Altus Cloud Electrification Study (ACES) Dataset Document

Introduction

The Altus Cumulus Electrification Study (ACES) was an uninhabited aerial vehicle (UAV) based project that investigated thunderstorms in the vicinity of the Florida Everglades in August 2002. ACES was conducted to investigate storm electrical activity and its relationship to storm morphology, and to validate satellite-based lightning measurements. In addition, as part of the NASA sponsored UAV-based science demonstration program, this project provided a scientifically useful demonstration of the utility and promise of UAV platforms for Earth science and applications observations. ACES employed the Altus II aircraft, built by General Atomics - Aeronautical Systems, Inc. Key science objectives simultaneously addressed by ACES were to:

(1) investigate lightning-storm relationships,
(2) study storm electrical budgets, and
(3) provide Lightning Imaging Sensor validation.

The ACES payload included electrical, magnetic, and optical sensors to remotely characterize the lightning activity and the electrical environment within and around thunderstorms. ACES contributed important electrical and optical measurements not available from other sources. Also, the high altitude vantage point of the UAV observing platform (up to 55,000 feet) provided "cloud-top" perspective. By taking advantage of its slow flight speed (70 to 100 knots), long endurance, and high altitude flight, the Altus was flown near, and when possible, over (but never into) thunderstorms for long periods of time that allowed investigations to be conducted over entire storm life cycles. An innovative real time weather system was used to identify and vector the aircraft to selected thunderstorms and safely fly around these storms, while, at the same time monitor the weather near our base of operations. In addition, concurrent ground-based observations that included radar (Miami and Key West WSR88D, NASA NPOL), satellite imagery, and lightning (NALDN and Los Alamos EDOT) enable the UAV measurements to be more completely interpreted and evaluated in the context of the thunderstorm structure, evolution, and environment. This marked the first time a UAV was used to conduct lightning research. All objectives were successfully met, opening the way for continued use of UAVs in pursuit of NASA's scientific endeavors.

Instrument Description

"Slow" and "Fast" antennas

This system acquires lightning waveforms and provides a measure of total lightning. The system consists of flat plate antennas and a broadband charge amplifier with a selectable time constant (by selecting the proper time constant, the decay rate of the signal can be switched from a long time period for the slow antenna, to a short time period for the fast antenna), a filter for removing undesired signals (e.g., radio frequency interference), a transient waveform recorder, and a data acquisition system. The primary use of the slow antenna is to measure the electric field transients associated with lightning events (i.e., return strokes, leaders, k-changes, etc.).

Electric Field Mill

One of the most important measurements of thunderstorm development and severity is the “static” vector electric field produced by the storm. For this investigation we installed five state-of-the-art, low-noise, high-dynamic range electric field mills (EFMs) on the ALTUS. With these sensors, the full vector components of the atmospheric electric field (i.e., Ex, Ey, Ez) was directly obtained, providing detailed information about the electrical structure within and around the storms overflown. The dynamic range of these instruments extends from the fair weather fields (a few tens of V/m) to large thunderstorm fields (thousands of V/m). Using these field mills, it was possible to detect both intracloud and cloud-to-ground lightning from the abrupt electric field changes in the data.Total lightning (i.e., CG and IC) was identified from the abrupt changes in the electric field data. Additionally, it was often possible to differentiate between the IC and CG discharges. The field mills also provided a measure of the electric charge (Q) on the aircraft. The EFMs incorporated self-calibration capabilities that reduced the time required to obtain a full aircraft calibration. In addition, with these mills the electric field signals are digitized at each mill and transmitted in a digital data stream, reducing signal noise and simplifying aircraft integration. The EFMs have relatively slow time response (~10 Hz) so they do not provide details associated with the fast transient electric field changes due to lightning. Storm electric currents can be derived using the electric field and the air conductivity measurements provided by the Gerdien conductivity probe.

Dual optical pulse sensor (DOPS)

The DOPS system consists of high time resolved optical pulse sensors with a very wide field of view, an all sky detector, a video camera with VCR tape storage, and associated data acquisition systems for the optical pulse sensors. Collecting optical energy and power statistics from the cloud-top emissions from lightning represented another important measurement priority in the ACES investigation. Dual multiple channel, calibrated, OPS, were used to determine the intensity, duration, and waveform characteristics of the different types of lightning discharges from thunderstorms. The DOPS were configured to detect lightning events in the visible and near-infrared portion of the spectrum. Each channel consists of a photodiode at the focus of a wide angle (60°) field-of-view lens. A narrow-band interference filter was placed in the front of the lens to restrict the measurement to a strong emission feature in the lightning spectrum. The DOPS had no problem detecting transit lightning events during daytime conditions. The DOPS were designed such that rapidly varying signals due to lightning were passed while slowly varying signals, such as sunlight reflecting off of the clouds, were strongly attenuated.

Search Coils

The magnetic field antennas for this mission were three orthogonal search coil magnetometers, each consisting of many turns of fine wire wound about a high-permeability core, along with preamplifier circuitry. This instrument was specifically designed to measure AC magnetic fields in the frequency range of 100 Hz–20 kHz. Such search coils have been successfully deployed by GSFC/LEP on rocket (Norwegian PULSAUR and Sporadic-E Layer) and UAV (Naval Research Laboratory’s Swallow) platforms in addition to use in ground-based campaigns (SPRITE 96).
The search coil measures dB/dt, or temporal changes in magnetic flux density, which couples to the instrument in two distinct ways. The intended mechanism is transformer coupling, whereby AC field lines from a distant source couple to the windings of the sensor. The secondary mechanism is generator coupling, whereby the sensor is physically displaced within a DC magnetic field (e.g. the geomagnetic field). This mode will produce interference on a small aerial vehicle as it operates in turbulent air. In order to discern transformer/generator emission types, a vector accelerometer was mounted in close proximity to the search coil. As such, physical displacement data was recorded and used as a correlation parameter, along with DC magnetic field data, during data analysis. In addition, this technique provided a cross-calibration mechanism for AC and DC magnetic field sensors.

Magnetometer

The three-axis magnetometer is a highly-sensitive, high-resolution, three-axis fluxgate magnetometer designed for deployment on aircraft for in situ measurements of magnetic fields due to locally driven currents and other perturbations. The magnetometer not only senses the steady component of the Earth’s field, but also measures very low frequency magnetic fields. The instrument measured changes in the ambient magnetic field during thunderstorm activities ranging from 0–100 Hz. Three analog output voltages were interpreted to determine the vector magnetic field at magnitudes in the range of one milligauss to one gauss. The ALTUS utilized a miniature three-axis fluxgate magnetometer, model APS533 by Applied Physics Systems.

Accelerometer

An accelerometer is a device measuring changes in velocity in x,y , and z directions (with respect to the aircraft). The accelerometer in the ALTUS has the capabality of measuring +/-4G acceleration.

Gerdien Conductivity Probe

The atmospheric conductivity measurements were made by a Gerdien condenser probe. This is a device designed to determine the electrical conductivity of the atmosphere. By using a capacitor exposed to a sample of air and measuring the time it takes for discharge, the electrical conductivity of the sample is determined. The Gerdien condenser probe has the ability to sense both positive and negative ion conductivities. The probe utilizes a concentric, cylindrical electrode geometry with the inner and outer electrodes serving as the collector/guard and return electrodes, respectively. The ion conductivity is determined by applying a voltage (V) that is swept linearly between the two electrodes over a 1-minute period, and measuring the resulting current (I). The slope of the I–V characteristic (dI/dV) provides the conductivity measurement. Special construction techniques were used at the collector input to reduce stray leakage currents and susceptibility to electromagnetic interference (EMI). The length of the collector is 6.4 cm. The inner electrode extends another 10 cm as a guard section that is used to mechanically support the center electrode and the inner electrode is recessed 3.8 cm from the leading edge of the outer cylinder. The whole assembly has a length of 20 cm and a mass of 0.286 kg. It was mounted via a 7.6 cm sidestrut to the underside of the aircraft. A separate electronics box contained the sweep and data amplifier electronics.

Data Products

Data from the above instruments were collected and stored via the Flight Payload Data System (FPDS).The FPDS was designed for the ingestion, digitization, and archival of the sensor and payload data. In addition, the FPDS provided a variety of ground control (via command uplink) command capabilities controlling power (on/off and redundant source) and trigger functions. The FPDS was able to transmit small amounts of data to the ground during flights which allowed the user to monitor select sensor output, as well as the health of all the instrumentation in real time. The transmitter/receiver system between the UAV and ground utilized a 9,600-baud (i.e., low bandwidth) connection; therefore, the amount of data transmitted during a mission was limited. The FPDS had an external ethernet port to effect fast download of the data after the plane landed. A medium-speed digitizer/frame grabber was used to record the fast transient response from the sensors (i.e., slow & fast antennas, search coil, DOPS). The data from this frame grabber was continuously stored in a buffer until a trigger signal determined that an event should be stored. The data were time stamped with Global Positioning System (GPS) timing and stored on a hard disk, and the system then reset to collect the next event. There was also a slow-speed digitizer that continuously downloaded data to the hard drive (i.e., field mills, conductivity probe, magnetometer). The slow-speed digitizer did not depend on a trigger event to occur, and therefore recorded data for the duration of the UAV flight. The FPDS also continuously ingested the digitized output of the electric field mills. The GPS was used to time-tag the data with Universally Coordinated Time (UTC). This time was also used to name the corresponding data files. During each mission, measurements from the onboard electric field mills were downloaded and monitored in real time to avoid areas of high electric field that might induce a lightning strike to the ALTUS. Table 1.1 below list the data channel assignments. Sampled data were gathered at two rates, slow speed (64 bit encoding) and medium speed (16 bit encoding). The medium speed data (first 16 channels) were the ones that could be transmitted to the ground.

