2015 Fifth International Conference on Advances in Computing and Communications
Design and Implementation of a Spread Spectrum based Radar Altimeter
Liz Zacharia, M.G. Jibukumar
M.J. Lal, T.J. Apren
Division of Electronics Engineering
School of Engineering, CUSAT
Kochi, India
e-mail: lzzacharia@gmail.com
RFSD/ AVN
Vikram Sarabhai Space Centre
Thiruvananthapuram, India
Abstract— Conventional radar altimeter systems such as
Pulsed or Frequency modulated continuous wave (FM CW)
have limitations on measurable range and security. In this
paper, we propose a Direct sequence spread spectrum
(DSSS) based radar altimeter that offers good resolution and
low probability of intercept. We carried out MATLAB
simulations of the system and verified the results with a
scaled down prototype model. Various system performance
measures including Bit error rate (BER) were computed and
compared with the theoretical results.
Rest of the paper is structured as follows. Section II
gives a mathematical model of the DSSS RA system. The
system design and range computation techniques are
detailed in Section III. Simulation results are provided in
Section IV, and conclusions in Section V.
II.
Keywords- Radar altimeter;Direct sequence spread
spectrum; BPSK; Costas loop; PN sequence;Correlation;
FFT;
I.
b(t + LTc ) = b(t)
(1)
where L represents maximum sequence length of PN
sequence generator. A sinusoidal carrier signal of
frequency i is BPSK modulated with b(t), as in (2).
INTRODUCTION
Radar altimeters (RA) are a critical component to low
flying aircrafts like helicopters and unmanned aerial
vehicles. It plays an important role in flight safety,
reliability and capability. When used in safety critical
applications, it must be ensured that the radar signals are
undetected by enemy Electronic Support (ES) receiver. If
intercepted, they may send the signals back with random
delays and confuse the radar system. Low probability of
intercept radars are thus a popular choice for such
applications.
Commonly used RA are of pulsed and FMCW types.
Pulsed radars use a repetitive train of short duration pulses
for range detection [1]. The peak transmitted power of
pulsed radars is usually high for long range applications,
thus making it vulnerable to ES receiver [2]. Linear
FMCW radars operate by linearly modulating the
frequency of a continuous carrier signal. Due to low power
transmission, these systems cannot be easily intercepted by
ES receiver. However, ES receivers can degrade the radar
performance by sending jamming signals over a band of
frequencies [3]. Radars with good LPI capabilities will
have low peak powers and broad spectrum [4].
In this work, we propose a direct sequence spread
spectrum (DSSS) based radar altimeter that has good
resolution and low probability of intercept. It is a form of
phase coded radar [5], in which the phase of a continuous
radio frequency carrier is varied according to a binary
sequence [6]. Here pseudo noise (PN) sequence is used to
spread the radar signal into a wide bandwidth signal. We
simulated this system in MATLAB and then built a
laboratory scale working model. The resolution of the
designed system is less than one meter. Bit error rate
performance of the system is verified for SNRs as low as
-4dB. Also, the results of prototype model and MATLAB
simulation are compared.
978-1-4673-6994-7/15 $31.00 © 2015 IEEE
DOI 10.1109/ICACC.2015.68
MATHEMATICAL MODEL
Let b(t) be a pseudo noise modulating signal, with chip
duration Tc . As per definition, it is a noise like signal,
which repeats itself after a finite duration [7].
s(t) = Ai sin (i t +[ b(t)· ] )
2
(2)
In simple terms, BPSK modulation is the process of
shifting the phase of a carrier signal by 180º for one data
symbol while maintaining the carrier phase for the other.
When the amplitude of b(t) is restricted to ±1, (2) can be
rewritten as follows,
s(t) = b(t)Ai sin (i t) .
(3)
The frequency spectrum of this BPSK signal is
centered at carrier frequency fi and occupies a bandwidth B
where,
(4)
B = 2/Tc
This signal is then power amplified and transmitted
towards ground from the aircraft. The echo signal will
contain channel introduced noise, time delay and
sometimes Doppler shift. Considering the ideal case with
no Doppler shift and delay, we can represent the received
signal as follows,
r(t) = A'i b(t) sin(i t +i )
(5)
where A'i is the new amplitude and i phase of the
received signal. To demodulate this BPSK signal, a
coherent carrier signal, i.e. a carrier with identical
frequency and phase is needed. Let u(t) be a coherent
carrier signal generated internally in the demodulator, with
amplitude A0.
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334
u(t) = Ao sin(i t +i )
(6)
When operated in continuous mode,
m
the spread signal b(t)
repeats itself after 28-1, i.e. 2555 data symbols [7]. If the
chip duration Tc is set to 40ns, then we get a periodicity of
10.2μs or a range of 1530m.. Fig. 2 shows the system
design of the prototype modeel. This model has all the
parameters scaled down to one fifth the original design. An
ADSP 21060 SHARC is used as
a the core processor.
A mathematically simple method to demodulate BPSK
signal is to multiply the received signal with
w u(t).
r(t)×u(t) = A'i Ao b(t) sin2 (i t +i )
(7)
Using trigonometric identity, (7) caan be rewritten as
follows.
r(t)×u(t) =
1 '
A A b(t) [ cos 0 - sinn(2i t +2i )]
2 i o
B. Transmitter
L
Device generates the
A Complex Programmable Logic
PN sequences continuously usiing an 8-bit linear feedback
shift register [7]. We have chossen CPLD instead of FPGA
as this is a moderately compleex system. A Direct Digital
Synthesis (DDS) chip BPSK modulates
m
the PN sequences.
The modulated signal is thhen power amplified, up
converted and fed to transmitteer patch antenna.
