CHILL weather radar

Last week I had the chance to visit the CHILL research weather radar in Greeley, CO, which is operated by University of Colorado. During my studies, I had the privilege to make some dual polarization SMPRF measurements with this radar on a weather radar course organized by V. Chandrasekar (known to most people in the field as Chandra) at my university. For this reason, the chance to finally go and see this radar in person was special to me.  

There are several things that make this radar special in the world of weather radars. It has a 9 meter dish, which might seem small for somebody coming from the incoherent scatter radar community, but it is large for a weather radar. The radar is dual polarized, with independent 1 MW peak power 2.725 GHz transmitters on both polarizations, which can operate at a 0.16% duty-cycle. The dish also hosts a low power X-band radar, which makes simultaneous S- and X-band measurements possible. 

The radome and the trailers containing the radar operating center, and the transmitters. 

Offset feed antenna, hosting the dual polarized S-band and X-band radars.  

From this perspective,  the radar dish actually seems smaller than it really is. This picture offers a better representation of the proportions. 

One of the Varian VA-87B/C klystrons, that provide the 1 MW peak power for the radar. 

The klystron cabinet.

Perfect Incoherent scatter radar jammer

On November 27th, somebody stepped on our band here at MIT Haystack Observatory. At first I thought that there must be something wrong with the new radar echo deconvolution program that I have been developing. But a month ago the same program worked perfectly, and no major changes were made since then.

On the next day there was a routine radar calibration run, which seemingly looked ok in terms of electron density. However, there was a suspiciously uniform non-zero Doppler velocity across the altitude profile during the whole run.

I went back to the raw voltage and calculated spectra from range gates at ranges where no ionospheric return would be expected. Sure enough, there was strong radio interference right in the middle of our radar band at 440 MHz.

 

The signal looked like it had ~0.5 ms baud lengths and frequency shift keying modulation on it.

As an emergency plan of action, we considered moving our center frequency to 440.4 MHz to navigate around the jammer. However, by the next day, the jammer had drifted in frequency to 440.4 MHz from 440.2 MHz. In the following days, the signal continued randomly walking  between 439.9 and 440.4 MHz, our whole licensed band. 

Not only was the signal stopping our operations, it also occasionally looked nearly like an F-region ionospheric return, a double humped ion-line spectrum. This is shown in the picture below.

Screen capture while war driving to locate the jammer. Hey, it has a double humped spectrum, just like the F-region ionospheric return, and it has approximately the same width! Frank's comment: "I don't like this jammer".

Because the signal was strong, we suspected something on the observatory grounds might be causing it. However, despite multiple attempts, we failed to pick up the signal with a yagi antenna anywhere on the Hill. Doing an azimuth scan with the MISA antenna suggested that the signal was coming from the south west direction. Because we failed to find the signal on the hill, it was possibly coming from further away.

Azimuth scan of interference on the MISA antenna, indicating that the signal is arriving from the Southwest direction.

At this point, we loaded up a black SUV with a bunch of gear: a directional antenna, an omnidirectional roof mounted antenna, an RF front end (hastily built in a shoe box), and some real-time spectrum analysis software.

Mobile RFI detection unit, with our expert RF gumshoe, Will Rogers.

On the first night, our sleuth Will drove around and caught a weak glimpse of the same jammer signal in Leominster on I-190, about 20 km from our radar. A yagi antenna scan indicated that the signal was coming from further West.

Instrumenting the vehicle. The MISA antenna in the background.

A few days later, a more instrumented war driving expedition took place with Frank Lind, Will Rogers, and Juha Vierinen. This time we had GPS logging and recording of spectra. After a day of direction finding and driving around, we were fairly confident that the source was between Leominster and Mt. Wachusetts in Leominster State Forest, about 30 km from our radar. However, we still failed to locate the source.

RFI signal strength as a function of geographic location.

On the afternoon of the next day, on his way home, Will Rogers stopped by at the hot spot that we had identified near Stuart Pond. He obtained several precise angle measurements using a yagi and a magnetic loop antenna. He also recorded observed signal power. The results are indicated in the picture below.

