Back in September the GNU Radio 2016 (GRCon16) conference was held. GRCon16 is an annual conference centered around the GNU Radio Project and community, and is one of the premier software defined radio industry events. GNU Radio is an open source digital signals processing (DSP) tool which is often used with SDR radios.
One of our favorite talks from the conference is Micheal Ossmanns talk on his idea to create a low cost $150 RX/TX radio. Micheal Ossmann is the creator of the HackRF which is a $299 USD RX/TX capable SDR. It was one of the first affordable general purpose wide frequency TX capable SDRs. Micheal also mentions his other projects including Neapolitan which will be an add on for the HackRF which will enable full-duplex communications and Marizpan which will essentially be a single board Linux SDR using the HackRF circuit.
GRCon16 - Low-Cost SDR Hardware, Mike Ossmann
Another is Balints talk on “Hacking the Wireless World” where he does an overview of various signals that can be received and analyzed or decoded with an SDR. Some applications he discusses include Aviation, RDS Traffic Management Channel, Radio Direction Finding, OP25, IoT, SATCOM and his work on rebooting the ISEE-3 space probe.
GRCon16 - Hacking the Wireless World, Balint Seeber
With perfect Halloween timing, SDR enthusiast and ghost hunter Doug Haber has released his RTL-SDR compatible software called “gqrx-ghostbox”. This software supposedly turns your RTL-SDR into a electronic voice phenomenon (EVP) tool a.k.a a “Ghost Box”. Douglas explains what a Ghost Box is in the following blurb:
A ghost box is a device sometimes used by paranormal researchers to talk to spirits, the dead, disembodied entities, shape shifting lizard people, and other intra-dimensional fauna.
Some ghost boxes have electronics that give them distinct properties, and others are effectively radio scanners. This tool is of the radio scanning style.
Many examples of ghost box usage can be found on youtube. Generally, it involves asking questions and then listening for a response. Some people believe a medium or trance state is necessary in order for it to work. If you search for “ghost box” or “spirit box”, you will find information on different usage styles.
We’re not 100% sure if this is a late April fools joke, or a serious tool, but the code is real (it appears to just use GQRX to scan through frequencies), and at least these days when almost everything possible has already been tried with an RTL-SDR, this is something new!
Back in March of this year we posted about the release of the FlightAware "Pro Stick". The Pro Stick is FlightAware's ADS-B optimized RTL-SDR dongle. It uses a low noise figure LNA on the RF front end to reduce the system noise figure, thus improving the SNR at 1090 MHz. Because the added gain of the LNA can easily cause overload problems if there are other strong signals around, FlightAware recommend using one of their 1090 MHz ADS-B filters in front of the dongle to prevent overload.
FlightAware.com is a company that specializes in live air travel tracking. Most of their data comes from volunteers running RTL-SDR ADS-B receivers.
The new Pro Stick Plus RTL-SDR based ADS-B Receiver from FlightAware.
Over on their forums and on Amazon, they announced the device and specs. They wrote:
FlightAware is excited to announce the next evolution of USB SDR sticks for ADS-B reception! The new Pro Stick Plus USB SDR builds on the popular Pro Stick by adding a built-in 1090 MHz bandpass filter. The built-in filter allows for increased performance and range of reception by 10-20% for installations where filtering is beneficial. Areas with moderate RF noise, as is typically experienced in most urban areas, generally benefit from filtering. By integrating the filter into the SDR stick, we are able to reduce the total cost by more than 40% when compared to buying a Pro Stick and an external filter.
Specifications:
Filter: 1,075 MHz to 1,105 MHz pass band with insertion loss of 2.3 dB; 30 dB attenuation on other frequencies
Amp: 19 dB Integrated Amplifier which can increase your ADS-B range 20-100% more compared to dongles from other vendors which can increase range 10-20% over a Pro Stick in environments where filtering is beneficial
Native SMA connector
Supported by PiAware
R820T2 RTL2832U chips
USB powered, 5V @ 300mA
Note that this dongle is only for ADS-B at 1090 MHz, and not for 978 MHz UAT signals, as the filter will cut that frequency out.
