Proceedings Article | 12 October 2021
Steven Hancock, Ludwig Prade, Gerald Bonner, Stephen Todd, Christopher Lowe, Ciara McGrath, Johannes Hansen, Ian Davenport, Iain Woodhouse, Brynmore Jones, Haochang Chen
KEYWORDS: LIDAR, Satellites, Photonics, Semiconductor lasers, Telescopes, Solid state lasers, Mirrors, Energy efficiency, Vegetation, Sensors
Satellites have become essential tools for providing information for weather forecasts, monitoring agriculture and studying climate. In recent years satellite lidar (laser ranging) has come of age, with three missions launched in 2018. These spaceborne lidars are collecting ground-breaking data. They are the only current in-orbit technology capable of directly measuring bare Earth topography under vegetation and of making non-saturating measurements of forest biomass, height and cover.
However, the energy requirements of a lidar lead to very sparse coverage. NASA's GEDI mission, the densest sampling lidar yet, is expected to directly sample between 2-4% of the Earth's surface, leading to sampling errors and preventing its use in applications that require continuous coverage without interpolation. The coverage of a lidar satellite is controlled by how much laser power they can emit, collect and make a usable measurement from. This is controlled by the payload power, telescope collecting area, energy per shot needed for an accurate measurement, laser efficiency and detector efficiency. Initial calculations using currently in-orbit technology suggest that global, continuous coverage could be achieved within 5 years at 30 m resolution using ten ICESat-2 class lidar satellites. This would be prohibitively expensive. The Global Altimeter MISSion: GLAMIS project aimed to see whether recent developments in photonics, deployable optics and small-satellites could make continuous coverage lidar more cost-effective.
All current lidar satellites use a solid-state laser. These produce short, powerful bursts of light but are only ~5% efficient. Tapered laser diodes are 25% efficient, but cannot emit the same amount of energy in as short a burst as solid state lasers can. Diode laser would need to spread the energy over a longer time, using pulse compressed lidar (PCL) to allow measurements, which would lead to very different noise behaviour to sold state lasers, possibly preventing their use in spaceborne lidar. A satellite lidar simulator was used to determine that PCL is suitable for satellite lidar over a range of biomes.
The mirror size and cost for fixed and deployable telescopes were estimated for three different satellite size classes; a 12U satellite, a 150 kg satellite and a 500 kg satellite. The electric power and cost of each of these satellites was also estimated and used, along with the laser efficiencies, to calculate the lidar coverage each could achieve. Orbital constellation simulations were used to determine how many satellites of each configuration would be needed to achieve global coverage within a given timeframe, accounting for loss due to clouds. It was found that deployable optics would allow a more cost effective coverage than fixed optics. It was also found that the larger satellites would be a more cost-effective solution than smaller satellites.
In conclusion, using tapered laser diodes in pulse compressed mode and deployable optics are promising technologies to increase the coverage of satellite lidar. For a future constellation of lidar satellites capable of covering the whole Earth, the optimum configuration was found to be a constellation of eleven 150 kg (roughly microwave sized) satellites fitted with foldable mirrors.
