Volume 16, Issue 10 p. 1561-1569
Research Article
Free Access

Five-Year Results From the Engineering Radiation Monitor and Solar Cell Monitor on the VanAllen Probes Mission

R. H. Maurer

Corresponding Author

R. H. Maurer

Space Sector, The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA

Correspondence to: R. H. Maurer,

[email protected]

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J. O. Goldsten

J. O. Goldsten

Space Sector, The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA

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M. H. Butler

M. H. Butler

Space Sector, The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA

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K. Fretz

K. Fretz

Space Sector, The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA

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First published: 24 September 2018
Citations: 3

Abstract

The Engineering Radiation Monitor measures dose, dose rate, and charging currents on the Van Allen Probes mission to study the dynamics of Earth's Van Allen radiation belts. Over 5 years, results from this monitor show a variation in dose rates with time, a correlation between the dosimeter and charging current data, and a comparison of cumulative dose to prelaunch modeling. Solar cell degradation monitor patches track the decrease in solar array output as displacement damage accumulates. The Solar Cell Monitor shows ~33% cumulative degradation in maximum power after 5.1 years of the mission. The desire to extend the mission to ~2,500 days from 800 days created increased requirements for the ionizing radiation hardness of spacecraft and science instrument electronics. We describe the investigations that insured compliance with these enhanced requirements.

Key Points

  • The Engineering Radiation Monitor dose and dose rate data versus time and shield depth are summarized
  • The Solar Cell Monitor shows ~33% cumulative degradation in maximum power after 5.1 years of the mission
  • The observatories electronic parts were qualified for a ~800-day mission; a radiation hardness survey gives a projection exceeding the 2,500-day extended mission goal

1 Introduction

The Van Allen Probes mission is part of NASA's Living With a Star Geospace program to explore fundamental processes that operate throughout the solar system, especially those that generate hazardous space weather near the Earth (Mauk et al., 2012). The two identical spacecraft are in nearly matching orbits with perigee of ∼600-km altitude, apogee of 5.8 RE geocentric, and inclination of 10°. The orbital period is ∼9 hr and a slight difference in orbital apogees causes their radial spacing to vary between ∼100 km and ∼5 RE. The probes access the most critical regions of the radiation belts. The mission launch date was 30 August 2012; the 5-year mission lifetime has allowed all local times to be studied repeatedly (Maurer et al., 2013).

Real-time monitoring of the Van Allen Probes dynamic radiation dose and dose rate near the location of sensitive electronics has been reported in the earlier Engineering Radiation Monitor (ERM) and solar monitor results (Maurer et al., 2013, after 260 days; Holmes-Siedle et al., 2014, after 365 days; and Maurer & Goldsten, 2016, after 971 days).

The present report covers 5.1 years (up to 1,858 days) of data from a GTO (Geosynchronous Transfer Orbit) through the Van Allen belts with an apogee in the outer belt. Sections 2 and 3 give very brief descriptions of the ERM and Solar Cell Monitor, respectively. More complete descriptions can be found in Goldsten et al. (2012) and Maurer et al. (2013). Section 4 contains the cumulative dose and dose rate data versus both elapsed mission time and shield depth. Section 4.1 discusses the fade of the Radiation-sensing Field Effect Transistors (RADFETs). Section 5 discusses the Solar Cell Monitor cumulative displacement damage results. Section 6 describes the ground-based effort to insure that the spacecraft and science instrument electronics could survive a substantial mission extension. Section 7 is the summary.

2 The Engineering Radiation Monitor

The Engineering Radiation Monitor (ERM) was developed as a replacement for a spacecraft balance mass and as an engineering experiment for the mission. The mass is 2.9 kg; the power 0.2 watts when operating; the dimensions are 18 × 18 × 6 cm. A detailed description of the ERM and its operation can be found in Goldsten et al. (2012). The ERMs are powered through the RBSPICE (Radiation Belt Storm Probe Ion Composition Experiment) instruments renamed as the Van Allen Probe Ion Composition Experiment), and the ERM data are in the RBSPICE data streams.

The ERM monitors total ionizing dose (TID) and dielectric charging current at each spacecraft in real time. The ERM contains an array of eight dosimeters and two buried conductive plates. The dosimeters are REM-Oxford Type RFT300 dual RADFETs (Holmes-Siedle & Adams, 2002; Holmes-Siedle et al., 2007, 2014) and are operated at zero bias (the gate held at 0 V) to preserve response when powered off. The extended range of the RADFETs is above 1,000 krad (Si) to avoid saturation.

