1.1

Lightning Phenomenology

A lightning discharge creates and excites atomic oxygen, which decays from its excited state by emitting photons at characteristic wavelengths. To detect a lightning flash, optical lightning mappers typically rely on a prominent oxygen triplet whose emission lines are near 777 nm. The transient optical signature of a lightning pulse diffuses through the surrounding cloud and illuminates a wide area of the cloud top, typically tens of km². The cloud medium is optically thick but absorbs very little at near-infrared wavelengths, so the resulting multiple scattering blurs the source geometry and delays and time-broadens the pulses. Observed on the cloud top, each lightning flash consists of a series of short (less than one millisecond) strokes separated by several milliseconds. An optical sensor positioned above the cloud top can thus sense the diffuse 777 nm glow from the individual optical pulses generated by the strokes without having a direct view of the lightning plasma channel itself.

2.1

Key Design Drivers

Lightning dissipates the very large quantities of energy in a convective storm cell. Only a small portion of this energy is converted to light that diffuses through the cloud and escapes to space, suffering 1/R² losses along the ~4e7 m path to geosynchronous orbit (GEO). As seen by GLM, the radiant energy of a typical lightning pulse is on the order of 10 μJ sr^-1m^-2. This pulse energy is what sizes the pupil area of the instrument, from which many other design parameters are derived.

Detection of lightning is complicated by the presence of bright sun light reflected from the cloud top. To distinguish the short, dim lightning signal at the top of the clouds from the relatively constant bright solar reflection, we apply temporal, spectral and spatial discrimination to extract the lightning signal. Achieving this discrimination, along with the wide field of view to cover the full disk, are the key design drivers for GLM.

2.2

Spatial Discrimination

The GLM CCD was designed such that the ground sample distance (GSD), i.e. the projected area of each pixel on the Earth’s surface, is approximately constant with a target value of 8 km at nadir, and a limit of < 14 km across the Continental United States, matching the typical size of a storm cell. When following the development of severe thunderstorms, it is important to track the lightning flash rate of individual storm cells, and therefore constant ground sample distance over the Earth is preferred.

Near the edge of the field of view, the CCD design (patented under U.S. Patent 8063968) uses reduced pixel pitch to compensate for the foreshortening as the view shifts away from nadir. This ensures that the cloud background signal (and its associated photon noise) is minimized while the lightning signal is maximized, thus maintaining a good SNR near the Earth’s limb.

Figure 5:

Measured ground sample distance (km) for GLM Flight Model 2 (on GOES-17). Pixel pitch boundaries are visible as vertical and horizontal discontinuities, at each of which the GSD steps down to compensate foreshortening of the pixel footprint. Black coast lines show a simulated view from 75°W longitude.

2.3

Temporal Discrimination

GLM detects the individual optical pulses caused by lightning, against a bright background of sunlit clouds. To detect these pulses with sufficient signal to noise, the frame rate is selected to be of the same order as the average duration of a typical pulse. If the frame rate is too low, then additional background is sensed, increasing photon noise without additional signal, thus lowering SNR. If the frame rate is too high, the signal can split into successive frames, again reducing SNR.

Considering only the phenomenology of lightning, the “sweet spot” is around 1500 fps. However, frame rate is strongly constrained by the performance of the analog readout chain and the data bandwidth of the digital processing. As a result, the frame rate selected for GLM was 503 Hz, resulting in a frame period ~4x longer than a typical lightning optical pulse.

2.4

Spectral Discrimination

While anyone can build a lightning detector that works well at night, the true test of a lightning mapper is how well it detects dim lightning events emanating from a bright, zenith-illuminated cloud top. Clouds are nearly Lambertian reflectors with an albedo that sometimes approaches unity, so a large amount of undesired reflected sun light is present in the spectral region of the oxygen triplet.

This background cloud radiance creates photon noise that can drown out dimmer lightning events. The background signal is reduced as much as possible by using optical filters that have the narrowest feasible band pass while still passing most of the lightning oxygen triplet. GLM contains three filters of increasingly narrow spectral width: a Solar Rejection Filter (SRF) at ~75 nm Full Width Half-Maximum (FWHM) that performs the task of rejecting the bulk of out-of-band radiation, a solar blocking filter (SBF) at ~3 nm FWHM, and the key narrow band filter (NBF) at ~1 nm FWHM. These filters are discussed in more detail below.

