Description
GOES-R Platform
The Geostationary Lightning Mapper (GLM) will be carried aboard the next generation of GOES satellites beginning with GOES-R. The next generation GOES satellites are 2800kg including payload and will have a power capacity of greater than 4000 W. The lifespan of this platform is expected to be at least 15 years. In addition to the GLM, the next generation GOES satellites will carry the Extreme UV and X-Ray Irradiance Sensor (EXIS), the Advanced Baseline Imager (ABI), the Space Environment In-Situ Suite (SEISS), the Solar Ultraviolet Imager (SUVI), and a magnetometer. The positions of these instruments can be located in Fig. 4 (MacKenzie, 2012). The first launch of the R series of GOES satellites is expected to take place in 2015.
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GLM Instrument
The GLM will continuously map IC and CG lightning over the Americas and adjacent Atlantic and Pacific oceans. According to MacKenzie (2012), the GLM will act similarly to a digital camera. The instrument will look for differences between a background image and a current image at a 2ms frame rate. To increase the ability to differentiate between lightning and non-lightning pulses, the sensors included in the GLM instrument will capture background images of ground and cloud features in the near-IR at 777.4nm. 777.4nm is a prominent oxygen emission line within the lightning spectrum (Koshak et al., 1999). Special filters and data processing techniques will be utilized so the GLM will be able to operate both day and night (See Fig. 5). The pixel field of view of the GLM will be 8km at nadir and 14km at the edge of the field of view of the instrument. The GLM is expected to have a flash detection efficiency of between 70 and 90% with a less than 5% false alarm rate. The data from the GLM will have a 7.7Mbps downlink rate and will be available for use in less than 20 seconds (Goodman, 2008). Fig. 5 depicts a basic schematic of the main components within the GLM instrument.
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Product
When the GLM detects a single pulse due to lightning, the pulse will be categorized as an event. For example, in Fig. 1 each individual bright spot or pulse may a lightning discharge illuminating one or more adjacent pixels in the GLM image. However, several pulses may be due to one single occurrence of lightning so lightning is categorized in an event, group, and flash system. Table 1 describes the event, group, and flash categorization system that will be applied to GLM data (See Fig. 6 for visualization of categorization). Because lightning is a 3D process, pulses observed a distance apart may actually be part of the same lightning flash, and thus an event, group, and flash categorization is necessary. Video 1 shows the extent of a single flash of lightning and is an example of the motivation behind grouping lightning in this manner (Goodman et. al, 2010).
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Once the lightning has been categorized into events, groups, and flashes, a time and location (lat,lon) is assigned to each flash. These flashes will be available for visualizations on various map types for operational uses. Fig. 3 depicts the coverage area of the GLM once GOES-EAST and GOES-WEST are upgraded to R series satellites. Also, the lightning climatology produced by NASA by the Optical Transient Detector and the Lightning Imaging Sensor is plotted in Fig. 3. GLM will produce similar lightning climatologies. Total lightning detected by the GLM will also be used on more local scales for mesoscale weather monitoring and will be available for forecasters using AWIPS in the future (NASA, 2012). See Fig. 7 for a sample future GLM product.
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Similar Instruments / GLM Advantages and Disadvantages
The main predecessors to the GLM are the NASA OTD\LIS instruments. Both the Optical Transient Detector (OTD) and the Lightning Imaging Sensor (LIS) were designed to be flown aboard polar orbiting satellites. OTD was in operation from 1995-2000 and LIS has been operating aboard the TRMM satellite since 1997. Because of the nature of a polar orbit, the sampling times over one specific region of interest is short for these types of instruments (Finke & Hauf, 2002). Because the LIS is aboard TRMM (Tropical Rainfall Measuring Mission) , only tropical latitudes are viewed by the lightning sensor (See Fig. 8). The GLM will have many advantages over the polar orbiting sensors that preceded it. The GLM will be capable of viewing most of the Americas and adjacent ocean region, and will only be limited in viewing very high latitudes. Also, weather nowcasting using total lightning data will be possible because the GLM will be able to view the same region continuously instead of only taking short snapshots of a region as OTD did and LIS does now. Ground based lightning detection networks can rival the GLM in the nowcasting sense because of their fixed position in a given location (Goodman, 2008). However, ground based lightning detection systems such as the World Wide Lightning Location Network (WWLLN) depend on the number of sensors on the ground, and have a lower detection efficiency than the GLM instrument (MacKenzie, 2012). The main disadvantage to the GLM is that it will not be operational for at least three more years, whereas the LIS is currently in operation and ground based lightning mapping arrays have been recently improving. Once the GLM becomes operational, it will be a clear leader in lightning detection because of its high detection efficiency, its ability to continuously image an extremely large region, and because it only requires a GLM instrument aboard GOES-EAST and GOES-WEST as opposed to the ground based networks which require many lightning sensors on the ground. Table 2 provides a side by side comparison of the GLM, OTD, LIS, and WWLLN lightning mapping systems.
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