Table 1.1
ACES TELEMETRY ITEMS

Data Format

Aces1 Triggered Data

The system digitally recorded 16 simultaneous channels at a 200 kHz rate. The channel-to-channel jitter and delay were both found to be less than one sample point. Triggering of the system was user selectable and was usually set to trigger on one of the electrical channels (while further away from the storm) or to one of the optical channels (when close to the storm). Pre-trigger settings enabled the collection of some data (user selectable) prior to the trigger time. The first 16 lines of the above chart (Table 1.1) show the data that could be sent to the ground when something ‘ intresting ’ was observed. This allowed a higher rate of data return versus the 64 bit data.
The triggered data files were combined into daily files, using the unix "tar" command. They are distributed in the following form:

aces1trig_yyyy.ddd_v2.50.tar (where yyyy.ddd= year and 'day of year', and v2.50 is the version number)

The files inside these daily files are in BINARY format, of the form:

ACES_TRIG_V2.50_yyyy-dddThh-mm-ss.dat (where yyyy=year, ddd=day of year, hh=hour, mm=minutes, ss=seconds)  

Aces1 Continuous Data

This dataset is the output of the slow-speed digitizer that was continuously downloading data to an on-board hard drive.The entire 64 bit data stream as shown in Table 1.1 above is consolidated into this dataset. The data files are in BINARY format, and were combined into daily files, using the unix "tar" command. They are distributed in the following form:

aces1cont_yyyy.ddd_v2.50.tar (where yyyy.ddd= year and 'day of year', and v2.50 is the version number.

The files inside these daily files are of the form:

ACES_CONT_V2.50_yyyy-dddThh-mm-ss.dat (where yyyy=year, ddd=day of year, hh=hour, mm=minutes, ss=seconds)

Aces1 Electric Field Mill Data

Electric field mills were used to measure the vertical component of the electric field as the aircraft flew in the vicinity of electrified clouds.
The data files were combined into daily files, using the unix "tar" command. They are distributed in the following form:

aces1efm_yyyy.ddd_v2.50.tar (where yyyy.ddd= year and 'day of year', and v2.50 is the version number.

The files inside these daily files are in BINARY format, of the form:

ACES_FM_V2.50_yyyy-dddThh-mm-ss.dat (where yyyy=year, ddd=day of year, hh=hour, mm=minutes, ss=seconds)

Aces1 Timing Data

This dataset consist of timing data, which can be useful in analyzation of other ACES datasets. The data files are in BINARY format, and were combined into daily files, using the unix "tar" command. They are distributed in the following form:

aces1time_yyyy.ddd_v2.50.tar (where yyyy.ddd= year and 'day of year', and v2.50 is the version number)

The files inside these daily files are of the form:

ACES_TIME_V2.50_yyyy-dddThh-mm-ss.dat (where yyyy=year, ddd=day of year, hh=hour, mm=minutes, ss=seconds)

Aces1 Flight Data Logs

This dataset consist of initalization log files from the Flight Payload Data System (FPDS). These log files are in ASCII format, and were combined into daily files, using the unix "tar" command. They are distributed in the following form:

aces1log_yyyy.ddd_v2.50.tar (where yyyy.ddd= year and 'day of year', and v2.50 is the version number)

The files inside these daily files are of the form:

ACES_LOG_V2.50_yyyy-dddThh-mm-ss.dat (where yyyy=year, ddd=day of year, hh=hour, mm=minutes, ss=seconds)

Data Processing

These data were processed using MatLab 7, which is a high-level technical computing language and interactive environment for algorithm development, data visualization, data analysis, and numerical computation. Learn more about MatLab at http://www.mathworks.com . The file, ACES_TOOLKIT.tar, is a bundle of sample MatLab programs that you may find useful when analyzing these data.

References

Chauzy, S., J.-C. Medale, S. Prieur, and S. Soula, Multilevel measurement of the electric field underneath a thundercloud: 1. A new system and the associated data processing, J. Geophys. Res., 66, 22319-22326, 1991
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Jacobson, E. A. and E. P. Krider, Electrostatic field changes produced by Florida lightning, J. Atmos. Sci., 33, 112-117, 1976.

