Geodesy
Presented by:
Sujan Sapkota
Global Navigation Satellite Survey Instructor,
Nepal Banepa Polytechnic Institute.
System (GNSS) sujansapkota27@gmail.com
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Learning Outcomes
You will be able to describe the basics of GNSS.
You will be able to know the principle on how GPS works.
You will be able to know about the field procedures for
conducting GPS surveys.
Finally will be able to know about the error sources in GPS
and way to mitigate those problems.
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Humans have always been interested in where things are…
One of the basic questions has always been…
where am I?….which leads to…
where am I going and how do I get there?
Early solutions:
• Marking trails with piles of stones (Remember Hansel and Gretel)
(problems when snow falls…or on ocean)
• Navigating by stars
(requires clear nights and careful measurements)
most widely used for centuries
…location within a mile or so
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United State Department of Defense finally said:
“We need something better: all-day and all-night; all terrain”
End-product of which is Global Positioning System (GPS)
Fig: GPS orbital system
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Modern solutions:
GNSS
GNSS is a satellite system that is
used to pinpoint the geographic
location of a user’s receiver,
anywhere in the world.
Each of the GNSS system employs a
constellation of orbiting satellites
working in conjunction with the
network of ground stations.
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What is GPS?
GPS, which stands for Global
Positioning System, is the
only system today able to
show you your exact position
on the Earth anytime, in any
weather, anywhere.
Not exactly!
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The GPS Constellation
31 Operational Satellites
(Nominal Constellation: 24)
• Satellites distributed in 6 inclined orbital
planes with 4 satellites on each
• Angle of inclination of 55 degree
• Orbital period is twice every 24 hours at an
altitude of app. 20,200km.
• Why 55 degree and given altitude?
• To ensure that no matter where you are on
the surface of the planet, there are at least
four satellites in the range of your GPS
receiver at any moment. 7
Segments of GPS
• Space Segment
• Control Segment
• User Segment
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Components of the System
Space segment
24 satellite vehicles
Six orbital planes
Inclined 55o with respect to
equator
20,200 km elevation above
Earth
Orbital period of 11 hr 55 min
Five to eight satellites visible
from any point on Earth
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Contd…
Control Segment
• Orbital position is constantly monitored and updated by the ground stations
• The ground control segment has one master control station, one alternative
master control station, 12 command and control antennas and 16 monitoring
sites
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Contd…
1. Master control system
• Located at Falcon Air Force Base in Colorado Springs, Colorado
• Responsible for overall management of the remote monitoring and transmission
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Contd…
2. Monitor Stations
• Each of the monitor stations checks the exact altitude, position, speed and overall
health of the orbiting satellites
• Control segment uses measurements collected by the monitor stations to predict the
behaviour of each satellites orbit and clock
• Prediction data is transmitted to the satellites for transmission back to users
• A station can track up to 11 satellites at a time
• Variation such as these caused by the gravity of the moon, sun and the pressure of
solar radiation are passed along the master control station
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Contd…
3. Ground Antennas
• Monitor and track the satellites from horizon to horizon
• Also transmit correction information to individual satellites
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User Segment
• GPS receivers are generally composed of an antenna, tuned to the
frequencies transmitted by the satellites, receiver processors and a
highly stable clock.
• Also include a display for showing location and speed information to
the user
• A receiver is often described by its number of channels this signifies
how many satellites it can monitor simultaneously.
• As of recent, receivers usually have between 12 to 20 channels
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Position determining procedure in GPS
Very famous resection principle in Surveying
C(ts-to) = 𝑥𝑠 − 𝑥𝑜 2 + 𝑦𝑠 − 𝑦𝑜 2 + 𝑧𝑠 − 𝑧𝑜 2
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Contd…
• GPS provides two levels of services to the user segment:
1. Standard Positioning Service (SPS)
Provides positioning and timing information to any user on a continuous worldwide
basis
2. Precise Positioning Service (PPS)
Provides position, velocity and timing information to authorized US and allied military,
federal government and civil users who satisfy specific US requirement
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How does GPS work?
Trilateration
Geometric Principle:
You can find one location if you know its
distance from other, already-known
locations.
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1. The receiver picks up the signals from the satellites
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2. Receiver triangulates to determine position of the receiver
•If only one satellite is visible, position location is impossible
as the receiver location can be anywhere on the surface of a
sphere with the satellite at its center.
