Journal of Space Technology, Vol 3, No. 1, July 2013

13

Abstract— The concept of pulsed RADARs is associated with Chirp Signal Transmission. Chirp signal provides high precision in target resolvability with the flexibility of lower average power. The transmitted Chirp Signal defines the resultant image resolution, which varies from mission to mission. The objective of this paper is to discuss the implementation of a FPGA based runtime user configurable Chirp Signal Generator using the Xilinx System Generator. The designed system is capable of transmitting three different types of Chirp Signal; i.e. Unidirectional Up-Chirp, Unidirectional Down-Chirp and Bidirectional Chirp. The system can generate an output Chirp of Bandwidth between 10 to 100MHz with a transmitted pulse duration of between 1 to 10 Micro Seconds and a Pulse Repetition Frequency of between 100 to 10000Hz. The sampling frequency of the system is kept constant at 150MHz.

Index Terms— Bandwidth, Chirp, FPGAs, Pulse Repetition Frequency, Pulsed RADAR, SAR, System Generator, Transmitted Pulse Duration

I. INTRODUCTION

ynthetic Aperture RADAR (SAR) is a common tool in remote sensing. It finds its applications in global positioning, disaster management, agricultural

monitoring e.t.c. SAR was first proposed by Carl Wiley in 1951 [1] which described the use of Doppler frequency analysis to improve radar image resolution. SAR has many advantages over optical imaging, for example SAR can be used for soil monitoring. Originally the concept of RADAR imaging was based on continuous wave RADARs. The echoes were hard to resolve due to overlapping and the average power of the system was very high. This gave birth to the idea of RADAR imaging based on pulsed RADARs. [2] However, with this new concept came many new problems, for example; the echoes from objects with lower RADAR Cross Section (RCS) in presence of objects with larger RCS became impossible to detect. Such problems were resolved by frequency modulated (FM) pulsed RADARs Chirp. This concept greatly relaxed the processing complexity as the echoes became relatively spaced out as compared to continuous wave RADARs. The concept of FM Pulsed RADAR has its own pros and cons. [3]

The focus of this paper is discuss a FPGA based Runtime User Configurable Chirp Signal Generator for multi mission Synthetic Aperture Radar Imaging Payload. The design requirements for the desired SAR Payload System are illustrated by the Table I.

TABLE I SAR PAYLOAD SYSTEM PARAMETERS

System Parameters Specifications

Mode of Operation Stripmap

Operating Frequency 4-6 GHz (C-Band)

Chirp Bandwidth 10-100MHz

Azimuth Resolution 0.05-1m

Range Resolution 1.5-15m

Platform Speed 100-150m/s

Altitude of Platform 1-5km

The requirement of a runtime user configurable chirp signal generator arises from the system requirements of the payload. The sampling frequency is kept 150MSPS to ensure that the resultant chirp signal is smooth. The systems is capable of generating chirp signal of all three type, bidirectional, unidirectional upward and unidirectional downward, however a degraded output is observed at bandwidths beyond 60MHz. The PRF is also kept user configurable because it is dependent upon platform speed, which is variable in this case. The average transmitter power of the system changes with a change in transmitted pulse width. When altitude is higher a higher average power is required to ensure that the SNR of the received signal is in allowable range, therefore, with a variable platform altitude a requirement for variable transmitted pulse duration arises.

II. RELATED WORK

A similar approach is presented in [4] which is based on the design of a FPGA based reconfigurable chirp signal generator for Unmanned Air Vehicle SAR Payload. However, the system is user configurable in the pre-synthesis phase and only generates an unidirectional upward chirp. The author discusses two approaches in this paper; Memory Based approach in which the signal is stored in a memory and the second a DDS compiler based approach. In this paper a non memory based approach is used. However, the signal generation is based on Cordic Algorithm.

FPGA Implementation of a Runtime User Configurable Chirp Signal Generator for Multi

Mission Synthetic Aperture Radar Imaging Hamza Mansoor Manya and Ashfaq Amjad Khan

S

FPGA Implementation of a Runtime User Configurable Chirp Signal Generator for Synthetic Aperture Radar Imaging

14

The pros and cons of using chirp signals in pulse radar imaging are discussed in [5]. The paper discusses the basic concepts of synthetic aperture radar imaging and matched filtering approaches. The issues in SAR imaging include processing latency, which is addressed in [6]. The idea is how to effectively divide the acquired data frame into tiles to increase the processing latency by processing these tiles in parallel without having an intense effect on the processed image quality. Similarly, SAR antenna simulation is a highly time consuming and mathematically complex process. This simulation performed in [7].

