GPS/DGPS History with Comparative Overview of GNSS Systems
Global Positioning System (GPS)
GPS, a satellite-based navigation system, was originally developed by the United States Department of Defense. The system became fully operational in 1995 after decades of development, starting in the 1960s. Its primary purpose was to provide accurate, real-time positioning for military applications, but it was later made available for civilian use.
Development and Early History (1960s – 1980s):
The origins of GPS date back to the launch of the first artificial satellite, Sputnik 1, in 1957 by the Soviet Union. Scientists realized that by monitoring the Doppler shift of radio signals from a satellite, they could pinpoint its location in orbit, which led to the idea of satellite-based navigation.
The U.S. Navy initially developed the TRANSIT system in the early 1960s for submarine navigation. This system laid the groundwork for GPS but had limitations in accuracy.
In the 1970s, the U.S. Department of Defense began to develop GPS with Navstar satellites, designed for continuous, global coverage. The first test satellite, Navstar-1, was launched in 1978.
By 1983, after a Korean Air Lines flight was shot down due to navigational error, President Ronald Reagan declared that GPS would be available for civilian use once fully developed.
Civilian Use and Global Deployment (1990s – Present):
In the 1990s, the system was enhanced and became widely available for civilian use. The system consists of a constellation of 24 satellites orbiting the Earth at an altitude of approximately 20,000 km, continuously transmitting signals.
GPS technology revolutionized multiple industries, including aviation, maritime, land surveying, transportation, and personal navigation.
The accuracy of civilian GPS signals was initially limited by the Selective Availability (SA) feature, which intentionally degraded accuracy for non-military users. This feature was disabled in 2000, significantly improving civilian GPS accuracy.
Today, GPS is used worldwide in smartphones, vehicles, aviation, and other applications, offering positioning accuracy within a few meters.
Differential Global Positioning System (DGPS)
DGPS is an enhancement to standard GPS, designed to improve the accuracy of position data by correcting GPS signals in real-time. It uses a network of fixed, ground-based reference stations to calculate the difference between the known precise location of the station and the position calculated using GPS.
Development and Purpose:
GPS signals can be affected by errors like atmospheric conditions, satellite orbit shifts, and timing discrepancies, leading to a margin of error in standard GPS readings (up to 10 meters in civilian use).
DGPS was developed to correct these errors. It was primarily used in areas requiring highly accurate positioning, such as maritime navigation, land surveying, and agricultural applications.
The first operational DGPS systems were introduced in the 1990s, using fixed stations that could transmit correction data to DGPS receivers via radio signals.
How DGPS Works:
DGPS uses a network of ground-based stations that are located at precisely known points. These stations receive GPS signals and calculate the error between the true location and the GPS-determined location.
The difference (or correction) is then broadcast to nearby DGPS-equipped receivers. The receiver applies the correction to its own GPS data, allowing it to improve accuracy, often reducing the margin of error to 1-3 meters.
Modern DGPS systems also use more advanced techniques like Real-Time Kinematic (RTK) and Wide Area Augmentation Systems (WAAS), offering accuracy levels down to centimeters.
Applications:
DGPS is used where high precision is critical, such as in geodetic surveys, precision farming, maritime navigation, and air traffic control.
As more satellite constellations like GLONASS, Galileo, and BeiDou are added to complement GPS, DGPS remains an essential tool for achieving high-accuracy positioning across many fields.
Today, both GPS and DGPS are integral to modern infrastructure and technology, supporting a wide range of industries and everyday applications.
Global Navigation Satellite Systems (GNSS) by Region
In addition to the U.S. GPS system, other countries and regions have developed their own satellite navigation systems to enhance global coverage, ensure independence, and improve the accuracy and reliability of positioning data. Below is a brief overview of the major GNSS systems from the U.S., China, Russia, Europe, and others.
- U.S. Global Positioning System (GPS):
Number of Satellites: The U.S. GPS currently consists of a full constellation of 24 satellites, though it often operates with more for redundancy. These satellites orbit the Earth at an altitude of approximately 20,200 kilometers in six orbital planes.
Functions: GPS satellites continuously transmit positioning signals, which are received by ground-based receivers to determine precise locations. GPS is used for military and civilian purposes, including navigation, mapping, and timing services.
