The most important hardware in a GPS surveying operation are the receivers. Their characteristics and capabilities influence the techniques available to the user throughout the work, from the initial planning to processing. There are literally hundreds of different GPS receivers on the market. Only a portion of that number is appropriate for GPS surveying and they share some fundamental elements. They are generally capable of accuracies from submeter to subcentimeter. They are capable of differential GPS (DPGS), real-time GPS, static GPS, and other hybrid techniques. They usually are accompanied by postprocessing software and network adjustment software. And many are equipped with capacity for extra batteries, external data collectors, external antennas, and tripod mounting hardware. These features, and others, distinguish GPS receivers used in the various aspects of surveying from handheld GPS units designed primarily for recreational use.

 

A portable GPS Tracker, like any electronic tracking devices, must collect and then convert signals from GPS satellites into measurements. It is not easy. The GPS signal has low power to start with. An orbiting GPS satellite broadcasts this weak signal across a cone of approximately 28º of arc. From the satellite’s point of view, about 11,000 miles up, that cone covers the whole planet. It is instructive to contrast this arrangement with a typical communication satellite that not only has much more power, but a very directional signal as well. Its signals are usually collected by a large dish antenna, but the typical GPS receiver has a very small, relatively nondirectional antenna. Fortunately, antennas used for GPS receivers do not even have to be pointed directly at the signal source.

 

Stated another way, a GPS satellite spreads a low power signal over a large area rather than directing a high power signal at a very specific area. In fact, the GPS signal would be completely obscured by the huge variety of electromagnetic noise that surrounds us if it were not a spread spectrum coded signal. The GPS signal intentionally occupies a broader frequency bandwidth than it must to carry its information. This characteristic is used to prevent jamming, mitigate multipath, and allow unambiguous satellite tracking.

 

The configuration of the GPS Space Segment is well-known. A minimum of 24 GPS satellites ensure 24-hour worldwide coverage. But today there are more than that minimum on orbit. There are a few spares on hand in space. The redundancy is prudent. GPS, put in place with amazing speed considering the technological hurdles, is now critical to all sorts of positioning, navigation, and timing around the world. It is that very criticality that requires the GPS modernization. The oldest satellites in the current constellation were launched in 1989. Imagine using a personal computer of that vintage today. It is not surprising that there are plans in place to alter the system substantially. What might be unexpected is many of those plans will be implemented entirely outside of the GPS system itself.

 

In fact, the goal of a single electronic tracking devices that can track all the old and new satellite signals with a significant performance improvement looks possible. But after all, the main attraction of interoperability between these systems is the greatly increased number of satellites and signals, better satellite availability, better dilution of precision, immediate ambiguity resolution on long baselines with three-frequency data, better accuracy in urban settings, and fewer multipath worries. Those are some of the things we look forward to. It is beginning to look like at least some of those things are achievable.

 

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The development tools available for GPS application design vary depending on the complexity of the target system and the GPS solution being used. Most GPS Solution vendors offer software tool suites that allow a developer to communicate with the GPS receiver through the serial port of a personal computer. These software tools typically use messages compatible with the standard NMEA (National Marine Electronic Association) format, but many vendors also offer their own customized sets of messages and message formats.

The more advanced development tools, available for some GPS chip sets, are intended to help the application developer integrate their software with the GPS tracking software running on the same MCU. Because of the hard real-time constraints typical of GPS software implementations, the most efficient way to enable the smooth integration of the GPS tracking device with the application software is through a clearly defined software API. With a standard interface to the GPS software and the necessary development/debugger tools to support it, an application developer can easily configure the GPS receiver software, enabling access to the appropriate PVT information by the application as needed. For an illustration of the basic software architecture of a GPS enabled application running on a single MCU that is supported by this type of tool suite.

