GPS: It stands for global positioning system and is a satellite based navigation system. It is works on the principle of timing and ranging using that fact that signals transmitted by satellites travel at known speed of 3x108 m/s.Time taken by signal to travel is measured to obtain distance of satellite from the earh.This way each satellite gives positions sphere which on interception with earth’s surface gives position circle. Two such circles give 2- D and three such circles give 3-D positions. An additional satellite measurement is taken by GPS receiver to account for receiver clock bias and thus correct all satellite “pseudo ranges” to correct ranges and make the position free of errors. This is needed because receiver clock is not very accurate like the expensive atomic clocks fitted on satellites. It is more accurate than any other existing system, provides worldwide coverage and is unaffected by weather conditions and man made or electrical interferences.
GPS signal consists of pseudo random code, ephemeris and almanac. Each satellite transmits two codes i.e. P (precision) code (gives 10 m accuracy) and C/A (coarse acquisition) code (gives 100 m accuracy). C/A code is for civilian users and P code is reserved for US military and its allies.
These codes are modulated by phase modulation technique on two carrier frequencies:
L1 = 1575.42 MHz (consists of both P and C/A codes)
L2 = 1227.60 MHz (consists of only P code)
The C/A code is made up of 0s and 1s called chip, having a frequency of 1.023 MBits/sec or 1.023 Bits/µ sec or 1023 Bits/msec.
Entire sequence is repeated ever 1 msec. i.e. after every 1023 Bits or every 3x 105 m
Thus each Bit (chip) is given by: 300000 / 1023 = 293 m = 1.023 µ sec
In case of P code the chip frequency is 10.23 MBits/sec, the duration of each Bit becomes 0.1023 µ sec or, in terms of distance 29.3 meters.
In addition each satellite transmits 2 types of orbit data Almanac and Ephemeris. Almanac data contains the health and approximate location of each satellite in the system. A GPS receiver gathers Almanac data from any available satellite; using information from the Almanac, the receiver then determines which set of satellites will give the best geometries for position fix. And then the receiver obtains Ephemeris from those satellites.
The receiver then assesses the transmission time and signal quality from each satellite and multiplies the difference in transmission time by speed of light to arrive at estimated satellite’s distance.
Ephemeris data contains the precise orbital parameters of each satellite.
GPS signal is extremely resistant to conditions that disturb other electronic navigation systems. In general GPS is not affected by:
Weather conditions
Passing ships
Onboard electronic installations
Onshore electronic installations
Onboard engine ignition
Portable radio receivers
INITILISATION OF GPS: Go to set up and feed:
1. Coordinate system for posn display ( UTM: Universal Transverse Mercator, Lat/Long etc. )
2. Elevation/Antenna Height: Select 2D or 3D mode. When in 2D mode, the receiver computes position only and to maximize accuracy in this mode you should enter the elevation/antenna height. In 3D, the receiver computes position and elevation.
3. Lat and Long of the vessel: Enter your approximate position. It will greatly shorten the time to fix the first position otherwise it will take 20 min to give first fix.
4. Time : Select UTC or local time
5. Velocity Averaging: Velocity averaging can be used to minimize the effects of SA on velocity output. The settings are off, 20 s, 60 s.
6. Compass Reference: Select magnetic or True north.
7. Chart Datum: Select the required datum , preferably WGS 84
8. Last Fix Interval: Sets the rate at which positions are automatically saved.Settings are: off, 10, 20, 30 min.
9. Plot Setup: Adjusts the track orientation and track history on the plot screen. For track select 0.1, 0.5, 1.0, 2.0 miles. For orientation choose between Head up and North Up.
10. Alarms: Select any or all of the following alarms: Anchor alarm; Arrival alarm; cross track alarm.
11. Light intensity and contrast settings.
GPS SEGMENTS: The GPS system has 3 segments:
GROUND BASED SEGMENT: It consists of 4 land based monitoring stations (Kwazalein, Hawaii, Ascension Island and Diego Garcia). It also has master control station and upload station located at Colorado Springs in USA. The monitoring stations track the satellites, obtain the data from these satellites and pass the information to master control station. After receiving the data from the monitoring stations, the master control station does the necessary computation to predict the future path and position of all the satellites. The master control station also determines the error of the atomic clocks in all the satellites. The updated data are fed to the upload station, which in turn transmits the same data to each satellite three times a day.
The master control station can selectively degrade satellite data.This degradation is known as Selective Availability and it can cause position errors of 100 meters.
