Q) What is Clock-Bias Error? (July-18, May-16)
Ans:- SATELLITE CLOCK BIAS ERROR:- Very small discrepancies in the accuracy
within atomic clocks of the satellite can result in travel time measurement errors
causing a degradation of about 1.5m in the final calculated position. For radio waves,
time difference of 1μs (microsecond) equals 300m in terms of distance.
Q) What are various segments of GPS? (Jan-19, Jan-18, Jan-17, Aug-16) OR
Q) Explain briefly: Why shipboard GPS receiver needs to track minimum of
three satellites for position? (Jan-20, March-16) OR
Q) With respect to GPS write short notes on: Space segment & ground
control segment. (July-18)
Q) Explain how the GPS is obtaining the position. (July-19, March-18)
Ans:- GPS or Global Positioning System is a satellite navigation system that
furnishes location and time information in all climate conditions to the user. GPS is
used for navigation in planes, ships, cars and trucks also. The system gives critical
abilities to military and civilian users around the globe. GPS provides continuous real
time, 3-dimensional positioning, navigation and timing worldwide.
GPS System Working:-
The GPS system consists of three segments:
1) The space segment: the GPS satellites
2) The control system, operated by the U.S. military,
3) The user segment, which includes both military and civilian users and their GPS
equipment.
Space Segment:
The space segment is the number of satellites in the constellation. It comprises of
29 satellites circling the earth every 12 hours at 12,000 miles in altitude.
The function of the space segment is utilized to route/navigation signals and to
store and retransmit the route/navigation message sent by the control segment.
These transmissions are controlled by highly stable atomic clocks on the satellites.
The GPS Space Segment is formed by a satellite constellation with enough
satellites to ensure that the users will have, at least, 4 simultaneous satellites in
view from any point at the Earth surface at any time.
GPS
GPS Control Segment:
The control segment comprises of a master control station and five monitor
stations outfitted with atomic clocks that are spread around the globe.
The five monitor stations monitor the GPS satellite signals and then send that
qualified information to the master control station where abnormalities are revised
and sent back to the GPS satellites through ground antennas. Control segment
also referred as monitor station.
User Segment:
The user segment comprises of the GPS receiver, which receives the signals from
the GPS satellites and determine how far away it is from each satellite.
Mainly this segment is used for the U.S military, missile guidance systems, civilian
applications for GPS in almost every field.
Most of the civilian uses this from survey to transportation to natural resources
and from there to agriculture purpose and mapping too.
User segment
How GPS Determines a Position:
The working/operation of Global positioning system is based on the „trilateration‟
mathematical principle.
The position is determined from the distance measurements to satellites. From the
figure, the four satellites are used to determine the position of the receiver on the
earth.
The target location is confirmed by the 4th satellite. And three satellites are used
to trace the location place.
A fourth satellite is used to confirm the target location of each of those space
vehicles. Global positioning system consists of satellite, control station and monitor
station and receiver.
The GPS receiver takes the information from the satellite and uses the method of
triangulation to determine a user‟s exact position.
A fourth satellite is used to confirm the target location of each of those space
vehicles. Global positioning system consists of satellite, control station and monitor
station and receiver.
The GPS receiver takes the information from the satellite and uses the method of
triangulation to determine a user‟s exact position.
GPS Circuit:-
GPS is used on some incidents in several ways, such as:
To determine position locations; for example, you need to radio a helicopter pilot
the coordinates of your position location so the pilot can pick you up.
To navigate from one location to another; for example, you need to travel from a
lookout to the fire perimeter.
To create digitized maps; for example, you are assigned to plot the fire perimeter
and hot spots.
To determine distance between two different points.
Q) Explain how a GPS receiver determines the ship‟s position. (Jan-20, March-
18, Jan-18, Nov-17, July-17, Aug-16)
Ans:- Position Fixing:
The receiver locks on to one satellite, and from this satellite it obtains the almanac
of all the other satellites, and thereby selects the most suitable satellites for
position fixing.
The position obtained by the receiver is basically by determining the distances
from the receiver to each of the selected satellites.
