Chips in Space: Let’s look inside ARISSat-1 (part 1)

It’s been a little over one week since ARISSat-1 was deployed from the International Space Station and it has been operating beautifully. Radio amateurs are submitting signal reports, telemetry and Slow-Scan TV (SSTV) pictures. See for yourself at the ARISSat-1 SSTV Gallery.

Many of you are curious about what’s inside ARISSat-1. In the next two to three blog posts, I’ll introduce you to each of the subsystems. Following that, I’ll relate some of the challenges we faced in the development of these. After examining the design challenges and how we overcame them, I’ll conclude the blog by filling you in on what we are learning from ARISSat-1’s journey through space, as we analyze the telemetry and SSTV photos. Though this was designed as a limited series blog, I’ll post from time to time after that, as interesting developments occur with ARISSat-1, and when it eventually burns up in the Earth’s atmosphere.

Let’s get started…

A cross-sectional diagram of our satellite.

From the cross section diagram, the box labeled IHU contains four PCB assemblies:

• Inter-Connect Board (ICB)
• Power Supply Unit (PSU)
• Integrated Housekeeping Unit (IHU)
• Software Defined Transponder (SDX)

The 'Stack' – Inter Connect Board, Power Supply Unit, Integrated Housekeeping Unit and Software Defined Transponder

IHU Lid

'The Stack'
In the above photos you can see 'The Stack', as we have come to affectionately call it, mounted on the lid of a Hammond Manufacturing 1590F die-case aluminum box. Connectors are mounted to the lid.

Inter-Connect Board (ICB)
The ICB started as a passive assembly, serving as a signal distribution and mounting system for the rest of the assemblies. But it quickly gained the active components required to drive the latching relays and part of the safety system (you can see the relays to the left in the photo). The safety system is a series of interconnects and timers to inhibit the electrical system and radio transmitter. The timers were set to inhibit the transmitter for sixteen minutes. This gave the cosmonaut time to flip the switches and deploy the satellite before it started transmitting.

Bottom view of the Power Supply Unit (PSU)

Power Supply Unit (PSU)
Next up in 'The Stack' is the PSU. As the name suggests, the PSU’s job is all things power related. It interfaces to the six Maximum Peak Power Trackers or MPPT’s (more on them in a moment) and 28-volt Silver-Zinc battery. The PSU monitors the charging of the battery and the performance of the power supplies, and is the primary recovery system if the IHU or SDX "latch up" due to the space environment. The PSU regulates +5 volts for the experiments, IHU, SDX and onboard MCU, and +12 volts for the cameras.

The PSU is commanded by the IHU via a serial communications link. The IHU can turn on and off, via the PSU, the experiments (up to four of them) and the cameras. (ARISSat-1 only has one experiment aboard.) The PSU monitors voltages and current to the various subsystems, and these values are reported in the telemetry stream transmitted down to Earth.

There are two 8-bit microcontrollers on the PSU: a Microchip PIC16F887 manages all of the PSU functions while a PIC16F690 acts as a serial expander interface for the PIC16F887 to the six MPPTs. Communications to the MPPTs are done through serial RS-485 communications links.

Top view of the Integrated Housekeeping Unit (IHU)

Integrated Housekeeping Unit (IHU)
Continuing to move up “The Stack,” the IHU is the brains of the satellite. It sequences all the events of the satellite, such as when to take a picture, transmit a greeting from space, or turn on the experiments. The IHU microcontroller is a 32-bit Microchip PIC32MX. The IHU encodes telemetry into the BPSK bit-stream, provides the raw audio data for all the FM missions (Voice, SSTV) and generates the CW on-off key from telemetry and a stored list of call signs.

The camera circuitry is resident on the IHU board and consists of a four-channel video input processor from Philips-NXP-Trident Microsystems (SAA7113H), an Altera MAX II CPLD (EPM570T144C5) and a 16 MB SDRAM device from Micron Technology (MT48LC8M16A2). The cameras are off-the-shelf security cameras. To take a picture, the analog output of the cameras is fed into the video input processor where the photo is digitized and stored in the SDRAM. The CPLD is the glue logic between the video input processor, the SDRAM and the PIC32MX MCU.  The PIC32MX reads out of the SDRAM the colors and converts them to Robot 36 SSTV tones that are transmitted on the downlink to Earth.

Finally, the greetings from space were recorded, edited and stored on an SD memory card. The IHU PIC32MX retrieves them and sends them to the SDX via a serial peripheral interface (SPI) link for transmission.

Top view of the Software Defined Transponder (SDX)

Software Defined Transponder (SDX)
On the top of “The Stack” is the SDX. The SDX gets its name from a combination of Software Defined Radio technology and its function as a satellite transponder. The SDR technology is a quadrature sampling detector (QSD) on the uplink (receive) and quadrature sampling exciter (QSE) on the downlink (transmit).

The SDX interfaces to the RF receiver and transmitter subsystems via the 10.7 MHz intermediate frequency (IF). The IF is sampled up/down to audio baseband frequencies and digitized by a Texas Instruments TLV320AIC23BIPW CODEC. The actual radio modulation and demodulation functions are processed by a Microchip PIC32 MCU.