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Tech Talk: Should You Be Using GNSS?

July 20, 2011
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GPS is considered a key technology by most surveyors. In trend studies such as POB’s annual Salary & Benefits Study, GPS continues to be cited as one of the technologies having the biggest impact on the profession more than 30 years after its initial introduction.



GPS is considered a key technology by most surveyors. In trend studies such as POB’s annual Salary & Benefits Study, GPS continues to be cited as one of the technologies having the biggest impact on the profession more than 30 years after its initial introduction. But is it GPS or GNSS? What’s the difference between the two? And as additional satellite systems come online and more technologies are marketed as GNSS, are GPS-only systems becoming obsolete?

The Global Positioning System (GPS) is a system designed, built, operated and managed by an arm of the United States government. It was the first system for rapid global positioning and satellite surveying using carrier-phase tracking to deliver centimeter-level positioning results. It was and is the most transparent system designed for navigation in terms of technical information. This transparency has enabled scientists to figure out how to capitalize on its features to deliver more accurate, more timely and more transparent features to specialized non-navigator developers of systems for surveying, mapping, construction and agriculture.

By itself, GPS is a robust system that enables navigators and non- navigators alike to get good results in the great majority of situations where satellite-based navigation systems might be used to determine position, navigation information and time. When global navigation satellite systems (GNSS) first started being developed, they seemed unnecessary. But as great as GPS is, and as redundant as the development of other GNSSs seemed to be at the time, many have discovered the benefits of multiple systems. It comes down to this: More satellites is better.

This doesn’t mean GNSS is better for everyone. GNSS-enabled receivers are more expensive and definitely more complex than GPS-only receivers. Many users of GPS, including those who survey and map, can get all the performance they need for the tasks they do without any additional satellites.

However, as other global systems are developed, the tendency is fast developing to consider all GNSS as well as all satellite-based augmentation systems (SBAS) as a cornucopia of data that can be observed, managed and synthesized to produce better results than with GPS alone. The first non-GPS GNSS to be initiated was the Soviet Union’s, now the Russian Federation’s GLONASS. There were many false starts since initially the Soviets had no desire to collaborate with either the United States or with scientists who wished to use the data from both systems. After many other obstacles along the way, including languishing near death for many years, GLONASS is finally being accepted as a good-enough system to convince many of the world’s leading manufacturers to develop systems that track data from both sets of satellites. Many customers now use the combined technology in a host of applications where the benefits are sometimes significant and sometimes relatively marginal over GPS alone but with hope for the future.

There are many benefits touted for each of the “additional” systems that have come or will come after GPS, and we can’t know yet, especially with Galileo and the other systems (Indian, Chinese, Japanese, etc.), how these systems will be integrated into the GNSS receivers of the future. We probably can be assured that not all will integrate the same way. But there are some general ideas about how a multiplicity of satellites can help. First, they make global satellite surveying and mapping easier, and in some cases possible, where it was marginal at best with GPS alone. These situations include urban and natural canyons, deep open-cut quarries and mines, areas with tree cover, and areas adjacent to buildings where half of the sky or more is blocked. While the 24 or 27 or 30 satellites of GPS sound like a lot, 60, 70, 80 or even 90 are much better for being able to get whatever minimum number is required to achieve the surveying or mapping operational objectives.

When the sky is mostly obscured, the chances of finding a satellite in the few patches of sky that are visible to the receiving antenna increase with the number of satellites in the sky. And it’s not just about being able to go where you couldn’t go with GPS in the past. GNSS compared to GPS alone can also improve accuracy and speed data collection. That’s because not only is more data available from more satellites, but it’s also better data--only the strongest satellites in the combined constellation can be used, with weighting as appropriate. GNSS therefore improves positioning accuracy, reduces the observation time required to get to required accuracy levels, and reduces the risk of being shut down due to local conditions.

So should you trade in your GPS-only device for a GNSS-enabled system? The answer depends on how you’re using the technology. The benefits of GNSS today really depend on whether you can benefit immediately from having access to the GLONASS satellites. It is wise to not even consider what the other systems can contribute. (Although Galileo test satellites have been launched, it is difficult to get a reliable estimate of how many satellites will be operating in the next two to five years.)

If you work in difficult environments (whatever that might be), if you need the highest possible performance at all times, and if you need to minimize delays due to periods of high dilution of precision (DOP) or obstructed satellites, then GNSS is for you today and perhaps even more tomorrow. However, keep in mind as fast as technology turns, the advances in electronics (miniaturization, memory capacity increases, processors speeding up, decreasing power consumption, advances in communications systems) are capabilities you are going to want to have when Galileo and other systems are truly available for your GNSS receiver to access and use. If you are quite satisfied with the performance your GPS-only system gives you today, even recognizing that you have delays and less than optimum performance at times because of the cost-benefit tradeoff, then don’t trade up.

In the end, this decision, like many others, comes back to this: As technology becomes more automated and easier to use, understanding how it works becomes more important to achieve the optimum operation level and financial investment that best suits your organization’s needs. Stay informed, and you’ll make the right decision at the right time.

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