The range measurement is achieved by measuring the propagation time from the
selected satellite to the receiver.
Not possible to precisely synchronize satellite and receiver clock hence the pseudo
ranges are obtained.
Hence an additional satellite is used to obtain the true ranges.
Equation for position fixing:-
R = C X (t-t2)
Range (R) of the satellite to the user.
Where C is the velocity of the radio waves and
(t-t2) = is the time difference (time taken for satellite signals to reach receiver.)
The satellite clock & the GPS clock may not be perfectly synchronized so this gives
rise to an error in range measurement and the obtained is termed as pseudo range.
Hence, there are four unknowns i.e. latitudes, longitude, altitude (x, y, z coordinates)
of the user as well as the user‟s clock error with respect to satellite clock.
The position of the satellite S1 (x1, y1 , z1) is known to the user by the 30 sec
navigational message and from this satellite the following equation is obtained:-
Where PR1 is the pseudo rage from satellite S1.
(C x Δt) is error in range measurement due to the error in the user‟s clock.
Since there are four unknown, they can be resolved from four equations obtained
from four different satellites, the other 3 equations will be following:
Where PR1 is the pseudo rage from satellite S1.
(C x Δt) is error in range measurement due to the error in the user‟s clock.
Since there are four unknown, they can be resolved from four equations obtained
from four different satellites, the other 3 equations will be following:
Q) With respect to GPS briefly explain the following: Precise Positioning
Service (Jan-19, May-17)
Ans:- Precise Positioning Service (PPS):-
Authorised users have access to Precise Positioning Service.
The L1 frequency, transmitted by all Navstar satellites, contains a
course/acquisition (C/A) code ranging signal, with a navigation data message, that
is available for peaceful civil, commercial, and scientific use; and a precision (P)
code ranging signal with a navigation data message, that is reserved for
authorized use.
PPS predictably is 30 meters.
Q) With respect to GPS briefly explain the following: Standard Positioning
Service (Jan-19, May-17)
Ans:- Standard Positioning Service (SPS):-
Civil Users worldwide use SPS.
The L1 frequency, transmitted by all satellites, contains a coarse/ acquisition (C/A)
code ranging signal, with a navigation data message, that is available for peaceful
civil, commercial, and scientific use.
SPS predictability Accuracy: within 35 meters.
It is subject to selective availability, intentional down gradation of accuracy.
Now it has been announced by US Govt, that intentional down gradation will not
be done.
Q) What are the errors possible in GPS? (July-19, July-17, May-17, Nov-16,
March-16) OR
Q) With respect to GPS. Explain the following: What factors affect the
accuracy of GPS positions. (Jan-17) OR
Q) Explain: Pseudo Range (March-18, Nov-18)
Ans:- ERRORS OF GPS:-
1) Atmospheric Error: Changing atmospheric conditions change the speed of the GPS
signals as they pass through the Earth's atmosphere and this affects the time
difference measurement and the fix will not be accurate.
Each satellite transmits its message on two frequencies and hence a dual
frequency receiver receives both the frequencies and correction is calculated and
compensated within the receiver thus increasing the accuracy of the fix.
a. Effect is minimized when the satellite is directly overhead.
b. Becomes greater for satellites nearer the horizon. The receiver is designed to
reject satellites with elevation less than 9.5 degrees.
2) User Clock Error: If the user clock is not perfectly synchronised with the satellite
clock, the range measurement will not be accurate. The range measurement along
with the clock error is called pseudo range. This error can be eliminated within
the receiver by obtaining pseudo range from three satellites and is done
automatically within the receiver.
3) Satellite Clock Error: This error is caused due to the error in the satellite‟s clock
w.r.t. GPS time. This is monitored by the ground based segments and any error in
the satellites clock forms part of the 30 seconds navigational message.
4) GDOP Error: The GDOP of a satellite determines the angle of cut which in turn
governs the quality of the position obtained. Wider the angular separation between
the satellites, better the accuracy of the fix. Or, conversely said, the lower the
GDOP value, the greater the accuracy of the fix. The GDOP value is indicated on
the display unit.
