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Are the GPS satellites fitted with any accurate clocks?
回答
GLONASS is an alternative to what system?
Navstar GPS.
Receiver for Navstar GPS.
e-Loran.
Satellite Tracking Stations.
Global Navigation Satellite Systems (GNSS) provides three services known collectively as PNT, what are they?
Position, Navigation and Timing.
Position, Navigation Data and Time.
Precision, Navigation and Time.
Position, Nautical and Terrestrial.
How can the adverse effects of the ionosphere and troposphere be minimized?
The effects of the ionosphere and troposphere can be reduced by not using the data from satellites below a certain altitude.
There is no method available to reduce the effects of the ionosphere and troposphere.
The effects of the ionosphere and troposphere can be reduced by not using the pseudo random signals from the satellites.
The effects of the ionosphere and troposphere can be reduced by only using bearings of the satellites and not ranges to fix the ship’s position.
How does the Differential GPS improve the accuracy of the output position?
The position from the Navstar GPS produced by the receiver is compared with a e-Loran signal to improve the accuracy of the ship’s position.
The two satellite signals L1 and L2 are received by the ship and the difference between the two signals produces a correction factor to the position output.
A combination of signals from the Navstar GPS satellites are sent to the ship’s receiver and compared to provide a more accurate position output.
An error of position message is provided by a differential reference station and sent to the ship.
How does the Navstar GPS receiver finds a position using the calculated distances from satellites?
The latitude and longitude are provided by calculating the frequency differences of the signals from the satellites.
The intersection of bearing lines from the satellites.
The intersection of circles of position with the satellites at their centres.
The intersection of spheres of position with the satellites at their centres.
In the Navstar GPS system there are normally 24 active satellites, how many different carrier frequencies are used for transmissions by all of the GPS satellites?
Each satellite uses 2 carrier frequencies.
There is a different frequency used by each satellite therefore 24 carrier frequencies used in total.
The same single carrier frequency is used by all satellites.
Each group of satellites uses the same frequency, therefore 6 carrier frequencies used in total.
In the Navstar GPS system there are normally 24 active satellites, how many different carrier frequencies are used for transmissions by all of the GPS satellites?
Each satellite uses 2 carrier frequencies.
There is a different frequency used by each satellite therefore 24 carrier frequencies used in total.
The same single carrier frequency is used by all satellites.
Each group of satellites uses the same frequency, therefore 6 carrier frequencies used in total.
In the Navstar GPS system, satellites orbit the earth at an altitude of approximately what distance?
5 000 km.
10 000 km.
15 000 km.
20 200 km.
In the Navstar GPS system, satellites orbit the earth in approximately:
3 hours.
6 hours.
12 hours.
24 hours.
It is commonly recognised, that the Navstar GPS system consists of three main segments. What are these segments?
Space, Control and User segment.
Latitude, Longitude and Time.
Satellite, Signal and Receiver.
Position, course and speed.
It is recognized, that Global Navigation Satellite Systems (GNSS) transmit signals, that can be used to provide three different services. What are these?
Position, Navigation and Timing.
Latitude, Longitude and Altitude.
Position, HDOP and Datum.
Position, course and speed.
The Navstar GPS provides two levels of service. What are they?
Standard SPS and Precise PPS.
Special SPP and Standard SSP.
Continuous CPI and Incremental (IPP).
Navigation and Time.
What actions should be taken by the Officer of the Watch when the Navstar GPS receiver is set to provide an output position of latitude and longitude using the datum of WGS 84, which is different to the datum on the paper chart?
Apply the corrections marked on the chart before plotting the latitude and longitude.
Change the datum used by the GPS receiver to the same datum as used on the chart.
There will be very little difference between the two datum’s and it can be ignored.
Plot the output latitude and longitude and insert warning note on the chart of possible inaccuracies.
What are the dangers of the Officer of the Watch being totally reliant on the Navstar GPS?
