PICPOT: A NANOSATELLITE FROM TURIN POLYTECHNIC

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PICPOT: A NANOSATELLITE FROM TURIN POLYTECHNIC S. Speretta (1), L. Reyneri (2), C. Sansoé (2), C. Passerone (2) (1) Politecnico di Torino, Corso Duca degli Abruzzi 24 10129 Torino Italy, stefano.

1 PICPOT: A NANOSATELLITE FROM TURIN POLYTECHNIC S. Speretta (1), L. Reyneri (2), C. Sansoé (2), C. Passerone (2) (1) Politecnico di Torino, Corso Duca degli Abruzzi Torino Italy, stefano.speretta@gmail.com (2) Politecnico di Torino, Corso Duca degli Abruzzi Torino Italy, leonardo.reyneri@polito.it ABSTRACT In 2004 the Turin Polytechnic started the PiCPoT project that led to the development of a nanosatellite. PiCPoT had been completely built by students of several engineering departments. The main constraint of this project was its cost, so every system was built using Commercial Off-The-Shelf (COTS) components and radiation-hardened by design. Since most components use C-MOS technology particular attention had been focused on creating an anti latch-up system that monitors the supply current of all ICs. The main goals of this satellite are the transmission of on-board telemetry measures (solar panel and battery temperature, voltage and current) and photos taken with commercial cameras. 1. INTRODUCTION Picpot is the first nanosatellite developed in the Turin Polytechnic. Since June 2004 the project had been carried out by students of different engineering departments, in particular Aeronautics and Electronics Department. From the beginning of the project more than 40 students took part to the development, construction and testing of the satellite. The main features of PiCPoT are: Size: 13cm x 13cm x 13cm Total power: less than 3 W Total mass 2.5 kg Orbit: sun-synchronous Altitude: 800 Km Mission minimum duration: 90 days No space-born components Redundancy is the key word for the project: in fact the satellite is divided into 2 independent sub-systems. Both have their own power supply (Li-Po and Ni-Cd batteries), OBC (On-Board Computer), time scheduler and RF module (437 MHz and 2.44 GHz). Solar power is granted by 5 GaAs solar panel with 5 MPPT (Maximum Power Point Tracker) to charge the batteries. The satellite hosts the following electronic boards: PowerSupply: it is dedicated to battery charging and analog signal conditioning (temperature, current, voltage). PowerSwitch: it has to schedule the power-up of the OBCs and control current consumption, to stop latch-up events. ProcA and ProcB: they are the two on-board processor, responsible of the acquisition of analog measures and data reception and transmission. TxRx: it is the RF module and is divided into two separate modules (437 MHz and 2.44 GHz) and handles RF modulation and demodulation. Payload: it is a board developed to take photos with three commercial cameras and to compress and store them. Figure 2 describes the structure of the satellite. 2. SATELLITE STABILIZATION Figure 1. PiCPoT At low orbits, the Earth magnetic field is strong enough to allow magnetic attitude control and we developed a passive control to reduce the power consumption of the system. Magnetic field lines over southern Europe have a 45 slope and passive control is expected to grant adequate direction control for the on-board cameras and antennas. Magnetic hysteresys bars are also used to increase the stability of the satellite.

Antennas are not influenced much by the spin-axis rotation since their irradiation diagram is symmetrical along that axis. 3.

2 Spin-axis rotation could not be reduced with passive control, so we developed an active control. A motor, controlled by ProcB board, with a reaction wheel is used to reduce the angular velocity while shooting photos with the on-board cameras. Antennas are not influenced much by the spin-axis rotation since their irradiation diagram is symmetrical along that axis. 3. ON-BOARD SYSTEMS On-board systems are: PowerSupply, PowerSwitch, ProcA, ProcB, Payload and TxRx. conversion units are hysteretic switching power supplies. We have chosen that type of supply to reduce complexity, also considering that we could not use COTS supply, as they all were based on C-MOS technology, therefore sensitive to latch-up. The battery charging process is controlled by both board processors to increase reliability. This board also houses all the analog signal acquisition: batteries and solar panels voltages, currents and temperatures are acquired and sent to both processor boards for the transmission to the ground station. We paid particular attention to the measure of supply currents of all C-MOS ICs to ensure latch-up free operation. In case of latch-up the power supply to the ICs is switched off until complete discharge of parastatic capacitance and then switched on again. The board is illustrated in Figure PowerSwitch This board is composed of two completely independent sub-systems (one for every OBC) with the following tasks: Select the battery to power the corresponding board processor Voltage regulation for OBCs Schedule the power up of OBCs Latch-up events count Figure 2. PiCPoT Structure The design of every sub-system followed different project strategies in order to maximize the chance of success of the mission. The first sub-system (connected to OBC A) uses a Microchip PIC microcontroller, while the other sub-system uses a Texas Instrument MSP430. The most important criterion of selection of the microcontrollers was the power consumption, since these processors are always active and the power budget is quite critical due to the extremely low solar panel surface PowerSupply This is the EPS system (Electronic Power Supply), which is responsible of powering all other sub-systems. The main sources of power are 6 batteries: 2 x 7.2V 900mAh Ni-Cd pack 4 x 7.2V 1500mAh Li-Po pack Batteries are divided into 2 groups (1 Ni-Cd and 2 Li-Po packs) which power the corresponding processor. Five GaAs solar panels (nominal voltage 4.9V, peak power 1.9W) are also present: all of them have a dedicated MPPT (Maximum Power Point Tracker) for redundancy. Generated power is used to recharge battery and to power the sub-systems. The power Figure 3. PowerSupply Board

