Unit-1 GNSS.pdf

Prerequisites for GNSS
•Students are expected to complete the courses AC
and DC
•They are expected to have the knowledge of
coordinate systems

Course Outcomes:
At the conclusion of this course, the student will be able:
• To describe the basic concepts and working of GPS and
estimate the spatial position and range of the object.
• To recognize the fundamental characteristics of GPS-
coordinate systems, time and GPS signal structure and
compute the errors in estimation.
• To interpret the augmentation systems and constellation
of satellite
• To describe the applications of GPS and reproduce the
navigation and observation data formats of GPS
• To apply the knowledge of GPS to modern navigation
systems.

List of text books
• Rao G.S., “Global Navigation Satellite Systems – with
Essentials of Satellite Communications”, Tata McGraw Hill,
2010.
• Satheesh Gopi, “ Global positioning System:Principles and
Application”,TNH,2005.
• Pratap Mishra and Per Enge,” Global positioning System
Signals,Measurement and performance,” Ganga-Jamuna
Press,2/e,Massachusetts,2010.
• B Hofmamann-Wellenhof,H Lichtenegger,and J Collins,” GPS
theory and practice,”Springer Verlog,2008
• Bradford W Parkinson and James J Spilker,”Global
positioning system : theory and application”Vol II American
Institution of Aeronautices and Astronautics
Inc.Washington,1996.

FREQUENCY SPECTRUM
(Radio Waves)
8
Frequency Band Frequency λ Application
Very Low Frequency
(VLF)
3 - 30 KHz > 10000m Telegraphy, human range frequency
Low Frequency (LF) 30-300 KHz
10000-1000
m
Point to point, navigation
Medium Frequency
(MF)
300K-3 MHz 1000-100m
AM radio broadcast, maritime/aeronautical
mobile
High Frequency(HF) 3 - 30 MHz 100 - 10 m Shortwave Broadcast Radio
Very high
Frequency(VHF)
30 - 300 MHz 10 - 1 m
Low band: TV Band1- Channel 2-6, Mid band:
FM radio, High Band: TV Band 2- Channel 7-13
Ultra High frequency
(UHF)
300M - 1GHz 1 m - 10 cm Mobile phone, Channel 14 - 70
Super high frequency
(SHF)
3-30 GHz
0.01-0.001
m
Satellite communication, C-band, x- band,
Ku-band, Ka-band.
Extremely High
Frequencies (EHF)
30 - 300 GHz < 0.01m
Satellite, radar system, IR, UV, X-rays, Gamma
Rays.

FREQUENCY SPECTRUM
(Microwaves)
10

L-Band signals
•All the GPS signals are in the L-band the frequency
spectrum.
•L-Band was chosen for several reasons, including:
• L-Band waves penetrate into clouds, fog, storms etc &
GPS units can receive accurate data in all weather
conditions, day or night
• Ionospheric delay is more significant at lower
frequencies
• Simplification of antenna design
• Minimize the effect that weather has on GPS
signal propagation

GNSS
(Global Navigation Satellite System)
Generic name for Satellite navigation system for
global coverage- 3D location
5 GNSS constellations
•GPS (US),
•QZSS (Japan),
•BEIDOU (China),
•GALILEO (EU), and
•GLONASS (Russia).

GPS (Global Positioning System)
•Invented to determine the position / location of an
object.

Earlier
solutions

• marking trails with piles of
stones (problems when
snow falls…or in ocean)
• navigating by stars
(requires clear nights and
careful
measurements…location
within a mile or so)
Earlier
solutions

• LORAN: radio-based; good
for coastal waters…limited
outside of coastal areas
• Sat-Nav: low orbit satellites;
use low frequency
Doppler…problems with small
movements of receivers
Modern
ideas

Current
ideas

•GPS
Current
ideas

NAVSTAR
•Combined Navy and Air Force systems in 1973
•NAVigation Satellite Timing And Ranging
(NAVSTAR)
•First used in combat during Operation Desert
Storm – 1991
•Full Operational Capability – April 27, 1995

Dept of Defense US
High Altitude orbits
24 satellites-
first one to be launched-1978 and 24th
satellite was launched in June 1993

Applications of GPS
•anything that involves location, motion,
or navigation

Applications of GPS
examples of some applications (users):
• navigation (very important for ocean travel)
• zero-visibility landing for aircraft
• collision avoidance
• surveying
• precision agriculture
• emergency vehicles
• electronic maps
• Earth sciences (volcano monitoring; seismic hazard)
• tropospheric water vapor

Applications –
Military
•In addition to basic navigation activities, military
applications of GPS include target designation of cruise
missiles and precision-guided weapons and close air
support.
•To prevent GPS interception by the enemy, the
government controls GPS receiver exports
•GPS satellites also can contain nuclear detonation
detectors.