3.1

Overall Optical Arrangement

The GLM optical assembly consists of seven powered lens elements subdivided into a front housing containing a two-element beam expander and a rear housing containing the five-element imaging optics, including one aspheric surface. The front and rear housing, both made of aluminum, can be separated to access the unpowered narrow band filter, which lies at the heart of the telescope and functions as the aperture stop. The central location of this filter also facilitates rigorous thermal control of this key component. Two additional unpowered filter elements are located at the front of the system. GLM has a total of 10 glass elements and 21 optical surfaces, including the focal plane. All elements are bonded in place with RTV566 and mechanically retained. Lens element 5 is a compensating element made of F2G12 glass (all other elements are fused silica). Its axial location is set to compensate for as-built variations and optimize image height and encircled energy performance of each assembly over temperature. The lens assembly weighs approximately 16 kg, and was fabricated and tested by II-VI Optical Systems (formerly LightWorks Optics) in Tustin, California.

Figure 6:

GLM telescope layout: SRF, SBF, 2-element beam expander, NBF, and 5-element imaging optics. There are no moving parts or mechanisms in the design.

3.2

Spectral Filters

GLM is a highly monochromatic design with a system band pass of 1.1 nm. The band pass must strike a delicate balance between passing enough of the oxygen triplet signal while blocking as much solar background radiation (and associated photon noise) as possible. The spectral filtering elements are therefore critical to achieving the desired SNR performance of the entire system. All spectral filtering elements of the telescope are unpowered, with interference coatings deposited on one or both surfaces of a fused silica substrate. Top manufacturing challenges included coating uniformity, center wavelength placement, and maintaining surface quality and cleanliness. Each filter has a similar mechanical arrangement, using an RTV566 bond to a flexure-mounted titanium ring mount. Each filter mount has a circumferential redundant patch heater to control its temperature.

The GLM instrument has three filters with progressively narrower band pass:

A key requirement for the telescope configuration was to limit the angle of incidence impinging on the narrow band filter to 5° or less. Beyond 5°, the center wavelength of the narrow band filter shifts too far away from the oxygen triplet and begins to cut off the lightning signal. Because the telescope was designed to cover the full Earth disk, a beam expander with expansion factor of 8/5 was required to reduce the angle of incidence at the narrow band filter to an acceptable range.

As a beneficial result of this arrangement, the differential spectral shifts of the SBF and NBF (varying at a 5:8 ratio with field angle) cut each other off beyond an angle of ~12°, which serves as a “virtual baffle” to reject off-axis stray light. While the virtual baffle is useful, it does not remove the need for a physical baffle to reduce access for out-of-band leakage at very high incidence angles, sunlight, and other sources of glint.

3.3

Focal Length Athermalization

The lens assembly image plane shifts with temperature due to thermal expansion of the aluminum housings and lens elements, and small thermal shifts in the index of refraction. The resulting image plane motion is about 6 μm/°C, a shift that is almost compensated by housing expansion. The focal plane is positioned by a composite metering tube with a layup designed for near zero CTE, ensuring that the image plane remains within 12.5 μm of the focal plane over all temperature environments. In practice, lens assembly temperature ranges by < 10°C, thanks to the strong thermal isolation between the telescope and the rest of the sensor unit.

3.4

CCD Focal Plane

The focal plane is an STA3900A backside-thinned 1372 x 2624-pixel split frame transfer CCD with an image area of 1372 x 1300 pixels and an image diameter of ~36 mm. The device, with the previously discussed variable pixel pitch, was designed by Semiconductor Technology Associates of San Clemente, California. The progressively clocked frame transfer architecture enables shutterless operation at 503 fps, with 56 parallel analog outputs operating at 2e7 pixels/s. The CCD has been described in a conference paper⁴.

