Global Navigation Satellite System (GNSS)

GPS ConceptsJune 21, 2013

2- Control Segment

1- Satellite/Space Segment

3- User Segment

3 Main Segments to any GNSS

Monitor Stations

GroundAntennas

Master Station

GPS – Global Positioning System

– The US developed and operated NAVSTAR Global Positioning System (GPS) was the first Global Navigation Satellite System (GNSS).

– GPS provides specially coded satellite signals that can be processed in a GPS receiver to compute Position, Velocity and Time.

– First satellite launched in 1978.

– Full constellation achieved in 1994.

Basic Functions of GPS

1. Position and coordinates (latitude, longitude, altitude)2. Direction of travel between any two points.3. Travel velocity4. Accurate time of the day

Space Segment

– GPS Operational Constellation consists of 24 + satellites orbiting at a height of 20,000 KM(Medium Earth Orbit)

– Constellation has spares– 6 orbital planes (with 4 to 5 Satellites in

each) equally spaced.

– 4 to 8 active Satellites visible from any point on the earth.

– GPS Satellites send Radio Signals.

– Every satellite orbits the Earth twice daily (one revolution in approx 12 hrs)

Control Segment

5 Monitor stationsTrack GPS signals and send them to master control station

1 Master Control Station located at Falcon AFB (air force base) ColoradoCorrect orbit and clock errors Create new navigation messages

Ground Antenna (upload station)4 ground antennas

Kwajalein Atoll

US Space Command

Control Segment

Hawaii

Ascension Island

Diego Garcia

Cape Canaveral

Ground AntennaMaster Control Station Monitor Station

User Segment – Consists of GPS receivers and user

community.

– GPS receivers convert Radio Signals into Position, Velocity, and Time estimates.

– 3 satellites are required to compute Position (X, Y, Z) .

– 4th satellite is used to recalibrate receiver clock (Time).

– Navigation in three dimensions is the primary function of GPS.

Each satellite has a unique pseudo-random code which helps for determining time difference (Travel Time) by comparing how late the satellite's pseudo-random code appears compared to our receiver's code. There are two types of pseudo-random codes:

• The C/A (Coarse Acquisition) code for timing for civilian GPS users and the status message are modulated on L1 frequency (1575.42 MHz).

• The more precise and complicated pseudo-random code (P-code) for military users is modulated on L1 and L2 (1227.60 MHz) frequencies. After encryption, it is called Y-code.

Pseudo-Random Codes and Carriers Frequencies

Ephemeris data including information about the satellite's orbits, their clock corrections and other system status.

GPS Signal

Navigation Message

Range DistanceGPS positioning based on Range Distance or RangeThis is the distance between a satellite in space and a receiver on or above the Earth’s surfaceRange distance is calculated as follows: Range = speed of light * Travel timeCoded signals are used to calculate signal travel time by matching sections of the codeDifference between transmission and reception times is the travel timeA range measurement from a single satellite restricts the receiver to a location somewhere on the surface of a sphere centered on the satellite

How GPS Works? The measurement principle of GPS is based on 3D

Trilateration (A mathematical method of determining the relative positions of objects using the geometry of triangles) from Satellites

To triangulate (loosely speaking), a GPS receiver measures distance using the travel time of radio signals coming from GPS Satellites.

To measure travel time, GPS needs very accurate timing

Fig A

Position Calculation - TrilaterationSurfaces of two spheres intersect at a circle (Fig A)Intersection of third sphere with the first two will

intersect that circle at two points (Fig B)For GPS carriers near Earth surface the position will

be at the intersecting point closest to the Earth

Fig B

Fourth Satellite Positioning and Correcting the GPS Clock

Correct position can also be determined by fourth intersecting sphere

Four satellites are usually required to reduce receiver clock errors (recalibrate)

Correcting Clock BiasIt's geometrically impossible for four mutually

intersecting spheres to merge at the same point unless the clock is spot on

A simple routine is run by receiver to adjust or reset its clock so that all four lines of position intersect the same point

4th satellite range is required for confirmatory test as well as Time.

GPS Error Sources1. Atmospheric delays - The satellite signal slows down as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.

2. Signal multi-path - This occurs when the GPS signal is reflected off objects before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.

3. Receiver clock errors - A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.

4. Orbital errors - Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.

5. Number of satellites visible - The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.

6. Satellite geometry/shading - This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.

7. Intentional degradation of the satellite signal - Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

Differential GPS– We discussed position measurements collected with a single receivers

known as autonomous GNSS positioning– Differential GPS involves the cooperation of two receivers:

– Stationary (with known coordinate points)– Rover

– When two receivers are fairly close to each other, the signals that reach both of them will have traveled through virtually the same slice of atmosphere, and will have similar type/amount of errors, and:

– Differential GPS can eliminate all errors that are common to both.

– These include everything except multi-path errors and any receiver errors.

Stationary (Known Position)

Rover (Un-known Position)

Differential GPS

DGPS Base-stations

Wide Area Augmentation System (WAAS)Based on a network of ground reference stations scattered

about North America (works only with NAVSTAR GPS)To provide accurate, dependable aircraft navigation.To provide real-time accuracies to reduce individual errors less

than 7 meters (tests shows errors less than 7 meters 95% of the time)

Signals from satellites received at each station and errors calculated

Correction calculated and transmitted to a geo-stationary satellite.

Correction signals collected by WAAS compatible roving receivers.

Substantial improvement over uncorrected GPS (errors above 15 meters)

Country Augmentation AbbreviationUSA: Wide Area Augmentation System WAASEurope: European Global Navigation Overlay System EGNOSJapan: MTSAT Satellite Based Augmentation System MSASCanada: Canadian Wide Area Augmentation System CWAASChina: Chinese Sat. Navigation & Augmentation Service SNASIndian: GPS and Geo Augmented Navigation System GAGAN

GNSS Augmentation

GPS Jamming Devices

Source: http://www.ac11.org/gps1.htm

Range: Few meters to 200 Km

Country Name No. of Satellites Operational Date

US NAVSTAR / GPS (Global) 24+ Operational since 17 July 1995

Russian Federation GLONASS (Global) 31 24 Operational

European Union GALILEO 302013

(2 launched in Oct 2011 and 2 in Oct 2012)

China COMPASS (Beidou) Beidou 1 = (3+1)Beidou2 = 35

14 satellites operational for Asia Pacific general users,

(2020 global coverage)

IndiaIRNSS (Indian Regional Navigational Satellite System)

72015

(First Satellite to be launched in June 2013)

JapanQZSS (Quasi-Zenith Satellite System)

3 2013 (1st launched in Sept. 2010)

FrenchDORIS (Doppler Orbitography and Radio-positioning Integrated by Satellite)

- Operational Since 1990

GNSS - Worldwide

- United States NAVSTAR Global Positioning System (GPS) has been the only fully operational GNSS till recently- GLONASS: In October 2011, the full orbital constellation of 24 satellites was restored, enabling full global coverage

GPS Uses/Applications• Search and rescue• Disaster relief• Surveying and Mapping• Geographic Information Systems (GIS)• Marine, aeronautical and terrestrial navigation• Remote controlled vehicle and robot guidance• Satellite positioning and tracking• Shipping• Military• Recreation

References

Lecture notes of Paul Burgess, University of Redlands, Redlands Institute. “Introduction to GPS”

http://www.spacetoday.org/Satellites/GPS.html http://www.astronautix.com/project/navstar.htmhttp://en.wikipedia.org/wiki/Global_Positioning_System