(8)
On low pass filtering the above prodduct signal, we get
the demodulated signal.
d(t) =
1 '
A A b(t).
2 i o
C. Receiver
The echo received is downn converted amplified and
then digitized through a higgh speed ADC. The echo
samples are latched to FIFO at fixed time intervals. CPLD
generates the read, write andd other control signals for
FIFO and latch, in sync with PN
N sequence generation. The
samples are then transferredd to DSP memory every
30.3ms, for demodulation annd range estimation. This
gives an altitude update rate off 33Hz.
Two well-known methods are available for coherent
demodulation of BPSK: a squaaring loop and Costas loop.
The latter is used in the propposed system as it is more
tolerant to Doppler shifts annd is suitable for systems
operating at high frequency. Thhe design of a digital Costas
loop is described in detail in [11] and [12]. Table I lists
the characteristics of the propposed design and prototype
model.
(9)
Autocorrelation of PN sequences yiield a single peak
and uniform side lobe level of -1/L [7]. If
I the PN sequence
is delayed in time, then the correlation peak will be shifted
by an equal amount of time. DS SS ranging system utilizes
this property of PN sequences, to estim
mate the round trip
delay time [8]. The radar range can theen be found using
(10),
1
Range = ×c×D
2
(10)
where c denotes the velocity of ligght 3×108m/s and
D denotes the time delay.
Recent works show that Almoost perfect autocorrelation sequences have better corrrelation properties
and lower side lobe levels [9]. Howeveer, PN sequence is
chosen in this work to keep the system siimple. Fig.1 shows
the proposed radar altimeter system.
III.
D. Range Computation
The round trip delay time of the echo is computed
using cross correlation of demoodulated data and reference
PN sequence [13]. Correlation can be done in time
domain or frequency domain. In time domain, it is done
using shift and add mechaanism. This operation is
mathematically defined as folloows,
N
SYSTEM DESIGN
A. Configuration
A system is set to
The resolution of the designed RA
0.75m. Applying (10), we see that the minimum
m
delay to
be detected in the echo signal is 5ns, i.e. the sampling
interval is 5ns [10].
The maximum detectable altitude depends on the
choice of spreading sequence and chip
c
rate. In the
proposed system, 8-bit maximal length PN
P sequences (msequences) are selected as the spreading sequence.
N-1
R12 [] = ෍ v1 [n]v2 [n+]
n=0
Figure 1. Proposed DSSS radar syystem
Figure 2. Systeem Design
335
337
(11)
TABLE I.
EXPERIMENTAL SPECIIFICATIONS
Characteristics
Proposed Design
Prototype Model
Specifications
Resolution
0.75
3.75m
Altitude
0 to 1530m
0 to 7650m
Sampling Rate
200MHz
40MHz
Altitude
Update Rate
33
measurements/second
33
meeasurements/second
where, v1[n] and v2[n] are two seequences of finite
duration. Here denotes the sequencce shift and take
values from 0 to N-1.
For two sequences of length N,, direct correlation
needs N multiplications and N-1 addditions. When the
length of sequences increases, compuutation complexity
increases drastically. Frequency doomain correlation
method exploits the fact that cross correlation
c
is the
inverse Fourier Transform of Cross Power Spectral
Density. This relation is shown in (12) and (13),
R12 []=F-1 ሾG12 ሿ
(12)
G12 = S1 ሺfሻ ൈ S*2 ሺfሻ
(13)
where S(f) denote the Fourier Transfforms and G12 the
cross power spectrum. The executionn times of direct
correlation and FFT methods are compaared in MATLAB
in Table II. It can be seen that FFT method
m
is far more
computationally efficient.
IV.
ULTS
SIMULATION RESU
Main building blocks of the altimeter system were
simulated in MATLAB. Fig. 3 shows paart of a transmitted
data, echo signal, demodulated data andd correlation result
for two different echo signals.
The BER performance of the designeed DS SS receiver
is compared with ideal BPSK demodulattion. It is seen that
the bit error rate of designed system is higher,
h
mostly due
to non-ideal filtering in Costas loop dem
modulation [11].
The prototype model was hardware co-simulated
c
using
Visual DSP++ emulator software. For testing the
prototype model the transmitter and receeiver sections were
directly connected through a 50m lonng coaxial cable.
Echo signal processed by DSP and itss cross correlation
result are shown in Fig. 5. The posittion of the largest
peak corresponds to the delay in signnal reception. The
range computed was matching with the expected
e
results.
TABLE II.
Figure 3. MATLAB simulation plots. (a)
( Generated PN sequence,
original carrier signal and BPSK moduulated signal (b) Echo received
without any delay, corresponding demoodulated data and cross correlation
result (c) Echo received after 1micro seecond. Cross correlation peak is
seen shifted by 1 microsecond.
COMPARISON OF CORRELA
ATION METHODS
Execution Times (in seconds)
No: of
samples
Direct Correlation
FFT method
1024
1.0078
0.000406
2048
4.2719
0.000687
4096
15.9219
0.001800
Figure 4. BER performance comparisoon between Ideal demodulation and
Costas loop demodulation implementedd in this work.
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338
Figure 5. Hardware test results. (a) Echo signal received through 50m long coaxial line. (b) Cross correlation computed by the processor.
Correlation peak is shifted by 14 samples. It corresponds to a range of 52.5m
V.
CONCLUSION
[5]
This paper presents an efficient design of a spread
spectrum based radar altimeter. Major computations are
done in software using a digital signal processor. Hence
this is a highly flexible RA design. The BER to SNR
performance of the designed Costas loop is also analyzed.
FFT method is chosen for correlation process to make the
system computationally efficient.
[6]
[7]
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