GIS/GPS Chapter 2

Geographical coordinates

origin at Greenwich Observatory and Equator, Greenwich London

Old School

determining an unknown position, replace and relocated with respect to NGS CORS and GPS reference stations

What do benchmarks represent?

highly accurate known reference positions

Example of benchmarks

brass disk, chisel, rock piles, buried monuments

Specifying an unknown position

respect to known features, triangulation of known features to determine an unknown point, national networks established

Datum with Survey Network

historically triangulation network, angle measurements

Why datums for surveying?

easy to measure, difficult to measure distance accurately, time consuming to measure point position accurately

Where is a datum surveying network located at Wilkes?

top of Cohen

GPS Original Design

started development in the late 1960s as NAVY/USAF project to replace Doppler positioning system, aim with real-time positioning to <10 m, fast moving vehicles, limit civilian users to 100 m positioning

GPS Design

innovations with multiple satellites, transmitting at the same frequency, signals encoded with unique codes generated by random sequence, dual frequency band transmission, uses wifi to locate a person

Dual frequency band transmission

L1-1.5 GHz, L2-1.25 GHz

Space segment

designed originally for military applications with allowing soldiers to keep track of their position, assisting in guiding weapons to targets, funding by the US government, controlled by the US Department of Defense ($12 billion)

User segment

military, amazon, search and rescue, surveying, GIS, satellite positioning

Measurements

time difference between transmission from satellite and arrival at ground station (pseudo-range), everywhere else uses military time except for US, atomic clocks

Range equation

speed of light x travel time

Range equation meanings

c(t1-t2), measurements relative to “clocks” in ground receiver and satellites (potential problems)

Atomic clocks

WW2 has nothing to do with GPS, vibrations of quartz crystal, run 30 millionths of a second faster than a clock at sea level

Signal

carrier signal translated into coded signal (modulated to help see what time is left), GPS C/A Code Chips read every 10 milliseconds, later version of code compared with early version to insure correlation peak is tracked

2D is how many satellites?

3

3D is how many satellites?

4

How many satellites does a phone need?

8-9

What does more satellites allow for?

more accurate one point

Selective availability

defense department dithered the satellite time to reduce position accuracy to some GPS users, designed to prevent US enemies from using GPS against us and allies, pentagon reduced to 0 m error, reactivated at any time by Pentagon

Sources of GPS error

SPS civilian users, satellite clocks, orbital errors, user error, erros cumulative and increased by PDOP, standalone positioning since May 1, 2000, standalone positioning by 2011

Standalone positioning

May 1 2000, C/A code on L1, no selective availability

Standalone positioning

2011, C/A code on L1, C/A code on L2, new code on L5

Knowing where a satellite is in space

air force launched satellites into known orbits, orbits known in advance and programmed into receivers, satellites monitored by DoD to identify errors in orbits (ephemeris errors), corrections relayed back to satellite “data message” about their “health”, lots of satellites in space and move due to things hitting them

PDOP/GDOP

measure of satellite geometry, precisional/accurate, can fix with differential positioning

High PDOP

satellites close together, large area of uncertainty

Good GDOP

bad visibility

Low PDOP

satellites widely spaced, low area of uncertainty, they’re good

What is the ideal PDOP?

1 overhead and 3 all at 120 degree intervals

Differential positioning

± a meter or so, =/- 5 cm in an ideal setting, pseudo ranges/code phases, long permanent stations (top of cohen) know the exact position to collect data constantly, real-time kinematics

What do the long permanent stations help with?

calculate using errors to find out correct positions

Real-time kinematics

base stations that fixed/not fixed with known positions and in real time collecting distances from those positions and trilaterating those positions, corrections in real time, 2-3 mm differences

Precise point positioning

main issue is those with surveying background think GIS has nothing to do with them, not exact GPS coordinates, useless if the markers go missing

Go to way point

where you want to go

Active from waypoint

where you are starting

Desired track

angle of where you want to go

Course made good (CMG)

true direction over a fixed reference point

Bearing

new angle used to get to place wanted to get to

Position fix

based on real-time satellite tracking, defined by a set of coordinates, no name, position represents only an approximation of the receiver’s true location, position isn’t static with changing constantly as the GPS receiver moves (wanders due to random errors)

What must be required for a receiver to e 2D/3D mode?

gives position fix, ¾ satellites acquired, 3D mode dramatically improves position accuracy

Way point

based on coordinates entered into a GPS receiver’s memory, either a saved position fix/user entered coordinates, created for any remote point on Earth, must have a receiver designated code/number/user supplied name, entered and saved remains unchanged in the receiver’s memory until edited/delete

Blue Receiver

potential circle of GPS error at each waypoint

Yellow Receiver

where you want to go

Green Receiver

where the GPS receiver may take you

DGPS Correction

x+(30-5) and y+(60+3), true coordinates of x+25 and y+63

True Coordinates

x+0 and y+0, correction of x-5, y+3

USCG NDGPS Ground Stations

yellow shoes overlap between NDGPS stations, green shows little to no coverage, topography limits some areas of coverage