3-2-5 fmcw ionosonde for the sealion project - nict · 3-2-5 fmcw ionosonde for the sealion project...

1 Introduction

Ionosondes typically use a wide range offrequencies in the MF-HF band depending onthe characteristics of the bottomside of theionospheric region. Stationary ionosondes forroutine observation at four domestic observa-tories and Antarctic Syowa Station have beensuccessively developed by the National Insti-tute of Information and CommunicationsTechnology (NICT)［1］–［3］. However, it is dif-ficult to promptly initiate observation at adesired location with such a non-portable sys-tem. In addition, as each ionosonde is a pulseradar system with transmitting power of 10kW, there requires accommodations betweenneighboring radio systems and making thesystem unsuitable for continuous observations.

As one of the pulse compression technolo-gy, FMCW (Frequency Modulated ContinuousWave) enables low transmitting power［4］andoffers continuous observation without mutualinterference with neighboring radio stations.Coupled with appropriately selected observa-tion sequence and data processing, FMCWcan provide exceptional flexibility in opera-

tion. The measurement technology of HF-band FMCW radar was first established in the1970s. Barry［4］designed practical models bymodifying a commercial synthesizer for fasterfrequency switching and later by adopting amicro-phase synthesizer in which the sweepvoltage signal applied to a voltage-controlledoscillator (VCO) apart from synthesizer feed-back loop.

FMCW exhibits its distinctive characteris-tic utility in bi-static observation in which thetransmitting and receiving antennas are locat-ed sufficiently apart from each other. One ofthe highly adaptive applications is long-rangeoblique sounding. FMCW sounding signalsfrom the icebreaker Fuji on the way to Antarc-tica and from Germany were monitored inJapan to examine propagation modes togetherwith maximum usable frequency (MUF) andlowest usable frequency (LUF). Ichinose etal.［5］analyzed seasonal changes of magneticdisturbance effects on the propagationbetween Japan and Germany. Bi-static over-the-horizon FMCW radar has also been usedto observe ocean waves by means of skywaves［6］.

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3-2-5 FMCW Ionosonde for the SEALION Project

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A portable low power FMCW ionosonde has been developed and deployed mainly atunmanned overseas observation points. Due to the low power of transmitting signals, there hasbeen very little interference from existing radio systems around the FMCW ionosondes. Thepost-observation data processing offers wide possibilities in manipulating the observed databecause the FMCW ionosonde records the entire baseband signal and measurement parame-ters are selected after observation. Output ionograms are equivalent to those of the routineionosondes operated by the National Institute of Information and Communications Technology.FMCW ionospheric observation technology has been transferred to Kyushu University to aidmainly in studying the electric field by Doppler observation.

KeywordsFMCW, Ionosonde, DDS

288 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009

For mono-static observation which trans-mits radio waves vertically, Poole［7］ andPoole and Evans［8］demonstrated that polar-ization mode separation, Doppler shift, andarrival angle observations could be establishedby means of an vertical incidence FMCWionosonde with a peak power of 20 W.Oblique sounding was also conductedbetween the Antarctic SANAE Station andGrahamstown, South Africa. The FMCWionosonde had thus been used in limited appli-cations until sweep frequency generators andpost-observation data processing became easi-ly accessible.

In Japan, an ionosonde attached to the MUradar of Kyoto University was developedbased on the FMCW technique［9］. Due to thelimit of effective sweep range of the sweepsynthesizer, observations were conducted bystepping up the sweep start frequency by 100kHz. The FMCW ionosonde assembled by theRadio Research Laboratories (predecessor ofNICT) for Antarctic observations［10］per-formed continuous observations at a fixed fre-quency as low as 20 W, and found wavy varia-tions in the bottomside of the ionosphericregion above Syowa Station［11］. Due to lowcapacity on the data processing, one observa-tion was divided into about 100-kHz sweepunits and it took about 15 minutes to obtainone ionogram.