3 Figure 4. PowerSwitch Board At power-up (when the satellite is separated from the launch pod) both PowerSwitch processors start a wait cycle of 5 minutes not to interfere with launcher electronic systems. After that every OBC is turned on for a maximum of 55 seconds over one minute. This way every OBC will not be influenced by SEU (Single Event Upset) occurred in the previous cycle (FLASH memory is considered not affected by radiation in an expected life period of 90 days). By request of OBCs, they could be turned off before the maximum available time of 55 seconds in case all the operation had been completed (this is important to extend battery life). The two processors power on cycles are time shifted by 30 seconds, in this way both transmitters should not interfere with each other, keeping also the peak power consumption low. Together with board processors, PowerSwitch powers also the transmitters and monitors their current consumption: a threshold comparator monitors the current and switches off both systems in case of excessive consumption (in case of latch-up, for example). When a latch-up event happens, a software event counter is incremented and the result is sent to the corresponding OBC to transmit it to the ground station. Also PowerSwitch processors are protected against latch-up. All data are collected to create housekeeping telemetry packets. From these packets, an extended telemetry packet is created which collects minimum, maximum, mean values and standard deviation of all the telemetry channels. Data are stored in a serial FERAM (Ferroelectric RAM), in order to test this new type of memory in space environment.after these operations the processor has to check for battery status and select the battery to charge with solar panels. After completing all housekeeping operations, the processor waits for incoming command from ground station for 5 seconds: if a correct command is received the corresponding operation is performed, otherwise the housekeeping telemetry packet is sent as a beacon and the Switch-Off command is sent to the PowerSwitch board to turn the OBC off. Here is a list of the commands the processor accepts: Send housekeeping telemetry Send extended telemetry Reset extended telemetry Shoot a photo Send a complete photograph Send one block of the photograph As can be seen from command list, this board is able to control the Payload board and to retrieve data from it. For a complete explanation of the functionalities and the image encoding and storage capabilities of that board see section 3.5. All telemetry streams are based on standard APRS protocol to ensure compatibility with commercial TNC ProcB This is the second board processor sub-system, built with a Texas Instrument MSP430. Although the two processors have similar functionalities, their design is quite different, to ensure a fault free operation of at least one of them ProcA This is one of the two board processor and is based on a Chipcon CC1010 (8051 core) with an integrated RF transceiver (437 MHz). The processor is responsible of the acquisition of analog signals coming from PowerSupply board and the number of latch-up events coming from PowerSwitch board. An analog multiplexer selects the signal that should be acquired using the integrated AD converter of the OBC; the multiplexer is controlled by the processor through an address decoder implemented with a Xilinx CPLD, mainly to test it in space environment. Figure 5. ProcA Board

Moreover, this board is connected to an electric motor with a reaction wheel to add active spin-axis stabilization: this motor can be controlled from the ground with appropriated commands.