SPACE SEGMENT: It consists of 24 satellites evenly placed in 6 different orbits. They move at the speed of 3.9 km/sec at a height of 20,200 km above earth’s surface. Each orbit is inclined at an angle of 55 deg to the equinoctial and the angle between 2 successive orbits is 60 deg. The satellites are arranged in such a manner that at any time signals can be received from at least 4 satellites having elevation of more than 9.5 degrees.
If any satellite emits erroneous data or is otherwise not operating properly, a ground station marks it “unhealthy”. The affected satellite broadcasts its status to the GPS receiver, which is programmed to ignore an unhealthy satellite and use the next best satellite.
USER SEGMENT: It consists of receiver with built in computer, display unit and antenna. The receiver locks on to one satellite and from this satellite it obtains the almanac of all the other satellites and thereby selects the four most suitable satellites for position fixing. The fix obtained is displayed on the display unit along with other information like COG, SOG and UTC.
Each satellite transmits a navigational message of 30 s in the form of 50 Bits/s data frame. This data, which is different for each satellite is supplied by MCS and is divided into 5 sub-frames. Each sub-frame commences with telemetry word (TLM) containing satellite status followed by hand over word (HOW) data for acquiring P code from C/A code. The 1st sub frame contains data relating to satellite correction. The 2nd and 3rd sub frames contain the satellite ephemeris defining the position of the satellite. The 4th sub frame passes alphanumeric data to the user and will only be used upload station has a need to pass specific message. The 5th sub frame gives the almanac of all other satellites which includes data on satellite health and identity codes.
ERRORS IN GPS POSITIONS: 1. Satellite clock error: this is caused by error in satellite’s clock wrt GPS time. This is monitored by ground based segment and any error in the Satellite clock forms part of the 30s navigation message. Though satellite clocks are highly accurate, they are not perfect. Clock errors can cause positional errors of about 1.5m.
2. Use clock error: It is caused by user clock not being synchronized with the satellite clock. Because of this range measurement will not be correct and is known as pseudo range. This error is eliminated within the receiver by extra satellite measurement (three for 2D and four for 3D positions).Extra measurement is used to find the extra unknown variable i.e. the error between user and satellite clock.
3. GDOP: The lower he value of GDOP, higher the accuracy of the fix obtained. The value of GDOP is indicated on the display. Since the receiver knows he position of all the satellites through 30s navigational message, it is programmed to select the best available satellites considering their elevation and geometry.
4. Multi path Error: This is caused signal coming to receiver after being reflected from other obstructions in addition to reaching the receiver directly. The two signals thus received simultaneously cause distortion of signal and in turn affect the range measurement. This problem is resolved by suitable siting of antenna. Multipath errors are difficult to detect but good quality sophisticated receivers can minimize the problem using intricate signal rejection techniques.
5. Ionospheric and troposphere error: Radio waves transmitted by satellites pass through different layers of atmosphere and space and undergoes change in its velocity which in turn affects the range measurement and hence the fix. Dual frequency receivers are able to receive both the frequencies and calculate the correction and do the compensation within the receiver. Ionosphere [position errors can be about 5m whereas troposphere conditions can cause rerrors up to 1 m.
6. Deviation of satellite from predicted path: The satellites are monitored and their paths predicted by ground based segment. Between two consecutive monitoring there may be small deviations from their respective paths. These deviations hen translate into errors in fixes. These errors can be up to 2.5 m.
DOP: A dimensionless number that takes into account the contribution of relative satellite geometry to errors in position determination. Positions tagged with a higher DOP value generally constitute poorer measurement results than with lower DOP.Thus DOP is a measure of the quality of GPS data received from the satellites. The main factors affecting DOP are the number of satellites being tracked and where these satellites are positioned in the sky. There are 5 types of DOPs:
HDOP: It is a measure of the accuracy of 2 dimensional positions. Higher HDOP values can be caused by satellites at high elevations.
VDOP: It is a measure of how well satellites are arranged to give vertical position .Higher VDOP means less certainty and can be caused if satellites have low elevations.
PDOP: is a measure of overall certainty in GPS position with TDOP not included in the estimated certainty.
PDOP = square root of (HDOP square + VDOP square)
A perfect PDOP implies one satellite directly overhead and 3 satellites positioned at 120 degrees intervals just above the horizon.PDOP less than 4 gives best accuracy. PDOP 4 -8 means acceptable accuracy and PDOP greater than 8 implies poor accuracy.
TDOP: is measure of certainty of how well GPS measures time.
GDOP: is overall certainty of position as well as time measurement i.e. including TDOP as well.