Angle of arrival measurements.

The results indicated that the source of the interference must be somewhere in the forest, possibly closer to locations 2 and 3, than 1 and 4. After some analysis of the data using satellite and aerial photographs, the only man made object in the forest, a radio tower, was identified. This was a huge relief, as one theory was that the source might be a broken animal tracking collar.

Incoherent scatter radar echo simulator.

This tower was soon identified as a local FM radio station. The engineer responsible for the station informed us that he had a few weeks ago installed a FSK telemetry link operating at 450 MHz. This was consistent with the appearance of the interference. The faulty telemetry link was aptly replaced by the engineer, and the interferer finally went away!

At about 2013.12.13 13:00 LT, the interference finally goes away as faulty telemetry radio link is replaced on the radio mast in Princeton.

During this episode, we learned a lot about radio direction finding in practice. This will come in handy in the future, as interference is abundant at Haystack.

For software, we used the gnuradio framework. We mostly used the waterfall and FFT spectrum graphical sinks, as the signal was drifting around in frequency a lot. For the war driving, I used a custom block that I wrote, which simply calculates power spectra using FFTW, and records averaged results to disk. I then used post processing of this to calculate the RFI power at any given location. I recorded the GPS coordinates with my phone.

Hastily built front end and rtlsdr digital receiver. The other output can be connected to another receiver, such as a USRP, a HAM rig, or a spectrum analyzer. 

For hardware, we used both rtlsdr dongles, and a USRP N200. The rtlsdr dongle and the gnuradio FFT spectrum sink actually worked surprisingly well for direction of arrival determination. The RF front end consisted of a low noise amplifier and a filter. As direction finding antennas, we used two different yagi antennas, and a magnetic loop antenna. For war driving, we first used a roof mounted discone antenna, and later a magnetically mounted quarter wave dipole.

Yagi antenna (above) used to direction finding by maximizing signal power. The magnetic loop antenna (below) is used to find the angle of arrival by minimizing the signal power, as the antenna radiation pattern has a sharp null in the direction of the loop axis.  

I had already prepared software to record absolute time of arrival of the interferer using GPS syncronized USRP receivers at several independent locations. However, good old fashion detective work with directional antennas managed to provide us with enough clues to solve the problem before we got to deploy our time of arrival setup. Well, maybe the software and receivers can be used to hunt down the spike-like interference that we see every now and then.

Dual antenna passive radar interferometry

Last week I tested passive radar interferometry with two USRP N200 devices and new K&R filters (this could in theory be done also with thee rtlsdr dongles with a shared clock). Three channels were recorded: the transmit waveform with a directional antenna pointed towards the FM radio transmitter, and two SKA (Square Kilometer Array) prototype log periodic prototype antennas pointed towards zenith to record echoes. 

To analyze the signals, I performed deconvolution of the transmit waveform, and estimated a cross-spectrum between the two channels at each range gate. I then plotted the range-Doppler-intensity of the echoes, using hue to encode relative phase difference between antennas. The results mainly show airplanes moving about in the two antenna interferometer with a ~18 lambda spacing, but there are occasional specular meteor echoes too. The phase gives some idea of where the signal is arriving from, but there is still ambiguity that can only be resolved by adding more antennas.    

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Here are some plots of some specular meteor trails that I spotted in the results:

Specular meteor echo at 200 km range.

Specular meteor echo at 150 km range.

Meteor at 240 km.

Meteor at 150 and 200 km.

Meteor at 110 km.

Meteor at 90 km

Meteor at 230 km.

Once we get more cables to the antennas, we should be able to start testing passive radar imaging and radio source calibration. There are already plenty more antennas awaiting cabling:

The rapid antenna field. The xmas tree like log period antennas are the SKA Cambridge prototype antennas, the loop is for HF, and the small white dome is the NASA space debris radar.

