Live Link Testing
Live Over-The-Air Link Testing
As a final test to ensure that the Ku downlink is fully functional across both Space and Ground segments, an end to end transmission test was conducted, with the expected attenuations from a space to ground link. An MPEG Transport stream would be encoded over DVBS2, and sent via the Ku band link at designed frequency and power to the ground station segment, where the data would be recovered. To simulate link conditions at orbital altitudes, attenuators will be inserted into the RF chain after amplification of the transmission signal by the HPA. This attenuation must be calculated to match the difference in the actual free space attenuation due to the test distance, and the expected attenuation during actual space to ground transmission.
Firstly, a space with direct line of sight to the ground station testing location (E2A rooftop) was identified. This was found at Utown Residences North Tower, at level 24. The distance was measured on Google maps and subsequently confirmed with a Range Finder, estimated at 753m
To ensure an accurate link test, an attempt was made to match the path attenuation between the test conditions and the actual space to ground link.
Free Space Loss was determined as 20 log (D) + 20 log (f) + 92.45
An additional 2.74 dB was added to account for atmospheric and rain losses, at a 99% link availability rate (calculated from ITU R-618). The final attenuation difference would be inserted into the link through the use of attenuators.
The space segment and the ground segment were then set up at UTR and E2A respectively. GPS coordinates were acquired at both sites, and programmed into the Ground Station control system. The ground station would then automatically calculate and command the ideal pointing Azimuth and Elevation to target UTR. The Patch antenna was manually aligned through the use of the range finder.
A series of transmission tests were then planned and conducted as a functional test of the link:
- Full Power Test
- 30 dB Attenuation Test
- 40 dB Attenuation Test
- 65 dB Attenuation Test
A test image was encoded and transmitted over DVBS2 at QPSK ¾ at 2 MSPS using a B205 Mini SDR. On the receive side, a B205 Mini was used to receive the transmitted data. The DVBS2 Modulator/Demodulator came in the form of open source software SDRAngel, an user friendly Windows based Open Source SDR software.This image was successfully received and decoded by the software. Link conditions were relatively stable at this stage, and quadrature was acquired.
The next test was conducted at 30 dB attenuation. While the signal was usable, it was observed to barely rise above the noise floor. The B205 Mini’s Variable Gain Amplifier (VGA) was used to raise the signal to a level where the demodulator could achieve a lock. Data was received but some frames showed indications of corruption, while other frames were fully received. .
The third test conducted was at a 40 dB attenuation level. The variable gain amplifier was configured to different settings to improve the signal strength, with limited effect. At this stage, the signal was almost completely overpowered by the noise floor. The demodulator was completely unable to achieve a lock on the signal.
Through the various tests conducted, it was noted that the signal strength would fluctuate - by up to 10 dB on SDR Angel’s scale - leading to a suspicion that the dish pointing was being affected by environmental conditions such as wind loading. While the SDR is not calibrated to return accurate power measurements, the observed variation in Signal strength was enough to warrant further investigation, as it would significantly affect the ability to close the link budget.
