View source for HacDC Spaceblimp 6/Projects/Science Payload: Radiation & FLASH memory

'''Science Payload: Radiation & Flash Memory'''

'''Introduction: Cosmic Rays'''

This is an experiment to detect and determine the vulnerability of contemporary consumer Flash memory media (eg SD cards, USB thumbdrives, SSDs, etc) to cosmic rays. This is useful to know whether we can rely on such media for future Spaceblimp payloads and in general whether this media is suitable and reliable for high altitude applications (weather balloon payloads, aircraft, low-earth orbit spacecraft, etc). Cosmic rays are high-energy particles, largely protons with energies 200MeV to 500MeV that bombard the Earth from all directions. 99.99% of the cosmic rays that reach the stratosphere (30km) are absorbed by air before reaching the ground (sea level). The ones that do reach the earth's surface are energetic enough to penetrate electronics, people and buildings largely unnoticed except by the occasional memory glitch or bad pixel in a photo. High-reliability servers use error-correcting memory to deal with errors caused by cosmic rays that might cause errors on cheaper consumer-grade computer systems. Electronics for critical applications like aircraft, spacecraft and even some medical devices must be either specially made as radiation-resistant chips or the systems designed to cope with occasional errors from cosmic rays.

Although the susceptibility of computer memory to cosmic rays has been tested before, technology changes quickly so there is value in repeating these experiments with contemporary technology. The nature, energy and density of cosmic rays is constant but as the components of computer systems are scaled ever further down (following Moore's Law), the smaller components are more likely to have their function disrupted by a cosmic ray hit. At the same time, the larger density of components means an ever smaller fraction of those components will hit by cosmic rays in any given time. 

'''Introduction: Flash Memory'''

Flash memory stores data as charge in a floating gate. Charge can be added or removed from the floating gate by quantum tunneling when a large voltage is applied. The charge stored on the gate affects the conductance of the transistor channel below it. The value zero (0) is the high voltage state and one (1) is the low-voltage state. Modern flash uses multi-level cells with various possible voltage values to store several bits per transistor. 
There is a great description of Flash memory here: 

http://www.anandtech.com/show/5067/understanding-tlc-nand/2
http://www.eetimes.com/author.asp?doc_id=1327904

Flash memory also has error-correction mechanisms in place to compensate for manufacturing flaws and wear. This experiment will test the media with all the normal error correction mechanisms in place, so is not a raw measure of bit errors. It also tests only the susceptibility of data stored statically on the media and does not test the CMOS readout circuitry in which cosmic rays could also produce errors during active data read.

'''Results & Discussion'''
Four contemporary microSD cards (16GB and 32GB) were filled with zero values (the highest voltage per cell) and lifted to 104,000 ft, spending about one hour above 35,000ft. The flight profile is shown below (pending data). The cosmic ray dose, based on literature measurements of flux at various altitudes is estimated (pending data). This is the equivalent dose of XX hours at ground level.

There were no errors on any of the four cards. There were 884,431,912,960 zero values before and after the flight. The probability of bit-flips during the exposure is therefore less than 1 in 8.84E11 per hour (pending exact flight time). It appears data stored on contemporary 16GB and 32GB microSD cards are not highly susceptible to data corruption by cosmic rays at high altitude. This measurement of static data does not test the effect of cosmic ray flux during active readout, where CMOS circuitry must operate correctly. However, since the readout errors are expected to be random from one read to the next, this result already points to possible mitigation strategies in case of CMOS readout errors. For example, the data could be read out multiple times and checked for errors before being utilized. Future radiation effects experiments on the Spaceblimp platform will test contemporary CMOS microprocessors and dynamic memory. 

'''Method & Details'''

We acquired four different reasonably contemporary microSD cards and wrote as many zeroes (0) as would fit, placing as many floating gates at their maximum voltage values. We read out the values to verify all the zeroes were written correctly at ground level before the launch of Spaceblimp6. The four cards were  placed in the Spaceblimp6 payload capsule and lifted to the stratosphere over a period of three hours. The flight reached 104,000ft following the flight profile shown below (pending data) before parachuting back to ground level. The microSD cards were recovered and immediately checked for errors by counting the number of zeroes present.


Methods - Writing and Checking Zero Values

   dd if=/dev/zeroes of=/dev/mmcblk0
   hexdump -C /dev/mmcblk0

The hexdump output prints the first line, represents identical lines with a * and outputs a final hex value.

Card 1 before (31914983424 bytes)
    00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    76e480000

Card 1 after (31914983424 bytes)
    00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    76e480000



Card 2 before (32010928128 bytes)
    00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    774000000

Card 2 after (32010928128 bytes)
    00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    774000000

Card 3 before (31104958464 bytes)
    00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    73e000000

Card 3 after (31104958464 bytes)
    00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    73e000000


Card 4 before (15523119104 bytes)
    00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    39d400000

Card 4 after (15523119104 bytes)
    00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    39d400000


1. 32GB SanDisk Ultra PLUS HC I, 6362CRAC40XD
2. 32GB Samsung EVO HC I, MB-MP32D, MBMPBGVEQDFW-F, KNATHNNRH628
3. 32GB PNY Elite HC I, E532G1637, TWLN003044812
4. 16GB MicroCenter Class 10 HC, E516GG1612, TWLKA63410A38