Space and satellites are something that only few people are fortunate enough to interact with. However, this is starting to change due to the rapid growth in capability of consumer electronics. In fact, you can receive and decode transmissions from satellites using only a Raspberry Pi, a USB software-defined radio receiver, and a few other cheap parts.
The orbits of all satellites are regularly measured by NORAD radar and their orbital parameters (known as TLEs or Keps) are publicly available. Using these orbital parameters, it's possible to predict the position of each satellite. Here's an example map showing 5 hours of FITSAT-1's orbit:
Since there are many satellites, it's useful to know when any of them will be passing above our station. Carpcomm's website has a system for this. For example, here you can see the upcoming passes one afternoon for my station:
To get the predictions for your location, sign into the Carpcomm website and create a entry for your station. You will need to specify the location, the elevation, and the direction of your antenna.
Now whenever a satellite passes over, we need to tune the radio to the correct frequency and record the pass. Then we can review the recorded data and decode information from it.
Plug the USB radio receiver into the Raspberry Pi. Connect it to the low-noise amplifier (LNA) with a coaxial cable and connector adaptor. Connect the LNA to the antenna with another coaxial cable.
The connectors depend on the specific radio and LNA. RTL-SDR radios usually have MCX connectors. The FUNcube Dongle uses SMA-female. The DG0VE LNA has SMA-female connectors on both ends.
It may be possible to use the FUNcube Dongle without an LNA. However, an LNA is definitely needed for the RTL-SDR dongles.
The hardest part is mounting the antenna. It has to be mounted somewhere with a clear view of the sky within its viewing angle. For small Yagi, like the one in the photo below, you'll need a clear view from 30° to 90° degrees elevation and ±30° azimuth. The Yagi should be mounted at roughly 30° elevation. As you can see below, I have attached my antenna to a pole on the roof of a house. However, setting it up on the balcony of an apartment should work too.
To test whether the connected RTL SDR works, run "rtl_test".
To test whether the connected FUNcube Dongle works, run "carpsd-fcd".
Go to your station page on the Carpcomm website. If the station is online, you should see a "Go to station console" link:
Click on it to get to the station console page:
Tune it to 434 MHz. This is in the ISM band in Europe and Africa so there are likely to be a lot of transmissions there. Click the "Start" button to start the waterfall. You should see something like this:
What does it mean? The x-axis is frequency and the y-axis is time. The color indicates the signal power. The gray part is just background noise. The white blotches are signals. You may see many or just a few signals depending on how many users of the 434 MHz ISM band there are nearby.
Check your pass predictions for one of these satellites. Wait until it is about to pass over, then tune to the appropriate frequency (as listed on the satellite page above), start the waterfall and stop it once the pass is finished. The recorded data will then be uploaded to servers and we'll try to decode it. The server will also generate a spectrogram of the recording that you can inspect.
Here's an example spectrogram from FITSAT-1:
Notice that you can read off the Morse code directly! (Also notice how the frequency changes over time. This is due to Doppler shift - more info about that here.)
The decoded telemetry will be displayed on the satellite's page:
Automatic operation can be enabled by clicking the "edit" button on your station page:
You should follow @Carpcomm on Twitter, +Carpcomm on Google+ or subscribe to this blog for updates.
There is also a mailing list for the CarpSD open source project.
In this blog post, I'll describe in detail how to put together such a system.
Experimenting with receiving satellite signals is a lot of fun. Also, by receiving data from these satellites and sharing it with the satellite owners, you can help their scientific missions.
Goal
Build a satellite receiving station for amateur radio band cubesats using a Raspberry Pi.Parts needed
- Raspberry Pi
- RTL-SDR or FUNcube Dongle Pro+ USB radio receiver
- Low-noise amplifier for the 70 cm amateur radio band e.g. DG0VE LNA70-1
- Yagi antenna for the 70 cm amateur radio band
- Coaxial cable and connectors between the antenna and LNA
- Coaxial cable and connectors between the USB radio and LNA
- Power supply for the LNA
Background
The satellites we are going to receive are typically orbiting between 300 and 700 km altitude. At these altitudes, satellites orbit the earth every couple of hours or so. A satellite will be within range for only perhaps 2 to 5 minutes per orbit.The orbits of all satellites are regularly measured by NORAD radar and their orbital parameters (known as TLEs or Keps) are publicly available. Using these orbital parameters, it's possible to predict the position of each satellite. Here's an example map showing 5 hours of FITSAT-1's orbit:
 |
| 5 hours of FITSAT-1's orbit. |
 |
| Satellite pass predictions at my station in Durban, South Africa. |
To get the predictions for your location, sign into the Carpcomm website and create a entry for your station. You will need to specify the location, the elevation, and the direction of your antenna.
Now whenever a satellite passes over, we need to tune the radio to the correct frequency and record the pass. Then we can review the recorded data and decode information from it.
Hardware construction
It's fairly easy to connect the parts together. Here's a diagram:
Plug the USB radio receiver into the Raspberry Pi. Connect it to the low-noise amplifier (LNA) with a coaxial cable and connector adaptor. Connect the LNA to the antenna with another coaxial cable.
The connectors depend on the specific radio and LNA. RTL-SDR radios usually have MCX connectors. The FUNcube Dongle uses SMA-female. The DG0VE LNA has SMA-female connectors on both ends.
