Tagged: direction finding

LimeSDR Angle of Arrival Experiments at 145 MHz

Two J-Poles used in LimeSDR Angle of Arrival Experiments.
Two J-Poles used in LimeSDR Angle of Arrival Experiments.

Together with some Spanish amateur radio operators, Daniel Estevez performed an experiment with the goal of detecting the angle of arrival of meteor reflections coming from the GRAVES radar at 143.05 MHz.

The GRAVES radar at 143.05 MHz is often used by amateur radio astronomers as a way to detect the echos of meteors entering the atmosphere. The basic idea is that meteors leave behind a trail of ionized air which is reflective to RF energy. This RF reflective air can reflect the signal from the powerful GRAVES space radar in France, allowing the radar signal to be briefly received from far away. Detecting the angle of arrival from these reflections could help determine where the meteor entered the atmosphere.

Their experiments used a pair of J-Pole antennas and a LimeSDR receiver. The LimeSDR has two channels and can receive the signal coherently from both channels. The phase difference in the received signals from the two antennas can then be measured, and the angle of arrival calculated.

In their testing the first tested with 145 MHz amateur radio satellites. Unfortunately due to the low elevation of the antennas and multipath from terrain obstructions an angle could not be calculated. In a second experiment they tried receiving terrestrial APRS signals. With APRS they were successful and were able to determine the angle of arrival from multiple stations. Unfortunately for GRAVES meteor echoes they were not entirely successful, citing multipath issues due to houses, and the need for a clear view of the horizon.

We note that it may be possible to perform similar experiments with our KerberosSDR coherent RTL-SDR unit.

KerberosSDR Direction Finding with Android App Demo and Tutorial

Over on our YouTube channel we've uploaded a short video that gives a tutorial and demo of the KerberosSDR being used as an RF direction finding system in a car. If you weren't aware, KerberosSDR is our recently released 4x Coherent RTL-SDR which can be used for tasks such as direction finding and passive radar. KerberosSDR was successfully crowdfunded over on Indiegogo, and we have recently completed shipments to all backers. Currently we are taking discounted pre-orders for a second production batch on Indiegogo.

In the video we use a Raspberry Pi 3 B+ running the KerberosSDR image as the computing hardware. The Pi 3 is connected to a high capacity battery pack. It is important to use a high quality battery pack that can output 3A continuously as this is required for the Raspberry Pi 3 B+ to run without  throttling. The battery pack we used has multiple outputs so we also power the KerberosSDR with it.

Once powered up we connect to the KerberosPi WiFi hotspot, and then browse to the web interface page. We then tune the KerberosSDR to a TETRA signal at 858 MHz, perform sample and phase calibration, set the decimation and FIR filtering, and then enable the direction finding algorithm. At this point we enter the Android app and begin direction finding and logging our data.

After driving for a few minutes we stop and check the logfile and find that the majority of the bearing lines point in one direction. With this info, a drive in the direction of the bearing points to gather more data is performed. Once additional data was gathered we open the log file up again, and see where all the bearing lines cross. Where they cross indicates the location of the 858 MHz transmitter. The heatmap data also gives us a second confirmation that the transmitter is located where we think.

NOTE: Some of the features shown in the video like the heatmap, confidence settings and plot length settings are not yet released in the current version of the app. They will be released next week.

Full instruction on using the KerberosSDR are available at rtl-sdr.com/ksdr.

KerberosSDR Direction Finding With Android App Demo and Tutorial

KerberosSDR Batch One End of Stock, Batch Two Preorders Available

If you weren't aware, KerberosSDR is our recently released 4x Coherent RTL-SDR which can be used for tasks such as direction finding and passive radar. KerberosSDR was successfully crowdfunded over on Indiegogo, and we have recently completed shipments to all backers. Currently there is only about 20 units of the batch one production left in stock.

We are currently offering discounted preorders for batch two units on Indiegogo which we expect will be ready to ship in July or hopefully earlier. If you are interested, please order soon to avoid missing out as the price will be raised again once we are shipping. Batch two will be the same as batch one except for some minor changes. For example we have decided to convert the microUSB port into a USB-C port as we have found that there are many very poor quality microUSB cables on the market which could cause issues for users. USB-C cables are generally of a higher quality.

More information about KerberosSDR is available on the Indiegogo page.

KerberosSDR Updates

Since our last post on this blog about KerberosSDR we have made some enhancements to the software.

