BRIDGE EQUIPMENT - GPS

GLOBAL POSITIONING SYSTEM

OVERVIEW
 Official name of GPS is NAVigational Satellite Timing And Ranging Global Positioning System
(NAVSTAR GPS). Global Positioning Systems (GPS) is a form of Global Navigation Satellite
System (GNSS). First developed by the United States Department of Defense
 Consists of two dozen GPS satellites in medium Earth orbit (The region of space between
2000km and 35,786 km)

GPS FUNCTIONALITY
GPS systems are made up of 3 segments:
 Space Segment (SS)
 Control Segment (CS)
 User Segment (US)

SPACE SEGMENT
It consists of 24 operational satellites evenly placed in 6 different orbits. The satellites have their
own propulsion system to maintain their orbital path and can be controlled remotely. The angle
between each of the 6 orbital planes and the equatorial plane i.e., Inclination, is 55°. GPS satellites
fly in circular orbits at an altitude of 20,200 km and at a speed of 3.9 km/s. It takes 12 hours to
complete one orbit. Each satellite makes two complete orbits each sidereal day. It passes over the
same location on Earth once each day. The satellites continuously orient themselves to point their
solar panels toward the sun and their antenna toward the earth. Orbits are designed so that at the
very least six satellites are always within line of sight from any location on the planet.

CONTROL SEGMENT
The CS consists of 3 entities:
 Master Control System
 Monitor Stations
 Ground Antennas
Master Control Segment
The master control station, located at Falcon Air Force Base in Colorado Springs, Colorado, is
responsible for overall management of the remote monitoring and transmission sites.
Monitor Stations
Eleven monitor stations are located at Falcon Air Force Base in
Colorado Springs (MCS), Hawaii, Ascension Island, Diego Garcia,
Kwajalein Island, Australia, Bahrain, Argentina, Washington
(USA), Equador and UK. Each of the monitor stations checks the
exact altitude, position, speed, and overall health of the orbiting
satellites. The monitoring stations track the satellites, obtain the
data and pass the information to the MCS. The MCS uses the data to compute and predict the future
path of all the satellites.
The MCS also determines the error of the atomic clocks in all the satellites. The updated data is then
fed to the upload station which in turn transmits the same data to each satellite three times a day.
Ground antennas:
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Ground antennas monitor and track the satellites from horizon to horizon. They also transmit
correction information to individual satellites.

USER SEGMENT:
It consists of a receiving antenna, receiver with built-in Computer and display unit. The receiver locks
on to one satellite and from this satellite it obtains the almanac of all other satellites and thereby
selects the four most suitable for position fixing. The fix obtained is displayed on the screen along
with the course and speed made good. Receivers designed to receive only one frequency are known
as single frequency receivers. To enhance position accuracy, there are dual frequency receivers that
can receive both frequencies.

SATELLITE NAVIGATION DATA STRUCTURE

GPS satellites broadcast three different types of data.
Almanac – The almanac contains information about the status of the satellites and their
approximate orbital information. The GPS receiver uses the almanac to calculate which satellites are
currently visible. This tells the GPS receiver where each satellite should be at any time through the
day. However, the almanac is not accurate enough to let the GPS receiver get a fix.
Ephemeris – To get a fix, GPS receiver requires additional data for each satellite, called the
ephemeris. This data gives very precise information about the orbit of each satellite and current date
& time. GPS receiver can use the ephemeris data to calculate the location of a satellite to within a
metre or two. The ephemeris is updated every 2 hours and is usually valid for 4 hours.
Pseudo Random Code –Each satellite broadcasts what is called a pseudo random code for
timing purposes (each satellite has its own distinct and unique pseudo random code). GPS satellites
and receivers are synchronized so they're generating the same code at exactly the same time. The
distance (or range) between the receiver and the satellite can be calculated from the time it takes
for the satellite signal to reach the receiver. To determine the time delay, all we have to do is receive
the codes from a satellite and then look back and see how long ago our receiver generated the same
code. The time difference is how long the signal took to get down to us.

