The DDS (Data Downlink System) is a system which sends the payload data to a ground station. It sends the pictures with a rate of 10 Mbps to the IRS ground station in order to be able to transmit the required amount of data in the limited transmission time. HAM amateur radio frequencies are used within the S-Band. 

In order to do so, 3 pieces of equipment are used:

  • Two redundant transmitters (on the left) modulate the signal into a SRRC (Square Root Raised Cosine Filter) QPSK (Quadrature Phase Shift Keying) waveform. The output power of the transmitter is about 0,5 Watt. The transmitter has been developed with COTS components and qualified at the IRS.
  • A space qualified switch (center) is used to switch between the two transmitters in case one of them fails.
  • Finally, an antenna (on the right) is used to transmit the signal into free space. This antenna has a gain of about 13,5 dB and is circular polarised in order to minimise the influence of the orientation and position of the satellite on the link budget. The antenna is based on waveguide technology and was developed at the IRS.



Picture: Measurement of the directivity of the DDS horn antenna