It is always dangerous to be totally reliant on one navigational system, because no single navigational system is 100 % reliable.
It is always dangerous to be totally reliant on one navigational system, because no system is accurate enough.
It is only dangerous because that system receiver may break down and be unavailable.
It is always dangerous to be totally reliant on the Navstar GPS, because it has periods when satellites are not visible to the ship and can’t provide a position.
What datum is used by the HO approved vector charts, as used by the Navstar GPS system?
WGS84.
OSGB.
MHWS.
PZ-90.
What geographical factors can affect the accuracy of the Differential GPS produced position?
Distance between ship and Reference Station.
The strength of the satellite signal.
The strength of the signal from the reference station.
The height of the DGPS receiver antennae.
What is a “Multipath Error” when related to GNSS?
A multipath error is when the satellite signal is reflected off something before arriving at the receiver.
A multipath error is when the satellite signal is received by the receiver as a second trace return.
A multipath error is when the acquired position from the GNSS shows the vessel is off the planned track.
A multipath error is when the configuration of the satellites is unsuitable to obtain an accurate position.
What is meant by the abbreviation RAIM?
Receiver Autonomous Integrity Monitoring.
Route Assisted Instruction Manual.
Receiver Aided Information Methods.
Regulation for the Area Information (Marine).
What is the affects of selective availability being switched off?
A greater need for Differential GPS.
More accurate positions.
Less accurate positions.
Less satellites available for receiving signals.
What is the approximate strength of the GPS transmitted signal?
100 watts.
500 watts.
1 000 watts.
5 000 watts.
What is the difference between the speed of a radio signal and the speed of light?
Nothing.
The speed of light is faster than the speed of a radio signal.
The speed of light is slower that the speed of a radio signal.
There is no direct comparison and it varies.
What is the nearest approximation of the shape of the earth?
Ellipsoid with diameter at the poles greater, than at the equator.
Ellipse.
Sphere.
Spheroid.
What is the pseudo-random code with reference to the Navstar GPS?
The complicated digital code sent from the satellite and mirrored by the GPS receiver.
The code sent by the tracking stations to the satellites.
The complicated code sent by the satellite, which is decoded by the GPS receiver to measure the range.
The complicated code, which is transmitted at random intervals by the satellite.
What is the reason for “clock error” with reference to the Navstar GPS system?
The difference between the time at the satellite clock and the time at the receiver clock.
The delay in the signal from leaving the satellite to reaching the receiver.
The error due to the interference of the ionosphere.
The difference of the receiver time and the Universal Standard Time.
What is understood by the abbreviation WAAS?
Wide Area Accuracy System.
World Accuracy Assistance System.
Wide Area Augmentation System.
Wide Augmentation Accuracy System.
What is “Jamming” when referred to GNSS?
Jamming is the interference of one satellite signal to another.
Jamming is the atmospheric interference of the satellite signals.
Jamming is the intentional blocking of the satellite signals.
Jamming is the problem associated with two satellite signals arriving at the receiver at the same time.
What output figure of HDOP indicates good accuracy?
Lower than 4.
Above 8.
Only when the figure is Zero.
The HDOP output is not related to accuracy.
What types of Global Navigation Satellite Systems (GNSS) receivers are used at sea for navigational purposes?
Navstar GPS, GLONASS, and combined Navstar GPS and GLONASS, while other GNSS are under development.
Only Navstar GPS.
Only the Global Positioning System.
Combined Navstar GPS and Decca navigator receivers.
When a Navstar GPS receiver loses the satellite signals, it reverts to what mode of operation?
DR mode.
2 dimensional mode.
3 dimensional mode.
None, it ceases to provide any position information.
When considering GPS what is the main function of RAIM?
To assess the integrity of the GPS signals.
To improve the DGPS position accuracy.
To select the satellites with best configuration.
To calculate ships COG and SOG.
