International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 3

Number 3

September 2009

311

1 INTRODUCTION

The CRM-203 type Coastal Surveillance Radar is

solid state Frequency Modulated Continuous Wave

(FMCW) sensor with low transmission power. One

of the most significant parameters of this coastal ap-

plication is small targets detection possibility in

heavy sea clutter conditions and high range resolu-

tion. The requirements perform FMCW technology,

which is rapidly advancing recently. Fully solid-state

transmitter design (due to the low radiated power)

ensures excellent Mean Time Between Failure

(MTBF) and practical without service continuous

operation.

FMCW transmitter produces a constant amplitude

linear frequency modulated signal. The principle of

FMCW radar is presented on Figure 1.

Figure 1. Principle of FMCW radar

Radar signal is transmitted, reflected by the sur-

face of the target and then received after a delay

time τ:

τ =2R / c (1)

where: c = speed of light; and R = distance.

The difference between the transmitting and re-

ceiving frequency f

is directly proportional to the

distance and is used to further FFT processing

(Wawruch & Stupak 2008):

= 2R∆f / cT (2)

where: ∆f = frequency deviation; and T = modula-

tion period.

Frequency Modulated Continuous Wave technol-

ogy offers low probability of intercept feature be-

cause of the low peak power and frequency modula-

tion.

2 RADAR GENERAL DESCRIPTION

The prime function of CRM-203 is detection and es-

timation of planar co-ordinates for sea surface tar-

gets and automated tracking the selected ones to per-

form the coastal surveillance tasks. The radar sensor

gives a presentation of the current sea situation and

calculates the future situation to accomplish the au-

tomated radar plotting aids.

Functional diagram of described radar is present-

ed on Figure 2. Radar sensor basically includes the

antennas integrated with FMCW transceiver, anten-

nas motor drive and Signal Processing & Control

Unit (SPCU) also including local interface and con-

CRM-203 Type Frequency Modulated

Continuous Wave (FM CW) Radar

S. Plata

Telecommunication Research Institute Ltd., Gdansk, Poland

R. Wawruch

Gdynia Maritime University, Gdynia, Poland

ABSTRACT: Paper presents description of the principle of work, structure and basic technical parameters of

the Maritime Coastal Surveillance Frequency Modulated Continuous Wave (FMCW) Radar CRM-203 type

constructed by Telecommunication Research Institute Ltd. in Gdańsk. Results of its tests in real conditions

and comparison with pulse ship radars with scanners installed in the same place will be presented during the

conference.

transmitter

receiver

min

max

T-

-T/2

T/2

f(t)

312

trol circuitry. Each radar transceiver is controlled by

the SPCU which is connected to the Operations Cen-

tre (OC).

The main functions performed by radar are:

− selection of the operative mode (local or remote);

reception from the OC of all controls/ commands

and selections needed for complete operation ca-

pability; in case of control line failure all the con-

trols are automatically put in a default condition -

in order to guarantee the antenna rotation, the ra-

dar emission and the automatic acquisition and

tracking of targets;

− processing of the radar video; compression and

transmission to the OC of the digitised video;

− automatic or on-demand acquisition of targets

falling inside predefined automatic acquisition

zones or acquired by the operator;

− automatic tracking of targets falling inside prede-

fined automatic tracking areas; and

− transmission to the OC, once per antenna revolu-

tion, of status and alarms from the sensor (BITE),

track data (position, speed/course) of targets un-

der tracking.

Figure 2. CRM-203 functional diagram

3 ANTENNAS

CRM-203 in the coastal surveillance application has

typical requirements as small target detection in

weather and sea clutters and high angular resolution.

To provide good angular resolution a narrow azi-

muth beam is required. A narrow azimuth beam is

desirable to reduce resolution cell size for three main

reasons:

− to provide accurate bearing information on the

target;

− to differentiate between targets which are close

together; and

− to reduce clutter returns.

The used system features 12 feet, X-band anten-

nas with horizontal polarisation and the following

electrical parameters:

− 3dB horizontal beam width = 0.7°;

− 3dB vertical beam width = 22°; and

− gain = 32 dB.

Each antennas group consists of a pedestal sup-

porting the rotating unit. The pedestal contains the

drive mechanism, the rotary joint and an 4096 pulses

encoder for transmission of antenna position data.

The power rating for motor controller is 1.5 kW. An-

tennas rotating speed is selectable between 12 and

30 rpm. The antennas group is designed to withstand

severe marine environmental conditions such as salt

spray, sun light, sand, etc.