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•If two satellites are visible the receiver location can be anywhere
on a circle where the surfaces of the two spheres intercept. So
position location is also impossible.
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•When a third satellite becomes visible, the GPS receiver can establish its position as being at one of
two points on the circle where the third satellite sphere intercepts it. So position fixing is possible, but
only in two dimensions (2D fixing): in latitude and longitude.
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• With at least four satellites visible, and their alignment
good, the receiver position can be accurately fixed in three
dimensions (3D fixing): in latitude, longitude and altitude.
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The GPS receiver compares the time a
signal was transmitted by a satellite
with the time it was received. The time
difference tells the GPS receiver how far
away the satellite is.
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Velocity x Time = Distance
Radio waves travel at the speed of light, roughly 186,000
miles per second (mps)
If it took 0.06 seconds to receive a signal transmitted by a
satellite floating directly overhead, use this formula to find your
distance from the satellite.
186,000 mps x 0.06 seconds = 11,160 miles
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GPS Codes
A GPS signal contains 3 different bits of information:
• Pseudorandom code, is simply I.D code that identifies which satellite is transmitting
information
• Ephemeris data, contains important information about the position of satellite, status
(health), current date and time. This part of signal is essential for determining a position.
These data is constantly transmitted by each satellite.
• Almanac data, tells the GPS receiver where each GPS satellite should be at time
throughout the day. Each satellite transmits almanac data showing the orbital information
for that satellite and for each other satellite in the system.
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Fig: Ephemeris data Fig: Almanac data
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Contd…
Carrier Signals
• Each GPS satellite transmits a microwave radio signal composed of two carrier frequencies
modulated by two digital codes and a navigational message
• L1 carrier – 1575.42 MHz (19cm)
• L2 carrier -1227.60 MHz (24.4cm)
• The second frequency allows measurement of the ionospheric signal delay.
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Contd…
Binary Codes modulated on the carrier signal
1. Coarse Acquisition (C/A) code
• Short pseudorandom noise (PRN)
• C/A code modulates the L1 carrier and is the basis for civil SPS
• GPS satellite may be uniquely identified by their PRN number
2. Precise (P) code
• PRN code
• P cod modulates both the L1 and L2 carrier phases and is the basis for the military PPS
• When Anti-Spooling (AS) mode is active, P code is encrypted into Y code
3. Navigational data
• Added to the C/A and P (Y) codes on the frequency and transmitted
• Contains low accuracy (almanac parameters) and high accuracy (broadcast ephemeris) GPS satellite orbit data as well as clock correction and other system
parameters
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Procedure of GPS Survey
• Workflow:
1. Preparation of plan of operation
2. Reconnaissance and Monumentation
3. Preparation of station Description card and diagram
4. Observation
5. Data Download and preliminary computation
6. Coordinate Transformation and adjustment
7. Data Achievement
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Plan of GPS Operation
• Keep in mind the no. of receivers, observers and observation schedule
• Also consider condition of satellites arrangements, known points and
selection of known ground points
• Select good window for observation time
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Overall Planning
• Consider for:
• No. of control points to be fixed and desired accuracy
• Connection to existing control points
• Base line should be kept as short as possible
• The best ways of observation and computation
• Use of two or more reference control points whenever possible
• Making possibility of forming loop so that loop/network adjustment could be
carried out.
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Office Planning
• Purpose of work • Instrument
• Type of observation – First order, second • Budget
order, third order etc
• Check availability of free space in Memory
• Duration of observation Card
• Map collection: Different Type of map ( • Check battery level
district map, topo sheet, other relevant
map and information) • Facilities of Transportation
• Marking on the map for Monumentation • Other accessories eg tape, compass etc
• Types of Monumentation • Facilities of Camping in the site
• Satellite availability
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Plan: Selecting good windows
• Not all conditions are suitable for
observation
• Sky plot panels in post processing
software help to select good window
for observation
• Following conditions are generally
preferred for observation:
• 5 or more satellites above 20°
• GDOP< 5
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Plan: Observation Duration
• Observation duration depends on baseline length, no. of satellites, satellite
geometry, ionosphere, etc.
• The normal collection rate (epoch) is 5 seconds for static observations, but for long
observation times of more than about 3 hours, 15 second epochs are acceptable.
