Helicopter Aerial Laser Ranging Håkan Sterner TopEye AB P.O.Box 1017, SE-551 11 Jönköping, Sweden 1 Introduction Measuring distances with light has been used for terrestrial surveys since the fifties. The instruments measured the distance between itself and a reflecting prism. When the Laser Range finder was introduced in military target acquisition systems a requirement to measure against basically any undefined surface with as few laser pulses as possible was introduced. The military requirement has driven the development of Laser Range Finders and hence generated an instrument technology that is well suited for capturing geographical information. Profiling systems, since 1970's airborne Laser Range Finders have been used to capture terrain profiles. The principle is basically that a geographically positioned aircraft measures the distance from the aircraft to the ground/object and consequently assigns a geographical position to the measured point on the ground. Scanning area/corridor capturing systems, basically creates a three dimensional digital terrain model (DTM) as the sensor is passing; push broom principle. The leading edge technology needed to build such a system has only recently been made available. Technology enhancements in the following areas have made it possible to make scanning systems: GPS positioning systems, accurate XYZ position. Inertial system, high resolution. Laser Range Finder; single pulse measurements with high Pulse Repetition Frequency. Scanning and stabilization systems. The advanced laser systems are made to be eye safe and have LRF's with a Pulse Repetition Frequency (PRF) up to 8000 Hz and a capability to distinguish multiple returns from each laser sounding. The absolute accuracy for a single point on the face of the earth can be better than 15 cm. 1.1 Key Applications Building digital terrain models is the principle application for any type of airborne Laser Range Finder. A key advantage versus alternate methods is that the laser, used as a "probe", makes it possible to penetrate through a tree canopy and hence finding the true ground. Advanced systems that can discriminate up to four returns from each laser pulse, provides a wealth of data that can be used to determine tree height, bushes, etc.
Applications are opened or augmented because the method circumvents previous obstacles. The advantages in this respect are possibly mainly the time and/or cost reduction concurrent with the technologies inherent advantages - reaching to the ground and measuring objects without texture such as tidal flats. Examples in this area are environmental modeling, related to natural disasters such as flooding, erosion and 3D city modeling for propagation models (telecommunication, noise, pollution..). Transmission lines can easily be mapped. One single survey flight can capture the data to: Map the location of the transmission towers and the right of way. Data to measure the line sag for each catinary - can be used in an engineering model to determine the transmission capacity of a segment ampacity. Identify potential danger trees - arcing between the conductor and a treetop can create forest fires and electricity outages. From the data set a work order can be generated to dispatch a field crew to take down a specific tree at a defined location (GPS coordinates). The same data set can be used for mapping, engineering and operational procedures and thus creates a new dimension in the use of spatial data. It is feasible to establish surveillance systems that on a regular basis monitor a transmission line for various parameters; encroachment, line sag, snow jeopardy, etc Danger Trees
2 Survey Process Any airborne survey system needs to take a number of factors into consideration. This paper discusses only the primary data acquisition process. Where the data capturing process ends and the mapping process starts determines to what extent data needs to be integrated with other information to produce the deliverable product. Survey Plan Survey Flight Post Proc. Airborne Equipment GPS Radio Link Post Processing Software Ground Reference Station 2.1 Survey Planning Part of the survey planning is standard procedure, for example; defining the survey area or object and planning the approach flight, refueling location, etc. Most are the same as for any aircraft operation. However, selecting reference points, plotting flight lines and way points requires an understanding of both flying and surveying. If the positioning system is based on GPS, the satellite availability and configuration must be checked for the period the survey flight is planned. The satellite configuration is correlated to achievable GPS position accuracy and is thus related to the required accuracy. The LRF sensor parameters need to be defined for systems using an advanced Laser Range Finder. Parameters such as beam divergence, ground separation between each laser pulse and how the returns shall be digitized. The selection is related to the purpose for which the data is to be used.
2.2 Survey flight - Capturing the Data The Laser Range Finder system is either designed for a special purpose dedicated aircraft or designed as a mobile system that can use a general-purpose aircraft. Saab has chosen the latter to accommodate maximum utilization of the system and to incorporate pilot guidance and support to make the survey flight itself as routine as ever possible. To achieve highest possible accuracy a ground reference station is needed to capture GPS data for the post processing. Commercial differential GPS services (e.g.rds; Oministar) or a radio link is needed to provide the pilot guidance system with better real-time accuracy. A Scanning Laser system is not affected by weather and lighting conditions. As the flying altitude is below 500 meters (1,600 feet) the cloud base is seldom a problem nor is turbulent air. However, rain and snowfall are conditions best avoided. 2.3 Post processing The post processing can be seen as two separate procedures. The first phase is related to determine the GPS position of the aircraft. Combining the GPS data from the aircraft with the data captured by the ground reference station does this. This will provide two very accurate positions for every second of the survey mission. Considering that up to 8000 laser soundings can be done per second, two GPS positions during the same time is far from sufficient. Henceforth, the second phase, is where the Inertial Navigation System (INS) data determines the exact position for each laser sounding. This position and the attitude of the scanner combined with the data from the LRF will produce up to four Geo-referenced XYZ positions for each laser sounding. To facilitate the subsequent processing of the data, each XYZ position is also tagged with a time stamp and quality information that defines what kind of laser echo was retained for every single position. Generating the DTM itself and extracting features is the next process and is not included in the primary data acquisition. Establishing the DTM is done according to common practice. Extracting a transmission line and determining the line sag between every tower is probably not that widespread practice and can be seen as one good illustration of what integration of advanced technology can do to be of service to the community. 3 Customer benefits 3.1 Lead time As the method is only to a limited extent affected by weather, tree canopy and other ground conditions, data can be captured when needed even if it is out of traditional season. The significantly shorter process time further reduces the total lead-time from identified need to available data.
