1. REAL-TIME SURVEYING WITH GPS

2. Important Phone Numbers
Trimble support
Technical Assistance Center ftp://ftp.trimble.com
(hardware and software support) SOS-4TAC
Computer Bulletin Board
System Operator (David Elms)
Coast Guard Navigation Center
Recorded message
Live voice
Computer Bulletin Board
National Geodetic Survey
Information Center
Computer Bulletin Board or 4182
Point out the handout in the grey folder.
Point out that all the about GPS Information Providers are accessable by voice, modem (BBS) or internet.
The Coast Guard provides information on the status of the GPS constelation. (Show the students a sample copy of a NANU)
The NGS provides you with the Precise Ephemeris, which can be used for very precise position over long baselines.

3. GPS some background
Satellite based positioning in development since mid 1960’s
NAVSTAR GPS
NAVigation Satellite Timing And Ranging
Global Positioning System
NAVSTAR GPS - Merging of two military programs in 1973
Naval Research Laboratory - TIMATION program
Air Force - 621B Project
Managed by the Department of Defense
System tested with Ground Transmitters (pseudo-satellites)
First test satellites (Block I) launched in 1978
Operational satellites began launching in 1989 (Block II & Block IIA)
Block II & Block IIA launched by Delta II rockets from Cape Canaveral
Next generation of satellites (Block IIR) are already on contract
Ground Transmitter ( GT ) testing started in mid 1970’s ?
U.S. Army Yuma Proving Grounds ( YPG ) Yuma, Arizona
Inverted Range Control Center ( IRCC ) with 4 GT’s
Proof of concept testing, Phase I, II, III testing.
Tanks, Jets, Helicopters, Jeeps.

4. GPS the segments Space Segment User Segment Monitor Stations
Diego Garcia
Ascension Is.
Kwajalein
Hawaii
Colorado Springs
Control Segment

5. Control / Monitor Segment
5 Stations world-wide
Monitored by Department of Defense
All perform monitor functions
Receive all satellite signals
Collect Meteorological data ( used for ionospheric modelling )
Transmit data to MCS
Master Control Station
Upload to Satellites
Orbital prediction parameters
SV Clock corrections
Ionospheric models
(Basically everything in NAVDATA)
SV commands
Colorado Springs -- MCS
Diego Garcia
Ascension Islands
Kwajalein
Hawaii
Onizuka - Backup Control Station
control SA/A-S selective availability/anti-spoof

6. Space Segment 25 satellites in final constellation Very high orbit
6 planes with 55° rotation
each plane has 4/5 satellites
Very high orbit
20,183 KM, 12,545 miles
approximately 1 revolution in 12 hours
for accuracy
survivability
coverage
Copied from “GPS Navstar User’s Overview” prepare by GPS Joint Program Office, 1984

7. User Segment Surveyors Anyone with GPS equipment
Hardware and Software can be application specific
Vehicle Tracking Ambulances
Navigation Police
Mapping Cruise Ships
Hydrographics Courier Services
Aircraft Approach and Landing Hikers
Dredging
Sunken ship salvage
Oil Exploration

8. Working surfaces
A Datum is described by a specifically oriented reference ellipsoid defined by 8 elements
Position of the network (3 elements)
Orientation of the network (3 elements)
Parameters of the reference ellipsoid (2 elements)
Ellipsoid fitting North America
Ellipsoid fitting Europe
Font change?? more Datum slides ??
Geoid
Regional Datums are designed so that the ellipsoid conforms to
the geoid over the desired region rather than the whole Earth

9. Earth-Centered, Earth Fixed System
Z axis = Mean rotational axis (Polar axis)
X axis = 0 longitude
X axis in plane of equator
Y axis = 90 E longitude
Y axis in plane of equator

10. Elements of an ellipse a = semi-major axis b = semi-minor axis
f = flattening = (a-b)/a
Parameters used most often: a and 1/f
SEMI-MAJOR AXIS
SEMI-MINOR AXIS

11. Ellipse in 3-D: an Ellipsoid
Rotate ellipse about semi-minor (polar) axis to obtain 3-d ellipsoid
Semi-major axis is equatorial axis
SEMI-MAJOR AXIS
SEMI-MINOR AXIS

12. Common ellipsoids in surveying
WGS (Datum = WGS-84)
a = b = /f =
GRS (Datum = NAD83)
a = b = /f =
Clarke (Datum = NAD27)
a = b = /f =
NOTE SIMILARITY BETWEEN WGS-84 AND GRS-80

