GPS: Components and Application

GPS
The Global Positioning System (GPS) is a
satellite-based navigation system developed by the
U.S. Department of Defense. Initially designed for
military use, GPS was made available to civilians in the
1980s.

● Global Satellite Network: The GPS network

consists of 24 orbiting satellites positioned at
11,000 nautical miles above Earth in six orbital paths. Each satellite
moves continuously, completing two full orbits every 24 hours, at
approximately 2.6 kilometers per second.

Background and Evolution of GPS

● Initial Developments:

○ During the 1950s and 1960s, the U.S. Navy developed two
satellite-based navigation systems: Transit and Timation.
■ Transit became operational in 1964 and was opened to public
use in 1969.
■ Timation was an experimental system, contributing to later
designs but never reaching operational status.
○ Concurrently, the U.S. Air Force explored a positioning system
known as System 621B. Testing proved its feasibility, but the
system was not fully developed due to military restructuring.

● Establishment of NAVSTAR GPS:

○ In April 1973, the Deputy Secretary of Defense appointed the U.S.

Air Force to unify Transit, Timation, and 621B into one coherent
 navigation system: the Defense Navigation Satellite System
(DNSS).
○ From this consolidation emerged NAVSTAR GPS (Navigation
System with Timing and Ranging), or simply GPS.

How GPS Works: Basic Principles

● Line of Sight and Satellite Signals: GPS receivers calculate a user’s

position by receiving signals from a minimum of four satellites. These
signals enable the receiver to compute: Latitude (X), Longitude (Y),
Altitude (Z), and Time (T).

● Obstacles: Physical barriers like buildings or mountains can obstruct

signals, limiting accuracy due to blocked line-of-sight paths to satellites.

● Dilution of Precision (DOP): A metric indicating the degradation of GPS

accuracy based on satellite positioning.

○ High vs. Low DOP:

■ High DOP occurs when satellites are clustered, reducing
accuracy.
■ Low DOP (spread satellites) offers higher precision.

GPS System Components/Segments

1. The Control Segment

The Control Segment is

the part of the GPS that
monitors and maintains
the satellite network. It
ensures that each satellite
remains in its designated
orbit, has updated data,
and provides accurate signals.

A. Master Control Station (MCS): The Master Control Station (MCS) is the
central command of the GPS system, located at Falcon Air Force Base in
Colorado Springs, Colorado.

Responsibilities of the MCS include:

● Tracking Satellite Health and Position: Ensures each satellite is in its

correct orbit (known as "station keeping").
● Data Processing and Uploads: The MCS collects data from monitoring
stations, processes it, and sends corrective information to satellites. This
includes updating the satellite’s clock, ephemeris (position data), and
navigation messages.
● System Backup: Two additional MCS locations in Sunnyvale, California,
and Rockville, Maryland, act as backup centers in case of a primary MCS
failure.

B. Monitor Stations: Monitor Stations are unmanned facilities located

strategically worldwide. They continuously track GPS satellites, collecting data
for the MCS to analyze.

● Locations: There are five primary monitor stations, including sites in

Hawaii, Ascension Island, Diego Garcia, and Kwajalein.
● Functionality: Monitor stations passively receive signals from satellites,
tracking satellite position (ephemeris) and clock timing. They transmit this
data to the MCS for further processing.
● New Stations (post-2005): Additional monitor stations in Washington
DC, England, Ecuador, Argentina, Bahrain, and Australia were added to
enhance global satellite tracking coverage.

C. Ground Antennas: Ground Antennas are used to transmit data from the
MCS to satellites. They are strategically positioned around the globe to ensure
optimal signal reach.
 ● Role: Ground antennas receive updated ephemeris and clock correction
data from the MCS and upload it to the satellites. This upload occurs
frequently to minimize positional and timing errors.
● Operational Frequency: Regular updates are made multiple times daily,
depending on satellite needs and error thresholds.

2. The Space Segment

The Space Segment is

the satellite network in
orbit around the Earth,
responsible for
transmitting signals to
users worldwide. Each
GPS satellite transmits
unique timing and
position data essential for
GPS receivers on Earth.

A. Satellite Constellation: The GPS network includes 24 primary satellites,

which orbit Earth at approximately 20,200 kilometers above the surface.