Back in April, we did a review of the original Pro Stick. We found its performance on ADS-B reception to be excellent, but only when a filter was used. The low NF LNA theoretically improves the SNR of ADS-B signals by about 7-8 dB, but in reality there is too much gain causing signal overload everywhere, thus making reception impossible without the filter. Rural environments may not need a filter, but in a typical urban or city environment strong FM/TV/GSM/etc signals are abundant and these signals easily overloaded the Pro Stick when no filtering was used. This new Pro Stick Plus dongle completely solves that problem at a low cost with its built in filter.
Remember that if you are using a run of coax cable between the LNA and RTL-SDR, then it is more optimal to use an external LNA, like the LNA4ALL. Only an external LNA mounted near the antenna can help overcome coax, connector, filter and other losses as well as reducing the system noise figure. The FlightAware dongles are the optimal solution when they are mounted as close to the antenna as possible. This is usually the case when running the FlightAware feeder software on a Raspberry Pi.
We hope to soon review the Pro Stick Plus, however we assume it will operate nearly identically to the Pro Stick + FlightAware ADS-B filter combination.
Many people with an RTL-SDR have had fun receiving NOAA and METEOR low earth orbit (LEO) weather satellite images. However, a step up in difficulty is to try and receive the geostationary orbit (GEO) weather satellites like GOES. These satellites are locked to a fixed position in the sky meaning there is no need to do tracking, however since they are much further away than LEO satellites, they require a 1m+ satellite dish or high gain directional antenna to have a chance at receiving the weak signal. The GOES satellites transmit very nice high resolution full disk images of the earth, as well as lots of other weather data. For more information see this previous post where we showed devnulling’s GOES reception results, and this post where we showed @usa_satcom’s presentation on GOES and other satellites.
The nice thing about Lucas’ post is that he documents his entire journey, including the failures. For example after discovering that he couldn’t find a 1.2m offset satellite dish which was recommended by the experts on #hearsat (starchat), he went with an alternative 1.5m prime focus dish. Then after several failed attempts at using a helix antenna feed, he discovered that his problem was related to poor illumination of the dish, which meant that in effect only a small portion of the dish was actually being utilized by the helix. He then tried a “cantenna”, with a linear feed inside and that worked much better. Lucas also discovered that he was seeing huge amounts of noise from the GSM band at 1800 MHz. Adding a filter solved this problem. For the LNA he uses an LNA4ALL.
To position the antenna Lucas used the Satellite AR app on his phone. This app overlays the position of the satellite on the phone camera making it easy to point the satellite dish correctly. He also notes that to improve performance you should experiment with the linear feeds rotation, and the distance from the dish. His post of full of tips like this which is very useful for those trying to receive GOES for the first time.
In future posts Lucas hopes to show the demodulation and decoding process.
GOES signals received with the dish, LNA4ALL, filter and an Airspy.
Over on YouTube well known SDR tester Leif (SM5BSZ) has uploaded a video that compares the performance of several HF receivers with two tone tests and real antennas. He compares a Perseus, Airspy + SpyVerter, BladeRF + B200, BladeRF with direct ADC input, Soft66RTL and finally a ham-it-up + RTLSDR. The Perseus is a $900 USD high end HF receiver, whilst the other receivers are more affordable multi purpose SDRs.
If you are interested in only the discussion and results then you can skip to the following points:
24:06 – Two tone test @ 20 kHz. These test for dynamic range. The ranking from best to worst is Perseus, Airspy + SpyVerter, Ham-it-up + RTLSDR, Soft66RTL, BladeRF ADC, BladeRF + B200. The Perseus is shown to be significantly better than all the other radios in terms of dynamic range. However Leif notes that dynamic range on HF is no longer as important as it once was in the past, as 1) the average noise floor is now about 10dB higher due to many modern electronic interferers, and 2) there has been a reduction in the number of very strong transmitters due to reduced interest in HF. Thus even though the Perseus is significantly better, the other receivers are still not useless as dynamic range requirements have reduced by about 20dB overall.
33:30 – Two tone test @ 200 kHz. Now the ranking is Perseus, Airspy + SpyVerter, Soft66RTL, BladeRF+B200, Ham-it-up + RTLSDR, BladeRF ADC.
38:30 – Two tone test @ 1 MHz. The ranking is Perseus, Airspy + SpyVerter, BladeRF + B200, ham-it-up + RTLSDR, Soft66RTL, bladeRF ADC.
