GPS – AN OVERVIEW
The GPS (Global Positioning System) is a “constellation” of 24 well-spaced satellites that orbit the earth and make it possible for people with ground receivers to pinpoint their geographic location. The location accuracy is anywhere from 100 to 10 meters for most equipment. Accuracy can be pinpointed to within 1 meter with special military-approved equipment .GPS equipment is widely used in science and has now become sufficiently low-cost so that almost anyone can own a GPS receiver.
The GPS has three components namely:
- The space segment: consisting of 24 satellites orbiting the earth at an altitude of 11000 nautical miles.
- The user segment: consisting of a receiver, which is mounted on the unit whose location has to be determined.
- The control segment: consists of various ground stations controlling the satellites.
The GPS is owned and operated by the U.S Department of Defense but is available for general use around the world. Briefly, here’s how it works:
- 21 GPS satellites and 3 spare satellites are in orbit at 10,600 miles above the earth. The satellites are spaced so that from any point on earth, 4 satellites will be above the horizon.
- Each satellite contains a computer, an atomic clock and a radio. With an understanding of its own orbit and the clock, it continually broadcasts its changing position and time. (Once a day, each satellite checks its own sense of time and position with a ground station and makes any minor correction).
- On the ground, any GPS receiver contains a computer that “triangulates” its own position by getting bearings from 3 or 4 satellites. The result is provided in the form of a geographic position- Longitude and latitude, for most of the receivers, within 100 meters.
- If the receiver is also equipped with a display screen that shows a map, the position can be shown on the map.
- If the 4th satellite can be received, the receiver/computer can figure out the altitude as well as the geographic position.
- If you are moving, your receiver may also be able to calculate your speed and direction of travel and give the estimated times of arrival to specified destinations.
The GPS unit contains a GPS module along with a GPS receiver antenna. The module functions according to its built and the antenna receives the information from the GPS satellite in NMEA (National Marine Electronics Association) format. This data is then sent to the microcontroller wherein it is decoded to the required format and sent further.
The AGA Series GPS antenna is a standard product for the GPS system. The circular polarization improves reception ability. The built-in low noise amplifier with very low DC power consumption enhances an already high performance patch array. The antenna has the following features:
- Low noise figure
- High gain
- Ceramic patch antenna
- Water-tight housing
- Temperature and vibration qualified
- Compact size
- Low cost
CPIT GPS module SA3618/SA3618P (patch on top) is a high sensitivity ULTRA LOW power consumption cost efficient, compact size; plug & play GPS module board designed for a broad spectrum of OEM system applications.
GETTING GPS DATA:
After the GPS Module computes the positioning and other useful information, it then transmits the data in some standard format. With differential GPS signal input the accuracy ranges from 1 to 5 m; however, without differential input, the accuracy can be 25 m.
About 60 s after the GPS module is cold booted it begins to output a set of data (according to the NMEA format) though port C once every second at 9600 bps, 8 data bits, one stop bit, and no parity. NMEA GPS messages include six groups of data sets: GGA, GGL, GSA, GSV, RMC, and VTG. We use only the most useful RMC message- Recommended Minimum Specific GNSS Data-which contains all of the basic information required to build a navigation system.
We only need position and time data, so the UTC position, longitude with east west indicator, and latitude with north/south indicator are picked out from the RMC message. All of this data will be formatted into a standard fixed length packet with some other helpful information. Next, this data packet will be transmitted to the control center and stored in the micro controller.
Here’s a sample of how the GPS receiver antenna receives information from the GPS satellite in NMEA format:
NMEA format sample:
The GPS module that we are using in this unit is SA3618 and the GPS receiving antenna used is G-501.
SA3618 NMEA Protocol
The SA3618 software is capable of supporting the following NMEA message
The general NMEA (National Marine Electronics Association) format consists of an ASCII string commencing with a. $. Character and terminating with a <CR><LF> sequence. NMEA standard messages commence with .GP. then a 3-letter message identifier. NemeriX specific messages commence with $PNMRX followed by a 3 digit number. The message header is followed by a comma delimited list of fields optionally terminated with a checksum consisting of an asterix .*. and a 2 digit hex value representing the checksum. There is no comma preceding the checksum field. When present, the checksum is calculated as a bitwise exclusive of the characters between the. $. and.*.. As an ASCII representation, the number of digits in each number will vary depending on the number and precision, hence the record length will vary. Certain fields may be omitted if they are not used, in which case the field position is reserved using commas to ensure correct interpretation of subsequent fields. The tables below indicate the maximum and minimum widths of the fields to allow for buffer size allocation.
$GPRMC: This message transfers recommended minimum specific GNSS data. The $GPRMC message format is as shown below