Two large-area (∼10 cm2) charge monitor plates are set behind 1.0- and 3.8-mm-thick covers of aluminum to measure the dynamic currents of penetrating electrons (minimum electron energies of >0.7 and >3 MeV, respectively). The charge monitors can handle large current density (∼3,000 fA/cm2) with sufficient sensitivity (∼0.1 fA/cm2) to characterize quiescent conditions. High time resolution (5 s) monitoring allows detection of rapid changes in flux (Maurer et al., 2013).

3 Solar Cell Radiation Damage Monitor

The +Y and –Y solar array panels of both spacecraft have solar cell degradation patches located on them to ascertain the loss factors and power generation margins of the array throughout the mission. Each patch, constructed the same as the main array, has two 28.3-cm2, 28.5%-efficient, Emcore BTJ, InGaP/InGaAs/Ge solar cells connected in series and covered with a 500-micron cerium-doped microsheet with an indium tin oxide (ITO) coating. The cover glass adhesive is 0.1 mm of DC93–500. Additional details of the Solar Cell Damage Monitor can be found in Maurer et al. (2013).

4 On-Orbit Data Results From ERM for Total Dose Effects

Figure 1 shows the current monitor data from ERM-A. The current monitor data show the variation in activity or dose rate over the Van Allen Probes mission up to 1,675 days including the dose depth data from ERM-B until day 1487 and the dose [19.4 krads (Si)] at the 9-mm shield depth from ERM-A for 1,675 days. The dose at the near-surface minimum shield depth of 0.05 mm Al at the top of the figure is >1 Mrad. Surface materials on the spacecraft were tested to an exposure level of 10 Mrads. Figure 2 has the latest available (1,487 days) TID versus depth curve from ERM-B. Subsequent dose depth curves were not available due to the sticking relays in the ERM-B electronics. Both ERMs have data gaps in total dose due to sticking relays (see Figure 3); however, the charge collection data in the unbiased RADFETs continue to accumulate and can be retrieved the next time that the relays are unstuck. ERM-A was producing corrupted data shortly after launch (Fall 2012). Since then ERM-A has functioned as a charge monitor only due to a stuck relay that prevented the internal switching required to read the seven dosimeters on the radiation detector bench. ERM-B has functioned normally until ~October 2016 due to a reduced read out cadence implemented to extend the relays' lives.

Details are in the caption following the image
The 1,675-day dose [rads (Si)] versus mission time history from ERM-A (at only the 9 mm shield depth) and 1,487-day ERM-B RADFETs at the seven shallower shield depths. The dosimeters are mounted under covers of varying shield thickness (0.05 mm Al, 0.39 mm Mg, 0.78 mm Mg, 1.16 mm Mg, 1.55 mm Mg, 2.32 mm Mg, 4.66 mm Mg, 9.00 mm Al) to obtain a dose depth curve (see Figure 2) and to characterize the electron and proton contributions to total dose. The dose rate or current data at the bottom of Figure 1 is from ERM-A. The left ordinate is the dose in rads (Si) from the RADFETs; the right ordinate is the current in picoamperes from the current monitor plate behind 1 mm Al shielding. ERM = Engineering Radiation Monitor; RADFETs = Radiation-sensing Field Effect Transistors.
Details are in the caption following the image
The 1487-day dose [rads (Si)] versus depth curve from the ERM-B TID data. The dosimeters are mounted under covers of varying shield thickness (0.05 mm Al, 0.39 mm Mg, 0.78 mm Mg, 1.16 mm Mg, 1.55 mm Mg, 2.32 mm Mg, 4.66 mm Mg, 9.0 mm Al) to obtain this dose depth curve. ERM = Engineering Radiation Monitor.
Details are in the caption following the image
Example of the effect of sticking relays on the ERM-A total dose data for the 9-mm shield depth RADFET on the ERM-A electronics board. ERM = Engineering Radiation Monitor; RADFETs = Radiation-sensing Field Effect Transistors.

However, the electronics board-mounted ERM-A detector for the 9-mm Al shield depth (eighth radiation detector) provided well-distributed readings over days 1,533 to 1,858 (see Figure 3). ERM-B has not provided any valid TID readings since day 1,533 (October 2016). A comparison of ERM-A dosimeters (when sporadically working) to the functional ERM-B dosimeters showed agreement within a few percent. Thus, there is only one complete set of dosimetry data at the 9-mm Al shield depth, representing the spacecraft electronics location for the two spacecraft (see the last two rows of Table 1).