GDOP = square root of ( PDOP square + TDOP square )
The position accuracy = DOP x Measurement Precision
E.g. if DOP is 5 and M.P. is 1 meter, then
P.A. = 5 x 1 = 5 meters
Geodetic Datum: Also called horizontal datum. It is a reference for specifying positions on earth’s surface. Each reference (datum) is associated with a particular reference spheroid. Positions referenced to different datum can differ by several hundred meters.
WGS 84 stands for World Geodetic Datum (system) 1984.All GPS positions are wrt WGS 84.If chart is not drawn to the same datum, position from GPS has to be reduced to the datum of the chart by the difference known as “datum shift”. This is given on charts under the title “satellite derived positions”. An example is also given showing how to apply datum shift to GPS positions before plotting them on the chart.Eventually all charts will be referenced to WGS 84 datum.But this is a lengthy process and one that can proceed when the relationships between existing surveys and WGS 84 datum have been established.Any differences in positions obtained between GPS and other source should be reported by mariner on Form H102b ( Form for Recording GPS Observation and Corresponding Chart Positions )
Most GPS receivers now have the facility to perform transformation of positions from WGS84 Datum to variety of local horizontal datums.The general parameters used in the software may differ from those used by the Hydrographic Office resulting in the possibility that positions may not agree with the chart, even if the horizontal datum is stated to be the same.It is therefore recommended that GPS be kept referenced to WGS84 datum and apply datum shift indicated on chart.
COLD START: The ability of a GPS Rx to start providing posn without the assistance of any almanac info stored in its memory.
Spheroid: It is a mathematically regular surface resembling a slightly flattened sphere, defined by the length of its axes and used to approximate the “geoid” in geodetic computations.
Geoid: An imaginary surface which is everywhere perpendicular to the plumb line and which on average coincides with MSL in open oceans. Its shape approximates to that of a spheroid but it’s irregular because of uneven distribution of earth’s mass.
Almanac: A set of parameters included in GPS satellite message that is used by receiver to predict the appropriate location satellite.
Pseudo random code: It is a fundamental part of GPS.Physically it is just a very complicated digital code i.e. a complicated sequence of “on” and “off” pulses. The signal is so complicated that it looks like random electrical noise. Hence the name “pseudo random”.
The complexity serves two purposes.
It makes sure that receiver doesn’t accidentally sync up to some other signal. The pattern is so complex that it is unlikely that a stray signal will have exactly the same shape.
Since each satellite has its own PR code this complexity also ensures that receiver won’t accidentally pick up some other satellite’s signal. So all satellites can use same frequency without jamming each other. And it makes it more difficult for a hostile force to jam the system. In fact PR code gives DoD a way to control the system.
Scale of charts: At a scale of 1:25000 plotting precision of 0.2 mm means an accuracy of about 5 meters on ground.
GPS signal consists of pseudo random code, ephemeris and almanac. Each satellite transmits two codes i.e. P (precision) code (gives 10 m accuracy) and C/A (coarse acquisition) code (gives 100 m accuracy). C/A code is for civilian users and P code is reserved for US military and its allies.
These codes are modulated by phase modulation technique on two carrier frequencies:
L1 = 1575.42 MHz (consists of both P and C/A codes)
L2 = 1227.60 MHz (consists of only P code)
The C/A code is made up of 0s and 1s called chip, having a frequency of 1.023 MBits/sec or 1.023 Bits/µ sec or 1023 Bits/msec.
Entire sequence is repeated ever 1 msec. i.e. after every 1023 Bits or every 3x 105 m
Thus each Bit (chip) is given by: 300000 / 1023 = 293 m = 1.023 µ sec
In case of P code the chip frequency is 10.23 MBits/sec, the duration of each Bit becomes 0.1023 µ sec or, in terms of distance 29.3 meters.
In addition each satellite transmits 2 types of orbit data Almanac and Ephemeris. Almanac data contains the health and approximate location of each satellite in the system. A GPS receiver gathers Almanac data from any available satellite; using information from the Almanac, the receiver then determines which set of satellites will give the best geometries for position fix. And then the receiver obtains Ephemeris from those satellites.
The receiver then assesses the transmission time and signal quality from each satellite and multiplies the difference in transmission time by speed of light to arrive at estimated satellite’s distance.
Ephemeris data contains the precise orbital parameters of each satellite.
GPS signal is extremely resistant to conditions that disturb other electronic navigation systems. In general GPS is not affected by:
Weather conditions
Passing ships
Onboard electronic installations
Onshore electronic installations
Onboard engine ignition
Portable radio receivers
INITILISATION OF GPS: Go to set up and feed:
1. Coordinate system for posn display ( UTM: Universal Transverse Mercator, Lat/Long etc. )
2. Elevation/Antenna Height: Select 2D or 3D mode. When in 2D mode, the receiver computes position only and to maximize accuracy in this mode you should enter the elevation/antenna height. In 3D, the receiver computes position and elevation.