● Orbital Configuration:
○ Satellites are distributed across six orbital planes, each with four
satellites.
○ Inclination: The orbits are tilted 55 degrees from the equatorial
plane, providing global coverage.
○ Orbital Period: Each satellite completes one orbit every 12 hours
(two orbits daily), ensuring that users can access multiple satellites
at any given time.
○ Redundancy: The system includes backup satellites that can replace
malfunctioning units.
 ● Physical Characteristics:
○ Weight: Each satellite weighs approximately 900 kilograms.
○ Dimensions: Around five meters wide when solar panels are fully
extended.
○ Power Source: Solar panels supply energy to the satellite,
supporting continuous operations.
○ Onboard Atomic Clocks: Equipped with highly precise atomic
clocks, the satellites maintain timing essential for accurate
positioning data.

3. The User Segment

The User Segment comprises the

individuals and devices (GPS receivers)
that use satellite data to calculate location,
velocity, and time. This segment is highly
versatile, supporting applications across
civilian, military, scientific, and industrial
fields.

A. GPS Receivers: GPS receivers are devices that process signals from multiple
satellites to compute the user’s Position, Velocity, and Time (PVT).

● Functionality:
○ To calculate accurate 3D positioning, a GPS receiver must receive
data from at least four satellites. This enables it to determine X
(latitude), Y (longitude), Z (altitude), and T (time).
○ GPS receivers operate passively, simply receiving signals and
performing calculations without needing to transmit data back to
satellites.

B. Applications of GPS Receivers

● Navigation:
 ○ Civilian and commercial use of GPS has expanded to include
navigation for vehicles, aircraft, ships, and personal use.
○ In cars, smartphones, and wearable devices, GPS provides real-time
directions and tracking capabilities.

● Military Use:
○ Military GPS receivers are enhanced to withstand jamming and
interference. These devices allow troops to navigate securely and
target positions with high accuracy.

Working Functions of GPS

Step 1: Triangulating from Satellites

GPS positioning relies on ranging (calculating

distance) from satellites using a method
called trilateration. Trilateration uses data
from multiple satellites to pinpoint the
receiver’s location.

1. Broadcasted Navigation Message: Each satellite broadcasts a navigation

message that includes:
○ Course Acquisition (CA) Code:
■ A pseudo-random code containing an Almanac with orbital
information about the satellite constellation.
○ Ephemeris Data:
■ Details of the satellite’s exact position, adjusted for minor
disturbances that can affect orbit.
○ System Time:
■ The GPS system time, which is based on an atomic clock in the
satellite, includes clock correction parameters to adjust for the
differences between UTC and GPS time.
○ Ionospheric Model:
 ■ A mathematical model to estimate signal delays caused by
the ionosphere (atmospheric layer).
○ Health Message:
■ Status information indicating if a satellite is functioning
correctly; unhealthy satellites are excluded from calculations.

2. Data Reception and Processing by GPS Receiver:

○ The receiver collects this data over a period of 12.5 minutes, during
which it matches the satellite’s CA code with a similar code stored
in the receiver’s database.
○ By shifting the CA code to align with the satellite’s code, the
receiver calculates the time delay, which determines the signal
travel time.

3. Pseudo-range Calculation:
○ The distance calculated by the GPS receiver is called a
pseudo-range since it’s derived from the time delay rather than a
direct distance measurement.
○ Pseudo-ranges are affected by atmospheric delays and multipath
errors (signal reflection), making them susceptible to errors.
However, they provide an initial approximation for the receiver’s
position.

4. Trilateration Process:
○ Using data from at least four
satellites, the GPS receiver can
determine its location through
trilateration:
■ One Satellite: Only indicates
the possible locations on a
sphere centered around the satellite.
■ Two Satellites: Reduces the possible locations to a circle
where two spheres intersect.
 ■ Three Satellites: Allows for 2D positioning (latitude and
longitude) by narrowing down to two potential points.
■ Four Satellites: Enables accurate 3D positioning (latitude,
longitude, and altitude) by pinpointing one unique location.
○ With more than four satellites, the GPS receiver can validate signal
accuracy, reject erroneous data, and enhance precision.

Step 2: Measuring Distance from a Satellite

GPS calculates distances based on the time it takes for a signal to travel from
the satellite to the receiver.

1. Signal Ranging:

○ Distance to the satellite is calculated using the formula: Distance

(d)= Speed of signal×Travel time

where the speed of the signal (radio waves) is approximately 3x

108 meters/second.