When considering GPS, a 3D position has three unknown parameters: latitude, longitude and altitude. Why are signals from four satellites required to find an accurate 3D position?
Because of the ionospheric delay.
Because of the receiver clock error.
Because of the ephemeris error.
Signals from four satellites are not required, three is enough since there are only three unknown parameters.
When considering GPS, how does the elevation mask angle influence the number of available satellites for position fixing?
The lower the elevation mask angle is set, the fewer satellites are available.
Normally, the higher the elevation mask angle is set, the fewer satellites are available.
The higher the elevation mask angle is set, the more satellites are available.
There is no difference in the number of satellites available when the elevation mask angle is changed.
When considering GPS, what is the result of receiving multipath signals?
System accuracy increases.
System accuracy stabilizes.
System accuracy decreases.
Signals are never deflected into multipath signals with GPS.
When considering Navstar GPS receiver, what is the minimum number of satellites, that must be available to calculate a 3 Dimensional (latitude, longitude and altitude) position?
2.
3.
4.
5.
When considering Navstar GPS, what is the meaning of pseudo-range?
Position of a satellite.
Relative position between two satellites.
Relative distance between satellites.
Initial calculated distance between the satellite and the receiver.
When considering Navstar GPS, what is the name of the common error, affecting the orbital satellite position information?
Fluxgate error.
Ephemeris error.
Kalman error.
Ionosphere error.
When considering Navstar GPS, what is the safest operating mode to select?
WGS84.
Three dimensional.
Two dimensional.
DR mode.
When considering Navstar GPS, which DOP is most important for a maritime user when checking GPS accuracy?
HDOP.
PDOP.
TDOP.
VDOP.
When considering Navstar GPS, which coordinate system (datum) is recommended to be used at all time?
WGS-84.
ED-50.
NAD-27.
NAD-83.
When considering a Navstar GPS receiver the output course and speed, provided by the receiver, provides what information?
The course and speed over the ground.
The course steered and the log speed.
The course and speed through the water.
The gyro course and the Doppler speed.
When considering a Navstar GPS receiver, what control should be operated when a member of the crew falls over the side of the ship?
The “Man overboard” button.
The Officer of the Watch should record the Latitude and Longitude position given on the GPS display as soon as possible.
There is no special controls on the GPS available for this type of incident.
The Officer of the Watch should sound four long blasts on the whistle as soon as possible.
When considering a Navstar GPS receiver, what is the minimum number of satellites, that must be available to calculate a 2 Dimensional (latitude and longitude) position?
3.
2.
4.
5.
When considering a basic GPS, which of the following statements is true?
If own ship speed is low, the GPS calculated speed over the ground is of high accuracy.
If own ship speed is low, the GPS calculated speed over the ground is of low accuracy.
If own ship speed is high, the GPS calculated speed over the ground is of low accuracy.
If own ship speed is low, the GPS calculated speed over the ground is not calculated.
Why are the calculated ranges known as “pseudo-ranges”?
They are referred to as pseudo-ranges, because they are based on an estimate of time difference.
They are referred to as pseudo-ranges, because they are not actually a measurement of range.
They are referred to as pseudo-ranges, because they are based on the transmitted pseudo-random codes.
They are referred to as pseudo-ranges, because they are never correct.
Why is it considered dangerous to totally rely on the Navstar GPS for obtaining positions?
Because it is not 100 % reliable.
It is not dangerous to totally rely on GPS.
Because there are other methods, that can be used for finding positions.
Because the GPS is now being degraded by Selective Availability.
With reference the Navstar GPS outputs, what does a low HDOP figure indicate?
A low HDOP figure indicates a probable accurate position, provided by broad angles of intersections between the position lines from the satellites in use.
A low HDOP figure indicates a poor inaccurate position, because of small angles of intersections between the position lines from the satellites in use.
A low HDOP figure indicates, that there are only a few satellites visible and suitable for use.
