## 7 Conclusions  
In this article, we have presented an algorithm that can measure frequency and time characteristics of in-orbit interference with low downlink data rate requirements. The method is validated through theoretical analysis, simulations and hardware-in-the-loop testing, and executed in-orbit measurements on the LUME-1 satellite. The studied frequency band was the UHF amateur radio frequencies commonly used for TT&C of small satellites. The in-orbit interference measurements were carried out over the South Pacific, South Atlantic, North America, Europe, North Africa, the Middle East, and the Arctic during the spring of 2021.  
As expected, higher interference levels were generally detected over populated areas. Three different time behaviours were distinguished: AWGN, narrow band CW, and pulsed. The pulsed behaviour was seen by analysing the variation of the local mean of the average envelope as a function of window length. High values of the coefficient of variation were estimated in regions with band-limited interference that have significant power over the general noise floor. For that interference, the peak power is much higher than the average power. This interference was observed over areas with known type A ground radars in the UHF band around 435 MHz, and the observed bandwidth of 300 kHz is consistent with the chirp bandwidth for radar search mode [21]. Furthermore, wide-band interference with pulsed behaviour was measured over Europe, but the bandwidth could not be estimated due to the limited bandwidth of 500 kHz in the measurements.  
A global measurement campaign is planned to provide a world overview of the interference environment in the selected band. Calibration measurements using a ground station transmitting to the satellite will enable the use of absolute power levels. Furthermore, different centre frequencies will also be studied, and ECDF measurements of the IQ samples will be planned to obtain the proper interference statistics to design the countermeasures required to improve the reliability of the satellite uplink in the UHF amateur radio band.  
Conceptualisation, G.Q.-D., T.E. and F.A.A.; methodology, G.Q.-D. and T.E.; software, G.Q.-D.; validation, G.Q.-D. and D.H.d.M.; formal analysis, G.Q.-D. and T.E.; investigation, G.Q.-D., J.M.L.A., A.G.M. and F.A.A.; resources, F.A.A., A.G.M., D.H.d.M. and J.M.L.A.; data curation, G.Q.-D.; writing--original draft preparation, G.Q.-D.; writing--review and editing, G.Q.-D., T.E., F.A.A., J.M.L.A. and D.H.d.M.; visualisation, G.Q.-D.; supervision, T.E. and F.A.A.; project administration, T.E.; funding acquisition, T.E. and F.A.A. All authors have read and agreed to the published version of the manuscript.  
The work of the Norwegian University of Science and Technology (NTNU) is supported by the Norwegian Research Council (Grant No. 270959), the Norwegian Space Agency, and the Centre of Autonomous Marine Operations and Systems (NTNU AMOS). The work of the University of Vigo has been funded by Programa de Cooperacion Transfronteria INTERREG V-B SUDOE (Fondos FEDER) under the project \"Wildfire picostatellite constellation & uavs remote sensing: active fire mapping and management\" (Grant No. FIRE-RS, SOE1/P4/E0437) and by Ministerio de Economia y Competitividad-- Programa Estatal de Investigacion, Desarrollo e Innovacion Orientenda a los Retos de la Sociedad under the project \"Development of a small satellite for spectrum monitoring and validation of robust communications\" (Grant No. ESP2016-79184-R).  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
[PERSON] wishes to acknowledge the help of her colleagues, [PERSON] and [PERSON], and [PERSON] for useful discussions of ideas and results.  
The authors declare no conflict of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.