4 While ProcA stores telemetry data in a FERAM device, ProcB uses the integrated FLASH memory of its microcontroller to save space in the board. ProcA has also an integrated RF transceiver, while ProcB uses an external transceiver, housed in TxRx board. Moreover, this board is connected to an electric motor with a reaction wheel to add active spin-axis stabilization: this motor can be controlled from the ground with appropriated commands. As ProcA, this board is capable of controlling the Payload board. Commands for this processor are exactly the same as for ProcA, with the only exception of the reaction wheel command, which is not implemented in the first board processor. Telemetry streams are encoded according to the APRS standard protocol. in a 2Mbyte FLASH, which is sufficient for holding the program and 5 different pictures. Before compression, the image is divided into 9 blocks and every block is separately encoded and stored. The advantages of this strategy are two-fold: i) avoid retrasmitting the whole image if any error occurs, as only the affected blocks will need to be resent; ii) allow a partial trasmission of an image when the trasmission window during a satellite pass over the ground station is too short (a 128 kbyte picture requires around 100 seconds at 10kbit/s, excluding the APRS overhead). When the ground station requests the transmission of the whole image, board processors send all the blocks of the image in sequence. This board is powered by two different SMPS (Switching Mode Power Supply) housed in ProcA and ProcB boards and communicates with them with an RS- 232 and SPI link respectively, for redundancy Payload Figure 6. ProcB Board The board is responsible of acquiring images of the Earth, compressing them using JPEG format, and storing them into non-volatile memory for subsequent transmission under ProcA or ProcB command. The board is designed around the Analog Devices Blackfin DSP and is connected to 3 commercial video cameras, with different focal length (see Table 1). The analog video stream is digitized using a low-power Texas Instruments video decoder IC, connected to the DMA channel of the DSP to ensure fast data transfer. Camera Focal Length Ground FOV Resolution T1 3.6 mm 750 km 800 m / pixel T2 6 mm 450 km 500 m / pixel T3 16 mm 170 km 180 m / pixel Table 1. Cameras resolution A 720x576 color frame is copied in ram (SDRAM or SRAM for redundancy), compressed in JPEG and stored 3.6. TxRx Figure 7. Payload Board The RF link had been studied with much attention since power consumption should be limited and antennas had to be very small to fit in the design. The 437 MHz channel is made of a power amplifier and a low noise amplifier: the modulator/demodulator is integrated in the board processor A (CC1010). The other channel is composed of 2 Chipcon CC2400 Transceiver, a low noise amplifier and a power amplifier. This is the best configuration for reduced RF power loss in transmission and sensibility loss in reception. RF Channel Sensitivity Output Power Bitrate 437 MHz -115 dbm 30 dbm 9.6 kbps 2.44 GHz -118 dbm 26 dbm 10 kbps Table 2. RF Power Budget

4. ASSEMBLY AND TESTING Students with professors supervision assembled all the and in particular soldered all the electronic circuits. Figure 9. 437 MHz antenna Figure 7.

temperature and illumination tests. Figure 10. 2.44 GHz antenna 6. CONCLUSIONS Figure 8. Final steps in PiCPoT assembly process 5.

Both antennas (see Figure 9 and 10) are equipped with an automatic tracking system, controlled by a PC, to follow the satellite.

The frenquency compensation is calculated using satellite position and velocity, obtained from the NORAD TLE (Two Line Element) set.

5 4. ASSEMBLY AND TESTING Students with professors supervision assembled all the and in particular soldered all the electronic circuits. Figure MHz antenna Figure 7. TxRx Board System testing had been done at Turin Polytechnic in June and July 2006: functional testing took the greatest amount of time, but attention had been also paid to vibration, vacuum, temperature and illumination tests. Figure GHz antenna 6. CONCLUSIONS Figure 8. Final steps in PiCPoT assembly process 5. GROUND STATION Together with the satellite, a ground station was built and installed on the Polytechinc roof in Torino. Both antennas (see Figure 9 and 10) are equipped with an automatic tracking system, controlled by a PC, to follow the satellite. Furthermore a computer controls the frequency of the two transmitters to compensate Doppler effect. The frenquency compensation is calculated using satellite position and velocity, obtained from the NORAD TLE (Two Line Element) set. Two main strength points can identified in the project: the educative character and the reduced costs. All the students involved in the project acquired great experience in the development of complex systems, since all the development, assembly and testing had been carried out by them, with the guide of professors and researchers. The cost of the project is very low with respect to a typical space program. COTS components allowed a reduction in cost and an acceleration in development time, with further reduction in cost. PiCPoT is now complete and waiting for a successful launch to space. 7. REFERENCES 1. See 2. L. Reyneri, C. Sansoè, C. Passerone, M. Tranchero, D. Del Corso, Teaching Complex Electronic System Design: A Small Satellite As Case Study, EWME June , Stockolm Sweden 3. R. Twiggs, J. Puig and C. Turner, Cubesat, The Development and Launch Support Infrastructure for Eighteen Different Satellite Costumers on One Launch, Small Satellite Conference Proceedings, August

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