GPS & AIS and Loran - C Flashcards
List 4 error affecting the range measurements of GPS
Ionosphere
Troposphere
Multipath error
Clock error
receiver error
State the GPS datum and describe what is meant by datum
WGS84
This is the horizontal datum related to latitude and longitude datum relating the the earths surface. It is the grid to which GPS position can relate to. This datum is based on a mathematical model to as far as possible resemble the shape of the earth.
Define DOP
Dilution of precision; The reliability of the date GPS data due to the geometry of the position of the available satellites
State one example of good DOP and one example of Poor DOP
Good : 2
Poor : 8
Explain how DGPS improves the accuracy of the GPS
A base station receives a signal from GPS satellite. The station knows its location on the earths surface accurately. In compares the calculated satellite range with the range calculated from the know position. The difference is the local error and this is then transmitted out to everyone in the area via VHF.
What does D stand for in DGPS and what are the two augmentation systems?
Differential
- Ground based augmentation (GBAS)
- Satellite based augmentation system (SBAS)
What are the three distint parts or segments that make up a GPS system
Space Segment
Control Segment
User Segment
Minimum number of satellites in a GPS system
25
What is the purpose of the a monitoring station?
To track satellites, collect data, feed it back to the master controller and make corrects in position in those area’s to assist with navigation
The receiver calculates a user position by from a number of satellites, these are pseudo ranges why?
Because the calculations includes a time valve with an error in it, so the ranges are not always accurate.
Explain how the range from a satellite is determined
The satellite transmits a distinct code and at the same time the receiver generates an internal code. When the receiver receives the satellites code the time different is calculated to give a distance. For a fix to be generated the unit needs 4 different ranges from four different satellites.
Describe how a multipath error may affect the position shown on a GPS unit.
Multipath errors occurs when the GPS satellite signal bounces off nearby structures like buildings and mountains before reaching the GPS receiver. These reflected signals travel a longer distance than the direct line of sight, which can cause the GPS unit to miscalculate its position.
Describe how a GPS receiver calculates its position from a satellite
There are 25 orbiting satellites that make up the GPS system. They use a signal which is imbedded with a time stamp to compare with time produced on board by the receiver unit. The two signals are compared to calculate a distance. To get a position on the surface of the earth the receiver needs 4 satellites signals to triangulate the vessels position.
What is the datum used by Navstar
Navstar Global Positioning system (GPS) uses WGS84 World Geodetic System 1984.
What does SOTDMA stand for?
Self Organised Time Division Multiple Access.
It’s a technology used in the Automatic dentification System (AIS), primarily for Class A units, to reserve their space in the AIS slot map.
List four piece of static data that may be transmitted by an AIS unit
MMSI number
IMO number
Ships name
Type of ship
List four pieces of dynamic data that may be transmitted by an AIS unit
GPS position
Course over ground
Speed over ground
Rate of turn
Discuss how AIS can potentially be considered in ‘all available means’ in terms of rule 5 and rule 7 of the IRPCS
AIS can be a valuable tool in fulfilling the requirements of both Rule 5 keeping a good look out and Rule 7 risk of collision of the IRPCS, as it provides real-time, dynamic information about the vessel’s own state and that of other vessels in the vicinity. However, it’s important to note that AIS should not be used in isolation, but as part of a comprehensive suite of navigational tools including radar and visual bearings.
Define the term E-Loran
E-Loran, short for Enhanced Long-Range Navigation. It is a terrestrial-based Positioning, Navigation, and Timing (PNT) system.
List five potential sources of error in a hyperbolic navigation system such as loran C.
Propagation errors
Sky wave effect
Atmospheric refraction
Geometry of intersecting hyperboloids.
Timing errors
With a simple diagram describe how a hyperbolic navigation system works.
A hyperbolic radio system on LoranC, allows a receiver (ship) to determine its position by listening to radio signals transmitted by fixed land based and additional radio beacons. Transmitters are in chains of 3 – 5 stations with one Master and 2 – 4 slave stations. The receiver can identify a line of position from the time difference between two or more stations. With three stations, two lines of position can be drawn. At their intersect you will get a fix.
How does E-Loran differ from Loran-C
- E-Loran is synchronized to UTC
- Is more accurate for high speeds vessels
- It can work in any type of water while loran C must be calibrated for water conductivity.
- It can also recived additional information including DGPS corrections and navigations messages.
What do the hyperbolic lines represent about the signals received from the two stations?
Lines of equal time difference between receiving the signals. They represent lines of position.
State the three types of data transmitted on AIS
- Static
- Dynamic
- Voyage related
How frequently is each of the above transmitted?
Static
Dynamic
Voyage related
- Every 6 minutes or on request
- Dependent on ship speed and whether altering course
- Every 6 minutes or on request
List two things that are safety related that are NOT to be sent on AIS
- Distress
- Personal messages
State three advantages of AIS over Radar
- It can see round corners
- It gives immediate indication of vessel altering course
- Targets do not get lost in clutter
Why should the OOW not over rely on AIS information for collision avoidance?
Information is provided from another vessel’s instruments, which we cannot checked. CPA and TCPA is from a different source and could be incorrect. It should be used in conjuction with radar and visual observations to confirm information is correct.
What 3 modes of operation should AIS be able to work in?
Autonomous and Continuous
Assigned
Polled
Define H-DOP
H-DOP stands for Horizontal Dilution of Precision. It is a measure used in GPS (Global Positioning System) navigation to quantify the effect of satellite geometry on the accuracy of horizontal position determination. H-DOP values indicate how accurately GPS satellites are positioned relative to each other in the sky, with lower values representing better geometric conditions and thus higher position accuracy. A lower H-DOP value indicates better accuracy, while higher values suggest poorer accuracy due to unfavorable satellite geometry.