4 TRANSCEIVER

Functional diagram of the transceiver unit is shown

on Figure 3. Direct Digital Synthesizer (DDS) pro-

duces a synthesized “chirp” (linear frequency modu-

lated) signal. Next this signal is up-converted by

multiplier. The last stage of transmitter circuit is sol-

id state power amplifier (PA), which feeds X-band /

10W FMCW signal to antenna. The receiver consists

of a low noise amplifier (LNA), image rejection

mixer and intermediate frequency amplifier (IFA).

Figure 3. Functional diagram of transceiver unit

The DDS advantages include very fast switching

(typically sub microseconds), excellent phase noise,

transient-free (phase continuous) frequency changes,

extraordinary flexibility as a modulator, and small

size, among others. Frequency changes look like

those of a Voltage Controlled Oscillator (VCO) –

smooth and without phase discontinuity sweep

across a defined frequency range with synthesizer

accuracy, but without the glitches and transient pro-

duced by any other synthesizer technique. Because

of the synthesis techniques, this characteristic is

unique to the DDS and enables it to produce a syn-

thesized “chirp”. It is very important in FMCW ap-

plications because frequency modulation accuracy is

313

directly influencing on accuracy of distance meas-

urements and frequency modulation non-linearity

decreases targets detection.

The proposed transceivers have some additional

features which make them specially suited for

coastal applications:

− sector blanking: emission can be inhibited within

an adjustable sector, so as to avoid undesired re-

turns (e.g. land clutter);

− 12 dB/okt. frequency curve slope of IFA amplifi-

er ensures equal intermediate frequency (IF) out-

put signals for targets in different ranges; and

− digital automatic receiver gain control function

ensures optimal IF signal output level, inde-

pendently of under detection target radar cross

section.

5 SIGNAL PROCESSING AND CONTROL UNIT

5.1 Functional diagram

Functional diagram of the signal processing is

shown in Figure 4.

Figure 4. Functional diagram of signal processing

The frequency measurement performed to obtain

the range measurement is made digitally using the

Fast Fourier Transformation (FFT). So the IF signal

is digitised and sent to the spectrum analyser that

performs FFT. On the input of the signal processing

an analog-to-digital converter samples the IF signal

with 8 MHz frequency and 12-bit resolution. Next

the spectrum analysis of the digitised IF signal is

performed on the base of 8192-point FFT. At the

output of the spectrum analyser a periodogramm

presenting 4096 range cells is obtained. Range cell

size is 5.6 m for radar scale range 12 NM.

The signal after the frequency analysis can be

best referred as the video signal. The signal is indeed

an exact analogue of the video of pulse radar. The

range data output from the spectrum analyser is fur-

ther processed like in pulse radar: CFAR (Constant

False Alarm Ratio) thresholding and binary integra-

tion during the dwell time on a target are performed.

Spectrum analysis

The IF signal is analysed using FFT transform. The

analysis is carried out in real time. The analysed sig-

nal can be modelled as a sum of sinusoids embedded

in noise and clutter. In FMCW processing scatters at

different ranges appear as different constant fre-

quency components at the IF output. The FFT re-

sponse to a sinusoidal input reveals a main lobe and

side lobes. The width of the main lobe indicates

Fourier Domain Resolution, which for CRM-203

application is very narrow and equal 1 kHz. This

Fourier Domain Resolution or differently bandwidth

of FFT frequency cell is very important parameter of

FMCW radar, because of detection performance.

Probability of detection depends on the ratio of the

target received signal level to the sum of clutter and

noise. FMCW transceiver noise power N

is function

of the FFT frequency cell bandwidth:

= kT

FFT

(3)

where: k = Boltzman’s constant; T

= effective noise

temperature; and B

FFT

= FFT frequency cell band-

width.

This relationship explains excellent CRM-203 ra-

dar noise properties allowing low transmitter power.

5.2 CFAR thresholding

The radar must detect a target against a changing

background of clutter and noise. The clutter reflec-

tivity and statistics will generally vary with range

and direction. The problem is how to set a threshold

to provide an acceptable probability of false alarm

whilst maximising the probability of detection

. Standard detection strategy is to fix the P

. In

CRM-203 application an automatic CFAR detector

is used. To control of the false alarms, the detector

must be able to estimate the parameters of the prob-

ability density function of the clutter and noise re-

turns. A well known method of estimating the clutter

mean level is the cell-averaging CFAR circuit. The

mean level of the cell under test is obtained from the

average of a number of surrounding clutter cells. A

gap between the cell under test and the surrounding

cells is method to ensure that a distant relative strong

target does not contaminate the clutter estimates. In

CRM-203 radar smallest off CFAR window is taken

to calculate threshold. This strategy helps to detect

small targets in neighbourhood of strong clutter re-

gion. Size of CFAR window is small so a threshold

can follow the local clutter mean and can give a

much better performance in detection in our case.

