ISSN (Online) 2321 – 2004 ISSN (Print) 2321 – 5526 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN ELECTRICAL, ELECTRONICS, INSTRUMENTATION AND CONTROL ENGINEERING Vol. 1, Issue 9, December 2013 Copyright to IJIREEICE www.ijireeice.com 422 DSSS Based Radar Altimeter Divya S.Menon 1 , Lal M J 2 , Tomson Devis 3 PG Scholar, Department of ECE, St. Joseph College of Engineering and Technology, Kottayam, India 1 Scientist, RFSD, AVN, Vikram Sarabhai Space Centre, Thiruvananthapuram, India 2 Asst. Professor, Department of ECE, St. Joseph College of Engineering and Technology, Kottayam, India 3 Abstract:The basic functions of a radar altimeter is to provide accurate altitude measurements above the Earth surface with a high degree of accuracy and integrity during the approach, landing, and climb phases of aircraft operation. In this paper a design of Direct Sequence Ranging System useful for the development of spread spectrum based radar altimeter is proposed. Radar altimeter measures altitude more directly, using the time taken for a radio signal to reflect from the surface back to space vehicle. In Spread Spectrum technique the spectrum of the modulated signal is spread to cover a wider range of frequency spectrum. It offers features like resistance to jamming, transparency to unfriendly receiver, high resolution ranging etc. Basic building blocks of a direct sequence spread spectrum based delay measurement system are simulated using Matlab. From the measured delay, the range is estimated and the accuracy/resolution of the ranging system is analysed from the results. Keywords: Spread spectrum, Direct Sequence Spread Spectrum, PN sequence, Gold sequence, Radar altimeter I. INTRODUCTION An altimeter is an instrument used to measure the altitude of a space vehicle above landing surface. Radar altimeter [1] measures altitude more directly, using the time taken for a radio signal to reflect from the surface back to space vehicle. Conventional radar altimeters use FM-CW ranging technique. FMCW radar [1][2] system is susceptible to interference from other radio devices as they are continuously transmitting across a frequency band. This is due to the larger range of frequencies encountered and due to the lower peak power, resulting in the returned signal being overwhelmed by other emissions. Also due to the lower power and continuous transmissions of FMCW systems, they may be more easily jammed by electronic warfare systems. As these radar systems continuously transmit, they are easily detected by electronic warfare systems. In order to overcome the disadvantages of FM-CW ranging a direct sequence ranging system [3] is proposed which can be used for spread spectrum based radar altimeter. Spread spectrum techniques [4] were initially developed by the military to send data and messages without being affected by jamming (intentional interference) or detection by the enemy. In addition it is used in satellite communications, cellular telephony, Global Positioning System (GPS) etc. Spread-spectrum techniques are methods by which a signal (e.g. an electrical, electromagnetic, or acoustic signal ) generated with a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. In the proposed system, the spread spectrum modulated signal is BPSK modulated and transmitted and on reception, Costas loop is used for demodulation and, the carrier and the data signals are recovered. The delay of recovered data signal can be estimated by correlating it with the transmitted signal and from the delay the range can be calculated. II. PROPOSED SYSTEM The proposed system consists of two sections: transmitting section and receiving section. In the transmitting side, the data sequence is spread using a PN sequence and the spread sequence is BPSK modulated and transmitted. In the receiving side the received signal is demodulated using a Costas loop and the carrier and data signals are recovered. And in order to estimate the delay the transmitted signal and the recovered signal are correlated. Figure 1 illustrates the different stages of Direct Sequence Ranging System. In the figure, g(t) represents the spread signal which is BPSK modulated and transmitted as s(t),a Direct Sequence signal with power Ps.  = 2  () 0  (1) The signal is reflected from the intended target and received T1+T2 seconds later as r(t).  = (− 1 − 2 ) = 2  (− 1 − 2 )cos ( 0  + ) (2) where α represents the signal attenuation and θ is a random phase caused by the time delay [3]. The carrier 2  0 (− 1 − 2 ) and the data sequence are