Communications

In satellite communication systems, two systems with different functions are commonly distinguished: The Telemetry, Tracking and Control (TT&C) System and the Payload Data Downlink System. The TT&C system is used to transmit housekeeping data, to receive commands and to determine the position and the orbit of the satellite. It has to perform with a high reliability, but the necessary data rates are not that high. Typical data rates are a few kbit/s for the uplink (ground station -> satellite) and about 100kbit/s for the downlink (satellite -> ground station). Sometimes it is also referred as the TM/TC system, if no tracking is used. The Payload Data Downlink System, on the other hand, is used exclusively to transmit payload data. Unlike TT&C and TM/TC system, the required reliability is lower as the required data rates are much higher.

Flying-Laptop Antenna CharacteristicIllustration: Measurement of the antenna characteristics of FLP. Top left: DDS LHCP, Bottom left: DDS RHCP, Top right: TMTC RHCP, Bottom right: TMTC LHCP

 

TM/TC-System of the Flying Laptop

The TMTC system of the Flying Laptop is a highly reliable commercial system operating on commercial S-Band frequencies (between 2.0 GHz and 2.3 GHz) at a data rate of 128 kbit/s for the TM downlink and 4 kbit/s for the TC uplink. In order to improve the reliability, both the receiver and the transmitter are built redundantly so that the TMTC system is still fully functional in case one unit fails. The FLP is equipped with two hemispherical turnstile antennas for the TM/TC system so that telemetry can be sent and telecommands can be received at all times, regardless of the attitude of the satellite.

  Downlink Uplink Antennas
Frequency 2263.5 MHz 2083.5 MHz  
Bandwidth 350 kHz 8 kHz  
Data rate (net) 56 kbps 4 kbps  
Modulation BPSK (NRZ-L) PM (16 kHz BPSK sub-carrier)  
Coding CCSDS (Conv. R=1/2, K=7 & R.S. 255/223) CCSDS (BCH)  
Sensitivity / Output power -118 dBm 28 dBm  
Required Eb/N0 4.5 dB 13.5 dB  
Polarization     RHCP
Max. directivity     5 dBi
Theta3dB angle     110 deg

 

Payload Data Downlink System

The payload Data Downlink System (DDS) is used to downlink payload data, especially larger amounts of image data generated by the cameras of the Flying Laptop (MICS / PAMCAM) to a ground station. The Flying Laptop DDS uses data rates of up to 10Mbit/s. Because the available space and the electrical power are limited, a directional horn antenna is used, which has a much higher gain in boresight than the TT&C rod antennas for example. Due to the antenna’s directivity, its boresight needs to be directed to a receiving ground station in order to transmit data to the ground.

The DDS is designed to operate on ham radio S-Band frequencies between 2.4 GHz and 2.45 GHz. It receives the data it shall transmit to the ground as well as the clock signal from the Payload On-Board Computer (PLOC). The whole system is being developed at the IRS.

  DDS Downlink Antenna
Frequency 2408.0 MHz  
Bandwidth 10 MHz  
Data rate (net) 5.5 Mbps  
Modulation QPSK (NRZ-L, SRRC α=0.4)  
Coding CCSDS (R.S. 255/223 & Pseudo-Random.)  
Output power 30 dBm  
Required Eb/N0 7.8 dB  
Polarization   LHCP
Max. directivity   13.5 dBi
Theta3dB angle   15 deg