Space – Page 36 – Daniel Estévez

Receiving Ku-band geostationary satellite beacons

After sorting out some problems with several connectors which caused huge phase noise in the external 27MHz reference, I have my 10GHz receiver up and running as it should. This station will be used to receive Es’hail-2 in the future. The station is composed of a 95cm offset dish, an Avenger PLL321S-2 Ku-band LNBF modified to use an external 27MHz reference, an OCXO/Si5351A kit used as the 27MHz reference, an RTL-SDR, and a cheap DVB-S2 receiver as a power supply (this allows me to change polarizations and LO frequency easily).

The dish is pointing to the 26ºE or 25.5ºE orbital position, where Es’hail-2 will be. Actually, I have pointed the dish to peak the beacon from BADR-5 the best I can. To test the performance of the station, I have tried to receive the beacons from several Ku-band satellites. Here are the results.

First signals from AAUSAT-4

Today I woke up early to receive the signals from AAUSAT-4 as it passed over Spain for the first time. This satellite was launched from Kourou yesterday at 21:02UTC into a Sun-synchronous orbit. The main payload for the launch was Sentinel-1B, a 5GHz Synthetic Aperture Radar satellite from the Copernicus project of the ESA. The remaining satellites that were launched by the Soyuz rocket were Microscope, from the French CNES, designed to test Einstein’s equivalence principle and the three cubesats in the Fly You Satellite! program: OUFTI-1, from the University of Liège, which carries a D-STAR amateur radio transponder, e-st@r-II, from the University of Torino, and AAUSAT-4, from the University of Aalborg, which carries an AIS receiver. Since the launch was into a polar orbit, the first pass of the Fly Your Satellite! cubesats over Spain was at 05:42UTC today.

Scanning Ku band satellites with the FUNCube Dongle

I’ve recently installed my satellite dish and modified LNBF in my garden. This equipment will be used to receive Es’hail 2, the first geostationary satellite carrying an amateur radio transponder. Here I’ll look at the hardware I’m using, how I did the alignment to the 25.5ºE geostationary orbital position where Es’hail 2 will be located, and how to have some fun scanning the direct broadcast satellites in the Ku band with a FUNCube Dongle Pro+.

Updating Gpredict transponder files

Gpredict stores the information for the satellite transponders in plain text files, one file per satellite. These files are called .trsp files. The files shipped with the stable version tend to become outdated pretty soon, as new satellites are launched. However, the project’s git repository is usually up to date.

The following shell script will download the files from git and install the .trsp files in Gpredict’s config path in the user’s home folder. This can be used in Linux and probably other Unix-like operating systems. Something similar can also be done in Windows.

This is just an easy and temporary workaround to keep the transponder files updated, as Alexandru OZ9AEC, who is Gpredict’s author, thinks of perhaps using an online database as a long-term solution.

Doppler correction with GNURadio

When receiving signals from a satellite, it can be important to correct for the Doppler shift in the signal. Normally, I use Gpredict to track satellites and compute the Doppler shift. Gpredict can control the frequency of a receiver using Hamlib to track the Doppler shift. When using an SDR receiver, there are several possible ways of using Gpredict’s frequency control.

Normally, the SDR software doesn’t support Hamlib control in a way that it’s useful and easy to use for this purpose. This is the case with Linrad, which is the software I use, and probably with many other popular SDR softwares. An easy solution is to let Gpredict completely control the frequency of the SDR receiver through Hamlib and prevent the SDR software from controlling the frequency. With the FUNCube Dongle Pro+, which is the receiver I normally use, this is easy to do. It can be controlled without problem with recent versions of Hamlib, and if you set the dongle in Linrad as an “Undefined” card instead of a FUNCube Dongle, then Linrad will not try to control its frequency.

The problem with this solution is that each time that the frequency gets updated, it does so in a non phase continuous manner, because the PLL of the receiver has to lock on to the new frequency, effectively losing reception for just a tiny amount of time. This supposes no problem for SSB, CW or FM reception, because your ears just don’t notice. However, if you want to receive any digital signal or SSTV, the frequency change usually messes with the decoder software, which loses sync and suffers decoding problems. An alternative solution is to leave the receiver frequency fixed and correct for Doppler shift in software.

LilacSat-2 downlink signal strength

In a previous post, I recorded and decoded LilacSat-2 telemetry. This satellite transmits telemetry on 437.200MHz and 437.225MHz using two different radios and antennas, as can be seen in the radio info page. The transmission on 437.200MHz is usually 9k6 BPSK telemetry, but this is the same frequency, radio and antenna that is used for the amateur FM transponder when it is active. Looking at the waterfall as I recorded the IQ, I had the impression that the signal on 437.200MHz was much weaker than the signal on 437.225MHz. Using my LilacSat-2 receiver and the IQ recording I did, I have plotted the signal strength on both frequencies to compare.

LilacSat-2 GNUradio receiver

I have made my own LilacSat-2 telemetry GNUradio receiver using the flowgraphs included in gr-lilacsat. The new features of this receiver are the following:

All the decoders and the frontend are run on the same flowgraph. I find this more practical than having to run all the flowgraphs separately. Also, some blocks can be reused in this way.
It uses gr-gpredict-doppler to compensate for Doppler. As I mentioned in a previous post, I prefer this to the Doppler correcting system included in gr-lilacsat.
It plots and outputs to a file the signal strength on 437.200MHz and 437.225MHz. This can be used for later analysis.
It supports recording file input, or live SDR using an ALSA source (for the FUNCube Dongle Pro+, for instance). Another SDR supported by GNUradio can be easily used.
It supports recording both the raw IQ data and the Doppler corrected IQ data. The raw IQ file can be then played back by simultaneously running Gpredict with the correct settings for the recorded pass. The Doppler corrected IQ file can be played back without running Gpredict.

This receiver can be downloaded from GitHub. The flowgraph is a bit crammed, but that’s what you get for having all the decoders in the same flowgraph. Several of the input/output blocks are disabled, so that you can choose which ones to use.

LilacSat-2 telemetry PDU decoder

Recently, I showed my experiences receiving and decoding LilacSat-2 telemetry. I managed to get 15 packets of 4k8 GFSK telemetry and 15 packets of 9k6 BPSK telemetry during a single pass. However, I couldn’t decode these packets because I didn’t know the telemetry format. It turns out that Mike DK3WN has released a decoder software. Here I show Mike’s software decoding the packets I received.

Decoding LilacSat-2 telemetry

LilacSat-2 9k6 BPSK packet (left) and 9k6 FSK packet (right)

After having my first QSO through the Harbin Institute of Technology amateur radio satellite LilacSat-2, I decided to give a serious try to the telemetry decoding software. This is available as a GNURadio module. A Linux distribution with all the proper software installed and configured is provided, for an easy use. However, I have used GNURadio in the past, so I wanted to try to setup the GNURadio module directly on my machine.

The web page for LilacSat-2 gives also a description of the different telemetry formats. The satellite has an SDR radio transmitting on 437.200MHz. This radio is used for the FM amateur radio transponder and also to transmit several different telemetry formats. The satellite also transmits telemetry on 437.225MHz, presumably using a different (non-SDR) radio and a different antenna, so that the satellite can keep transmitting telemetry even if the SDR system fails. Typically, when the FM transponder is disabled, the satellite will transmit 9k6 BPSK telemetry on 437.200MHz and 4k8 GFSK telemetry on 437.225MHz. These can be seen in the picture above, which was made using my RF recording and baudline. The packet on the upper right is 4k8 GFSK and the packet on the lower left is 9k6 BPSK. Notice the slight slant due to Doppler.