2. Calculating Time Difference:

○ The GPS receiver records the sending time (t1) of the signal from
the satellite and the receiving time (t2) upon arrival.
○ The time difference (Δt) is given by: Δt=t2−t1
○ By multiplying this time difference with the signal speed, the GPS
calculates the distance to the satellite.

Step 3: Getting Perfect Timing

Timing is critical for GPS calculations, as even a minor error can result in
significant positioning inaccuracies.

1. Importance of Precise Timing:

○ GPS requires precise timing, as an error of even one-thousandth of a
second can cause a 200-mile error in position.
 ○ Atomic Clocks: Each satellite is equipped with atomic clocks,
ensuring nearly perfect time synchronization.
○ Synchronization Process:
■ GPS relies on calculating the distance to an additional satellite
to check timing accuracy.
■ If three satellite distances define a position in three
dimensions, a fourth measurement acts as a check for timing
consistency.
2. Role of the Fourth Measurement:
○ The fourth measurement corrects any inaccuracies from the
receiver’s clock and confirms the exact 3D location.

Step 4: Knowing the Satellite's Position in Space

Accurate positioning relies on the GPS system’s knowledge of each satellite's

location in orbit.

1. Satellite Positions:
○ Satellite positions are meticulously calculated and monitored by the
Control Segment, ensuring that each satellite serves as an accurate
reference point.
○ Monitoring Stations: Ground-based stations track satellites and
upload corrective data, adjusting for orbital shifts and other
influences.
2. Reference Points:
○ Each satellite's location is calculated and updated regularly,
allowing GPS receivers to use them as reliable reference points in
space.

Step 5: Correcting Errors

While GPS calculations are precise, several factors can still introduce errors in
positioning data.
 1. Types of Potential Errors:
○ Ionospheric and Atmospheric Delays: Variations in the atmosphere
can delay GPS signals.
○ Multipath Errors: Signals may reflect off surfaces before reaching
the receiver, creating inaccuracies.
○ Satellite Geometry: The relative positions of satellites in the sky
(referred to as Geometric Dilution of Precision, or GDOP) can affect
accuracy.

2. Error Correction Techniques:

○ Mathematical Models: The Control Segment uses ionospheric and
atmospheric models to correct delays.
○ Signal Processing: The receiver applies adjustments based on the
corrected data from satellites, reducing the impact of multipath and
atmospheric effects.

Types of GPS Signals

GPS satellites broadcast two main types of

signals, each serving different purposes and
user groups.

1. Pseudo-Random Code (PRC)

The pseudo-random code is a digital sequence embedded in GPS signals,

allowing receivers to identify the satellite’s unique transmission.

Types of PRC Signals:

● Coarse Acquisition Code (C/A):

○ Used for civilian applications, it is broadcast on the L1 frequency.
○ The C/A code repeats every 1023 bits and modulates at a 1 MHz
rate.

● Precise Code (P):

 ○ Designed for military applications, the P code modulates on both L1
and L2 frequencies at a 10 MHz rate, providing more precise
information.
○ The P code is more complex, enhancing security and precision.

2. L-Band Signals

GPS uses two main frequencies, L1 and L2, to transmit information:

● L1 Frequency:
○ Broadcasts at 1575.42 MHz and carries the status message and
pseudo-random code, essential for civilian users.
● L2 Frequency:
○ Broadcasts at 1227.60 MHz, primarily for military applications. It is
more resistant to atmospheric disturbances, enhancing accuracy for
military users.

Applications and Uses of GPS

1. Satellite Imagery and Mapping: GPS aids in topographic surveys and

thematic mapping through satellite images.
2. Traffic Management: GPS navigation devices detect road congestion,
recalculating routes based on real-time traffic data.
3. Defense and Security: Used in military operations for tracking, defense
strategy, and rescue operations.
4. Accident Response: Assists in locating and rescuing crashed ships and
aircraft.
5. Seismology and Tectonics: Measures crustal movements and seismic
strain for earthquake monitoring.
6. Counterterrorism: GPS helps locate terrorist activities, as in surgical
strikes by the Indian Army.
7. Mining Operations: RTK GPS offers centimeter-level accuracy for drilling,
tracking, and surveying.
 8. Climate Science: Supports weather mapping and atmospheric data
collection.
9. Tourism: Provides location-based content for points of interest.
10. Navigation: Essential for accurate route planning, finding services, and
identifying optimal travel paths.