Table 1. Engineering Radiation Monitor Dose and Dose Rate History
Data end date Days 9-mm cumulative dose [rads (Si)] 9-mm interval dose [rads (Si)] 9-mm cumulative, interval dose rate [rads (Si)/day]
Oct 2013 ERMB 427 6,154 6,154 14.4, 14.4
April 2014 (B) 607 6,934 780 11.4, 4.3
Oct 2014 (B) 790 7,900 966 10.0, 5.3
April 2015 (B) 971 9,700 1,800 10.0, 9.9
Oct 2015 (B) 1,150 12,400 2,700 10.8, 15.1
April 2016 (B) 1,334 14,200 1,800 10.6, 9.8
Sept 2016 (B) 1,487 15,900 1,700 10.7, 11.1
Mar 2017 ERMA 1,675 19,400 2,700 11.6, 14.4
Sept 2017 (A) 1,858 23,000 3,600 12.4, 19.7

The data in Table 1 show that the ERM RADFET dose rate at the 9-mm Al equivalent shield depth for nominal spacecraft electronics varies from a minimum of 4.3 rads (Si)/day to 19.7 rads (Si)/day over the review intervals for the 5-year Van Allen Probes mission. Other than the latest 6-month interval that includes the September 2017 flares, the dose rate varies from ~5 to 15 rads (Si)/day during quiet and active solar times, respectively. These latter two dose rates are used for the extended mission extrapolation to 2,500 days in Figure 7. The latest accumulated total ionizing dose after 1,858 days of the mission is 23 krad (Si), which represents a mission overall average dose rate of ~12.4 rads (Si)/day. The charging current or dose rate versus spacecraft location is shown in Figure 8 of Maurer et al. (2013).

A TID of 36.2 krads (Si) was the worst case pre-CDR (Critical Design Review) prediction (RBSPICE science instrument electronics box) using the NOVICE (https://empc.com/about/thomas-m-jordan/) ray tracing Monte Carlo shielding material transport code for the AP8/AE8 static environment models with an RDM (Radiation Design Margin) of two extrapolated to 1,858 days from the baseline 805-day mission. RDM's are used for insuring conservatism from the static models of the near-Earth radiation environment, in this case AE8/AP8. One of the goals of the Van Allen Probes mission is to produce dynamic models of said environments. The measured dose at 9-mm shield depth is a factor of 1.27 times [23.0 krads (Si)/18.1 krads (Si)] the RDM = 1 worst-case predicted dose. Thus, the traditional rule-of-thumb RDM = 2 is conservative but necessary.

4.1 RADFET Fade Dose After Low Dose Rate Irradiation

Small exposures [~100 rad (Si)] of flight RADFETs confirmed proper response and good matching of the flight devices but revealed higher fade rates than expected. Investigations by the manufacturer confirmed that the fade rate for unbiased devices is higher than for biased devices (Holmes-Siedle et al., 2014). The effect of fade will be to reduce dose accuracy; but assuming the effect can be mathematically fit, an appropriate correction can be made in the calibration to determine more accurate dose estimates.

To gauge the magnitude of the effect, fade data were collected at room temperature (23–24 °C) for several months following a 100-krad (Si) laboratory irradiation at low dose rate (LDR). After ~100 days annealing, the reported dose had dropped ~17% (Figure 4). The overall characteristic appears logarithmic in time; a noticeably higher fade rate at the beginning suggests a multiexponential characteristic that contains both fast and slow components (see Figure 4). We determined that the fade rate is less at colder temperatures, but not significantly less until we approach 0 °C. So it is likely that the RADFETs are underreporting dose by ~10% or more. The operating temperatures observed in flight are lower (15–20 °C) than room temperature, which reduces fade (Maurer et al., 2013). However, there are also other uncertainties (matching, temperature, etc.) associated with the RADFETs response.

Details are in the caption following the image
The fade at room temperature for ~100 days of the Van Allen Probes REM-Oxford RADFETs after the ground test at 0.0244 rads (Si)/s (low dose rate) to 100 krads (Si). RADFETs = Radiation-sensing Field Effect Transistors.