5) Multipath Error: This error is caused by the satellite signals arriving at the ship‟s
antenna both directly from the satellite and those that get reflected by some
objects. Thus two signals are received simultaneously which will cause the
distortion of signal from which range measurement is obtained. Siting the antenna
at a suitable place can minimize this error.
6) Orbital Error: The satellites are monitored and their paths are predicted by the
ground based segment. However, between two consecutive monitoring of the
same satellite, there may be minor drifts from their predicted paths resulting in
small position inaccuracy.
Q) With respect to GPS explain the following: Contents of navigation
message (Sept-19, Sept-17, July-16, Jan-16) OR
Q) Describe the contents of navigation message and structure of the C/A
Code transmitted by the GPS satellites. (May-19) OR
What message is contained in each sub-frame of the GPS Navigational
Message (March-17)
Ans:- Navigation Message: Essential purpose of the navigation message transmission
by satellites is to determine its position by the GPS receiver. Each satellite transmits
a navigational message of 30 seconds in the form of 50 bps data frame. This data,
which is different for each satellite, is previously supplied to the satellites by master
control station 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 sub-frames are:
The 1st sub-frame contains data relating to satellite clock correction.
The 2nd and 3rd sub-frames contain the satellite ephemeris defining the position
of the satellite.
The 4th sub-frame passes the alpha-numeric data to the user and will only be used
when upload station has a need to pass specific messages.
The 5th sub-frame gives the almanac of all the other satellites which includes the
identity codes thus allowing the user the best choice of satellites for position fixing.
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Q) With respect to GPS briefly explain the following: Geometric Dilution of
precession (May-17) OR
Q) With respect to GPS explain the following: Various DOP‟s used (Sept-17,
Jan-16) OR
Q) Explain Geometric Dilution of Precision (GDOP) (Sept-19, Nov-18, Sept-18)
Ans:- Explanation:-
Dilution of precision (DOP), or geometric dilution of precision (GDOP), is a term
used in satellite navigation and geomatics engineering to specify the additional
multiplicative effect of navigation satellite geometry on positional measurement
precision.
DOP can be expressed as a number of separate measurements:
o HDOP – horizontal dilution of precision
o VDOP – vertical dilution of precision
o PDOP – position (3D) dilution of precision
o TDOP – time dilution of precision
These values follow mathematically from the positions of the usable satellites.
Signal receivers allow the display of these positions (skyplot) as well as the DOP
values.
The term can also be applied to other location systems that employ several
geographical spaced sites. It can occur in electronic-counter-counter-measures
(electronic warfare) when computing the location of enemy emitters (radar
jammers and radio communications devices). Using such an interferometry
technique can provide certain geometric layout where there are degrees of
freedom that cannot be accounted for due to inadequate configurations.
The effect of geometry of the satellites on position error is called geometric
dilution of precision and it is roughly interpreted as ratio of position error to the
range error. Imagine that a square pyramid is formed by lines joining four
satellites with the receiver at the tip of the pyramid. The larger the volume of the
pyramid, the better (lower) the value of GDOP; the smaller its volume, the worse
(higher) the value of GDOP will be. Similarly, the greater the number of satellites,
the better the value of GDOP.
Q) With respect to GPS, write in brief about: HDOP (March-19)
Ans:- HDOP: Acronym for horizontal dilution of precision. A measure of the geometric
quality of a GPS satellite configuration in the sky. HDOP is a factor in determining the
relative accuracy of a horizontal position. The smaller the DOP number, the better the
geometry.
Q) Describe how a GPS receiver determines the speed of the ship. (Sept-18,
Nov-16)
Ans:- SPEED DETERMINATION
The carrier frequency is also used to determine the speed of the user by the
measurement of Doppler shift, i.e. change in the frequency of radio waves
received when the distance between the satellite and user is changing due to the
relative motion between the two.
The position and velocity of the satellite as well as the position of the user are
known to the user‟s receiver.
