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FMCW Phased Array Radar for AutomaticallyTriggered Measurements of Snow Avalanches
M. Ash, P. V. BrennanDept. of Electronic & Electrical Engineering
University College LondonLondon, UK
{m.ash,p.brennan}@ee.ucl.ac.uk
N. M. Vriend, J. N. McElwaineCentre for Mathematical Sciences
University of CambridgeCambridge, UK
{nv253,jnm11}@cam.ac.uk
C. J. KeylockDept. of Civil & Structural Engineering
University of SheffieldSheffield, UK
Abstract— Radar has emerged as a means of measuringavalanche dynamics for validating models to predict avalanchebehaviour for improved calculation of risk zones. However,current radar measurements do not provide a true representationof an entire avalanche flow. An FMCW, multi-chirp, 8 receiverchannel phased array radar has been developed in attempt toprovide 2-D field measurements in unparalleled detail. In thispaper, we summarise the design of the radar and discuss furtheradditions to the radar design since a previous report. The radar isdeployed in a bunker at well equipped avalanche test site where ithas been successfully automatically recording naturally occurringavalanches. A large wet snow avalanche which occurred on the7th December 2010 has already been recorded. We present anencouraging MTI image of this avalanche from one of the receiverchannels which reveals a wealth of information embedded withinthe data. In the future we hope to cohere the 8 channels of datato provide cross-range information, and also resolve the rangeand Doppler of various components of the avalanche.
I. INTRODUCTION
Avalanches pose a significant threat to human life andsettlements. Hazard mapping of avalanches has historicallyrelied on observations of avalanche occurrence and run-out distances. This means that large, and possibly extreme,avalanches are missed [1]. Recently, the emergence of remotesensors has led to an interest in formulating models of theunderlying dynamics of an avalanche to help counter thisweakness. However, we currently struggle to validate modelsof avalanche behaviour due to insufficient field data. Con-sequently our ability to predict avalanche run-out distancesand impact pressures, parameters that are extremely importantin formulating risk zones for settlements [2], is limited. Thegathering of high quality, quantitative field data to gain greaterinsight into avalanche dynamics is crucial for progress in thisarea [3].
Radar has been used to make non-invasive depth andvelocity measurements of avalanches. There are three kinds ofradar in general use: continuous wave (CW), pulsed Doppler(PD), and buried FMCW radar [4], [5], [6]. CW and PD radarprovide similar information but of course PD radar providesbetter localisation of its velocity measurements. However, PDradar in their current form they only provide single dimensionrange measurements, and their available transmit power limitsthem to a range resolution of the order of 50 m, too coarseto provide a true representation of the avalanche dynamics.
Buried FMCW radar are limited to making point measure-ments of the avalanche and so to gain a reasonable measureof the avalanche’s progress, several of these are deployed atdifferent points along the avalanche track and the averagevelocity between them is calculated.
The aim of this project is to study the underlying dynamicsof avalanche flows with the aid of an advanced FMCW phasedarray radar [7]. In a previous paper we have introduced anewly developed FMCW phased array radar for imaging fullavalanches, a single-channel prototype of which was used tomeasure smaller scale snow movements [8]. The radar devel-opment is now complete and the radar has been successfullyinstalled at a well equipped avalanche test site: Vallee de laSionne, Switzerland. In this paper we describe some of thedesign challenges experienced throughout the development ofthe radar. We also include an MTI image of a recent avalanchewhich occurred in the early hours of the 7th December 2010.In our final submission, we hope to show some images post-beamforming with cross-range information and also calculatethe avalanche front velocity using the embedded Dopplerinformation.
II. RADAR DEVELOPMENT
A. Design Summary
The radar specification and design has been previously de-scribed in [8] and is summarised in table I. The key advantagesof this newly developed radar over other radar for full-scaleavalanche measurement is its ability to produce 2-D data withthe use of an 8-channel receiver array, and it’s superior rangeresolution of c.0.75 m. The 8 channel receiver array is spreadover a wide aperture to give a 0.6o beamwidth on receive. Theradar operates at 5.3 GHz to illuminate larger blocks of snowin the underlying dense region of the avalanche which can bemasked at higher frequencies by a low density powder cloudduring a dry snow avalanche. Our radar performs derampingin hardware by mixing the local FMCW radar signal with thereturn FMCW radar signal. This results in a 1 MHz base-bandwidth to give a 3750 m maximum range. The output ofthis process is a baseband signal whose frequency spectrumcan related to target range using Eq. (1) [9].