  • The KerberosSDR code is now fast enough to run at 1-2 Hz update rates for direction finding and passive radar on a Raspberry Pi 3 B+.
  • There is now a web interface, so the KerberosSDR can be controlled via a WiFi hotspot and internet browser. Useful for use on the Pi 3 and Tinkerboard.

For future updates we are currently working on several new features:

  • Filters to remove low confidence DoA results on the Android app.
  • A secondary heatmap type display on the Android app based on signal strength, for two direction finding indications.
  • Methods to determine the center of multiple bearing intersection points.
  • Further enhancements to processing speed, possible improved results from processing gain and possible better accuracy from improved DoA algorithms.

Within the next few weeks we will also release full tutorial videos that will show how to set up and use the KerberosSDR for direction finding and passive radar with a Raspberry Pi 3 or Tinkerboard. If you prefer a text based explanation we already have a guide up at rtl-sdr.com/ksdr.

Below is an image that demonstrates the KerberosSDR direction finding Android app. A user of KerberosSDR has also submitted two of his own screenshots that show that he was able to determine the location of a GSM transmitter with a linear antenna array.

KerberosSDR Direction Finding Results
KerberosSDR Direction Finding Results. Multiple data points collected during a drive, with bearings pointing towards the TX tower (red marker). Circular array of whip antennas used at freq. 858 MHz.

KerberosSDR Updates: Demo Software Speed Improvements, Android App, Manufacturing Updates

If you weren't already aware, over the past few months we've been working with the engineering team at Othernet.is to create a 4x Coherent RTL-SDR that we're calling KerberosSDR. A coherent RTL-SDR allows you to perform interesting experiments such as RF direction finding, passive radar and beam forming. In conjunction with developer Tamas Peto, we have also had developed open source demo software for the board, which allows you to test direction finding and passive radar. The open source software also provides a good DSP base for extension.

If you're interested and missed out in the early campaign, don't worry we still have about 250 units left from this batch for sale at a price of $140 + shipping over on our Indiegogo Campaign.

Demo Program Updates

Over the past few weeks we've been working on a few code speed improvements to the demo software, and we now believe that it should be fast enough to run on a Pi 3 B+ at decent update rates.  In particular the passive radar display frame rate has been improved and we're able to get about 1 FPS on a Tinkerboard now.

We will soon release the full code, but for now you can see the main two libraries developed by Tamas' that are used in the KerberosSDR code. These libraries contain the direction finding and passive radar processing algorithms.

pyAPRIL - Python Advanced Passive Radar Library. Available on PyPi and GitHub

pyArgus - Python Beamforming and Direction Finding Algorithms. Available on PyPi and GitHub.

Android Direction Finding Companion App Updates

Over the holidays we've been working on a simple companion Android app for the direction finding feature. Using the GPS and/or compass sensors on the Android phone, and the transmitter bearing given by the KerberosSDR we can plot a bearing towards the transmitter that we are tuned to.

The phone connects to a laptop/SBC WiFi hotspot running the KerberosSDR Linux software, and reads the bearing via a simple php HTML server.

Driving around with the KerberosSDR gives better results than when stationary as we can take multiple readings at different points which helps to average out multipath distortions.

In the image below we used a linear antenna array of four dipoles attached to the windscreen of a car. KerberosSDR was tuned to a TETRA transmitter at 858 MHz.

We drove down a street and then back up it. The red lines indicate the direction of the car as determined by GPS, the blue lines indicate the forward direction towards the transmitter, and the green lines the reverse direction. (a linear antenna array won't know if the transmitter is in front or behind it). 

You can see that the majority of blue/green lines point towards the TETRA transmitter which we've marked with a red location marker at the known location.

KerberosSDR Results from a Linear Antenna Array of Dipoles
KerberosSDR Results from a Linear Antenna Array of Dipoles

Getting a bearing from GPS requires that you are moving. However if you are stationary it is also possible to use the compass sensor in the Android app, but Android compass sensors are not particularly accurate.

We also tested the app with a circular array of antennas and found it to work well too. A circular array has the benefit over a linear array of providing only one direction towards the detected signal, but may be more susceptible to multipath issues. In our test the circular array was simply four magnetic whips placed on top of a car.

KerberosSDR using Whip Antennas in a Circular Array on a Vehicle
KerberosSDR using Whip Antennas in a Circular Array on a Vehicle

This time we then drove around for a longer time while logging the data in the Android app. We can see that the majority of blue lines point towards the known transmitter location. Blue lines pointing away from the transmitter may be due to multipath or a briefly incorrect GPS heading (e.g. during a turn). Sometimes reflections or refractions of the signal can be more likely to be picked up if the direct path to the transmitter is really blocked. However if you have enough data points from driving around, it becomes much more clear where the actual transmitter is. 