With the onset of the 1990s, the direct dig-ital synthesizer (DDS) operable in the HFband became increasingly popular, enablingan entire frequency band sweep with high pre-cision. Advances in computer technology havealso increased greater flexibility to the scopeof observation control and data processing.Because of the strict regulation on transmit-ting radio wave qualities, it took a long time tonegotiate before the Japanese Telecommunica-tions Bureau agreed to issue the radio license,regardless of output power［12］and spuriousintensity［13］that satisfied the Radio Act overthe entire frequency range. For this reason,overseas observations took the lead. Aportable ionosonde, developed for the West-Pac campaign near the geomagnetic equator in

1997［14］and observations in Fukui Prefecturein collaboration with the MU radar in1998［15］, employed domestically producedDDSs. The transmitter and receiver containedin trunk cases were transported as the handbaggage of one person. An ionosondedesigned for the SEALION Project［16］ in2002 had its receiver/controller unit sizablydigitized, featuring PLD (ProgrammableLogic Device) implementation in the secondIF and subsequent circuits.

This paper provides preliminary insightinto the principles of operation of the FMCWionosonde in Section 2, system construction inSection 3, and some observational examplesin Section 4, mainly on the FMCW ionosondedeveloped for the SEALION Project.

2 Principles of operation

When the observation radio wave with fre-quency f varying linearly as f ＝ f0＋ f

·· t

with time t is reflected by a moving target atrange r and with line of sight velocity v asr = r0＋ v · t , frequency difference fb betweenthe transmitting and receiving signals isexpressed, under the condition of v≪c, as

（1）

where f·

is the frequency sweep rate and c isthe velocity of light. Frequency and timerelation is shown in Fig. 1. The first term inthe parenthesis on the right side of Equation(1) gives the frequency shift due to the range;the second term denotes the Doppler shift. Intypical observations, f

·is set at around

100 kHz/s, and if the Doppler term isdisregarded, the baseband frequencycorresponding to 1000 km, the maximumheight of the bottom of the ionospheric regionwould equal 667 Hz, thus enabling a narrowerbandwidth of the IF stage than a pulse typeionosonde, and alleviating the effects ofinterference.

Poole［7］and Poole and Evans［8］operatedan FMCW ionosonde using three-cell sound-

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2.2 Data processing gainThe FMCW is one of the spread spectrum

methods. Frequency analysis on the basebandsignal corresponds to the matched filter ofspread spectrum communications. Signalamplitude is increased by the pulse compres-sion ratio G after passage through the matchedfilter. G is defined as:

（4）

where F and τ are the frequency scan widthand the sampling duration, as shown in Fig. 1respectively［19］. Assuming typical valuesof f

·＝100 kHz/s and Δr =1 km, Eq. (2) gives

τ=1.5 sec and F=150 kHz, then G=2.25×105

can be derived. Therefore, a FMCW radarsystem with a transmitting power of 10 Wtheoretically equals a pulse radar system of2.3 MW peak power.

2.3 Transmit/receive switchingIn mono-static FMCW observations, cou-

pling between the transmitting and receivingantennas is inevitable in the MF-HF band usedby ionosonde, such that a considerable part ofemitted power from the transmitting antennasneaks into the receiving antenna and sup-presses the receiver. When a transmittingpower of 40 dBm (10W) is fed to a conven-tional delta antenna, the typical ionosphericreflected signal strength is about －100 dBm atthe receiver input. Because the coupling lossbetween the transmitting and receiving anten-nas placed perpendicularly is about 30 dB,direct waves sneaking into the receivingantenna from the transmitting antenna havestrength of 10 dBm and by far exceed thereceiver s dynamic range. As a solution to thisproblem, so-called Frequency ModulatedInterrupted Continuous Wave (FMICW orPulsed Chirp) is commonly used by alternat-ing transmission and reception as shown inFig. 2.

The signal reflected from the target atrange r (and hence the delay 2r/c) is effective-ly received upon arrival in the receiving win-dow, but the signal arriving at the receiver

ing to confirm its applicability to polarizationseparation, Doppler observations, and arrivalangle measurement. In contrast, our ionosondeis designed on the assumption that the Dopplervelocity is obtained by converting the base-band signal twice［17］［18］.