50:40 – Real antenna night time SNR test @ 14 MHz. Since the Perseus is know to be the best, here Leif uses it as the reference and compares it against the other receivers. The ranking from best to worst is Airspy + SpyVerter, ham-it-up + RTLSDR, BladeRF B200, Soft66RTL, BladeRF ADC. The top three units have similar performance. Leif notes that the upconverter in the Soft66RTL seems to saturate easily in the presence of strong signals.
1:13:30 – Real antenna SNR ranking for Day and Night tests @ 14 MHz. Again with the Perseus as the reference. Ranking is the same as in 3).
rx1compare
In a previous video Leif also uploaded a quick video showing why he has excluded the DX patrol receiver from his comparisons. He writes that the DX patrol suffers from high levels of USB noise.
Earlier this month Akos from the RTLSDR4Everyone blog reviewed a prototype of the latest ThumbNet N3 RTL-SDR. In this post we will also give a quick review of a prototype of their new unit which was kindly provided to us by ThumbNet. ThumbNet is a company that is hoping to provide low cost satellite deployments, and make use of volunteers around the world with RTL-SDR’s to help track them. The RTL-SDR’s and antenna kits are provided to schools and educational institutions for free by ThumbNet, in exchange for students setting up and monitoring a satellite tracking station.
To help with the needs of their project they have designed and manufactured the ThumbNet N3, which is a redesigned RTL-SDR dongle. As they must order in bulk, they are also selling surplus units to the RTL-SDR community with the hope that any profits will help fundraise for other related projects.
The ThumbNet N3 is a redesigned RTL-SDR with a focus on lowering the noise floor and spurs for optimal reception of their satellites. The N3 uses a 0.5 PPM TCXO for low frequency drift and a common mode USB choke for reduced USB noise. The PCB size is also increased allowing for better thermal dissipation. Since F-type is more common in the areas they intend to donate the units to, an F-type antenna connector is used on the dongle. A full list of the changes and improvements they’ve made can be found on their N3 details page.
One way in which they have reduced the noise is by disabling the internal switching voltage regulator within the dongle, and instead using a linear regulator. Linear regulators are much quieter than their switching counterparts, however they do draw significantly more power. The N3 draws 450mA of current, wheras a standard RTL-SDR draws closer to 270mA. Since many USB ports have a 500 mA limit this gets close to problematic to run directly from the USB port. To get around this, the N3 has an external power port, so it can be powered by an AC->DC power pack (like what you use for charging your phone), or more ideally with a quieter linear power supply or batteries. This has the added advantage of avoiding noisy USB power lines.
Review & Testing
The N3 feels very sturdy, and all the connectors are mounted strongly and are unlikely to break off. The top of the receiver shows the power port, the USB port, the shielding can and the F-type connector. The USB connector uses the older depreciated “USB mini” standard (which is different to USB micro found on most phones).
The bottom shows a few components as well as the two linear regulators. In order to power the unit we used an AC to DC 5V power supply (normally used for mobile phone charging) which we soldered on to the bottom of the PCB. Ideally we’d use a battery or linear power supply, but we’ll test that later with the actual production unit.
The standard N3 unit comes with no enclosure or RFI shielding cans (these are paid add-ons). Our prototype unit came with a shielding can covering the components which was enough to block most interfering signals. We did not see many unwanted signals being received with the antenna unplugged which is a good sign that the shielding can is doing its job.
Thumbnet N3 TopThumbnet N3 Bottom
We gave the unit a quick test on a noise floor scan. The results show that the noise floor has been significantly reduced. The clock spurs are still there but they are reduced in strength vs the standard RTL-SDR.
A standard RTL-SDR vs RTL-SDR Blog V3 vs ThumbNet N3 noise floor scan
On L-band at around 1.5 GHz the standard RTL-SDR dongles tend to fail at receiving after they heat up a little. The ThumbNet N3 showed no problems in this region, probably thanks to its larger PCB with better heat dissipation.
Conclusion
Once the production model is released we intend to do a more in depth review, but as it stands right now the N3 is looking very good especially for those who use RTL-SDR’s in monitoring applications that can benefit from very low noise floors, or for those who like the idea of being able to externally power the unit.
