Global Navigation Satellite System
(GNSS)
GLOBAL POSITIONING SYSTEM
Satellite Positioning System is a space based, microwave, 24 hour, all
weather, navigation system designed , deployed, financed and managed by
different countries.
1.Designing a Satellite Positioning System
Satisfy as many users as possible , military as well as civilians
• accuracy level
• for those require velocity and time information
• all air, land and sea users
2. Relatively low user cost and easy operation
• minimal complexities
• low power requirement
• small bulk for the user equipment
3. Unrestricted access by all users
4. Satisfy military positioning requirements
suitable for all classes of platforms
- aircraft ( jet to helicopter)
- ship
- land (vehicle to hand held equipment ) and
- space ( missiles and satellites).
• level of real-time accuracy available to non-military users to be controllable
• Resistant to jamming and careful attention to other “survivability” issues.
• Non-detectability of the user by the enemy , hence no requirement to transmit
signals of any kind.
Consequences for Geodetic Datum Definition
• a single global geodetic datum is ensured
• National Datum have no importance in satellite positioning except when
the measurements are compared with the conventional ground based
technique.
• most nations have no control on the satellite datum. If the system operator
decides to change the datum , other countries have to follow that.
• the computation of Satellite ephemerides – an important part –the
responsibility of the system operators. The quality of ephemerides –
effects on accuracy and reliability of positioning results.
Consequences of Measuring to Satellites
Satellite Positioning Systems are global. The signals can be “seen” over
a large area, and are not interfered with by terrain or geography to the
same extent as conventional ground-based positioning systems.
To make the technology widely accepted by the users , the user equipment
must be relatively inexpensive – the following consequences-
• Most of the complexities should be placed in the satellites as far as
possible
• Receivers should be small, with low power units, essentially
passive
• the most appropriate technology is the microwave , one way
(passive) ranging satisfying all weather, 24 hour night and day
operation for the civilians as well as the military requirement for
non-detectability of the user by the enemy.
The measurement technology to be used
A microwave satellite-based system:
- Can transmit signals that can be “seen” over a far larger area than
ground- based systems
- They can be most readily implemented within the satellites themselves
- Can transmit signals through cloud and rain
- Can be used day or night, as long as the transmitting satellite is above
the user’s horizon
- Recognises no national boundaries and refers positions to a
global datum uniquely defined for that system
- The position information is three dimensional.
- They are similar to conventional terrestrial survey and navigation
technologies , such as range and range-difference systems.
The positioning strategies
Range + multi-satellites
Range-Difference + multi-satellites
NAVSTAR GPS
Satellite Constellation
Global Positioning System is a space based, microwave, 24 hour, all
weather, global military navigation system designed , deployed,
financed and managed by the U.S. Department of Defense.
Charlie Leonard, 1999
(revised 2001, 2002)
NAVSTAR – Navigation Satellite
Timing And Ranging
The History of GPS
i. Feasibility studies begun in 1960’s.
ii. Pentagon appropriates funding in 1973.
iii. First satellite launched in 1978.
iv. System declared fully operational in
April, 1995.
The NAVSTAR Global Positioning System- a satellite
based
positioning system , is revolutionising the task of
navigation and surveying
Relatively high positioning accuracy
Determination of velocity, time to an accuracy level with
position
Available to users anywhere on the globe :
relatively low cost system with no user charges
All weather system, available 24 hours a day
The position information is in three dimensions – horizontal as
well as vertical information is provided
Main Characteristics of
NAVSTAR
Orbit altitude – 20,200 km
Number of Orbital planes – 6
Inclinations – 55
Frequencies – 1575 MHZ , 1228 MHZ
Geodesic Coordinate – WGS 84
Navigation data – 4D, X , Y, Z, t and velocity
Availability – continuous
Accuracy – 15m and above
Repeatability – 1.3 m relative
NAVSTAR
Satellite Constellation – 24
(21 +3 spares )
Number of Orbital Planes – 6
Geometry – Repeating
Satellite Clock – Rubidium , Cesium
GLONASS - a Russian Global
positioning System
Orbit Altitude – 19,100 km
Orbital period – 11 hours 15 m
Number of Orbital planes – 3 Planes, 8
satellites per Plane
Inclinations – 64.8
Frequencies – 1602.5625 MHZ - 1615.5
MHZ
Geodesic Coordinate – SGS 85
Accuracy – more than 100 m horizontal
and 150 m vertical
GLONASS
Satellite Costellation – 24
(21+3 spares)
NAVSTAR – the 1st GPS constellation- replaced TRANSIT Doppler Satellite
Navigation System and other nav-aids.
Development started in 1973 merging several R&D Projects, two important -
Navy’s TIMATION and Air Force’s 621 B Project
The development and production programme – managed by US Air Force
Systems Command, Space Systems Division, Joint Program Office (JPO) at the
Los Angeles Air Force Base, California.
The JPO - the USAF, US Navy, US Army, US Marine Corps, US Coast Guard, US
Defense Mapping Agency, Nato Nations and Australia.
The aim of the JPO - to develop an all-weather, 24 hour, truly global navigation
system to support the positioning requirements for the armed forces of the US
and its allies.
A great change made on system’s utility, reliability and survivability- with a
number of stringent conditions.
Basic Functions of GPS
•To find out location – determining a basic position
•Navigation – going from one location to another
•Tracking - monitoring the movement of people, vehicles and things
•Timing – bringing precise timing to the world (required by Astronomers, Power
Companies, Computer Network, Communication Companies, Banks, Radio and
Television).
How GPS Works
The Space Segment
The Control Segment
The User Segment
