GNSS – Why is that important for Mobile Mapping?
Thanks to Jeff Fagerman at Lidar USA for this article.
Mobile mapping systems (MMS) are gaining rapidly in popularity and their use is spreading into many fields beyond the more technical areas of surveying and engineering. Applications for archeology, forensics and accident scene reconstruction, riot scene analysis, a multitude of agricultural and forestry applications, golf courses, game parks, and city simulations are just a few of these areas. The standard applications for roadway, railway, bridges, tunnels, erosion, topography, hydrology, GIS – applications all centered about the surveying, mapping and engineering fields – are still a very big part of mobile mapping. Nevertheless, the more atypical uses are definitely driving a lot of mobile mapping system development. Of course, the use of such systems on UAVs, or drones, is also at the forefront of this push as well.
Of key importance to a mobile mapping system is position and this is typically done via GNSS (or GPS). In many cases, these newer users think of GPS in terms of travel (GoogleMaps, etc.) or recreation (GeoCaching, recording a hike, racing). For these users, the GNSS (Global Navigation Satellite System), or simply GPS, is assumed to be a minor component and an inconsequential investment relative to their main focus. They expect more and better results all the time for less and in less space. GNSS just adds a cool “wow” factor but isn’t absolutely required.
Meanwhile, in the surveying and mapping realm, users expect similar things but they also expect the investment will be quite costly and is probably the biggest investment they may ever make. For such an investment, they generally think the equipment should be large and somewhat difficult to operate. Until recently, many of the mobile mapping systems definitely met their expectations by being quite large and with a price tag of $500,000 to $1,000,000. This high cost completely eliminated the possibility of mobile mapping by all but the larger firms with healthy clients and a willingness to pay top dollar for their work.
Time has a way of bringing about change. In just the past few years the price of mobile mapping systems has dropped dramatically. A number of new systems are available for under $500,000 and some under $100,000. Likewise the size has been greatly reduced and the ease of use has improved. This has opened a doorway for the new users to venture into.
As the price of the systems comes down, the burden to at least superficially educate the user on certain geospatial aspects often falls to the surveying and mapping profession. Just as with GPS prices dropping, many non-licensed, self-taught, would-be surveyors thought they could properly perform and locate property boundaries. The same holds for the new mobile mapping user. Owning the technology is not the end of the line. In some cases, you need a professional license. In other cases, it is enough to gain a better understanding of the limits and capabilities of the new equipment. We will look at the technical side only from this point forward.
To begin, let’s consider just a few aspects of the GNSS component in a mobile mapping system without giving a full discourse on any one item. The GNSS is one of two main parts of the INS (Inertial Navigation System), a key part of a mobile system. Virtually all commercial INS solutions utilize GPS which has two frequencies of interest to use for positioning: L1 and L2. An L1-only system can deliver results at best to 50 to 100 cm while an L1/L2 system can deliver just a few centimeter results. Furthermore, GPS is not the only system in the sky. There is also GLONASS, Galileo, and Beidou – all working towards a GPS-like solution at least for certain areas in the world. Generally speaking, adding more systems and capable receivers will improve the final result (in this case, better position).
Generally we would use the GNSS data in a post processed solution. Normally this is done with some sophisticated software and a local base station (a private base station or a public system such as CORS). These post processed solutions with a base station logging at 1Hz allow for the best possible solution to be computed. An alternative to using a base station is to use Post Processed Positioning (PPP). In this case, the receiver must be running for 2.5 to 3 hours with uninterrupted GPS lock. In both cases, centimeter level results are achievable via post processing.
There are other solutions available though that offer real-time solutions (meaning no post processing to improve results). If the application does not require such tight positioning, but must roam over very large areas, then a space based augmentation system (SBAS) solution using something like OmniSTAR or NovAtel CORRECT are available (usually on a subscription). These systems utilize a geostationary satellite (meaning they are geographically dependent) to introduce additional information into the computation of the receivers positioning. Generally these systems can achieve at best 10 cm accuracy. Initialization and re-initialization after loss of GPS signal is of key concern with these systems. Again, there is no post processing to improve the positioning.
Likewise, there is often a free SBAS solution (using GPS only) that does not require a subscription (and is nearly global). As you might expect, the results are not as reliable as the subscription systems. Still, results as good as 60 cm are possible (but expect 1+ meter). Since SBAS uses geostationary satellites, availability is limited and the quality of the results will vary depending upon the receiver location.
Lastly, there is the RTK solution. This solution uses a local base station (which may be a virtual base station) to provide live communication between itself (the base) and the rover. The communication includes update information to provide improved positioning based upon local parameters. This can rival a post processed solution for positioning accuracy but is limited in range and highly susceptible to loss of lock issues due to obstructions. Due to the increase of base station availability this solution improves almost daily.
Range of coverage of each of these methods varies. A post processed solution using a base station is limited by the range of the base station and required accuracy. In most cases a 10km range provides the highest accuracy with the accuracy dropping of linearly after that by 1 or 2 ppm. A PPP solution is virtually unlimited as long as conditions remain suitable (no loss of lock, stable weather). The SBAS solutions virtually cover continents. The RTK solution is very limited in range in most cases and is highly susceptible to loss of lock.
Reviewing, there are L1 and L1/L2 systems using GPS. GPS is not alone in the sky as there are several other constellations available at least in part. There are ground stations and PPP alternatives for positioning improvement. There are subscription SBAS solutions and the free GPS SBAS solution. Lastly, there is the RTK solution. These all fill certain needs at various prices, hardware configurations, range, and accuracy. Post processing solutions will invariably offer better results just as a proof-read, highly edited, manuscript is better than the initial or rough draft.
As you can see, positioning of a mobile system using GNSS is not trivial. There are many contributing factors (cost, range, availability, accuracy). If we include additional means of positioning such as the Locata solution, SLAM, and others, there is even more to be of concern.
We’re out of space now. Maybe next time we can continue to look a little deeper into this subject.