GPS_measure_continue1.ppt satellite data gps measuememnt

Consists of GPS receivers, which can be found in devices like smartphones, car navigation systems, and specialized GPS units.
Receivers decode the signals from the satellites to determine their position, velocity, and time.
How GPS Works
Signal Transmission:

Each GPS satellite continuously transmits a signal containing the satellite's position and the exact time the signal was sent.
The signal travels at the speed of light and can be received by any GPS receiver within its range.
Triangulation:

The GPS receiver needs signals from at least four satellites to determine its three-dimensional position (latitude, longitude, and altitude) and time.
By calculating the time it took for each satellite's signal to reach the receiver, the distance to each satellite is determined.
Using these distances, the receiver employs a process called trilateration to pinpoint its exact location.
Error Correction:

Various factors can affect the accuracy of GPS signals, such as atmospheric conditions, signal reflection (multipath), and clock discrepancies.
The control segment updates the satellites' information to account for these errors.
Some GPS systems use additional techniques like Differential GPS (DGPS) and Wide Area Augmentation System (WAAS) to improve accuracy.
Applications of GPS
Navigation:

Widely used in cars, airplanes, ships, and smartphones for turn-by-turn navigation and route planning.
Enables precise location tracking, which is essential for modern logistics and delivery services.
Mapping and Surveying:

Provides accurate geolocation data for creating maps and conducting land surveys.
Utilized in Geographic Information Systems (GIS) to collect and analyze spatial data.
Timing and Synchronization:

Atomic clocks in GPS satellites provide highly accurate time signals.
Used in telecommunications, banking, and power grid operations for precise time synchronization.
Recreational and Scientific Uses:

1. Last time:Terrestrial Positioning
• Levelingmeasures difference in height from an instrument
to a level rod or staff, relative to the geoid; can be accurate
to ~ 1 mm or less…
• Electronic Distance Measurementaccurate to ~ 1 cm
(less for e.g. two-color laser which reduces refraction error).
• Space-based positioning (GPS, InSAR) is cheaper and as or
more accurate, so dominates modern tectonic geodesy.
GPS Positioning
• Resection(location of intersecting ranges) uses 3 comp’ts:
• Space segment(satellite vehicles active transmission)
•Control segment(orbit determination & communication)
•User segment(antenna/receiver passive)
Geology 6690/7690
Geodesy & Crustal Deformation
06 Sep 2013
© A.R. Lowry 2013Read for Mon 9 Sep: Wahr§1 (1-3); §2.5 24-75

2. (The view from Earth Science/Engineering, KAUST)

3. Reminder: Read for next Wed (11 Sep)
Luttrell, K., Mencin, D., Francis, O., & Hurwitz, S. (2013).
Constraints on the upper crustal magma reservoir beneath
Yellowstone Caldera inferred from lake‐seiche induced
strain observations. Geophysical Research Letters40(3)
501–506.
Discussion lead should be prepared with
• Slides with each of the important figures from the paper
• Summary slides of the observations, methodology,
& results
• Supporting slides from other sources that help to illustrate
important or unfamiliar tools, concepts, ideas
• Critical thinking skills switched to “ON”
Who wants it?

4. Critical Thinking Skills (I):
When reading ANYpaper, it’s important to make certain
you understand the terminology being used, e.g.:
• Seiche
• Borehole strainmeter
• Load
• Spectrogram

5. Critical Thinking Skills (II):
Always Look Carefully at the Data Analysis:
• Is there a better way to quantify the data?
• Is there a potential for multiple signal sources?
• Is there an approach to analysis that might remove
signals that aren’t relevant for processes you wish
to understand?
• Are there quantitative tools that might have been
used to extract more information from the data?

6. Critical Thinking Skills (III):
When reading a paper that involves dynamical modeling,
it’s always helpful to think about:
• What are the assumptions of boundary conditions
(and do they matter?)
• What are the assumptions of initial conditions
(and do they matter?)
• What physical processes are being modeled
(and are there neglected physical processes that
could conceivably be important?)
• What observations are being modeled
(and how are they related? Qualitative or quantitative?
Comparison or inversion? What is the criterion for a
“Good Match” to the observations? Are there other
observations that might be relevant?)
• Does the model make testable predictions? How might
you test it?

7. Critical Thinking Skills (IV):
Assumptionsmatter!
• What was assumedabout the system being studied in
order to simplify or approximate the dynamics? Are
these assumptions reasonable?
Numbers matter!
• What are the assumptionsof rock material properties?
Do they gibe with laboratory measurements?
Do they gibe with geophysical measurements of
in-situ properties?
• What are the numerical values of other physical
properties? Are they reasonable? Is there
observational support?

8. Components I. Space Segment
•Constellation of 24+ satellites (21
navigational plus 3+ active spares) in
12-hour circular orbits at 20,200 km
(currently 31 + 3-4 decommissioned)
•Six planes (4+ space vehicles–SV’s–
each) inclined 55°from equatorial
•Six to twelve satellites in view from
anywhere on Earth, at any given time
•SV’s transmit low-level microwave
signals
•Four atomic clocks each (2 cesium;
2 rubidium)
(Courtesy US DOD!)

9. Components II. Control Segment
•Satellites are controlled by US Air
Force satellite command near
Colorado Springs
•Five global monitoring stations track
SV’s, calculateephemerides
(SV orbital position and velocity)
•Master control facility in CO transmits
ephemerides, clock corrections,
almanac to SV’s (so that these can
be relayed to GPS receivers) &
performs maneuvers

10. Components III. User Segment
Receivers havecodesto recognize GPS
signals; use ranges to calculate (x,y,z)
location+ time(1 m of range = 3 ns!)
Commonly used for:
•Navigation (e.g., air traffic, ships,
autos, hikers!)
•Surveying (property boundaries, GIS,
crustal deformation)
•Time-keeping (e.g. seismo stations)
•Meteorology (trop. water vapor, space
weather)
Civilianusers have now wrested some
control away from military…

11. GPS Signal Structure
Cesium clocks on SV’s havefundamental frequency
f
0= 10.23MHz (offset 5 mHz for relativistic effects
between SV and surface)
Clocks are stable to 10
-13
s/day (= 0.03 mm range)
Cesium oscillator drivessignalsandcodes:
•Signalsare unmodulated carrier frequenciesL
i=a
icos(f
it)
(Recallf =2p/T =c/l)
L
1154f
0= 1575.42MHzwavelengthl= 19.0cm
L
2120f
0= 1227.60MHzwavelengthl= 22.4cm
(AlsoL
3for NTV;L
4, L
5in preparation)

12. GPS Signal Structure Cont’d
Carrier frequencies are modulated (multiplied) by +1 or –1
using three binarycodes:
•C/A (Coarse Acquisition) codef
o/10= 1.023MHz
(l= 293m)
–Unique, public pseudo-random noise (PRN) code for each SV
–PRN repeats once per millisecond
–Receivers use known PRN tocross-correlatewith antenna
measured microwave signal and pull out individual SV signals!
+1
–1
1 11100 0 “chips”

13. GPS Signal Structure Cont’d
Othercodesinclude:
•P (Precise) codef
0= 10.23MHz(l= 29.3m)
–Unique ENCRYPTED PRN for each SV (Also called
Y-code); an anti-spoofing (AS) measure to ensure
selective availability and to prevent jamming (which
it ultimately cannot do).
–PRN would repeat once every 266.4 days!
BUT key is changed once per day…
•Message codeat 50 Hz contains:
–satellite health
–almanac (approx. orbits = first 6 of 21 ephemerides)
–satellite clock correction term
–time tag
• M-Code: A military only code about which little is known.