02 Nov 2005
Mobile LIDAR came under the spotlight at SPIE Europe's recent Remote Sensing event held in Bruges, Belgium. James Tyrrell caught up with Environment Canada's Kevin Strawbridge to discover the benefits of taking a high-power laser on the road.
Vehicle-mounted LIDAR units that can travel cross-country to monitor air quality are proving to be a real asset to environment agencies worldwide. One such resource is Environment Canada's Rapid Acquisition Scanning Aerosol LIDAR. Dubbed RASCAL, the custom apparatus has been busy analysing the country's pollution hot-spots.
Featuring an Nd:YAG laser supplied by US firm Continuum, RASCAL fires 0.5 J pulses of infrared (1064 nm) and green (532 nm) light into the atmosphere at a rate of 20 Hz. Two 24-inch mirrors steer the beam through the sky, targeting particles up to 15 km away. A telescope equipped with a highly sensitive detection system, based on avalanche photodiodes and photomultiplier tubes, captures any backscattered light.
Time-of-flight algorithms convert the data sequence of backscattered light into a particle map with a resolution of 3 m along the beam axis.
"Primarily RASCAL is different from other [mobile] scanning systems because we can put more power out and because we have a large optical path coming back, which gives us our high resolution," said Strawbridge. "You begin to see a lot more information than you could ever see from other LIDAR systems that are either pointing straight up or averaging over 1, 5, or 10 mins."
With a scan time of just 6 s, Strawbridge and his colleagues are able to make LIDAR "movies" that show in striking detail the complex dynamics of particles circulating in the air. Being highly sensitive, RASCAL can even measure the low aerosol concentrations in the free troposphere (pristine air).
What's more, extra detail is available thanks to the LIDAR's dual wavelength configuration. "Being able to measure both in the infrared and in the visible allows us to compare [our data] with other instrumentation," said Strawbridge. "[However] the real advantage from a LIDAR point of view is that you can ratio the two wavelengths in a space-time graph to give information about particle size."
This means that along with pinpointing the altitude and range of an aerosol layer, the system can distinguish between different sized particles, discriminating large dust particles from fine smoke haze.
"If you want to come up with a size distribution then you would probably use six or seven wavelengths, but that becomes impractical on a scanning system," explained Strawbridge. "We are interested in being able to look at gross features and [here] just two wavelengths can be very useful."
Preserving RASCAL's rapid scanning capability is important as this is one of the mobile unit's key strengths. Based on a two mirror design, with the first mirror fixed and the second free to rotate horizontally and vertically, the assembly provides 180 ° azimuth operation and elevation scans up to 100 °. "You can more or less see half the sky at any given time with this unit," explained Strawbridge.
RASCAL is able to scan at speeds of up to 24°/s, although typical operation is around 0.2-2° /s. Computer controlled and featuring 18-bit digital to analogue conversion, the scanning units have a resolution of 1/1000° .
Highly reproducible and GPS equipped, RASCAL can revisit a specific site and repeat a series of measurements with ease. "We have GPS interaction to log where we are," said Strawbridge. "Sometimes we'll point the beam straight up and drive around an area, for example from an urban [neighbourhood] into a rural zone, to look at the dynamics on a different scale."
Recently, the RASCAL team took its measurement expertise to Golden, British Columbia. Surrounded by mountains, the Canadian town is located in a geological bowl and has the second highest concentration of airborne pollution in the province. "When we got there I was surprised at the lack of wind," said Strawbridge. "Particles are transported through complex re-circulation effects."
The town has four major pollution sources - a plywood processing plant, a national rail depot, winter wood burning and truck emissions. The contribution from heavy goods vehicle fumes is exacerbated in winter months due to bad weather.
"You can have anywhere from 500 to 800 of these trucks sitting there percolating for up to four days at a time waiting for the roads to open up," explained Strawbridge. "We were there to look at source apportionment and [to find out] how the meteorology fits in."
Supporting the town's existing measurement stations and operating alongside a mobile chemistry lab, RASCAL's strong visual data took little time to make an impact. "We are already seeing new policies being implemented as a result of the LIDAR study," said Strawbridge. "The town council has now banned solid-fuel-burning appliances, such as wood stoves, in new homes being built in the area."
The Golden campaign also threw up some unexpected results, such as the presence of Saharan sand. "For us it was a really significant measurement because we had never seen Saharan dust come all the way across and impact in British Columbia," said Strawbridge.
Predictably, the use of laser systems in the open is becoming increasingly regulated. However, Strawbridge and his colleagues have more than 15 years of experience in maintaining a safe operating environment and the group has strong links with the relevant governing authorities.
"Transport Canada evaluate the safety concerns based on when we are going to operate, how we are going to operate and what our safe distances are," explained Strawbridge. "They will sometimes limit our direction, for example if we are shooting over an airport runway, or they might move traffic around us through communications with the local air traffic control centre."
RASCAL's array of on-board safety equipment includes a TCAS transponder unit that is widely used by the aircraft industry to prevent mid-air collisions. "The advantage of [TCAS] is that it gives you bearings as well as range, so you know where the aircraft is and whether the aircraft is increasing or decreasing in altitude," said Strawbridge. "We also operate a modified marine radar that looks for hard targets [without TCAS transponders]."
When it comes to studying particle dynamics on a larger scale, Environment Canada is also able to deploy an airborne unit flying at 12,000-18,000 ft that maps areas such as valleys and whole cities in a matter of hours.
Squeezed into a Cessna light aircraft, the agency's Small Aircraft LIDAR System for Aerosols (SALSA) benefits from the ongoing reduction in size and weight of lasers. Strawbridge and his colleagues were keen to develop a small footprint LIDAR that would fit into small planes.
"This is a huge advantage for us from a funding point of view, because we can reduce the flight cost from CAN $6000 (€4210) to $300 per flight hour, including a pilot," said Strawbridge. "You can also deploy small aircraft so much faster than large aircraft - you can be airborne in 20 mins."
Reliability is another reason for the group to adopt the latest laser technology. "Diode pumped lasers are a huge improvement in terms of maintenance," said Strawbridge. "They now have lifetimes of 3-4 years, which [also] makes them ideal for a satellite based platform."