Applications – Civilian
•Automobiles are often equipped GPS receivers.
• They show moving maps and information about your
position on the map, speed you are traveling, buildings,
highways, exits etc.
• Some of the market leaders in this technology are Garmin
and TomTom.

Applications – Civilian (cont’d)
•For aircraft, GPS provides
• Continuous, reliable, and accurate positioning information
for all phases of flight on a global basis, freely available to
all.
• Safe, flexible, and fuel-efficient routes for airspace service
providers and airspace users.
• Reduction of expensive ground based navigation facilities,
systems, and services.
• Increased safety for surface movement operations made
possible by situational awareness.

•Agriculture
• GPS provides precision soil sampling, data collection, and
data analysis, enable localized variation of chemical
applications and planting density to suit specific areas of
the field.
• Ability to work through low visibility field conditions such
as rain, dust, fog and darkness increases productivity.

•Disaster Relief
• Deliver disaster relief to impacted areas faster, saving lives.
• Provide position information for mapping of disaster
regions where little or no mapping information is available.
• Example, using the precise position information provided
by GPS, scientists can study how strain builds up slowly
over time in an attempt to characterize and possibly
anticipate earthquakes in the future.

•Marine applications
• GPS allows access to fast and accurate position, course,
and speed information, saving navigators time and fuel
through more efficient traffic routing.
• Provides precise navigation information to boaters.
• Enhances efficiency and economy for container
management in port facilities.

•Other Applications not mentioned here include
• Railroad systems
• Recreational activities (returning to the same fishing spot)
• Weather Prediction
• Skydiving – taking into account winds, plane and dropzone
location
• Many more!

Internet, data, voice,
Communication satellites,
Earths observation satellites,
Spy satellites etc.
24 hrs Location , navigation
Satellite TV

GPS system segments
•Space segment
•Control segment
•User segment
49

Space segment
•Satellites
• Platform for radio transceivers, atomic clocks, electronic
equipment to measure range
• solar panels for power supply
• Propulsion system- orbit adjustments & stability control
• Satellite identification-launch sequence no
-PRN code
-orbital position no.
-NASA Catalogue no.
- International designation

Space segment
GPS Constellation

Space segment
•Satellites • 845 Kg
• 4.5 year design lifetime
Block I
• >1500Kg
• 7.5 years lifetime
Block II
• A-Additional capability
• Mutual communication
Block IIA
• R-Replenish
• >2000Kg
• Communication & inter satellite tracking
• 10 years
Block IIR
• F-Follow on
• >2000Kg
• I5 years design life
• Onboard capabilities
Block IIF
• new signals and broadcasting at higher power levels.
Block III A
• To be launched by 2026
Block III F

Location of Satellites in space
•GPS satellites orbit at an altitude of 20,200 Km
above Earth’s surface
•Radius of GPS satellite orbit is 26,560 Km
•Satellites are placed in 6 orbits
• Each orbit has 4 satellites
• The inclination angle is 55 deg
(Angle made by the orbit with equator)
Orbits are separated by 60deg to cover 360 deg

Location of Satellites in space

Orbital Period
•The orbital period of a satellite is the time it takes
to return to the same place in the orbit.
•Solar day: Time taken by the earth to rotate once
relative to sun
•Sidereal day: Time taken by earth to rotate exactly
360deg

•A solar day is of 24 hours
•A Sidereal day is slightly shorter- 23 hours 56 minutes 4.1 secs

Orbital Period
Significance of orbital period: the visible positions of
the satellites repeat themselves every day

Orbital Period
Significance of orbital period: the visible positions of
the satellites repeat themselves every day
•Hence the orbital period is exactly half the sidereal
day = ½ (23 hours 56 mins 4.1 secs)
= 11 hours 57 minutes 57.26 secs

68
GPS segments
•Space segment
•Control segment
•User segment

Control Segment
•Task
• Tracking the satellites and predicting their positions
• clock determination
• Time synchronization
• Uploading of data message to satellites
•Consists of 3 different stations
• Master control station (1 no.)
• Monitor stations (5 no.)
• Ground control / antenna stations (4 no.)

