L3 GEOGRAPHIC COORDINATES

GNSS

Global Navigation Satellite System

• generic term for space-based radio-navigation system

• provides capability to determine geographic location @earth

• 1ry tool for real-time determination of field locations

Current GNSS

• GPS (USA) = Global Positioning System

  • 1978

• GLONASS (Russia) = GLObal NAvigation Satellite System

  • 1982

• GALILEO (EU)

  • 2005

• BeiDou (China) = BeiDou Navigation Satellite System

  • 2000

Multi-GNSS Support

• today's hardware = access multiple global GNSS

• track @+challenging environm

GPS-NAVSTAR

Global Positioning System NAVigation System for Timing And Ranging

• US DoD positioning system

• Full constellation = 24 satellites

• Orbital planes = 6

• Inclination = 55 degrees

• Altitude = 20k km (11k nautical miles)

• Orbital period = 11h58 > back to same position after that amount of time (engineered that way)

• Coordinate system = WGS-84 (MEMORIZE)

Sources of Error

*Vertical usually 50% + error than Horizontal !!

1. Atmospheric error = distortions affect values detected by satellites

   1. Imposed by ionosphere (accountable) = charged particles > freq delays (solar activity)

   2. Imposed by troposphere (non-accountable) = lower atmosphere > neutral gases + water vapor delay

• Solution - Carrier-Phase position = high-freq (shorter wavelength) for ++precision

2. Clock error = inaccurate time measurement

   1. clock @satellite (drift, relativistic effects > alter time flow), GPS receiver

   2. clock @compu system (incorrect setup) (ArcGIS)

   3. error of 0.000001s > 1000 ft

• Solution - GPS Almanac = ++long GPS ON > ++info collected > ++accurate

3. Multipath error = signal goes thru multiple paths (reflections off surface) > travel longer distances > arrives @receiver antenna w delay > receiver miscalculate position

• Solution - Mask angle = set a min elevation angle > filter out distorted signals from satellites @very low on horizon (10-15 degrees)

4. Dilution of Precision (DoP) = measures how geometric arrangement of satellites > affects accuracy of position measurement

   1. Low DOP = good satellite geometry

   2. High DOP = poor satellite geometry (clustered @1 part of sky)

• Solution - Post-processing = 2 GPS > 1 “base” mode, 1 moving to collect positioning > corrections from fixed base station/gov network

  • @harsh envrionm when real time corrections (RTK) impossible

5. Selective availability = satellite availability (depends on time of day)

• Solution - Differential correction = real-life processing (automatic), receive LIVE error to deal with LIVE and automatically corrected

*other issues = envrionm (underwater, cave, clouds, magnetic changes)

Solutions

• Solution - Carrier-Phase position = high-freq (shorter wavelength) for ++precision

• Solution - GPS Almanac = ++long GPS ON > ++info collected > ++accurate

• Solution - Mask angle = set a min elevation angle > filter out distorted signals from satellites @very low on horizon (10-15 degrees)

• Solution - Post-processing = 2 GPS > 1 “base” mode, 1 moving to collect positioning > corrections from fixed base station/gov network

  • @harsh envrionm when real time corrections (RTK) impossible

• Solution - Differential correction = real-life processing (automatic), receive LIVE error to deal with LIVE and automatically corrected

Differential Correction Methods

• Software data filtering = X external signal

• Beacon (Coast Guard) = first network

• Private Satellite Service (OmniStar, StarFire) = private services

• WAAS (Wide Area Augmentation System) + EGNOS (European Geostationary Navigation Overlay Service) = public services

• Portative Station/Netwrok (RTK) = independent or private service

  • @MAC !!

  • @small stationary receiver shown in class

/!\ today's issue = accessibility $$$

Claimed Accuracy (of satellites)

*'“claimed”, some close to standard they claim, others aren't

*measurements change !! depend on many factors > tectonic movement, satellite movement

• Option - Correction source - Short-term accuracy (15 min) - Long-term accuracy (next day) > measurements change bc satellites move, etc.

