| LIDAR Versus Radar
Some insights
By Robert A. Fowler A reader writes: I have read each of Mr. Fowler's two LIDAR articles in EOM and found them both informative and refreshing in that he has given many people an insight into how things really work with LIDAR, not just the advertising claims. However, I have one comment in regard to radar mapping. Mr. Fowler mentioned that radar is not able to penetrate through foliage. In keeping with his clarity on the limitations and capabilities of LIDAR, I am surprised that he omitted the extraordinary capabilities of radar. I do not know where he got the notion that radar is so limiting, but there is a wealth of information out there to counter these claims.
I can understand trying to downplay radar. After all, it is taking over what was going to be the LIDAR market. Timing is everything! Thanks again for the articles. Gary Grieve Renton, Wash. The author replies: Mr. Grieve raises some interesting points that are worth examining in greater detail. Let's start with a few words of explanation. First, I previously worked for Intermap Technologies - owners and operators of the STAR 3i interferometric radar system - so I do have some knowledge of radar technology, although I am far from an expert. Second, my earlier article was not a comparison of different technologies, and I didn't intend it as such. That article was specifically about using LIDAR for flood risk mapping. I truly believe that each type of technology has its advantages and disadvantages, and both radar and LIDAR have significant amounts of each. Look Angle
There are a number of factors that come into play regarding how much penetration of vegetation is possible through radar. The most significant of these is the "look angle." Most commercial systems use side-looking radar. These signals are sent out at a considerable angle away from the vertical. While there is some penetration of foliage by this radar, because of its side-looking geometry, there is a greater propensity for signals to hit tree trunks and branches, as they offer a bit more resistance. In densely forested areas radar tends to hit more tree trunks, and this produces a scattering effect that makes for a very noisy return. It becomes very difficult for the people processing this radar data to determine what is producing these returns: some foliage, some branches and tree trunks, and maybe even the ground. If the vegetation being surveyed is parkland with scattered trees, this is not a problem. But in heavily forested areas, it is. I have seen enough radar processing and imagery to know that, in heavy tree cover, radar doesn't work all that well with the types of wavebands used by commercial systems.
To a lesser extent, this effect also occurs with LIDAR. When the laser beam is pointing straight down, if there is a hole in the tree canopy, the system will receive a return. In a forested area, the wider the angle of scan, however, and the further the beam is off vertical, the greater is the chance of hitting other objects, including tree trunks and branches. In a forested area, my company typically flies survey lines closer together to avoid some of this effect.
Now, when we consider P-band radar, this holds the promise of much greater penetration of vegetation. I use the phrase "holds promise" because there are still limitations. To my knowledge, P-band is currently being used only in research situations for elevation production. It is not commercially available on a broad scale. There are also wide variations of opinion as to whether this technology works or ever will work effectively. Secondly, this band has a tendency to knock out wireless communications in areas where it is being used. With ubiquitous, worldwide cell phone and other wireless communication in use today, this is a major technological hurdle to overcome. Accuracy
Claims of accuracy for some radar systems are also contentious. Without looking to upset various people in the industry - and I still have many friends and contacts in the radar business - it is fair to say that most companies enjoy bragging about the top results they achieve. However, obtaining sub-meter accuracy with radar on a consistent basis is not easy. Everything must work close to perfectly. To be fair, extravagant claims occur with LIDAR service providers, too. Many companies' marketing personnel will advertise exceptional accuracy, basing their claims on relative accuracy between data points rather than on absolute accuracy. For me, an accuracy statement should relate to what is consistent and repeatable. Other Factors
There are a host of other factors that come into play when using any sort of airborne system. For example, wind shifts or wind shear can pose a problem. Most people use a one-second GPS position epoch, that is, they record GPS data once every second. If there are sudden positional changes of the aircraft due to being buffeted by wind, or if there is a sudden change in atmospheric pressure that results in a rapid drop or rise in aircraft position in between GPS epochs, this can play havoc with GPS positioning and inertial systems. Obviously, the faster the aircraft is traveling the worse is the effect on accuracy.
There are also minor influences between computed and real position, and attitude, due to the time lag between changes in the aircraft roll-and-tracking when detected by its inertial system. Inertial systems themselves have biases that are usually resolved through calibration flights. But if they are banged about a lot, these calibrations can change.
In the case of interferometric radar, the alignment of the two radar antennae with each other must be rigid, and the alignment must also be known in relation to the aircraft and the IMU. Obviously, this alignment should be consistent during survey flights, just as the alignment of a laser in a LIDAR system is likewise a calibration requirement.
Atmospheric interference and radiation interference (sunspot effects) can result in degraded GPS accuracy. The geometry of the satellite constellation plus having a sufficient number of satellites in view is also important for high-accuracy surveys. Locating ground GPS stations on or near the project area for translocation computations is also significant when sub-meter accuracy is required.
This partial listing indicates some of the phenomena that can have minor, and perhaps not so minor, effects on system performance, regardless of the technology being used.
Technology continues to evolve, and radar systems have a definite place in the market. I don't believe that LIDAR service providers are currently in direct competition with radar, but that could change. Intermap Technologies claims that, within a few months, it will achieve one-foot accuracy. Aerosensing Radarsysteme GmbH claims something similar. Time will tell if this accuracy will be consistent, repeatable, and achievable over the same area with different survey flights.
LIDAR systems are uniquely suited to low-level, high-accuracy surveys. As long as the cloud ceiling is sufficiently high, they can be flown under clouds whereas aerial photography cannot. Radar systems are ideally suited to flying in conditions and areas where cloud cover is persistent and low-lying to the ground. This makes radar ideal for many tropical areas, but it doesn't mean that either technology can't be flown when there are no clouds. Still, these are their respective significant advantages.
Airborne radar surveys are well suited for covering large areas quickly. Because radar is typically quite expensive to operate, it is not really suitable for small-area surveys. LIDAR surveys are flown aboard slower-moving aircraft and are typically a little less expensive to operate, thus making them ideal for covering smaller areas.
As I have said many times before, limitations on LIDAR's achievable accuracy do not reside with the laser. This part of the device is far more accurate than any client generally needs it to be. Instead, the limitations reside in the inertial system and the GPS, and airborne radar has these problems as well. Until we can defeat the problems, everyone is in the same boat. On the other hand, if we take a step backward and look at reality, the accuracy level achievable today with airborne technology is truly remarkable, so much so that deciding who is right on any accuracy issue can prove difficult.
About the Author:
Robert Fowler is vice president of sales and marketing for Lasermap Image Plus. Our readers can join in the debate through the pages of EOM, or by contacting the author personally at [email protected]. Back |
