1. GPS: MAJOR COMPONENTS AND
Part III
GPS: MAJOR COMPONENTS AND
THE SIGNAL STRUCTURE
GS608

2. GPS Satellite System
24 satellites
altitude ~20,000 km
12-hour period
6 orbital planes
inclination 55o

3. GPS Time System Precise time measurement is behind the success of GPS
GPS uses its own time system that is based on the atomic time scale
Basic units: second of the week (second since the beginning of the week) and a week number
The initial GPS epoch (week 0) is 0h UTC of January 6, 1980
Universal Coordinated Time (UTC) is the time scale based on atomic second that corresponds to Greenwich time, and is the basis for most radio time signals and legal time systems

4. Time Systems
Since TAI (atomic time) is independent of the Earth’s rotation, the concept of Coordinated Universal Time (UTC), that is in some prescribed way connected to the rotational time, was introduced in 1961, taking advantage of the stability, predictability and almost immediate accessibility of TAI.
UTC is based on the atomic second, thus its rate is uniform. Also, its epoch is manipulated accordingly so that the difference between the time based on Earth diurnal rotation and UTC is maintained on a level less than or equal to 0.7 s.
For that purpose UTC is modified by introducing a leap second, when required, e.g., on December 31 and/or June 30. As a result, UTC and TAI always differ by an integer number of seconds that can change only every year or one-half year

5. GPS Time System
Since UTC is altered to keep it synchronized with the rotational time (based on Earth rotation rate), the difference (in seconds) between UTC and GPS time grows
Consequently, what you see on most of GPS receiver displays is the GPS time, which is close to UTC (Greenwich time), which is 5 hours ahead from our time zone
One can usually set up the receiver to display local time if needed

6. GPS Time System
The Global Positioning System (GPS) experienced the first rollover of its internal clock, termed the End of Week (EOW) Rollover, on August 21, 1999
The EOW rollover exists because the largest increment for counting GPS system time is one week, and weeks are accumulated in a 10-bit register
GPS time started Jan. 6, 1980 with week "0000" and continued until 23:59:47 Universal Time Coordinated (UTC), Aug. 21.
After the rollover, the GPS clock reset itself to "0000." This was the first EOW rollover since the GPS constellation was established.

7. GPS Satellite System continuous signal transmit
fundamental frequency MHz
almost circular orbit (e = 0.02)
at least 4 satellites visible at all times
from any point on the Earth’s surface (5-7 most
of the time)

8. GPS - Major Components
Space Segment - responsible for satellite development, manufacturing and launching
Control Segment - continuous monitoring and controlling the system, determining GPS time, prediction of satellite ephemeris and the clock behavior, as well as updating the navigation message for every satellite
User Segment - numerous types of GPS receivers, providing navigators, surveyors, geodesists and other users with precise positioning and timing data

11. GPS Operational Modes
Precise Positioning Service (PPS) - only for authorized users, provides 2D point positioning accuracy of about below 10 to 20 m (real-time), and 3-5m for static (abut 1 hour) observation
Standard Positioning Service (SPS) - available for numerous civilian applications, provides 2D point positioning accuracy of about 40 m, and 3D accuracy of about ~70 m (much worse under SA);
However, the currently achievable accuracy, even with a hand-held receiver, is
Horizontal Accuracy (50%) meters
Vertical Accuracy (50%) meters
Horizontal Accuracy (95%) meters
Vertical Accuracy (95%) meters

12. Restricting the Accuracy of the Standard Positioning Service
Department of Defense (DoD) has established a policy for the implementation of Selective Availability (SA) and the Anti-Spoofing (AS) for the GPS signal to limit the number of unauthorized users and the level of accuracy for nonmilitary applications. This results in the degradation in the positioning performance and, in general, complicates the solution strategy.
Under AS, the P-code gets encrypted by adding (modulo 2 sum) a W-code, which results in the Y-code, not known to the civilian users.

13. The fundamental frequency of GPS signal
10.23 MHz
two signals, L1 and L2, are coherently derived from the basic frequency by multiplying it by 154 and 120, respectively, yielding:
L1 = MHz (~ cm)
L2 = MHz (~ cm)
The adaptation of signals from two frequencies is a fundamental issue in the reduction of the errors due to the propagation media, mainly, ionospheric refraction and SA

14. GPS Signals Two carrier frequencies (to remove ionospheric effects)
L1: MHz (154  MHz)
wavelength cm
L2: MHz (120  MHz)
wavelength cm

15. New GPS Signal FOR Civilian Users
Planned for Block IIF satellites (2005)
L5: MHz (115  MHz)
wavelength – 25.5 cm
Signal L2 will remain a civilian signal as well