5 On-Orbit Solar Cell Monitor Cumulative Degradation

Figure 5 shows the normalized (initial orbit, 1 AU, 28 °C) degradation history in percent for 1,858 days mission time of the InGaP/InGaAs/Ge solar array patches behind 500-micron cerium-doped cover glass with ITO coating on Spacecraft B. Earlier data was presented in Butler (2016). The top four data plots are for Voc (open circuit voltage), Isc (short circuit current), Vmp (voltage at maximum power) and Imp (current at maximum power) for the solar cell degradation patch on the array. The four model predictions (Model in the Figure 5 legend) are also included for comparison. These data values degrade by 15–20% over the ~5 years of the Van Allen Probes mission—for the most part more than the predictions. However, note that Voc follows the model fairly well. Voc is a direct measurement by the spacecraft and is not affected by small losses in light transmissibility through the cover slides and adhesive as is the case for currents. The bottom two curves are for maximum power data (the solid curve) and the prediction (dotted line) of maximum power degradation (Pmp). Again the data degrade slightly more (33%) than the prediction (31%).

Details are in the caption following the image
Normalized (initial orbit, 1 AU, 28 °C) degradation history in percent for 1,858 days mission time of the InGaP/InGaAs/Ge solar array patches behind 500 micron cerium-doped cover glass with Indium Tin Oxide coating on Spacecraft B. The eight curves at the top of the figure are for the Voc, Isc, Vmp, and Imp data (solid curves) and model predictions (dashed curves). The lowest two curves are for maximum power data (the solid curve) and prediction (dotted line) of maximum power degradation. Day 0 is 30 August 2012.

The prediction model used integral fluences generated from the SPace ENVironment Information System (SPENVIS) suite of codes (https://www.spenvis.oma.be/) for the Van Allen Probes GTO. The AE8/AP8 trapped radiation models were used with an 85% level of confidence; no additional factors were added for margin. The integral fluences were run through the NRL (Naval Research Laboratory) Solar Cell Radiation Environment Analysis Models (SCREAM https://www.researchgate.net/profile/Scott_Messenger/publication) software (Messenger, 2010), modeling the entire covered interconnected solar cell (CIC) and substrate module for both the front and rear of the solar panel assembly. The combined front and rear displacement damage dose were calculated for the mission. The solar cell triple junction coefficients required by SCREAM were computed and provided by SolAero Technologies. The prediction model uses a linear propagation of the displacement damage dose with respect to mission day to generate the appropriate remaining factors for Voc, Isc, Vmp, and Imp.

The damage/darkening of the ITO, cover-glass, and cover-glass adhesive were separated into three parts. The cover-glass adhesive darkens due to UV exposure, with most of the damage occurring within the first year of exposure (Hoang et al., 2012; Matcham et al., 1998). The ultraviolet (UV) exposure affects the current generation capability of the cell. The model propagates the UV reduction using a first-order exponential decay based on mission day.

The radiation damage to the cover glass and ITO layers both affect the light transmissibility through the materials reducing the current generation of the cell (Hoang et al., 2012; Meshishnek et al., 2006; Russell & Jones, 2003). The Meshishnek et al. report has a distribution limited to the DOD (Department of Defense and contractors). Proton exposures of 20, 40, 100, 200, 300, and 400 keV were used with no illumination. Sample types had several kinds of bare glass, antireflective (AR) coating over bare glasses or AR coating over ITO over bare glasses. ITO is used to control spacecraft surface charging. For the Van Allen Probes spacecraft no additional AR coating was used over the solar cells since the AR coating adds to the total proton displacement damage degradation that is already significant for the ITO.

The degradation of the ITO coating is spectrally quite different than AR-coated or uncoated glasses. It is not simply a loss in the blue portion of the solar spectrum; rather, it is a loss across the entire solar spectrum mimicking a neutral density filter. Reduction of the ITO from proton exposure to indium and tin metal would account for the results. The degradation model uses a linear interpolation of cover-glass darkening, and ITO darkening over the mission life; the remaining factors are proportional to the average loss in transmissibility over the solar cells spectral response region.

The model prediction curves start out lower than the data for about the first 2 years of the mission when the radiation environment was quieter. The data curves cross over and become lower than the model predictions after ~2–2.5 years when the radiation environment becomes more active and dynamic (see dose rate currents on the lower part of Figure 1 and data and projections in Figure 7).

