International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 5
Number 2
June 2011
233
1 ASSUMPTIONS OF DESIGNED SYSTEM
The starting point is to build a ground-based system
which is mainly associated with the hyperbolic
localization systems. They are based on the
differential measurement method called Time
Differential of Arrival (TDOA). The first hyperbolic
system (Gee) appeared during Second World War. It
has evolved (DECCA, OMEGA), but the moment
satellite navigation appeared, they have practically
gone out of use. Up to now only LORAN C system
is still operational.
Our goal has been to create a system of
hyperbolic localization but made in modern
technology. The designed system uses spread
spectrum signals. The second element is an
asynchronous operation. The system resigns chain
relationship between stations. With this approach,
our system has gained new features and new
functionality compared to traditional solutions.
The first task is to determine the basic
parameters, i.e.: frequency, bandwidth, modulation,
etc. After careful consideration, the following
parameters has been set:
− spread spectrum signals (using DS-CDMA)
− reliance on a hyperbolic system (TDOA method)
− frequency: 431.5 MHz
− the width of the transmission channel – 1 MHz
− transmission speed of navigational information -
1 kb/s.
− modulation: QPSK
2 HYPERBOLIC SYSTEMS – TDOA METHOD
The TDOA method, as mentioned before, is based
on a calculation of the time difference between
stations. Suppose there are N ground stations, the
coordinates for the i-th station are
( )
,,
SiSii
yxS =
where i = 1, ..., N, and the search object's
coordinates are
( )
MM
yxM ,=
.
If you define a signal propagation time between
the i-th station and the searched position in the point
M as T
i
, so the distance between the i-th station and
the point M is as follow:
( ) ( )
,
22
MSiMSiii
yyxxcTd −+−=⋅=
(1)
where:
c - velocity of wave propagation (3 * 10
8
m / s)
Ground-based, Hyperbolic Radiolocation
System with Spread Spectrum Signal - AEGIR
S.J. Ambroziak, R.J. Katulski, J. Sadowski, W. Siwicki & J. Stefanski
Gdansk University of Technology, Poland
ABSTRACT: At present the most popular radiolocation system in the world is Global Positioning System
(GPS).As it is managed by the Department of Defence of the U.S.A., there is always the risk of the occasional
inaccuracies or deliberate insertion of errors, therefore this system can not be used by secret services or ar-
mies of countries other than the U.S.A. This situation has engender a need for development of an autono-
mous, ground-based radiolocation system, based on the hyperbolic system with spread spectrum signals. This
article describes the construction and operation of such a system technology demonstrator which was devel-
oped at the Technical University of Gdansk. It was named AEGIR (god of the ocean in Norse mythology).
This paper presents preliminary results and analysis of its effectiveness.
234
T
i
- the propagation delay between the i-th station
and the point M,
d
i
- distance between i-th station and the point M.
Timing differences between the i-th station and a
first one, can be written as:
T
i1
= T
i
– T
1
(2)
Differences in the distances between those
stations, can be described by the following
relationship:
(3)
After putting equation (1) in equation (3) we
obtain hyperbolic equation:
(4)
Equation 4 presents the difference in distance
between the first and i-th station.
Determination of the distance difference between
another pair of base stations generates more
hyperbolas and a point of their intersection gives us
a position. There are many algorithms [1-4], which
allow to determine the coordinates, however for the
purpose of the system the Chan method was chosen
[1].
The principle of TDOA method can be illustrated
as follows. Assume that we have three reference
stations positioned as in Figure 1.
Figure 1. Deployment of ground stations to illustrate the
method of TDOA
Propagation time from the station to your desired
position in the point M is respectively T
1
, T
2
and T
3
and the distance between them is d
1
, d
2
and d
3
. Each
station has coordinates as follows: S1=(x
S1
, y
S1
),
S2=(x
S2
, y
S2
) and S3=(x
S3
, y
S3
).
Determination of temporary differences between
the stations is illustrated in Figure 2. It has been
assumed that each station transmits at the same time
an impulse signal. Figure 2a shows the moment of
broadcasting signals by the station. Figure 2b shows
the time of receipt of the impulses at the point of
searched position.
Analyzing Figure 2 it can be observed that when
the impulses are transmitted at the same time from
each ground station, the time difference at the
receiver side is easily measured. Unfortunately, such
a synchronization is difficult to obtain.
For this reason, the system has been designed as
asynchronous one. This allows switching off and on
any station without resynchronization the system. In
order to implement this feature, it has been
necessary to create a reference station, which not
only transmits, but is also able to receive signals
from neighbouring stations. With this approach, the
reference station measures the time differences in
synchronization between the reference signal and its
neighbouring stations so the calculated time
differences are sent to the receiver. This mode of
operation is illustrated in Figure 3 [8].
Figure 2. Timing between signals broadcasted by ground
stations a) the moment of broadcasting impulses by the stations
b) the time of receipt of impulses by the receiver
As in the previous example, stations transmit a
reference signal as an impulse, but time of
broadcasting these impulses, as shown in Figure 3, is
random. The stations have the ability to "listen to”
neighbouring stations. This is illustrated in Figure
3b. Reference station designated as S1 receives
signal from other two stations: S2 and S3, and