Monitoring Stations
•These unmanned monitor stations track all GPS
satellites visible to them at any given moment and
collect the signal data from each satellite.
•They check the exact altitude, position, speed and
overall health of the orbiting satellites.
•These measurements are used to predict the
behaviour of each satellite’s orbit and clock.
•This information is passed to the master control
station at colorado via a secure defense satellite
communication system.
•The predicted data is transmitted to the satellite’s
for transmission back to the users.

Master Control Station
•At master control station ,the satellite position,
clock timing data etc are estimated and predicted.
•Then the master control station periodically sends
the corrected position and clock timing data to
appropriate ground antennas, which they upload to
each of the satellites.

The Ground Station
•The ground antennas uplink the data to the
satellites.
•This data includes ephemeris and clock correction
information.
•This information is uploaded to each satellite three
times per day i.e. every 8 hours.
•The satellites use that corrected information in the
data transmission down to the end user.
•This sequence of events occurs every few hours for
each of the satellites to ensure that any possibility
of error creeping into satellite positions or their
clocks is minimised.

76
GPS segments
•Space segment
•Control segment
•User segment

User segment
• User segment consists of the user and his GPS receiver.
• GPS receivers are composed of
• an antenna (Internal or external), tuned to the frequencies
transmitted by the satellites,
• receiver-processors,
• a highly stable clock (often a crystal oscillator).
• they also include a display for providing location and speed
information to the user.
• A receiver is often described by its No. of channels: this
signifies signals’ from how many satellites it can process
simultaneously.
• Originally limited to 4 or 5, this has progressively
increased over the years.

User Segment
•Receiver types –applications
GPS receivers measurement
C/ A code Pseudo range 3-D position determination
(hikers & boaters)
C/ A code carrier Phase Carrier phase(L1 carrier)
(static, kinematic surveys)
P -code Carrier Phase L1 & L2 carrier phases
(surveying ,point positioning &
navigation)
(military related developments)
Y –code carrier Phase L1 & L2 carrier phases

Principle of operation of GPS
•Principle of operation –
TRILATERATION / TRIANGULATION
•It determines the position of an object by measuring its
distance from other objects of known locations.

TRILATERATION
•Trilateration is a process that uses distance from at
least three known locations to determine position.
•Trilateration can be either two dimensional or
three dimensional.

TRILATERATION
•To determine location of receiver, it has to find:
1. Where the satellites are (in space)?
2. How far are the satellites?

To find where the satellites are:
•The GPS receiver uses 2 kinds of coded information
for this estimation:
•Almanac data
•Ephemeris Data

•On the ground, all GPS receivers have an almanac
programmed into their computers that tells them
where in the sky each satellite is.
•This data helps the receiver to track the satellite
faster.
•Ephemeris data is required to know the exact
positions of each visible satellite and clock error for
the precise calculation of the receiver location.

Almanac data
•Almanac data is a coarse orbital model for all
satellites.
•This almanac data is not very precise and
considered valid for unto several hours.
•This data is continuously transmitted and stored in
the memory of GPS receiver so that it knows the
orbits of satellites and where each satellite is
supposed to be.
•The almanac data is periodically updated with new
information as the satellite move around.

Ephemeris Data
•It is very precise orbital and clock correction for
each satellites and is necessary for precise
positioning
•any satellite can travel slightly out of orbit, so the
ground monitor station keep track of the satellite
orbits altitude, location and speed.
•Each satellite broadcasts only its own ephemeris
data. Subsets of ephemeris data are broadcast by
each satellite continuously, which remains valid for
the orbit for the few minutes.

