STP-H5-Radiation Hardened Electronic Memory Experiment (STP-H5 RHEME) - 06.20.18

Overview | Description | Applications | Operations | Results | Publications | Imagery

ISS Science for Everyone

Science Objectives for Everyone
Spacecraft are continually exposed to radiation from the sun and other cosmic sources, but radiation can harm spacecraft computers by causing glitches and erasing data in their memories. This can affect a spacecraft’s ability to work properly, including its ability to store data and communicate with ground controllers. The STP-H5-Radiation Hardened Electronic Memory Experiment (STP-H5 RHEME) studies how frequently electronic memory experiences a glitch after being struck by high-energy particles in space.
Science Results for Everyone
Information Pending

The following content was provided by Keith Avery, Ph.D., and is maintained in a database by the ISS Program Science Office.
Experiment Details


Principal Investigator(s)
Keith Avery, Ph.D., Air Force Research Laboratory, Kirtland AFB, NM, United States

Alonzo Vera, Ph.D., COSMIAC/University of New Mexico, Albuquerque, NM, United States
David Alexander, M.S., COSMIAC/University of New Mexico, Albuquerque, NM, United States

COSMIAC Research Center/ University of New Mexico, Albuquerque, NM, United States

Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)

Sponsoring Organization
Technology Demonstration Office (TDO)

Research Benefits
Space Exploration, Earth Benefits

ISS Expedition Duration
September 2016 - August 2018; -

Expeditions Assigned

Previous Missions
Information Pending

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Experiment Description

Research Overview

  • Electronic memories operating in space are exposed to high energy protons and galactic cosmic rays, which are atoms that have been stripped of their surrounding shell of electrons. When these particles strike a memory or other microcircuit, they create a very brief current pulse that can change the information store in a memory bit from a logical 1 to a 0, or vice versa. In some cases, multiple bits of memory can be changed. The loss of this information can cause the electronics to malfunction and jeopardize the mission. We need to know how frequently these errors occur, how many memory locations are affected, and how we can efficiently correct the errors.
  • The STP-H5-Radiation Hardened Electronic Memory Experiment (STP-H5 RHEME) instrument includes memories that were specifically designed to detect single errors and clusters of errors. Companion electronics count the number of errors and determine how often correction procedures need to be applied to prevent errors from being passed to other circuits.
  • The results of STP-H5 RHEME permit the design and building of memories that can detect and correct errors efficiently to eliminate errors in the spacecraft electronics without using more power than necessary.


The STP-H5-Radiation Hardened Electronic Memory Experiment (STP-H5 RHEME) instrument includes two types of SRAM (static random access memory) -- four 16 Mbit devices and a 72 Mbit device composed of a stack of four 18 Mbit SRAM. The memories were designed by Silicon Space Technology, Inc. for space applications. The mapping of the logical to the physical address is known for each memory type. The memories incorporate EDAC (error detection and correction) which may be turned on or off. The 16Mbit SRAMs incorporate a scrubbing option that may be turned on or off, and the scrub rate may be selected. The 18 Mbit SRAM incorporates EDAC and an automatic read and write back which effectively scrubs the memory if the entire memory space is read. Both types of memory have undergone cyclotron testing to determine an upset cross section as a function of ion LET (linear energy transfer). The characterizations have been done for the beam impinging at normal incidence to the memory and at angled incidence. The occurrence of multiple bit upsets as a function of LET and angle of incidence have been determined.
These terrestrial test results are compared with the on-orbit results to determine the fidelity of the terrestrial tests in simulating the space environment. The EDAC implementation allows for single error correction and double error detection. Its correction capability can be defeated if two or more bits in the memory word are corrupted. The multiple bit error can occur from independent ion strikes that upset bits in the same word. This accumulation of errors can be defeated by scrubbing (i.e., reading, correcting, and re-writing) the memory at a rate that is faster than error accumulation. The 16 Mbit SRAM has scrubbing circuitry built into the memory that may be activated at different scrub rates or turned off. Since the scrubbing procedure consumes power, it should be performed at the lowest possible rate needed to prevent error accumulation.
The STP-H5 RHEME experiment can vary the scrub rate and assess the effectiveness of scrubbing. The STP-H5 RHEME experiment is divided into four segments:  (1) no EDAC and no scrubbing, (2) EDAC and no scrubbing, (3) EDAC and low rate scrubbing, and (4) EDAC and high rate scrubbing. The results in each case are compared to the terrestrial tests. The identification and frequency of occurrence of multiple bit upsets from a single heavy ion strike are especially interesting. The frequency of upsets can be correlated to the environment through STP-H5 RHEME's dosimeter sensor developed by Radiation Monitoring Devices (RMD). It incorporates a proton/electron discriminating detector and is capable of determining the percentage of dose due to protons and electrons, respectively. Thus, there is an independent measure of the proton environment that will permit and separation of proton induced memory upset from galactic cosmic ray (i.e., heavy ion) upset rates.

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Space Applications
Spacecraft computers can experience problems after they are struck by tiny high-energy particles, such as protons and cosmic rays from the galaxy. These particles can delete computer memory, interrupt communications or cause other glitches. The STP-H5 RHEME investigation studies two radiation-hardened types of computer memory that can quickly detect errors caused by these particles. The memory can also recover from errors by correcting itself faster than the rate at which errors can pile up.

Earth Applications
Integrated circuits are getting smaller, which benefits consumers and industry but also makes the circuits more vulnerable to errors introduced by radiation and high-energy particles. This investigation provides new insight into error correction strategies, improving electronics and computers on Earth.

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Operational Requirements and Protocols

STP-H5 RHEME's dosimeter experiments generate data packets at irregular intervals, depending on the space radiation conditions at a point in time. This experiment is responsible for most of the data that STP-H5 RHEME generates. An average of 70KBytes per second of data from STP-H5 RHEME is expected. STP-H5 RHEME's memory experiment consist on interrogating the onboard SRAMs once every hour about any upset detected. This experiment generates about 1 Kbyte of data each time the memories are interrogated. Memory testing results are transmitted alongside the dosimeter data. To successfully complete both experiments STP-H5 RHEME should be facing open space, and data generated by both experiments should be transmitted down to Earth for its analysis. No return of samples or equipment is necessary.
STP-H5 RHEME has five modes of operation that are used at different periods of the mission. Upon booting, STP-H5 RHEME enters a default operation mode which includes having all onboard SRAM memories configured without scrubbing or EDAC protection and the dosimeter's data acquisition subsystem ready to gather and processes data. The other four modes consist of different combinations of memory configurations providing different levels of protection against radiation induced upsets. On all five modes the dosimeter experiment does not change its default operation. STP-H5 RHEME switches modes sequentially each six months throughout the mission's end of life. Changing STP-H5 RHEME's operation mode requires a command packet being sent to the experiment. Power cycling is not considered as a normal experiment procedure, but can be considered if the experiment fails at any point in time due to radiation induced upsets on the control and communication subsystems.

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Decadal Survey Recommendations

Information Pending

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Results/More Information

Information Pending

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Related Websites
The Radiation Hardened Electronic Memory Experiment (RHEME) is UNM’s first flight on the International Space Station.

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Front View RHEME Flight Experiment prior to lid attachment. Image courtesy of University of New Mexico COSMIAC Research Center.

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Top View of RHEME Flight Experiment prior to lid attachment. Image courtesy of University of New Mexico COSMIAC Research Center.

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NASA Image: ISS050E052652 - Space Test Program-H5 (STP-H5). Photo taken during Expedition 50.

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