• Abstract
• Introduction
• Oersted Memory Circuits
• Oersted High-Energy Charged-Particle Detector Experiment
• Oersted Observations
• CPD Data
• SIM Blinding
• Discussions
• Modeling of Bit Flip Events
• Conclusions

Introduction.

This note describes some of the radiation-induced malfunctions experienced by the instruments onboard the Danish Oersted satellite launched into a near-polar orbit on 23 February 1999. The orbit is slightly eccentric with the height above ground varying between 640 and 880 km. At launch the ascending node was at 14:11 LT but drifts toward morning at 0.88 min/day. The satellite is gravity-gradient stabilized such that the 8-m boom is always pointing vertically upward. The rotation around the boom axis is controlled such that the SIM star imager co-located with the CSC vector magnetometer in the gondola look away from the sun.

Oersted CDH Memory Circuit

The satellite carries two main computers, CDH1 and CDH2, who are partially redundant and have separate memory circuits. These memory units are protected by Error Detection And Correction (EDAC) systems which adds cyclic redundancy check to the data bytes. In case of an erroneous bit flip the EDAC system may detect and correct the error. If undetected or not corrected, such an error may cause the computer to halt with a memory parity error. The memory circuit is shown to the right (click on the figure or here for a larger version).
In addition to checking every Read-Write operation the EDAC system scans through the memory independent of the computer processor. The cycle lasts 10 minutes. When an error has occurred the time of detection is noted. However, the time of the error event remains uncertain to within the 10-min cycle.

There are two main causes for erroneous bit flips in spacecraft memory circuits:

High-energy particles penetrating through the enclosures (incl. spacecraft structure). This type of problem is typically encountered in the inner radiation belt characterized by intense fluxes of hard radiation (in the MeV range).

Electrical impulses from sparking discharge of accumulated dielectric charging within the insulation of cables or other poorly conducting compounds, This problem is typically met in the auroral zone during events of intense precipitation of energetic electrons (in the tens of keV range).

Oersted High-Energy Charged-Particle Detector Experiment

The Orsted High-Energy Charged-Particle Detector (CPD) experiment aims at detecting high-energy radiation in a Low-Earth Orbit (LEO) of the following kinds and energies:

- Electrons : 50 keV - 1 MeV
- Protons : 250 keV - 30 MeV
- Alfa particles: 1-100 MeV

The experiment uses an array of 6 solid-state silicon detectors with different shieldings and depletion depths. The amplified signals from particles entering the detectors are connected to pulse-height analyzers to provide energy resolution of the detected particles. Four of the analyzer units have 8 discriminator level; two have 4 levels only. The counts in the total of 40 different channels are collected by the experiment computer, that communicate the data and status to and receives commands from the satellite Central Data Handling (CDH) computer.
The detectors with their high-voltage power supplies, the pulse amplifiers, the pulse-height analyzers counter circuits and the experiment computer are all built into a box that may fuction as a separate unit. It will need standard supply voltages and it communicates data and commands with an external computer system via a RS485 signal line.

Four of the CPD particle detectors look vertically upward along the 8-m boom while the remaining two look horizontally to the side. The detectors look through cut-aways in the top and side polar panels. The pitch angle for each detector can be determined from the vector magnetometer measurements combined with the rotation of the boom-mounted gondola with respect to the satellite body. In near-polar regions (above some 60 deg in latitude) with the high inclination of the geomagnetic field the 4 upward-looking detectors will all look into the loss cone which at the altitude of the satellite (around 700 km) is 60 deg. The 2 horizontally-looking detectors, correspondingly, will always look at the radiation outside the loss cone. For further technical info click here

Observations of Particle Radiation and EDAC Effects

The EDAC observations through March-August 1999 are displayed in the world map diagram below. The diagram also presents counting rates for one of the CPD detector channels. The time of the occurrence of the event is uncertain to within the EDAC cycle period of 10 min. Each case has been marked by plotting a large dot, circular for CDH1 and square for CDH2, corresponding to the approximate satellite location at the EDAC event. From the map three regions are distinguished:

The South-Atlantic anomaly region. (majority of cases)

The northern and southern auroral regions.