The ground clutter has been removed from the above pictures, to avoid self-noise. Out of curiosity, I also plotted that:

Ground clutter phase difference-range-time-intensity picture. The clutter phase difference is fairly stable, indicating that the ground clutter angle of arrival distribution is also fairly constant over time.  

The bottom line is that the 16 channel usrp receiver works fine for passive radar with the K&R filters to remove aliasing.

Update: added a picture of the antennas, and more meteor echoes. Fixed typos.

GNU Chirp Sounder 1.23

During the gnuradio conference hackfest last week, I migrated all of my gnuradio blocks to the cmake build environment and gnuradio 3.7. As a consequence of this, GNU Chirp Sounder was also updated. I also wrote a new adaptive filter block to remove HF broadcast band signals.

The new version can be downloaded here: http://www.sgo.fi/~j/gnu_chirp_sounder/

I've attached a plot and a video that resulted from testing the migrated code. It shows chirp sounders presumably in Virginia and Puerto Rico. The receiver was located in Boston.

A sounder possibly in Puerto Rico. The new version has a feature that reduces HF broadcast band noise in amplitude domain before chirp downconversion. 

ePOP / Cassiope launch is successful!

Congratulations today are in order for our Canadian colleagues and Elon Musk's company Space X. Space X conducted a successful launch of their Falcon 9 rocket for the first time from the west US coast out of a new facility at Vandenberg Air Force Base. Although much of the payload was devoted to technology demonstration packages, the space science community is thrilled to have on orbit the ePOP platform aboard the Cassiope satellite, another important orbiting platform for study of space weather and upper atmospheric variations.  Early reports are that the six-sided Cassiope satellite had a nominal separation and is healthy in orbit following its first ground station pass over Antarctica.

Led by principal investigator Professor Andrew Yau (University of Calgary), ePOP stands for Enhanced Polar Outflow Probe and is a significant milestone for the Canadian Space Agency.  The e-POP team is comprised of scientists and engineers from seven Canadian universities and three research organizations: the University of Calgary; York University; the Universities of Alberta; Athabasca; Saskatchewan; Western Ontario; and New Brunswick. The Communications Research Centre, located in Ottawa, as well as the Institute of Space and Astronautical Science of Japan and the U.S. Naval Research Laboratory are also partners in the project.

The instruments on ePOP will all help to continue investigations into the nature of heavy, cold ion outflow from the polar regions of Earth's ionosphere into the magnetosphere. These often oxygen ion rich flows come up from the ionosphere near noontime in a region known as the cusp, and represent a significant mass load for the coupled ionosphere-magnetosphere system. Understanding them is an important key to understanding the entire Earth geospace system's response to space weather variations, or changes in characteristics of the flow of the solar wind streaming outward continually from the sun's corona.  This is because heavy cold ion outflows significantly change the 'stiffness' of the magnetic field lines forming the magnetosphere surrounding earth, and therefore change resonant frequencies of a common wave mode coupling geospace.  The Alfven mode, named for Hannes Alfven, can be thought of as a longitudinal mode similar to plucking a string - except this time, instead of the string being one on say a musical instrument, the string is the magnetic field.  More heavy mass on that string changes the frequency when it's plucked, just like using heavier gauge strings lowers their resonant frequency.

For radio physics and geospace studies, ePOP has several important instruments:

• The Coherent Electromagnetic Radiation tomography experiment
(CER) will perform radio transmission from e-POP to ground for
radio propagation and ionospheric scintillation measurements.

• The Fast Auroral Imager (FAI) will measure the large-scale auroral
emissions in the 630-1100 nm wavelength range.

• The Global Position System (GPS) receiver-based Attitude, Position,
and profiling experiment (GAP) will be used for spacecraft position
and attitude determination and for ionospheric radio occultation
profiling measurements.

• The Imaging and Rapid-scanning ion Mass spectrometer (IRM) will
measure the composition and 3-dimensional velocity distributions of
ions in the ionosphere.

• The MaGnetic Field instrument (MGF) will investigate the localization
and characterization of field-aligned currents in the high-latitude
auroral zones and polar caps.