P Code and C/A Code: Each satellite

transmits two codes, i.e. the Precision (P)
code and the Coarse Acquisition (C/A)
code. All civilian GPS receivers decode the
C/A code and it also helps the military
receivers to access the more accurate,
encrypted, P code. These codes are
modulated by phase modulation technique
on two carrier frequencies, L1 and L2. The
L1 signal consists of both the codes i.e. the
P and the C/A codes while the L2 consists of only the P code. Data transmitted are as follows:
 L1 (1575.42 MHz) - Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted
precision (P) code.
 L2 (1227.60 MHz) – Precision (P) code only.

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
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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.

OPERATION OVERVIEW
A GPS receiver can tell its own position by determining the ranges to three satellites and using
Trilateration. To get the distance, each satellite transmits a signal.
These signals travel at a known speed. The system
measures the time delay between the transmission of the
signal by the satellite and its reception by the receiver.
The signals are carrying information about the satellite’s
location. The GPS receiver determines the position of and
distance to, at least three satellites (to reduce error) and
computes the position using Trilateration.

POSITION CALCULATION:
The coordinates are calculated according to the World
Geodetic System WGS84 coordinate system.
The satellites are equipped with atomic clocks while GPS receiver uses an internal crystal oscillator-
based clock that is continually updated using the error correction signals from the satellites. To make
the measurement we assume that both the satellite and our receiver are generating the same
pseudo-random codes at exactly the same time. Receiver identifies each satellite's signal by its
distinct C/A code pattern, then measures the time delay for each satellite.
The distance is obtained by multiplying the travel time by the speed of light. Orbital position data
from the Navigation Message is used to calculate the satellite's precise position. Knowing the

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position and the distance of a satellite indicates that the receiver is located somewhere on the
surface of an imaginary sphere centered on that satellite and whose radius is the distance to it.
When distances from four such satellites are measured at the same time, the point where the four
imaginary spheres meet is recorded as the location of the receiver. This method is called
Trilateration.

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:
i) In the direction towards the user
ii) In the direction perpendicular to (i).
nd
The 2 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 from their respective satellites. These
velocity vectors are resolved into three other vectors, i.e. x, y and z co-ordinates and with these
three vectors the course and speed of the user is calculated.

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.
 Effect is minimized when the satellite is directly overhead.
 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
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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.

ACCURACY OF GPS
The accuracy of the position your GPS reports is influenced by a number of factors, such as the
positions of the satellites in the sky, atmospheric effects, satellite clock errors and ephemeris errors
etc., as explained above. Under ideal conditions, this may be 5, or even 3 metres. GPS units often
show on the screen an accuracy figure.

The accuracy of a GPS can be improved by using secondary data from external reference stations.
One such method is DGPS.

DIFFERENTIAL GPS (DGPS): DGPS uses a network of ground stations located at precisely known
positions. The DGPS reference station is situated at a fixed location and from this position the GPS
receiver tracks all the satellites within its sight, obtains the data from them and calculates the
corrections for each satellite.

These corrections are then transmitted

to the GPS receiver. There are two
methods employed for enhancing DGPS
position accuracy.

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In the first method, the reference station knows it’s exact position and compares it to the position
obtained by the GPS and determines the corrections based on the actual and GPS positions. These
corrections are then broadcast to the users in terms of co-ordinates (i.e. x, y and z). The accuracy
decreases as the distance of the user from the reference station increases.

Besides, the reference station and the user must select the same satellites, which is practically not
possible as the user does not have the option of manually selecting the satellites.

In the second method, the reference station receives signals from all the visible satellites and
measures the pseudo range to each of them. The reference station also calculates the true range to
all the selected satellites. By comparing the calculated true range and the measured pseudo range,
corrections to each satellite can be determined. These corrections are then broadcast to the users
and applied to their pseudo range measurements before calculation of the position.

As the above explanations warrants, the DGPS will be working only in coastal areas where such
services are provided.