Over on YouTube Leif (SM5BSZ) has uploaded two videos showing some of his tests with the new Airspy Mini. Leif is fairly well known within the SDR community for writing the program Linrad and for doing various tests on different SDR’s on YouTube and his website. Recently Leif also did some testing on the SDRplay on a previous video.
In his video he shows how to improve the shielding on the Airspy Mini enclosure by ensuring that a good electrical connection is made between the SMA connector and aluminum enclosure. Improving the shielding reduces out of band interference and USB noise.
The Airspy Mini is a $99 USD software defined radio with up to 6 MHz of bandwidth, a 12 bit ADC and tuning range from 24 MHz to 1800 MHz. If you are interested in the Airspy Mini we will also be doing a review of this SDR soon.
The RTL-SDR has a maximum available stable bandwidth of about 2.4 MHz. Many people have had the idea to combine multiple RTL-SDR dongles together to implement a wider band or multi channel RX device, but very few successful implementations have been seen. The biggest challenge is time synchronization between the multiple RTL-SDR units. Even if a common clock is used, there is no guarantee that the samples streams are synchronized, which can cause problems for the decoding of many signals. The most successful implementations so far have used a common clock, and an external synchronization signal from a generator in addition to other hardware like switches.
In his Multi-RTL block he implemented a method of a discovery he made that allows a way to time synchronize the dongles by using a signal that is already being broadcast over the air. He writes that his method is the following:
tuning the RTL-SDR dongles to the same frequency where some transmission is present,
recording a short signals with all of the dongles,
computing cross-correlation of the signals (i.e. with respect to a one selected channel),
finding position of maximums of cross-correlations in order to estimate relative delays of the channels,
correcting the delays so the channels are time-synchronized,
switching the dongles to their target frequencies,
changing other parameters of the channels (like gains) to target values.
With his Multi-RTL GNU Radio block Piotr was able to successfully monitor a GSM uplink and downlink channel pair that were spaced 45 MHz apart. Whilst monitoring the signals he sent an SMS to his phone, and then using his recovered encryption key was able to use gr-gsm to decode his message.
The successful implementation of this tool opens the door for many more RTL-SDR based projects, such as the reception of GSM uplink and downlink channels simultaneously, reception of frequency hopping signals, passive radar, and the receiving and decoding of signals with a bandwidth wider than 2.4 MHz.
Two dongles with a common clock.Synchronizing two dongles by using an external signal.
Unfortunately patients who are interested in taking a more active approach to their health (such as one member of the team who herself has an implanted defibrillator) do not get to see this data. The team are hoping to use an RTL-SDR to sniff this data which is transmitted in the 402 – 405 MHz ISM band, and then implement a decoder. So far they have successfully been able to capture some signals, and are working on decoding them into data.
By reverse engineering the signal they hope to draw attention to the fact that healthcare providers are not providing real time body data to the patient, preventing them from making their own informed decisions about their health. They write:
It’s all about making informed decisions. A patient knowing about arrhytmias episodes that occured to him/her has the power to change his lifestyle accordingly, by deducing the factors that have influenced his recent attacks and eliminating them – i.e. observing his/her heart condition according to his/her sleep schedule, work rhythm, food choices and participation in sports. As for now, the patients can only hope to get some information on ICD-prevented arrhytmias on scheduled appointments with their doctor, which often occur once a year or even less often. This eliminates any possibility of making informed choices by using patient’s lifestyle data for future arrhythmia episode prevention.
RTL-SDR.com reader Jean Marie Polard (F5VLB) recently wrote in to let us know about a useful document that he has put together which covers beginners amateur radio astronomy. The document includes various introductions to the types of antennas and electronic tools often used in radio astronomy, the software used and an introduction to all the different types of observable objects. There are also a few mentions of the RTL-SDR dongle which is known to be a useful tool for amateur radio astronomy.
The document is available in pdf form in English, as well as in French. If you are looking at getting started in amateur radio astronomy then this is a good starting guide.
The LimeSDR is a new SDR $299 USD currently seeking crowdfunding over on CrowdSupply. At the time of this post the LimeSDR is currently 60% funded, with 29 days left to go. The LimeSDR is a RX/TX capable device, with a 100 kHz – 3.8 GHz frequency range, 12-bit ADC and 61.44 MHz bandwidth. From the price and specs, we consider it to be a potential next generation HackRF type device which will have vastly improved RX performance.
Over on the MyriadRF blog, Alexandru Csete has received a demonstration unit, and has written about his first thoughts on the LimeSDR. Alexandru was able to easily set the device up by using SoapySDR and GQRX on Linux. He first did the “hello world” of SDR and was able to successfully receive broadcast FM signals. Next he tried to receive Amateur Satellite signals and was successful in receiving the FO-29 satellite. Finally he was also successful in receiving NOAA weather satellite images.
From a quick judge of the waterfall images it looks as though the LimeSDR has a very clean spectrum with a low noise floor, which looks good for RX. In future posts Alexandru hopes to test out the transmit capabilties of the LimeSDR, as well as its shortwave RX performance.
It is well known that the NOAA satellites broadcast weather satellite images which can be received and displayed with an RTL-SDR and computer. What is less known is that there is a telemetry beacon that is also transmitted by the same satellites. The telemetry not only contains data such as the current spacecraft time, day and ID, but also contains scientific data from on board instruments such as:
The HIRS/3 and HIRS/4 instruments which is a high resolution infrared sounder which can be used to create a low resolution multi-spectral scan of the earth. (more info)
The Space Environment Monitor (SEM-2) which has a Medium Energy Proton and Electron Detector (MEPED), and a Total Energy Detector (TED). This experiment is used to measure the effect of the sun on satellite communications. (more info)
The experimental DCS/2 transmitter which retransmits signals from 401.65 MHz sea buoys, arctic fox collars, sea ice monitors, weather balloons and more. (more info pdf)
The ARGOS Advanced Data Collection System (ADCS) which amongst other uses is used in research for tracking animal GPS collars around the world.
A satellite tracker is a motorized unit that points a directional antenna towards passing satellites. Most satellites are not in a fixed orbit, and will fly over your head a few times a day and will be receivable for a few minutes, and a directional antenna is usually recommended since the signals can be weak. The goal of the SatNOGS project is to set up various volunteer satellite tracker stations around the world, and network the received data on the internet, so that satellite data is always being received and shared.
Although the tracker works, he admits that there are some problems and that it is probably not as good as the SatNOGS recommended build, which is a more permanent solution. But the SatNOGS build requires access to a 3D printer and higher quality components, so Paul’s solution is a much cheaper solution to implement at least for experimentation.
Over on YouTube user FMDX HUN (Luc1f3rk0) has uploaded a video showing how useful the SDR# IF Processor and Notch Filter Plugin can be when attempting to DX FM broadcast stations. He shows that it can be used to listen to stations that are almost overlapping by cutting out the unwanted signal.
