This work has been carried out during the period from March 2015 to June 2018 as a part of the 4DSpace Strategic Research Initiative at the University of Oslo. During that period, I have been lucky to work with many passionate and talented people. Just some lines to express my gratefulness to important ones who inuenced me the most. I would like to express my sincere gratitude to my main supervisor Ketil Røed for all things he has done for me. Thank you for your all the valuable insight, discussions and support. I would like to thank my co-supervisor Philipp D. Häige for introducing me to the research group as well as valuable discussions on ASIC development of our current measurement electronics. I would also like to thank Tore André Bekkeng and Arne Pedersen, who both have guided me into the world of space instrumentation. Many thanks to Lasse B. N. Clausen, Jøran I. Moen, Wojciech J. Miloch and Andres Spicher for fruitful discussions and also support throughout last three and half years. Jøran, thank you for your eort in obtaining the nancial support for this work. Special thanks also to Lasse and Jøran for your guidance in the thesis writing process. Espen Trondsen and Bjørn Lybekk, your help and assistance have been greatly appreciated. Further thanks to Halvor Strøm, David M. Bang-Hauge, Erlend Bårdsen and Stein S. Nielsen at ELAB for the support of PCB production and for sharing your practical experiences in electronics. I have also been fortunate to study and work with many brilliant students in the 4DSpace group and the electronics group of the Department of Physics. Finally, I am very grateful for all the unconditional support and encouragement I have received from family during the PhD studies. I would also like to acknowledge the Norwegian Research Council, ESA PRODEX programme, Nano Network, Norwegian Space Center for nancial support of projects and test campaigns I have been involved in.

In this paper we evaluate two data analysis techniques for the multi-needle Langmuir probe (m-NLP). The instrument uses several cylindrical Langmuir probes, which are positively biased with respect to the plasma potential in order to operate in the electron saturation region. Since the currents collected by these probes can be sampled at kilohertz rates, the instrument is capable of resolving the ionospheric plasma structure down to the meter scale. The two data analysis techniques, a linear fit and a non-linear least squares fit, are discussed in detail using data from the Investigation of Cusp Irregularities 2 sounding rocket. It is shown that each technique has pros and cons with respect to the m-NLP implementation. Even though the linear fitting technique seems to be better than measurements from incoherent scatter radar and in situ instruments, m-NLPs can be longer and can be cleaned during operation to improve instrument performance. The non-linear least squares fitting technique would be more reliable provided that a higher number of probes are deployed.

NorSat-1 was launched on 14 July 2017 as a satellite carrying, among other instruments, the multi-Needle Langmuir Probe (m-NLP), an instrument which, on NorSat-1, is capable of measuring the ionospheric plasma electron density with the high sampling frequency of 1000 Hz. The m-NLP instrument operates by analyzing the current-voltage diagram resulting from the measurements from each individual probe. In principle, the m-NLP operation methodology should be insensitive to spacecraft charging. However, this is not always the case. In this paper, we present an overview of the instrument response to passes into and out of eclipse. When the satellite exits eclipse, we observe a collapse in the collected probe currents. This acute drop is unaccounted for by the theoretical operation of the instrument. We present a statistical analysis of the phenomenon based on several months of NorSat-1 data, and we suggest a plausible reason for the observed drop in the current, namely spacecraft charging by solar cell arrays upon eclipse exit. We briefly discuss how satellite orientation and plasma wake affect the current drop. With this paper, we address Langmuir probe current susceptibility to spacecraft potential.

The QB50 mission is a satellite constellation designed to carry out measurements at between 200 -380 km altitude in the ionosphere. The multi-needle Langmuir probe (m-NLP) instrument has been mounted on board eleven QB50 satellites in order to characterize ambient plasma. The distinct feature of this instrument is its capability of measuring the plasma density at high spatial resolution without the need to know the electron temperature or the spacecraft potential. While the instrument has been deployed on many sounding rockets, the QB50 satellites offer the opportunity to demonstrate the operation of the instrument in low-earth orbit (LEO). This paper provides a brief review of the m-NLP instrument specifically designed for the QB50 mission and the case studies of the instrument's performance on board the Ex-Alta 1 and Hoopoe satellites. The system has also been functionally verified in a plasma chamber at the European Space Research and Technology Center (ES-TEC). Although the QB50 mission's scientific goals have not been reached yet and some uncertainties still remain, there are some optimistic in-orbit preliminary results which could be helpful for the system improvement in future campaigns. Particularly, the electron emitter as part of the m-NLP science unit has demonstrated its capability in the plasma chamber and in orbit to mitigate spacecraft charging effects.

A method for evaluating electron density using a single fixed-bias Langmuir probe is presented. The technique allows for high-spatio-temporal resolution electron density measurements, which can be effectively carried out by tiny spacecraft for multi-point observations in the ionosphere. The results are compared with the multi-needle Langmuir probe system, which is a scientific instrument developed at the University of Oslo comprising four fixed-bias cylindrical probes that allow small-scale plasma density structures to be characterized in the ionosphere. The technique proposed in this paper can comply with the requirements of future small-sized spacecraft, where the cost-effectiveness, limited space available on the craft, low power consumption and capacity for data-links need to be addressed. The first experimental results in both the plasma laboratory and space confirm the efficiency of the new approach. Moreover, detailed analyses on two challenging issues when deploying the DC Langmuir probe on a tiny spacecraft, which are the limited conductive area of the spacecraft and probe surface contamination, are presented in the paper. It is demonstrated that the limited conductive area, depending on applications, can either be of no concern for the experiment or can be resolved by mitigation methods. Surface contamination has a small impact on the performance of the developed probe.