The ThumbNet N3 can be bought from their store. It costs $25.75 (with no shielding can), $27.75 (with shielding can), $31.50 (with aluminum case and no shielding can) or $33.50 (with aluminum case and shielding can) + $4.50 worldwide shipping. ThumbNet write that initial demand for the N3 unit has been high, so if you are interested in the unit, you need to order early to ensure that you can get one. They are due to ship out by the end of October. We’ve received a note that the delivery date is rescheduled for no later than November 11.
Also in a recent email from ThumbNet they wrote:
First of all, let me thank you from the whole team, for your support of ThumbNet and helping to promote STEM education around the globe with your purchase. We have sold more of the surplus N3’s than we expected to at this point, so if you have friends that are hesitant, tell them that time is running out! 🙂
Secondly, I wanted to take a moment to update you on the status.
The production run testing of the N3 was completed on time, but testing found three small improvements we could put into effect immediately, to produce a better receiver. Those changes have been submitted to the manufacturer and we are currently waiting for a revised ship date.
We still anticipate that the N3 will ship in the month of October, but I will send a follow up email with a more accurate schedule, when I get one in a day or two, from the manufacturer.
I thank you all for your understanding and patience. All of our testing so far indicates that the N3 is performing very well, and we hope you’ll agree it was worth the wait, when it arrives.
Outernet is a satellite based file delivery service. Currently they’re beta testing their service and they are using RTL-SDR’s as the receiver. In previous posts we’ve seen that they’re now regularly transmitting weather updates, wikipedia files and more files like images and books. Over time the service is becoming more and more useful. If you’re interested in receiving their service we have a tutorial available here.
While most of the Outernet software is open sourced, the signal protocol itself is closed source, which ties you into needing to use the official Outernet software. Over on his blog, Daniel Estévez has been working on reverse engineering the Outernet signal with the goal of publishing the results and building a fully open source receiver.
So far he’s managed to fully reverse engineer the modulation, coding and framing. He’s also been able to build a GNU Radio program that receives the Outernet frames and a Python program called free-outernet which does the decoding. His post goes into greater details on how he reverse engineered the signal and what his finding are.
Coherent-receiver.com is a company which is a customer of our RTL-SDR V3 dongle and they have been working on creating a multi-channel coherent receiver product based on the RTL-SDR. An RTL-SDR multi-channel coherent receiver is at its most basic, two or more RTL-SDR dongles (multi-channel) that are running from a single clock source (coherent). A multi-channel coherent receiver allows signal samples from two different antennas to be synchronized against time, allowing for all sorts of interesting applications such as passive radar and direction finding.
The team at coherent-receiver.com have used the new expansion headers on our V3 dongles to create their product. In their receivers they attach a control board which has a buffered 0.1 PPM TCXO (buffered so it can power multiple RTL-SDR’s). They also added an 8-bit register and I2C connection capabilities which allows for control of future add-on boards. The I2C capability is useful because it means that several RTL-SDR dongles can be controlled and tuned from the same control signal. More information on the registers and build of the receiver control board can be seen on their technical support page.
A ten channel RTL-SDR coherent receiver.The Coherent Receiver block diagram.
One example application of a multi-channel coherent receiver is passive radar. Coincidentally, we’ve just seen the release of new GUI based Passive Radar software by Dr. Daniel Michał Kamiński in yesterdays post. Passive radar works by listening for strong signals bouncing off airborne objects such as planes and meteors, and performing calculations on the signals being received by two antennas connected to the multi-channel coherent receiver.
A second example is direction finding experiments. By setting up several antennas connected to a multichannel coherent receiver calculations can be made to determine the direction a signal is coming from. An interesting example of direction finding with three coherent RTL-SDRs can be seen in this previous post. A third example application is pulsar detection which we have seen in this previous post.
Coherent-receiver.com sent us a prototype unit that they made with four of our V3 dongles. In testing we found that the unit is solidly built and works perfectly. We tested it together with Dr. Kamiński’s passive radar software and it ran well, however we do not have the correct directional antennas required to actually use it as a passive radar yet. In the future we hope to obtain these antennas and test the coherent receiver and the software further.
Currently they do not have pricing for these models as it seems that they are first trying to gauge interest in the product. If you are interested in purchasing or learning more they suggest sending an email to [email protected]. It seems that they are also working on additional RTL-SDR ecosystem products such as filters, downconverters, antennas and LNAs.
We hope that the release of this product and Dr. Kamiński’s software will give a boost to the development of coherent multi-channel receivers as we have not seen much development in this area until recently.