3.2 Cost efficiency Capturing the data is less costly than alternate methods. The saving is due to elimination of activities such as ground control, model orientation, etc. The primary data is closer to the desired finished result that further enhances the cost efficiency compared with alternative methods. 3.3 Penetration to the ground Using photogrammetric methods requires that a given point be represented in two images constituting a stereo pair. This can be very difficult to achieve in both rural forested areas as well as in urban areas with high rise buildings and narrow streets. A single probing laser pulse will reach the ground in these types of areas and produce the XYZ coordinates from the ground. 3.4 High XYZ accuracy for small objects Measuring electrical transmission line sag has been a cumbersome task that with this method can be done in a highly automated process. Other survey and surveillance applications will surface as currently un-served needs are efficiently addressed. 4 Accuracy Defining accuracy is by itself a rather large task. The achieved absolute accuracy for an individual point is a result of the precision of a number of sub-systems: How accurate is each GPS position Precision and resolution of the INS How well can the LRF determine the distance between the sensor and the target. How precisely the scanner is moving. How fast and precise the stabilization system compensates for aircraft movements. XYZ 30cm 10 cm + 0 - Absolute Accuracy Altitude Relative Accuracy The actual achieved accuracy is related to the quality of the engineering work, manufacturing of each component and the assembly of the system. The result will also be dependent of the professional skill of the team operating the system. The relative accuracy will be significantly better than the absolute due to the error contribution from the INS platform. The INS platform has a drift in accuracy that is stable in a short term perspective while the accuracy in GPS and LRF systems are more random.
An absolute accuracy of 10-30 cm can be achieved with components available today. To reduce the absolute accuracy to better than 5 cm is probably beyond reach. The relative accuracy is significantly better and can be further enhanced by various filtering methods. 5 Past Technology Progression 5.1 Positioning GPS positioning systems that allows for 5-10 centimeter accurate XYZ positioning of the sensor platform that is today readily available. 5.2 Inertial Platforms - Laser Ring Gyro An inertial system that provides accurate position between the GPS fixes (2 Hz) and also controls the stabilization system is a required subsystem. The modern Laser Ring Gyros that is sturdy and fast enough to be used in-stable fighters planes, accommodates the need for fast and high precision inertial data. 5.3 Laser Range Finder A Laser Range Finder (LRF) using diode pumped lasers that allow for high pulse repeat frequencies is used to obtain high accuracy measurement and multiple distances from one single laser pulse. 5.4 Stabilized scanning Saab has during a number of years developed scanning and stabilization systems for optical systems. The technology has been used to produce a scanner that delivers a consistent laser scan pattern. 6 Helicopter versus Fixed Wing A Helicopter can operate from any reasonable flat area and does not need the access of an airfield. This is increases the operational efficiency and directly reduces operational expenses. It is easy for a Helicopter to follow any winding corridor. It can even stand still in the air and make turns without any banking - so called flat turns. A Helicopter is a remarkably stable aircraft even in turbulent air. Flying is normally done in uncontrolled airspace that further facilitates the ease of operation. A Helicopter will always be more expensive per flying hour versus a fixed wing aircraft. If the specific operational advantages of a Helicopter are not needed, then a fixed wing airplane is a better alternative.
The principles for rotary and fixed wing systems are the same. However, the engineering consideration that comes into play when designing a system for a Helicopter is quite different to a fixed wing aircraft. Other considerations that have a bearing on the system integration are related to the target applications. These are primarily: corridors versus area, desired map scale and accuracy together with different operational characteristic. As usually, there will never be a sole solution that is the best for everything 7 Future Developments Future developments are primarily to be expected in two areas; added sensor capabilities and to a larger extent, automated post processing of the captured data that produces the finished product faster and for less cost. Envisioned prospective sensors are better digital imaging devices such as infrared cameras, high resolution digital cameras, low light cameras, radar, etc. All combined with better storage systems. The most useful for most infrastructures related applications are expected to be infrared sensors and the high-resolution digital cameras.