6 Soft Part Survey for Mission Extension

The Van Allen Probes observatories were launched together on a single Atlas V-401 Evolved Expendable Launch Vehicle (EELV) from Kennedy Space Center on 30 August 2012. The observatories were originally designed for an on-orbit life of 2 years and 74 days. This included a 60-day commissioning period post launch, a 2-year science mission, and time at the end of the mission to disable the observatories.

The success of the Van Allen Probes mission despite the harsh radiation environment of the GTO, the interest generated by the science measurements, and the years remaining in Solar Cycle 24 especially near Solar Minimum created a desire for a mission extension.

Such a mission extension necessitated the review of the radiation hardness of the electronic parts to see how long an extension could be tolerated by devices originally qualified for an ~800-day mission (Maurer & Goldsten, 2016). Both the spacecraft and science instrument electronics were surveyed in Maurer (2013) and Maurer (2014), respectively.

6.1 Spacecraft Electronic Device Survey

The original prelaunch TID evaluations showed that the AD7943 12-bit Serial DAC (Digital to Analog Converter) in the Radio Frequency (RF) transceiver was hard to only 15 krads (Si) and the pacing item on the spacecraft by a margin of ~X2.

After the spacecraft electronics part survey (Maurer, 2013), discussions among the spacecraft subsystem and radiation engineers resulted in a more detailed TID evaluation in which dose/anneal cycles would expose the devices in 2.5 krad (Si) and 5 krad (Si) steps immediately followed by a 1-week anneal at 100 °C. The AD7943 is heavily shielded in the RF (radio) subsystem and the radiation dose it experiences is almost solely due to trapped protons for about 2 hr near the orbit perigee. The transceiver experiences minimal radiation flux for the remaining 7 hr of the orbit. The dose/anneal test was designed to simulate this exposure scenario for the AD7943.

The AD7943 12 bit serial DAC completed this more detailed test. The dose/anneal cycles were accumulated in 2.5 krad (Si) and 5 krad (Si) steps as shown in Figure 6 and completed after 25 krads (Si). The AD7943 devices behaved in a consistent manner through 20 krads (Si) with the supply current increasing linearly and then decreasing after the annealing interval. After 25 krads (Si) all four devices had zero output and were functionally dead drawing significantly lower supply current. After the 1-week anneal at 100 °C the devices were alive but with a significant increase in the supply current. We concluded that the AD7943 was a 20-krads (Si) hard part, a value that is 5-krads (Si) harder than the prelaunch high dose rate evaluation that did not include annealing intervals (Maurer & Goldsten, 2016).

Details are in the caption following the image
Supply current versus accumulated total dose and anneal cycles for the AD7943 Digital to Analog Converter. The four devices under test survived 20 krad (Si); functional behavior became erratic after 25 krad (Si).

6.2 Science Instrument Electronic Device Survey

The Van Allen Probe science instrument parts lists were also surveyed (Maurer, 2014). All devices met the revised 30 krads (Si) minimum total dose hardness requirement established by the program for the mission extension. The softest part was the AD822 in the Electric Field and Waves (EFW) instrument, which demonstrated 30-krads (Si) hardness for the MESSENGER program in 2002. The AD822 hardness level is likely 40 to 50 krads (Si), because all DC (Direct Current) parameters annealed significantly after 30 krads (Si) (Maurer & Goldsten, 2016).

6.3 Mission Extension Analysis

We concentrate the discussion on the AD7943 DAC in the RF transceiver, the least hard part with respect to radiation and thus the pacing item with respect to total ionizing dose lifetime.

The NOVICE Adjoint Monte Carlo Radiation Transport/Shielding Code (https://empc.com/about/thomas-m-jordan/) pre-CDR prediction for the RF transceiver electronics containing the AD7943 DAC was 10.2 krads (Si) maximum (RDM = 2) with a shield depth of 9.9 mm (390-mils Al equivalent) for the original 805-day mission, 51% of the dose/anneal procedure AD7943 TID test hardness (Figure 6). Extrapolating the 805-day 10.2-krads (Si) value to 1,858 days gives 23.5 krads (Si) maximum that would exceed the AD9743 hardness.

However, a FASTRAD three-dimensional Radiation Shielding Ray Tracing Code (https://www.fastrad.net/publications/) simulation after launch revealed that in the as-built hardware configuration the median ray trace path length was actually 19.2 mm (757 mils). The additional shielding was due to an electromagnetic interference shield added during spacecraft level testing. The consequent maximum dose predicted at 1,858 days for the AD7943 with an RDM = 2 is now a lower value: 14.2 krads (Si), only 71% of the AD7943 hardness.

