The advancement in miniaturization and microfabrication technology has led to the development of sensitive and selective electrochemical devices for field-based and in situ environmental monitoring. Electrochemical sensing devices have a major impact upon the monitoring of priority pollutants by allowing the instrument to be taken to the sample (rather than the traditional way of bringing the sample to the laboratory). Such devices can perform automated chemical analyses in complex matrices and provide rapid, reliable and inexpensive measurements of a variety of inorganic and organic pollutants. Although not exhaustive due to the vast amounts of new and exciting electrochemical research, this review addresses many important advances in electrochemical sensor design and development for environmental monitoring purposes. Critical design factors and development issues including analytical improvements (e.g. detection limits), microfabrication and remote communication are presented. In addition, modern environmental applications will be discussed and future perspectives considered.

development and applications

The aviation community has very stringent navigation integrity requirements that apply to a variety of manned and Unmanned Aerial Vehicle (UAV) operational tasks. This paper presents the results of the research activities carried out by the Italian Air Force Flight Test Centre (CSV-RSV) in collaboration with the Nottingham Geospatial Institute (NGI) and Cranfield University (CU) in the area of Avionics-Based Integrity Augmentation (ABIA) for mission- and safety-critical Global Navigation Satellite System (GNSS) applications. Based on these activities, suitable models were developed to describe the main causes of GNSS signal outage and degradation in flight, namely: antenna obscuration, multipath, fading due to adverse geometry and Doppler shift. Adopting these models in association with suitable integrity thresholds and guidance algorithms, the ABIA system delivers integrity caution (predictive) and warning (reactive) flags, as well as steering information to the pilot and electronic commands to the aircraft/UAV flight control system. These features allow real-time avoidance of safety-critical flight conditions and fast recovery of the required navigation performance in case of GNSS data losses. This paper presents the key ABIA concepts, architecture and mathematical models. A successive paper will address the ABIA integrity thresholds criteria and detailed results of a TORNADO simulation case-study.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/475FE99E4420FE6B33F9870AD9C151B7/S0373463313000027a.pdf/div-class-title-a-new-avionics-based-gnss-integrity-augmentation-system-part-1-fundamentals-div.pdf

A New Avionics-Based GNSS Integrity Augmentation System: Part 1 – Fundamentals

https://nottingham-repository.worktribe.com/preview/894312/Progress%20in%20Aerospace%20Sciences%20-%20GNSS%20Paper_.pdf

Global navigation satellite systems performance analysis and augmentation strategies in aviation

The Local Area Augmentation System (LAAS) is the Federal Aviation Administration's ground-based augmentation system (GBAS) for local area differential GPS (DGPS). It will support all categories of precision approach. LAAS is currently under advanced development and specification. The purpose of this paper is to provide a system description of LAAS and to describe important related developments for the aviation and navigation communities. Both top-level and intermediate-level descriptions are given, with emphasis on the Ground Segment. Three distinguishing advances are described in more detail: Multipath Limiting Antenna, Airport Pseudolite, and a method for transforming integrity parameters from the pseudorange to the position domain.

Description of the FAA's Local Area Augmentation System (LAAS)*

Both WAAS and LAAS receivers use carrier-smoothing filters to reduce the effects of multipath and thermal noise at the aircraft. For the majority of users this reduces the magnitude of the errors and leads to improved accuracy. However, the presence of a significant ionospheric gradient can introduce a bias into the output of this filter. If unmitigated, this bias can grow to be significantly larger than the noise and multipath effects the filter is employed to reduce. Such gradients are rare at midlatitudes; however, they are much more common in polarregions and sometimes a daily occurrence in equatorial regions. Therefore, this problem which may be rare in the United States or Europe will be much more significant in other parts of the world. The bias arises because the GPS code and carrier measurements are affected differently by the ionosphere. A positive delay for the code creates an equal, but opposite advance for the carrier. Therefore, as the ionospheric Total Electron Content (TEC) changes along a user’s line of sight to the satellite, the code and carrier measurements diverge from each other at twice the rate of ionospheric change. Nominally, the rate of change over a few hundred seconds is very small (less than a few millimeters per second) leading to decimeter-sized biases or less. However, if the user’s line of sight traverses a significant ionospheric gradient, the TEC can change by tens of meters in just a few minutes. This in turn can create a biased filter estimate of greater than 20 meters. This error is large compared to the overall error budget less than 0.4 m one-sigma. This paper investigates the combined effect of the TEC change and the filter bias on the WAAS/LAAS user’s differential range error and suggests a remedy to lessen the impact. This solution includes changes to the airborne algorithms to detect large differences between the code and carrier. The ability of the airborne receiver to detect this bias depends on the amount of thermal noise and multipath it experiences. This paper will review the current expected levels of noise and multipath and indicate level of performance that should be achievable.