• Sub-meter - Beacon, WAAS, etc. - 15-33cm - 76-100 cm

• Decimeter - John Deere SF2, OmniSTAR XP, etc. - 5-10cm - 10-25 cm

• Centimeter - Local Base RTK - 2.5cm - 2.5 cm > ultra precise

Geographic Location

• 3 numbers characterizing it

  • Latitude (0-90)

  • Longitude (0-180)

  • Altitude (elevation)

Geometrical Model of Earth

• almost perfect sphere (r = 6M m)

• ellipticity = 0.003353 > 0

  • many ellipsoid models

• GPS = uses WGS 84 model

  • equatorial r (semi-major axis) | a = 6M m

  • polar r (semi-minor axis) | b = 6M m

  • ellipticity | f = (a-b)/a

NMEA-0183 Communication Standard

National Marine Electronics Association (NMEA)

Simple ASCII serial communication

Sentence communicated to computer

• Sentence ID (comma delimiter) Field 1-6 (asterisk delimiter) Checksum

• $GP(ID), (UTC time), (latitude), N, (longitude), W, ?, (# satellites), ?, (m above mean sea lvl), M, (geoidal separation (m))

Decimal degrees (DD) > Decimal Minutes Seconds (DMS)

• Angular subdivisions (X time units)

• Degrees = Degrees + Minutes/60 + Seconds/3600

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• UTC time = universal

• Lon/lat = min.sec (high precision)

  • longitude = starts w 0 or 1 bc 0-180 degrees

  • latitude = ddmm.mmmm (output) (dd.dddddd = dd + mm.mmmm/60)

• #satellites used for the calculations

• sea lvl = elevation above geoid

  • determined diff ways globally

• geoidal separation = ellipsoid - sea lvl

  • locally defined (same value @mtl region)

  • 0 = sea lvl

• gps elevation = sea lvl + geoidal separation

  • aka elevation above ellipsoid

Global Coordinate System

• (-) South latitude

• (-) West longitude

Longitude and Latitude Conversion

Equations *exercises

• Conversion factors = how many min @1 degree of lat

  • Distance b/w 2 points > cartesian coords

  • Travel direction (heading)

  • Perimeter (sum of distances b/w consecutive pts) + area

GPS lat/lon processing

• Convert | lat/lon → decimal degrees

• Make longitude (-) (West) and latitude (-) (So

• Compute | conversion factors (for specific lat)

• Plot + analyze data

DMS → DD

• Degrees + min/60 + sec/3600 = add them up!

• »»» 00.00000 degrees N

DD → DMS

• Degrees = whole #

• Minutes = multiply remaining x60 > keep whole #

• Seconds = multiply remaining x60 > keep whole #

• »»» _ degrees _ ‘ _'' W

/!\ (-) @dd VS W/E @dms

Travel direction (heading)

Equations

• @math (CCW), @geo (CW) !!

Spatial Data Display (datum, projection)

Spatial data display @GIS = 2 coords systs

GNSS = provide systems

• Datum = global coord syst, fundational reference (shape, size (ellipsoid), origin point)

  • Diff systs = diff maps (shift when superpose them)

    • NA Datum NAD 27

    • NA Datum NAD 83

    • World Geodetic System WGS 84 (GPS data)

  • Crucial define it to display data, have to define the one associated to collected data (I have X data in X coord syst!)

  • (-) South latitude (-) West longitude

• Projection = map display > lab 3

  • Universal Transverse Mercator (UTM)

  • Modified Transverse Mercator (MTM)

  • Local coordinates

  • Not projected (Lat & Long)

  • if equatorial country > well preserved projection

  • if X (like Canada) > distortion

  • Always have to give up smth to display smth

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• tectonics moving > X @same place all the time (seen by satellite)

UTM & MTM Projections

• UTM = Universal Transverse Mercator

  • when @middle of a zone (like mtl) > good bc projection distortion = minimal

• MTM = Modified Transverse Mercator

  • Metric grid-based method of specifying locations

  • Uses transverse Mercator projection + zones spaced 3 degrees of long apart

  • Used @Eastern Canada

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• Unspoken practice = UTM + NAD 83