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

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.

Chips in Space: Let’s look inside ARISSat-1(part 2)

Chips in Space: Let’s look inside ARISSat-1 (part 2)

Welcome to Part 2 of our dive into each of ARISSat-1’s subsystems, where I will focus on the solar and battery power systems that are managed by the Power Supply Unit board discussed last week. It’s going to take a total of three parts to cover everything, and next week’s blog post will wrap up the remaining subsystems.

ARISSat-1 has been in operation for two weeks now. Last week the battery failed, causing the satellite to go silent during eclipses. However, once its solar panels are back in the sun, the systems power up and it begins operation. The Mission Elapsed Time (MET) sent in the telemetry is reset to zero each time the satellite falls into eclipse. I explain more about the battery below. All other subsystems are working nominally.


X, Y and Z Axes Labeled

In previous blog posts, I showed you the cross-sectional drawing of ARISSat-1. Above, I am showing you the axes labeled. In the telemetry transmitted down, several of the subsystems are labeled -X, +X, -Y, +Y, -Z and +Z. The axes are often associated with the solar panels, MPPTs and cameras. This way you will know which way is up (and for those that have read Ender’s Game, remember, the enemies’ gate is down).


Solar panel on the - X axis

Solar panels
The six solar panels were donated to the project by NASA. These are space-rated solar panels that were left over after the Small Explorer (SMEX) satellite program ended. Each panel measures 19-inches by 10.5-inches and consists of 50 cells. In full sunlight, each panel can produce about 50 volts open-circuit and more than 19 watts of electrical power. A panel is mounted on each of the six surfaces of the space frame.


Maximum Peak Power Tracker (MPPT)

Maximum Peak Power Trackers (MPPTs)
The six MPPTs are intelligent, SEPIC, switching power supplies that will either boost up the solar-panel voltage (when the sun is low) or buck down the solar-panel voltage (when the sun is high) to the battery voltage of approximately 28 volts. The minimum operating voltage is 15 volts, and the maximum is 100 volts. The peak operating point of the panels is specified as 45 volts. So far, telemetry is showing that the panels are running between 15 and 46 volts. Each MPPT uses a PIC16F690 8-bit MCU as its SMPS controller. The algorithm running in the PIC16F690s quickly shifts the operating point of the SEPIC up and down, to hunt for the peak power point. As the satellite was expected to tumble, the MPPTs must hunt very quickly and then track the fast-moving peak power point. Each panel is expected to have six seconds between sunrise and sunset.


Silver-Zinc (AgZn) type 825M3 battery

Battery
The battery was donated by RSC Energia. This is a type 825M3, and is the same exact type used to power the Russian Orlan space suits. It internally consists of eighteen rechargeable, Silver-Zinc (AgZn) cells and is specified for 14 ampere hours at 28 volts.

The battery has failed much earlier than we expected. This particular battery is not constructed to be a long-life battery. It is only rated for five charge cycles. We knew this going in. Since this was the battery we were given, we did our best to prolong its life through shallow charging. Now the battery is not holding a charge, and thus goes silent and resets all of its circuitry during eclipse. When ARISSat-1 returns to sunlight, the satellite begins operating after the 16-minute safety timers expire. The Mission Elapsed Time (MET) telemetry is reset to zero each time. This can be seen by monitoring the telemetry.

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

Chips in Space: Let’s look inside ARISSat-1 (Part 3)

Welcome to my third and final chapter on the ARISSat-1’s subsystems, which covers all of the Cs:  communications, cameras, control and cabling, along with the university experiment that hitched a ride. Next week’s blog post will begin a discussion of the challenges we encountered while designing the satellite—and how we solved them—followed in later posts by a summary of the lessons learned from ARISSat-1’s deployment and operation.

ARISSat-1 has been in operation for three weeks, now. The most up-to-date status information can be read at http://www.arissat1.org/v3/ and the AMSAT Bulletin Board. The battery is surely dead. ARISSat-1 orbits the Earth every 90 minutes. On each orbit, when it enters eclipse, no power is generated by the solar panels and the systems effectively reset. Otherwise, operations continue to be nominal.

Let’s finish up the description of the subsystems…


Interior View of the Receiver RF PCB

RF
The RF module has a 2-meter-band communications transmitter for the downlink, and produces a total of 500 milliwatts of power. The input to the downlink transmitter is a 10.7 MHz intermediate frequency (IF) signal that is generated by the Software Defined Transponder (SDX). (See Part 1 for more info on the SDX.)The RF module also has a 70-centimeter-band communications receiver, and its output is a 10.7 MHz IF signal that is fed to the SDX.


Concept Drawing Showing the 2-meter Antenna at Top and 70-cm Antenna at Bottom
Antennas
There are two antennas. The 2-meter downlink antenna is mounted to the top, and the 70-cm uplink antenna is mounted to the bottom. As mentioned in my deployment update blog of August 3, 2011, the 70-cm antenna appears to be broken off in the video of the deployment. We may never know what happened to that antenna, but to our pleasant surprise, radio amateurs are still able to communicate with ARISSat-1 just fine using 1 Watt on the uplink.

Interior view of the camera module

CamerasWe used Hunt Electronics’ HTC-2N3 Series CCD Sensor type cameras. There are four cameras, each pointing in a different axis.  If you take a look at the ARISS SSTV Gallery site, note that the call sign RS01S is in four different colors:

•    Red: -Y pointing camera, side view, mirror reverses image
•    Green: +Z pointing camera, top view (you can see the tip of the 2-meter antenna)
•    Blue: -Z pointing camera, bottom view
•    Magenta: +Y pointing camera, side view, mirror reverses image

The output is NTSC video that is digitized by the four-channel video input processor on the Internal Housekeeping Unit (IHU), which was also discussed in Part 1.


Exterior view of control panel

Control panel

The control panel allowed the cosmonauts to activate the satellite. It is an important component of the safety system. Upon flipping the three toggle switches, power was applied to the satellite and the safety timers were enabled, giving the cosmonauts 16 minutes to safely deploy the satellite before it started transmitting.


One of the First Pictures ARISSat-1 Took (captured by Mike Rupprecht, DK3WN)

Once ARISSat-1 was powered up, it started taking pictures. Two of the photos captured the cosmonauts handling the satellite. Here’s one that was captured by Mike Rupprecht, DK3WN, of Germany.


Photo of the Kursk University experiment

Kursk experiment
To the right of the control panel on the top plate is the Kursk science experiment. This experiment was developed by students at the Kursk State University in Russia, and is intended to measure the vacuum of space. The experiment was started 30 minutes after deployment, and will run once each day for a complete orbit. Telemetry from the experiment is transmitted on the downlink.


Interior View of ARISSat-1 Showing the Cabling

Cabling
No satellite is complete without cabling. Cables are something that you wish to minimize because they are not easy to assemble, are very labor intensive and take a long time to assemble. They are also prone to vibration failures, if not carefully laced with connectors secured in place. The cable harness was handmade. Individual strands of insulated wire and connectors were assembled according to the length of the cable run and the placement of the connectors. This makes for a nice, neat installation. It also facilitates the cable-harness tie downs, which keep the cable harness in place.