Electric Field Measurements with an Aircraft Douglas Mach, UAH Monte Bateman, USRA Richard Blakeslee, Hugh Christian, & William Koshak, NASA/MSFC
How to Make Electric Field Measurements with an Aircraft • Three external field components (Ex, Ey, & Ez), plus aircraft charge (Eq) • Four unknowns requires four independent measurements • In practice, we need 6 or more (redundancy, equipment failures, etc.) • Need wide dynamic range and high sensitivity • Where to place instruments?
Placement Ideas Careful mill placement allows us to retrieve each component of the field • The mills should be distributed so that no two mills have similar sets and polarities of dominant field components • Both polarities of each component should be represented • Each mill should contribute to the measurement of the external fields • Mill placement examples?
Possible Locations for Mills on A-10 • Top • Pods • Side panels • Bottom • As far forward as possible • Now, how to calibrate…
Koshak Method Expose aircraft to “known” electric fields Use those fields to determine C matrix via a Lagrange Multiplier method Use C matrix to determine electric fields Can use external constraints to “drive” the solution Theoretically driven Does not require mills to be symmetrically placed One fewer bullet point Calibration E(t) = Cm(t) m(t) = ME(t) C is “inverse” of M E is the vector electric field, m is the vector mill output Mach Method • Expose aircraft to “known” electric fields • Use those fields to determine M matrix via a least-squares iterative process • Invert M matrix to determine appropriate C matrix • Use C matrix to determine electric fields • Inversion process can be used to eliminate “bad” mill outputs • Empirically driven • Does not require mills to be symmetrically placed
Koshak Method 1. Fly aircraft roll/pitch maneuvers in fair weather, and collect the mill outputs. 2. Specify an initial estimate of the vertical fair weather field profile (FWFP, typically the Gish profile). 3. Specify an rms error tolerance (in V/m) of how much the to-be-retrieved fair weather field is allowed to deviate from the above FWFP estimate.* 4. Solve the Lagrange Multiplier equation set to obtain the “C-matrix" 5. Fly aircraft in storm and obtain storm field as E(t) = Cm(t) , where m(t) = mill outputs as a function of time, and E(t) = (Ex(t), Ey(t), Ez(t)) as a function of time. * This step specifies the familiar “side-constraints” of the text-book Lagrange Multiplier method. • Some benefits: • No inversion of an M or K matrix is required; and no iterations. • No estimate of aircraft charge is required; no stinger required either. • Flexible way to add side-constraints to the retrieval process. • Does not require mills to be symmetrically placed.
NASA/MSFC-UAH Mill Design • Individual microprocessors that digitize the electric field signal at the mill • High sensitivity (1 V/m per bit on aircraft) • Wide dynamic range (115 dB) • Very low noise (1 LSB on aircraft) • Mounted on an aircraft, these mills can measure fields from 1 V/m to 500 kV/m (on aircraft) • Once per second commanding from the data collection computer to each mill allows for precise timing and synchronization • The mills can also be commanded to execute a self-calibration in flight, which is done periodically to monitor the status and health of each mill • Data packet easily fits in a 2400 baud data stream
NASA/MSFC-UAH Mill Design • Proven set of mills used in 8+ field programs and 5 aircraft • Full set of mills and spares are available now • Dedicated set could be built for A-10 • Using same design • Redesign possible • smaller • use alternative data I/O method • add more “intelligence” • Data collection system/design available (SAMPLE/ISC)