Some days ago, Hans Hartfuss DL2MDQ sent me an email about some frequency measurements of the QO-100 NB transponder BPSK beacon that he had been doing. The BPSK beacon is uplinked from Bochum (Germany) through the transponder, and as the beacon is generated using a very good Z3081A GPSDO as a reference, the frequency drift observed on the beacon downlink is caused by Doppler and the drift of the local oscillator of the transponder.
In his measurements, Hans observed some small oscillations or “wiggles” that didn’t seem to be caused by Doppler. Decided to investigate this, I started to do some measurements of my own. This post is an account of my measurements and findings so far.
In previous posts, I have talked about the attempts of decoding LES-5 telemetry done by Scott Tilley VE7TIL and me. Now Taylor Bates KN4QGM has joined us in our efforts, and with their help I think I have figured out most details of how the telemetry of the RFI experiment works. One of the payloads of LES-5 was a radio frequency interference experiment that scanned the 255-280 MHz band and made spectrum measurements. The receiver of this experiment was also used as the telecommand receiver for the spacecraft. We are very interested in studying the telemetry of this RFI experiment to see to what extent the receiver is working and if the spacecraft could actually receive commands.
Yesterday I posted about decoding the data in an X-band recording of BepiColombo. I only made a very shallow analysis of the data, which consisted of CCSDS TM Space Data Link frames. However, I showed that most of the data was transmitted on virtual channel 7. A few hours later, Oleg_meteo in Twitter noted that this data in virtual channel 7 was just a 511 bit PN sequence. After some analysis I’ve confirmed what Oleg_meteo said and shown another interesting and unexpected property of this data.
All the Space Data Link frames in virtual channel 7 have a first header pointer field of 2046, which means “idle data only”. When the payload in these frames is concatenated (there are 8792 payload bits per frame) we obtain an infinite sequence that fits the following description.
BepiColombo is a joint mission between ESA and JAXA to send two scientific spacecraft to Mercury. The two spacecraft, the Mercury Planetary Orbiter, built by ESA, and the Mercury Magnetospheric Orbiter, built by JAXA, travel together, joined by the Mercury Transfer Module, which provides propulsion and support during cruise, and will separate upon arrival to Mercury. The mission was launched on October 2018 and will arrive to an orbit around Mercury on December 2025. The long cruise consists of one Earth flyby, two Venus flybys, and six Mercury flybys.
The Earth flyby will happen in a few days, on 2020-04-10, so currently BepiColombo is quickly approaching Earth at a speed of 4km/s. Yesterday, on 2020-04-04, the spacecraft was 2 million km away from Earth, which is close enough so that Amateur DSN stations can receive the data modulation sidebands. Paul Marsh M0EYT, Jean-Luc Milette and others have been posting their reception reports on Twitter.
Paul sent me a short recording he made on 2020-04-04 at 15:16 UTC at a frequency of 8420.535MHz, so that I could see if it was possible to decode the signal. I’ve successfully decoded the frames, with very few errors. This post is a summary of my decoding.
In GNSS, when considering the propagation of signals from the satellites to a receiver, it is easier to work in an ECI reference frame, since (ignoring the gravitational potential of Earth), light travels in straight lines in ECI coordinates. However, it is often common to do all the calculations in an ECEF frame, as the final goal is to obtain the receiver’s position in ECEF coordinates, and the ephemerides also use ECEF coordinates to describe the satellite positions. Therefore, a non-relativistic correction needs to be applied to account for the fact that light no longer travels in straight lines when one considers ECEF coordinates. Often, the correction is done as some kind of approximation. These types of corrections are known in the GNSS literature as the Sagnac effect.
The goal of this post is to discuss where the corrections arise from, the typical approximations that can be made, and how these corrections affects the calculation of range and range-rate. I didn’t find a good source in the literature where this is described in detail and in a self-contained way, so I decided to write it myself.