3. Lat and Long of the vessel: Enter your approximate position. It will greatly shorten the time to fix the first position otherwise it will take 20 min to give first fix.
4. Time : Select UTC or local time
5. Velocity Averaging: Velocity averaging can be used to minimize the effects of SA on velocity output. The settings are off, 20 s, 60 s.
6. Compass Reference: Select magnetic or True north.
7. Chart Datum: Select the required datum , preferably WGS 84
8. Last Fix Interval: Sets the rate at which positions are automatically saved.Settings are: off, 10, 20, 30 min.
9. Plot Setup: Adjusts the track orientation and track history on the plot screen. For track select 0.1, 0.5, 1.0, 2.0 miles. For orientation choose between Head up and North Up.
10. Alarms: Select any or all of the following alarms: Anchor alarm; Arrival alarm; cross track alarm.
11. Light intensity and contrast settings.
GPS SEGMENTS: The GPS system has 3 segments:
GROUND BASED SEGMENT: It consists of 4 land based monitoring stations (Kwazalein, Hawaii, Ascension Island and Diego Garcia). It also has master control station and upload station located at Colorado Springs in USA. The monitoring stations track the satellites, obtain the data from these satellites and pass the information to master control station. After receiving the data from the monitoring stations, the master control station does the necessary computation to predict the future path and position of all the satellites. The master control station also determines the error of the atomic clocks in all the satellites. The updated data are fed to the upload station, which in turn transmits the same data to each satellite three times a day.
The master control station can selectively degrade satellite data.This degradation is known as Selective Availability and it can cause position errors of 100 meters.
SPACE SEGMENT: It consists of 24 satellites evenly placed in 6 different orbits. They move at the speed of 3.9 km/sec at a height of 20,200 km above earth’s surface. Each orbit is inclined at an angle of 55 deg to the equinoctial and the angle between 2 successive orbits is 60 deg. The satellites are arranged in such a manner that at any time signals can be received from at least 4 satellites having elevation of more than 9.5 degrees.
If any satellite emits erroneous data or is otherwise not operating properly, a ground station marks it “unhealthy”. The affected satellite broadcasts its status to the GPS receiver, which is programmed to ignore an unhealthy satellite and use the next best satellite.
USER SEGMENT: It consists of receiver with built in computer, display unit and antenna. The receiver locks on to one satellite and from this satellite it obtains the almanac of all the other satellites and thereby selects the four most suitable satellites for position fixing. The fix obtained is displayed on the display unit along with other information like COG, SOG and UTC.
Each satellite transmits a navigational message of 30 s in the form of 50 Bits/s data frame. This data, which is different for each satellite is supplied by MCS and is divided into 5 sub-frames. Each sub-frame commences with telemetry word (TLM) containing satellite status followed by hand over word (HOW) data for acquiring P code from C/A code. The 1st sub frame contains data relating to satellite correction. The 2nd and 3rd sub frames contain the satellite ephemeris defining the position of the satellite. The 4th sub frame passes alphanumeric data to the user and will only be used upload station has a need to pass specific message. The 5th sub frame gives the almanac of all other satellites which includes data on satellite health and identity codes.
ERRORS IN GPS POSITIONS: 1. Satellite clock error: this is caused by error in satellite’s clock wrt GPS time. This is monitored by ground based segment and any error in the Satellite clock forms part of the 30s navigation message. Though satellite clocks are highly accurate, they are not perfect. Clock errors can cause positional errors of about 1.5m.
2. Use clock error: It is caused by user clock not being synchronized with the satellite clock. Because of this range measurement will not be correct and is known as pseudo range. This error is eliminated within the receiver by extra satellite measurement (three for 2D and four for 3D positions).Extra measurement is used to find the extra unknown variable i.e. the error between user and satellite clock.
3. GDOP: The lower he value of GDOP, higher the accuracy of the fix obtained. The value of GDOP is indicated on the display. Since the receiver knows he position of all the satellites through 30s navigational message, it is programmed to select the best available satellites considering their elevation and geometry.
4. Multi path Error: This is caused signal coming to receiver after being reflected from other obstructions in addition to reaching the receiver directly. The two signals thus received simultaneously cause distortion of signal and in turn affect the range measurement. This problem is resolved by suitable siting of antenna. Multipath errors are difficult to detect but good quality sophisticated receivers can minimize the problem using intricate signal rejection techniques.
