Learning GPS devices and data

ArcGIS Runtime for Windows Mobile leverages Global Positioning System (GPS) technology to provide location-awareness to mobile enterprise applications. While most people are now familiar with the term GPS and have some idea of what it is and how it works, it is a large and complicated system. Most GPS consumers use the system they are provided as an end-user application such as in car navigation. These focused applications are delivered as closed systems on a stand-alone device with the GPS receiver and mapping application. ArcGIS Runtime for Windows Mobile has its own library that contains the components used to integrate GPS and mobile applications. The GPS library is exposed to application users as a series of dialog boxes to configure the connection and tools to use the GPS data. GIS applications developers are exposed to the GPS library through the GPS namespace, which contains the components used to integrate GPS and mobile applications. The GPS namespace allows developers access to fully control the connection between the GPS receiver and the mobile application. The ArcGIS Runtime SDK for Windows Mobile includes several GPS components, which have been designed to simplify the work for developers to incorporate GPS into a mobile mapping application, while providing access to the detailed GPS properties if needed. To fully leverage the power of a GPS, it is necessary to have a good understanding of the system and its associated technologies.

GPS navigation

GPS is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS uses radio signals broadcast from orbital satellites (or man-made stars) to calculate positions accurate to better than a centimeter with the right hardware. GPS has become a vital global utility, indispensable for modern navigation on land, sea, and air around the world.

The foundation of GPS is receiver trilateration from the measured distances between at least three satellites. To achieve accurate positioning, the satellites broadcast their orbital position allowing a GPS receiver to measure the individual distances using the travel time of each radio signal.

There are numerous error sources introduced as part of the system or from the environment. The autonomous (self-contained) GPS system provides accuracies in the order of 5 to 20m ideal for navigation and large scale positioning. To improve the accuracy of the autonomous GPS system, many differential GPS technologies are available allowing users to remove the errors and improve accuracy.

For a more in depth understanding of GPS than what is presented in these topics, the following sources are recommended:

GPS NMEA standard

Within the GPS industry, there is one truly global standard that GPS receivers leverage to report their position information. The National Marine Electronics Association (NMEA) currently maintains and distributes the standards that relate to the marine electronics industry. Today the NMEA 0183 standard is widely accepted as the common protocol reported by GPS receivers. There are many other GPS protocols from a range of vendors; however, in most cases, all receivers report their proprietary protocol and the NMEA 0183 protocol. The NMEA 0183 standard was last revised in January 2002 and is denoted as Version 3.01.

The NMEA protocol reports sentences of information specific to the device that is reporting them. The first part of the sentence defines the interpretation of the rest of the sentence. Each sentence from a GPS receiver is tagged with a GP prefix followed by three characters denoting the type of sentence. For example, GPGGA is the GPS sentence for reporting GPS fix information. There are a standard set of sentences for GPS; however, each vendor can also report their proprietary sentences as necessary. GPS components within the ArcGIS Runtime SDK for Windows Mobile do not read vendor sentences. All sentences are ASCII text. The NMEA standard calls for the interface speed to be 4800 baud with 8-bits of data, no parity, and a one stop bit. While most GPS receivers adhere to this, it might not always be the case. It is important to understand exactly the settings your GPS receiver is leveraging.

Example NMEA sentence:

$GPGGA,010725.753,3403.4073,N,11711.4305,W,1,04,6.0,370.6,M,-32.5,M,0.0,0000*48

A Differential GPS (or DGPS) provides corrections to autonomous GPS positions providing accuracies ranging from 5m to sub-centimeter. A DGPS relies on at least one GPS receiver located at a fixed point. The DGPS calculates corrections by measuring the difference between the known distance and the calculated distance of the GPS receiver and each satellite in orbit.

DGPS corrections are disseminated in real time or by post-processing. Real-time DGPS is achieved by broadcasting the DGPS correction over a radio, satellite, or IP-based medium. Post-processed DGPS is achieved by storing the DGPS correction in a file or database, and processing them against autonomous GPS data collected in the field.

Real-time DGPS is ideal for applications in the field that require high accuracy to navigate to features or re-position them on-the-fly. However, for applications that require the highest possible accuracy it is ideal to use post-processed DGPS. This ensures that the likelihood of all GPS positions being differentially corrected is high, as often in the field, real-time DGPS can be patchy as a result of poor communication coverage to the DGPS source, and also removes any latency in the processing of the positions that occurs in real time. This is critical to ensuring the most accurate data possible for your GIS.

GPS devices

Since the early 1990s, GPS has been leveraged as a tool for managing the accuracy and quality of GIS. It is extremely important to understand the relationship between the purpose of your mobile deployment and the GPS technology applicable to your needs. There are three main types of GPS solutions currently used in the GIS market; those targeted at navigation, data collection, and surveying. Each type of GPS solution meets a specific need:

Specific markets require different GPS systems and accuracy needs. The following section outlines how GPS accuracy is achieved and the types of GPS systems common in the market today. The device type is a factor that plays the core role in differentiating a GPS. Those that provide the highest accuracy often cost the greatest due to the research and development needed to build systems to achieve such accuracies. Data collection systems provide ideal methods for collecting new GIS data in the field, and taking existing data into the field and updating the accuracy, while other less costly systems can be used for navigation successfully, even though they do not produce highly accurate measurements.

The accuracy and yield of a GPS is determined by a number of factors. Depending on the needs of your application, consider the hardware capabilities, DGPS availability, and environmental factors when selecting a GPS .

Higher quality GPS receivers generate more accurate GPS positions. In most cases this is the result of more advanced technology employed to receive GPS signals, process GPS signals, and also protect the GPS receiver from any interference. Data collection and survey systems often employ these technologies to ensure they generate the most accurate GPS positions possible. Navigation systems often leverage lower quality GPS receivers to ensure their price point is ideal for the mass market. The trade off is that these systems are not ideal for collecting data at small scales but are well suited to provide in car and foot-based navigation solutions.

Leveraging a DGPS solution increases the accuracy of the GPS positions dramatically. As outlined earlier, an autonomous GPS although much greater in accuracy than any other position system available in the past, still carries a lot of error inherent in the system. Differential GPS removes the majority of this error and can result in major improvements to the accuracy of the GPS positions. Today, the majority of GPSs in every market look to leverage DGPS. The data collection and survey systems often provide real-time and post-processed DGPS to ensure the greatest possible accuracy; whereas, the navigation systems employ only real=time DGPS as an added benefit in the field.

Environmental influences can degrade the accuracy of a GPS position. Higher quality GPS receivers employ more advanced mitigation techniques ensuring the best possible GPS positions by filtering out poor signals. The trade off with mitigating GPS signals is that it can reduce the positional yield, if your applications require that high accuracy mitigating signals are beneficial; however, if the converse is true, accepting poor signals ensure the best possible yield. Data collection systems provide the flexibility to operate in either mode. Generating highest possible accuracy by filtering out poor signals or best yield by accepting all signals when accuracy isn't critical, or when getting a position is more important than no positions. Navigation systems are typically tuned to produce the best yield in tough urban canopy environments. Survey systems provide the highest accuracy possible and as a result, produce low yields especially in regions with urban or vegetative canopy. To overcome this, other more traditional survey systems relying on optical technologies are employed to generate high accuracy in these environments.

Accuracy or yield is often achieved at the expense of the other. 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.

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1/7/2015