GPS uses 24 satellites in MEO orbit to provide positioning services to users. The GPS system consists of three segments - space, control, and user. The space segment comprises the GPS satellites. The control segment consists of monitoring and control stations that track satellites and upload navigation data. The user segment contains GPS receivers that use satellite signals and navigation messages to determine location. Each satellite continuously broadcasts navigation messages containing ephemeris, clock, and almanac data. The messages are structured in frames, subframes, and pages to efficiently transmit necessary information to users.

1. GPS Introduction & GPS Messages
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2. Topics to be Covered
Architecture of GPS
SPS and PPS
GPS frequency overview
GPS Satellite Signals
GPS data/time/week rollover
GPS message Structure
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3. At the end of this session, trainee will be able to:
Explain GPS architecture, frequency, satellite signals,
data, and time
Describe GPS message, its structure, frame, sub-frames
and pages
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4. Space
Segment
Control
Segment
User
Segment
GPS ARCHITECTURE
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5. 1. Constellation of 24 GPS satellites(more than 24 operational satellites as new
ones are launched to replace older satellites)
2. Near circular Orbits of approximately 20,200 Km above the earth (MEO)
3. Orbits are equally separated at a separation of 60°
4. Six Orbital Planes – Four in each Plane
5. Orbital inclination relative to Equator is 55º
6. Satellites travel at a velocity of 3.9 Km/Sec
7. Nominal Orbital Period – 11 hrs 58 minutes
8. This constellation provides the user with between five and eight SVs visible
from any point on the earth
9. Signal frequency L1=1575.42 MHz & L2=1227.6 MHz
SPACE SEGMENT
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6. SPACE SEGMENT
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7. •The GPS control segment consists of a global network of ground facilities that
track the GPS satellites, monitor their transmissions, perform analyses, and send
commands and data to the constellation
Monitor Stations – 16 no’s
 Tracks GPS Satellites by making pseudo range measurements to each
satellite in view using both L1 and L2 GPS satellite down link frequencies.
Master Control Station (MCS) – 2no’s
 Process the data received from monitor stations and is used to form satellite
clock corrections, ephemeris and almanac data and other indicators in the
navigation message .
Ground Uplink Antennas – 12 no’s
 Provides a means of commanding and controlling the satellites and
uploading navigation messages and other data.
CONTROL SEGMENT
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8. The Control Segment consists of a system of tracking stations located around
the world
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9. Monitor Stations pickup
Satellite Navigation Signals.
[Known Position]
Master Control Station Computes
GPS Position & Time Errors, and Relays
Corrections to Ground Antenna
Ground Antennae
Upload Ephemeris Co-ordinates
and Time Bias Factors
24 Hour Cycle
S-band Upload
Control Segment Operation
L-band
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10. User Segment
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11. 1. The GPS User Segment consists of the GPS receivers and the user community
2. GPS receivers convert SV signals into position, velocity, and time estimates
3. Four satellites are required to compute the four dimensions of X, Y, Z
(position) and time
4. Precise positioning is possible using GPS receivers at reference locations
providing corrections and relative positioning data for remote receivers.
5. Technique called trilateration is used to determine position or “fix”
USER SEGMENT
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12. • GPS offers two levels of services
1. Standard Positioning Service (SPS)-(Civil users worldwide use
without charge or restrictions)Less accuracy. For Civilian Use.
Broadcast of C/A code using L1 frequency.
2. Precise Positioning Service (PPS)- (Authorized users with
cryptographic equipment and keys and specially equipped receivers)
High accuracy. For Military use. Broadcast of P code using L1 and L2
frequencies.
• Expected Positional accuracy of GPS services
Error SPS PPS
SA active SA off
Horizontal 100 m 10 m Better then SPS
(S/A off)
Vertical 156 m 15 m
Time 340 (Nano Seconds) 200 (Nano Seconds)
GPS SERVICES
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13. Band Frequency Description
L1 1575.42 MHz Coarse-acquisition (C/A) and
encrypted precision (P(Y))
codes, plus the L1 civilian
(L1C) and military (M) codes
on future Block III satellites.
L2 1227.60 MHz P(Y) code, plus the L2C and
military codes on the Block
IIR-M and newer satellites.
L3 1381.05 MHz Used for nuclear detonation
(NUDET) detection.
L4 1379.913 MHz Being studied for additional
ionospheric correction.
L5 1176.45 MHz Proposed for use as a civilian
safety-of-life (SoL) signal.
GPS FREQUENCIES OVERVIEW
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14. 1. The L1 frequency Carries
navigation message and
SPS code signals
2. The L2 frequency is used to
measure Ionospheric delay
by PPS equipped receivers
C/A CODE(basis for SPS)
1. Modulates L1 Carrier
phase
2. 1023 bits long
transmitted at 1.023
mbps and repeats
once a millisecond
3. Different C/A code for
each PRN allowing
receivers to recognise
multiple satellites with
same frequencies
P CODE(basis for pps)
1. Modulates L1 & L2
2. 6.1871 × 1012 bits long
(720.213 gigabytes)
transmitted at 10.23 mbps and
only repeats once a week.
3. In Anti spoofing Pcode is
encrypted into Y code
Navigation Message
1. Modulates L1-C/A code signal
2. 50 Hz signal consisting of data
bits that describe the GPS
satellite orbits, clock corrections,
and other system parameters
GPS SATELLITE SIGNALS
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15. GPS DATA
The GPS Navigation Message consists of time-tagged data bits marking the time of transmission of
each subframe at the time they are transmitted by the SV.
A data bit frame consists of 1500 bits divided into five 300-bit subframes. A data frame is transmitted
every thirty seconds.
