Emmanuel Kollie

humans have always been interested in where things are.

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

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

orbit ~ 12 hours

27 satellites: 24 operational and 3 spare ground tracks

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

examples of applications

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

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

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

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

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

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

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…

SVs transmit two microwave carrier (carry information) signals L1 (1575.42 MHz): carries navigation message; SPS code (SPS: standard positioning servic) L2 (1227.60 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.

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

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

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

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

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

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

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

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

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

step 5: identifying errors ionosphere: electrically charged particles 80-120 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)

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

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

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

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

http://www.youtube.com/watch?v=XukJgsnKzro Youtube song about GPS