Feature: The US Global Positioning Satellites (GPS) are an integral part of the world of inflight connectivity. If it were not for GPS an inflight connectivity system wouldn’t know where or how to point to the correct satellite. But what is GPS, how does it work and what is its future?
Let’s look at some Q&As about how GPS works and why it is so important.
What is the Global Positioning Satellite (GPS) system?
The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. The full constellation of 24 satellites was achieved in 1994 although each satellite is built to last about 10 years and replacements are constantly being built and launched into orbit.
Are the GPS satellites in geostationary orbit?
No, this is a common misconception. They are actually about 12,000 miles above us, making two complete orbits in less than 24 hours and travelling at speeds of about 7,000 miles an hour. This puts them about halfway to a geostationary orbit.
What frequency do they transmit on?
GPS satellites transmit two low power radio signals with civilian GPS using the L1 frequency of 1575.42 MHz in upper UHF (L band). The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects.
What does the signal contain?
A GPS signal contains three different pieces of information – a pseudorandom code, ephemeris data and almanac data. The pseudorandom code identifies which satellite is transmitting information.
Ephemeris data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time.
The almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system.
How can an aircraft use this information to locate its position?
Its GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. With distance measurements from a further two or more satellites, the receiver can determine the user’s position. This is obviously a complex procedure performed by computer.
How does the aircraft know where to point its satellite antenna?
The KANDU (Ku/Ka-band Aircraft Networking Data Unit) is what physically controls the satellite antenna. It interfaces with the aircraft navigational systems to control its movement.
The KANDU is also responsible for making sure the system reconnects to the right satellite as a single satellite can only cover a certain area, which means longer flights may switch between two or even three different satellites (known as a hand-off) especially with Ku-band systems.
Does GPS have any pitfalls?
Yes, a few. The mains ones are:
- Ionospheric and tropospheric delays – The satellite signal slows as it passes through the atmosphere, depending on the moisture content. Ionisation caused by the sun’s UV and X-ray output can also disturb the signals.
- 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.
- Orbital errors – Also known as ephemeris errors, are inaccuracies of the satellite’s reported location.
- Number of satellites visible – The more satellites a GPS receiver can “see,” the better the accuracy.
- Satellite geometry – 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.
- Intentional degradation of the satellite signal – jamming can be an issue with GPS.
How can you improve the accuracy of GPS?
A system called local or wide area “augmentation” can be used. With a local area augmentation systems (LAAS) you have a GPS receiver at a fixed known position. You then compare its known position with the GPS-calculated one. Any difference between the two can then be re-broadcast as a correction signal.
Wide Area Augmentation (WAAS) works in a similar way but uses a whole host of fixed stations over a large area. The WAAS correction signal can then be transmitted via another, usually geostationary, satellite.
The US WAAS specification requires it to provide a position accuracy of 7.6 metres (25 ft) or better (for both lateral and vertical measurements), at least 95% of the time. Actual performance measurements of the system at specific locations have shown it typically provides better than 1.0 metre (3 ft 3 in) laterally and 1.5 metres (4 ft 11 in) vertically throughout most of the contiguous United States and large parts of Canada and Alaska.
In Europe a system called The European Geostationary Navigation Overlay Service (EGNOS) has been developed by the European Space Agency, the European Commission and EUROCONTROL. It supplements the GPS, GLONASS and Galileo systems by reporting on the reliability and accuracy of the positioning data.
It specifies that its horizontal position accuracy should be better than seven metres. In practice, the horizontal position accuracy is at the metre level. The EGNOS system consists of four geostationary satellites for its satellite-based augmentation system (SBAS), and a network of ground stations.
Interestingly, inflight connectivity provider Inmarsat was providing EGNOS correction signals from three of its I-3 and I-4 satellites, although the current status is unknown.
Japan has its own Multi-functional Satellite Augmentation System (MSAS). India has launched its own SBAS programme named GPS and GEO Augmented Navigation (GAGAN) to cover the Indian subcontinent. Both Korea (2013) and China (2014) have announced plans to start their own SBAS implementation.
What is the future of GPS?
The U.S. Air Force and Lockheed Martin are developing a next-generation satellite system, known as GPS III.
GPS III will improve position, navigation and timing services and provide advanced anti-jam capabilities with superior system security, accuracy and reliability. GPS III will also deliver signals that are three times more accurate than current GPS spacecraft.
Lockheed Martin says it will demonstrate the value of its flexible GPS III design over the next 26 months, as part of the Air Force’s GPS III Space Vehicles 11+ Production Readiness Feasibility Assessment. On May 5 2016, the Air Force awarded Lockheed Martin a $5 million contract for Phase 1 of this procurement for future GPS III satellites.
Lockheed Martin’s says its GPS III production line is in full swing. In December 2015, the first GPS III satellite completed system-level Thermal Vacuum (TVAC) testing, validating Lockheed Martin’s design and manufacturing processes.
In May 2016, seven more GPS III satellites were currently following the first one in production at Lockheed Martin’s GPS Processing Facility in Denver, Colorado.