andrea ghirardini

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

Andrea Ghirardini

Business Development

Spazio ZeroUno

An efficient and effective method of conveying Differential

GPS data to on-board positioning devices for train control.

Cell Broadcast (CB) as a bearer for GPS augmentation

in GSM-Railway applications

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

Agenda

The study

GPS

GPS Error sources

correction systems

CB in the game

The system

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

A feasibility study carried out by Spazio ZeroUno and LABEN for improving GPS accuracy in railways systems

The “DGPS on GSM-R CB” project

Project submitted for financing to the Italian Space Agency (ASI)

Next step: prototype development with real and simulated elements for concept assessment

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

GPS basics

• 24 satellites orbiting earth in 12 hours

• Constellation provides 5 to 8 visible satellites from any point on the earth

• 4 satellites are required to compute the 3 dimensions of position and time stamp

• Precision ranges from 10 m to 100 m depending mainly on Selective Availability and Ionospheric effects

GPS positioning is affected by error

sources Error importance is related to the

application

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

GPS error sources

Error source Error range Correlation Impact

Ionospheric delay High correlation in Time & Space

3 – 4 m (night)20 – 30 m (day)

Tropospheric delay

High correlation in Time & Space3 m

Ephemeris error3 m

Satellite Clock

These errors can be corrected by

“Augmentation Systems”

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

Error reduction efficiency largely depends on Receiver-Station distance and transmission

delays

Error correction = f ( T , S )

GPS

Receiver (mobile)

1

DGPS: an augmentation system

Known referenc

eposition

DGPS Station

2

Calculates correction

factors

3

Bearer system

4Calculate

s position

5

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

Cell Broadcast can be an

additional bearer based on the

GSM network infrastructure

Traditional bearers

• RDS/FM

• Radiobeacons

• Special LF-transmitters

• EGNOS

Current bearers No bearer

system has been

standardized

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

Augmentation algorithms

Static Survey

Kinematic Survey

Dynamic Carrier DGPS

Wide Area Code Differential

Local / Regional Area Code Differential

SPS

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

Augmentation algorithms performance

1mm 1cm 10cm

1m

10m

100m

< 50 Km

< 30.000 Km

Global (no DGPS)

S

S

Position accuracy

CB bearer

Static Survey

Kinematic Survey

Dynamic Carrier DGPS

Wide Area Code

Differential

Local / Regional Area Code Differential

SPS

<200 Km

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

Accuracy 1 – 10 m

Summary

Accuracy 20 – 30 mPositioning without DGPS correction

Positioning with DGPS correction

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

Is CB a suitable bearer for DGPS in Railways applications?

• DGPS payload: 480 bit per 6 satellites

• 12 satellites at most are in visibility,

hence 960 bit

• 1 CB message bears 82x8 bits, hence

656 bit (equivalent to 8,2 satellites)

• CB air interface manages 1 message

every 1,8 seconds

• Expected network delays are

estimated in the order of 10 seconds

Error within 1 to 5 m

REQUIREMENTS

PayloadPayload

BandwidtBandwidthh

T < 30 secS < 100 Km

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

Terrestrial network

GSM-R network

Elements and architecture

GPS Rx

DGRS

GSM Rx

BTS

Known

Position!

Bearer

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

GPS Simulatorposition

Correction and position data generator

(1)

Configuration parameters

Prototype architecture

Cell Broadcast Center Server

GSM network simulator

Terminal (simulator)

RX GPS RX GSM/CB

Content management

(Cell Broadcast Entity)

correction

GSM network simulator & CB bearer

RX DGPS

correction

(2)

Correction log files for all DGPS stations

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

Validating the concept

11

22TRAIN

Correction range for DGPS

station 1

11

R-DGPSR-DGPS 50 – 100 Km

0.5 – 5 m accuracy

Spazio ZeroUno Cell Broadcast Forum Plenary, Milan, May 28-29th 2002

Project summary

Feasibility study carried out by Spazio ZeroUno and

LABEN Project submitted for financing to the Italian Space

Agency (ASI) Objectives:

•validate CB as a bearer for DGPS over GSM-R

•identify performance in worst case conditions

• identify performance ranges (accuracy depending on

speed, distance, delay)

•define optimal deployment of DGPS stations with respect

to GSM-Railway BTSs

•identify evolution path towards GALILEO positioning

system

•structure a commercial offer to the Railways industry