W-band for CubeSat Applications

The increasing data amount generated by current and future earth observation missions requires high data rate downlinks to avoid data loss. The large bandwidth available in the W-band (75–110 GHz) serves the need for the high throughput of scientific data but also allows a wide range of commercial broadcast and Internet services. Aside from the advantages of large bandwidth, reduced interference and decreased equipment size the impairments by the troposphere are insufficiently modeled as models are only validated up to 50 GHz. This stresses the need for spaceborne atmospheric attenuation measurements concurrent with rain measurements to enhance these models and set up statistics on the rain fades in W-band. The objective of this phase 0/A feasibility study was to close this gap with a Cube-Sat W-band channel characterization mission. First a list of requirements for attenuation measurements and data transmission experiment shall be derived. The question how a first order payload concept and corresponding low earth orbit (LEO) satellite concepts look like shall be addressed. From climate observations in Munich, for every month five distinct weather scenarios were selected. Based on these climate variables the tropospheric attenuation was simulated with the radio frequency propagation models recommended by the International Telecommunication Union (ITU). With the strongest attenuation occurring in July, 17.5 dB in zenith direction, the link calculations were performed for a beacon transmit power of 3.3 W. Binary phase-shift keying (BPSK) with a coding rate of 7/8 ensures at least a 1 Mb/s link for total losses below 197.6 dB and 10 Mb/s for total losses below 187.6 dB. The selected optimal conical horn fits in a 33 mm wide instrument and is supported by commercial off-the-shelf (COTS) subsystems. Utilizing the Space Mission Analysis and Design (SMAD) procedures resulted in three satellite concepts orbiting in a sun-synchronous repeat track at an altitude of 570 km. A magnetic torquer controls the attitude of the 1U satellite concept A, which comprises a nadir looking instrument. In concept B (1U) magnetic torquers point the instrument at the ground station. An integrated attitude control system points the satellite of concept C (1.5U). During the study it became clear, that the selected power subsystem limits the mission life time to 2.2 years. Therefore, the planned step-wise measurement procedure comprising both, attenuation measurements and data transmission experiments is not realizable. The author suggests to further investigate two separate missions. The first mission dedicated to attenuation measurements and the second to data transmission. For concurrent attenuation measurements and rain observations a nadir pointing concept is sufficient to quantify atmospheric phenomena linked to the ground station location. For the data transmission it is inevitable to point at the ground station to maximize the transmission time.