The Evolution of LiDAR Field Operations and Preferred Methods

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In the past twenty years, LiDAR field operations have changed significantly. The advances in LiDAR technology played a significant role. The improvement in the GPS constellation, the addition of GLONASS and other available systems make it possible to essentially fly around the clock. There are still precautions that need to be taken as it relates to GPS during collection. The access to significant Continually Operating Reference System (CORS) data, State CORS networks provided by various agencies make collection much more efficient than it used to be. There is still the need for GPS base stations setup based on accuracy requirements and configuration components of a given project. Additionally, technology innovations such as repetition rates and terrain following are a welcomed development in how field operations are done today. Lastly, the manufactures have done an excellent job providing flight planning software packages that provide expedited flight plans and the ability to adapt the flight plans as needed on the fly.

Roughly twenty years ago, when LiDAR became commercially available, field collection was tedious at best. One 4 hour mission was a significant exercise. The flight planning would require creating flight lines and offsetting each flight line in CAD software and then creating waypoints at the ends of each line in a distinct order, then exporting the waypoints so that they could be uploaded in an aviation GPS such as a Garmin. There was extensive positional delusion of precision (PDOP) planning that was required based on the availability of the GPS constellation as it related to a given projects location. For example, a project location in New Mexico that could be flown at any time today took several days to fly in 1996 because the availability of satellites and acceptable PDOP only occurred between 2am and 4am and then again at 2pm and 4pm. There was also selective availability to contend with which was the military scrambling the GPS signal randomly which made the GPS solution much less accurate then it is today. Additionally, the repetition rates of the LiDAR sensors at the time where far less then they currently are. Imagine collecting a project site today with a sensor that could only operate at 5 to 10khz versus 500knz to 1MHz. Additionally, the time to verify coverage required processing the data completely in the field which averaged about double the time as collecting the data. Currently, most LiDAR manufactures have systems in place to verify collection and coverage instantly. This doesn’t mean that we would have a 100 percent confidence in the collection, so typically the data is verified by processing it but it takes much less time than double that of the flight time.

The evolution of GPS and additional services is positively staggering especially as it relates to LiDAR collection. During the Gulf War from 1990-1991, was the first time the U.S. military used GPS in conflict. Before that it was primarily used for satellite launches, testing and creating a command and control center for the U.S. military to monitor satellites. In the late 1990’s, the government saw it to be useful for civilians as well as the military and created a dual-use system that eventually led to the creation of 2 signals for civilian use only that ultimately strengthened accuracy and reliability. The date that changed everything was May 2, 2000 when selective availability was discontinued and the non-degraded signal opened the opportunity for flying more frequently.

While the U.S. was making improvements with GPS, there were other nations doing the same. The European Union (EU) had created their own system called Galileo, also known as Global Navigation Satellite System (GNSS) and had made an agreement with the U.S. in 2004 that provided cooperation as it pertains to shared satellites. Furthermore, Russia had "GLObal NAvigation Satellite System" (GLONASS) in the works. After some ups and downs in the early 2000’s, the Russian government made restoring this system a top priority and was globally available by 2010. With the combination of these satellite systems, there is rarely a time when there are not enough satellites to fly therefore forecasting PDOP became irrelevant.

Even with all the satellite availability, there is still a need for ground networks to dial in even closer accuracy and precision for clients. A GPS base station that is usually set-up by the flight operator prior to flying is still relevant. The best set-up location are inside the client’s boundary as the most accuracy data will come from this configuration and the station is the closest to the aircraft during flight. Additionally, setting up a base station at the airport of operation is always good for redundancy regardless of using CORS, state CORS or other base stations in the event that something happens to these other reference stations. Continuously Operating Reference Station (CORS) are a network of stations available to use from the U.S. National Geodetic Survey which provide accuracies for local GPS readings. While CORS can be very useful, it is not available everywhere, state CORS can also provide precise accuracies using a network of reference points. There are varying degrees of State CORS networks throughout the U.S. State agencies such as Department of Transportations set up CORS networks for their uses and these systems can be accessed for free or a small subscription fee for collection.

