Challenges and Solutions for GPS Receiver Test · GNSS Related Systems Satellite Based Augmentation Systems (SBAS) Geostationary satellites transmit correction and integrity data

Challenges and

Solutions for GPS

Receiver Test

Presenter: Mirin Lew

January 28, 2010

© 2010 Agilent Technologies

Agenda

• GPS technology concepts

• GPS and GNSS overview

• Assisted GPS (A-GPS)

• Basic tests required for GPS receiver verification

• Test solutions• Test solutions

• Signal creation for GPS receiver test

• A-GPS test systems

GNSS = Global Navigation Satellite Systems


GPS and GNSS Overview

GPS: Global Positioning System

• System owned and operated by the U.S. government

• Civilian service freely available to users worldwide

• Military service available to selected agencies onlyagencies only

GNSS: Global Navigation Satellite System

• General term for any satellite-based navigation system

• Includes multiple systems worldwide


Global Navigation Satellite Systems (GNSS)

Galileo

– Joint effort of European Community and European Space Agency

– 2 test satellites in orbit, contracts awarded for first 14 satellites, up to 32 satellites operational by 2014

– Interoperability agreement signed with GPS

– 4 services (open service, paid commercial service, safety of life service, public regulated service) as compared to 2 GPS services (public and private)

Global Orbiting Navigation Satellite System (GLONASS)

– Russian system first launched by Soviet Union in 1982

– Became non-functional for most applications in the 1990’s

– Currently being restored, 22 satellites in orbit as of Dec. 2009

– Particularly good coverage over upper latitudes (Northern Europe)

Compass (Beidou-2)

– Chinese system

– 3 satellites are up, 12 satellites by 2012 to provide regional service

– Eventually 30 satellites


GNSS Related Systems

Satellite Based Augmentation Systems (SBAS)Geostationary satellites transmit correction and integrity data for GNSS

system over the GNSS frequency. Provides increased positioning accuracy.

North America:

Wide Area

Augmentation System

(WAAS)

Europe:

European Geostationary

Overlay Service (EGNOS)

Japan:

Multifunctional Satellite

Based Augmentation

Regional Navigation Satellite Systems (RNSS)Intended for improved coverage over limited areas

(WAAS)Based Augmentation

Satellite System (MSAS)

Japan:

Quazi Zenith Satellite

System (QZSS) - 2013

India: Indian

Regional Navigation Satellite System

(INRSS) - 2012

India:

GPS and GEO Augmented

Navigation (GAGAN)


GPS Technology Overview

• Constellation of 24 active satellites in orbit (up to 32 satellites total)

• Each satellite transmits its current location and time

• Each satellite transmission is synchronized to the rest by atomic clock

• Minimum of 4 satellites required for 3D location calculation

• Major segments of the system

• Space: Satellites or Space Vehicles (SV) orbiting the

Earth twice a day at 20,200 km

• Control: Ground stations provide navigation

information update and SV control

• User: GPS receiver


How Does GPS Work in the Real World?

Space Segment

Uplink data:Satellite orbital information

Position constantsClock correction factors

GPS Data

Monitor Stations

MasterControl

Station

User

Clock correction factorsAtmospheric data

Almanac

Control segment


GPS Transmitted Signal

L1 Carrier 1575.42 MHz

C/A Code 1.023 MHzL1 Signal

(Civilian use)+Σ

90˚

Satellite GPS signal has 3 components:• Carrier wave: 1575.42 MHz (L1) & 1227.60 MHz (L2)

• Ranging (pseudo-random) codes: Coarse acquisition (C/A) code and precise (P) code

• Navigation message: 50 bit/s contains ephemeris data (detailed orbital information for the

transmitting satellite) and almanac data (more general orbital information for all satellites)

Navigation Data 50 Hz

P Code 10.23 MHz

L2 Carrier 1227.6 MHz

(Civilian use)

L2 Signal

(Military use)

modulo 2 adder

modulator

+

+

+


Navigation Message

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Navigation message

25 pages/frames37,500 bits12.5 minutes

Frame (page)

