Thomas Dautermann

Global Positioning System detection and energy estimation of the ionospheric wave caused by the 13 July 2003 explosion of the Soufrière Hills Volcano, Montserrat

[1] Volcanic explosions or shallow earthquakes are known to trigger acoustic and gravity waves that propagate in the atmosphere at infrasonic speeds. At ionospheric heights, coupling between neutral particles and free electrons induces variations of electron density detectable with dual-frequency Global Positioning System (GPS) measurements. Using GPS data collected in the Caribbean, we identified an ionospheric perturbation after a major volcanic explosion at the Soufriere Hills Volcano (Montserrat, Lesser Antilles) on 13 July 2003. Spectral analysis reveals peaks centered at 1 and 4 mHz, similar to those in previous observations and consistent with theory, suggesting both gravity and acoustic wave components. We retrieve a horizontal velocity of ∼624 m/s for the acoustic component, which implies upward propagation at ∼33°, consistent with ray-tracing results. We model the acoustic wave using an N-wave pressure source at ground level combined with ray tracing to propagate the neutral pressure wave; this accounts for the dispersive characteristics of the atmosphere while conserving total acoustic energy. Plasma velocity is derived from neutral velocity using a finite difference solution of the magnetohydrodynamic momentum equation. The continuity equation for charge densities is used to compute corresponding electron density variations, which are then numerically integrated along satellite-to-receiver line of sights, simultaneously accounting for the satellite displacements. We minimize the misfit between observed and model waveforms to estimate a total acoustic energy release of 1.53 x 10 10 J for the primary explosion event at Soufriere Hills Volcano associated with the peak dome collapse. This method can be applied to any explosion of sufficient magnitude, provided GPS data are available at near to medium range from the source.

Approach service type D evaluation of the DLR GBAS testbed

Ground-based augmentation systems (GBAS) for satellite navigation are intended to replace the instrument landing system for precision approach of aircraft into an airport in the near future. Here, we show an evaluation of data collected during flight trials with the GBAS testbed of the German aerospace center with respect to requirements for the GBAS approach service type D. This service will permit approach and landing down to the zero visibility conditions of category IIIc approaches. We show output of all airborne monitors and the results of an integrity analysis. During all flight trials, the system performed within the required criteria for integrity, continuity, and availability.

The spread F Experiment (SpreadFEx): Program overview and first results

We performed an extensive experimental campaign (the spread F Experiment, or SpreadFEx) from September to November 2005 to attempt to define the role of neutral atmosphere dynamics, specifically wave motions propagating upward from the lower atmosphere, in seeding equatorial spread F and plasma bubbles extending to higher altitudes. Campaign measurements focused on the Brazilian sector and included ground-based optical, radar, digisonde, and GPS measurements at a number of fixed and temporary sites. Related data on convection and plasma bubble structures were also collected by GOES 12 and the GUVI instrument aboard the TIMED satellite. Initial results of our analyses of SpreadFEx and related data indicate 1) extensive gravity wave (GW) activity apparently linked to deep convection predominantly to the west of our measurement sites, 2) the presence of small-scale GWactivity confined to lower altitudes, 3) larger-scaleGWactivity apparently penetrating to much higher altitudes suggested by electron density and TEC fluctuations in the E and F regions, 4) substantial GW amplitudes implied by digisonde electron densities, and 5) apparent direct links of these perturbations in the lower F region to spread F and plasma bubbles extending to much higher altitudes. Related efforts with correlative data are defining 6) the occurrence and locations of deep convection, 7) the spatial and temporal evolutions of plasma bubbles, the 8) 2D (height-resolved) structures of plasma bubbles, and 9) the expected propagation of GWs and tides from the lower atmosphere into the thermosphere and ionosphere.

Non-Gaussian Error Modeling for GBAS Integrity Assessment

Four basic error sources exist for residual pseudo-range errors in a single frequency differential GPS system for ground based augmentation (GBAS): signal multipath, increased receiver noise (carrier-to-noise density ratios (C/N0)) due to interference, residual differential troposphere error, and the error induced by ionosphere gradients. Without restricting ourselves to classical Gaussian overbounding, we combine their probability density functions (pdfs) to a total pseudo-range error distribution. This distribution is propagated through the GBAS Hatch filter and then mapped into the position domain using a worst case (selected by maximum vertical dilution of precision (VDOP)) of a full 31 satellite constellation with the two most critical satellites failed observed at Braunschweig Airport, Germany. Our calculations yield a significant reduction amounting to 46% of the position domain error at the 1.5 × 10-7 integrity risk level when compared with the classical Gaussian overbounding approach.

GBAS landing system – precision approach guidance after ILS

Purpose – The purpose of this paper is to show the performance during flight tests of the proposed GBAS Approach Service Type D navigation – intended to support autoland operations – in comparison to ILS.Design/methodology/approach – An experimental GBAS station was installed at the research airport in Braunschweig. Data processing complied with the currently proposed requirements to support automatic landings. Corrections for GPS measurements and integrity parameters were sent to a research aircraft which was equipped with an experimental GPS receiver providing raw measurement data. The received data and measurements were then processed on board in real‐time and provide approach guidance information to the experimental pilot in form of a flight director indication. To evaluate system performance the authors create a truth reference track from a post processed carrier phase solution. Finally, the GBAS outputs and the received ILS signals are compared to the truth reference.Findings – The system performed ...

