A Novel Approach in Detection and
Characterization of CW Interference of GPS
Signal Using Receiver Estimation of C/No
A. T. Balaei, A.G. Dempster and J. Barnes
Cooperative Research Centre for Spatial Information System (CRC-SI)
and the School of Surveying and Spatial Information Systems at
The University of New South Wales
ABSTRACT Narrowband interference can
severely degrade the performance of GPS
receivers. Different narrowband interference
suppression techniques also degrade this
performance correspondingly. Detecting the
presence of interference and then characterizing
it can lead to its removal. Knowledge that can be
useful is: the location of the source or direction
of arrival of the interference (spatial-directional
characteristics), the time specification and
frequency and power of the interference
(temporal-spectral characteristics). Our focus in
this work is on the second type. Using the post-
processing capability of a software GPS receiver,
CW interference is detected and characterized.
This is achieved by passing the GPS signal and
the interference through the correlator. After
correlation, using the definition of carrier to
noise density ratio (C/No), a mathematical
expression for C/No is given in which the
temporal and spectral parameters of interference
are found. Then, using the conventional
definition of C/No as the squared mean of the
correlator output divided by its variance, the
actual C/No is calculated. Finally comparing
these two values and considering the structure of
the GPS C/A code, the existence and
characterization of the existing interference is
determined.
KEYWORDS: GPS, Interference, C/No,
Correlator
I. INTRODUCTION
Radio frequency interference (RFI) is amongst
the most disruptive events in the operation of a
GPS receiver. It affects the operation of the
automatic gain control (AGC) and low noise
amplifier (LNA) in the RF front-end [1, 2] and
depending on how much of it passes through
these primary modules, it can also affect the
carrier and code tracking loops [3, 4] which
results in deterioration of all the GPS
observables or in complete loss of lock in severe
cases. Continuous-wave (CW) interference has
been proved to have effects for the GPS C/A
code signal [5], related to the characteristics of
the frequency spectrum of this signal. To make
the receiver less vulnerable to interference or to
monitor the quality of the signal, interference can
be detected in the receiver either before the
correlation (pre-correlation detection) or after it
(post-correlation detection). In the pre-
correlation techniques, antenna [6], AGC [2] and
ADC [7] have been used to detect and
characterize the RFI and in post-correlation
techniques, the observables of the receiver that
are affected by RFI have usually been used to
detect and characterize the interference. In [8],
using a statistical approach, it has been shown
that correlator output power shows consistent
performance under varying levels as well as
types of interference and carrier phase vacillation
was used as a backup indicator. In [9], correlator
output is calculated in a multi-correlator receiver
to estimate the frequency of the CW RFI. In this
paper, AGC level together with correlator output
power is used to detect and characterize the RFI
but the difference is that the receiver simply uses
a standard correlator. The other advantage of this
algorithm is that it is capable of detecting and
characterizing CW RFI even it is not close in
frequency to any of the C/A code spectral lines.
The problem is described in section 2. In
section 3, a mathematical expression for the
C/No which is introduced. We derived it in [4],
using the spectral analysis of both the signal and
the RFI in passing through the correlator. The
actual C/No is calculated using received I and Q
data from a software GPS receiver in section 4.