The well depth of the CCD is matched to the frame rate and spectral bandwidth of the optical filters. For the focal plane designer, an unfortunate feature of lightning is that it most often occurs in optically thick and very reflective clouds, in the afternoon when the clouds are well illuminated by the Sun. The CCD well depth must accommodate the background spectral radiance of bright clouds (~35 mW sr^-1cm^-2μm^-1 at 777 nm) integrated over the pupil area, over the frame period, and over the spectral band pass, before the lightning signal is added. While most lightning pulses as sensed by GLM produce only 5 to 10 thousand photoelectrons, the CCD requires a well depth of ~2 million electrons to swallow the cloud background signal while leaving sufficient head room to measure the radiance of the brightest lightning events. Under noon time cloud illumination, the photon noise from the background is of the same order as the read noise of the camera.

The focal plane is sub-divided into 56 physical regions, each 49 pixels tall by 650 pixels wide, known as subarrays. Each subarray is read out in parallel and is associated with an independent signal chain consisting of amplifier, ADC, and RTEP event detection logic.

3.5

Stray Light Control

The solar disk has ~5e4 times greater spectral radiance than the brightest background clouds that GLM ever sees. Keeping sunlight out of the image is the primary concern of the stray light mitigation measures in the GLM optics, since the viewing geometry from GEO makes it unavoidable that the sun will intrude near the edge of the field of view. The first line of defense against stray light is the baffle assembly, a 50 cm long cone seen in Figure 3 with 11 internal vanes achieving a solar exclusion angle of 24.5°. Inside of this angle, some sunlight begins to enter the optics. The second line of defense is the virtual baffle formed by the cutoff of the spectral filters, effective up to ~12°. The third line of defense consists of numerous internal features (such as steps and vanes) and black coatings to absorb light that (by design) enters the lens barrel. At a field angle of 8.7°, which corresponds to the Earth limb, sunlight reaches all the way back to the rear end of the lens assembly where a knife edge masks off the Earth limb before light can continue to the focal plane.

This layered approach to stray light rejection reduces the stray light by several orders of magnitude and allows GLM to continue operating through eclipse sunrise / sunset events. The system requirements allow for two 5° radius Zones of Reduced Data Quality (ZRDQ) centered on the sun and on its ghost image symmetrically opposite the optical axis, where the instrument is temporarily not required to meet performance requirements.

So far, the discussion of stray light has centered on undesired ray paths that bounce from some feature of the optics onto the focal plane, for which the traditional methods of stray light control are effective. Unfortunately, the viewing geometry of GEO features a second source of unwanted light where the ray bounce occurs on the Earth, which can be an excellent specular reflector. The resulting glint is an unavoidable feature of full-disk imaging from GEO and may occur anywhere in the image area. Excessive glint causes blooming of the CCD (despite anti-blooming features) resulting in undesired image artifacts. For an event detector like GLM, the moving edges of a bloomed area are the source of very high rates of false events that must be accommodated in the downlink and removed in ground processing.

The last line of defense for rejection of stray light is the ground processing algorithm that removes blooming events. This algorithm is based on a state machine formulation where each pixel in the image may have one of four states: not bloomed, flooding, bloomed, or ebbing. Flooding is characterized by a rapid increase in the background radiance to a level that is unusually high; bloomed state occurs when the pixel is saturated, therefore causing successive samples to be equal, a condition that results in zero event detections; and ebbing is characterized by a rapid decrease of the background radiance back to a normal level. The transitions between states are derived entirely from event data and may occur on timescales from a few seconds (e.g. for a brief river glint) to over an hour (e.g. sunrise glare from a calm cloud-free ocean). The blooming filter was designed and implemented after GLM was deployed on orbit, as the detailed event signature of solar glare would have been extremely difficult to simulate or test on the ground.

4.1

Focus and Alignment

When the focal plane assembly is mated to the optics, it must be aligned and focused to produce good imagery. The GLM instrument does not include any focus mechanisms; therefore, the correct focus must be achieved by shimming during assembly of the instrument. Three shims placed around the circumference of the aft end of the metering tube set the focus piston, tip, and tilt. Centration is achieved using three push/pull screws. Once the alignment and focus are set, the shims are staked, and the push/pull screws are removed.

The fast optics of the GLM lens assembly mean that the detector sits approximately 6 mm behind the last optic, and the allowable focus error is ±12.5 μm across the entire 36 mm image diameter. This makes the focus difficult to set properly; the procedure must account for air-to-vacuum shift as well as for non-flight-like temperatures and temperature gradients that can displace the image plane.