After CFAR the binary integrator is used with the

“M-of-N” rule in accordance with formulas valid for

Gaussian noise.

314

The Signal Processing & Control Unit includes a

local display facility, in order to allow local mainte-

nance and set-up operations. All radar controls are

available on the local panel. Moreover SPCU feeds

video signal to Radar Display Unit, which accom-

plish:

− video acquisition and processing;

− plot extraction; and

− tracking.

Track data are sent to the OC for further pro-

cessing. Also plot information can be routed through

the same communications channel. All the radar

controls, including those available at the local panel,

can be also sent by the OC via the remote interface.

The radar continuously sends the status information

to the OC, together with target data.

6 TECHNICAL DATA

Basic technical are presented in Tables 1-7.

Table 1. Transmitter

__________________________________________________

Parameter Value

__________________________________________________

Output power 1mW-2W (switched)

Carrier frequency 9.3 – 9.5 GHz

Frequency deviation switched according to the required

scale range:

54 MHz at 6 NM

27 MHz at 12 NM

13.5 MHz at 24 NM

Range scales 0.25 NM – 48NM

Modulation DDS based linear FMCW

Sweep repetition period 1 ms

__________________________________________________

Table 2. Receiver

__________________________________________________

Parameter Value

__________________________________________________

IF bandwidth 4 MHz

Noise factor 2 Db

Maximum gain 120 Db

Frequency curve slope of IF amplifier 6 dB/oct; 12 dB/oct; 18

dB/oct.

__________________________________________________

Table 3. Antennas

__________________________________________________

Parameter Value

__________________________________________________

Antenna length 3.6 m

Beamwidth (3 dB) horizontal/vertical 0.70°/22°

Polarisation Horizontal

Gain 32 dBi

Rotation speed min/max. 12/30 rpm

Drive motor 1.5 kW

__________________________________________________

Table 4. Signal processing

__________________________________________________

Parameter Value

__________________________________________________

FFT signal processing 8192-points FFT

Sampling frequency 8 MHz

Number of range cells 4096

Signal thresholding CFAR

Signal integration binary, number of detections de-

pendent on antenna rotation speed

Sea clutter reduction signal correlation from 2 antenna

rotations.

__________________________________________________

Table 5. Display unit

__________________________________________________

Parameter Value

__________________________________________________

Display size 22 inch

Resolution 1280 × 1024 pixels

Acquisition automatic up to 100 targets

Tracking automatic of all acquired targets

Zones 2 guard zones

Target information target number, target range and bearing

from radar position, target course

Options ARPA anti-collision functions

__________________________________________________

Table 6. Range and angle measurements

__________________________________________________

Parameter Value

__________________________________________________

Scale range [NM] 12 / 24 / 48

Range cell size [m] 5.6 / 11 / 22

Range measurement accuracy 1% of selected range or 50 m

(whichever is greater)

Angle resolution 0.1°

Bearing accuracy 0.7°

__________________________________________________

Table 7. Environmental conditions

__________________________________________________

Parameter Value

__________________________________________________

Wind operational 30 m/s

Wind survival 50 m/s

Humidity 98 %, 25 °C

Temperature operational from -10 °C to +50 °C (inside operat-

ing room) and from -30 °C to +50 °C

(outside operating room)

Temperature survival from -40 °C to +65 °C

__________________________________________________

7 RECAPITULATION

Described radar was installed in the radar laboratory

of the Gdynia Maritime University this year. Its an-

tenna is located on the roof of the university building

nearby the south entrance to the Gdynia Harbour.

Operational tests of the radar will be conducted in

December 2008. Its detection possibilities, accura-

cies and clutters resistance will be checked during

measurements in real hydro-meteorological condi-

tions. Results will be compared with information

about positions, courses and speeds received from

Automatic Identification Systems (AIS) installed on

board detected and tracked objects and data about

these objects received at the same time from four

different ship pulse radars installed in the same la-

boratory. Results of these tests will be presented on

the conference.

REFERENCES

Wawruch R. & Stupak T. 2008. Charakterystyka radaru na falę

ciągłą. Prace Wydziału Nawigacyjnego Akademii Morskiej

w Gdyni No. 21, p.120-130.