KerberosSDR Results from the Circular Array
KerberosSDR Results from the Circular Array

Manufacturing Updates

We now have some pictures of the boards being manufactured at the factory. Unfortunately we are behind our initial shipping target of mid-Jan due to the previous unexpected payment delays from Indiegogo, and because of this we may hit the Chinese New Year holidays which could delay us further as factories take a 2 week holiday starting late Jan. We're really hoping to have them shipped off just before then, but we don't know if we can beat the clock. I know some of you are anxious to get started with KerberosSDR, and so I do apologize for the delay.

KerberosSDR in it's metal case (no screen printing yet)
KerberosSDR in it's metal case (no screen printing yet)

 

An Overview on RF Direction Finding with RTL-SDRs

Thanks to K2GOG of the Hudson Valley Digital Network for writing in a sharing with us his latest blog post which is a useful overview of some direction finding techniques that can be used with RTL-SDR dongles. RF direction finding is the act of using a radio to determine the physical location of a signal.

In his post K2GOG mentions our successfully crowd funded KerberosSDR which will be shipping in January next year. KerberosSDR is our 4x coherent RTL-SDR, and one possible application is to use it as a four antenna phase coherent direction finder. K2GOG explains the phase coherent concept in his post quite elegantly.

While looking over KerberosSDR, K2GOG was also reminded of another direction finding technique called heat mapping which can be performed with a single RTL-SDR. This process involves driving around with an RTL-SDR and GPS logger, measuring the signal power as you drive and combining it the current GPS coordinates. From that data a heat map can be generated, which shows where the signal is the strongest, and therefore where the likely source is. The RTLSDR Scanner application by eartoearoak makes doing this easy, and in his post K2GOG provide a short tutorial on setting it up.

A heatmap generated by K2GOG with an RTL-SDR, GPS and RTLSDR Scanner.
A heatmap generated by K2GOG with an RTL-SDR, GPS and RTLSDR Scanner.

KerberosSDR: One Week of Discounted Preorders Remaining

Just a reminder that one week remains in the KerberosSDR Indiegogo campaign. This is your last chance to grab a KerberosSDR at a discounted preorder price. And at the time of posting there are still 50 "second early bird" units remaining at a discounted price of only $115 USD.

If you weren't already aware, over the past few months we've been working with the engineering team at Othernet.is to create a 4x Coherent RTL-SDR that we're calling KerberosSDR. A coherent RTL-SDR allows you to perform interesting experiments such as RF direction finding, passive radar and beam forming. In conjunction with developer Tamas Peto, we have also had developed open source demo software for the board, which allows you to test direction finding and passive radar. The open source software also provides a good DSP base for extension.

More information available on our KerberosSDR page, and the Indiegogo page.

Updates

Due to the higher than anticipated number of preorders, we have been able to immediately fund further work on improving the demo software, and will be able to continue to work on improving it throughout this and next year. First on the agenda is improving the code buffering structure and DSP processing speed. Shortly after we'll be looking at adding additional features to aide with calibration and direction finding.

We have also now begun ordering parts, begun prototyping the metal enclosure, and have finalized the PCB. Manufacturing is on track to begin shortly after the campaign ends.

KerberosSDR with Calibration Board Attached (Metal Enclosure with SMA connectors Not Shown)
KerberosSDR Prototype with Calibration Board Attached (Metal Enclosure with SMA connectors Not Shown)

Locating a Radio Transmitter with Direction Finding Techniques and KerberosSDR our 4-Tuner Coherent RTL-SDR

KerberosSDR is our upcoming low cost 4-tuner coherent RTL-SDR. With four antenna inputs it can be used as a standard array of four individual RTL-SDRs, or in coherent applications such as direction finding, passive radar and beam forming. More information can be found on the KerberosSDR main post. Please remember to sign up to our KerberosSDR mailing list on the main post or at the end of this post, as subscribers will receive a discount coupon valid for the first 100 pre-order sales. The list also helps us determine interest levels and how many units to produce.

In this post we'll show an experiment that we performed which was to pinpoint the location of a transmitter using KerberosSDR's coherent direction finding capabilities. RF direction finding is the art of using equipment to determine the location of a transmitting signal. The simplest way is by using a directional antenna like a Yagi to try and determine the bearing based on signal strength. Another method is using a pseudo-doppler or coherent array of antennas to determine a bearing based on phase information.