Other regions

CPD Data

The CPD observations are illustrated in the figure. The top fields display data from the two upward looking detectors shielded by 1 mm Al and 1 mm Cu, respectively. The middle field displays data from three of the channels of the upward looking detector shielded by 1.25 um Ni while the bottom field displays data from the corresponding side-looking detector.
The horizontal time axis spans a semi-orbit from equator to equator. The region void of radiation in the middle of the diagrams is the polar cap delimited by the auroral zones.
The occurrence of a typical "hard" EDAC event occurring at the South-Atlantic Anomaly (SAA) region is marked by the vertical dashed line. Note here the high count rates in the shielded detectors (I1, I2)
The CPD observations for the same specific channels (out of the 40 available channels) are displayed in the figure to the right for a typical "soft" EDAC event occurring in the auroral regions. Note now the low count rates for the shielded detectors (I1, I2). In this case the EDAC event occurs at a spike in the lowermost energy channel which could indicate the occurrence of "splash" of auroral particle radiation (few keV). This would be a situation where dielectric charging of cables and other external insulating parts might occur. In such cases the possible sparking discharging may cause disturbing electrical pulses in the cabling harness.
SIM Blinding

An example of corresponding problems encountered by other units of the Ørsted satellite is presented by the Star Imager (SIM) system used to provide the very precise attitude needed for the geomagnetic mapping mission. The construction of the SIM sensor system is shown in the schematic drawing of a cross section of the boom-mounted gondola displayed in the leftmost figure. Presumably the radiation penetrates the aluminum cylinder enclosing the SIM and CSC sensor units and hits the CCD chip from the rear rather than penetrating the lenses constituting the optical system.
The SIM system uses a CCD camera chip and optics to generate an image of the sky. The images are processed by a PC-486 computer to recognize the star constellation in view. The system is sensitive to the above space conditions in several ways. In addition to the problems experienced by the electronic circuits, the CCD chip itself is exposed to the radiation which may cause the appearance of "false stars" giving difficulties in the processing of the image. Too many false stars in the images may invalidate the SIM attitude data.

The SIM observations of radiation induced anomalies are illustrated in world maps like the example shown in the figure to the right for the interval from 1 to 10 September 1999. In these maps the Oersted orbit has been plotted in a geographical coordinate system with full line (pen down) when SIM data are valid. At invalid SIM data the pen was raised. The plot is made for nighttime passes only in order not to be influenced by the occasional blinding by sunlight.
A blank spot is seen in these data at a location a little below and to the left of the center of the diagram. This is the region of the South-Atlantic anomaly. There are invalid or missing SIM data at other locations but they do not form a clear pattern like the SAA region. The satellite also crosses through intense radiation at higher latitudes in the vicinity of the auroral zones. Here, apparently, the radiation is not hard enough to cause problems for the SIM instrument.

Discussions of Observations

The geographical distribution of EDAC events is clear from the world map in the above figure. The observations by the CPD instrument clearly reveals the dominant regions of energetic radiation, the South-Atlantic anomaly (SAA) region, and the northern and southern auroral zones. After analysis of the CPD data we can more precisely distinguish the cases on basis of the radiation characteristics.
In the South-Atlantic anomaly region the magnetic field is weaker than in other regions of the world with the result that the radiation belts come closer to the Earth and may reach down to the altitude of the satellite (640-880 km). For the SAA cases the CPD instrument measures high fluxes of hard, penetrating radiation which is most likely protons and electrons in the MeV range. For the auroral zone cases the CPD instrument measures low to moderate fluxes of high-energy radiation, much less than in the SAA cases. The charged particles causing the events are probably softer than the CPD range starting at 50 keV. Hence the events are more likely related to dielectric charging by intense fluxes of low energy particles than to penetrating high-energy particles.

There was this grouping based on data for 82 events from April-August,1999 :

SAA cases: 46 EDAC events

Auroral Zones: 21 EDAC events

Other regions: 15 EDAC events

Modeling of EDAC bit flip events

We have attempted to model the occurrences of bit flips detected by the EDAC system by using the radiation model of the ESA/NASA Space Environment Information System (SPENVIS) (Heynderickx, D., B. Quaghebeur, E. Speelman, Space Environment Information Service ( http://www.spenvis.oma.be/spenvis/ ).
For the S-RAM memory component Micron MT5C1008C-35-L883C we have a Single Event Upset (SEU) Linear Energy Transfer (LET) threshold of ~3 MeV cm2/mg and a radiation cross section of 2 10-2 cm2 . The CDH1 and CDH2 S-RAM memory circuits are mounted in electronics boxes at the middle of the satellite body. We have derived approximate shieldings for the two CDH units (3.50 and 2.0 g/cm2). Using the SPENVIS web service we have then derived the LET spectra for typical Ørsted satellite orbits. These spectra provide integral fluxes above the LET threshold of particles capable of producing SEU in the circuit. Multiplying these fluxes with the device cross section and the exposure solid angles gives the anticipated SEU rates. They are shown in the table below.

Table. Comparison of predicted (SPENVIS) and observed (Ørsted) Single Event Upsets