSP430 a good MCU option for the Floripasat modules. In addition, these microcontrollers have already been used and validated in space applications, which allows greater reliability for this purpose.

3. What failproof techniques are in OBDH? How do they work?

OBDH's failure strategy reflects both in hardware and software to ensure system reliability. At hardware level redundancies were projected on critical resources such as memory, watchdog timer and IMU. In software, the Cyclic Redundancy Check (CRC) was adopted in communications between modules for validation of data received and transmitted. Also, using a RTOS improves the reliability, since it has mechanisms to ensure that the timing requirements will be met. As a preventive action, there is a periodic reset that occurs every 12 hours.

4. Why store data in a non-volatile memory? How big that should be?

Since the main function of the satellite is to capture and send data, a non-volatile memory becomes crucial so that the data can be retained until they are sent to ground. Also, it is important to note that the size of this memory infers the amount of data that can be collected. Then, for a correct data storage, OBDH saves the data in memory in a cyclic way, i.e. when there is no more available space, the memory begins to overwrite the older data.

5. How much data can be stored in memory? How many days of data?

The OBDH has a SD Card which can stored a 2 GB (current size, but could be increased) of data, as explained above. The data of the system is stored in a rate of 1 block (512 bytes) per second, this means that over time, the memory will be full of data in 45 days. For more details about this memory time cycle and how the memory is organized click here.

6. What is RTOS? Why we are using a RTOS?

A Real Time Operating System(RTOS) is an operating system intended to serve real-time applications that process data as it comes in without buffer delays. The OBDH has a RTOS for synchronize and coordinate its tasks that has several time constraints. In other words, the RTOS ensures that the tasks are executed in specific intervals and manages possible conflicts of resources (peripherals, communication channels and others, ...). For more, click here.

7. Why have an external Watchdog timer? Why it has a voltage monitor?

The OBDH has an external Watchdog timer, that cause a reboot if the main execution exceeds 1 second, to work in redundancy with the microcontroller one. In addition to this function, the external Watchdog has a voltage monitor to ensure that the OBDH is working above a 2.6V threshold. This constraints consists in a strategy to avoid deadlocks (software lock in some code stretch) and to ensure correct data interpretations.

8. Why IMU is important and why use 2 of them?

The IMU, through an accelerometer, a gyroscope and a magnetometer, captures important informations about the CubeSat's rotation and the earth's gravity and magnetic fields. Furthermore, two IMU's are used to have redundant readings and ensure accurate data. This data is useful to the Attitude Control and Determination System (ADCS).

9. Which protocols are used to make the communications with EPS and TTC?

The FloripaSat team developed a protocol for communications between modules, the FloripaSat Protocol (FSP). In both cases, the OBDH module uses the FSP to establish communication. With the TTC is used a SPI channel made by software through GPIO's to communicate with the MCU and a hardware SPI to communicates with the radio. With the EPS an I²C channel. Click in the topics for more informations, FSP, Radio interface, Beacon interface, EPS interface.

10. How does OBDH send data to Ground Stations?

The OBDH retains all relevant data generated by the CubeSat. When this data needs to be sent to the ground station, the module has two options: send through the main radio or send data to the TTC MCU for be sent through the Beacon signal. Through the radio, the last block of data is sent periodically and more data blocks can be sent when requested by the ground station. Over the Beacon, only some data is sent periodically.

11. What is a shutdown mode? How is it done?

The shutdown mode is a state when the CubeSat ceases its transmissions for 24 hours. In this state, the modules continue to operate normally, except for the TTC that does not perform the sending operations. The CubeSat enters the shutdown mode when it is requested by a ground station through a public command. This has been done to comply with regulations that seek to reduce possible radio interference caused by excessive and uncontrolled communications.

12. How much is the OBDH power budget?

The OBDH power budget can be splitted on static, typical components consumption, and dynamic, feature execution that changes the consumption. This separation uses worst case cenarios to allow the board power behavior description and estimate peaks of consumption. The microcontroller and IMUs are the most significant consumers of the static segment and the memories represents the most relevant peaks in the dynamic budgets. Also, the OBDH has an important indirect quota of consumption through the radios. So, the maximum estimated OBDH static power budget is approximately 0,11W. Adding the dynamic power budget we get approximately 0,24W of mean consumption and could reach 2,59W in a peak (considering the indirect radio consumption).