16. GPS Signals Carrier L1 and L2 Codes superimposed on carrier
P-code (precise/protected code, under AS it’s replaced by a Y-code) on L1 and L2
C/A – code (clear/coarse acquisition) on L1
The fourth type of signal transmitted by GPS satellites is the broadcast message (navigation message) on L1 and L2 (identical)

17. GPS Signal Structure
Code modulation (sequence of binary values: +1 or –1)
L1: P1 & C/A code, navigation message
L2: P2 code, navigation message
P-code frequency MHz (i. e., million binary digits or chips per second)
P-code repetition rate: days, 7-day long portion of the code are assigned to every satellite; codes are restarted every week at midnight from Saturday to Sunday.
P-code “wavelength” m
C/A-code frequency MHz (i.e., million binary digits or chips per second; codes are repeated every millisecond)
C/A-code “wavelength” m

18. How do we get the numbers right?
Assuming MHz frequency for C/A-code, and repetition rate of 1 millisecond:
1,023,000 Hz * 10-3 sec = 1023 bits (or chips); this is the length of the C/A code
For 1023 chips in 1 millisecond we get separation between two chips equal to (roughly) 1 microsecond
1 microsecond separation between the chips corresponds to ~300 m chip length (for 300,000 km/sec speed of light)
Check it out the same way for the P-code!!!

19. GPS Signal Structure The epochs of both codes are synchronized
In civilian receivers, the short C/A code is acquired first to allow access to the P-code
Carrying two codes on L1 is achieved by phase quadrature
unmodulated L1 carrier is split off and shifted in phase by 90º, then mixed with C-code and then added to the
P-modulated signal – see Figure 7.8 below

20. APD(t)P(t)sin(1t)

22. GPS Signal Summary Table

23. GPS Message
Data File - carrier phase, pseudorange, and range rate (Doppler)
Navigation Message (broadcast ephemeris) - provides information about satellite orbits, time, clock errors and ionospheric model to remove the ionospheric delay (error) from the observations)
Provided in binary-receiver dependent format
Usually converted to RINEX - Receiver Independent Exchange format (ASCII file)

24. GPS Navigation Message
TLM = Telemetry Word HOW = Handover Word (contains Z-count)

25. TLM, telemetry word – contains a synchronization pattern which facilitates the access to the navigation data
HOW, handover word allows direct access to the P code; but first the C/A code must be acquired to allow for time synchronization; this allows an access to HOW from the navigation message, and then the P-code can be acquired
P-code can be accessed only after the C/A code-supported receiver time synchronization with GPS time through the Z-count
HOW contains so-called Z-count
Z-count is defined as integer number of 1.5-second periods since the beginning of the GPS week, and thus identifies the epoch of a data record in GPS time
If one knows the Z-count, one can acquire the P-code within the next six seconds

26. But we don’t know the actual P-code (under AS)
We already discussed how a GPS receiver measures the range (or pseudorange) to the satellite by measuring the time delay between the incoming signal and its replica generated by the receiver
Signal synchronization (correlation) provides the signal travel time measure
The PRN code (P-code) carried by the signal allows to achieve that (if its known; currently, civilians know only C/A code)
But how do we get an access to the precise code under AS policy, if the Y-code (replacing the P-code) is not known, and thus, the time synchronization scheme will not work?

27. Techniques to recover L2 signal under AS

28. GPS Navigation Message (RINEX)
NAVIGATION DATA RINEX VERSION / TYPE
DAT2RIN 1.00e The Boss JUN98 17:59:25 GMT PGM / RUN BY / DATE
COMMENT
.1118D D D D ION ALPHA
.9011D D D D ION BETA
D D DELTA-UTC: A0,A1,T,W
LEAP SECONDS
END OF HEADER
D D D+00
D D D D+00
D D D D+04
D D D D-07
D D D D-08
D D D D+00
D D D D+03
D+06
D D D+00
D D D D+00
…………………….

29. GPS Observation File Header (RINEX)
OBSERVATION DATA RINEX VERSION / TYPE
DAT2RIN 1.00e The Boss JUN98 17:59:19 GMT PGM / RUN BY / DATE
Mickey Mouse CFM OBSERVER / AGENCY
TRIMBLE 4000SSI Nav 7.25 Sig REC # / TYPE / VERS
ST L1/L2 GEOD ANT # / TYPE
____ MARKER NAME
____ MARKER NUMBER
APPROX POSITION XYZ
ANTENNA: DELTA H/E/N
WAVELENGTH FACT L1/2
4 L1 C1 L2 P # / TYPES OF OBSERV
INTERVAL
TIME OF FIRST OBS
TIME OF LAST OBS
# OF SATELLITES
PRN / # OF OBS
PRN / # OF OBS
PRN / # OF OBS
……………………… (rest of the SV is given here)………………………………… PRN / # OF OBS
END OF HEADER

30. GPS Observation File (RINEX)
………………………………………………………………………………. continues

31. RINEX 2 description: