Gps survey

DEPARTMENT OF CIVIL ENGINEERING
PRESENTATION BY
SHANMUGASUNDARAM N
ASSISTANT PROFESSOR
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CE6404-Surveying II/Unit III by,
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GLOBAL POSITIONING SYSTEM

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GLOBAL POITIONING SYSTEM
 World wide radio navigation system
 Formed from a constellation of 24 satellites and their ground
stations.
 GPS uses satellites as reference point to calculate positions in
meters and also CM
 GPS plays a vital in cars, boats, planes, construction equipments,
movie making gears, farm machinery, even laptop computers,
phones etc.....

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Basic concepts of GPS
 Trilateration from satellites
 Relative positions of objects using the geometry of triangles.
 GPS receiver measure distance using the travel time of radio
signals
 Used atomic clock to achieve Accurate timing
 Along with distance the exact location of the satellites in space
must be known.
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 Different segments of GPS
1. Space segments
2. Control segments
3. User segments
Space segment
Deal with GPS satellites system
Control segment
Ground based time & orbit control predication
User segments
Types of existing GPS receiver and its applications
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 Different segments

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 Different segments

SEGMENT INPUT FUNCTION OUTPUT
Space Navigation message
Generate
Transmit code
Carrier phase
Navigation message
P-Code
C/A-Code
L1,L2 Carrier
Navigation message
Control P-Code operation time
Produce GPS time
predict Ephemer is
manage space vehicles
Navigation message
User
Code operation carrier
phase operation
navigation message
Navigation solution
surveying solution
Position velocity time
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SPACE SEGMENT
Consists of 21 GPS Satellites with an addition of 3 active
spares
Placed in almost 6 circular orbits with an inclination of
55 degree
Orbit height 20200km corresponding to about 26600km
from the semi major axis.
Normal orbit period is 12Hrs.

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SPACE SEGMENT
24 hours global navigation and time determination capacity
Each satellite send a full description of its own orbit and an
approximate guide to orbits of other satellites
Location of a satellites are established by their own orbit
data
Transmitted also health of satellites, parameter for
propagation, error correction Etc,...

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SPACE SEGMENT
GPS satellites divided into 3 types
1. Block 1 – 1 to 11 – 1978 to 1985 – 5 years- development
purpose
2. Block 11 – 28 – 1989 – 5 to 7 years- production satellite
3. Block 11R – 20 satellites – Spare

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CONTROL SEGMENT
Vital link in GPS technology.
Monitoring and controlling the satellite system continuously
Determine GPS system time
Predict the satellite ephemeris and behaviour of each satellite
clock
Update periodically the navigation message for each particular
satellite

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CONTROL SEGMENT
Its consists of
1. Master control station (MCS)
2. Several monitor station (MS)
3. Ground antennas (GA) (All our world)
4. Operation control segments (OCS)

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CONTROL SEGMENT
 MCA located in Colorado (USA)
3 MS and GA – Kwajalein, Ascension and Diego Garcia
2 MS in Colorado and Hawaii
Monitor station receives all visible satellite signals by using
antennas.
Antennas are contact to satellite at least 3times per day
automatically.

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USER SEGMENT
Ground station equipments consists of an antenna and a
receiver for surveying purpose.
In small unit(work) using poles and large unit using tripod
over a control station
Transmitted signals are received by the antenna, processed
electronically and passed by the cable to the receiver where
microprocessor reduces the data.

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USER SEGMENT
 Method of establish the distance b/n the satellite and receiver
1. Psuedo-range
2. Carrier phase measurement method.
Psuedo-range method
 Distance measurement is depends upon accurate time
measurement and precise synchronization of clock in both
satellite and receivers
 Its almost impossible to achieve the technique is known as
Psuedo-range.
 Satellite continuously transmits its code at every milliseconds

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Psuedo-range method
 Due to the travel of signal the receiver received the code
delay and its converted into a distance by multiplying by the
speed of lights
 Due to the atmosphere the error will be occurred in speed of
light
Carrier phase measurement method.
 Similar to the operational of EDM
 Problem creating by two clock
 To prevent this problem by taking two reading satellite by
simultaneously and operation by single satellite by two
station.

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Types of GPS
1. Absolute positioning
Single receiver station -50 to 100m accuracy
2. Differential or Relative positioning (DGPS)
Two receiver station – 0.5 to 5m accuracy
3. Real-time kinetic float (RTK float)
More precise-Dual receiver-20cm to 1m accuracy
4. Real-time kinematic fixed (RTK fixed)
Dual receiver- 7 to 5cm-very accuracy

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Signal structure:
 One way ranging system
 distance is calculated through the knowledge of signal
propagation velocity
 clock readings at transmitted and receiver antennas are
compared
Two clocks are not strictly synchronized
The observed signal travel time is biased with systematic
synchronization error.