2.1 ResolutionsThe range resolution Δr and velocity res-

olutionΔv are determined by the conditions off b（Δf , 0）・τ＝ 1 and f b（0, Δv）・T＝1, andcan be expressed as follows:

（2）

（3）

As can be seen from Fig. 1, F = f·

·τ is afrequency width scanned at each samplinginterval. On the basis of Eq. (2), Δr is deter-mined solely by the frequency width used inthe analysis, so that it can be established afterthe observation. Pulse radar requires a narrow-er pulse width for fine resolution, and thenhigh transmitting peak power to achieve therequired signal strength, but FMCW radardoes not necessarily require the high transmit-ting power for fine resolution observation.Similarly to pulse radar, FMCW radar has itsvelocity resolution determined by Dopplerobservation time T.

Fig.1 FMCW radar system schematic


during transmission does not contribute to theobservation. Assume transmit/receive switch-ing signal g (t) is as follows: g (t)=1 at trans-mission, and g (t)=0 at reception, then, theeffective receiving time rate ρ(r) of the echofrom a target located at range r can beexpressed as follows:

（5）

With transmit/receive switching in a fixedrectangular waveform, ρ(r) would vary peri-odically between a maximum of 0.5 and aminimum of 0.0 with the range . A constantρ(r) independent on the range is obtained byrandom T/R alternation［20］. When using anM-sequence pseudorandom code having chiprate t0 and n-stage shift register sequencelength N=2n－1, Equation (5) becomes

（6）

Therefore, ρ（r）would be 25% (constant)beyond range r0 = ct0/2 for sufficiently large N.

Figure 3 shows the range variation ofρ（r）fort0 = 0.5 ms. The thin and bold lines indicatecases of transmit/receive switching in a fixedrectangular waveform (Fig. 2 a) and a pseudo-random code with n = 10 (Fig. 2 b), respec-tively. Poole［20］showed thatρ（r）varies withrange when the Q-sequence code signal,which is derived from the M-sequence code, isemployed for alternating transmit/receiveoperations.

According to Iguchi［21］, spectrum ofinterrupted frequency sweep signal S (f) con-sists of monotonous carrier fc and the side-band due to switching as:

（7）

where, A denotes the amplitude of the carrier.The first term in the parentheses on the rightside of Eq. (7) represents the carrier compo-nent, that is, the effective signal, and the sec-ond term represents a sideband due to alternat-ing transmit/receive operations by the pseudo-random pulses. When the carrier is interrupted

Fig.2 Transmit/receive switchingschematic(a) Transmit/receive switching signal.(b) Transmitting signal interrupted according

to (a).(c) Transmitting signal with shaped envelop.

Fig.3 Changes in receiving time ratio ontransmit/receive switchingThe thin line denotes a repetition frequencyhaving a symbol transmission interval of0.5 ms, and the bold line designates transmit/receive switching with an M-sequencepseudorandom code by using a 10-stage shiftregister.

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by an M-sequence pseudorandom code of n =10 (Fig. 2 b), the sideband spreads widelyfrom the level 30 dB below the carrier in theform of (sinf/f)2, thereby being considered aconstant level over the observing range asindicated by a black line in Fig. 4. The signalreflected from the target also has a spreadspectrum around the carrier given by Eq. (7),and all signals lower than 30 dB from thedominant echo are shielded. Even if the trans-mitting power is increased, the sideband levelwould be also lifted, leaving the apparentdynamic range unchanged at 30 dB. Thespread echo spectrum disturbs the detection ofscattered signals having significantly changinglevels according to the range as with oceanwave radar, in contrast to harmless echoes thatdo not vary as much as reflections from thestratified ionospheric region. The ocean waveradar alternates transmission and reception byusing monotonous signals in spite of rangevariation in ρ(r) shown with the thin line inFig. 3. Transmit/receive switching at a fixedfrequency would turn the sideband into isolat-ed peaks of switching frequency harmonics,thus allowing a weak received signal to beobserved by flushing the switching frequency

out of the IF band［21］［22］.As the spread of spectrum by T/R switch-

ing is merely an obstruction to neighboringradio systems, excess spectrum is suppressedby smooth shaping of the switching signal asshown Fig. 2 (c) (see the green line in Fig. 4).The received signal is modulated with inverseof the T/R switching signal in the receiver.Iguchi［21］showed further increase in the side-band of the baseband signal.