5. Ionospheric and troposphere error: Radio waves transmitted by satellites pass through different layers of atmosphere and space and undergoes change in its velocity which in turn affects the range measurement and hence the fix. Dual frequency receivers are able to receive both the frequencies and calculate the correction and do the compensation within the receiver. Ionosphere [position errors can be about 5m whereas troposphere conditions can cause rerrors up to 1 m.
6. Deviation of satellite from predicted path: The satellites are monitored and their paths predicted by ground based segment. Between two consecutive monitoring there may be small deviations from their respective paths. These deviations hen translate into errors in fixes. These errors can be up to 2.5 m.
DOP: A dimensionless number that takes into account the contribution of relative satellite geometry to errors in position determination. Positions tagged with a higher DOP value generally constitute poorer measurement results than with lower DOP.Thus DOP is a measure of the quality of GPS data received from the satellites. The main factors affecting DOP are the number of satellites being tracked and where these satellites are positioned in the sky. There are 5 types of DOPs:
HDOP: It is a measure of the accuracy of 2 dimensional positions. Higher HDOP values can be caused by satellites at high elevations.
VDOP: It is a measure of how well satellites are arranged to give vertical position .Higher VDOP means less certainty and can be caused if satellites have low elevations.
PDOP: is a measure of overall certainty in GPS position with TDOP not included in the estimated certainty.
PDOP = square root of (HDOP square + VDOP square)
A perfect PDOP implies one satellite directly overhead and 3 satellites positioned at 120 degrees intervals just above the horizon.PDOP less than 4 gives best accuracy. PDOP 4 -8 means acceptable accuracy and PDOP greater than 8 implies poor accuracy.
TDOP: is measure of certainty of how well GPS measures time.
GDOP: is overall certainty of position as well as time measurement i.e. including TDOP as well.
GDOP = square root of ( PDOP square + TDOP square )
The position accuracy = DOP x Measurement Precision
E.g. if DOP is 5 and M.P. is 1 meter, then
P.A. = 5 x 1 = 5 meters
Geodetic Datum: Also called horizontal datum. It is a reference for specifying positions on earth’s surface. Each reference (datum) is associated with a particular reference spheroid. Positions referenced to different datum can differ by several hundred meters.
WGS 84 stands for World Geodetic Datum (system) 1984.All GPS positions are wrt WGS 84.If chart is not drawn to the same datum, position from GPS has to be reduced to the datum of the chart by the difference known as “datum shift”. This is given on charts under the title “satellite derived positions”. An example is also given showing how to apply datum shift to GPS positions before plotting them on the chart.Eventually all charts will be referenced to WGS 84 datum.But this is a lengthy process and one that can proceed when the relationships between existing surveys and WGS 84 datum have been established.Any differences in positions obtained between GPS and other source should be reported by mariner on Form H102b ( Form for Recording GPS Observation and Corresponding Chart Positions )
Most GPS receivers now have the facility to perform transformation of positions from WGS84 Datum to variety of local horizontal datums.The general parameters used in the software may differ from those used by the Hydrographic Office resulting in the possibility that positions may not agree with the chart, even if the horizontal datum is stated to be the same.It is therefore recommended that GPS be kept referenced to WGS84 datum and apply datum shift indicated on chart.
COLD START: The ability of a GPS Rx to start providing posn without the assistance of any almanac info stored in its memory.
Spheroid: It is a mathematically regular surface resembling a slightly flattened sphere, defined by the length of its axes and used to approximate the “geoid” in geodetic computations.
Geoid: An imaginary surface which is everywhere perpendicular to the plumb line and which on average coincides with MSL in open oceans. Its shape approximates to that of a spheroid but it’s irregular because of uneven distribution of earth’s mass.
Almanac: A set of parameters included in GPS satellite message that is used by receiver to predict the appropriate location satellite.
Pseudo random code: It is a fundamental part of GPS.Physically it is just a very complicated digital code i.e. a complicated sequence of “on” and “off” pulses. The signal is so complicated that it looks like random electrical noise. Hence the name “pseudo random”.
The complexity serves two purposes.
It makes sure that receiver doesn’t accidentally sync up to some other signal. The pattern is so complex that it is unlikely that a stray signal will have exactly the same shape.
Since each satellite has its own PR code this complexity also ensures that receiver won’t accidentally pick up some other satellite’s signal. So all satellites can use same frequency without jamming each other. And it makes it more difficult for a hostile force to jam the system. In fact PR code gives DoD a way to control the system.
Scale of charts: At a scale of 1:25000 plotting precision of 0.2 mm means an accuracy of about 5 meters on ground.
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