• The Neutral Mass and velocity Spectrometer (NMS) will measure
mass composition and velocity of neutral atmospheric species.

• The Radio Receiver Instrument (RRI) will measure wave electric fields
in the 10Hz - 18MHz range, at magnitudes from 1 μV/m to 1 V/m.
These measurements will provide information about the morphology
and dynamics of ionospheric density structures, auroral wave-particle
interactions, plasma nonlinear processes created by intense high
frequency waves, and the mechanism of coherent wave backscatter.

• The Suprathermal Electron Imager (SEI) will measure the electron
energy and pitch angle distribution over the energy range of 1 to
200 eV, with particular emphasis on photoelectrons in the 1 to 50 eV
range, which are believed to play an important role in the polar wind
outflow.

Cassiope has a polar, highly elliptical orbit (324 x 1500 km altitude).  At MIT Haystack, we expect to have many opportunities for science using in particular the CER beacon experiment with ground receivers during overflights.  These overflights will also allow us to make multipoint observations of subauroral ionospheric features through combining ePOP data with wide field ionospheric measurements using our megawatt class large aperture incoherent scatter radar system, HF backscatter radar measurements of convection using the SuperDARN array, and other orbiting satellites such as the DMSP platform.  

Congratulations to all involved, and we're looking forward to a long and successful mission.  You can follow the status of the mission at SpaceFlight Now's page, and more Cassiope information is at the project home page.

It's Retro Friday at MIT Haystack Observatory.. by Necessity

Hello from MIT Haystack in the northeast US.  Here, as with any production quality geospace observatory, sometimes we have to reach deep and use whatever tools are available to get the job done.  Today, this took the form of a scramble all-hands hardware debugging session on a legacy Sun disk server powering user space
files for the Atmospheric Sciences Group.  The server's external RAID storage array abruptly turned off due to a bad uninterruptible power supply after it exhausted its batteries.  Unfortunately, its pitiful beeping noise (if there was any) was drowned out by whirring fans in the network closet.

Since no modern computing hardware has real 9 pin serial ports any more, and since the Sun server was designed to run headless with serial only console access, we ended up making use of some .. legacy .. equipment lying around.  The 486DX 33 MHz laptop (8 MB memory) used to be a debug platform for an embedded antenna controller on our 46 meter UHF steerable antenna, talking in that role to a system running embedded DOS 1.0.  The 486DX saw new brief life since it had a working, real serial port.  Somehow, I managed to remember where the Terminal program was on its spiffy, working Windows 3.11 installation.  As you can see, unfortunately the keyboard had a non-functioning "Enter" key (rather critical) so we had to steal a PS/2 external keyboard from somewhere.   See the annotated photo for other qualities of our situation.

The real world intrudes on scientific thoughts.

2 hours later (and several kibbitzers involved too), we had a working system again!  This is actually a good analogy for what goes on regularly at any large facility - lots of improvising with equipment of various vintages, sometimes ancient.  All those things that you forgot might be useful again someday.  However, in this case I'm not hoping for a repeat!

Thanks to Juha Vierinen for the photo at a crucial moment, when things were finally looking a bit better after some despair.

Passive radar with $16 dual coherent channel rtlsdr dongle receiver

My previous post describes the $16 dual channel rtl_sdr dongle hack. In the last few days I've done some more testing and it turns out I can use the system for passive radar! I didn't expect this, because the receiver only has 8 bits and passive radar requires a lot of dynamic range.

Airplanes and occasional specular meteor echoes. 

I hooked up the two channels into yagi antennas that we have used with Echotek and USRP receivers for passive radar. One of the antennas was measuring the transmit waveform, and the other was measuring the echoes. I ran a measurement, and to my great surprise, it worked just fine.

I did tweak the signal levels a bit in order to ensure that I optimally use the dynamic range. I also had the bandwidth set to 2.4 MHz, giving me about 4.5 bits extra dynamic range after filtering the signal to 100 kHz in single precision floating point.