It may be possible to use the FUNcube Dongle without an LNA. However, an LNA is definitely needed for the RTL-SDR dongles.
The hardest part is mounting the antenna. It has to be mounted somewhere with a clear view of the sky within its viewing angle. For small Yagi, like the one in the photo below, you'll need a clear view from 30° to 90° degrees elevation and ±30° azimuth. The Yagi should be mounted at roughly 30° elevation. As you can see below, I have attached my antenna to a pole on the roof of a house. However, setting it up on the balcony of an apartment should work too.
Software setup
For the software, we'll use the CarpSD ground station control software. It's an open-source program with the source hosted on GitHub. It runs as a background process and connects to the Carpcomm server, so that you can control your station from the Carpcomm website. Thus, there is no need to connect a display to your Raspberry Pi and you can leave your station running continuously in the background. The instructions below can be executed entirely over SSH.Preparation
Firstly, make sure you have a recent version of Raspbian on your SD card. Then SSH into your Raspberry Pi.Setup tmpfs
To avoid wearing out the flash storage, we'll configure CarpSD records radio data to tmpfs. Therefore, we need as much RAM as possible. Run$ sudo raspi-configand choose the maximum RAM/video split. Next, increase the tmpfs size by editing /etc/default/tmpfs:
$ nano /etc/default/tmpfsFind the line "#TMPFS_SIZE=20%VM" and change it to "TMPFS_SIZE=90%VM".
Install CarpSD
CarpSD .deb packages are available from the Carpcomm apt repository. First add the repository public key:$ wget http://carpcomm.com/apt/public.key$ sudo apt-key add public.keyThen edit /etc/apt/sources.list to add this line:
deb http://carpcomm.com/apt/ lucid mainNow install the package with apt:
$ sudo apt-get update
$ sudo apt-get install carpsd carpsd-fcd rtl-sdr
Check that your USB receiver is detected
First, check that your receiver is being seen correctly by typing$ lsusbIt should display something like:
Bus 001 Device 004: ID 04d8:fb31 Microchip Technology, Inc.
for the FUNcube Dongle Pro+, or
Bus 001 Device 004: ID 0bda:2838 Realtek Semiconductor Corp. RTL2838 DVB-T
for the RTL-SDR.
To test whether the connected RTL SDR works, run "rtl_test".
To test whether the connected FUNcube Dongle works, run "carpsd-fcd".
Configure CarpSD
If you haven't already done so, log into the Carpcomm website and create an entry for your station with its latitude, longitude and elevation. On the station page, click the "Get login info" link and note your station's id and secret.
Then edit the configuration file in /etc/carpsd/carpsd.conf so that it looks like this (for the FUNcube Dongle):
[Client]
id: <your station id>
secret: <your station secret>
server: mux.carpcomm.com:1234
ca_certificate: /etc/carpsd/ca_cert.pem
[FCDReceiver]
recording_dir: /dev/shm
alsa_device: hw:1
or like this (for RTL-SDR):
[Client]
id: <your station id>
secret: <your station secret>
server: mux.carpcomm.com:1234
ca_certificate: /etc/carpsd/ca_cert.pem
[RTLSDRReceiver]
recording_dir: /dev/shm
device_index: 0
tuner_gain_db: 0.0
sample_rate_hz: 96000
Start CarpSD
Now start CarpSD by running:$ sudo /etc/init.d/carpsd startCheck the logs to see if it connected successfully:
$ tail /var/log/carpsd/carpsd.latest.stderrIf you go to your station page on the Carpcomm website, it should say "Online".
Testing the station
Now that all the hardware and software is setup, we can finally try it out!Go to your station page on the Carpcomm website. If the station is online, you should see a "Go to station console" link:
Click on it to get to the station console page:
Tune it to 434 MHz. This is in the ISM band in Europe and Africa so there are likely to be a lot of transmissions there. Click the "Start" button to start the waterfall. You should see something like this:
What does it mean? The x-axis is frequency and the y-axis is time. The color indicates the signal power. The gray part is just background noise. The white blotches are signals. You may see many or just a few signals depending on how many users of the 434 MHz ISM band there are nearby.
Receiving a satellite
Now that we know the station works, we can try to pick up a signal from a satellite. Some satellites are easier to receive than others. Usually the easiest are ones that have a morse code beacon. Some examples: STRaND-1, SwissCube, FITSAT-1, Masat-1.Check your pass predictions for one of these satellites. Wait until it is about to pass over, then tune to the appropriate frequency (as listed on the satellite page above), start the waterfall and stop it once the pass is finished. The recorded data will then be uploaded to servers and we'll try to decode it. The server will also generate a spectrogram of the recording that you can inspect.
Here's an example spectrogram from FITSAT-1:
Notice that you can read off the Morse code directly! (Also notice how the frequency changes over time. This is due to Doppler shift - more info about that here.)
The decoded telemetry will be displayed on the satellite's page:
Automatic operation
Once you've managed to receive a few satellite manually, you can enable automatic operation for your station. It will then record and try to decode every satellite that passes overhead, without you needing to do anything. This is useful especially for passes during the night or when you're busy with other things.Automatic operation can be enabled by clicking the "edit" button on your station page:
Concluding remarks
I hope this guide will encourage more people to try building satellite receiving stations. Please do let us know your experiences in the comments.
There is also a mailing list for the CarpSD open source project.
