Altitude Measurement Based
on Beam Split and Frequency
Diversity in VHF Radar
BAIXIAO CHEN
GUANGHUI ZHAO
SHOUHONG ZHANG
Xidian University
China
A new beam split altitude interferometry based on altitude
diversity and frequency diversity for very high frequency (VHF)
radar is proposed in this paper. As opposed to microwave rada r,
VHF radar has a wide beam which is usually split when ground
reflection occurs. The reflected wave which is coherent with the
direct wave will always be contained in the target echo. In this
paper, several antennas of different heights are utilized. Owing to
the certain relationship among the phases of the split beams, the
elevation region where the targets are located can be determined
using the phases of echoes. Using “amplitude-comparison”
of echoes from various antennas to get the normalized error
signals, the elevation of the target can be obtained by looking
it up in the error signal table. The effect of roughness on the
altitude measurement accuracy is also analyzed. The real data
processing on flat and slope terrains testifies to the validity of the
proposed method. The altitude measurement precision, especially
in low-elevation regions, is greatly improved by using frequency
diversity.
Manuscript received September 17, 2006; revised August 2, 2007,
January 26, May 27, and July 11, 2008; released for publication
July 18, 2008.
IEEE Log No. T-AES/46/1/935925.
Refereeing of this contribution was handled by P. Lombardo.
This work was supported by the National Science Foundation of
China under Grant 60772068 and the Program for New Century
Excellent Talents in University under Grant NCET-06-0856.
Authors’ addresses: B. Chen and S. Zhang, National Lab for Radar
Signal Processing, Xidian University, Xi’an, Shaanxi, China; G.
Zhao, Intelligent Perception and Image Understanding Key Lab of
Ministry of Education, Xidian University, Xi’an, Shannxi 710071,
China, E-mail: (ghzhao@mail.xidian.edu.cn).
0018-9251/10/$26.00
c
° 2010 IEEE
I. INTRODUCTION
With the rapid development of concealment and
ARM (antiradiation missile) technology, current radar
often experiences difficulties in tracking low-flying
targets, i.e., targets that are within a beamwidth of
the horizon. The difficulty is due to the reflections
from the surface of the Earth. In general, the
reflected energy consists of the specular and diffuse
components. The specular component is usually more
difficult to deal with since it is by far the stronger
of the two; it is highly correlated with the direct
component, and it also lies within the beamwidth
of the receiving antenna. Thus, detecting the target
of low altitude and avoiding the influence of the
multipath are becoming more challenging. Nakatsuka
[1] and Zoltowski [2] introduce multiple beams to
separate the paths respectively. Recently Boman [3]
presents a new low-angle target-tracking method
with comprehensive target models. Considering
the complex reflection over the sea, E. Bosse [4]
investigates the Swerling fluctuating targets tracking.
Recently array signal processing has been used in this
field, and much effort has been spent in developing
high-resolution techniques for direction-of-arrival
(DOA) estimation of multiple signals [5—7]. These
methods, such as the multiple signal classification
(MUSIC) [8] algorithm, min-norm [9] algorithm, are
based on the subspace, which can provide an excellent
performance at high signal-to-noise ratio (SNR), long
data records, and in a spatially uncorrelated sources
field. However, when some of the signals are fully
coherent, e.g., when the target flies at a low altitude,
the echo received contains multipath signals, and
these techniques encounter serious difficulties. The
maximum likelihood (ML) [10] method, which is
based on analyzing the joint probability distribution
of received data, can be used in the case of coherent
signals with better statistical performance than
MUSIC. However, more computation is required,
which restricts the ML algorithm from practical
applications. Motivated by computational efficiency
and the need for real-time tracking, a number of ML
estimation schemes have been proposed for low-angle
radar tracking [11]. Some other methods based on
multifrequency tracking are also underdeveloped
[4, 12]. In this paper, we propose a new method for
altitude measurement based on beam split.
In 1939, an American made the first radar, CXAM,
that could estimate the altitude of a target by using
the multipath signals. The performance of this radar is
greatly affected by many aspects such as the condition
of the sea, the reflection of the atmosphere, and the
SCR of the target. During World War II, a new set of
radar called CH was built in England which measures
the altitude of a target by comparing the amplitude
of the main beams of two receiving antennas. By
switching in aerials mounted at different heights above
IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 46, NO. 1 JANUARY 2010 3
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