We measure the focus of the GLM instrument by filling the pupil with a steerable collimated infrared laser beam; the laser is tuned to pass through the filters. We then adjust the input beam from slightly converging, through collimation, to slightly diverging, while monitoring the spot size on the CCD at 16 points around the field of view. We analyze the data to generate a shim prescription to best fit the position error between the focal plane and the telescope image plane, neither of which are planar on such small scales. Each of the three shims is hand lapped and precision cleaned. We then iterate the process until the shim prescription converges. Once the focus is set and the instrument fully assembled, the focus is verified during thermal vacuum testing to ensure that the instrument remains in focus under flight-like conditions over the temperature environments expected on orbit.

4.2

Static Calibration

The static response of GLM is measured using a calibrated, NIST-traceable integrating sphere with a diameter of 50 cm and an aperture diameter of 20 cm. The integrating sphere is used to fully illuminate the GLM aperture and provides a flat field at illumination levels from 0 to 30 mW sr^-1cm^-2μm^-1 per the calibrated monitoring photodiode. We collect background images at 33 illumination levels, which provides an absolute static response of each pixel. Due to out-of-band leakage at very high angles of incidence, the large integrating sphere must be placed at 150±10 mm from the first optic. This distance places the sphere at the furthest distance from the instrument while still filling the pupil at all angles in the FOV. With the sphere at the largest distance possible from the instrument, we minimize the impact of out of band light on the measurement.

Figure 7:

views of the Static calibration setup (left) and Transient calibration setup (right)

The GLM data product consists of calibrated, geolocated lightning events. To calibrate transient events, we generated a look up table that consists of the lightning signal gain (J/DN) for each pixel at each of 32 levels of background illumination. The calibration factor already accounts for optics transmission, filter cut-off, pupil area and pixel solid angle; it represents the time-integrated energy of the pulse just before it enters the optics. The gain is calculated using the slope of the response curve measured for each pixel during static calibration. Because the response curves are not perfectly linear, higher radiometric accuracy is achieved by determining the gain against a 32-segment piece-wise linear curve. In this way, the static calibration data set (acquired using the simplest of methods using the minimal amount of processing) is the only data used to calibrate the GLM data product. The next section describes the transient calibration, which checks the calibration for pulses imparted to a small number of pixels across the field of view.

4.3

Transient Calibration

The goal of transient calibration is to “test like you fly” by measuring the performance of GLM in detecting lightning-like optical pulses. In practice, it is extremely difficult to recreate a controlled, high-fidelity lightning signal in the lab, with the same spatial, spectral and temporal characteristics as the real phenomenology. The transient calibration uses pulsed infrared LED light to simulate lightning events to the extent possible with standard calibrated lab equipment. A significant portion of the final lightning event calibration is calculated analytically using the static and transient calibration data along with sub-assembly level data, most importantly filter bandpass measurements.

Transient calibration verifies event detection capability of the GLM instrument and provides event intensity calibration data to compare to the values derived from the static calibration data. The transient calibration is accomplished using small integrating sphere to simulate cloud background illumination, and a pulsing LED to inject calibrated events, both of which are mounted on a gimbal system to point the calibrated optical pulses to the required locations across the field of view, ensuring that all 56 readout channels are exercised. The LED is pulsed with constant amplitude and a pulse width that is varied over two orders of magnitude. Pulse width ranges from ~10 μs to ~1.5 ms and is synchronized with the GLM frame time to prevent frame splitting of the pulse energy.

During the test, three parameters are varied: the cloud background radiance simulated by the small sphere, the position of the pulser, and the width of the pulses. For each small sphere illumination setting, the pulser is steered to a variety of positions on the CCD, covering at least one position in each of the 56 subarrays; at each position, the width of the pulse is varied from sub-threshold to saturation. During event testing, the energy input is known from the measured amplitude and width of the LED pulse.

During static calibration, we evaluate the radiometric response of every pixel in the array. We calibrate the transient response of a selected subset of pixels (several hundred) within each subarray and evaluate the consistency of their transient response to their static radiometric response.