Krider, E. P., Spatial distribution of lightning strikes to ground during small thunderstorms in Florida, Proc. Int. Aerospace and Ground Conf. On Lightning and Static Electricity, April 19-22, 1988.

Laroche, P., A. Delannoy, and H. Le Court de Beru, Electrostatic field conditions on an aircraft stricken by lightning, Int. Conf. On Lightning and Static Electricity, University of Bath, UK, 1989.

Mach, D. M., Shuttle lightning threat analysis, Preprints, Third Int. Conference Aviation Weather System, Jan 30-Feb 3, 1989 .

Mazur, V., Triggered lightning strikes to aircraft and natural intracloud discharges, J. Geophys. Res., 94, 3311-3325, 1989a.

Mazur, V., A physical model of lightning initiation on aircraft in thunderstorms, J. Geophys. Res., 94, 3326-3340, 1989b.

NASA Facts, Lightning and the space program, FS-1998-08-16-KSC, NASA John F. Kennedy Space Center, Florida, 11 pp, August, 1998.

Peckham, D. W., M. A. Uman, and C. W. Wilcox, Jr., Lightning phenomenology in the Tampa Bay area, J. Geophys. Res., 89, 11789-11805, 1984.

Pierce, E. T., Triggered lightning and some unsuspected lightning hazards, 138th Annual Meeting of the AAAS, Philadelphia, 1971.

Soula, S. and S. Chauzy, Multilevel measurement of the electric field underneath a thundercloud: 2. Dynamic evolution of ground space charge layer, J. Geophys. Res., 66, 22327-22366, 1991.

Willett, J. C., D. A. Davis, P. Laroche, An experimental study of positive leaders initiating rocket-triggered lightning, Atmospheric Research, 51, 189-219, 1999.

Blakeslee, R. J., C. L. Croskey, M.D. Desch, W. M. Farrell, R. A. Goldberg, J. G. Houser, H. S. Kim, D. MMach, J. D. Mitchell, and J. C. Stoneburner, The ALTUS Cumulus Electrification Study (ACES): AUAV-based science demonstration, Preprints, 12thInt. Conf. on Atmos. Electr., June 2003, this issue.

Christian, H. J. and S. J. Goodman, Optical observations of lightning from a high altitude airplane, J.Atmos. Ocean. Tech., 4, 701-711, 1987.

Christian, H. J., Blakeslee, R. J., and Goodman, S. J., Lightning Imaging Sensor (LIS) for the EarthObserving System, NASA TM-4350, Available from Center for Aerospace Information, P.O. Box 8757,Baltimore Washington International Airport, Baltimore, MD 21240, 44 pp., 1992.

Christian, H. J., R. J. Blakeslee, D. J. Boccippio, W. L. Boeck, D. E. Buechler, K. T. Driscoll, S. J.Goodman, J. M. Hall, W. J. Koshak, D. M. Mach, and M. F. Stewart, Global frequency and distribution of lightning as observed from space by the Optical Transient Detector, J. Geophys. Res., 108 (D1),4005, 10.1029/2002JD002347, 2003.

Farrell, W. M., R. A. Goldberg, M.D. Desch, J. G. Houser, J. D. Mitchell, C. L. Croskey, R. J. Blakeslee,and D. M Mach, ACES: A unique platform for electrodynamic studies of upward currents into themiddle atmosphere, Preprints, 12thInt. Conf. on Atmos. Electr., June 2003, this issue.

Goodman, S. J., H. J. Christian, and W. D. Rust, Optical pulse characteristics of intracloud and cloud-to-ground lightning observed from above clouds, J. Appl. Meteor., 27, 1369-1381, 1988.

Koshak, W. J., M. F. Stewart, H. J. Christian, J. W. Bergstrom, J. M. Hall, and R. J. Solakiewicz, Laboratory calibration of the Optical Transient Detector (OTD) and the Lightning Imaging Sensor(LIS), J. Atmos. Ocean. Tech., 17, 905-915, 2000.

Thomason, L. W. and E. P. Krider, The effects of clouds on the light produced by lightning, J. Atmos. Sci.,39, 2051-2065, 1982

Contact Information

The data producer is:

Dr. Richard Blakeslee
NSSTC/NASA
320 Sparkman Dr.
Huntsville, AL  35805

To order this data or for further information, please contact:

Global Hydrology Resource Center
User Services
320 Sparkman Drive
Huntsville, AL 35805
Phone: 256-961-7932
E-mail: support-ghrc@earthdata.nasa.gov
Web: http://ghrc.nsstc.nasa.gov/