DGPS

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Table of Contents

Multiple Choice Questions (MCQs)

Answers

True False Statements

Answers

Short Quires

Answers

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Multiple Choice Questions

1. When satellite are close together the chance of ______ increased in the position

a) Accuracy

b) Duplication

c) Uncertainty

d) None of them

2. Three different satellites intersect each other at a point is the basic concept of

a) Positioning

b) DGPS

c) Remote sensing

d) All of them

3. We always measure the unknown point from some referenced bench mark is the

main concept of

a) GPS

b) DGPS

c) Positioning

d) b and c

4. If we required the accuracy of positioning result within 10-15 meters then we use

a) Measuring tape

b) DGPS

c) Theodolite

d) GPS

5. Triangulation results consider to be more accurate while using _____ frequency

mode in DGPS

a) Single

b) Dual

c) Random

d) a and c

6. While using DGPS in single frequency mode best triangle is obtained between

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a) two receiver and one rover station

b) two receiver and one satellite

c) one receiver and two satellites

d) b and c

7. When we perform surveying to establish control point in any location with

precision is in mm then we use DGPS ______ type

a) Stop and go surveying

b) Rapid static surveying

c) Static surveying

d) All of them

8. Post processing of DGPS is very complex and difficult because it deal with the

a. Geometry of satellites

b. Signals travels from satellite to receivers

c. Time delay issues

d. All of them

9. Accuracy degradation in the three dimensional position is obtained from

a. GDOP

b. TDOP

c. PDOP

d. HDOP

10. To establish a base line from known point to unknown point we need two things

a) Length between stations and Azimuth

b) Travelled time from one station to other and bearing

c) Distance from known point to unknown point and angle from north

d) None of them

11. If there are 7 stations possible number of non-trivial baselines are

a) 7

b) 6

c) 5

d) 4

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12. A radio link is maintain between the base receiver and the rover, and the base

receiver supplies the correct measurements without post processing.

a) static survey

b) rapid static survey

c) real time survey

d) dilution of precision

13. Surveying type to replace short range traversing and local triangulation with

accuracy 5-10 mm + 1 ppm.

a) static survey

b) rapid static survey

c) real time survey

d) kinematics survey

14. Master station is included in________.

a) Space segment

b) Control segment

c) user segment

d) all of them

15. _________ is a method of determining the relative positions of objects using the

geometry of triangles.

a) Triangulation

b) Trilateration

c) Trivial

d) Resection

16. Point positioning using four satellites is similar in principle to _______.

a) Triangulation

b) Trilateration

c) Resection

d) cross-sectional method

17. Latitude, Longitude and Height gives________ accuracy.

a) Geometrical

b) Positional

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c) Horizontal

d) Vertical

18. Which of the following is not true about reference station?

a) Computes corrections

b) Transmits corrections

c) receives the corrections

d) none

19. In point positioning the accuracy in each component is_______.

a) 5 to 10m

b) 10 to 20m

c) 10 to 30m

d) 5 to 20m

20. __________ is used to create control points by using raw GPS data from the

satellites.

a) Pre Processing

b) Post Processing

c) Real Time

d) none

21. ________ Survey is Suitable for highly accurate survey e.g. geodetic control

network.

a) Static

b) Rapid static

c) kinematic

d) stop and go

22. No post processing is required in _______.

a) Static survey

b) Real Time

c) both a & b

d) none

23. _______ Survey is usually done by Triangulation.

a) Static

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b) Rapid static

c) kinematic

d) stop and go

24. Static survey is usually done by________.

a) Triangulation

b) Trilateration

c) Trivial

d) Resection

25. A _________is the difference between the transmission time at the satellite and

the reception time at the receiver.

a) orbital error

b) Pseudo range

c) ionosphere error

d) tropospheric error

26. Which is not true about point positioning?

a) Accuracy 10 to 20m in each component

b) Dependent on DoP Selective Availability

c) Navigation Applications

d) Not suited for Surveying or Precise Navigation

Answers

1. c 2. a 3. b 4. d 5. b

6. d 7. c 8. d 9. b 10. c

11. b 12. c 13. b 14. b 15. b

16. c 17. b 18. c 19. b 20. b

21. b 22. b 23. a 24. a 25. b

26. b

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True False

1-Radio signals are used from a constellation of earth-orbiting satellites to determine the

3D position of a receiver? (True/false)

2-GPS signals must travel through a number of layers making up the atmosphere.

(True/false)

3-When the GPS signal arrives at earth it may reflect off various obstructions.