13. Datum (WGS 84)

14. Datum (NAD 27)

15. Datum
One point can have different sets of coordinates depending on the datum used x

16. Coordinate Systems Z Cartesian coordinates (X, Y, Z)P H
Cartesian coordinates (X, Y, Z)
Ellipsoidal coordinates (f, l, H) Z Y f l X Y X

17. Altitude Reference Ellipsoid Geiod
A smooth, mathematically defined model of the earths surface
Geiod
A surface of equal gravitational pull (equipotential) best fitting the average sea surface over the whole globe
HAE
MSL
Earths Surface
Notes on Heights:
GPS gives heights, or changes in heights, above the WGS-84 ellipsoid
Conventional elevations and levels are referenced to mean sea level, or the geoid
Conventional elevations generally considered to be orthometric heights
Ellipsoid and geoid are not necessarily coincident or parallel
Difference between the two surfaces is Geoid Separation, or N
Estimates of N can be obtained from geoid models
Geoid 90 and Geoid 93 based on 5-km grid
Basic equation: Ortho hgt = Ellipsoidal hgt - Geoid separation
or h = H N
BEWARE: use of h and H is not consistent throughout geodetic and GPS literature. KNOW WHAT YOU’RE LOOKING AT!
BEWARE also the use of term “Geoid Hgt”: it is height of geoid above or below the ellipsoid
Ellipsoid
Geoid

18. Notes about the geoid The geoid approximates mean sea level
The geoid is a function of the density of the earth
The geoid is a level surface which undulates
Conventional levels are referenced to the geoid

19. Reference Surfaces Earths Surface 50 ft Ellipsoid Height = H H = 41 ft
B.M. “A” elevation 84 ft
B.M. “ B ” elevation 73 ft
Earths Surface
50 ft
Ellipsoid Height = H
H = 41 ft
Ellipsoid
84 ft
h = 73 ft
Orthometric
Height = h
Notes on Heights:
GPS gives heights, or changes in heights, above the WGS-84 ellipsoid
Conventional elevations and levels are referenced to mean sea level, or the geoid
Conventional elevations generally considered to be orthometric heights
Ellipsoid and geoid are not necessarily coincident or parallel
Difference between the two surfaces is Geoid Separation, or N
Estimates of N can be obtained from geoid models
Geoid 90 and Geoid 93 based on 5-km grid
Basic equation: Ortho hgt = Ellipsoidal hgt - Geoid separation
or h = H N
BEWARE: use of h and H is not consistent throughout geodetic and GPS literature. KNOW WHAT YOU’RE LOOKING AT!
BEWARE also the use of term “Geoid Hgt”: it is height of geoid above or below the ellipsoid
34 ft
N = 32 ft
Geoid
Geoid
Height = N
DE = B.M “A” - B.M. “B” =
ORTHOMETRIC 84 ft - 73 ft = 11 ft
ELLIPSOID ft - 41 ft = 9 ft

23. Conditions for surveying with GPS
At least 2 receivers required
At least 4 common SV’s must be tracked from each station
Visibility to the sky at all stations should be sufficient to track 4 SV’s with good geometry
Data must be logged at common times (sync rates, or epochs)
Receivers must be capable of logging carrier phase observables (not just C/A code)
At some point in the survey, at least one point must be occupied which has known coordinates in the datum and coordinate system desired
2 horizontal and 3 vertical control points are required for complete transformation to the desired datum

24. What the surveyor gets in GPS
2 Types of Measurements:
Change in phase of the code
Change in phase of the carrier wave
2 Types of Results:
Single point positioning and navigation -- from code
Baseline vectors from one station to another (post-processed or processed as “real time”)--from carrier wave
Note that this begins a confusing discussion: code vs. carrier
Ultimately we don’t care about single-point positioning, but we need to get the basics anyway

25. WHAT IS A VECTOR? Notes on Vectors:
GPS vectors computed from carrier phase observations
Vectors have magnitude and direction
Vectors expressed as dx, dy, dz in ECEF coordinate system, from one station to another

26. Satellite Signal Structure
Two Carrier Frequencies
L Mhz
L MHz
Three modulations
Two PRN codes
Civilian C/A code
L dBw
Option for L2 in future
Military P code (Y code if encrypted)
L dBw
L dBw
Navigation message (NAVDATA) L1 L2
Spread Spectrum
Spread Spectrum: Use wider bandwidth than needed to transmit data - protects against jamming and noise interference.