3 System construction

As ionosondes are often installed in ruralsites under inconvenient power and communi-cation conditions, the system is designed towithstand self-sustained operation for a longperiod without maintenance. Remote controland data archiving are practicable, providedthat network access is available. One entireobservation system includes a main FMCWionosonde and such peripherals as a pair oftransmit/receive antennas, an antennaswitch/attenuator, an RF wattmeter, coaxialarresters, a control PC, a network PC, and oth-ers. As an example, Figure 5 shows a blockdiagram of the system installed at ChumponCampus, King Mongkut’s Institute of Technol-ogy Ladkrabang (KMITL), the Kingdom ofThailand (CPN: 10.73ºN, 99.38ºE). Figure 6shows photographs of the antenna and the sys-tem, respectively. Table 1 summarizes the keyspecifications. Observations normally takeplace at a frequency sweep rate of 100 kHz/sbetween 2 MHz and 30 MHz, and therefore,each session of observation takes 4 minutesand 40 seconds to complete.

A pair of crossed folded dipoles with ter-minators or delta loop antennas is stretchedfrom the top of a 30-m tower. Since theobserving frequency range is 10 times or morewide, traveling-wave antennas, though poor inefficiency, are used to achieve flat frequencycharacteristics in a simple setup.

The antenna selector SW/attenuator inter-connects the transmitter/receiver and antennasonly when the ionosonde is at work. Theionosonde is disconnected from the antenna

Fig.4 Spectrum spread around the carrierby transmit/receive switching withpseudorandom codeThe black line denotes interruption of the carrier(Fig. 2 b), and the green line designates theresult of amplitude modulation with a smoothenvelope (Fig. 2 c). The blue line marks the lim-its to out-of-band radiation imposed by theRadio Regulatory Laws［13］.


while stand by state to protect from lightningdamage. The attenuator compensates for thedifference in echo intensity between daytimeand night.

As shown in Fig. 7, the receiver/controllerhouses two identical receivers as backups,allowing additional observational functions ofdirection-finding, polarization separation if anappropriate antenna is installed. The receiverfirst IF is 70.1 MHz and the second IF is100 kHz. The second IF is digitally sampled at

1 MHz with 14 bits, followed by digital fre-quency conversion and bandwidth limiting onthe PLD to generate a baseband signal. Thesecond IF and subsequent stages are digitizedto eliminate the bulky analog components andlaborious adjustment tasks. As there is no tun-ing at the input stage of the receiver, all envi-ronmental radio waves of broadcasting andcommunications pile up above the echo sig-nals at receiver input. Only MF broadcastingis eliminated by a 2 MHz high pass filter(HPF) at the front end of the receiver. A highintercept level module is selected for the HeadAmplifier. Spiky pulses, generated whensweep frequency passes the strong interfer-ence carrier frequencies, are picked up by thenoise blanker and eliminated by turning offthe transmit/receive switch (T/R SW) placeddownstream of the signal flow. During thetransmitting sequence in mono-staticionosonde mode, the receiver input signal iscut off by turning off the gate switch and alsothe D/A converter of the DDS, so that thetransmitting signal never leaks into the receiv-er. The switched signal is then envelope-shaped for suppressing ringing by the gainweighter, an analog multiplier subsequent toT/R SW.

Commands, baseband data, and house-keeping data are sent to and from the controlPC via a USB interface. The control unit inter-prets a command and then settles DDS, T/Rswitching, and envelop shape, while process-ing the received baseband signal. As the con-trol PC sets parameters and issues an observa-

Table 1 FMCW ionosonde key specifica-tions

Fig.5 FMCW ionosonde system block dia-gram

Fig.6 FMCW ionosonde installed atChumphon (CPN), ThailandUpper: 30 m antenna tower.Lower: (Right) from top to bottom: transmission-

type wattmeter, transmitter, receiver,antenna selector switch/attenuator.(Left) quick-look ionogram displayedon the control PC.

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tion start signal for each session of observa-tion, the controller starts observation in syncwith 1-second pulses from the GPS clock, andthe baseband signal is transferred from thecontroller to the control PC. The GPS clocksends time codes to the control PC via an RS-232C interface every second. Observation canbe triggered by any delay time from the 1-sec-ond pulse in order of microseconds so thatinterference between neighboring stations canbe avoided by adjusting each offset time.