(True/false)

4-RANGE = Time Taken x Speed of Light. (True/false)

5-To "triangulate," a GPS receiver measures distance using the travel time of radio

signals. (True/false)

6-Differential GPS or "DGPS" can yield measurements good to a couple of meters in

moving applications and even better in stationary situations. (True/false)

7-GDOP (Geometrical), Includes Lat, Lon, Height & Time (True/false)

8-HDOP (Horizontal) Includes Lat only. (True/false)

9-Single frequency as well as Dual Frequency GPS receivers are used in static survey.

(True/false)

10-Every station in the network is connected to at least one other station by a non-

trivial baseline. (True/false)

11-The new control points established relative to known control points by static /

kinematic survey.

(True/false)

12- Post processing required for real time code /phase (True/false)

1. T 2. T 3. T 4. T 5. T

6. T 7. T 8. F 9. T 10. T

11. T 12. F

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SHORT QUESTIONS

Q1: Define pseudo-range for GPS and its formula?

A pseudo range (code observation) is the difference between the transmission time at

the satellite and the reception time at the receiver (Erickson, 1992). Pseudo range

between the satellite and the receiver is obtained by scaling it using the speed of light.

The observation equation for a pseudo range is given AS:

Where,

p is the observed pseudo range, is the unknown satellite-receiver range,

c is the speed of light,

dt is the satellite clock error,

dT is the receiver clock error,

d is the orbital error,

dion is the ionospheric error,

dtrop is the tropospheric error,

p is the code measurement noise and multipath.

Q2: How satellite clock error effects the position of GPS satellites?

Though they use atomic clocks, they are still subject to small inaccuracies in their time

keeping. These inaccuracies will translate into positional errors: “Orbit Uncertainty”.

The satellites position in space is also important as it’s the beginning for all calculations.

They drift slightly from their predicted orbit

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Q3: What is the source of error in distance calculation of satellite-receiver?

GPS signals transmit their timing information via radio waves. It is assumed that a radio

wave travels at the speed of light. GPS signals must travel through a number of layers

making up the atmosphere. As they travel through these layers the signal gets delayed.

This delay translates into an error in the calculation of the distance between the satellite

and the receiver.

Q4: On which factors selection of GPS survey depends?

Desire level of accuracy

Use of survey

Size of survey

Type of equipment used

Available software

Q5: Why to use DGPS?

Eliminates errors in the sat. and receiver clocks

Minimizes atmospheric delays

Accuracy 3mm - 5m

Q6: Define DOPs?

A description of purely geometrical contribution to the uncertainty in a position fix. It

is an indicator as to the geometrical strength of the satellites being tracked at the time

of measurement

Q7: How RTIME Code works?

At Reference Station

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Reference Station on a Known Point

Tracks all Satellites in View

Computes corrections for each satellite

Transmits corrections via a communication link in either propriety format or in

the RTCM format

At the Rover Station

Rover unit receives the corrections via the communication link

Rover position corrected by applying the received corrections

Q8: Write down the merits and de-merits of Real Time GPS?

Merits De-merits

Good Accuracy for short surveys Not good accuracy for detailed survey

One man operation Short rang network

Ease of operation

No post processing

Q9: What are the major steps for planning a Real Time GPS survey?

Accuracy Requirements

Code = meter / sub-meter

Phase = centimeter

Availability of Control

Horizontal

Vertical

Both

Type of Transformation

Local Grid

WGS84

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Q10: Define DOPs by its rating?

Q11: What are the required steps for planning a GPS survey?

• Available Planning tools

• Satellite availability

• Satellite visibility

• Sky-plots

• Elevations of satellites

• Predicted DOP values (HDOP, VDOP, GDOP,TDOP)

Q12: Write down the nomenclature for GPS data processing files and write down its

extensions?

File name nomenclature AAAABBBC.dat

AAAA = last 4 numbers of receiver serial number

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BBB = Julian date

File number starting from 0 to F

Extensions are:

DC01, DC02, DAT

Q13: What are the 5 segments of the GPS system?

Space Segment

Master Control Station

Monitor Station

Ground Antennas

User Segment

Q14: Differentiate between static and stop n go methods?

Static Stop n Go

More accurate Less Accuracy

Wide Range area coverage Shot range

Not real time Semi -real time example

Longer observation time Shorter observation time

Q15: List out all type of errors in GPS system?

Satellite clock uncertainties

Multipath errors

Receiver clock drift

Internal receiver noise error

Obstruction from buildings, mountains etc.

Signal out of sight