27. Who uses the code? Code-based applications: Rough mapping
GIS data acquisition
Navigation
Any applications able to tolerate accuracies in range of sub-meter - 5 meters

28. Measure Ranges to the satellites
Use the simple formula: Distance = Rate X Time
Distance = RANGE to the satellite (Pseudorange)
Time = travel time of the signal from the satellite to the user
When did the signal leave the satellite?
When did it arrive at the receiver?
Rate = speed of light
SV Time
SV Time
redo slide
1. Use the simple formula Distance = Rate X Time
Rate = speed of light
Time = travel time of the signal from the sv to the user
2. We need the travel time
to get travel time we need : time that the signal left the sv and time that the signal arrived at the user.
User Time

29. How do we know when the signal left the satellite?
One of the Clever Ideas of GPS:
Use same code at receiver and satellite
Synchronize satellites and receivers so they're generating same code at same time
Then we look at the incoming code from the satellite and see how long ago our receiver generated the same code
from satellite
from ground receiver
measure time
difference between
same part of code

30. The Integer Ambiguity--What is it?
Receiver measures partial wavelength when it first logs on
Receiver counts successive cycles after this
Receiver does not know whole number of wavelengths behind that first partial one, which exists between the receiver and the SV
This unknown, N, is called the integer ambiguity or bias (also called phase ambiguity or bias)

31. How carrier waves produce baselines
At least 4 common SV’s must be observed from at least 2 separate stations
The processor uses a technique called “differencing”
Single difference compares data from 2 SV’s to 1 station, or from 1 SV to 2 stations
Double difference combines these two types of single differences
Single and double differences performed at specific epochs in time
Triple difference combines double differences over successive epochs in time (every 10th epoch normally)

32. Sequence in processing carrier waves
Begin with a code estimate of receiver locations
First generate the triple difference solution
Based on triple difference processing, find and correct cycle slips
Using improved estimate of dx,dy,dz from triple difference solution, compute double difference float solution
Set estimates of N from float solution to integers and re-compute baseline: double difference, fixed integer solution
Final result of processing is baseline vector, dx,dy,dz, estimated to centimeter-level or better precision

33. Calculate your position
With range measurments to several satellites you can figure your position using mathematics
One measurement narrows down our position to the surface of a sphere
11,000 miles
We are somewhere
on the surface of
this sphere.
4 unknowns
Latitude
Longitude
Height
Time
Need 4 equations

34. Calculate your position cont’d
Second measurement narrows it down to intersection of two spheres
11,000
Miles
12,000
Intersection of two
spheres is a circle

35. Calculate your position cont’d
Third measurement narrows to just two points
Intersection of three spheres is only two points
In practice 3 measurements are enough to determine a position.
We can usually discard one point because it will be a ridiculous
answer, either out in space or moving at high speed.

36. Calculate your position cont’d
Fourth measurement will decide between the two points.
Fourth measurement will only go through one of the two points
The fourth measurement allows us to solve for the receiver
clock bias.

37. Dilution of precision (DOP)
An indication of the stability of the resulting position
DOP is dependent upon the geometry of the constellation
DOP is a magnification factor that relates satellite measurement noise (input) to solution noise (output)
The lower the DOP, the more accurate the position is.
The higher the DOP, the less accurate the position is.
In surveying, we care most about PDOP and RDOP
PDOP = Position dilution of precision--refers to instantaneous SV geometry
RDOP = Relative dilution of precision--refers to change in SV geometry over the observation period
For all DOP’s, the lower, the better
GDOP: Geometry of SV’s cause the mathematics that relate the satellite measurements to user position to be indeterminant or a divergent set of equations.
Need explaination of URA

38. Dilution of precision (DOP)
Relative position of satellites can affect error
4 secs
6 secs
idealized situation

39. Dilution of precision (DOP)
Real situation - fuzzy circles
6 ‘ish secs
4 ‘ish secs
uncertainty
uncertainty
Point representing position is really a box

40. Dilution of precision (DOP)
Even worse at some angles
Area of uncertainty
becomes larger as satellites get closer together

41. Dilution of precision (DOP)
Can be expressed in different dimensions
GDOP - Geometric dilution of precision
PDOP - Position dilution of precision
HDOP - Horizontal dilution of precision
VDOP - Vertical dilution of precision
EDOP - East dilution of precision
NDOP - North dilution of precision
TDOP - Time dilution of precision
GDOP2 = PDOP2 + TDOP2
PDOP2 = HDOP2 + VDOP2
HDOP2 = EDOP2 + NDOP2