 

For those developers that have the skill of designing the entire GPS receiver circuit into their application, several semiconductor manufacturers now offer GPS chip set solutions. These chip sets, offered with either complete or partial reference designs and control software, enable the designer to integrate GPS into an application at the lowest possible cost, while also conserving power, board space and system resources. However, this high level of integration is achieved at the expense of doing the RF and IF circuit layout and software integration in-house, which can take significant resources and effort. The custom chip sets used for the original GPS receivers often had up to seven ICs, including the external memory chips, amplifiers, downconverter, correlator ASIC and system processor, in addition to a variety of discrete components. Continuous advances in the performance and integration level of MCUs have greatly increased the performance of the newer GPS chip sets while reducing the power consumption and physical size of the complete system. System-on-a-Chip (SoC) technology has resulted in the integration of the GPS correlator directly onto the MCU, along with embedded RAM, ROM and FLASH memory. In some cases, this increased level of integration has reduced the device count down to a mere two ICs and a handful of discrete components, further decreasing the cost and development effort required.

Even more recently, high-performance RISC MCUs have begun showing up in low-cost GPS chip set solutions. These powerful processors have many more MIPS available for GPS computations, which in turn increases the overall performance and reliability of the GPS tracking solution  . This level of computational power is making it possible to execute dead reckoning or WAAS algorithms on the same processor as the GPS algorithms, further improving the accuracy of the positioning solution at little or no increase in chip set cost. This diagram illustrates all of the functional blocks required by a basic GPS system, including an active antenna, a downconverter with an integrated temperature sensor, and a correlator integrated onto a basic microcontroller, along with the additional MCU peripherals required to perform a basic tracking loop routine and calculate a PVT solution.

 

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With the recent completion of the Global Positioning System constellation and the appearance of increasingly affordable spaceborne receivers, GPS is moving rapidly into the world of space flight projects. Indeed, owing to the great utility and convenience of autonomous onboard positioning, timing, and attitude determination, basic navigation receivers are coming to be seen as almost indispensable to future low earth missions. This development has been expected and awaited since the earliest days of GPS. Perhaps more surprising has been the emergence of direct spaceborne GPS science and the blossoming of new science applications for high performance geodetic space receivers.

 

Applications of spaceborne GPS to Earth science include centimeter-level precise orbit determination (POD) to support ocean altimetry; Earth gravity model improvement and other enhancements to GPS global geodesy; high resolution 2D and 3D ionospheric imaging; and atmospheric limb sounding (radio occultation) to recover precise profiles of atmospheric density, pressure, temperature, and water vapor distribution. Figure 1 offers a simplified summary of the Earth science now emerging from spaceborne GPS.

 

Conventional single- and dual-frequency GPS tracking device have been flown in space for basic navigation and (increasingly) attitude determination on a number of recent missions. Consistent with these different uses, there has developed in recent years a two-tiered user community for GPS in space: those seeking basic, moderate-performance GPS navigation, timing, and (in some cases) attitude determination, and those pursuing the more demanding science activities requiring the highest performance dual-frequency receivers. As the mission-dependent requirements within each group are diverse, a variety of receiver models for space use has emerged.· While that healthy situation is likely to continue, from the standpoint of the scientists it may be hoped that in the future the high end instruments will reach levels of size, cost, and generality of function that will allow them to serve both user classes economically, thus converting the most utilitarian satellites into potentially powerful science instruments. As GLONASS becomes established as a reliable navigation system we can expect to see considerably more commercial resources devoted to developing the technology for both the ground and space. A high performance spaceborne GPS/GLONASS receiver for navigation and science applications is currently under development by the European Space Agency and may fly within two years.

 

The utilitarian spaceborne GPS applications represent, in essence, a fulfillment of the GPS vision. They exploit GPS( tracking device), sometimes in clever ways, for purposes for which it was expressly intended. For the growing class of high-precision spaceborne science users surveyed here, the same cannot be said. GPS was not conceived with such uses in mind (indeed, their feasibility was generally recognized only after GPS deployment was well underway), and has not been altered in any way to accommodate them. Within these diverse scientific enterprises we find many exampleas in which GPS innovators have, through ingenuity and industry, coaxed a reluctant system to perform unexpected feats, thereby expanding the GPS mission. In the face of the seriously confounding security features known as selective availability and anti-spoofing, they have extracted from GPS levels of performance undreamed of by its architects.

