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Global Positioning System: Difference between revisions
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==== User segment ==== | ==== User segment ==== | ||
The user's GPS receiver is the user segment (US) of the GPS system. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock (often a [[crystal oscillator]]). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, | The user's GPS receiver is the user segment (US) of the GPS system. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock (often a [[crystal oscillator]]). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2006, receivers typically have between twelve and twenty channels. | ||
GPS receivers may include an input for differential corrections, using the [[RTCM]] SC-104 format. This is typically in the form of a [[RS-232]] port at 4,800 bps speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even low-cost units commonly include [[Wide Area Augmentation System|WAAS]] receivers. | GPS receivers may include an input for differential corrections, using the [[RTCM]] SC-104 format. This is typically in the form of a [[RS-232]] port at 4,800 bps speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even low-cost units commonly include [[Wide Area Augmentation System|WAAS]] receivers. | ||
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While most clocks are synchronized to [[Coordinated Universal Time]] (UTC), the [[Atomic clock]]s on the satellites are set to '''GPS time.''' The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain [[leap seconds]] or other corrections which are periodically added to UTC. GPS time was set to match [[Coordinated Universal Time]] (UTC) in 1980, but has since diverged. The lack of corrections means that GPS time remains synchronized with the [[International Atomic Time]] (TAI). | While most clocks are synchronized to [[Coordinated Universal Time]] (UTC), the [[Atomic clock]]s on the satellites are set to '''GPS time.''' The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain [[leap seconds]] or other corrections which are periodically added to UTC. GPS time was set to match [[Coordinated Universal Time]] (UTC) in 1980, but has since diverged. The lack of corrections means that GPS time remains synchronized with the [[International Atomic Time]] (TAI). | ||
The GPS navigation message includes the difference between GPS time and UTC, which | The GPS navigation message includes the difference between GPS time and UTC, which as of 2006 is 14 seconds. Receivers subtract this offset from GPS time to calculate UTC and 'local' time. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) which, at the current rate of change of the Earth's rotation, is sufficient to last until the year 2330. | ||
As opposed to the year, month, and day format of the [[Julian calendar]], the GPS date is expressed as a week number and a day-of-week number. The week number is transmitted as a ten-[[bit]] field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on [[January 6]] [[1980]] and the week number became zero again for the first time at 23:59:47 UTC on [[August 21]] [[1999]] (00:00:19 TAI on [[August 22]], [[1999]]). In order to determine the current [[Gregorian calendar|Gregorian]] date, a GPS receiver must be provided with the approximate date (to within 3,584 days) in order to correctly translate the GPS date signal. To address this concern the modernized GPS navigation messages use a 13-bit field, which only repeats every 8,192 weeks (157 years), and will not return to zero until near the year 2137. | As opposed to the year, month, and day format of the [[Julian calendar]], the GPS date is expressed as a week number and a day-of-week number. The week number is transmitted as a ten-[[bit]] field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on [[January 6]] [[1980]] and the week number became zero again for the first time at 23:59:47 UTC on [[August 21]] [[1999]] (00:00:19 TAI on [[August 22]], [[1999]]). In order to determine the current [[Gregorian calendar|Gregorian]] date, a GPS receiver must be provided with the approximate date (to within 3,584 days) in order to correctly translate the GPS date signal. To address this concern the modernized GPS navigation messages use a 13-bit field, which only repeats every 8,192 weeks (157 years), and will not return to zero until near the year 2137. | ||