Almanac
• Coarse data
• Each satellite
broadcasts data for
all the satellites
• valid for several
months
Ephemeris
• Precise data
• Each satellite
broadcasts its own
data
• valid for 4-6 hrs

Determining how far the sat’s
are:
•The principle of operation is TOA (Time of arrival).
d = s x t
distance = velocity of light x travel time of satellite signal
3 x 108
m/s

Travel time of satellite signal
PRN Code-Pseudo Random noise code
--Unique ID for each satellite

Travel time of satellite signal

if receiver applies different PRN code to SV signal
…no correlation
when receiver uses same code as SV and codes begin to align
…some signal power detected

when receiver and SV codes align completely
…full signal power detected
Receiver compares the two codes to determine how much
it needs to delay its code to match the sat’s code-- Δt

•To determine location of receiver, it has to find:
1. Where the satellites are (in space)?
2. How far are the satellites?
•Now using this data the Rx
Position can be
determined in 2D or 3D

Determining Rx
Position in 2D
x
y
D2=t2x c
D1=t1x c
(x, y)

Determining Rx
Position in 3D
•Position determination using One Sat
The receiver is
somewhere on
this sphere.
D=t x c

Determining Rx
Position in 3D
•Position determination using Three Sat’s
D1=t1x c
D3=t3x c
D2=t2x c

Determining Rx
Position in 3D
•Position determination using Four Sat’s

Basic equation to find user/Rx
Position (4 satellites)
x
y
z
.(x1,
y1,
z1
)
.(x2,
y2,
z2
)
.(x3,
y3,
z3
)
.(xu,
yu,
zu
)
S1
S2
S3
R
D2
D1
D3
S4 .(x4,
y4,
z4
)
D4

(xu,
yu,
zu
) can be solved
4 equations- - - 4 unknowns

Pseudo range measurement
If user clk’s are perfectly synchronized with sat’s clk’s
then distance travelled is
D=C.(tu
- ts
)
•Practically it is difficult to obtain correct time .
d = s x t
Offsets of satellite
and receiver clocks

X
what if receiver wasn’t perfect?
…receiver is off by 1 second
“real” time
wrong time
XX position is wrong;
caused by wrong time
measurements
XX

How accurate a clock do we need?
• Electromagnetic waves travel at the speed of light
(c). In a vacuum is 299,792,458 m/second.
• Pseudorange is c * Δt.
– A clock accurate at 10-4
yields an error of 299,792 meter
error.
– A clock accurate at 10-9
yields an error of 3 meters.
– To obtain millimeter level precision we need a clock
accurate to what level?

Orbital Mechanics (Kepler’s laws)

Kepler’s Three Laws
•Laws of Planetary Motion
• Law 1 - Law of Ellipses
• Law 2 - Law of Equal Areas
• Law 3 - Harmonic Law
•Kepler’s laws provide a concise and simple
description of the motions of the planets

Kepler’s First Law:
Planets follow elliptical
orbits with the Sun at one
focus of the ellipse.
As the foci get closer and
closer together, the ellipse
approaches a circle.

Kepler’s 1st Law: Law of Ellipses
The orbits of the planets are ellipses with the sun at one focus.
Or, the orbits of satellites around the earth are ellipses with
the earth at one focus…..

Kepler’s Second Law: As a planet orbits the Sun, it moves
in such a way that a line drawn from the Sun to the planet
sweeps out equal areas in equal time intervals.

Kepler’s 2nd Law: Law of Equal Areas
As a planet orbits the Sun, it moves in such a way
that a line drawn from the Sun to the planet
sweeps out equal areas in equal time intervals.
t0
t3
t1
t2
Area 1
Area 2
t1
-t0
= t3
-t2
Area 1 = Area 2 Satellites travel at varying
speeds!!

Kepler’s 3rd Law: Law of Harmonics
The ratio of square of the planets’ orbital
period and the cubes of the mean distance
from the sun is const:
T1
2
/T2
2
= a1
3
/a2
3
= const

ORBITAL ELEMENTS
(Keplerian elements)
Purpose
•Discriminate one satellite from other satellites
•Predict where a satellite will be in the future or
has been in the past
•Determine amount and direction of maneuver

ORBITAL ELEMENTS /
The Six Keplerian Elements
• Size/Period (“a” is the semi-major axis of the orbit)
• Shape (Circular or Ellipse)(“e” is the orbit’s eccentricity)
• Inclination(“i “is the orbit’s inclination with respect to the
central body’s plane )
• Right Ascension (“Ω “is the right ascension of the ascending
node)
• Argument of Perigee(“ω” is the argument of perigee)
• True Anomaly (“ν” is the spacecraft’s true anomaly)