A low HDOP figure indicates, that there is a Clock error discrepancy, which needs to be resolved before accurate positions will be obtained.
With reference to Navstar GPS system used in conjunction with a Differential Reference Station, on what is the accuracy of the resultant position mainly dependant?
The distance between the observer and the reference station.
The relative angle between the satellites.
The number of satellites available.
The price of the equipment.
With reference to Navstar GPS system, what happens to the GPS radio signal speed when it passes through ionosphere?
The speed increases.
The speed decreases.
The speed remains unchanged.
It only passes through the ionosphere when the angle of elevation of the satellite is small.
With reference to Navstar GPS, how many ground based control stations are used to monitor the satellites?
The Master control station and 4 regional control stations.
Three control stations.
The Master Control Station in USA and six regional tracking stations.
There is only the Tracking Station in USA, which tracks all satellites every 24 hours during which times they will all be visible to that location.
With reference to Navstar GPS, why is the initial calculated range of the satellite call “pseudo-range”?
The initial range is calculated uses an assumption to make the calculation.
The range calculated is not the information that is required for a position, but it is used to determine the course and speed of the ship.
The range is called pseudo-range to ensure it is not confused with the radar range.
The pseudo-range is only one of several ranges from a satellite required to plot a position of latitude and longitude.
With reference to a standard ships Navstar GPS receiver, how would the probable accuracy of the output position be indicated?
HDOP.
Probability in metres.
Probability in cables.
Probability in decimals of a nautical mile.
With reference to the Navstar GPS system, how are the inaccuracies of position due to “clock error” removed?
By making use of the signal from one more satellite than the minimum required.
By correcting the receiver clock to the same time as the satellite clocks.
By correcting the satellite clock to the same time as the receiver clocks.
By applying a correction to the acquired position.
With reference to the Navstar GPS system, which statement is true?
GPS provides good global coverage.
GPS provides good coverage between latitudes 55° N and 55° S.
The coverage provided by GPS is exactly the same anywhere on earth.
GPS provides good coverage on the northern hemisphere but poor on the southern hemisphere.
With reference to the Navstar GPS, what are the effects on the ships position, resulting from an incident of “spoofing”?
Spoofing can deliberately produce a feasible, but incorrect position.
Spoofing may result in the GPS not being able to provide any form of position.
Spoofing may increase the amount of ionospheric interference and reduce the accuracy of the GPS position.
Spoofing is the deliberate downgrading of the Navstar GPS signals, resulting in less accurate positions.
With reference to the Navstar GPS, what is the result of the height of the GPS antennae being incorrectly entered into the GPS receiver?
When in the 2 dimensional mode of operation the GPS position may be less accurate.
The antennae height input will never make any difference to the GPS positional output.
The antennae height being incorrectly entered will only affect the GPS output of altitude.
When in the 3 dimensional mode of operation the GPS position of latitude and longitude may be less accurate.
With reference to the Navstar GPS, what is the significance of the “pseudo-random code” of the satellite transmissions?
The signal is so complicated, that it almost looks like random electrical noise.
The transmitted signal from the satellite changes content randomly to avoid being duplicated.
The transmitted signal from the satellite changes frequency randomly to reduce the ionospheric interference.
The satellite send a false signal prior to the correct signal to reduce the possibility of spoofing.
With reference to the Navstar GPS, what is “selective availability” and does it affect the GPS position accuracy at this time (2011)?
Selective Availability is when the GPS accuracy is intentionally degraded to a maximum of 100 meters, 95 % of the time. It ceased to be operational in the year 2000.
Selective Availability is the automatic system where the GPS receiver selects the available satellites which are above the horizon and is continuously operational.
Selective Availability is the method used by a combined GPS/GLONASS receiver to select which satellites to use to obtain a position and is continuously operational.
With the Navstar GPS system, how many monitoring stations are there?
2.
3.
4.
5.
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