1. Emmanuel Kollie

2. humans have always been interested in where things are.

3. one of the basic questions has always been…
where am I?….which leads to…
where am I going and how do I get there?
early solutions:
• marking trails with piles of stones
(problems when snow falls…or on ocean)
• navigating by stars
(requires clear nights and careful measurements)
most widely used for centuries
…location within a mile or so
modern ideas:
• 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

4. Department of Defense finally said:
“we need something better: all-day and all-night; all terrain”
end-product is Global Positioning System (GPS)
• system (constellation) of 24 satellites in high altitude orbits
(cost ~ $12 billion)
• coded satellite signals that can be processed in a GPS
receiver to compute position, velocity, and time
• parts of system include:
space (GPS satellite vehciles, or SVs)
control (tracking stations)
users
first one launched in 1978
….June 26, 1993
Air Force launched 24th SV

6. orbit ~ 12 hours

7. 27 satellites: 24 operational and 3 spare
ground tracks

8. basic concept is that the GPS constellation replaces “stars” and
gives us reference points for navigation
examples of some applications (users):
• navigation (very important for ocean travel)
• zero-visibility landing for aircraft
• collision avoidance
• surveying
• precision agriculture
• delivery vehicles
• emergency vehicles
• electronic maps
• Earth sciences (volcano monitoring; seismic hazard)
• tropospheric water vapor
anything that involves location, motion, or navigation

9. examples of applications

10. we will break system into five conceptual pieces
step 1: using satellite ranging
step 2: measuring distance from satellite
step 3: getting perfect timing
step 4: knowing where a satellite is in space
step 5: identifying errors

11. GSP satellite vehicles (SVs):
two generations: block I and block II
GPS block II
weigh ~1900 lbs.
built by Rockwell
GPS block I

12. three can be enough to determine position…
one of the two points generally is not possible (far off in space)
two can be enough if you know your elevation
…why?
one of the spheres can be replaced with Earth…
…center of Earth is “satellite position”
generally four are best and necessary….why this is a little later
this is basic principle behind GPS…
…using satellites for triangulation

13. step 2: measuring distance from satellite
because GPS based on knowing distance from satellite
…we need to have a method for determing how far
away the satellites are
use velocity x time = distance
GPS system works by timing how long it takes a radio signal
to reach the receiver from a satellite…
…distance is calculated from that time…
radio waves travel at speed of light: 180,000 miles per second
problem: need to know when GPS satellite started
sending its radio message

14. requires very good clocks that measure short times…
…electromagnetic waves move very quickly
use atomic clocks
came into being during World War II; nothing to do with GPS
-physicists wanted to test Einstein’s ideas about gravity and time
• previous clocks relied on pendulums
• early atomic clocks looked at vibrations of quartz crystal
…keep time to < 1/1000th second per day
..not accurate enough to assess affect of gravity on time
…Einstein predicted that clock on Mt. Everest
would run 30 millionths of a second faster
than clock at sea level
…needed to look at oscillations of atoms

15. principle behind atomic clocks…
atoms absorb or emit electomagnetic energy in discrete amounts
that correspond to differences in energy between different
configurations of the atoms
when atom goes from one energy state to lower one,
it emits an electromagnetic wave of characteristic frequency
…known as “resonant frequency”
these resonant frequencies are identical for every atom
of a given type:
cesium 133 atoms: 9,192,631,770 cycles/second
cesium can be used to create extraordinarily precise clock
(advances also led to using hydrogen and rubidium)
GPS clocks are cesium clocks

16. now that we have precise clocks…
…how do we know when the signals left the satellite?
this is where the designers of GPS were clever…
…synchronize satellite and receiver so
they are generating same code at same time
analogy:
2 people separated by some distance both start yelling
one, two, three…at same time
person 2 hears “one” shouted by person 1 when
person 2 says “three”
…if you both said one at same time,
the distance away person 2 is from person 1
is time difference between “one” and “three”
times the velocity of the sound
let us examine GPS satellite signals more closely…

17. SVs transmit two microwave carrier (carry information) signals
L1 ( MHz): carries navigation message; SPS code
(SPS: standard positioning servic)
L2 ( MHz): measures ionospheric delay
3 binary codes shift L1 and/or L2 carrier phases
C/A code (coarse acquisition) modulates L1 carrier phase
…repeating 1 MHz pseudo random noise (PRN) code
…pseudo-random because repeats every 1023 bits or
every millisecond…each SV has its own C/A code
…basis for civilian SPS
P-code (precise) modulates both L1 and L2
…long (7 days) pseudo random 10 MHz noise code
…basis for PPS (precise positioning service)
…AS (anti-spoofing) encrypts P-code into Y-code
(need classified module for receiver)
navigation message modulates L1-C/A; 50 Mhz signal
….describes satellite orbits, clock corrections, etc.

19. GPS receiver produces replicas of C/A and/or P (Y) code
receiver produces C/A code sequence for specific SV

20. C/A code generator repeats same 1023 chip
PRN code sequence every millisecond
PRN codes defined for
32 satellite ID numbers
modern receivers usually store complete set
of precomputed C/A code chips in memory

21. receiver slides replica of code in time until
finds correlation with SV signal
(codes are series of digital numbers)

22. 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

23. when receiver and SV codes align completely
…full signal power detected
usually a late version of code is compared with early version
to insure that correlation peak is tracked

24. each SV sends amount to which GPS time is offset from
UTC (universal time) time…
correction used by receiver to set UTC to within 100 nanoseconds

25. position determined from multiple pseudo-range measurements
4 satellites…3 (X, Y, Z) dimensions and time
when clock offsets are determined, the receiver position is known

26. this leads us to why 4 GPS satellites are necessary and to…

27. sites have co-located:
• VLBI (very long baseline interferometry);
• lunar laser-ranging (from instrument left by Apollo astronauts)
…primarily for length of day considerations
• satellite laser-ranging

28. step 5: identifying errors
ionosphere: electrically charged particles miles up;
affects speed of electromagnetic energy
…amount of affect depends on frequency
…look at differences in L1 and L2
(need “dual-frequency” receivers to correct)

29. user community…
primary application is GPS navigation
X, Y, Z (position) and time from 4 satellites to calculate position

30. differential GPS: improves accuracy
correct bias errors at one location using
measured bias errors at known position (base station)
…requires software in reference receiver that can track
all SVs in view and form individual pseudo-range
corrections for each

31. can also use carrier phase (L1; L2)
two receivers must be < 30 kms from one another
(ionospheric delay must be less than one wavelength);
requires special software
…real-time kinematic (RTK) processing

33. old slide (1994): currently, dual-phase geodetic receivers ~$10K

34.
Youtube song about GPS