 

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Your GPS receiver is pretty darned useful even if you can’t connect it to your computer. With your receiver, you can find your way around as well as your way back, even if you don’t know what a computer is. The ability to connect your GPS receiver to your computer, however, can make your GPS receiver even more useful . . . heck, a lot more useful. Essentially, a connection between your GPS receiver and your computer allows them to talk and share information, such as uploaded maps and waypoints to your receiver as well as downloaded receiver information that you store while on your adventures. In this Blog, I show you why this interconnectivity is a fantastic addition to your receiver. You’ll discover the ins and outs of the physical connections, how to get your computer and receiver to speak the same language, the lowdown on great receiver utility programs, and how to upgrade your receiver firmware.

If you choose to use a mapping GPS receiver (one that you can upload maps to from a PC), you’re in the right chapter. And kudos to you to getting a model that really lets you maximize using your GPS receiver. You’ll be outdoor navigating and geocaching in no time. Here are the very cool things you can do with a PC-compatible receiver:

1. Back up and store GPS receiver waypoints, routes, and tracks on your computer.

2. Download waypoints, routes, and tracks from your GPS receiver to your computer to use with computer mapping programs.

3. Upload waypoints, routes, and tracks to your GPS receiver from other sources such as Internet sites, other GPS users, or mapping programs.

4. Upload maps from your computer to your GPS receiver (if your receiver supports mapping).

5.Provide GPS data to a moving map program on a laptop for real-time travel tracking.

 

6. Update your GPS receiver’s firmware.

Before I talk about how to interface a GPS receiver to a PC, you need to understand the types of data that can be passed between the two devices:

GPS receiver to PC: Saved waypoints, routes, tracks, and current location coordinates.

PC to GPS receiver: Maps (if the GPS receiver supports them), way points, routes, and tracks.

You can interface a GPS receiver to a computer and transfer data in two ways:

Cable: Most GPS receivers use a special cable, with one end that plugs into the receiver and the other that plugs into the serial or Universal Serial Bus (USB) port of your computer.

Memory card: Some GPS receiver models use Secure Digital (SD) or MultiMediaCard (MMC) memory cards to store data. You transfer data between the GPS receiver and your computer with a card reader con nected to the computer. ( If you use a Bluetooth wireless GPS receiver, you don’t need a cable or memory card reader to transfer data. These are designed to be used exclusively with laptops and PDAs.

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GPS was originally designed to guide bombs, aircraft, soldiers, and sailors. In all cases, the GPS receiver was expected to be outside with a relatively clear view of the sky. The system was designed to require a start-up time of approximately 1 min, and after that it would operate continuously. Today GPS is used for many more civilian than military purposes. Counterintuitively, the system demands of these civilian applications far exceed those seen before. GPS is now expected to work almost anywhere, even, sometimes, indoors; push-to-fix applications have emerged where a single position is expected almost instantly; and all of this must be delivered in a way that adds little or no cost, size, or power consumption to the host device.  You probably recognize the ways that your car's GPS rearview mirror and telematics system track you for your own good. Their tracking gives you directions. It means you will be found in the event of a crash.

 

What can A-GPS do for you? The PLGR was the GPS receiver most widely used by the U.S. military. It is a five-channel, L1-only receiver, with a typical time to first fix of over a minute, and a cost of about $2,000. The PLGR receives encrypted P-code military signals, is waterproof, weighs three pounds and is far more robust than any modern mobile phone. But many of those mobile phones today have A-GPS, which can compute a position within a second and acquire satellites at signal levels 100 times lower than the PLGR, and adds less than $5 to the cost of the phone.  Nowadays, GPS enabled phones are widely available and are viable options for a dedicated handheld or an in-car GPS. These requirements are what drove the development of A-GPS. Assisted GPS (A-GPS) improves on standard GPS performance by providing information, through an alternative communication channel, that the GPS receiver would ordinarily have received from the satellites themselves.

 

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