III. PRINCIPLES OF SIGNAL GENERATION IN LINEAR

FREQUENCY MODULATED PULSED RADARS

Synthetic Aperture RADAR systems are frequently associated with the use of pulsed linear frequency modulated signals (chirp). Chirp signal incorporates numerous sinusoid frequency components in a single pulse. The frequency slope of a Chirp signal can be of three different types i.e. unidirectional upward frequency slope, unidirectional downward frequency slope and bidirectional frequency slope. Generally in SAR the preferred Chirp type is bidirectional frequency slope. The advantage of using this type of chirp signal is, to attain a particular bandwidth in a Chirp

pulse the highest frequency component theoretically required is half of bandwidth edge while in a unidirectional chirp the highest frequency component should at least be the edge of the bandwidth. The use of a bidirectional frequency slope therefore immensely relaxes the sampling frequency requirements and consequently the hardware requirements. The generation of all three types of signals only differs in instantaneous phase generation. The preceding section elaborates the processes of phase generation of all three types.

IV. DESIGN DESCRIPTION

The Fig. 1 (B) illustrates the block level detail of the runtime user configurable chirp signal generator unit design. The runtime user configurable chirp signal generator unit will become a integral part of the Runtime User Configurable SAR Acquisition Controller. The block level detail of the Runtime User Configurable SAR Acquisition Controller are illustrated in Fig. 1 (A). The scope of this paper is to discuss the design detail of the runtime user configurable chirp signal generator unit. The system was designed using the Xilinx System Generator. The details of the different blocks of the runtime time user configurable chirp signal generator are illustrated below:

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REFEREN

[1] Wiley, No. 3, 196,[2] Mehrda[3] A.A. KQuality of Emerging T[4] M. Y. UAV SAR2009 [5] M. Y.Proceeding[6] Einar-A[7] Matthe2009.

Fig

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l Generator for

the evolution anystems, like FPion and signal prgital systems oparts when it coms paper an archi

enerator is discudic Sine generatibidirectional chid. requirements wn and runtime rith a wide range

ment. system is howevwhich currently

L equivalent andransceiver design

NCES

C. A., "Pulse Dopple, 436, Filed, August ad Soumekh “Synthe

Khan and H.M. Manyf Synthetic Aperture Technologies 2012, 8Chua and V. C. K

R", Proceedings of th

. Chua and V. C. gs of the Progress In Arne Herland “ERS-1ew Schlutz “Synthetic

g 16: Unidirectiona

15: Unidirectional

r Synthetic Ap

CONCLUSIO

nd availability oGAs, the trend rocessing systemffer a greater

mes to reconfiguitecture of a runssed. The archition and is non mirp signal in SAR

which led to threconfigurabilty

e of stimuli and h

ver just a integraly in design phasd is now being en.

er RADAR Methods1954, 1965.

etic Aperture RADARya, "Analysis of the Radar", Proceeding

89-92, 2012 Koo, "FPGA-Based Che Progress In Electro

Koo, " An introduElectromagnetics Re1 SAR Processing wic Aperture Radar Im

al Up 10MHz Chi

l Down 10MHz Ch

erture Radar Im

ON

of off the shelf rfor implementin

ms has increaseprecision than

uration. ntime user configtecture of the symemory based. R digital transce

his work requiry. The designedhas proven to ful

l part of the syntse. This system employed in the

s and Apparatus," Un

R Signal Processing”Effect of Chirp Par

g of the Internation

Chirp Generator Foromagnetics Research

uction to Synthetic esearch, Vol. 2, 27–6ith CESAR”, 2007.

maging Simulated in

rp with 3 Micro S

hirp with 3 Micro

maging

reconfigurable ng digital data ed by the day.

their analog

gurable digital ystem is based The merits of eivers are also

red pin point d system was lfill all system

thetic aperture was ported to

e overall SAR

nited States Patent,

”, 1999. rameters on Image nal Conference on

r High Resolution h, PIER 99, 71-88,

Aperture Radar", 60, 2008

MATLAB”, June,

econds Tp

Seconds Tp