## 1 Introduction
Channel interleavers can be used in combination with error correction codes for burst noise. A window covering the full temporal dynamics of the interference can be used for the interleaver length since the burst length will be shorter. Moreover, if the interference has a specific time structure, such as pulsed signals, windows of opportunity could be estimated to transmit in between pulses. To understand the current issues experienced in the TT&C links to small satellites using the UHF radio amateur band, new measurements that combine time and frequency information are needed. Furthermore, the time variability characteristics of the interference can be exploited to improve the communication system design in the band. Ideally, a continuous spectrogram and the probability density function of the interference should be measured to obtain accurate frequency and temporal dynamics. Due to the limited downlink rate of small satellites using the UHF amateur radio band (430-440 MHz) and their power constraints, a low complexity algorithm is desired to measure time and frequency characteristics. The UHF amateur radio band (430-440 MHz) is shared with other radiocommunication services, such as radiolocation and Earth Exploration Satellites [20]. As a part of the radiolocation service, there are different types of ground radars (type A, B, and C) in the 420-450 MHz band that transmit up to 5 MW of power [21]. Radar type A is used for space object tracking and could be the source of interference of satellite communication in the amateur radio band. In Reference [13], a strong pulsed interference was identified as the ground radar operating in the United Kingdom (UK). Measurements from the SERPENS satellite [12] also suggest that these ground radars can be interfering in the satellite uplink since high power interference was detected over scarcely populated areas. These type A radars transmit pulse frequency modulation in the form of linear chirps. For the search mode (when surveying space objects), the chirps have a bandwidth of 100-350 kHz, while, for tracking, the bandwidth is 1 or 5 MHz [21]. The pulse width can be 0.25, 0.5, 1, 2, 4, 8, or 16 ms, the average duty cycle is 25%, and the pulse repetition frequency is up to 41 Hz [21]. In Reference [22], the characteristics of the Precision Acquisition Vehicle Entry (PAVE) Phased Array Warning System (PAWS) early warning radars in the United States (US) are described for an environmental impact statement. These radars are located in Cape Cod (Massachusetts), Beale (California), and Clear (Alaska). For these radars, the pulse width for tracking mode can be 0.25, 0.5, 1, 2, 4, 8, and 16 ms, and, for surveillance, 0.3, 5, and 8 ms. The pulse repetition rate can be between 18 and 72 pulses per second. The 3 dB antenna beamwidth is 2.2\({}^{\circ}\), the same as in Reference [21]. The PAVE PAWS radars also transmit high power (around 0.5 MW peak power), so they are an expected source of interference in space. In this article, we present measurements of the radio environment in the 435 MHz band for satellites in Low Earth Orbit (LEO) using the LUME-1 satellite in the spring of 2021 to support interference characterisation. This satellite has an approximately 98\({}^{\circ}\) inclination orbit allowing measurements over the polar areas. The LUME-1 is a CubeSat with limited on-board data processing capabilities and low data rates for communication. This called for the development of simple on-board estimation procedures to obtain the time-frequency interference statistics with low downlink data rate requirements. We performed spectrum measurements over five seconds and estimated the time variability of the interference over windows between 0.21 ms and 27.3 ms, measuring the first-order stationarity by estimating the variance of the Local Mean Envelope (LME) with different lengths of averaging windows.