ORBITAL ELEMENTS
or
• Size/Period (“a” is the semi-major axis of the orbit)
• Shape (Circular or Ellipse)(“e” is the orbit’s eccentricity)
• Inclination(“i “is the orbit’s inclination with respect to
the central body’s plane )
• Right Ascension (“Ω “is the right ascension of the
ascending node)
• Argument of Perigee(“ω” is the argument of perigee)
• True Anomaly (“ν” is the spacecraft’s true anomaly)

ORBIT CLASSIFICATION
Size/Period
•Size is how big or small your satellite’s orbit is….
•Defined by semi-major axis
•There are basically 4 sizes of orbits satellites use:
• Low Earth Orbit (LEO):
• Medium Earth Orbit (MEO) or Semi-synchronous Orbit:
• Highly Elliptical Orbit (HEO):
• Geo-synchronous or Geo-stationary Orbit
(GEO):

ORBIT CLASSIFICATIONS
Elliptical Orbits
Characteristics
• Varying speed
• Varying altitude
• Asymmetric Ground Track

Eccentricity
• It fixes the shape of satellite’s orbit.
• It tells us how flat the orbit is.
• This parameter indicates the deviation of the orbit from
a perfect circle.
• A circular orbit has an eccentricity of 0, while a highly
elliptical orbit is closer to (but always less than) 1.

ORBITAL ELEMENTS
or
• a – Semi major axis of the orbit
• e – orbits eccentricity
Size and
shape of the
orbit
• i – Orbits inclination
• Ω – right ascesion of the ascending
node
• ω – argument of perigee
Orbital
plane
• v – the true anamoly
Position
in plane

ORBITAL ELEMENTS
Inclination
Orbital Plane
Equatorial Plane
Inclination
• Inclination is the tilt of the orbit
• At 0 degrees of inclination, you are orbiting the equator
• At 90 degrees of inclination, you are in a polar orbit
Inclination:
Is this angle,
measured in
degrees

ORBITAL ELEMENTS
Inclination
Equatorial: i = 0 or 180
Polar: i = 90
Prograde: 0 ≤
i < 90
Retrograde: 90 <
i ú 180

ORBITAL ELEMENTS
Right Ascension
Inclination
L
i
n
e
o
f
N
o
d
e
s
First Point
of Aries
(♈)
• Right Ascension will determine where your satellite will
cross the Equator on the ascending pass
• It is measured in degrees
Right Ascension
is this angle,
measured in
degrees
You will not have a
Right Ascension if your
Inclination is 0, why?

Right Ascension: It is an angle measured in the equatorial
plane from a reference point in the sky ( the vernal equinox)
and the ascending node.

ORBITAL ELEMENTS
Argument of Perigee
♈
Inclination
L
i
n
e
o
f
N
o
d
e
s
Ω
Perigee
Argument of
Perigee: Is
this angle,
measured in
degrees
• Argument of Perigee is the angle between the ascending
node and the perigee
• It is measured in the direction of motion of the satellite
Apogee

ORBITAL ELEMENTS
or
• a – Semi major axis of the orbit
• e – orbits eccentricity
Size and
shape of the
orbit
• i – Orbits inclination
• Ω – right ascension of the
ascending node
• ω – argument of perigee
Orbital
plane
• v – the true anamoly
Position
in plane

ORBITAL ELEMENTS
True Anomaly
Direction of satellite
motion
•True anomaly is the angle measured in the direction of motion
from perigee to the satellite's position at epoch time.
True Anomaly:
Is this angle,
measured in
degrees
Fixed point in
space

Time Representations
•Hour, Minute, Second
•Year, Month, Day
• Day of Year

GMT and UTC
• Greenwich Mean Time (GMT) was adopted as the
time standard around the world in 1884.
•It is the time calculated by the Greenwich
Observatory in London by reference to the Earth’s
rotation and by the position of the sun.
•Coordinated Universal Time (UTC) was developed
(1960’s) as a means of measuring time
independent of the Earth’s rotation and the sun
and the stars.
•It is the measure of rotation angle of Earth (360
deg) as measured astronomically

•GMT—Time Zone (24 hour format)
•UTC – Time standard (time scale for all time based
laboratories around the world)
•UTC is the same time all over the world because it
is the time measured by the vibration of caesium
atoms in atomic clocks
•Atomic clocks were set by reference to Greenwich
Mean Time
•UTC is the same as Greenwich Mean Time, but they
are not exactly the same because the Earth is
slowing down in its rotation. UTC is corrected
periodically with an integer number of leap secs

