The Communications subsystem ensures that payload, telemetry and telecommand data are successfully exchanged between the AcubeSAT (space segment) and the ground station (ground segment) through the following:
- Link Budget Analysis
- Antenna Design
- Digital Signal Processing with Software Defined Radios (SDRs)
- Radio Frequency (RF) Front-End Design
- Ground Station design and setup
- Embedded systems programming
The AcubeSAT makes use of radio amateur frequencies and consists of two radios, one operating on 2.4-2.45 GHz (S-band) for downlinking the payload data and a half-duplex radio on 435-438 MHz (Ultra High Frequency (UHF) band) for housekeeping telemetry (TM) transmission and telecommands (TC) reception.
A board currently being developed by Libre Space Foundation (LSF) as an European Space Agency (ESA) funded open-source project is employed for our design (you can find the GitLab repo here). It is based on the reconfigurable AT86RF215 transceiver chip, which provides two independent transmission (TX) and reception (RX) chains and will be used as the base for both the UHF and S-band radios.
S band Radio
Since the scientific payload data comprises mainly of images, a high data rate is necessary in order to acquire the data in its entirety, while maintaining the minimum power consumption possible. The radio has the following general characteristics:
- Modulation: Offset Quadrature Phase-Shift Keying (OQPSK)
- Bit rate: 160 kbps
- Forward Error Correction (FEC): Low-Density Parity-Check (LDPC)
- Framing: CCSDS 132.0-B-2
- Tx Power: 1 W
For the 2.4-2.45 GHz band we have designed (based on this paper) and are currently testing a circular polarization, multi-layered patch antenna, with a measured gain of approximately 4 dBi and a beamwidth of 104 degrees. Below you can see the CAD of the antenna, the 3D pattern as measured in the anechoic chamber of the Radar and Microwaves Unit of the Telecommunications Lab and a photo of the patch during the measurement.
The main parameters of the UHF radio are the following:
- Modulation: Gaussian Minimum-Shift Keying (GMSK)
- Bandwidth: 20 kHz
- Bit rate: 20 kbps
- FEC: Convolutional
- Framing: CCSDS 132.0-B-2 (TM), CCSDS 232.0-B3 (TC)
- Tx Power: 1 W
A GMSK-modulated beacon in the same form as a TM frame will be transmitted every 30 seconds. Apart from the CCSDS framing, AcubeSAT will also be transmitting a Carrier Wave (CW) beacon (as seen in the figure bellow) with essential TM every 3 minutes at a 20 Words Per Minute (WPM) rate.
All the TC will be HMAC-signed to prevent accidental access and attacks.
In order to reliably control the satellite in all cases of orientation, a turnstile dipole, circular polarization antenna to be mounted on UPSat’s deployment mechanism has been designed. The antenna has a simulated 2.14 dBi gain and beamwidth of 141 degrees.
The ground station, currently under development, will be based on Aristotle University of Thessaloniki and be a part of the SatNOGS global network of satellite ground stations.
In the current design, telecommunications will be handled by a Raspberry Pi, programmed using a custom image provided by SatNOGS and interfaced with two Adalm Pluto SDRs, one used for the S-band reception operations and the other for the UHF TX and RX operations.
Encoding and decoding of the data will be implemented digitally through GNURadio flowgraphs. More precisely, there are some ready-made blocks in the software enabling quick development of the RF chain, while sometimes a custom block is required. An OQPSK receiver has been implemented, with the corresponding encoder for testing. A CCSDS Space packet decoder will be implemented, along with an LDPC decoder and Convolutional encoder/decoder.
The ground station will include two antennas covering the needs for both operational bands. The antenna for the S-band downlink will be a parabolic dish type from RF HAMDESIGN, utilizing a helical feed and having a gain of 23.6 dB and 9.7 degrees HPBW. UHF link operations will be conducted through a custom-made helical antenna, with a simulated gain of 12 dB and a beam width of 36 degrees. Both antennas will be mounted on the SatNOGS Rotator v3 and controlled by the SatNOGS Rotator Controller.