Subsequent to mission day 1533 the ERM on Spacecraft B (ERM-B) continued to acquire data, but the sticking of its relay had increased in frequency such that there have been no new dose readings since day 1533; that results in loss of Dose versus Depth data (or updates to Figure 2). Figure 2 after 1,487 days is the last one we can calculate. The extrapolated dose at the 19.2-mm shield depth of the AD7943 DAC is about 0.45 that of the 9-mm RADFET measurement. However, ERM-A continues to provide an accurate dose at the single 9-mm Al box electronics shield depth. Since ERM-A and ERM-B TID values match within ~5%, we can continue to estimate the future life for the AD7943 DAC by using the shielding extrapolation factor of 0.45 from the earlier dose depth curves.

Figure 7 shows two different mission dose rates depending on solar activity at the maximum 9-mm RADFET shield depth (the nominal depth of most electronic box devices). Active Sun produces ~15 rads (Si)/day; Quiet Sun produces ~5 rads (Si)/day (see data in Table 1). Two split extrapolations are made: one from the 260-day point of Maurer et al. (2013) and a second from the 1,858-day point from the latest ERM data. The End Of Mission extrapolated dose from the 1,858-day data is expected to lie in the range of 26 to 32 krads (Si) depending on whether environmental conditions return to the previous quiescent level or remain at the latest active level. The current TID measurement after 1,858 days is 23 krads (Si) which represents a mission average dose rate of ~12.4 rads (Si)/day.

Details are in the caption following the image
Long-term ERM total ionizing dose data in krads (Si) at 9-mm Al shield depth extrapolated from 1,858 days to the end of the desired 2,500-day extended mission. The uppermost dashed line is the active time extrapolation from the 260-day data of Maurer et al. (2013); the lowest dashed line is the quiet time extrapolation from the same day. The middle dashed line follows the ERM measured dose data and has active and quiet extrapolations from the 1,858-day point. The two ordinates on the plot are the same as in Figure 1. ERM = Engineering Radiation Monitor; TID = total ionizing dose.

For the AD7943, extrapolation from the last two proton-dominated shield depth points at 3 mm and 9 mm from ERM-B RADFET measurements of the 1,487-day proton dose-depth curve in Figures 2 to 19.2 mm gives 7.2 krads (Si) (the factor of 0.45). After 1,858 days the time extrapolated measured dose from ERM-A is 10.4 krads (Si) (using the factor of 0.45 on the 9 mm measurement of 23 krads (Si) from Table 1). The AD7943 has a margin of 1.92 [20.0 krads (Si) from the dose-anneal laboratory test hardness/10.4 krads (Si) from ERM-A measurement at 9 mm extrapolated to 19.2 mm].

This margin of 1.92 multiplied by the present 1,858 days Van Allen Probes lifetime yields a 3,600-day (9.8-year = June 2022) mission. This value well exceeds the 2,500-day (~June 2019) extended mission goal shown in Figure 7.

7 Summary

On-orbit data from the ERM and Solar Cell Monitor show the accumulation of total ionizing dose and displacement damage, the variation in dose rates versus time (a factor of ~4 in Table 1), the decrease in dose versus increased shield depth (Figure 2), and the degradation of the solar arrays (~33% in maximum power over 5.1 years as shown in Figure 5) due to the high radiation exposure of the Van Allen Probes mission. We briefly discuss the fade of RADFETs (Figure 4) and the sticking relays problem in the ERMs (Figure 3).

Radiation hardness assurance surveys, laboratory TID test evaluations, and results from the ERM flight data (Figure 7) project an actual 3,600-day mission for the AD7943 DAC at 19.2-mm Al equivalent shield depth. This projection exceeds the 2,500-day extended mission goal (versus the originally planned 805-day mission). The electromagnetic interference shield material added to the RF transceiver because of a noisy amplifier during subsystem and spacecraft ground qualification testing supplied enough additional radiation shielding to protect the most radiation-sensitive AD7943 DAC and enabled the extended mission.

Acknowledgments

This work was supported by the NASA Living with a Star Program Office at NASA Goddard Space Flight Center. The official ERM data are included with the RBSPICE science instrument data at http://rbspicea.ftecs.com/Level_0/ERM/. The Solar Cell Monitor data used in this study were provided courtesy of JHU/APL Van Allen Probes operations center and are provided in the supporting information.