All signals within the receiver/controllerare generated in sync with the signal from thebuilt-in, 10-MHz high-stability crystal oscilla-tor. Phase continuous sweep frequency sig-nals, the fundamental elements of FMCWradar, are generated in 1 Hz steps using DDSsindividually for transmission and reception.The transmit and receive frequencies can bestaggered to generate pseudo-echoes at test-ing. The DDS clock frequency is 80 MHz; thetransmitting frequency is generated directlybetween 2 and 30 MHz. For the first receiverlocal frequency, DDS output frequency is dou-bled and then converted with a fixed frequen-

cy to 72.1 ～ 100.1 MHz. The DDS is 32 bitswide for frequency and 12 bits wide for ampli-tude. Spurious level of the D bit D/A conver-sion is not more than

（8）

according to Reference［20］. Therefore, thespurious strength attributable to the DDS isheld to -70 dBc or below.

As shown in Fig. 2, the swept-frequencysignal generated in the transmit DDS is inter-rupted by a pseudorandom signal and enve-lope shaped before being directed to the trans-mitter. The transmitter amplifies the input sig-nal of 0 dBm and sends it through the LowPass Filter (LPF) block to the transmittingantenna with 20 W peak level. Because of thewide frequency range, a tune-less type ampli-fier is employed. A C-MOS FET Class A-Bpush-pull module is employed for the finalstage, expecting low spurious level althoughlow efficiency. A simple transmitting antennais assumed as mentioned above, and theamplifier works under a mismatched condi-

Fig.7 Receiver/Controller block diagram


tion. However, the amplifier is further protect-ed by a relay at the extremely degradedVSWR. The LPF block consists of low passfilters varying in cutoff frequency by timesfrom each other in order to suppress harmon-ics. As the observed frequency is increased, anappropriate filter is selected from the con-troller.

The control PC issues commands to thecontrol unit according to the observationschedule. After an observation sequence, abrief ionogram is drawn for a quick look andthe data compressed file of the baseband sig-nal is stored in a storage device. Althoughentire baseband signal consumes a large sizeof strage, it provides greater freedom in thework of generating ionograms, h’-t plots andmore for analysis. As the control PC focuseson the tasks of controlling the ionosonde andreceiving data, the network PC takes charge oftransmitting data to and from the outside ofthe site, updating the observation schedule,providing the Internet firewall protection, andother tasks.

4 Observations

4.1 Ionosonde observationsThe FMCW ionosonde affords greater

freedom in post-processing by recording theentire baseband signal. Ionograms equivalentto the domestic routine observations are gen-erated and then the ionospheric parameters areextracted by commonly utilizing the scalingsystem for the stationary ionosondes(http://wdc.nict.go.jp/IONO/index.html). Fre-quency scan F used to draw a frequency line isdetermined as F = c/2Δr from Eq. (2) forgiven range resolution Δr . Frequency step inan ionogram is discretionally selected byduplicating a data sample as shown in Fig. 1.

Figure 8 shows the spread F phenomenonobserved at CPN on July 8, 2003. Ionogramsdrawn every 20 minutes for 3 MHz ～ 9 MHzare put in order. The time is given by UT. Sun-set time on that day was 11:49UT. It is clearthat F-region echo rose up due to E×B driftafter sunset and developed into the spread Fnear the apex. Vertical streaks in each iono-gram indicate intense interferences. Sufficientecho strength is obtained under the stratifica-tion conditions before and after spread F.

Fig.8 Spread F sequence above CPN

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Under the spread F condition, however, thesmall cross section of each ionized patchrequires higher transmitting power or moredetailed threshold processing in drawing anionogram.

4.2 h’-t observationsCollaborative observation with MU radar

at Shigaraki, Shiga Prefecture (34.9ºN,136.1ºE) was conducted during the summersfrom 1998 to 2002 when sporadic E (Es) was

active. The FMCW ionosonde was installed atSabae, Fukui Prefecture (36.0ºN, 136.3ºE) justbelow the E region field aligned irregularity(FAI) illuminated by the MU radar［15］.Because the FAI has a structure along geo-magnetic field line B0 as shown in theschematic diagram of location in Fig. 9, thecross section of FAI scattering has a maximumvalue in the plane perpendicular to the geo-magnetic field line. The radio waves transmit-ted northward from MU radar are stronglyscattered by the E-region FAI just aboveSabae. The MU radar has a beam width of4.5º. Although the FMCW ionosonde has abeam width of about 60º, it only observesecho signals reflected from the surface per-pendicular to the line of sight.