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Signal structure:
Biased ranges are known as pseudoranges
 Need four pseudoranges are necessary to determine X,Y and Z
coordinates of user antenna and clock bias.
 Used two different codes p and C/A
P means precision C/A means Clear/acquisition
Satellite have highly precise oscillators with a fundamental
frequency of 10.23 MHz. Its consists of 3 components

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Three components
1. Two micro wave L-band (carrier) waves
L1 carrier : 1574.42 MHz
L2 carrier : 1227.60 MHz
2. Ranging codes modulated on the carrier waves
C/A code modulated at 1.023MHz
degraded code for civilian users modulated on L1
P (Y) code modulated at 10.23 MHz
Authorized military users used both L1 and L2

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3. Navigation message:
 Modulated on both L1 and L2 and contains satellite positions
and constants

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Signal structures:

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Orbit determination (OD) :
 process of estimate the position and velocity of a satellite at a
specific epoch based on models of forces acting on the satellite,
integration of satellite orbital motion equations and measurements
to the satellites
1. Preliminary orbit determination (improved as
simplified orbit determination (SOD))
2. Precise orbit determination (POD)

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1.Preliminary orbit determination:
 Geometry method to estimate orbit elements from a
minimum set of observations before the orbit is known
from the traditional source.
A. Gaussian orbit determination
 Determining the orbital parameters for three sets of
widely spaced direction observations
B. Laplacian orbit determination
 To drive the initial velocity at a time instant from
different combination of observation.
 Used to find the orbit from different position

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2. Precise orbit determination:
 Dynamic or combined geometric and dynamic method, a
process completed with two distinct procedures
 Orbit integration
 Orbit improvement
Orbit representation:
 Representing a satellite orbit as a continuous trajectory
with discrete observation data at the time of interest
Keplerian elements method is simplest orbit
representation- ellipse shape

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Anti- spoofing and selective availability:
 In US GPS is a military navigation system, in civilians used
limited accuracy system.
Standard positioning system (SPS) for civilians
Precise positioning service (PPS) for authorized users
SPS accuracy is 100m, 2DRMS and PPS accuracy is 10 to 20m in
3D.
Additional limitation Viz, Anti-spoofing (AS) and selective
availability (SA) was further imposed for civilian users.

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Anti- spoofing and selective availability:
 AS only authorized users to get access to the P-code
By imposing SA condition positional accuracy from block II
satellite was randomly offset for SPS users.
Since may 1, 2000 according to declaration of US president SA is
switched off for all users.

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Task of control segment:
Resolves satellite anomalies,
Make pseudo-range and delta-range measurements to determine
satellite clock corrections, almanac and ephemeris
In order to perform the above functions, the control segment is
comprised of three different components
1. Master control station (MCS)
2. Monitor station (MS)
3. Ground antennas

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Task of control segment:
MS tracks the satellite and relay their positions to the master
station.
Used to determine the precise location of satellite constellation.
Accurate atomic clocks on the satellites are also monitored and
compared with the master clock
Each satellite is capable of storing data that would accurately relay
its position for next 14 days via the carrier waves.
Its data bank is updated at every hour by the control station

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Task of control segment :
 MS while passing of satellite make pseudo-range and delta-range
measurements.
Measurements are made the two L band frequencies (L1 and L2)
C/A code has a 1 ms period and repeats constantly
P code transmissions is a 7 days sequence
Repeat at every midnight Saturday/Sunday.
Each satellites transmits this frequencies but with different ranging
codes.

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Task of control segment :
 Navigation data helps the receiver to determine the location of the
satellite at the time of signal transmission
Ranging code used to determine the satellite time to the users
Used in computer programmes to assist in position solutions.
Receivers:
1. Hand – held receivers
2. Geodetic receivers
3. Single - frequency receivers
4. Dual – frequency receivers

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 Data recorder which may be capable of tracking up to 6 satellites
Leica MX 8600 series is a typical of this type of equipment.
For differential work use the MX 8650 base station, tracking with
12 satellite
Using mapping by the utilities and local authorities
Base station would be permanently mounted at the office or depot.

