Figure from Mike Rymer, USGS

Transcription

1 Ge111A Winter /5/2009 1

2 Figure from Mike Rymer, USGS Ge111A Winter /5/2009 2

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4 SWIR image made from ASTER data Ge111A Winter /5/2009 4

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9 Intersection of 2 faults just SE of Box Cyn Rd Ge111A Winter /5/2009 9

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11 Some info on GPS and datums GNSS = Global Navigation Satellite System GPS US Department of Defense ( 325) GLONASS Russian Space Agency ( GALILEO - European Commission ( Ge111A Winter /5/

12 CDMA FDMA CDMA Ge111A Winter /5/

13 GPS determines locations in Earth centered, Earth fixed (ECEF) Ge111A Winter /5/

14 need to convert to latitude, longitude, and height above ellipsoid Ge111A Winter /5/

15 need to use datum descriptions of Earth s surface depends on projections flat Earth for short distances ellipsoidal models for whole Earth GPS uses WGS-84 (ellipsoid) geoid: surface resulted from gravity alone Ge111A Winter /5/

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17 Global Datum e.g. WGS84, NAD83 Ge111A Winter /5/

18 Local Datum e.g. NAD27 CONUS Ge111A Winter /5/

19 other reference ellipsoids exist Ge111A Winter /5/

20 can convert from one datum to another (standard equations) Ge111A Winter /5/

21 note position shifts important to be consistent Ge111A Winter /5/

22 What is the difference between NAD27 and NAD83? Latitude Longitude Datum shifts in seconds of arc Ge111A Winter /5/

23 Datum shift in meters smallest in the center of the USA From Ge111A Winter /5/

24 What is the difference between NAD27 and NAD83? Change in reference ellipsoid from Clarke 1866 (origin at Meade s Ranch, KS) to GRS 1980 (origin at the center of the earth) Clean up of nearly 200 years worth of survey data No simple formula to convert from one to the other Use the computer program on the web site above Ge111A Winter /5/

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26 See the UNESCO training manual: Ge111A Winter /5/

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28 Ge111A Winter 2009 orbit ~ 12 hours 3/5/

29 basic concept is that the GNSS 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 Ge111A Winter /5/

30 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 Ge111A Winter /5/

31 step 1: using satellite ranging GPS is based on satellite ranging, i.e. distance from satellites satellites are precise reference points we determine our distance from them we will assume for now that we know exactly where satellite is and how far away from it we are if we are lost and we know that we are 11,000 miles from satellite A we are somewhere on a sphere whose middle is satellite A and diameter is 11,000 miles Ge111A Winter /5/

32 if we also know that we are 12,000 miles from satellite B we can narrow down where we must be only place in universe is on circle where two spheres intersect if we also know that we are 13,000 miles from satellite C our situation improves immensely only place in universe is at either of two points where three spheres intersect Ge111A Winter /5/

33 three can be enough to determine position one of the 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 Ge111A Winter /5/

34 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 Ge111A Winter /5/

35 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 Ge111A Winter /5/

36 principle behind atomic clocks atoms absorb or emit electromagnetic 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 Ge111A Winter /5/

37 GALILEO clocks are rubidium clocks and hydrogen maser clocks on board the satellite, synched to a cesium clock on the Earth at regular intervals Frequency is around 6 GHz for the rubidium clock and around 1.4 GHz for the hydrogen clock. Ge111A Winter /5/

38 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 Ge111A Winter /5/

39 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. Ge111A Winter /5/

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41 step 3: getting perfect timing electromagnetic energy travels at 186,000 miles per second an error of 1/100th second leads to error of 1,860 miles how do we know that receiver and satellite are on same time? satellites have atomic clocks (4 of them for redundancy) at $100,000 apiece, they are not in receivers! receivers have ordinary clocks (otherwise receivers would cost > $100K) can get around this by having an extra measurement hence 4 satellites are necessary three perfect measurements will lead to unique, correct solution.four imperfect ones also will lead to appropriate solution Ge111A Winter /5/

42 illustrate this in 2D instead of referring to satellite pseudo-range in distance, we will use time units two satellites: first at distance of 4 seconds second at distance of 6 seconds this is if clocks were correct X location of receiver is X what if they weren t correct? Ge111A Winter /5/

43 what if receiver wasn t perfect? receiver is off by 1 second real time X XX XX position is wrong; caused by wrong time measurements wrong time Ge111A Winter /5/

44 how do we know that it is wrong? measurement from third satellite (fourth in 3D) 3rd satellite at 3 seconds all 3 intersect at X if time is correct X if time is not correct Ge111A Winter /5/

45 add our one second error to the third receiver circle from 3rd SV cannot intersect where other 2 do purple dots are intersections of 2 SVs XX define area of solutions receivers calculate best solution (add or subtract time from each SV) Ge111A Winter /5/

46 finally step 4: knowing where a satellite is in space Air Force injected satellites into known orbits orbits known in advance and programmed into receivers satellites constantly monitored by DoD identify errors (ephemeris errors) in orbits usually minor corrections relayed back to satellite data message about their health Ge111A Winter /5/

47 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 Ge111A Winter /5/

48 step 5: identifying errors ionosphere: electrically charged particles miles up; affects speed of electromagnetic energy amount of effect depends on frequency look at differences in L1 and L2 (need dual-frequency receivers to correct) Ge111A Winter /5/

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50 tropospheric water vapor: affects all frequencies; difficult to correct multipath: reflected signals from surfaces near receiver noise: combined effect of PRN noise and receiver noise bias: SV clock errors; ephemeris errors selective availability: SA; error introduced by DoD; turned off May, 2000 blunders: human error in control segment user mistakes (e.g. incorrect geodetic datum) more on this in a minute receiver errors geometric dilution of precision (GDOP): errors from range vector differences between receiver and SVs (pictures coming ) Ge111A Winter /5/

51 geometric dilution of precision (GDOP) SVs occupy a small volume in the sky Ge111A Winter /5/

52 SVs occupy a large volume in the sky Ge111A Winter /5/

53 when measuring must have good GDOP and good visibility may not always be possible Ge111A Winter /5/

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55 Homework due Thursday March 6: Write a one-page introduction to the field project and the field area. This will be the introduction that you use in your field report for Ge111B. It is the last homework for Ge111A and will be returned to you before the field trip so you will have that part done on your report ahead of time. Ge111A Winter /5/