LIDAR verses RADAR?

By Robert A. Fowler

I have read both of Mr. Fowler's LiDAR articles in EOM magazine and found them informative and refreshing in that he has given many people an insight into how things really work with LiDAR - not just the advertising claims. I have one comment though in regards to radar mapping. He mentioned that radar is not able to penetrate the foliage. In keeping with his clarity on the limitations and capabilities with 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 his 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, WA.)

Gary raises some interesting points worth looking at in more detail. However, first maybe a couple words of explanation: 1) I previously worked for Intermap Technologies (owners and operators of the STAR 3i interferometric radar system) and so I do have some knowledge of radar technology, although I am far from an expert. 2) The former article was not a comparison of different technologies as such, and I didn't intend it to be so. It was specifically about LiDAR for flood risk mapping. I also truly believe each type of technology has its advantages and disadvantages, and both RADAR and LiDAR have some significant of each.

Look angle

There are a number of factors which come into play regarding how much penetration of vegetation is possible by radar; the most significant being the look angle. Most commercial systems use a side looking radar. The signals are sent out at a considerable angle off vertical. While there will be some penetration of foliage by the radar, because of this side looking geometry there is a greater propensity for the signal to hit tree trunks and branches which offer a bit more resistance (it doesn't pass through them). In densely forested areas the radar tends to hit more tree trunks and this produces a scattering effect making a very noisy return. It is then very difficult for the people processing the radar data to determine what is giving them returns: some foliage, some branches and tree trunks and maybe some ground. If the vegetation is parkland with scattered trees, this is not a problem, but in heavy forest it is. I have seen enough radar processing and imagery to know that in heavy tree cover it doesn't work 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 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 the chance of hitting other objects (tree trunks and branches). Typically in a forested area, our company flies survey lines closer together to avoid some of this effect.

Now, when we consider P band radar, this holds promise of a lot more penetration of vegetation. I say holds promise because, again, there are a couple of limitations. To my knowledge, this band is currently only being used in a research situation for elevation production, and is not commercially available on a wide scale. (And there are some wide variations of opinion on whether it works or will really work effectively.) Secondly it tends to knock out a lot of wireless communications in the area while it is in use. With the ubiquitous cell phone/wireless communications in use today world wide, this is a major technological hurdle to overcome.

Accuracy

The accuracy claims for some radar systems are also contentious. Without wishing to upset various people in the industry, and I still have many friends and contacts in the radar business, it is fair to say most companies like to brag about the best results they achieve. However, obtaining sub meter accuracies on a consistent basis with radar is not easy. Everything needs to be working pretty near perfectly. To be fair, extravagant claims also happen with LiDAR service providers too, and many companies' marketing people will frequently advertise exceptional accuracies - basing their claims on relative accuracy between data points rather than absolute accuracy. For me, an accuracy statement should relate what is consistent and repeatable.

Other factors

There are a host of other factors which also come into play when using any airborne systems. For example, wind shifts/wind shear can be a problem. Most people use a one-second GPS position epoch: that is, they record their GPS data every one second. If there are sudden positional changes of the aircraft due to being buffeted by wind or there is a sudden change in atmospheric pressure resulting in a rapid drop or rise in aircraft position occurring between GPS epochs, this can cause havoc with GPS positioning and inertial systems. Obviously the faster the aircraft is travelling the worse the effect.

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 and when this is picked up by the inertial system. Inertial systems themselves have biases which are usually resolved through calibration flights, but if they are banged about a lot, the calibrations can change.

In the case of interferometric radar, the alignment of the two radar antennae with each other must be rigid and their alignment known in relationship to the aircraft and the IMU. And obviously this alignment should be consistent during the survey flights. (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 and, of course, the geometry of the satellite constellation and a sufficient number of satellites is also important for high accuracy surveys. Having the location of ground GPS stations on or near the project area for translocation computations is also significant when accuracies are required in the sub-meter range.

This partial listing indicates some of the phenomena which can have minor and some not so minor effects on system performance, regardless of the technology used.

Technology continues to evolve, and radar systems have a definite place in the market. I don't think the LiDAR service providers are in competition directly with radar at the moment - but of course that can change. Intermap Technologies claims within a few months it will achieve 1 foot accuracy. Aerosensing Radarsysteme GmbH claims something similar. Time will tell if these will be a consistent, repeatable accuracy, 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 when 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 them ideal for many tropical areas. This doesn't mean either technology can't be flown when there are no clouds, but these are the significant advantages.

Airborne RADAR surveys are suited to cover large areas quickly. Because they are also typically quite expensive to operate, they are not really suited for small area surveys. LiDAR surveys are flown on slower aircraft and are typically a little less expensive to operate making them ideal for smaller areas.

As I have said many times when talking about LiDAR, the limitations on the accuracy achievable by LiDAR are not the laser. That is generally far more accurate than most of our clients need it to be. The limitations are the inertial system and the GPS, and airborne radar has those problems too. Until we can beat those problems, everyone is in the same boat. On the other hand, if we take a step backwards and look at the reality, the accuracies achievable with airborne technology today are truly remarkable; to the extent where we have, I believe, reached the point where deciding who is right on an accuracy issue is not always easy.

First published in EOM, September 2000
Robert Fowler is Vice President of Sales and Marketing for Lasermap Image Plus. You can join the debate through the pages of EOM or contact him personally at: bobf@lasermap.com.