zaterdag 20 februari 2016

Hearing the sound of a satellite

So, the last few weeks were quite exciting at the DopTrack satellite tracking station. We have updated the software, such that the station is now able to automatically record any satellite we tell it to record. Also, the radio antennas are installed higher in the sky for better visibility. Moreover, TUDelft students have designed software to automatically extract the carrier signal from raw recordings. And last but not least, the development of the website for the Virtual Laboratory DopTrack is started, but I will report on that later on, when we have a working website. All major steps to have our project become a real educational tool for satellite tracking. 

Software update: Automated satellite tracking
The DopTrack station is now able to automatically compute, when a satellite is flying over and start recording it accordingly. We just have to set a line of code in the rec.list file and DopTrack is doing the rest. An example recording list is:

Delfi-C3                          32789                145870000                  250000
Delfi-n3Xt                      39428                 145870000                  250000
UKUBE-1                      40074                 145870000                  250000
CANX-2                         32790                 437478000                  250000

The list just needs the name, NORADID number, tuning frequency for the radio, and bandwidth for the recording. Everyday, this file is checked and new recordings are added to the waiting list. We then record the selected satellite when it is flying over, which is determined by propagating its updated TLE file. An example result: the radio downlink spectrogram of the UKUBE-1 (FunCube-2 module) is seen in the following figure.
Spectrogram of the recording UKUBE-1 at tuning frequency 145.870 MHz during the satellite pass on 18th of February 2016. Start of recording was 12:12 CET.
The shift in the frequency is due to the relative motion of the satellite as seen from the ground station. The goal of the DopTrack project is to determine the actual orbit of satellites by extracting this Doppler shift and convert it to range-rate observables. These can be used in orbit determination studies.

Positioning of the radio antennas
Last week, the antennas have received new locations. Three,three meter high, RVS poles were placed on the roof of the EWI building. We have placed the GPS and radio antennas (VHF and UHF) on the top of those poles. This gives the antennas 360 degree visibility of the sky.
Final inspection of the radio antennas by the DopTrack team.
The installation of the antennas was a good opportunity to have our intern do some work on the station and have some team bounding going on. Martin helped me installing the antennas on the poles, while Joao was busy downstairs. He installed a new power switch, which makes it possible to remotely turn off and on hardware components in the station. The new location of the antennas are awesome. Its almost as if they reach for the moon!
The VHF antenna with amplifier on the left and the UHF antenna on the right, both reaching for the moon.
After some small software hiccups, the station is back up and records the radio downlink of Delfi-C3.

DopTrack's Range-Rate Extraction software: DRRE
This semester DopTrack played a role in the new Space Minor of our faculty. For ten weeks long, two groups of students were asked to develope software that is able to extract the carrier signal of Delfi-C3 from the raw recorded data of DopTrack. At the end of the project, the students were able to delivered this software. Yet again it proves that our students are not to be under-estimated.
The red dots are carrier signal data records automatically extracted from the raw recording. The gray-scale picture is the spectrogram of the raw recorded electromagnetic spectrum.
Here, the red dots are the extracted data from the raw recording. Despite, the horizontal and vertical unwanted signal, the students were able to let the computer do all the work without them telling it too much specific information. Great job, because now we are able to post-process all the recorded data we have. So, I did this and made a plot of all the frequency at Time-of-Closest_Approach, or FCA. This is exactly at the bending point in the Doppler S-curve. Here, the relative velocity of the satellite is zero compared to the ground station and we are recording the real frequency of the satellite. In other words, we are hearing the true sound of the satellite.

Six months of data showing the variation of the transmitted frequency of Delfi-C3 is around 800 Hz, with some outliers.
For now, we have six months of data and all we can deduce from this data is that the onboard oscillator has an influence on the transmitted frequency of around 800 Hz. But after more recordings, we eventually hope to see a long trend in the data that we can attribute to degradation and/or temperature change onboard the satellite.

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