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