The velocity vector of the satellite can be resolved in two ways:
o In the direction towards the user
o In the direction perpendicular to (i).
The 2nd component is not considered as speed in this direction will not cause
Doppler shift.
The receiver calculates the velocity vector of the satellite in the direction towards
the user.
If the relative approach speed between the satellite and the user‟s speed (based
on the Doppler shift measurement) is not equal to the satellite speed vector
towards the user; the difference can only arise due to user‟s speed towards or
away from the satellite.
Similarly with the help of the other two satellites, the receiver can calculate two
additional speed vectors and these speed vectors will be towards or away fromtheir respective satellites.
These velocity vectors are resolved into three other vectors, i.e. x, y and z coordinates
and with these three vectors the course and speed of the user is
calculated.
Q) With respect to GPS explain the following: System configuration and
frequencies used (Sept-17, Jan-16)
With respect to GPS, explain the following: P & C/A code (Sept-19, Sept-18,
July-16)
Ans:- Codes:- Each satellite transmits two codes:-
P Code (Precession Code) that is only available to US military and its allies.
C/A Code (Coarse Acquisition Code) available for use to all civilian users.
Frequencies:- 1575.42 MHz (L1 signal) and 1227.6 MHz (L2 signal).
The L1 carrier consists of both the C/A and P codes, while the L2 carrier
consists the P code only.
Each satellite transmits pseudo random noise signals on these two different
frequencies.
Function of these codes is as follows:
For satellite identification since each satellite has a unique code.
For measurement of the propagation time from the satellite to user.
The C/A code:-
The C/A code is different for every satellite.
The C/A code is made up of sequences called chips.
Sequence repeats itself every millisecond.
The C/A code is for the civilians.
P code:-
The full code length is of 267 days.
The extremely long code length makes it difficult to lock on to the P code.
P code is available only for US & allies.
P code is different for every satellite.
Q) Explain briefly: Alarms of GPS (March-16)
Ans:- Alarms of GPS:- There are seven alarm conditions which generate both audible
and visual alarms. When an alarm setting is violated, the buzzer sounds and the
name of the offending alarm appears on the display. The alarm icon also
appears on the Plotter 1, Plotter 2 and Highway displays.
1) Arrival Alarm, Anchor Watch Alarm:-
a. Arrival alarm:- The arrival alarm informs you that own ship is approaching a
destination waypoint. The area that defines an arrival zone is that of a circle
which you approach from the outside of the circle.
b. Anchor watch alarm:- The anchor watch alarm sounds to warn you that own
ship is moving when it should be at rest.
2) Cross Track Error (XTE) Alarm :- The XTE alarm warns you when own ship is off its
intended course.
3) Ship‟s Speed Alarm:- The ship‟s speed alarm sounds when ship's speed is lower or
higher (or within) the alarm range set.
4) Trip Alarm:- The trip alarm sounds when the distance run is greater than the trip
alarm setting.
5) Water Temperature Alarm:- The water temperature alarm sounds when the water
temperature is higher or lower (or within) the preset temperature. This alarm
requires temperature signal from external equipment.
6) Depth Alarm:- The depth temperature alarm sounds when the depth is higher or
lower (or within) the preset depth. This alarm requires video sounder connection.
7) WAAS/DGPS Alarm:- The WAAS/DGPS alarm sounds when the WAAS/DGPS signal
is lost. This alarm may be enabled or disabled as below.
Q) Explain the functioning of DGPS (March-19, July-17) OR
Explain the working principle of DGPS and its limitations. (July-16) OR
Explain briefly: Working principle of DGPS (March-16) OR
Q) Explain the errors of the GPS system. (May-19)
Ans:- Differential GPS (DGPS) is a system in which differences between observed and
computed co-ordinates ranges (known as differential corrections) at a particular
known point are transmitted to users (GPS receivers at other points) to upgrade the
accuracy of the users receivers position.
Differential Correction:- Differential correction is a technique that greatly increases
the accuracy of the collected DGPS data. It involves using a receiver at a known
location - the "base station“ and comparing that data with DGPS positions collected
from unknown locations with "roving receivers."