42. Selective Availability (S/A)
Govt. introduces artificial clock and ephemeris errors to throw the system off.
Prevents hostile forces from using it.
When turned up, it's the largest source of error
Selective Availability is the sum of two effects:
Epsilon, or data manipulation term - ephemeris “fibbing”
Epsilon term changes very slowly - rate change once/hour
Dither, or clock variations
Dither term has cyclical variations from 1 cycle every 4 minutes to once every 15 minutes

43. Error Budget Typical observed errors No S/A Total (rt sq sum) 13 feet
Tropo/Iono
Receivers
Ephemeris
Satellite Clocks 20 40 60 80
100
Typical observed errors
satellite clocks feet
ephemeris error feet
receiver errors feet
tropospheric/iono feet
S/A Range error 100 feet
No S/A Total (rt sq sum) feet
Then multiply by HDOP (usually 2-3) which gives a total error of:
typical good receiver feet (7-10 meters)
with S/A Total (rt sq sum) feet
Multiply by HDOP (usually 2-3)
which gives a total error of:
typically feet (60 to 100 meters)

44. DGPS DGPS = “Differential” GPS
Generally refers to real-time correction of code-based positions
Real-time capabilities presume radio link between receivers
The “differential” is the difference between a GPS code position and a known position at a single receiver

45. Differential Correction
BASE
If you collect data at one location there are going to be errors
Each of these errors are tagged with GPS time .
t+1
Time, t

46. Differential Correction (Cont.)
ROVER
Time, t
t+1 ?
At the same time, the errors occurring at one location are occuring everywhere within the same vicinity

47. Differential Correction (Cont.)
ROVER
Time, t
t+1 ?
BASE .
t+1
Time, t
Satellites Used
Satellites Seen
Any Combination of Base SV's

48. GPS Error Sources Dilution of Precision (DOP)
Satellite ephemeris removed by differencing
Satellite clock drift removed by differencing
Ionospheric delay removed by differencing
Tropospheric delay removed by differencing
Selective Availability removed by differencing
Multipath
Receiver clock drift
Receiver noise
Unhealthy Satellites
ephemeris, sv clock drift, ionospheric, and tropospheric delay - seen over a relatively short period (10-20 min) are seen as relatively constant bias errors in receiver position data
Can be removed by DGPS
Ephemeris: errors in the satellite position ( usually <3 m, can be >30 m)
Ionospheric delay: Modeled for L1 or taken out for L1/L2 (20-30 m day
3-6 m night
Tropospheric delay: 2-3 m in zenith direction, m at 5 deg elevation - depending on temperature, humidity, pressure, user height, terrain below SV signal
Models can take out most of it
1 -3 m difference from Ref to User
S/A: C/A code 100 m accuracy absolute
Dither - alter freq and phase of SV signal
Epsilon - alter ephemeris data
Can be eliminated or reduced by DGPS
Corrections lose accuracy over time
Multipath interference
Signal bounces off other objects and interferes with straight line signal
Good receivers use advanced signal processing and good antenna

49. Summary 3 Segments of GPS GPS Signals Differentials Integer Ambiguity
Space
Control
User
GPS Signals
L1 - c/a code, P-code
L2 - P-code
Differentials
Code - sub-meter precision
Carrier - cm precision
Integer Ambiguity

50. Real-Time vs. Post-Processed
Results are available in the field, so checks can be verified immediately
Staking out is now possible
One base receiver supports multiple rovers (unlimited)
No post-processing time required in office
Transformation parameters needed prior to survey, for proper relationship between GPS WGS84 and local system

51. Conditions for Real-Time Surveying
At least 2 receivers required
At least 5 common SV’s must be tracked from each station
Visibility to the sky at all stations should be sufficient to track 5 SV’s with good geometry (4 SV’s required for baseline solution, but 5 are required for initialization)
Initialization must take place at beginning of survey
Radio link must be available between base and rover
“Lock” to SV’s must be maintained, or re-initialization must occur
Transformation parameters must be available to get from GPS WGS84 LLH to local NEE

52. What Happens in Real-Time
Data is logged simultaneously at base and rover
Base data is transmitted via radio link to radio antenna at rover
Survey is “initialized” using data from both base and rover (data is processed inside roving receiver)
Survey is conducted, with processing within roving receiver continuing throughout
Results of processing are sent to TDC1 for logging and viewing (results normally 2 seconds behind actual reception)
Results viewed may be either lat/long/ellipsoidal height or northing, easting, elevation, depending on whether sufficient information exists in TDC1 for transformation

53. What is Initialization?
Determination of integer wavelength counts up to the satellites
Required at beginning of all real-time surveys
Required in the middle of surveys, if continuous tracking of at least 5 SV’s (in common with the base) has been interrupted

