Downlink
Downlink is a communication link between a satellite and a ground station. In general, downlink contains HM data and payload related data primarily.
Flow of Data for Downlink
- Acquisition and Encoding of data: Different types of data gathered by the OBC which need to be transmitted to the ground-station. Further, this data needs to be encoded/packeted.
- Modulation of data: Now this data needs to be modulated on the desired carrier frequency.
- After the acquisition of data, we have to do baseband processing. Here 'baseband' refers to the original frequency range of a transmission signal before it is converted, or modulated, to a different frequency range. For example, an audio signal may have a baseband range from 20 to 20,000 Hz which is modulated to a frequency in between 80 MHz to 108 MHz for FM modulation (all the popular radio stations operate in this range). Here depending on your data and modulation type you may need to use Digital to Analog Converter (DAC) to achieve your desired data type that you need to transmit.
- Further, we have to do up-converting of signal to carrier frequency.
- Conditioning the signal: Here we have to accomplish two basic tasks related to conditioning of signal:
- Filtering: We have to filter out any unwanted signals present outside the signal bandwidth, which have been generated on transmission side due to hardware.
- Amplification: Further, we need to amplify the signal such that when receiving the signal at the ground-station its gain should be reasonably above the noise level.
- Transmission of the signal: We need to transmit the signal by radiating it through an antenna. You can select antenna using your requirement criteria.
Note: Most of the Student Satellites use Commerical Off-The-Shelf (COTS) transceivers. So, the entire modulation part which contains baseband processing, upconverting, DAC are part of the transceiver.
Case Study - Pratham
In Pratham, the on-board computer (OBC) gathers HM data from various sensor readings and stores it in the on-board memory. The OBC microcontroller accesses this data and packets it using AX.25 protocol and pushes it towards the downlink board microcontroller. On the downlink board, we have the CC1101 transceiver which gets data from downlink board microcontroller and then modulates it on the carrier frequency using 2-bit FSK modulation scheme. Now for conditioning, we amplify the signal gain using RF5110G Power Amplifier and then transmit the signal using monopole antenna onboard the satellite to ground.
If you are done reading this page, you can go back to Communications Subsystem