Limitation & Errors of DGPS:-
a) International Limitation of Accuracy
b) Receiver Independent Exchange Format
c) Reference System Co-ordinates
Methods used to Transmit Corrections:-
Computing & transmitting – a position correction in terms of Lat, Long & altitude
i.e. x, y, z co-ordinates.
Computation of pseudo range correction to each satellite which is then
broadcasted to the user and applied to the user‟s pseudo range measurement
before the position is calculated by the onboard receiver resulting in a higher
accuracy of position fix.
DGPS removes common-mode errors, those errors common to both the reference
and remove receivers (not multipath or receiver noise). Errors are more often
common when receivers are close together (less than 100 km). Differential
position accuracies of 1-10 meters are possible with DGPS based on C/A code SPS
Signal.
Q) How the DGPS calculate even more accurate position than the GPS? (Nov-
17)
Q) Explain how DGPS enhances the accuracy of a GPS receiver. (Sept-18,
March-18)
Ans:- Explanation:-
Differential Global Positioning System (DGPS) is an enhancement to Global
Positioning System that provides improved location accuracy, from the 15-meter
nominal GPS accuracy to about 10 cm in case of the best implementations.
DGPS uses a network of fixed ground-based reference stations to broadcast the
difference between the positions indicated by the GPS satellite systems and the
known fixed positions.
These stations broadcast the difference between the measured satellite pseudo
ranges and actual (internally computed) pseudo ranges, and receiver stations may
correct their pseudo ranges by the same amount.
The digital correction signal is typically broadcast locally over ground-based
transmitters of shorter range.
Q) What do you understand by the term „chart datum‟? Which default datum
is used in GPS? Can a fix obtained from the GPS receiver be plotted directly
onto a navigational chart? (March-20)
Ans:- Chart Datums:- Chart Datum (CD) is defined simply in the Glossary as the level
below which soundings are given on Admiralty charts. CDs used for earlier surveys
were based on arbitrary low water levels of various kinds.
Modern Admiralty surveys use as CD a level as close as possible to Lowest
Astronomical Tide (LAT), which is the lowest predictable tide under average
meteorological conditions. This is to conform to an IHO Technical Resolution which
states that CD should be set at a level so low that the tide will not frequently fall
below it.
The actual levels of LAT for Standard Ports are listed in Admiralty Tide Tables.
On larger scale charts, abbreviated details showing the connection between chart
datum and local land levelling datum are given in the tidal panel for the use of
surveyors and engineers, where those connections are known.
Datums in use on charts:-
Large scale modern charts contain a panel giving the heights of MHWS, MHWN,
MLWS and MLWN above CD, or MHHW, MLHW, MHLW and MLLW, whichever is
appropriate, depending on the tidal regime in the area concerned. The definitions of
all these terms are given in the Glossary. If the value of MLWS from this panel is
shown as 0·0 m, CD is the same as MLWS and is not therefore based on LAT. In this
case tidal levels could fall appreciably below CD on several days in a year, which
happens when a CD is not based on LAT.
Other charts for which the UKHO is the charting authority are being converted
to new CDs based on LAT as they are redrawn. The new datum is usually adopted in
Admiralty Tide Tables about one year in advance to ensure agreement when the new
charts are published. When the datum of Admiralty Tide Tables thus differs from that
of a chart, a caution is inserted by Notice to Mariners on the chart affected drawing
attention to the new datum.
Where foreign surveys are used for Admiralty charts, the chart datums adopted
by the hydrographic authority of the country concerned are always used for Admiralty
charts. This enables foreign tide tables to be used readily with Admiralty charts. In
tidal waters these CDs may vary from Mean Low Water (MLW) to lowest possible low
water. In non–tidal waters, such as the Baltic, CD is usually Mean Sea Level (MSL).
Caution. Many CDs are above the lowest levels to which the tide can fall, even under
average weather conditions. Charts therefore do not always show minimum depths.
For further details, see the relevant Admiralty Tidal Handbook.
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