the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Jun 2026
A Cubesat-optimized Magnetic Field Measurement Concept for Operation Beyond Low Earth Orbit
Abstract. This paper presents a CubeSat-optimized measurement concept for magnetic field, which was designed for missions beyond low Earth orbit targeting novel radiation belt studies. The instrument combines a miniaturized three-axis fluxgate sensor on a low‑mass deployable boom together with a compact sensor control unit that implements fully synchronous excitation, sampling, and digital control. A hybrid feedback module integrates three prototype fluxgate feedback microchips that provide low‑noise current sources with selectable full‑scale ranges (±3/±9 mA), enabling vector measurements over ±70 μT while retaining sub‑nT resolution. The boom provides 65 cm standoff from the CubeSat and pointing knowledge of 1°. The designed boom length limits the tolerable magnetic dipole moment inside the CubeSat to 0.05 Am2. Extended housekeeping includes voltage supply and thermal monitoring, an additional magneto‑inductive sensor on the sensor control unit for disturbance discrimination, and total ionizing dose tracking. Laboratory characterization demonstrates noise densities of 20–30 pT√Hz at 1 Hz, with 128 vectors per second science data and linearity suitable for near‑Earth fields. As it is a pivotal element in the instrument, the prototype feedback microchip was tested for its radiation response. Heavy‑ion testing showed no single‑event latch‑up up to 100 MeV cm2mg. Single‑event transients as well as single-event upsets were observed and are consistent with rare functional interrupts in the digital part of the signal chain. They are addressable by standard redundancy techniques. X‑ray tests up to 1 Mrad confirm functionality with worst‑case gain and offset drifts of approximately 600 ppmkrad and 1 μAkrad within 0–100 krad. The prototype meets the performance requirements derived from the Foresail‑2 mission concept and demonstrates a path to high‑quality, resource‑efficient magnetometry on 3U–6U CubeSat platforms suitable for operation in the Van Allen belts and other harsh radiation environments.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-1579', Anonymous Referee #1, 07 Jul 2026
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AC1: 'Reply on RC1', Raphael Steinhoefler, 20 Sep 2026
We kindly thank the Referee for the constructive comments on the manuscript.
We agree with the points provided in the comment and want to emphasize that the ASIC is still under development. Nevertheless, first performance tests in context of magnetic field measurement on cubesats could be carried out.
The open points in the comments are adressed by the following actions:
1. One of the main advantages of the delta-sigma DAC approach is its ability to achieve excellent linearity. However, the manuscript does not include an evaluation of the instrument's linearity. A measurement of the total harmonic distortion (THD) would provide valuable evidence of the instrument's performance. I recommend including such a characterization in the paper.
Measured values for the total harmonic distortion in both Low Range and High Range mode have been added to the paper in section 3.1
2. At the current stage of development, the performance of the current-feedback ASIC in terms of gain and offset stability over temperature is not overwhelming. The investigation of this chip and its application in fluxgate electronics is valuable, but the manuscript should more clearly emphasize that the ASIC is still under development. It should explicitly state that this is a newly developed device that requires further optimization before it can provide a viable solution for space applications.
The statement in Section 4 has been updated to explicitly state that further development is needed.
3. The tested temperature range is relatively narrow. The paper would benefit from measurements over a broader temperature range, even if these are performed only at the component (prticularoily the ASIC) or subsystem level rather than on the complete instrument. This is particularly important for CubeSat missions, where the limited thermal mass of the spacecraft generally results in larger temperature variations than those experienced by larger satellites. It is therefore essential to demonstrate that the ASIC remains operational and meets the required performance over a temperature range representative of a CubeSat environment.
An additional test over an extended temperature range was carried out and the results have been added to the paper in section 3.1
We hope that the changes to the manuscript sufficiently adress the open points in the comment.
Citation: https://doi.org/10.5194/egusphere-2026-1579-AC1
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AC1: 'Reply on RC1', Raphael Steinhoefler, 20 Sep 2026
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RC2: 'Comment on egusphere-2026-1579', Anonymous Referee #2, 05 Aug 2026
This manuscript presents a miniature fluxgate magnetometer concept optimized for small-satellite platforms. The noise performance demonstrates outstanding results considering the mass and size constraints. However, several aspects must still be addressed.
