NAVIGATION AIDS Flashcards
In the operation of a Global Navigation Satellite System what are the three distinct parts (In GPS known as segments) (3)
Space Segment
Control Segment
User Segment
- What is the minimum number of satellites that GPS is designed to operate with? (1)
24
- What is the purpose of monitoring stations? (2)
To track satellites, collect data and feed back to the master control station
- The receiver gives the user a position by calculating its ranges from satellites, these are called pseudo- ranges, why? (2)
Because the calculation includes a time value with an error in it, so the ranges are not accurate
- Explain how the range from a satellite is determined (6)
The satellite transmits a distinct code
At the same time the receiver generates the code internally
The receiver receives the code from the satellite
The time difference between the two codes is measured
The range from the satellite is calculated using this time difference in the .
equation D = Sx T where S is the speed of an electromagnetic wave
- List 4 errors which can affect range measurement (4)
Any of these six:
Ionospheric delay error
Ephemeris (orbit) error
Clock error
Tropospheric delay error
Multipath error
Receiver error
- What must the OOW do when plotting a satellite position on a chart? (4)
Check whether satellite positions can be plotted directly on the chart
If not, then check the note that states the corrections must be applied
Correct the satellite position
Plot corrected position on the chart
- State GPS datum and describe what is meant by datum in this context
WGS84
This is a horizontal datum relating to latitude and longitude. lt can be thought of as a reference point or grid to which GPS positions can be related. This datum is based on a mathematical model to as far as possible resemble the shape of the earth
Define DOP (4)
Dilution of precision: The reliability of the data due to the geometry (azimut and elevation) of satellites in view of the receiver.
It can Horizontal an vertical.
It is measured on a numerical scale 0-10
- State one example of a good DOP value and one example of a poor DOP value (2)
Good: 2
Poor: 8
- Which type of DOP is the OOW most interested in? (2)
HDOP - Horizontal Dilution of Precision
- What does the ‘D’ stand for in DGPS and what two augmentation systems are there? (5)
Differential
Ground based Augmentation System (GBAS)
Satellite based Augmentation System (SBAS)
- Explain how DGPS improves the accuracy of GPS (8)
A base station receives a signal from a GPS satellite.
The station knows its location on the Earth’s surface accurately.
It compares the calculated satellite range with the range calculated from its known position
The difference between the two allows the time error to be worked out
This error is transmitted to users within the area
The user’s receiver then corrects the GPS data to give a more accurate position
Explain, with the aid of a diagram, why a cross-track error alarm should be set when following a GPS route (4)
When following a GPS track the receiver gives the user an update on brg to next waypoint. If the vessel is off track there could be an obstruction between the (2) vessel and the waypoint. Therefore an alarm should be set to alert the user.
- What does LORAN stand for? (3)
Long Range Navigation
- It is a terrestrial hyperbolic navigation system, sketch the lines of hyperbolae between two transmitter (4)
- What do the hyperbolic lines represent about the signals received from the two stations? (3)
Lines of equal time difference between receiving the signals 2
They are Lines of Position 1
- What does ‘e’ in eLORAN stand for? (1)
Enhanced
- State three errors which affect the system (6)
Atmospheric attenuation of ground waves 2
Ionosphere effect (Skywave) 2
Land effect (fixed errors) 2
- Outline the echo-ranging principle (4)
A signal is transmitted
The signal hits an obstruction, sending an echo back The time difference between sending and receiving is measured The Range of the obstruction can be found by D=S xT divided by 2