At Sabae site, an aluminum pole about15 m high was erected on top of an elevatortower on a five-story building, and foldeddipole antennas with terminators werestretched on the rooftop as ionosonde trans-mitting and receiving antennas. The antennaswere removed prior to snowfall in winter. Tostudy the details of Es, the ionosonde wasswept through a narrow frequency rangeminute-by-minute, and altitude changes withcertain fixed frequencies were plotted with aresolution of 1.5 km. Simultaneous observa-tions of the ionosonde h’-f and MU radar FAIscattering are shown in Fig. 10［15］. Quasiperiodic (QP) echoes were observed as fol-lows: 2000/06/01 19:45～20:25 LT, 20:40～21:30 LT, and 06/02 00:10～02:10 LT. The Esabove Sabae, descended slowly from 120 kmto 100 km during the period. The echo altitudeobserved by the ionosonde coincided with thestrongest QP echo scattering altitude observedby the MU radar. Echo altitude variations at 4,5 and 6 MHz well corresponded to the genera-tion of QP echoes, but not with 3 MHz, there-by suggesting that the generation of QPechoes requires a certain electron density.

4.3 Doppler observationWhen the eastward electric field appears

in the ionospheric region, charged particlesmove perpendicular to the geomagnetic field

Fig.10 Changes in altitude of Es by MUradar (a), (c) and multiple fre-quency h’-t plots (b), (d)［15］

Fig.9 Antenna configuration of MU radarand FMCW ionosonde in collabora-tive observations［15］


due to E×B drift. Doppler observations makeit possible to measure the electric fieldthrough the drift velocity. The ionosondessimultaneously observe the range and velocity,while HF standard-wave Doppler observationonly provides velocity information. Figure 11shows Doppler variations at the geomagneticSC event observed at Sasaguri (SSG: 32.56ºN,129.24ºE), Fukuoka Prefecture［24］. The uppercase is SC phenomena at SSG in the daytime,indicating an eastward electric field. At SC,the magnetosphere is compressed, resulting inan increased geomagnetic field strength atboth Kuju (KUJ: 32.71ºN, 133.19ºE) nearSSG and at Santa Maria, Brazil (SMA:29.97ºS, 57.86ºW), on the opposite side of theearth. The lower case shows SC phenomena atSSG in the nighttime, indicating that the elec-tric field is impressed in the direction oppositeto that in the daytime.

5 Concluding remarks

Because FMCW observations transmit thesignal energy spread into time and frequency,a low-power advantage is exhibited on fine

range resolution and a long interval observa-tion. Because transmit/receive alternation isindispensable to HF mono-static observation,vertical incidence ionosonde cannot be totallyutilized the FMCW pulse compression advan-tage. Yet, the FMCW ionosonde permits low-power observation and retains great usefulnessas a compact portable observation apparatus.Low transmitting power results in no claimsfrom nearby radio systems in Japan and inEast Asia. Results of the mobile observationswere utilized to decide the permanent observa-tion point in the SEALION Project［16］. Theionosonde is designed to satisfy the require-ments for transmitting power and unwantedspurious radiation stipulated in Japan’s RadioAct over the observing frequency range, there-by making observation possible when andwhere required.

The FMCW ionosonde has also been usedat Kyushu University, which conductedDoppler observations at Sasaguri, FukuokaPrefecture, as well as in Russia and the Philip-pines, for correlative observations with vari-ous geomagnetic and ionospheric phenomenasuch as propagation of the DP2 field to low

Upper case: SSG in daytime.Lower case: SSG in nighttime.

Fig.11 Electric field observations in FMCW radar Doppler mode at geomagnetic SC［24］

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latitude and geomagnetic pulsations alongwith SC events［24］［25］.

Appreciating the FMCW advantages oflow power, low EMC, and high reliability, astationary FMCW ionosonde is being devel-oped for the next generation of Antarcticobservation apparatus. Interference will be

highly eliminated by a front end band-pass fil-ter varying in sync with the frequency sweep.The observation frequency range is planned toreduce the lower limit to 0.5 MHz and upperlimit to 16 MHz according to the ionosphericcharacteristics above the Antarctica.

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