- While not explicitly detailed in the text, Fig. 1 shows that the SCU is located near the CubeSat outer wall. Since platforms in LEO undergo steep thermal gradients during eclipse entries and exits, have the authors assessed the dynamic thermal behavior of the front-end electronics? For instance, any thermal lag between the housekeeping thermistor and the ASIC might introduce transient calibration offsets that require characterization.
- The feedback drive circuitry utilizes a Howland current source. However, this topology is sensitive to resistor ratio matching and differential thermal coefficient, which can easily unbalance the circuit and degrade its output impedance. Could the authors clarify the rationale behind selecting a Howland current source over alternative topologies that offer higher inherent stability against component tolerances? Additionally, given that the voltage reference often dominates the noise of the current source (especially at low frequencies), could the authors specify the chosen voltage reference and evaluate its contribution to the overall magnetometer noise budget?
- Pag 15, line 246: The authors state that "this phenomenon may be explained by a higher energy used to flip the magnetization of the soft magnetic cores in this design." Has this mechanism been experimentally verified?
- Pag 17, line 262: The substantially larger thermal drift reported on a single axis warrants further clarification beyond a potential "measurement error". If the behavior stems from a batch variation, discussing this in more detail would help reassure readers regarding the instrument's overall symmetry and reliability. The authors are encouraged to provide additional insights.
Citation: https://doi.org/10.5194/egusphere-2026-1579-RC2 -
AC2: 'Reply on RC2', Raphael Steinhoefler, 20 Sep 2026
We kindly thank the referee for the review and the comments on the manuscript.
We also want to emphasize that the ASIC is still under development and not a finalized device. This concept study includes the first test results and it is expected that several performance parameters of the instrument will improve with further semiconductor iterations.The following reasoning and actions address the open points raised in the comment:
While not explicitly detailed in the text, Fig. 1 shows that the SCU is located near the CubeSat outer wall. Since platforms in LEO undergo steep thermal gradients during eclipse entries and exits, have the authors assessed the dynamic thermal behavior of the front-end electronics? For instance, any thermal lag between the housekeeping thermistor and the ASIC might introduce transient calibration offsets that require characterization.
While important for operation in LEO, the dynamic thermal behavior is beyond the scope of this concept investigation. At this stage, thermal lag is not characterized. A model of the magnetometer that is scheduled for flight would account for thermal lag by placing a housekeeping thermal sensor close to or on the ASIC and it would undergo thorough temperature and thermal-vacuum testing, also characterizing any thermal lag calibration offsets.
The feedback drive circuitry utilizes a Howland current source. However, this topology is sensitive to resistor ratio matching and differential thermal coefficient, which can easily unbalance the circuit and degrade its output impedance. Could the authors clarify the rationale behind selecting a Howland current source over alternative topologies that offer higher inherent stability against component tolerances? Additionally, given that the voltage reference often dominates the noise of the current source (especially at low frequencies), could the authors specify the chosen voltage reference and evaluate its contribution to the overall magnetometer noise budget?
At the time of development of the ASIC, the Howland topology provided the most suitable current source for the application. Given the observed temperature dependence, further developments in the ASIC will also explore alternative feedback-current driver topologies. A statement is added to section 4 of the manuscript.
The voltage reference used is a ISL71090SEH25 Ultra-Low Noise Precision Voltage reference featuring an output voltage noise of 2 uV peak-peak in a band of 0.1 Hz - 10 Hz. With this voltage reference, the overall noise of the instrument of the instrument is consistent with the sensor noise in LR mode, meaning that the SCU is able to deliver the maximum noise performance with this sensor. Further investigation into the noise contribution of the voltage reference will be carried out with more recent developments in sensor technology, as they are not a limiting factor for this instrument. The noise figure of the reference is added to the manuscript in section 3.1.Pag 15, line 246: The authors state that "this phenomenon may be explained by a higher energy used to flip the magnetization of the soft magnetic cores in this design." Has this mechanism been experimentally verified?
No, it has not been experimentally verified. The statement is removed from the manuscript.
Pag 17, line 262: The substantially larger thermal drift reported on a single axis warrants further clarification beyond a potential "measurement error". If the behavior stems from a batch variation, discussing this in more detail would help reassure readers regarding the instrument's overall symmetry and reliability. The authors are encouraged to provide additional insights.
