From beginning of UWE-3 operations the downlink quality from the satellite to ground was excellent, showing high signal strengths even at low elevations. However, the telecommand uplink was initially difficult showing temporarily significant transmission failure rates. From beginning of 2014 the uplink quality degraded even further.
After excluding hardware problems from the list of potential failure sources it could be revealed that the uplink quality significantly depends on the location of the groundstation and the frequency used, due to local interferences on parts of the amateur radio spectrum. A change of uplink frequency could significantly improve operations. In beginning of June 2014 a software update has been uploaded to one of the redundant onboard-computers of UWE-3. Among other features, the update included several new experiments targeting the analyzation of the link quality. In order to further analyze the spatial distribution of in-situ interference levels for the whole amateur radio spectrum, an RSSI frequency sweep experiment has been implemented. The experiment allows to monitor and log measured RSSI noise levels while automatically scanning over a specified frequency range.
Several frequency sweeps have been recorded in third quarter of 2014, each lasting several days to ensure global coverage. The results discussed in the following base on measurements recorded between 06.08.2014 and 11.08.2014 and are representative for the observations made. The recording comprises more than 100.000 data points downloaded from the satellite in a 1.5 MB compressed data file.
Figure 1: Extract from the raw RSSI [dBm] measurements over time [UTC]
Figure 2: Orbit coverage of measurement recording.
The downloaded measurements have been mapped to their instantaneous sub-satellite points. The recorded data points show sufficient coverage of the entire globe and can be properly used as a grid for spatial interpolation to generate a global interference map for each frequency under investigation. It can be seen that the average interference levels vary significantly with location and observed frequency. Especially orbital regions connected to central Europe seem to be affected significantly on a wider frequency band between 437.000 MHz and 437.600 MHz.
Figure 3: RSSI [dBm] average interference world plot between 6.8.2014 and 11.8.2014.
The results reflect our experiences when operating UWE-3 during the first six month after launch. They further underline the importance of proper frequency selection for satellites using the radio amateur bands. Moreover, the results emphasize the requirement for in-orbit frequency re-configuration to be able to react to changes of inevitable global interferences caused for example by military space surveillance radars.
These and further results are published in:
Busch, S., Bangert, P., Dombrovski, S., Schilling, K., UWE-3, In-Orbit Performance and Lessons Learned of a Modular and Flexible Satellite Bus for Future Picosatellite Formations, 65th International Astronautical Congress, Toronto, Canada, Online Reference: 14.B4.6B.6