Navigating the World of GNSS

Editor’s note: This article was orginally written in April 2006. Although the basic premise of the article stands, some of the information is dated. We plan on updating it in the next couple of months.

The world of satellite positioning is changing . . . a lot. Global Navigation Satellite System (GNSS) is slowly but most certainly replacing GPS as the acronym used when discussing satellite navigation. The annual ION (Institute of Navigation) GPS conference is now known as the ION GNSS conference. The International GPS Service is now called the International GNSS Service. For the time being, GPS is still the only game in town. However, a revamped Russian GLONASS (GLObal NAvigation Satellite System) program and the European Galileo program will change the landscape of satellite positioning forever. What’s more, GPS itself is undergoing significant enhancements over the coming decades that will improve the integrity and accuracy of the data it produces.

With respect to surveying and mapping, these new systems and enhancements raise many questions about accuracy, tracking performance, cost, upgrades, and other issues: “Should I wait for a full L2C constellation before upgrading my receivers? Should I wait for Galileo to be operational before making my next major GPS equipment purchase decision?”

Predicting the Future

First of all, Galileo, GPS L5 and GPS III (L1C) are still several years away from having constellations substantial enough to make a difference. A lot can happen with receiver technology before then, so placing any significant weight on the “L5/Galileo-ready” feature of a receiver today may not make much sense. In fact, they are sort of “vaporware” because no L5 signal exists yet to test with and the Galileo folks have been reluctant to even release the signal specification to developers.

Additionally, the competitive landscape could change dramatically. L2C opens the market for other companies to design dual-frequency receivers without having to develop the L2 semicodeless techniques used today (techniques that are technically challenging and filled with a minefield of patent blocks). Granted, it’s not just receiver hardware that makes a GPS survey solution, but L2C certainly eliminates
a major roadblock for companies interested in competing in the survey market space.

Whereas the survey-grade GPS equipment manufacturers have been quick to announce products that are L2C/Galileo/GLONASS-capable, it’s interesting to note that very few mapping-grade (meter-level differential GPS) and no consumer-grade (autonomous) equipment companies have given GNSS the same attention. Why? Because today’s GPS is good enough for consumers. They can live with intermittent GPS coverage and still navigate from Point A to Point B. Survey-grade GNSS doesn’t work that way. It’s a satellite-hungry technology.

For mapping-grade product lines, there is technical value in using the GPS, GLONASS and WAAS, such as being able to work more productively under trees and around buildings. As I wrote in the original 2006 article, mapping-grade receivers will eventually exploit GNSS and that has started. Manufacturers have introduced GPS+GLONASS mapping-grade receivers in the past year. But remember that there are limited sources of differential correction data for GLONASS data. WAAS doesn’t support it. OPUS doesn’t support it yet. NDGPS doesn’t support it. There are a few CORS stations that support GLONASS and the National Geodetic Survey (NGS) said it’s going to offer GLONASS orbits by the end of 2009. But, essentially, one would have to setup their own reference station in order to post-process GLONASS data, or use an RTK Network that is broadcasting GPS+GLONASS corrections.

The killer app for GNSS — and what has driven companies to buy GNSS equipment now — is centimeter-level, real-time positioning (or RTK, RealTime Kinematic, as it’s commonly referred to). GPS/GLONASS receivers have existed in the survey and precise-positioning market space for many years, and they have finally proven their value after years of GLONASS uncertainty. Using only GPS, RTK still has “brownout” times during the day, especially in tough GPS environments in which satellite visibility is limited. Simply put, there are not enough operational
GPS satellites to fully meet the demands of real-time, centimeter-level positioning in many cases.

A System Primer

The following will discuss the different GNSS implementation benefits and rough schedules.

GLONASS: The RTK Partner. GLONASS is Russia’s version of GPS. It may surprise you to know that the first GLONASS satellite was launched more than 25 years ago (1982), but due to political and economic issues in Russia, the system never reached maturity as a standalone system. In recent years, however, GLONASS has earned consumer confidence and has proven to be a useful augmentation to GPS for applications using RTK.

Outside of RTK applications, GLONASS hasn’t been shown to add value to GPS. Markets such as survey-grade postprocessing,  mapping-grade GIS don’t have the cost–benefit payoff for GLONASS that RTK does. Therefore, very few GPS/GLONASS receivers are sold outside of the RTK market space.

For a long time, GLONASS was stagnant in terms of market acceptance. Without going into too much history, credit the increased popularity of GLONASS in RTK applications to the marketing of Javad Positioning Systems’ technology by Topcon Positioning Systems beginning early this decade. Javad and Topcon have since split (2004) but enough GPS/GLONASS systems had been fielded by then that the GPS/GLONASS RTK solution had proven to be effective and reliable. Today, nearly all major GPS manufacturers offer a GPS+GLONASS RTK system.

There are currently 17 operational GLONASS satellites and six more are scheduled for launch by the end of 2009. A full 24-satellite GLONASS constellation is scheduled to be in orbit by the end of 2010. The legacy and unreliable GLONASS satellites have been retired. The current GLONASS-M satellites have a design life of seven years. GLONASS-K satellites, with a design life of 10 years and sporting a new CDMA design for close compatibility with GPS, is due to launch at the end of 2010.

Since the 2006 article, Russia made an historic announcement that they will begin including CDMA signal structure on their next generation GLONASS-K satellites that are scheduled to begin launching at the end of 2010. This is a very significant development that will ease the complexity of designing GPS+GLONASS receivers. It will also result in newer

There is one serious technical consideration regarding GLONASS. Unlike GPS and Galileo, which use CDMA (Code Division Multiple Access) signal structure, GLONASS uses FDMA (Frequency Division Multiple Access) signal structure. With GPS and Galileo providing 57 satellites using CDMA, will the additional receiver complexity required to process GLONASS data motivate manufacturers to ignore GLONASS in the future? It’s a valid question.

Galileo Gets Started. Galileo is the European counterpart to GPS. Unlike GPS and GLONASS, which are financed and controlled by their countries’ respective military organizations, Galileo is controlled by civilians; the majority of the system development and all of the system maintenance are funded with commercial capital. It is a GNSS that has been targeted at commercial applications since its inception. It’s
designed to have a 30-satellite constellation (27 plus three spares), as well as a complement of groundstation equipment. The satellites orbit at slightly higher altitudes than GPS, but the operating principles are the same. The proposed constellation is designed so at least eight satellites are in view at all times.