Global Positioning System

The global positioning system (GPS) is a space-based satellite navigation system providing location data coupled with time information worldwide. This coverage is predicated on an unobstructed line of sight between the user and the dedicated satellites. The service is funded by U.S. taxpayers and maintained by a department within the U.S. government. Anyone with a GPS receiver can access the system without charge.

As a navigational resource, the orbiting satellites provide critical capabilities to military, civil, and commercial users around the world. It is intended to become the backbone for modernizing the global air traffic system.

The project was conceived in 1973 to overcome the limitations of previous ground-based navigation systems. GPS was created and undertaken by the U.S. Department of Defense (DoD) in 1994 and was fully commissioned with 24 satellites. In recent years, three additional orbiting units have been added to enhance worldwide coverage. As of December 2010, there were 30 operational satellites orbiting the earth actively broadcasting positioning, navigation, and timing messages to users, 24/7, around the globe. In addition, five older satellites are maintained in orbit in a standby mode that can be brought back to operational status if required.

Next generation

Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS system and implement the next generation of GPS III satellites and Next Generation Operational Control System (OCX).In 2000, U.S. Congress authorized a modernization effort. Renovation of the constellation, which should enhance the performance and capabilities of the system, began with the launch of eight GPS Block IIR-M satellites during 2005-2009 and the first of 12 GPS Block IIF satellites in May 2010. The next generation of satellites, GPS III, is currently in development and on schedule for a first launch in 2014.

In addition to the U.S. owned network, the Russian GLObal NAvigation Satellite System (GLONASS) was in use by only the Russian military, until it was made fully available to civilians in 2007. A Galileo project is in works by the European Union along with the Chinese Compass Navigation System, and Indian Regional Navigational Satellite System.

Signals from satellites

The design of GPS is based partly on similar ground-based radio-navigation systems, such as LORAN. A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the earth. Each satellite continually transmits messages that include the time the message was transmitted, precise orbital information (the ephemeris), and general system health including the orbits of all GPS satellites (the almanac).

The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite. These distances along with the satellites’ locations are used to compute the position of the receiver. This position is then displayed, perhaps with a moving map presentation or latitude and longitude. Elevation information may also be included. Many GPS units show derived information such as direction and speed, calculated from position changes.

Three satellites might seem enough to receive information to resolve a position since space has three dimensions and a position near the earth’s surface can be assumed. However, even a very small clock error multiplied by the very large speed of lightcoupled withthe speed at which satellite signals propagate would result in a large positional error. Therefore, receivers use four or more satellites to solve the receiver’s location and time. The accurately computed time is not displayed in all applications. A few specialized devices do use the time; these include time transfer, traffic signal timing, and synchronization of cell phone base stations.

Having four satellites in view is required for most operations, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. For example, a ship or aircraft may have a known elevation. Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a less accurate (degraded) position when fewer than four satellites are visible.

Interference

Like all radio-based services, GPS is subject to interference from both natural and human-made sources. A civilian GPS unit can lose reception in the presence of a device designed for intentional radio jamming. This can also occur during a solar flare. For this reason, the U.S. government strongly encourages all GPS users to maintain backup capabilities for positioning, navigation, and timing. In addition, new GPS signals that are more resistant to jamming are being developed. Even conditions within the earth’s Ionosphere impact satellite transmissions.

All satellites broadcast on several frequency bands termed L1 through L5. Basic military applications use two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal) while civilian receivers monitor the L1 transmission. The satellite network uses a Code Division Multiple Access (CDMA) technique where the message data is encoded with a pseudo-random (PRN) sequence that is different for each satellite. The receiver is programmed to be aware of the PRN codes for each satellite to reconstruct the message and extract the required information.

The L5 frequency band at 1.17645 GHz was added in the process of GPS modernization. This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under normal circumstance.

One recent situation involving communications service provider LightSquared resulted in a reassessment of a plan to launch a nationwide broadband service using frequencies bordering those of GPS. LightSquared is trying to build a cell phone network out of satellites, but the technology may, potentially interfere with GPS. LightSquared wants the military and other federal agencies to refit its equipment with filters. There have been reports of aircraft onboard GPS anomalies where data transmissions broadcast through the Inmarsat satellite communications systems have been identified as the culprit. This is another system utilizing filters to prevent interference.

Troubleshooting

When troubleshooting reports GPS problems it is often beneficial to employ techniques common in diagnosing basic radio problems. Signal interference is often difficult to detect but can render the GPS useless so finding out when and where the problem occurred is a good starting point. Many aircraft are equipped with dual receivers so questioning the status of both systems can provide direction to problem resolution. An aircraft inside a metal hangar is often blinded to satellite signals so installing a GPS repeater unit can be a value. This device includes an antenna that can be affixed to a building exterior, a receiver transmitter unit that will accurately reproduce satellite transmissions, and a transmit antenna that can be located within the hangar. This is a means to allow system alignment while the aircraft is indoors plus a valuable tool when on a fault-finding mission. Solar activity is another known detractor of GPS reception. An improperly bonded receiver antenna can render the navigation feature useless. Portable receivers or even remote antennae located in the cockpit may become blinded by electrically anti-iced windshields which may result in signal error.

GPS is no more than measuring time and distance to determine where you are. But when it comes to the actual measuring, there are so many external factors to be accounted for that without the benefit of high tech you could easily be thrown off by half a continent.

Jim Sparks has been in aviation for 30 years and is a licensed A&P. He can be reached at sparks-jim@sbcglobal.net.

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