GPS Overview

What is GPS?

The Global Positioning System (GPS) is a navigation system using satellites, a receiver and algorithms to synchronize location, velocity and time data for air, sea and land travel.

The satellite system consists of a constellation of 24 satellites in six Earth-centered orbital planes, each with four satellites, orbiting at 20,000 km above Earth and traveling at a speed of 14,000 km/h.

While we only need three satellites to produce a location on earth’s surface, a fourth satellite is often used to validate the information from the other three. The fourth satellite also moves us into the third-dimension and allows us to calculate the altitude of a devic

1. Location — Determining a position.
2. Navigation — Getting from one location to another.
3. Tracking — Monitoring object or personal movement.
4. Mapping — Creating maps of the world.
5. Timing — Making it possible to take precise time measurement

Some new GPS modules

• ZED-F9P 1cm High Precision GPS
• NEO-M8P-2 2.5cm High Precision GPS
• SAM-M8Q 1.5m 72 Channel GPS
• ZOE-M8Q 1.5m Compact GPS
• NEO-M9N 1.5m GPS

NMEA Message Structure

$GPGGA,181908.00,3404.7041778,N,07044.3966270,
W,4,13,1.00,495.144,M,29.200,M,0.10,0000*40

All NMEA messages start with the $ character, and each data field is separated by a comma.

• GP represent that it is a GPS position (GL would denote GLONASS).
• 181908.00 is the time stamp: UTC time in hours, minutes and seconds.
• 3404.7041778 is the latitude in the DDMM.MMMMM format. Decimal places are variable.
• N denotes north latitude.
• 07044.3966270 is the longitude in the DDDMM.MMMMM format. Decimal places are variable.
• W denotes west longitude.
• 4 denotes the Quality Indicator:
• 1 = Uncorrected coordinate
• 2 = Differentially correct coordinate (e.g., WAAS, DGPS)
• 4 = RTK Fix coordinate (centimeter precision)
• 5 = RTK Float (decimeter precision.
• 13 denotes number of satellites used in the coordinate.
• 1.0 denotes the HDOP (horizontal dilution of precision).
• 495.144 denotes altitude of the antenna.
• M denotes units of altitude (eg. Meters or Feet)
• 29.200 denotes the geoidal separation (subtract this from the altitude of the antenna to arrive at the Height Above Ellipsoid (HAE).
• M denotes the units used by the geoidal separation.
• 1.0 denotes the age of the correction (if any).
• 0000 denotes the correction station ID (if any).
• *40 denotes the checksum.

• $GPGSA – Detailed GPS DOP and detailed satellite tracking information (eg. individual satellite numbers). $GNGSA for GNSS receivers.
• $GPGSV – Detailed GPS satellite information such as azimuth and elevation of each satellite being tracked. $GNGSV for GNSS receivers.
• $GPVTG – Speed over ground and tracking offset.
• $GPGST – Estimated horizontal and vertical precision. $GNGST for GNSS receivers.