TABLE IRADAR SPECIFICATION.
Centre Frequency 5.3 GHz
Waveform Bandwidth 200 MHz
Base-bandwidth 1 MHz
Transmit Power 15 W
Maximum Chirp Length 5 ms
Maximum Range 3750 m
Array Beamwidth (Az.) 0.6o
MINICIRCUITS
ZX76-15R5-PP+
DIG. STEP. ATT.
ECLIPSE
MICROWAVE
IQ2040NP4
IQ MOD.
TEKTRONIX
AWG5002B
AWG
MINICIRCUITS
ZX60-6013E+
AMPLIFIER
MINICIRCUITS
ZX10-2-42+
POWER SPLIT
MINICIRCUITS
ZX10-2-126-S+
POWER SPLIT
MINICIRCUITS
ZX05-153MH+
MIXER
LORCH
MICROWAVE
CUSTOM
FILTER
MINICIRCUITS
ZX60-6013E+
AMPLIFIER
DC digital gain
control
15W POWER
AMPLIFIER
I Q
MINICIRCUITS
ZX60-6013E+
AMPLIFIER
LORCH
MICROWAVE
CUSTOM
FILTER
MINICIRCUITS
ZX05-153MH+
MIXER
MINICIRCUITS
ZX60-6013E+
AMPLIFIER
MINICIRCUITS
ZX05-C42MH+
MIXER
BASEBAND
ACTIVE
FILTER
MINICIRCUITS
ZRL-3500+
AMPLIFIER
MICROWAVE
AMPS AL7 SERIES
AMPLIFIER
MCLI PS8-11
8-WAY
POW. SPLIT.
MCLI PS8-4
8-WAY
POW. SPLIT.
NI ADC
PXIe-6366
(2 MS/s)
3 GHz
8.3 GHz
SOLWISE
NET-WL-ANT58-012PN
SOLWISE
NET-WL-ANT58-012PN
x8
2m cable 3m cable
MINICIRCUITS
ZX60-6013E+
AMPLIFIER x2
8m cable
Transmitter
Module
Receiver
Module
Receiver
Chain x8
Front-end
Module x8
±200 MHz
Fig. 1. System block diagram of the FMCW avalanche radar.
R =fdcT
2B(1)
where fd is baseband frequency, c is propagation speed, Tis chirp duration, and B is chirp bandwidth. The full radarsystem is shown as a block diagram in Fig. 1. In general, theradar components were commercially sourced.
B. Design Modifications
Since previous reports on the radar, the radar design hasundergone some changes as challenges were encounteredthroughout the development. This section briefly describessome of these changes.
Firstly, the radar is exposed to a low temperature and humidenvironment. Hence, a heating system was fitted to keep theambient temperature at the radar operating temperature. A pairof 400 W fan heaters are positioned on either side of the radarwith two series-connected thermostats. In addition, the radarand heating system are covered with an insulating blanket.The heating system keeps the ambient temperature within theinsulation at a warm 10oC.
The FMCW radar signal is designed to aid the processof resolving range and Doppler information unambiguously.The repeating signal is composed of 6 successive long linearfrequency modulated (chirp) waveforms with differing chirprates. Each of the chirps sweeps across of bandwidth of 200MHz. The longest chirp period is 5 ms giving a maximumradar range of 3750 m and a ( 1
T =) 200 Hz spectral resolutionwhich equates to a Doppler resolution of 5.7ms−1 providing afine measurement of sub-surface velocities. The full design of
the radar waveform is shown in Fig. 2. The total period of therepeating signal is 20 ms; this equates to 50 images per second.The 6 different chirp signals follow a sawtooth frequencymodulation to provide measurement redundancy which willaid the process of resolving range and Doppler informationassuming the target response stays reasonably constant over20 ms.
