Understanding GPS/GNSS

GPS/GNSS overview

The Global Positioning System (GPS) is a satellite-based navigation system operated by the U.S. Department of Defense that provides location information. GPS was originally intended for US military applications, but in the 1980s, the US government made the system available for civilian use. As GPS has become a vital global utility, other similar satellite systems have been launched. Collectively, these systems are referred to as Global Navigation Satellite Systems (GNSS). Examples of other GNSS are the Russian GLONASS, European Union GALILEO, and Chinese Beidou. As these systems become fully functional, many GNSS receiver manufactures are releasing devices that leverage multiple GNSS to increase the number of visible satellites for more accurate and reliable positioning.

GPS/GNSS works on the basis of trilateration. Each satellite in a system continuously transmit messages that include the time the message was sent, precise orbital information and the orbits of all satellites in the system (the almanac). A GPS/GNSS receiver calculates its position by precisely timing these messages from at least four satellites.

GPS/GNSS communication protocol (NMEA)

Position information from GPS/GNSS receivers can be obtained in many different formats. One of the most commonly supported protocols is the National Marine Electronics Association (NMEA) 0183 standard. NMEA is an electrical and data specification for communication between marine electronic devices including GPS/GNSS receivers. It uses ASCII text in a sentence format to transmit data from one talker to multiple listeners at the same time. The standard also defines the format of individual sentences. For serial communication, NMEA specifies an interface speed of 4800 baud (38400 baud for the high speed specification) with 8 bits of data, no parity, and one stop bit. It is important to note that while many GPS/GNSS receivers adhere to specification, not all do. It is important to consult the receiver manufacture documentation to understand your device.

NMEA sentences

Generally, NMEA 0183 sentences have the following structure:

1. Start of the sentence, denoted by a dollar sign ($).
2. A five character address field consisting of:
   1. A two character talker identifier. For GPS/GNSS sentences, the talker identifier generally includes: GP for GPS information; GL for GLONASS information; and GN for mixed GPS and GLONASS information.
   2. A three character sentence formatter.
3. One or more data fields separated by commas.
4. A checksum delimiter and value.
5. Sentence terminators (carriage return and line feed).

ArcGIS for Windows Mobile supports five standard NMEA 0183 sentence formatters:

• GGA—Global Positioning System Fix Data
• GSA—GNSS Dilution of Precision (DOP) and Active Satellites
• GSV—GNSS Satellites in View
• RMC—Recommended Minimum Specific GNSS Data
• VTG—Course Over Ground and Ground Speed

Connecting to a GPS/GNSS receiver

For the ArcGIS for Windows Mobile to use a GPS/GNSS receiver, a serial port, physical or virtual, is required for the application to connect to. This connection can be established in many different ways and varies from device to device; however, it will generally be one of the following:

• Directly connected with a serial cable or integrated into the device.
• A USB connection—generally, a driver is required from the device manufacture to create a virtual serial port.
• A Bluetooth connection using the Serial Port Profile (SPP).

Consult your receiver's manufacture documentation to understand your device.

GPS/GNSS simulator

In addition to supporting a connection to a receiver, ArcGIS for Windows Mobile supports playback of NMEA sentences stored in an ASCII text file. These sentences can be logged through the client applications, or obtained from other sources and replayed later for testing.

GPS accuracy and quality filter in ArcGIS for Windows Mobile

The position is not 100 percent accurate. There are numerous error sources introduced either as part of the system or from the environment, such as standing under trees or close to buildings, and thus the satellites are obstructed. So the accuracy and yield of a GPS system is determined by a number of factors. Depending on the needs of your application, you should to take into consideration the hardware capabilities, DGPS availability, and environmental factors when selecting a GPS system. The autonomous (self-contained) GPS system provides accuracies in the order of 5 to 10 meters, ideal for navigation and large-scale positioning. To improve the accuracy of the autonomous GPS system, many Differential GPS technologies are available in some of the devices, allowing users to remove the errors and improve accuracy. The commonly free to use differential correction service is from Satellite Based Augmentation System (SBAS). In the United States, Wide Area Augmentation System (WAAS) is one type of SBAS systems. Once the device enables WAAS and receives a DGPS fix type, the accuracy could be 2 to 5 meters.

Also, the geometry of GPS satellites takes effect on accuracy. The measurement of this effect is called Dilution of Precision (DOP). From different perspectives, it includes positional (3D) Dilution of Precision (PDOP), Horizontal Dilution of Precision (HDOP), and so on. One is the ideal value for DOP, and the higher it is the worse it is. DOP values with 1 to 5 are good for most applications. If the device has additional Russian GLONASS capabilities, it combines both GPS and GLONASS to improve the DOP value with more satellite visibility.

Lower PDOP value with a better GPS fix type means higher quality GPS positions. The trade-off is that it can reduce the positional yield. ArcGIS for Windows Mobile provides the GPS quality filter as a tool for the flexibility to operate in low-yield, higher accuracy mode or high-yield, lower accuracy mode. A navigation system is typically an example that requires it to be tuned to produce the best yield instead of higher accuracy in a GPS difficult environment. When deciding on the ideal GPS system for your mobile application, it is important to first understand what the accuracy and yield requirements are before making a purchasing decision as it can have a huge impact on the success of your mobile deployment.