Global Navigation Satellite Systems

(GNSS)

GNSS are satellite-based systems that

provide autonomous geo-spatial positioning
with global coverage. These systems allow
users worldwide to determine their exact
location, velocity, and time, under any
weather conditions, with continuous
availability.

1. Global Positioning System (GPS)- USA

The Global Positioning System (GPS) is the oldest and most widely used
GNSS, developed and maintained by the United States Department of Defense.
GPS has become the primary navigation tool globally, due to its early
availability, high accuracy, and widespread adoption.

● Satellite Constellation:

○ Constellation Size: GPS includes up to 32 medium Earth orbit

(MEO) satellites.
○ Orbital Planes: These satellites are distributed across six orbital
planes at an altitude of approximately 20,200 km.
○ Satellite Replacement: Satellites are continuously monitored and
replaced as they reach the end of their operational life, ensuring
uninterrupted service.
2. GLONASS (Global Navigation Satellite System) – Russia

GLONASS is a satellite-based navigation system developed by Russia. It serves

as an alternative to GPS and is used by both civilian and military sectors.

● Operational History:
○ Development of GLONASS began in the 1970s, with the system
becoming operational in 1993.
○ After several years of partial functionality, GLONASS reached full
operational capability with an expanded satellite constellation.

● Satellite Constellation:
○ Current Constellation: GLONASS consists of 27 operational
satellites in two orbital planes.
○ Orbit: Satellites orbit at 19,130 km above Earth.

3. Galileo – European Union

Galileo is the GNSS developed by the European Union to provide an

independent, civilian-operated navigation system, enhancing Europe’s strategic
autonomy.

● Operational History:
○ The European Union and the European Space Agency launched the
Galileo project in 2002.
○ Galileo achieved Early Operational Capability (EOC) in 2016, with
full operational capability expected by 2020.
● Satellite Constellation:
○ Planned Constellation: Galileo is designed for 30 MEO satellites,
with 22 satellites currently in orbit.
○ System Compatibility: Galileo is compatible with GPS, allowing
receivers to combine signals from both systems for improved
accuracy.
4. BeiDou – China

BeiDou is China’s GNSS, which has evolved from a regional navigation system
to a global network, offering comprehensive navigation capabilities.

● Operational History:
○ The first phase, BeiDou-1, provided limited regional coverage and
was decommissioned in 2012.
○ BeiDou-2 (COMPASS) extended coverage to the Asia-Pacific region,
while BeiDou-3 expanded to global coverage by 2020.

● Satellite Constellation:
○ Structure: BeiDou-3 consists of 35 satellites (30 MEO, 5
geostationary and inclined geosynchronous orbits).
○ Coverage: BeiDou-3 provides global navigation capabilities and has
integrated enhancements for secure military applications.

5. DORIS- France

Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS)

is a French GNSS used for high-precision geodesy and orbitography.

● System Overview:
○ Unlike traditional GNSS, DORIS relies on ground-based transmitters
with receivers on satellites, achieving high accuracy in orbital
positioning.

Regional Navigation Satellite Systems

Regional navigation systems complement global systems by providing

enhanced coverage and accuracy in specific regions. They are primarily
developed for national needs, including defense, agriculture, and urban
planning.

1. NavIC (Navigation with Indian Constellation) – India

NavIC (Navigation with Indian Constellation), also known as IRNSS (Indian
Regional Navigation Satellite System), is India’s regional navigation system
developed by the Indian Space Research Organization (ISRO).

● Operational History:
○ NavIC was approved in 2006 and became fully operational in 2018.
● Satellite Constellation:
○ Constellation Size: NavIC consists of seven satellites, three in
geostationary orbit and four in geosynchronous orbit.
○ Coverage: Covers India and regions up to 1,500 km around the
subcontinent, with plans for extended coverage in neighboring
areas.

2. QZSS (Quasi-Zenith Satellite System) – Japan

QZSS is Japan’s regional navigation system, designed to enhance GPS accuracy

and reliability over Japan and the Asia-Oceania region.

● Operational History:
○ The first QZSS satellite was launched in 2010, with services
available from 2018. The full constellation of seven satellites is
expected by 2023.

