Navigation system like GPS has become an inevitable part of our daily life by helping us to drive across unknown roads, find the nearest Starbucks outlets and play interesting games on our Smartphone. Let’s see what are satellite navigations systems, how does it works and what are the applications.
Global Positioning Systems – GPS is one of the most popular and globally available navigational systems consist of group of satellites orbiting above the earth. Satellite navigations systems are primarily designed to use in military applications but it gained popularity in civilian applications especially in road navigation. Four decades after introduction, many of the functions in aviation, logistics, shipping etc… cannot be performed without a proper navigational system like GPS.
Accuracy of the satellite navigation system has been improved significantly from 100 meters in the early generation devices to less than 1 meter in modern devices. Russia, European Union, China and India have developed their own version of satellite navigation systems to have a good grip on the technology and being self sufficient in satellite navigation. However, GPS is still one of the widely used navigations systems available in billions of devices today.
GPS enabled devices only receives signals from satellites, it does not send any information back to the navigation satellites.
Satellite navigations system like GPS consists of constellations of satellites orbiting earth at 20000 kilometers above earth. These satellites are equipped with highly accurate atomic clocks which broadcast the timestamp and position of the satellite to earth. At any given moment, orbiting satellites are positioned in such a way that the devices on earth will be able to receive signals from three to four satellites.
Signals from each satellite will have a tiny time delay from other satellites while receiving at devices. Those devices receive signals from three or more satellites frequently and compare distance to calculate the exact position or co-ordinates accurately.
Trilateration Method
GPS satellites broadcast it precise location and clock time continuously using radio frequency signals which travels at speed of light. Trilateration requires minimum three signals from different satellite, location of the receiver can be calculated using the intersect point of three signal circles like in below image.
The location and clock time received from the satellite are used to determine the precise location by comparing the delay time of three signals at the receiver.
There are different satellite navigation systems developed by United States, Russia, European Union, China, India and Japan. All these systems works based on similar principle except the frequency bands used to broadcast clock and position information.
GPS – Global Positioning System
GPS is satellite based navigation system introduced in 1978 by United States military and operated by US air force. It was primarily used as a military tool for location based operations and later became popular in many applications.
| Country | United States of America |
| Introduction | 1978 |
| Number of satellite | 31 |
| Frequency | 1575.42 MHz and 1227.60 MHz |
| Modulation | BPSK (Binary Phase-Shift Keying) |
| Satellite orbital height | 20,180 KM |
| Availability | Globally available |
GLONASS is a Russian satellite navigation system introduced in 1982 by Russian Federal Space Agency. Initially, GLONASS was designed to serve Russian territory and they have added more satellites to extend the service coverage. In 2011, GLONASS became a global navigation system has a constellation of 24 satellites in orbit above 19,100 KM above earth.
| Country | Russia |
| Introduction | 1982 |
| Number of satellite | 24 |
| Frequency | 1602 MHz and 1246 MHz |
| Modulation | BPSK (Binary Phase-Shift Keying) |
| Satellite orbital height | 19,100 KM |
| Availability | Globally available |
Galileo
Galileo is an initiative by European GNSS – global navigation satellite system created by European Union. First satellite was launched in 2005 and currently 18 active satellites are in orbit. Galileo expected to launch more satellites and it will be fully operational by 2020 with 30 satellites in the system.
| Country | European Union |
| Introduction | 2005 |
| Number of satellite | 18 |
| Frequency | 1575.42 MHz, 1176.42 MHz, 1207.14 MHz and 1278.75 MHz |
| Modulation | BPSK, CBOC, BOCcos and AltBOC |
| Satellite orbital height | 23,222 KM |
| Availability | Globally available |
BeiDou
BeiDou is a satellite navigation system by China which consists of geostationary and geosynchronous satellite systems. The first phase BeiDou-1 was introduced in 2000 with three satellites in operation and the project has decommissioned in 2012. Later in 2012, BeiDou-2 system launched 10 satellites covering mainly China and neighboring geographical areas.
Currently active system BeiDou-3 is global system with 9 more satellites in orbit. During completion of this project, it will have total of 35 satellites in orbit by end 2020 period.
| Country | China |
| Introduction | 2000 |
| Number of satellite | 23 |
| Frequency | 1575.42 MHz, 1191.795 MHz, 1268.52 MHz |
| Modulation | BPSK, BOC, MBOC and AltBOC |
| Satellite orbital height | 21528 KM and 35786 KM (geostationary satellites) |
| Availability | Globally available |
IRNSS is an Indian version of satellite navigation system developed by ISRO (Indian Space Research Organization) to support Indian military services in the continent and neighboring regions. This project launched first satellite in 2013 and currently consists of 7 satellites in orbit.
| Country | India |
| Introduction | 2013 |
| Number of satellite | 7 |
| Frequency | 1576.45 MHz and 2492.028 MHz |
| Modulation | BPSK and BOC |
| Satellite orbital height | 36000 KM |
| Availability | Indian sub continent and 1500 KM from borders |
Quasi-Zenith Satellite System
QZSS is a satellite-based augmentation and time transfer system developed by Japan to provide precise location based services in the region similar to GPS navigation. It consists of 4 active satellites in the orbit and not fully operational yet.
| Country | Japan |
| Introduction | 2010 |
| Number of satellite | 4 |
| Frequency | 1576.45 MHz, 1227.60 MHz, 1176.45 MHz and 1278.75 MHz |
| Modulation | BPSK and CSK |
| Satellite orbital height | 32000 to 40000 KM |
| Availability | Regional coverage in Japan |
- Road and Rail navigation
- Logistics and shipping services
- Marine application
- Military and commercial aviation
- Precision agriculture
- Autonomous driving (self driving cars)
- Drone operation
- Security and surveillance applications
- Fleet tracking and management
- Interactive gaming
- Search and rescue operations
- Medical application (tracking of patients requires special care)
- Weather prediction and broadcast
- Disaster management
- Field mapping and survey applications
- Efficient road traffic control and management
Limitations
- Accuracy limitation due to atmospheric conditions
- Jamming by other RF sources could interrupt GPS services
- Malfunction of atomic clock on satellite could result wrong location information
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