1500 bits30 seconds

Sub-frame 2

300 bits

6 secondsSub-frame 1 Sub-frame 3 Sub-frame 4 Sub-frame 5TLM

HOW

1 2 3 4 5 6 7 8 9 10

Satellite health

and clock

correction data

Ephemeris Ephemeris Partial

almanac &

other data

Almanac

TLM

HOW

1 2 3 4 5 6 7 8 9 10

TLM

HOW

1 2 3 4 5 6 7 8 9 10

TLM

HOW

1 2 3 4 5 6 7 8 9 10

TLM

HOW

1 2 3 4 5 6 7 8 9 10

8 bits

pre

am

ble

16 bits

reserved

6 bits

parity

7 bits

ID

17 bits

Time of week

TOW

6 bits

parity

30 seconds

Telemetry word (TLM)30 bits0.6 seconds

Handover word (HOW)30 bits0.6 seconds

TLM

HOW correction data

TLM

HOW

TLM

HOW

TLM

HOW

TLM

HOW

Worst Case: 30 seconds to receive full ephemeris data

12.5 minutes to receive full almanac data


Almanac and Ephemeris Files

Almanac File

Contains data on the health and general orbital information for every satellite in the constellation.

Updated weekly.

http://www.navcen.uscg.gov/GPS/almanacs.htm

Ephemeris File

Contains detailed information on the orbit of an individual satellite. Updated every 2 hours.Contains detailed information on the orbit of an individual satellite. Updated every 2 hours.

http://cddis.gsfc.nasa.gov/gnss_datasum.html#brdc

• Data from the almanac can be used to create a scenario file that contains the

satellite information for a specific date, time, and location.

• Ephemeris data files can be used to create a GPS signal that more accurately

represents the actual signals broadcast at that date, time, and location.


Agenda

• GPS technology concepts

• GPS and GNSS overview

• Assisted GPS (A-GPS)

• Basic tests required for GPS receiver verification

• Test solutions

• Signal creation for GPS receiver test

• A-GPS test systems

GNSS = Global Navigation Satellite Systems


Assisted GPS (A-GPS)

• Technique for cellular network to assist mobile phone’s GPS receiver to lock to

satellites and achieve location fix more quickly

– Fulfills U.S. FCC’s E911 directive which mandated fast and accurate location of mobile

phones by emergency services

– Needed due to low GPS signal levels that may be seen by mobile phones when indoors or

in areas without direct view of sufficient satellites

– Allows mobile phone’s GPS receiver to acquire location fix much more quickly

• Base station provides “assistance data” to mobile phones. Data includes:• Base station provides “assistance data” to mobile phones. Data includes:

– Navigation: precise satellite orbital information

– Almanac: coarse orbital information

– Time of Week: GPS time

– Ionosphere: single frequency (L1) correction factors

– Reference location: initial estimate of location

– Acquisition assistance: data to aid in locating or tracking satellites

– Real-time integrity: list of bad satellites

– UTC model: leap second time correction for GPS time


How Does A-GPS Work in the Real World?

Space Segment

Uplink data:Satellite ephemeris

Position constantsClock correction factors

Monitor Stations

MasterControl

Station

User

Clock correction factorsAtmospheric data

Almanac

Control segment

GPS AssistanceServer

Cellular Network

Network Downlink:Coarse TimeEphemeris Data

Coarse Location(100m accuracy)Almanac


A-GPS Operation

• Assistance Data Transportation

• Control plane: Uses dedicated messaging on network control channels

• User plane: Uses existing standard Internet protocol (IP) based data

connections; also called Secure User Plane Location (SUPL)

• A-GPS Modes

• Mobile station/user equipment (MS/UE) Assisted (older method)• Mobile station/user equipment (MS/UE) Assisted (older method)

– MS/UE supplies GPS measurements to network

– Network combines with assistance server data, calculates and transmits

location back to mobile

– Typically used with control plane

• MS/UE Based (newer method)

– MS/UE uses assistance data to calculate location

– Transmits location back to BS

– Used with user plane (less network dependent)


Documents

Challenges and Solutions for GPS Receiver Test · GNSS Related Systems Satellite Based Augmentation Systems (SBAS) Geostationary satellites transmit correction and integrity data