Ionospheric threat simulation for GNSS using the Spirent hardware signal simulator

Abstract Ionospheric disturbances present a considerable hazard to single-frequency satellite navigation systems for airborne users. We discuss our implementation of three ionospheric threat models in the DLR “multi-output advanced signal test environment for receivers” global navigation satellite system simulator, which is based on Spirent GSS 7780/7790 signal generator. These threat models include the standard front-based threat model developed for the integrity assessment of ground-based augmentation systems (GBAS), a simplified plasma bubble model, and ionospheric scintillation, which can be combined with either of the two previously mentioned models. These effects can now straightforwardly be simulated at the German Aerospace Center’s research facilities. As an example, we simulate a GBAS ground facility with code–carrier divergence monitoring, affected by an ionospheric front, and we show the results of a simulation with coincidental occurrence of a plasma bubble and scintillation with an S4 index of 0.4.

GBAS ionospheric threat analysis using DLRs hardware signal simulator

Ionospheric fronts pose a significant threat to single frequency ground based augmentation systems (GBAS) for airplane precision approach. Here, we show our implementation of two ionospheric threat models in the DLR MASTER GNSS Simulator, a modified Spirent GSS7780/779. These threat models include the standard GBAS ”front” threat model as required for CAT-1 certification and a simplified plasma bubble model intended to represent equatorial conditions. These models can now easily be simulated at the German Aerospace Centers facilities. As an example, we simulate a GBAS ground facility (with a code carrier divergence monitor) affected by both a plasma bubble with a constant differential delay of −2 m and fronts of various gradients and speeds. Results confirm that a plasma bubble can lead to worst case position biases that may cause misleading and hazardously misleading information to be used by an approaching aircraft. Moreover, we confirm that ionospheric fronts at a speeds close to the airplane approach speed and propagating into the approach direction cause the most severe range biases. These, mapped to the position domain, can also cause hazardously misleading information.

GNSS based relative navigation for intentional approximation of aircraft

In aviation, satellite navigation is generally only used to determine the absolute position of aircraft. We show that the signals can also be used for safe relative navigation provided that a data link exists between the two aircraft. The link can be used to form a double difference combination of code phase measurements and determine a three dimensional baseline vector. The baseline vector is protected by protection levels which determine the 3×10−7 error bound of the baseline estimation. Thus, the distance vector can be used to perform safe approximation maneuvers in instrument weather conditions. We derive the protection level expression and test the baseline vector estimation using data from two real satellite navigation receivers on the ground. Moreover, we simulate an intercept mission using a Spirent GNSS7790 simulator and show that with the derived protection bounds an approximation up to 10 m is possible.

Extending Required Navigation Performance to IncludeTime Based Operations and the Vertical Dimension

In this manuscript, we derive a concept for a four dimensional Required Navigation Performance by extending the existing lateral RNP into the vertical and along-track dimensions. Based on the target level of safety desired by ICAO and traffic on a given route, an along track requirement can be formulated for the traffic using the airway. In the vertical, accuracy requirements at the 99.5% level are already specified within the performance based navigation concept, however monitoring and alerting is not foreseen. Here, we suggest a monitoring technology based on satellite navigation that can also be used to ensure vertical separation in RVSM airspace.

GBAS based autoland system: A bottom up approach for GAST-D requirements

Ground Based Augmentation Systems (GBAS) for the precision approach of aircraft have never been so close to support automatic approach and landings under CAT IIIc conditions as they are today. However, as one key requirement for certification, it has to be demonstrated that the required integrity, continuity and availability allocations for GBAS are met, even under the most unfavorable circumstances. As for ILS/MLS, the performance of the total system (GBAS, the aircraft and its automatic landing system) is the key to eventual certification. Contrary to ILS/MLS-based guidance, at this moment no standard models and parameters for navigation errors and disturbances have been defined yet for assessing automatic landing performance based on GBAS. This raises the questions (1) how well does the integrated system perform with state of the art GBAS error models, and (2) for a given performance level reached with ILS/MLS, how much error could be tolerated when using GBAS instead? In this paper we have made a preliminary step to find an answer to the first question. To this end we have taken an existing, flight tested automatic landing system, tuned for use with DLRs fly-by-wire test bed ATTAS (Advanced Technologies Testing Aircraft System), which is representative for EASA Part 25-class aircraft. The landing system was adapted for use with GBAS and an advanced GBAS error model was incorporated in the simulation model. With this set-up, we performed the same Monte-Carlo simulations as were originally done with ILS. Initial results indicate that, based in the adopted error models, CS-AWO criteria can be met with more ease than with ILS/MLS-based guidance. It is expected that performance of GBAS-autoland systems can be further improved by exploiting new features that GBAS has on offer, tolerating larger NSE errors or possibly allowing operational autoland limitations, like maximum crosswind, to be relaxed