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.
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.
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 firstname.lastname@example.org.
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