Accuracy: After the removal of Selective Availability in 2000, GPS became widely used for commercial and personal applications, with positioning accuracies ranging from 3-5 meters in open areas. With enhancements such as DGPS and WAAS (Wide Area Augmentation System), accuracy improves to within 1 meter.
- Russian GLONASS (Global Navigation Satellite System):
Development: GLONASS is Russia’s counterpart to GPS, developed by the former Soviet Union and operational since the early 1980s. It was restored to full operational capacity in 2011 after a period of degradation following the dissolution of the USSR.
Number of Satellites: GLONASS operates with a full constellation of 24 satellites, orbiting at an altitude of around 19,100 kilometers. The system provides global coverage, though its accuracy is higher in northern regions due to Russia’s higher latitude.
Functions: GLONASS supports both civilian and military users, with an accuracy similar to GPS. For civilian use, the accuracy is typically within 2.8–7 meters, and for military use, it is much more precise. Dual-frequency receivers, capable of receiving signals from both GPS and GLONASS, are commonly used to enhance positioning accuracy.
- Chinese BeiDou Navigation Satellite System (BDS):
Development: China’s BeiDou (BDS) system is one of the newer GNSS systems. It began development in the early 2000s and became fully operational globally in 2020, following the completion of the BeiDou-3 constellation. The system was built to reduce China’s reliance on GPS and other foreign systems.
Number of Satellites: BeiDou consists of 35 satellites, operating in three distinct orbital planes: medium Earth orbit (MEO), geostationary orbit (GEO), and inclined geosynchronous orbit (IGSO). This combination provides global coverage, with enhanced accuracy over the Asia-Pacific region.
Functions: BeiDou offers global positioning services similar to GPS and GLONASS, with an accuracy of 2.5–5 meters globally, and better accuracy (up to centimeters) in China and surrounding regions, due to regional augmentation systems.
Special Features: BeiDou satellites have a unique two-way communication capability, allowing users to send short messages directly via the satellite, a feature not available in GPS or GLONASS.
- European Union Galileo:
Development: Galileo is the satellite navigation system developed by the European Union. It was designed to be independent of U.S. and Russian systems, offering civilian control and ensuring high levels of accuracy and reliability.
Number of Satellites: Galileo currently has a constellation of 30 satellites (24 operational, 6 spares), orbiting at an altitude of 23,222 kilometers. It became operational with Initial Services in 2016, and full services are expected soon.
Functions: Galileo provides global positioning services for civilian and commercial applications, with accuracy comparable to GPS. When fully operational, Galileo is expected to offer meter-level accuracy for civilian users, with even better precision for authorized (government and emergency) users.
Compatibility: Galileo is designed to be interoperable with other GNSS systems like GPS, GLONASS, and BeiDou. Most modern receivers are capable of receiving signals from multiple GNSS systems, improving overall accuracy and reducing signal loss.
- Other Regional Satellite Systems:
Indian Regional Navigation Satellite System (IRNSS or NavIC): India has developed its own regional navigation system, known as NavIC (Navigation with Indian Constellation), primarily covering the Indian subcontinent and surrounding regions. The system consists of 7 satellites in geosynchronous orbit, offering regional positioning accuracy within 10 meters.
Japan’s Quasi-Zenith Satellite System (QZSS): Japan operates a regional augmentation system called QZSS, which improves GPS accuracy in Japan and nearby regions. Its constellation consists of 4 satellites that provide augmentation for high-precision positioning services.
Comparative Overview of GNSS Systems:
| System | Country/Region | Satellites | Accuracy | Coverage |
| GPS | United States | 24+ | 3-5 meters | Global |
| GLONASS | Russia | 24+ | 2.8-7 meters | Global (better in northern regions) |
| BeiDou | China | 35 | 2.5-5 meters | Global (enhanced in Asia-Pacific) |
| Galileo | European Union | 30 | 1 meter | Global |
| NavIC | India | 7 | 10 meters | Regional (Indian subcontinent) |
| QZSS | Japan | 4 | Sub-meter | Regional (Japan and surrounding areas) |
Each of these systems, individually or combined, plays a crucial role in providing accurate, reliable, and robust positioning data for a wide variety of global applications, from smartphones to scientific research and from autonomous vehicles to space exploration. The interoperability of these systems allows users to achieve optimal accuracy by receiving signals from multiple constellations simultaneously.