After deramping, the baseband signal passes through aactive filter designed to perform frequency gain control. Thefilter is designed to compensate for a r3 drop in signal strengthwith increasing range. In an FMCW radar system, this isequivalent to varying the active filter gain in proportion tobaseband signal frequency. This r3 signal loss model is basedon the assumption that an area of snow is illuminated by theradar alluding to a distributed surface target radar equation[10]. This gain control makes best use of the receiver analogto digital converter (ADC) dynamic range [11].
Laboratory loop tests revealed that low frequency disconti-nuities at the beginning and end of the waveforms generatedby the AWG were heavily amplified by the baseband activefilter. Depending on the receiver input signal strength, this wasproducing signal levels that would saturate the receiver ADC.This problem was rectified by applying an amplitude taper tothe beginning and end of the FMCW waveforms as seen inthe lower amplitude plot in Fig. 2. In addition, the central lowfrequency regions (±1 MHz) are blanked out of each chirp.This measure was implemented to remove the effect of non-linearities in the deramping mixer reproducing the basebandchirp within the ADC band. Both of these measures haveimproved system sensitivity.
The receiver array consists of 8 elements spaced sparselyover a 5.3 m linear aperture. The average separation betweeneach antenna is 15λ. The antenna separations are randomisedto mitigate the effect of grating lobes [12]. Using this ran-domising method the response close to boresight (±15o) isreduced to 5.5dB below the mainlobe. However, it is under-stood that the radar cross range unambiguous resolving poweris still limited.
The radar operates in a sleeping state throughout the winterawaiting a trigger. The trigger is activated when the signalsfrom acoustic sensors within the avalanche track go abovea threshold. When a trigger occurs, it activates software(developed in LabView) that begins acquiring data over the8 receiver channels and also switches on a 5V DC signalto activate an external solid state relay. The solid state relayswitches the power supply of the transmitter PA. Hence, whena trigger occurs, data is acquired and the transmitter PA isturned on. Data acquisition proceeds for 3 minutes to capturethe entire avalanche event, storing the data on a fast but limitedcapacity solid state hard drive (SSD). Every night at 1 am, orif the SSD becomes full, the data is then transferred to a large-capacity external hard drive which is not expected to fill upover the course of the winter season. Hence, an entire seasonof avalanches may be recorded automatically.
Fig. 2. Frequency and amplitude over time of the 20 ms FMCW signalcomposed of 6 up and down chirps of different durations.
III. INSTALLATION AT VDLS TEST SITE
The radar was installed at the VDLS avalanche test site inlate November 2010 for the 2010/11 winter campaign. Thepurpose of the VDLS test site is to study the basic processesin avalanches for the development of improved avalanchedynamics models [13]. The site has been used to make awealth of measurements of large scale natural avalanches andartificially triggered avalanches. The avalanche track is c. 2500m long and is populated with various sensors including radar.Our radar operates in a reinforced-concrete bunker set onthe foot of the slope opposing the avalanche track at VDLS.Indeed, it has been operating throughout the 2010/11 winter.This bunker allows us to take non-invasive measurementsof entire avalanche events and provides protection for ourequipment.
Installation of the radar began with installing the radarantennas. The transmitter antenna is positioned on the southwall of the bunker, separated from the receiver antennas toavoid direct coupling of the transmitted signal to the receivers(see Fig. 4). The radar has two different receiver array options(of the same design) for different purposes; one spread over theinside of the bunker’s 1.3 m windows, and the other mountedoutside on the roof of the bunker.
The indoor array is used for artificial releases, i.e. people arepresent in the bunker and able to operate the window shutters.During these experiments, the shutters are open so that all ofthe sensors within the bunker can measure the avalanche. Theshutters are manually closed when the avalanche reaches thefoot of the avalanche track to protect the equipment.
During periods when the bunker cannot be accessed (themajority of the winter), the shutters are left closed to protectthe equipment within the bunker from naturally occurringavalanches and the weather. Thus, a further outdoor arraywas installed for measurement of natural avalanches. Thedisadvantage of this array relative to the indoor array is thatit introduces more attenuation due to longer cable lengths toconnect with the receiver module.