● Satellite Constellation:
○ Structure: The initial constellation comprises four satellites,
expected to expand to seven.
○ Coverage: Focuses on Japan and nearby Asia-Pacific regions.

GNSS Augmentation Systems

GNSS Augmentation involves systems that improve the accuracy, reliability,

and availability of GNSS data by adding external information.

1. WAAS (Wide Area Augmentation System):

 ○ Developed by the United States for aviation, WAAS corrects GPS
errors and provides precise positioning for safe air navigation.

2. EGNOS (European Geostationary Navigation Overlay Service):

○ The EU’s regional augmentation system for GPS, providing accuracy
improvements for European users, especially in aviation and
maritime sectors.

3. MSAS (Multi-functional Satellite Augmentation System):

○ Japan’s GNSS augmentation system, enhancing GPS signal accuracy
in the Asia-Pacific region for critical applications.

4. GAGAN (GPS Aided GEO Augmented Navigation):

○ India’s satellite-based augmentation system, providing accuracy for
GPS in aviation over the Indian region.

DGPS
Differential Global Positioning
System (DGPS) is an enhancement to
the traditional Global Positioning
System (GPS) that provides much
greater accuracy, reducing errors to
within a few centimeters. DGPS
achieves this high level of precision by
utilizing fixed ground-based reference
stations to correct positioning data provided by GPS satellites.

Types of DGPS Systems

DGPS can operate with both ground-based and satellite-based correction

systems:

● Ground-Based DGPS:
 ○ Utilized by entities like the U.S. Coast Guard (USCG) and Canadian
Coast Guard (CCG), these systems operate on long-wave radio
frequencies (285-325 kHz) near coastlines and waterways,
providing enhanced positioning for maritime applications.

● Satellite-Based DGPS (Wide Area DGPS or WADGPS):

○ Also known as Satellite-Based Augmentation Systems (SBAS),
this system broadcasts correction data from satellites rather than
ground stations, enabling broader coverage. Examples include the
United States' WAAS and Europe’s EGNOS.

Key Differences Between GPS and DGPS

Basis for GPS DGPS
Comparison

Number of One (stand-alone GPS Two (Rover and Stationary

Receivers receiver) Receiver)

Accuracy 10–15 meters 10 cm

Range Global Local (up to 100 km)

Cost Generally affordable More expensive due to

infrastructure

Frequency 1.1 - 1.5 GHz Varies by agency (285–325

Range kHz for USCG)

Accuracy Affected by satellite timing, Affected mainly by distance

Factors atmospheric conditions, between rover and
ionosphere, etc. transmitter

Coordinate WGS84 Local coordinate systems

System

Applications of DGPS

DGPS is utilized in numerous fields where high accuracy is essential:

● Air Navigation:
 ○ DGPS is commonly used in aviation to provide pilots with precise
3D positioning, enhancing flight safety and efficiency.
● Precision Farming:
○ Farmers use DGPS to map fields, monitor crop yields, and apply
chemicals precisely, which maximizes resources and minimizes
environmental impact.
● Hydrographic Surveying:
○ DGPS enables accurate mapping of water bodies and coastal areas,
supporting marine navigation and environmental monitoring.
● Weather Forecasting:
○ By analyzing atmospheric effects on satellite signals, DGPS
contributes data for improved weather models.
● Coastal and Environmental Monitoring:
○ DGPS is employed to monitor beach erosion, coastal morphology,
and environmental changes over time.

Advantages of DGPS

● High Accuracy:
○ DGPS achieves positioning accuracies as low as 1–3 cm, enabling
applications that require high precision.
● Error Correction:
○ DGPS minimizes errors caused by atmospheric conditions, satellite
timing, and multipath effects, leading to consistently accurate data.
● Reliability in Adverse Conditions:
○ DGPS systems are less affected by signal obstructions like trees or
urban structures, making them suitable for varied terrains and
conditions.

Limitations of DGPS

● Range Constraints:
 ○ DGPS’s accuracy diminishes beyond 100 km from the reference
station, making it less effective for widespread applications without
additional infrastructure.
● Cost:
○ DGPS infrastructure (reference stations, transmitters) and the
required equipment are more expensive compared to standard GPS
devices.
● Dependency on Local Reference Stations:
○ DGPS accuracy depends on the proximity to reference stations,
limiting its scalability and flexibility in remote areas without
coverage.