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 1 to 5m accuracy in MX 8601 using pseudo-range measurements
2 cm accuracy in MX 8612 and 8614 use both ranges.
2. Geodetic – receivers
Currently little available
Used in geodetic surveying and precise navigation
Used to start with single frequency C/A code receivers with four
channels
L2 added and tracking capability was increased

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2. Geodetic – receivers
Leading manufacturers have gone for code-less, non sequencing
L2 technique.
WILD/LEITZ (heerbrugg, Switzerland ) and MAGNAVOX
(Torrance, California) have jointly developed WM 101 geodetic
receiver in 1986.
Codes are observed once per seconds
4 channel C/A code receivers
3 channels are track 6 satellites and 4 channels are collect the
satellite messages periodically

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Data processing:
It is most important consideration
Collect the good quality data, for example avoiding multipath
including environments, correct measurement of antenna height,
adequate battery power, no signal observations (tree etc..) or
interference (microwave transmission , etc...) appropriate length
observation sessions, etc...
Repair, detect are appropriately pre-processed.

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Appropriate observation modelling and processing software:
Matched to the accuracy required so that the correct strategy is
used to account for the GPS measurement biases and in particular the
cycle ambiguities
Appropriate processing procedure:
Recognises that GPS phase data reduction involves a numbers of
steps executed in sequence.

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Degree of sophistication of data modelling and processing:
Used at wide range and verity of GPS modelling and software
options that are available
The surveying applications may be categorised according to three
general ranges of accuracy
Class A ( Scientific) – better than 1 ppm – precise engineering
Class B ( Geodetic) – 1 to 10 ppm – Geodetic densification,
mapping, resource development app...
Class C ( General surveying) – lower than 10 ppm – urban,
cadastral, general purpose survey)

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1. Data pre-processing :
a. Initial data transfer and decoding
b. Data screening and editing
c. Data reporting and data base creation and entry
d. Point positioning using pseudo – range data.
 Carried out on a single-station basis, and can therefore be carried
out in the field office.
 The result should be a set of appropriately formatted and pre-
processed , observations, together with ephemeris information
and approximate station co ordinates

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2. Initial data analysis:
 Final data adjustment
Using two or more GPS in field observation
Cycle slip detection and repair
Preliminary baseline solution based on triple – differenced phase
data or double-differenced pseudo-range data
Final adjustment:
 Formation of the phase data differences
Definition of the apriority weight matrices
Estimation of relative station coordinates
Estimation of carrier beat phase ambiguities and fixing them to
integers
Output of estimated parameters and covariance matrix

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Final adjustment :
Adjustment of GPS observations can be performed in two ways
1. By combining single baseline solutions into a network
adjustment of the baseline components
2. Bt a direct simultaneous solution involving all the GPS
observations for a session or complete network.

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Traversing and Triangulation:
1. General information
GPS used to other surveying tool
It can accomplish certain goals if we are conscious of its strengths
and limitations.
When surveying with GPS we do not need to have inter-visibility
b/n the stations to measure a baseline.
The only constraint to receive the signals is having a clear of the
sky

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Network design on map:
To make a map of the station in a good geometric figure, both
fixed control points and unknown points for the entire project area.
Scale and distance between the two stations are important factor
Confidence level is not only the accuracy level also the
configuration
To fulfil the basic role a strong and reliable reference frame work,
Its must be homogeneous
Individual figures should be well-shaped

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Stations should be evenly spaced as possible
Adjacent pairs of stations in the network should preferably be
connected by direct measurement.
Ratio of longest length to the shortest length should never be
greater then 1:5 and usually should be much less
Higher difference in heights b/n two stations will be avoided in the
network
Designed several small closed loops within the large network

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 The areas in which the ground does not permit to design the
network like traverse
There are two types of control
1. Horizontal control
2. Vertical control
Horizontal control
 Minimum 3 to 4 fixed control points for average size of project
with complete adjustments
 More number of control produce better redundancy with higher
quantity of check
 Four control points two at both ends of the network are required
to established for network adjustments

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Vertical control:
 MSL height are not to be confused with GPS height
 GPS height is based on the WGS 84 ellipsoid
 MSL height is based on an equipotential surface coinciding with
MSL called the geoids
 Need to know the geoidal undulation at that point for convert the
GPS and MSL height (ex: separation b/n geoid & ellipsoid)
 Connect maximum number of points to connect the height and
level adjustments
 GPS control points which is not connected with levelling line
will be adjusted by adjustment program with the help of fixed
vertical control points and EGM96 geoid model, used to find
unknown vertical points on to the same datum as fixed point.

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