54. Types of Initialization
Fixed Baseline
Survey Controller (SC) option: “RTK Initializer” (“mini” fixed baseline)
SC option: “Known point” -- should be previously surveyed with GPS
Automatic Initialization
While static -- SC option: “New point”
While moving -- SC option: “Moving” (often referred to as “OTF”, or on-the-fly)
NOTE: “Survey Controller” is firmware inside TDC1

55. Fixed baseline vs. Automatic
Fixed baseline initialization may be performed with all receivers
Automatic initialization requires dual frequency receivers (4000 SSE)
Automatic initialization while moving is additional option to basic 4000SSE real-time configuration
Survey Controller recognizes capability of receivers in survey, and presents only those options supported by your receivers

56. Components of RTK system
BASE
Receiver with RTK firmware -- may be single or dual frequency; internal memory (GPS data logging capability) not required
GPS antenna
TRIMTALK 900 radio
Radio antenna (7db recommended)
Battery
Cables
2 Tripods (one for GPS antenna, one for TRIMTALK antenna) and 1 tribrach (radio antenna has mounting bracket with 5/8 thread)

57. Components of system -- cont.
ROVER
Receiver -- may be single or dual frequency; internal memory not required
RTK firmware
TDC1 with Survey Controller firmware
TRIMTALK 900
GPS antenna
Radio antenna
Battery
Cables
Recommended: backpack and range pole with bipod

58. The Radio Link
Range of TrimTalk 900, with average conditions, is 1-3 km
Maximum range, with idealconditions, up to 10 km
Repeaters can be used to extend range
Base and rovers set on “Reference/Rover”settings
Repeaters must be set on separate, individual settings
All radios, including repeaters, must be on same (internal) channel
One base radio can be received by unlimited rovers
Rover can receive real-time data from only one base

59. Real Time Surveying Applications
Control
Topographic mapping
Construction stakeout
Cadastral surveying

60. Sources of error in RTS
Multipath (deflected GPS signal which can give erroneous results -- watch for reflective surfaces in survey area)
Poor PDOP -- weak satellite geometry (PDOP should be less than 7)
Erroneous antenna heights
Interference with radio link -- select a different channel within the TrimTalk

61. Multipath at Station
Direct Signal
Reflected Signal

62. Cycle Slips and Loss of Lock
Cycle slip = interruption of GPS signal, due to:
Obstructions
Radio or other electromagnetic interference
Loss of lock = Known integer biases on fewer than 4 SV’s
i.e. Cycle slips on so many SV’s that fewer than 4 integer biases are resolved
NOTE: if satellite tracking is reduced to 4 SV’s, then resulting PDOP may be too poor (i.e. high) to resolve integer biases on other SV’s -- may require a re-initialization

63. Grid coordinates
Initial result of GPS survey is precise network based on (possibly) inaccurate coordinates
WGS-84 coordinates must be transformed to meaningful local system
2 horizontal and 3 vertical control points with values in desired coordinate system and vertical datum are minimum required for transformation
In RTS, 4-6 control points are minimum number recommended, and up to may be desirable for large areas

64. Grid coordinates -- continued
Control points are first located with GPS to determine WGS-84 values
WGS-84 values and known NEE on control points are used to generate proper transformation parameters from GPS system to local grid
After transformation parameters have been determined (in the office), they are uploaded to TDC1 and used for all subsequent field work, which can now be performed in local grid system

65. Steps in Calibration Locate control points in the field
Occupy control points using GPS
Enter control (NEE) and GPS-derived coordinates (WGS-84 LLH) into TRIMMAP
Perform GPS calibration in TRIMMAP
Upload results of GPS calibration (by creating a new “DC”, or data collector, file) to TDC1
Continue field survey, which can now be performed in local grid system

66. GPS Calibration in TRIMMAP

67. Another view of GPS Calibration
CALIBRATION IS 2-STEP PROCESS
1. Deriving GPS coordinates for local control points (in the field)
2. Computing calibration parameters for the project using TRIMMAP (in the office)
4 POSSIBILITIES:
GPS to LLH on Local Datum: Datum Transformation
Local LLH to Local NEH: Mapping Projection
Local NEH to Local NEE: D Transformation
and Height Adjustment
NOTE: a Mapping Projection must be selected when creating a project in TRIMMAP, while the remaining three are optional (and will normally be performed)

68. Summary Carrier waves and integer ambiguity Real Time Surveys
Process of the Real Time Surveys
Initalizations
Fixed
Automatic
Components of RTS
Base
Rover
Radio