GPS Time
•GPS Time is the time scale used for referencing, or
time tagging, the GPS signals.
•It is computed based on the time scales generated
by the atomic clocks at the monitor stations and
onboard GPS satellites.
•There are no leap seconds introduced into GPS
Time, which means that GPS Time is a continuous
time scale.
•GPS Time scale was set equal to that of the UTC on
January 6, 1980

GPS time
•Primary time of reference for all operations. Unlike
UTC,GPS is a continuous time scale.
•GPS system time is always ahead of UTC by a
defined integer number of leap secs.
•Time recording for various standards at 2:06PM on
30th
march 2021

GPS time
•As of today GPS time is 18 secs ahead of UTC.
•GPS time is represented in GPS weeks and GPS
seconds from the start of the epoch.
•GPS epoch (=UTC) is
• 00hrs:00min:00secs of Jan 5th
/6th
1980
•The largest unit used in stating GPS time is one
week, defined as 604,800 seconds.
•Julian Dates (Day Number)
•Modified Julian Dates (MJD)

GPS time
• GPS week
• SOW
• 0 to 604800—largest unit in stating the GPS time
• Storage of such high values limits the efficiency/ storage
• DOW
• 1 to 7
• SOD
• 0 to 86400
• To calculate correct GPS satellite position , we need to
use the GPS time at the navigation message
transmission (Satellite internal clock time)

Julian Date and Modified Julian date
• GPS software uses these dates to keep a track of data.
• Julian Date (JD) - expressed as no. of days/ fractions of
the day
• Origin is fixed at Jan 0 ,4713 BC at noon
• Julian day denotes a day in this continuous count
• The Julian Day for 30 th march 2021 is 2459304.5
—Huge number
• Hence if often replaced by Modified Julian date (MJD)
• MJD= JD-2400000.5

Dilution of Precision(DOP)
•It is the measure of the geometry of the visible
satellite constellation.
•GPS errors resulting from the geometry of the
satellites can expressed in terms of the DOP.

Dilution of Precision(DOP)
•The best spread of satellites makes the best
trilateration
•The configuration of satellites in view to a receiver at
any given point can affect the accuracy of position
determination.
•The satellite geometry effect can be measured by a
single dimensionless number called the dilution of
precision (DOP).
•The lower the value of the DOP number, the better the
geometric strength, and vice versa

•The changes in the DOP value, however, will
generally be low except in the following two cases:
• a satellite is rising or falling as seen by the user’s
receiver, and
• There is an obstruction between the receiver and the
satellite (e.g., when passing under a bridge).

DOP can be expressed in the following
terms:
GDOP(Geometric
dilution of precision)
PDOP(Position
HDOP(Horizontal
VDOP(Vertical
TDOP(Time dilution
of the precision)

GDOP(Geometric dilution of precision
•It is the measure of the accuracy in 3D position and
time.
Positional accuracy= GDOP x total range error

GDOP cont.
•GDOP can be interpreted as the reciprocal of the
volume of the tetrahedron that is formed from the
4 satellites and the receiver position.
•Hence the best geometric situation for point
positioning is when the volume is which therefore
requires GDOP to be minimum

PDOP(Position dilution of precision
•PDOP represents the contribution of the satellite
geometry to the 3-D positioning accuracy.
•PDOP can be broken into two components:
• horizontal dilution of precision (HDOP) - represents the
satellite geometry effect on the horizontal component
of the positioning accuracy
• vertical dilution of precision (VDOP) - represents the
satellite geometry effect on the vertical component of
the positioning accuracy.

•VDOP will always be larger than HDOP --because
a GPS user can track only those satellites above the
horizon, the GPS height solution is expected to be less
precise than the horizontal solution.
• The VDOP value could be improved by supplementing
GPS with other sensors, for example, the pseudolites

•GDOP: geometric DOP (combination of PDOP &
TDOP)
•PDOP: positional DOP -Measure of accuracy in 3D
(combination of HDOP & VDOP)
•HDOP: horizontal DOP- Measure of accuracy in 2D
•VDOP: vertical DOP- Measure of height
•TDOP: time DOP - Measure of time

DOP Value Rating
1 Ideal
1-2 Excellent
2-5 Good
5-10 Moderate
10-20 Fair
>20 Poor

Unit-1 GNSS.pdf

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