• RTK(Real Time Kinematics) is done at 1second.
• Longer baselines will require longer observations on end points.
• Disturbances due to ionosphere is minimum at night time
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Plan: Satellite Geometry
• Satellite geometry factors to consider when planning a GPS survey are:
• Number of satellites available
• Minimum elevation angle for satellites (elevation mask)
• Obstructions limiting satellite visibility
• Positional Dilution of Precision (PDOP)
• Vertical Dilution of Precision (VDOP) when performing vertical GPS
surveys
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Plan: Weather conditions
• Generally, weather conditions do not affect GPS survey procedures with the
following exceptions:
• GPS observations should never be conducted during electrical storms.
• Significant changes in weather or unusual weather conditions should be
noted in the observation log (field notes). Horizontal GPS surveys should
generally be avoided during periods of significant weather changes. Vertical
GPS surveys should not be attempted during these periods.
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Plan: Reconnaissance and Monumentation
• Always make choice of best position for instrument setting situated in locations
relatively free from horizon obstructions (Cut off angle = 15⁰ )
• No reflecting surfaces to reduce the risk of multipath
• Locations near strong radio transmissions should be avoided.
• Away from traffic, possible to leave the receiver unattended
• Tentative location of stations planned previously on topographic map w.r.t. existing
higher order control points
• Preparation of station descriptions explained in D-Card.
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Monumentation
• Consists of two marks, upper and
lower mark
• Lower mark a slab 30 cm x 30 cm x 15
cm
• Upper mark concrete pillar of base
22*22cm and top 15*15 cm, height 55
cm
• Separated at 30 cm gap
• The location noted in D-card
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• Check GPS receivers along with its
accessories are carried
• Check the battery level and memory Card
Check list along with its free space for data
collection. Format memory card if
needed.
• Check the settings of receiver. The
format of coordinates must be in decimal
degrees in World Geodetic System of
1984 (WGS84) datum.
• Check that four satellites at least are
detected and that the mode 3D is set
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Observation & Data Acquisition
• Receiver Setup
• Antenna Setup
• Measurement of Height of Instrument
• Field GPS observation Recording Procedure
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Receiver Setup
• Receiver has to be set with different parameters according to the type of working
mode.
• If for base; one has to setup the following parameters like:
• Differential type
• Record raw data or not
• Base coordinate setting
• Delay
• Cut off angle
• PDOP Threshold
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Receiver setup
• If working on static mode, one has to setup the following parameters:
• Cut off angle
• PDOP value threshold
• Point Name
• Antenna height
• Time interval
• Automatic Record
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Data Processing
• Open the New project • Selection of reference and rover points
• Data Download • Data processing
• Editing • Check the residual
• Backup the data • Determination of Transformation
Parameter
• Import the data on the Project
• Checking the overlap and duration of
• Conversion of the coordinate on local
system
observation
• Input the coordinate of known station • Checking and verifying the data
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Overlap and Observation Duration
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Factors affecting the accuracy of GPS data
• Number of visible satellites
• Satellite Geometry
• Multipath
• Satellite clock Errors
• Ephemeris Errors
• Atmospheric Effects
• Receiver Errors
• Operator knowledge and Awareness
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• At least 4 satellites are required.
• Typically more than 7 satellites are
Number of visible satellite
preferred for accuracy
• When the number of satellites that can be
seen by users is increased or decreased,
the positioning results vary. If there is a
small number of satellites, the effects of
each satellite are large, and this variation
grows.
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Dilution of Precision (DOP)
• An arrangement of satellites in the sky also affects the accuracy of GPS
positioning.
• The ideal arrangement (of the minimum four satellites) is one satellite
directly overhead, three others equally spaced nearer the horizon (but above
the mask angle).
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Dilution of Precision (Contd..)
• Different types of DOP can be calculated as:
• GDOP (Geometric Dilution of Precision) gives accuracy degradation in 3D position
and time
• PDOP (Positional Dilution of Precision) gives accuracy degradation in 3D position
• TDOP (Time Dilution of Precision) gives accuracy degradation in time
• HDOP (Horizontal Dilution of Precision) gives accuracy degradation in horizontal
directions.