At the repetition of the gain deviation measurement, further anomalous behavior of the Y axis was observed. At several intervals throughout the output current range, increased noise and spiking in the output current occurs. The Additional information is provided in section 3.1 and a statement is added in section 4.
It is our hope that the explanations and modifications to the manuscript will satisfactorily address the open points.
Citation: https://doi.org/10.5194/egusphere-2026-1579-AC2
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EC1: 'Comment on egusphere-2026-1579', Marina Díaz-Michelena, 21 Sep 2026
Dear Dr. Steinhöfler, dear authors,
Thank you for the careful revision of the manuscript and for the detailed responses to the comments raised by the two referees.
I have reviewed the revised manuscript together with your responses. Overall, I consider that the referee comments have been adequately addressed. In particular, the additional THD characterization, the extended temperature testing, the clarification of the prototype status of the ASIC, and the more detailed discussion of the anomalous behaviour observed on the Y axis substantially improve the manuscript. I also appreciate that the revised manuscript now explicitly discusses the remaining limitations and the developments required before the ASIC can be considered for a mature space application.
I found three minor details that I list below:
- Please check the THD value reported for the Low Range in Table 4. The table currently gives a value of “75 dB”, whereas the High Range value is reported as “≤ −97 dB”. Please verify the original measurement and correct the value if necessary.
- Please check the sentence describing the expected nominal ionizing dose in Section 2.5. The current text reads “up to 200 100 krad”. This appears to contain a typographical error and should be corrected for clarity and consistency with the subsequent discussion of the 100 krad mission dose.
- Please clarify the statement in the Abstract that “The prototype meets the performance requirements derived from the Foresail-2 mission concept.” Since the manuscript also states that the temperature dependence, linearity in the Low Range, and the anomalous Y-axis behaviour still require improvement, please specify which Foresail-2-derived performance requirements are demonstrated by the present prototype.
These are minor editorial clarifications and will not require another round of external peer review.
Sincerely,
Handling Editor
Geoscientific Instrumentation, Methods and Data SystemsCitation: https://doi.org/10.5194/egusphere-2026-1579-EC1 -
AC3: 'Reply on EC1', Raphael Steinhoefler, 26 Sep 2026
Dear Dr. Díaz-Michelena,
Thank you very much for providing your comments that help to complete the manuscript.
Please check the THD value reported for the Low Range in Table 4. The table currently gives a value of “75 dB”, whereas the High Range value is reported as “≤ −97 dB”. Please verify the original measurement and correct the value if necessary.
The THD value was re-measured and Table 4 was updated. Test fields of 1/2 and 2/3 of the full range were used to provide meaningful THD estimates.
Please check the sentence describing the expected nominal ionizing dose in Section 2.5. The current text reads “up to 200 100 krad”. This appears to contain a typographical error and should be corrected for clarity and consistency with the subsequent discussion of the 100 krad mission dose.
Please have a look at the manuscript again. It appears to me that the "200" in "200 100 krad" is the line enumeration and does not belong to the text.
Please clarify the statement in the Abstract that “The prototype meets the performance requirements derived from the Foresail-2 mission concept.” Since the manuscript also states that the temperature dependence, linearity in the Low Range, and the anomalous Y-axis behaviour still require improvement, please specify which Foresail-2-derived performance requirements are demonstrated by the present prototype.
The statement in the abstract was updated for more detail. Furthermore, a link to the requirements of the mission concept was added in Section 2.6 and Section 3.1
We hope that these changes round off the manuscript and provide the full answer to the raised details.Citation: https://doi.org/10.5194/egusphere-2026-1579-AC3
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The paper is well written and provides a good overview of the development of a CubeSat fluxgate magnetometer. It demonstrates the use of a newly developed current-feedback ASIC with high radiation tolerance in a compact instrument. However, there are several points that need to be addressed before publication.
Overall, the instrument design follows a sound approach and presents promising initial results. Although the performance of the feedback ASIC is not yet at the level required for a space mission, the paper provides a valuable demonstration of its integration into a compact fluxgate magnetometer. If the points raised above are adequately addressed, I recommend the paper for acceptance. I look forward to seeing the future development of this ASIC.