The most appropriate group to review this topic would be a Senior Amateur Satellite Communication Review Board (AMSAT-RB), due to the specialized nature of amateur radio operations, satellite link budget calculations, technical system integration under severe time constraints, and the interface with commercial space operations.
Abstract:
This presentation details the complex operational and technical challenges faced by the DK0TU amateur radio group at TU Berlin in successfully establishing a voice contact with astronaut Rabia A. (Fram 2 mission) from a commercial spacecraft in Low Earth Orbit (LEO).
The attempt, structured similarly to ARISS educational contacts, necessitated rapid system engineering to overcome highly uncertain parameters, including unknown transmit power, antenna gain, and polarization characteristics of the spacecraft. The link budget analysis indicated path losses between 140 and 160 dB, requiring an estimated 20 dB margin above minimum decoding sensitivity.
The final ground segment deployed three parallel stations: (1) an SDR-based monitoring station for real-time frequency/Doppler tracking; (2) a legacy 50W SatNOGS station (known gain, right-hand circular polarization); and (3) a high-power station featuring 300W DIY amplification and high-gain X-quad antennas configured for left-hand circular polarization. Despite facing logistical delays, critical hardware failures (LNA destruction due to sequencing error), and incomplete information (non-disclosure agreements on spacecraft RF systems), the contact was successfully initiated 6 minutes and 27 seconds into the 10-minute pass, achieving a strong signal report (S9+20, -53 dBm) for 1 minute and 9 seconds before transmission ceased. Post-analysis confirms the high-power station was crucial for the successful uplink.
Amateur Satellite Communication Review Board (AMSAT-RB) Summary
0:37 Amateur Radio Definition: Amateur radio is governed by law, supporting experimental and scientific studies, international understanding, disaster relief, and space radio station (satellite) utilization.
1:46 DK0TU Overview: The Amateur Radio Group at TU Berlin (DK0TU) has operated since 1972, managing a station on the university's main building roof. Activities include contesting, antenna construction/maintenance, and offering courses in amateur radio licensing, radio astronomy, and satellite communication.
3:09 Satellite Ground Station Asset: The foundation for the contact was an existing high-gain four-element Yagi antenna array, built from a student project and integrated into the SatNOGS network, capable of tracking and receiving faint satellite signals.
4:11 Opportunity Initiation: The contact opportunity arose after DK0TU participated in a Slow-Scan Television (SSTV) competition associated with the Fram 2 mission, prompting contact from Rabia A.'s representatives. The official request for a voice contact was received on March 15 (approximately two weeks prior to the target date).
6:10 Logistical Planning: Due to the complexity and short timeline, TU Berlin's press office provided significant support, handling public relations and livestream production, allowing the radio team to focus on technical and operational readiness.
7:50 Operational Parameters: The fixed contact parameters were: April 1st at 09:04 CEST; S/R frequencies in the 70cm band; maximum duration 10 minutes (due to LEO visibility window).
8:24 Critical Unknowns (Link Budget): Key technical specifications of the spacecraft radio system (antenna polarization, transmit power, antenna gain) were unavailable or undisclosed, complicating the link budget calculation.
8:41 Satellite Communication Basics: Analysis covered fundamental RF propagation factors: Link Budget (Übertragungsbilanz), high antenna gain utilization, Polarization Mismatch (noting 3dB loss for linear-to-circular cross-polarization, or theoretically infinite attenuation for opposite circular polarizations), Free-space Path Loss (140 dB at zenith, 160 dB at horizon, averaging $10^{-15}$ power factor), and significant Doppler Shift compensation (up to several kHz due to 7000 m/s velocity).
14:48 Information Disclosure Failure: A critical communication meeting 7 days prior confirmed the lack of key RF specifications (antenna type, gain, polarity, TX power) due to confidentiality constraints. However, internal discussions suggested the ground station should utilize at least 100W TX power (DK0TU is licensed for 750W Class A operation).
15:48 FCC Filing Data: Public FCC filings revealed a 10W radio, estimated 2.2 dBi gain, and implied left-hand circular polarization, confirming the challenging operating environment.
16:17 Link Margin Analysis: Theoretical analysis showed that 10W TX power resulted in -116 dBm received signal at the ground station. To achieve a secure 20 dB margin (targeting -80 dBm), the ground station required an estimated 16 dB receiving gain.
17:27 Three-Station Strategy: Three systems were deployed to ensure mission success:
Station 1 (Monitoring): Non-directional antenna, Software Defined Radio (SDR) with filtering and pre-amplification, used solely for real-time spectrum tracking and Doppler correction validation.
Station 2 (Backup/Legacy): SatNOGS antenna (proven, 16 dB gain, right-hand polarized), paired with a standard 50W transceiver.
Station 3 (Primary/High Power): Dual X-quad antennas (high gain, left-hand polarized), paired with a DIY 300W power amplifier ("The Brate") and a hardware sequencer for RX/TX switching.
19:19 Station 1 Implementation: Use of cascaded filters and low-noise amplifiers (LNA) was necessary to combat high RF noise and interference in the dense urban environment before digitalization via the SDR.
20:13 Station 3 Implementation (High Power): The 300W DIY amplifier utilized repurposed DVB hardware. Critical sequencing logic was implemented via simple hardware relays and delay circuits (built late at night) to manage the transmit path switching, including a critical attenuator to protect components.
23:46 System Failure and Recovery: 10 hours before the pass, a critical operational error occurred: the sequencer power supply was off. Activating the transceiver resulted in 75W of RF transmitting directly into the unprotected Low Noise Amplifier (LNA), causing it to fail instantly (estimated €200 component loss). An emergency LNA replacement was sourced and built overnight from university laboratory parts.
25:31 Contact Parameters: The contact window was 7:29:33 to 7:39:52 UTC. Primary RX: 437.56 MHz; TX: 437.925 MHz.
26:18 Contact Initiation: After 6 minutes and 27 seconds of transmitting, contact was established. The received signal strength was very high (S9+20, -53 dBm).
27:19 Contact Outcome: Two out of 17 prepared questions were answered by Rabia A. The contact was lost after 1 minute and 9 seconds due to an abrupt frequency shift on the spacecraft, transitioning off the primary downlink frequency.
28:34 Post-Contact Analysis: Station 3 (High Power/Left-Hand Circular Polarization) achieved the successful link and was clearly audible to the astronaut. Station 2 (50W/Right-Hand Circular Polarization) received no signal, suggesting a high degree of polarization mismatch (potentially the theoretical 20 dB loss) and/or an insufficient power margin for that specific antenna configuration. The spacecraft’s frequency deviation remains unexplained (technical or human error).