• VDOP (Vertical Dilution of Precision) gives accuracy degradation in vertical directions
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Satellite Geometry
Horizontal Dilution of Precision (HDOP)
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Vertical dilution of precision (VDOP)
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Position Dilution of Precision (PDOP)
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Navigation satellites with poor geometry for
Geometric Dilution of Geometric Dilution of Precision (GDOP).
Precision (GDOP)
Effect of navigation satellite geometry on positio
nal measurement precision.
Navigation satellites with good geometry for
Geometric Dilution of Precision (GDOP).
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Elevation Cut- off
angle
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TURNED OFF ON MAY 2, 2000
Selective availability
• To limit GPS position accuracy available to
non military users, the DoD established a
policy of selective availability (SA).
• Time varying biases into the GPS signals to
intentionally degrade performance.
• Resulting time error can be as much as 340
nanoseconds (95 percent) resulting in change
in positions up to 100 meters.
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Anti-spoofing (A-S)
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Multipath
• “Multipath” means that satellite signals do
not just arrive in a straight line; they are
transmitted in multiple routes while being
reflected off mountains, buildings, etc. It
takes time for the reflected signals to arrive
at their destination. This results in a more
distance calculation, and is a factor that
negatively impacts accurate satellite
positioning.
• Satellites with high elevation angles are
included, which can improve positioning
errors caused due to this error.
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Satellite & receiver clock
errors
• In spite of the synchronization of the
satellite and receiver clocks, small amount
of inaccuracy in timing remains
• This can result in errors up to 2.1m
• To keep clock errors to 1m or less, the
time error must be limited to 20-30
nanoseconds
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Orbital/Ephemeris Error
• Even though the satellites are positioned
in very precise orbits, slight shifts are
possible due to the gravitational influence
of sun and moon
• Orbital errors can be high as 2m
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• Radio signals travel at the speed of light in space,
but are slowed down by the atmosphere
• The majority of this effect can be eliminated by
Atmospheric Effects receiver
• Lower frequency signals are slowed down
more than high frequencies
• The receiver can determine the difference in
the arrival time of high and low frequency
signals and calculate the correction
Ionosphere effect 4m
Troposphere effect 0.7m
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Ionosphere effect and its
solution
• A region of the upper atmosphere
from 100 to 1,000 kilometers that is
charged with electricity.
• When satellite signals from satellites
pass through the ionosphere, their
speed decreases.
• Estimated and mostly resolved by
simultaneously transmitting satellite
signals of multiple frequencies from
one satellite.
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For Tropospheric effect
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SNR (Signal to Noise Ratio)
• A measure of the information
content of a GPS signal relative to
the signal’s noise, or the ratio of
good information to degraded
information.
• GPS position is degraded if the
SNR of one or more satellites in
the constellation falls below certain
range
• A value above 20 is very good (20
parts good for every one part
degraded).
• The quality of a position is
degraded if the signal strength of
any satellite in the constellation is
below 6.
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Summary of GPS errors
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GPS Positional Accuracy
This accuracy value is of normal handheld GPS device whereas the accuracy in case of
DGPS is a few centimeters to millimeters.
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QZSS
GLONASS COMPASS GPS
IRNSS
Interoperability
Interoperability is defined in the ICG
(International Committee on Global
Navigation Satellite Systems) Forum as:
“Interoperability refers to the ability of global
and regional navigation satellite systems and the
services they provide to be used together to
provide better capabilities at the user level than
would be achieved by relying solely on the open
signals of one system.”
Interoperable = Better Together than Separate
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Navigation
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References
• Adhikari, K.R. Geodetic Survey and Geodetic Positioning System, Geodetic Survey Branch, 2061
• El - Rabbany, A. (2006). “Introduction to GPS : The Global Positioning System”. (Second
Edition). Artech House.
• Chao, C.H. (1998). “High Precision Differential GPS”. MSc Dissertation. Institute of Engineering
Surveying and Space Geodesy (Institute of Engineering Surveying and Space Geodesy (IESSG)) –
University of Nottingham.
• Hofmann, B., et all. (2008). “GNSS – Global Navigation Satellite Systems : GPS, GLONASS and
more”. Springer Wien New York.
• (2010). “Guidelines for the use of GNSS in land surveying and mapping”. (second edition). Royal
Institution of Chartered Surveyors (RICS)
• (2010). “Education Curriculum : Global Navigation satellite Systems”. English, Publishing and
Library Section, United Nations Office at Vienna
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Any Queries
?
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