The system noise was measured in the field to be c.-85dB on each receiver channel. Background measurements
Fig. 3. Radar in the VDLS bunker before application of insulation.
Fig. 4. Transmitter antenna, and indoor and outdoor receiver antenna arrayat the VDLS bunker.
of the lightly-covered avalanche track were used to adjustthe signal strength for both the indoor and outdoor array. Ahealthy maximum baseband signal strength of c.1.6Vpk−pk)was achieved for each array making good use of the ADCdynamic range (16-bit ADC full scale voltage is 2Vpk−pk).
IV. NATURAL AVALANCHE EVENT
A large wet snow avalanche occurred on the 7th December2010, soon after the radar was installed at the VDLS bunker.The radar was successfully triggered and recorded a 3 minutedataset. As a means of checking the data for the presence of anavalanche (false triggers can occur), MTI is performed on thedata having split the baseband signal into returns from eachof the 6 chirps. Each chirp is MTI processed separately bycoherently subtracting the spectra of neighbouring matchingchirp pulses [11]. The resultant images reveal a wealth ofinformation present within the data. A sample image createdby performing MTI on the data from channel 4 using thelongest up-chirp from each frame is shown in Fig. 5. To theauthor’s knowledge, this is the first time an entire avalanche’sprogress has been seen in such detail. In this image we can
Fig. 5. Range-time image from MTI processing of receiver channel 4 usinglongest period up-chirp returns.
clearly see the avalanche front and many components of theavalanche taking different trajectories down the avalanchetrack. Unfortunately, it can be seen that the avalanche con-tinues progressing after 3 minutes, at which point our dataacquisition stops. This was an unexpectedly long avalancheand in future the acquisition period will be extended to 4.5minutes.
For our final submission we will include images of the dataafter further processing has been carried out. This includes co-hering the measurements from each of the 8 receiver channelsand combining them to provide cross-range information. It isbelieved that the avalanche split into two paths near the top ofthe mountain and we hope to confirm this with the cross-rangeinformation. We also hope to produce some measurementsof the avalanche-front Doppler information, which is buriedwithin the baseband signal. We will use all 6 chirps to aidthis process as all 6 chirps are carrying the same information.During this process, the effect of the geometry of the radar andthe topography of the avalanche track will be removed fromthe Doppler measurements for accurate velocity calculations.
V. CONCLUSIONS AND FUTURE WORK
Understanding the behaviours of avalanches is of great im-portance in formulating risk zones for settlements. Our abilityto validate models of avalanche flows is limited due to a lack ofhigh quality field data. Current radar for measuring avalanchessuffers from coarse single dimension range resolution. In thispaper, we have described the design and installation of anewly developed FMCW radar for automatically measuringavalanches that outperforms existing radar in terms of rangeand cross-range resolution. Since previous reports we havemade several modifications to the radar design including atriggering system for automatic measurement of naturallyoccurring avalanches. The radar has been installed in a bunkerpositioned at the foot of the slope opposing the avalanche trackof a well equipped avalanche test site; Vallee de la Sionne,
Switzerland. It has been gathering data throughout the 2010/11winter.
A large natural wet snow avalanche event was successfullyrecorded by the radar in the early hours of the 7th December2010. MTI images have shown the radar data contains awealth of information. It is clear that the radar is sensitiveto snow movements over the entire specified range. For ourfinal submission, we intend to begin applying methods ofcohering and combining the data from each receiver channel togain cross-range information. Following this we will also startto look at resolving the range and Doppler of the avalanchefront. In the future, our velocity estimates from the Dopplerinformation will be validated against the data also collectedby the buried FMCW radars. Also, we plan to perform micro-Doppler analysis on specific components of the avalanche. Byanalysing the micro-Doppler signatures we hope to be ableto evaluate individual blocks of snow within the avalancheand provide statistical data on velocity and flow vectors ofunderlying snow movements from natural avalanche events,for comparison with flow models.
ACKNOWLEDGMENT
The authors would like to thank the Swiss Federal Instituteof Snow and Avalanche Research for their help throughoutthe duration of this work. The authors also thank the NaturalEnvironment Research Council for their support, grant ref.NE/F004621/1.
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