satellites – Page 13 – Daniel Estévez

Simulating delta-range observations in GMAT

During the DSLWP-B (Longjiang-2) mission, we made a number of VLBI observations of the spacecraft’s UHF signal by performing GPS-synchronized recordings at Dwingeloo (The Netherlands), Shahe and Harbin (China), and Wakayama (Japan). The basic measurement for these observations is the time difference of arrival (TDOA), which measures the differences between the time that it takes the spacecraft’s signal to arrive to each of the groundstations. This can be interpreted in terms of the difference of distances between the spacecraft and each groundstation, so this measurement is also called delta-range.

One very interesting practical application of the VLBI observations is to perform orbit determination. The delta-range measurements can be used to constrain and determine the state vector of the spacecraft. This would give us an autonomous means of tracking Amateur deep-space satellites, without relying on ranging by a professional deep-space network. Even though the measurements we made showed good agreement with the ephemerides computed by the Chinese deep-space network, during the mission we never ran orbit determination with the VLBI observations, mainly due to the lack of appropriate software.

While GMAT has good support for orbit determination, it doesn’t support delta-range measurements. Its basic orbit determination data type is two-way round-trip time between a groundstation (or two) and the satellite, as shown in the orbit determination tutorial.

I have started to modify GMAT in the gmat-dswlp Github repository to implement the support for this kind of VLBI observations. As a first step, I am now able to create and simulate delta-range observations.

Wiggles in the QO-100 local oscillator

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.

LES-5 RFI experiment telemetry

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.

Idle data in BepiColombo X-band signal

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.

Decoding BepiColombo

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.

Earth rotation corrections for range and range-rate in GNSS

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.

More data from LES-5

Yesterday I looked at decoding some data transmitted by LES-5. Today I have analysed a longer recording made by Scott Tilley VE7TIL to perform an eclipse timing on 2020-03-25. The study has been done in this Jupyter notebook, which looks at the sequences of symbols extracted before and after the eclipse (they are kept as two separate sequences because the transmit frequency changed slightly after the eclipse, so decoding required two separate passes).

Decoding LES-9

After decoding a recording of the LES-5 236.7MHz telemetry beacon made by Scott Tilley VE7TIL, I have decoded an older recording made by Scott of the S-band beacon of LES-9. This satellite was launched in 1976 and it has a 100 baud BPSK beacon at 2250MHz. Scott twitted about it in April 2019, and in January 2020 he reported that the modulation had stopped and the beacon was now a CW carrier.

I have used this recording made by Scott in 2020-01-13. The GNU Radio demodulator, which is very similar to the one for LES-5, is here and the Jupyter notebook with the results is here. Below, I make a brief summary of the results.

Decoding LES-5

LES-5 is a satellite launched in 1967. It was built by the MIT Lincoln Laboratory and its main payload was an experimental transponder for the military 230MHz band. It was placed in a subsynchrounous orbit with an altitude of around 33400km (GEO altitude is 35786km). Its operations ceased in 1971.

A couple days ago, Scott Tilley VE7TIL discovered that LES-5 was still transmitting, and was able to receive its beacon at 236.749MHz. Scott reports that LES-5 is the oldest GEO-belt object that he knows to be still transmitting.

The beacon is modulated, rather than being a CW carrier, so Scott sent me a short recording for analysis. This post is a summary of my study.

Decoding images from AMICal Sat

AMICal Sat is a 2U cubesat developed by the Space Centre of the Grenoble University, France, and the Skobeltsyn Institute of Nuclear Physics in the Lomonosov Moscow State University. Its scientific mission consists in taking images of auroras from low Earth orbit. The satellite bus was built by SatRevolution. Currently, the satellite is in Grenoble waiting to be launched on a future date (which is uncertain due to the COVID-19 situation).

A few weeks ago I was working with Julien Nicolas F4HVX to try to decode some of the images transmitted by AMICal Sat. Julien is an Amateur radio operator and he is helping the satellite team at Grenoble with the communications of the satellite.

This post is an account of our progress so far.