Three six-second subframes contain orbital and clock data
set of twenty-
five frames (125
subframes)
makes up the
complete
Navigation
Message that is
sent over a 12.5
minute period.
Sub frames four
and five are
used to transmit
different pages
of system data
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16. GPS Message
The Navigation Message provides all the necessary information that allows the
user to perform the positioning service. It includes
Ephemeris parameters, needed to compute the
satellite coordinates
Time parameters and Clock Corrections, to
compute satellite clock offsets and time
conversions
Service Parameters with satellite health
information
Ionospheric parameters model needed for single
frequency receivers, and the Almanacs, allowing
the computation of the position of all satellites in
the constellation
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17. GPS Message
The ephemeris
and clocks
parameters are
usually updated
every two hours
almanac is
updated at least
every six days
 structure of the navigation message
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18. FRAME
Sub
frame 1
Sub
frame 2
Sub
frame 3
Sub
frame 4
Sub
frame 5
To transmit a
complete
almanac, 25
different frames
are required
(called pages).
Transmission time
for the entire
almanac is
therefore 12.5
minutes
1500 Bits, 30 Seconds
300 bits, 6secondsEach frame divided into 5 Sub frames
 Each Sub frame
divided into 10 words
each containing 30
bits
 30 bits, 0.6 seconds
GPS Message
Each sub-frame
begins with a telemetry
word and a handover
word (HOW)
TLM HOW
DATA
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19. TLM and HOW
GPS Message
 The telemetry word (TLM) the first word of every Sub frame contains
1. preamble sequence 8 bits in length (10001011) used for synchronization
purposes
2. 16 bits reserved for authorized users
3. final 6 bits are parity bits
8 Bits 16 Bits 6 Bits
Preamble Reserved Parity
Telemetry Word
(TLM)
30 bits
Preamble
10001011
TLM Message 1 1 Parity
TLM Word
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20. GPS Message
The handover word (HOW) immediately follows the telemetry word in each sub frame
1. The handover word is 17 bits in length ranging from 0 to 131071
2. It Contains start time for the next sub frame, which is transmitted as time of the week (TOW)
3. TOW count begins with the value 0 at the beginning of the GPS week and is increased by a value
of 1 every 6 seconds, the count runs from 0 to 100,799 before returning to 0
4. Bit Nos. 20 to 22 is used in the handover word to identify the sub frame just transmitted.
5. Bit 18 is an "alert" flag. When this flag is raised(bit 18 = "1"),it shall indicate to the unauthorized
user that the SV URA may be worse than indicated in sub frame 1 and that he shall use that SV at
his own risk.
6. Bit 19 is an anti-spoof (A-S) flag. A "1" in bit-position 19 indicates that the A-S mode is ON in that
SV
17 Bits 7 Bits 6 Bits
Time of week Div ID Parity
Time of week Sub
frame ID
0 0
Alert Flag
AS Flag
3bits
Parity
Solved for bits to preserve
parity check with 0’s in
bits 29,30
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21. GPS Message
Subdivision of the 25 pages
1. A complete navigation message requires 25 pages and lasts 12.5 minutes.
2. In the case of subframes 1 to 3, the information content is the same for all 25 pages. This
means that a receiver has the complete clock values and ephemeris data from the
transmitting satellite every 30 seconds
3. The sole difference in the case of sub-frames 4 and 5
pages 2, 3, 4, 5, 7, 8, 9 and 10 relay
the almanac data on satellite
numbers 25 to 32. The almanac
data for one satellite only is
transferred per page
Page 18 transmits the values for
correction measurements as a
result of ionospheric scintillation,
as well as the difference between
UTC and GPS time
Page 25 contains information on the
configuration of all 32 satellites (i.e.
block affiliation) and the health of
satellite numbers 25 to 32.
Sub Frame
4
pages 1 to 24 relay the almanac
data on satellite numbers 1 to
24. In each case, the almanac
data for one satellite only is
transferred per page
Page 25 transfers information
on the health of satellite
numbers 1 to 24 and the
original almanac time.
Sub Frame
5
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22. GPS Message –Sub Frame Contents
Sub Frame 5
almanac data on satellite numbers 1 to 24 , All 25 pages are transmitted together with information
on the health of satellite numbers 1 to 24.
Sub Frame 4
almanac data on satellite numbers 25 to 32
difference between GPS and UTC time and
information regarding any measurement errors
caused by the ionosphere
Sub Frame 2 & 3
ephemeris data of the transmitting satellite
Sub Frame 1
time values of the
transmitting satellite
parameters for
correcting signal transit
delay and on board clock
time
satellite health and an
estimation of the
positional accuracy of
the satellite
10-bit week number
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23. GPS TIME
1. GPS Time is a "paper clock" ensemble of the Master Control Clock and the SV clocks
2. GPS Time is measured in weeks and seconds from 24:00:00, January 5, 1980 and is
steered to within one micro second of UTC
3. GPS Time has no leap seconds and is ahead of UTC by several seconds. GPS is now
ahead of UTC by 17 seconds
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24. Time in Universal Coordinated Time (UTC) is computed from GPS Time using the UTC
correction parameters sent as part of the navigation data bits.
At the transition between 23:59:59 UTC on December 31, 1998 and 00:00:00 UTC on
January 1, 1999, UTC was retarded by one-second. GPS Time is now ahead of UTC by 18
seconds
Note: Latest GPS
roll-over happened
in 6th April, 2019.
Firmware in all G-II
receiver of INRES
station were
updated, to nullify
its impact in GAGAN
performance.
GPS Weak rollover
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