In the past it was required to do a static initialization of the system for the IMU and GPS to find itself. Typically, this initialization took 10 minutes and was required before and after the flight. Over time this was reduced to 5 minutes and some times this wasn’t required. The hard 5 to 10 minutes is no longer required but it doesn’t hurt to spend the extra time to get a better solution. Now, the sensor system lights indicate that the system is initialized the plane can start moving. Another factor that should be considered is the KP-index. The KP-index is the measure of geomagnetic activity from the sun, commonly referred to as Solar Flares. The KP-index affects GPS signals. The higher the geomagnetic activity the worse the GPS signal. The cycle of geomagnetic activity is about 7 years. In some cases geomagnetic activity can cause the GPS to be very bad and the accuracy of the resulting solution is much worse than when there is little solar activity. It is still good to check the KP-index and more so when the solar flares are acting up on the upward swing of the seven year cycle.

Not having to worry about PDOP takes some daily pre-planning of flights off the table. Still, considering the project site and weather conditions is something that won’t evolved and is the most unpredictable part of aerial LiDAR acquisition.

Prior to mobilization, doing research on the project site is critical. First and foremost, for flight planning reasons, it’s necessary to know the terrain around the area as well as the airspace restrictions. And not only within the desired client boundary, but the area outside the boundary needs to be considered as well to avoid complications during the turns in between each line. During flight planning, considering terrain and the safety of the pilot and operator will help avoid the airplane getting too close to high terrain. This is very important and should be common sense but when planning a project at a work station real life considerations are not always realized. Restrictions could be set for a number of reasons but most occur for high air traffic conditions, military activity and air space restrictions for given events and governmental reasons. Knowing where these are located and what time of day they are active could significantly affect collection and efficiencies. These factors should always be considered when planning a project.

The knowledge of weather patterns and how atmospheric pressure can affect winds at flying altitude can ultimately help determine the best time of day to fly. For example, when flying the front range of the Rocky Mountains in the summer season. Considering the predictable afternoon showers and rapidly warming temperatures throughout the day, it’s almost certain that there will be clouds developing and winds worsening almost every day at a certain time. To avoid this, it’s best to take advantage of the night-time cooling temperatures that make the early mornings calm, clear and ideal for flying conditions. The time of day, for this area and time of year, also helps to avoid winds at altitude that come with the quick rising temperatures around mountain tops that could ultimately mean landing, if turbulence becomes too much to continue collection.

Technology malfunctions can be a huge hindrance in field acquisition and completing a project. Worst case scenarios may require on-site engineer support from the sensor manufacturer, but most problems encountered can be handled by a field operator who is already on-site. The support provided by the manufactures is sometime difficult to deal with because they have a protocol to follow regarding the sensors as directed within their organization. It is important to understand the sensor technology you use and develop an intimate understanding of the technology. It is clear as to why manufactures have protocol but it is not always the best course of action for a LiDAR provider because it causes delays that impact completion and revenue. More often than not it is a worn out cable that creates the most issues.

With all the technology advancements over the years, checking coverage onsite is still necessary. It can take roughly 2 hours of computer time if a full 5 hour mission is collected. With the sensors getting more powerful, downloading the data can take longer. Although, along with those sensor betterments comes computer upgrades that can easily handle the processing of any powerful sensor that ultimately counteracts the longer download time.

In the end, the technology continues to change rapidly, adapting at the same rate is crucial to be successful. Field operations is the first step in creating useful data for clients to utilize but can also be expensive especially when factoring in weather conditions and other considerations. Having an extensive knowledge of weather patterns, restricted air space, terrain, rapidly troubleshoot, sensor components and operation, and the ability to check data quickly on-site is required to be successful.

Jill Wrenn is currently a Senior Field Operator for Merrick & Co. located in Greenwood Village, Colorado. Her experience includes extensive field experience including GPS survey, LiDAR and Digital Imaging Sensor operation, testing and troubleshooting sensors. She has a thorough understanding of the LiDAR production process as a result of past experience as a LiDAR production lead.

A 1.597Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE