My LimeSDR board arrived on April 26, a day before the crowdfunding campaign was launched. I made an agreement to test the board and post my experiences here on the MyriadRF blog. This is the first out of hopefully many posts I will make about the LimeSDR.
Setting up the LimeSDR
I was a bit worried that the software support might not be quite ready for on-air testing. I wanted to test the LimeSDR using an existing SDR application like Gqrx. To that end, I was looking for driver library similar to librtlsdr or libairspy, which provides simple API to the hardware.
The LimeSDR uses the LMS7002M programmable RF transceiver with digital interface. Consequently, it requires more setup than what we are used to from the simpler RF tuner chips.
Fortunately, my worries were unnecessary. Josh Blum has already added LimeSDR support to SoapySDR and the board could be accessed by Gqrx as a SoapySDR device. You can read more about this in the Lime Suite driver architecture blog post by Josh.
Thus, my task was limited to figuring out what sample rates and bandwidth settings I could use in Gqrx. I could use sample rates up to 10 Msps limited by my computer. More than enough for what I had in mind.
I guess the first on air test we always perform with a VHF SDR is reception of FM radio. FM radio stations are always there and strong, so they provide a useful “signal generator” when testing receivers. Needless to say, I was very happy when I saw the first pixels on the FFT plot and could hear the music from the radio station.
Amateur radio satellite reception
The next item on my agenda was to try receiving an amateur radio satellite. I connected a handheld Arrow II Yagi antenna directly to the receiver input of the LimeSDR, which is sufficient for receiving satellites in VHF and UHF.
All current amateur radio satellites are in low Earth orbit and have an altitude between 600 and 1000 km. Thus, the range to the satellite is usually few thousand km while it is in line of sight and the signals can become very weak.
The waterfall below shows my very first satellite reception done with the LimeSDR. The satellite was FO-29, which has a beacon sending Morse code and a linear transponder used as a “flying repeater”.
The signals appear to drift in frequency because they are subject to Doppler shift. I have left the Doppler correction off to better illustrate that this is signal coming from an orbiting satellite.
A few days later I made another reception of FO-29. This time I observed a lot of extra carriers on the satellite downlink.
These carriers have a different Doppler shift than the satellite beacon. Thus, they must be interference on the satellite uplink.
Weather satellite reception
After the FO-29 reception I have decided to also try receiving weather satellites.
The NOAA weather satellites are also in low Earth orbit. They continuously scan the Earth using various sensors and transmit the data down on VHF and L-band frequencies. The VHF transmissions are in the APT format, which is very easy to receive using the following workflow:
- Record WAV file with gqrx using the built-in APT FM mode.
- Convert the WAV file to 11025 Hz mono using sox.
- Process WAV file with wxtoimg.
And here are the first NOAA weather satellite images received with the LimeSDR. The first image shows the raw sensor data, which at the time of the reception was near infrared and thermal infrared.
The next images is a false colour image generated by wxtoimg using HVC enhancement.
You really don’t need a big setup for receiving satellites. NOAA APT transmissions are very strong and a simple Turnstile antenna is sufficient.
The video below shows a reception in progress together with the simple antenna setup I was using. I recorded this video few days after I captured the image. Just to illustrate that receiving weather satellite images can be done with a simple setup.
There are two things I would really like to test before the crowdfunding campaign ends:
- Transmitting something
- Shortwave reception
Transmitting should not be a problem — all I need to do is create some GNU Radio programs.
On the other hand, tuning to shortwave frequencies requires some configuration on the LMS7002 that is not yet available to Gqrx, so that may take a little longer.