For the test we tuned the KerberosSDR RTL-SDRs to listen to a signal at 858 MHz and then drove to multiple locations to take direction readings. The antennas were set up as a linear array of four dipole antennas mounted on the windshield of a car. To save space, the dipoles were spaced at approximately a 1/3 the frequency wavelength, but we note that optimal spacing is at half a wavelength. The four dipole antennas were connected to KerberosSDR, with a laptop running the direction finding demo software. 

Low cost direction finding array mounted to vehicle windshield.
Low cost direction finding array mounted to vehicle windshield.

Our open source demo software (to be released later when KerberosSDR ships) developed by Tamás Peto gives us a graph and compass display that shows the measured bearing towards the transmitter location. The measured bearing is relative to the antenna array, so we simply convert it by taking the difference between the car's bearing (determined approximately via road direction and landmarks in Google Earth) and the measured bearing. This hopefully results in a line crossing near to the transmitter. Multiple readings taken at different locations will end up intersecting, and where the intersection occurs is near to where the transmitter should be. 

KerberoSDR SDR Directing Finding DOA Reading
KerberoSDR SDR Directing Finding DOA Reading

In the image below you can see the five bearing measurements that we made with KerberosSDR. Four lines converge to the vicinity of the transmitter, and one diverges. The divergent reading can be explained by multipath. In that location the direct path to the transmitter was blocked by a large house and trees, so it probably detected the signal as coming in from the direction of a reflection. But regardless with four good readings it was possible to pinpoint the transmitting tower to within 400 meters.

In the future we hope to be able to automate this process by using GPS and/or e-compass data to automatically draw bearings on a map as the car moves around. The readings could also be combined with signal strength heatmap data for improved accuracy.

This sort of capability could be useful for finding the transmit location of a mystery signal, locating a lost beacon, locating pirate or interfering transmitters, determining a source of noise, for use during fox hunts and more.

KerberosSDR pinpointing a transmitters location
KerberosSDR pinpointing a transmitters location
KerberosSDR Prototype
KerberosSDR Prototype

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KerberosSDR Running RF Direction Finding on a Tinkerboard

KerberosSDR (formerly HydraSDR) is our upcoming 4-input coherent RTL-SDR. It's designed for coherent applications like RF direction finding, passive radar, beam forming and more, but can also be used as a standard 4-channel SDR for monitoring multiple frequencies. In this post we demonstrate the direction finding application running on the TinkerBoard. 

Reminder: If you have any interest in KerberosSDR, please sign up to our KerberosSDR mailing list. Subscribers to this list will be the first to know when KerberosSDR goes on preorder, and the first 100 sales will receive a discounted price.

Subscribe to our KerberosSDR Announcement

Please select all the ways you would like to hear from RTL-SDR Blog:

You can unsubscribe at any time by clicking the link in the footer of our emails. We use MailChimp as our marketing platform. By clicking below to subscribe, you acknowledge that your information will be transferred to MailChimp for processing. Learn more about MailChimp's privacy practices here.

KerberosSDR Updates

This week we've managed to get the KerberosSDR demo software made by Tamás Peto functioning on a TinkerBoard. The TinkerBoard is a US$60 single board computer. It's similar to a Raspberry Pi 3, but more powerful. We've also tested the app running on the Raspberry Pi 3 and Odroid XU4. The Pi 3 is capable of running the software but it is a little slow, and the Odroid XU4 is a little faster than the TinkerBoard. In the future we hope to further optimize the code so even Raspberry Pi 3's will be smooth.

In the video below we used a circular array of four whip antennas connected to KerberosSDR. The TinkerBoard is connected to KerberosSDR and is set up to generate a WiFi hotspot, which we connect to with an Android phone and a Windows laptop. The Windows laptop connects to the TinkerBoard's desktop via VNC, and the Android phone receives an HTML/JavaScript based compass display via an Apache server running on the Tinkerboard. With this setup we can wirelessly control and view information from KerberosSDR and the TinkerBoard.

We've also tested the KerberosSDR system on a real signal, and have found it to work as expected. More demo's of that coming later.

For more info on KerberosSDR please see our previous announcement post.

KerberosSDR Direction Finding Test 2: Tinkerboard + Circular Array

KerberosSDR Prototype
KerberosSDR Prototype with TinkerBoard Running Computations