9.2 - Global Positioning System (GPS)

The Global Positioning System (GPS) is a navigation system which gives information on location and time to its users in all weather conditions.

How GPS Functions

GPS consists of three segments:

1. Control segment
2. Space segment
3. User segment

Control Segment

The control segment consists of:

• a master control station;
• an alternative master control station;
• command and control antennas;
• monitor stations.

Flow of information:

1. Monitor stations receive signals from GPS satellites.
2. The signals are sent to the master control station.
3. The master control station generates navigation messages on Earth.
4. An antenna on Earth sends information to the GPS satellite, including:
   • GPS satellite position;
   • time correction factor for the GPS satellite clock;
   • atmospheric data;
   • almanac.

Space Segment

• GPS consists of a satellite network.
• GPS satellites orbit Earth at an orbital height of $20000\text{km}$.
• This orbit is usually known as a semi-synchronous Earth orbit.
• The orbital period is approximately 12 hours.
• At least four GPS satellites can be seen at an angle of $15^{\circ }$ or more from the horizontal axis at any time from all locations on Earth.
• Information sent from a GPS satellite to a GPS receiving device includes:
  • the position of the GPS satellite;
  • the time the signal is sent.

User Segment

• A GPS user is anyone who uses a GPS receiving device, such as a smartphone.
• The receiving device uses information from GPS satellites to determine location and time.

GPS Coordinate System

A location can be written in two formats:

Format	Form
Degrees, minutes and seconds (DMS)	Latitude and longitude expressed using ${}^{\circ }$, ${}^{\prime }$ and ${}^{\prime \prime }$
Decimal degree (DD)	Latitude and longitude expressed as decimal values

Direction and Sign Conventions

Position	DMS	DD
Latitude in the northern hemisphere	N	Positive
Latitude in the southern hemisphere	S	Negative
Longitude east of the Greenwich Meridian	E	Positive
Longitude west of the Greenwich Meridian	W	Negative

The directions East and West in DMS coordinates are based on the Greenwich Meridian.

Coordinate Examples

National Planetarium, Kuala Lumpur

• DMS: $3^{\circ }08^{\prime }22.04^{\prime \prime }\text{N},101^{\circ }41^{\prime }22.53^{\prime \prime }\text{E}$
• DD: $3.139456,101.689593$

Copacabana, Rio de Janeiro

• DMS: $22^{\circ }58^{\prime }14.60^{\prime \prime }\text{S},43^{\circ }10^{\prime }56.51^{\prime \prime }\text{W}$
• DD: $-22.970722,-43.182365$

Applying GPS Coordinates for Navigation

Activity 9.8 — Navigate within the School

1. Work in an open area, such as the school field, so the GPS receiver can receive satellite signals clearly.
2. Mark five locations with a minimum distance of $10\text{m}$ between locations.
3. Use a GPS receiving device, such as a smartphone, to determine each location’s coordinates.
4. Record each coordinate in both DMS and DD formats.
5. Use the coordinates to navigate from one location to another.

Uses of GPS

• GPS is used for navigation in land, sea, air and space transport.
• Navigation applications that use GPS coordinates include Google Maps and Waze.
• A car GPS tracker is another application of GPS.
• Road-accident notifications in navigation applications help road users:
  • reduce speed and stay alert to possible obstacles;
  • choose an alternative route to avoid traffic congestion.