Two log periodic antennas used to passive radar with the dual coherent RTLSDR R820T dongle.

This really does give us a glimpse of the future where high end digital receivers will cost $10 per channel. The low end ones are already in that price range. Think of all the potential science that can be done!

$16 dual-channel coherent digital receiver

I have been playing around with the cool RTL dongles (more on rtl-sdr dongles on superkuh's web page or rtl-sdr.com) that you can buy on e-bay for about US$8 (including shipping). These are very capable 8-bit digital receivers that have up to 2.4 MHz bandwidth and can tune anywhere between 24 MHz and 1850 MHz.

I recently came up with a trivial hack to build a receiver with multiple coherent channels using the RTL dongles. I do this basically by unsoldering the quartz clock on the slave units and cable the clock from the master RTL dongle to the input of the buffer amplifier (Xtal_in) in the slave units (I've attached some pictures).

I originally drove the master crystal with both dongles, which also worked. However, Ian Buckley pointed out to me that a more typical way of doing this is feeding the signal into Xtal_In (in the pictures below). So I tried that too, and it also worked. I'm still not sure what the optimal setup is, as there is no schema for the dongle, but both methods I've tried so far have worked in practice.

This is how you make a dual coherent channel digital receiver with $16.  The clock drive probably won't be enough for many of these, but this can be fixed with a buffer or some other active splitter. 

The oscillator is wired using a piece of 75 Ohm antenna coax that came with the dongle. It's like they designed the dongle for  multi-channel coherent applications. 

This has some implications for low cost geophysical instruments. It will be possible to use this receiver for the 150/400 MHz beacon satellite receiver, as this only requires that the receivers have clocks that are locked with each other. Interferometry and passive radar are other application examples. With more than two locked channels, applications such as imaging start to become possible.

I've made some relative phase noise measurements, and the systems don't have detectable sample drift over two hours, and their relative phase is also pretty stable.

Spectrum at 1 Hz sample rate of the relative z_1/z_2 phase signal going into two receivers. 

IQ plot of the z_1/z_2 relative phase signal over ~6000 seconds at 1 Hz sample rate. 

And oh, by the way, I found this nice usb hub, which I'm going to use to hopefully get a 7 channel coherent rtl system.

Hub with the right usb port orientation for rtl dongles. 

Stay tuned for more results. I already have some pretty nice passive radar results using the system, which I'll be posting in a few days.

Update: Apparently three dongles will also run fine from one master clock. I know the clock isn't split correctly, but adding any components would increase the total cost and the whole point of this exercise is to determine what the lower bound is for software defined radios.

Three channel coherent RTLSDR receiver. 

Passive Radar Update

This week, I spent a few more days trying to understand the errors related with strong radar targets in the analysis of weak targets in passive radar. As a consequence of this, I wrote a linear least squares estimator to remove this contribution more effectively (and consistently). I also optimized my code a bit, and got a massive speed-up. Now the signal processing and plotting all runs in real time.

This video shows a lot of aeroplanes around the New England area detected using a simple passive radar setup, consisting of: one USRP and two yagi antennas, a quad core Linux PC. Every now and then an occasional specular meteor echo is observed too. Because this is FM radio, the waveform is not always optimal for inverting the echoes, which results in a blow up of the solutions when the waveform is narrow band (eg., somebody talking, or silence). This is not a problem with digital modulation schemes, which will be explored next. I updated my code. I came up with a way to efficiently estimate ground clutter and transmitter self-noise using a linear least squares matrix equation, which works better than the previous attempt. I also made some optimizations that allow me to run this in real-time using a normal PC. Next up: imaging using the midas-mini cube.

3 mW ionogram

I've been playing with my low cost, low power, low footprint ionosonde. I'm still missing some parts for the system, but I thought I'd give it a try. Here is an ionogram using 3 mW of power fed into a badly mismatched antenna.

Hopefully very soon in the near future, I will have the prototype ready, and I can convince somebody to fund a large network of these things.