1. Introduction

Nowadays altimeters that are extensively used in aircrafts with (and without) passengers and rockets utilize radio altimeters. In fact, these altimeters operate like range measurement radars. Radio altimeters are found in various types of pulse and FMCW altimeters. The first altimeters used in American systems were pulse altimeters. The basic deficiency of this kind of altimeter is its very high power, for example a 300 watt power of AN/APN-171 altimeter [1]. This power is proportional to technology of that age, was produced by tube and can not be decreased. In addition, the weight of this altimeter is about 7Kg and more.

Pulse altimeters of 60 decade usually use 100 watt cavity tubes and operate at an altitude of 5000 feet. By using solid state transmitters, transmitted power is remarkably reduced which improves the security factor, by reducing the probability of intercept (LPI) and size, while increasing the accuracy.

90 decade altimeters, with 5 watt peak power and weight between 1.8Kg and 4.5Kg can measure altitude to 1700m with accuracy equal to 2% of height [2].

Modern altimeters can reduce their consumed powers by power management methods and advanced techniques, increasing their jamming immunity. Also the maximum measurable altitude of these altimeters is more than previous ones.

Beside the pulse altimeters, FMCW altimeters are generally used by European companies. The great advantage of these altimeters is their low peak power. Since low peak power from different aspects such as LPI and the use of solid state transmitters is important, pulse altimeter manufacturers try to decrease peak power of their transmitters by pulse compression techniques and improvement in other system sections. It is noted that when we discuss altimeter specifications, electronic defense capabilities are very important [3]. Since n altimeter is a type of radar, designed perturbation techniques for radars can be used for similar altimeters. Also electronic Counter-Counter Measure (ECCM) techniques used in radars can be implemented in altimeters. Studies were performed on different altimeters from manufacturers such as Honeywell, Alcatel and ELTA, and various optimizations were accomplished [4]. Characteristics of a modern altimeter are shown in table (1).