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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
© 2010 Agilent Technologies
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
© 2010 Agilent Technologies
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
© 2010 Agilent Technologies
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)
© 2010 Agilent Technologies
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
© 2010 Agilent Technologies
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
© 2010 Agilent Technologies
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
+
+
+
© 2010 Agilent Technologies
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
© 2010 Agilent Technologies
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.
© 2010 Agilent Technologies
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
© 2010 Agilent Technologies
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
© 2010 Agilent Technologies
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
© 2010 Agilent Technologies
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)
© 2010 Agilent Technologies
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