Introduction to GPS Navigation

Introduction to GPS

• GPS is a satellite-based navigation system composed of satellites orbiting the earth
• The US Department of Defence (DOD) developed GPS in 1978 and continue to monitor it
• GPS offers precise global navigation unaffected by weather conditions, allowing for point-to-point navigation
• Global Navigation Satellite System (GNSS) is a general term encompassing GPS and other satellite navigation systems
• GPS operates within the L-band frequency range of 1100 - 1600 MHz.
• The distance is indirectly measured using time calculations
• Certified GPS equipment can serve as a supplemental IFR navigation tool for en route, terminal operations, and specific instrument approach procedures (IAPs)

GPS System Overview

• GPS functions as a satellite-based radio navigation system.
• Receivers use broadcasting signals to determine precise global positions.
• Originally for military use, it became available for civilian use in the 1980s.
• The 24-satellite constellation ensures global is coverage, with at least five satellites visible at any time.
• Receivers need signals from at least three satellites for two-dimensional positioning (latitude and longitude)
• Four or more satellites are required for three-dimensional positioning (latitude, longitude, and altitude).
• GPS operates in all weather conditions, 24/7.
• The Department of Defense (DOD) oversees the satellite constellation and its operations.
• Satellites broadcast orbital parameters (ephemeris data) as part of the GPS signal message.
• The GPS coordinate system is based on the World Geodetic System 1984 (WGS-84).

Global Positioning System (GPS)

• GPS navigation is the fastest-growing form of aviation navigation
• Navigation is achieved through NAVSTAR satellites maintained in orbit around the Earth

GPS Segments

• Space segment
• Control segment
• User segment

Space Segment

• Consists of a constellation of 24 satellites
• Arranged in six separate orbital planes, with four satellites each, in circular orbits
• Orbits characteristics include a 55° inclination to the Equator
• Altitude of approximately 20,200 km
• An orbital period of 12 sidereal hours
• Satellites position (ephemeris of the constellation)
• Satellites transmit constellation data (almanac)
• Atmospheric corrections

Satellite Positioning

• Employs twenty-four satellites, with 21 active and 3 spares.
• Located approximately 20,000 km above the Earth's surface.
• Satellites are positioned so that at least four are above 15° above the horizon at any time.
• Typically, between 5 and 8 satellites are accessible

Signal Transmission

• Each satellite transmits two signals with digitally coded information
• The L1 channel, operating at 1575.42 MHz, caters to civilian aviation.
• The L1 channel transmits Satellite ID, position, time, status, and other data to aircraft GPS receivers
• The L2 channel, operating at 1227.60 MHz, is designated for military use

GPS Frequencies

• GPS began with L1 and L2 signals
• L1 operates at 1575.42 MHz
• L2 operates at 1227.60 MHz
• GPS signals include P (Y) or Precision code and C/A (Carrier Acquisition) code
• Precision code is for military use
• Carrier Acquisition code is for civilian use
• Range codes are used to calculate the distance to the satellite
• The codes uniquely identify the navigation message

L5 Signal

• A significant upgrade for improved accuracy, reliability, and signal robustness
• It operates at 1176.45 MHz
• Enhanced resistance to interference
• Offer better performance in challenging environments.
• Designed for safety-of-life applications, like aviation.

GPS L1

• Operates at 1575.42 MHz
• Key Features include operating as a primary civilian frequency
• Carries C/A (public) and P(Y) (military) codes
• Transmits essential navigation data, including time, satellite health, ephemeris, and almanac
• Satellites crucial for initial satellite acquisition
• Military Advantage: P(Y) code offers superior interference rejection

GPS L2 Frequency

• Operates at 1227.60 MHz
• Key features include lower frequency for better penetration
• Originally for the military, now includes civilian L2C code on newer satellites
• Utilized for ionospheric error correction in dual-frequency receivers
• Provides redundancy
• L2 improves accuracy in high-precision applications

GPS L5 Frequency

• Operates at 1176.45 MHz
• It is the newest civilian frequency
• Key features include higher transmission power and advanced signal design
• Improved accuracy, reliability, and robustness
• Designed for aviation safety-of-life applications
• Wider bandwidth and longer spreading codes improve processing gain

GPS Signal Transmission

• GPS employs CDMA (Code Division Multiple Access) to differentiate between satellites.
• Despite transmitting on the same frequency, satellites each have a unique "pseudorandom" code.
• GPS receivers identify and decode signals using these codes, to pinpoint the source
• This allows the GPS receiver to extract information effectively, even when signals are weak.

Control Segment

• Monitors each satellite to guarantee precise location and time, using five ground-based receiving stations
• Consists of a master control station
• Three transmitting antennas
• Receiving stations forward status information from satellites to the master control station.
• Calculations and corrective instructions are then transmitted to the satellites.

User Segment

• The principle is founded upon measuring the transmission time of GPS signals from at least four satellites
• The segment is constituted by the GPS receiver, facilitating signal acquisition
• Allowing for distance calculation
• Navigation computation (Satellite choice, positioning, propagation corrections)
• The ability to detect and isolate failed satellites

Trilateration

• A method uses distances to determine a location
• Measures distances from known points.
• GPS uses trilateration.

GPS User Segment

• Encompasses all GPS receivers, especially those in aircraft
• Components include the GPS control panel/display, receiver circuitry, and antenna
• Components are integrated into a single unit alongside navigation and communication systems.
• Modern "glass cockpit" aircraft display GPS data on multifunctional screens.

GPS Receivers measuring Signal time

• A GPS receiver measures the time a signal takes to arrive from three transmitting satellites
• Radio waves travel at 186,000 miles per second, allowing the distance to each satellite to be calculated
• The intersection of these ranges provides a two-dimensional aircraft position.
• This position is expressed in latitude/longitude coordinates
• Incorporating a fourth satellite allows for the calculation of altitude
• Additional satellites refine the precision of the position

Benefits of GPS

• Immunity from service disruption due to weather
• GPS receivers for IFR navigation in aircraft must be built to TSO-129A

GPS Accuracy & WAAS

• Current GPS accuracy (20 meters horizontal) is sufficient for general navigation, but not for precise aircraft departures and landings
• WAAS improves accuracy to 7.6 meters
• Future upgrades like L2C and the aviation-dedicated L5 channel will improve accuracy, precision landings, and modern air traffic systems
• Newer satellite supporting this are in the process of implementation.

Wide Area Augmentation System (WAAS)

• Increases GPS accuracy aircraft navigation
• Employs approximately 25 precisely surveyed ground stations to receive signals and transmit correction data to the aircraft.

WAAS Operation

• Ground stations receive GPS signals and forward position errors to two master stations.
• Time and location data is analyzed, and correction instructions are sent to communication satellites in geostationary orbit over the NAS.
• Satellites broadcast GPS-like signals, which WAAS-enabled GPS receivers use to correct position data received.
• An aircraft with a WAAS-enabled GPS receiver can perform precision approaches into airports without ground-based equipment.
• Separation minimums are reduced between WAAS-equipped aircraft.
• WAAS reduces position errors to 1–3 meters laterally and vertically.