A network of satellites that continuously transmits coded information, which makes it possible to

precisely identify locations on earth by measuring distance from the satellites

GPSGlobal Positioning System

(NAVSTAR - DOD)

Used for military initially

now heavily used in civilian world

(satellites)

(tracking stations)(receivers)

The first GPS satellite was launched in 1978

constellation of 24 satellitessince 1994

each satellite is built to last about 10 years

2,000 pounds, 17 feet across with the solar panels extended

powered by solar energy

continuously broadcast coded radio signal

High orbit satellites (about 12,000 miles above earth surface)

traveling 7,000 miles per hour

allows them to circle earth once every 12 hours

Arranged in orbit so as to provide

coverage by 4 satellites at once

Each satellite transmits low power radio signals

on several frequencies (L1, L2)

Civilian GPS receivers listen on L1 frequency

Need to be able to receive the signalso not in buildings, underwater, caves

Signal will pass through clouds or glass, but not solid objects (line of sight)

L1 contains two pseudo-random signals

The Protected (P) code and the Coarse/Acquisition code (C/A)

Each satellite transmits a unique code

scrambling of the P code

Use these coded signals to calculate travel time from the satellite to the GPS receiver

Time of Arrival

Ground based Control Stations track the GPSsatellites and provide them with corrected

orbital and clock (time) information

Four unmanned and one master control station

Unmanned stations receive info and send to master

Master corrects satellite data and sends uplinks to GPS satellites

Updated datais transmitted to users

Ground stations monitor and update satellite locations

Space Segment

How GPS Works…

1 Distance fromsatellites needsto be known

4 Correct for atmosphericand ionospheric errors

3 Need to knowSatellite position

2 Accurate(Atomic)Clocks arerequired

5 Selective Availability

To calculate our position we need to know satellite location

and satellite distance

Along with the radio signal, the GPS receiver picks up two kinds of coded

information from the satellites

Almanac data contains the approximate position of the satellites

Stored in the memory of the GPS receiver so it knows where each satellite is suppose to be

From the almanac and ephemeris data GPS receiver knows location of satellites at all times

Master sends corrected info to satellites

Ground stations send orbital info to master station

Corrected and exact position is ephemeris data

Receiver compares the two codes to determine how much it needs to

shift (delay) its code to match the satellite code

When satellite is generating code so is receiver

Delay time is multiplied by the speed of radio wave

Speed of light (186,000 mps) less any delay as signal travels

through the atmosphere

Speed of radio wave?

Use model to account for delays

Uses measurements from 4+ satellitesdistance = travel time x speed of light

Sources of Error

1. Atmospheric Interference

signal slows as it passes through atmosphere

Use model to correct

troposphere

ionosphere

Multipath means that the same radio signal is received several times through different paths.

For instance, a radio wave could leave a satellite and travel directly to the receiver, but it also

bounces off a building and arrives at the receiver at a later time.

2. Multipath Errors

The internal satellite and receiver clocks have limited accuracy, and they are not precisely

synchronized. Since position computations are highly dependent on accurate timing information, small clock errors can cause significant errors in

position computations.

3. Clock Limitations

4. Ephemeris Error (Orbital errors)

inaccuracies in reported position of satellite

5. Satellite Configuration

The configuration of the satellites in view to a receiver at any given time can affect the accuracy of position determination. For instance, if all of the

visible satellites happen to be bunched close together, the triangulated position will be less

accurate than if those same satellites were evenly distributed around the visible sky.

6. Selected AvailabilityScrambling of signal by military

Place a GPS receiver (reference or base station) at a known location. This base station receiver will calculate receiver

errors by comparing its actual location to the location computed from the signals. This error information is sent to

the rover receiver, which uses it to correct the position information it computes from the signals. Accuracies of

DGPS systems can range from 15 feet to 3 feet depending on system configuration.

Differential GPS

Differential GPS in Action

1. Compares field data to datacollected at the same time at a nearby base station

2. Error at base station known and subtracted from field data

Known base station location Unknown field locationsData corrected in office

GPS Error BudgetTypical Error in Meters (per satellite)

Standard GPS Differential GPSSatellite clocks 1.5 0.0Orbital errors 2.5 0.0Ionosphere 5.0 0.4Troposhpere 0.5 0.2Receiver noise 0.3 0.3Multipath 0.6 0.6Selective availability* 30 0.0

Typical Position Accuracy

Horizontal 50 1.3Vertical 78 2.03-D 93 2.8

* No longer used

Latitude and LongitudeDefining a Location

units of measurement are Degrees

equator

Prime Meridian

Degree is divided into 60 Minutes Minute is divided into 60 Seconds

To convert coordinates from degrees, minutes, seconds format

to decimal format, use this easy formula:

degrees + (minutes/60) + (seconds/3600)

Latitude 42° 23’ 50.4” NLongitude 71° 7’ 32.8” W

Latitude 42.39733 N

Longitude 71.12578 W