The Effects of Large Ionospheric Gradients on Single Frequency Airborne Smoothing Filters for WAAS and LAAS

LAAS Signal-in-Space Integrity Monitoring Description and Verification Plan

The International Civil Aviation Organization (ICAO)
Global Navigation Satellite System (GNSS) Panel (GNSSP) has developed Standards and Recommended Practices (SARPs) for Ground Based Augmentation Systems (GBAS) and Space Based Augmentation Systems (SBAS). The RTCA Special Committee 159, Working Group 4A (WG-4A) has also developed the Minimum Operational Performance Standards (MOPS)
for airborne GPS/Local Area Augmentation System (LAAS) and GPS/Wide Area Augmentation System (WAAS) receivers. These standards include an airborne error allocation for airframe multipath defined by a
common error model. This model is intended to apply to all GBAS (i.e. LAAS) or SBAS (i.e. WAAS) receivers
installed on any aircraft within a reasonable set of
installation guidelines. The motivation for standardizing
the error model is to fix the allocation for receiver
tracking performance and limit test requirements to
evaluating that tracking performance. Testing of airframe
multipath on each aircraft model would be very
impractical and expensive. This paper describes the
validation activity that has been done to show that the
standard error model for airframe multipath is adequate.
The standard multipath error model is presented and
discussed in the context of the allocation of SARPs and
MOPS requirements. The expected magnitude of
airborne multipath based on theoretic considerations is
discussed. The paper then presents the methods used to
estimate the multipath error as well as the difficulties
experienced in isolating the multipath error component
from in-flight measurements. The in-flight measurements
were recorded during long duration flights of a number of
aircraft including the Boeing 727, 737, 747, 757, 767 and
777 airplanes. The paper concludes with the results and
interpretation of the validation activities.
This activity is significant because it represents the most
comprehensive study of airframe multipath on
commercial air transport airplanes to date. The study
shows that for commercial applications, an allocation for
airborne multipath that is independent of the specific
aircraft type is feasible.

Validation of the Airframe Multipath Error Allocation for Local Area Differential GPS

A method, apparatus, and computer program product for detecting anomalies in a landing system. In one embodiment, a magnitude difference between a blended inertial deviation magnitude and a raw deviation magnitude is identified to form a magnitude difference. The magnitude difference is compared to a magnitude threshold. If the magnitude difference exceeds the magnitude threshold, an anomaly in the data is detected.

Method and apparatus for detecting anomalies in landing systems utilizing a global navigation satellite system

The development of Ground Based Augmentation Systems (GBAS) to support aircraft Category II/III approach and landing operations is underway. Given improvements in GBAS ground stations, airborne multipath is apparently the largest error source affecting the accuracy and integrity of GBAS. This paper describes a program which has been designed and launched to investigate and quantify the effects of multipath from the airframe as well as from short delay ground bounce multipath during landing operations. The program includes development of an electromagnetic modeling capability which predicts amplitude, time delay and phase characteristics of airframe multipath. The program also includes the collection of a very large amount of GPS code and carrier phase measurement data recorded during flight testing of the 777-300ER and 737-NG airplanes. The flight test observations will ultimately be used to validate the modeling capability. Then, the modeling capability will be exercised to develop an Airborne Multipath Model (AMM) suitable for standardization. The paper describes the overall program, describes the electromagnetic modeling and gives details of the test setup and data collection program. This program is expected to be an important step in the development of GBAS to support CAT II/III approach and landing operations.

Program for the Investigation of Airborne Multipath Errors

This paper presents the development of a model of the
errors associated with the output of an airborne GNSS
Ground Based Augmentation System (GBAS) receiver.
The development of such a model is a necessary step in
the development and validation of satellite based landing
systems.
Standards groups such as the International Civil Aviation
Organization (ICAO), RTCA and the European
Organization for Civil Aviation Equipment (EUROCAE)
are maturing and harmonizing standards for ground based
augmentation (GBAS) of the Global Navigation Satellite
System (GNSS) for aviation use including precision
approach operations. Within the US National Air Space
(NAS), the GPS/LAAS system (Global Positioning
System augmented with a local area augmentation
system) will provide this service. RTCA, functioning as a
Federal Advisory Committee, has developed a concept of
Performance Type to classify requirements for the
combined performance of the signal-in-space and fault
free airborne GPS/LAAS. The concept currently defines
three levels of performance. Requirements for
Performance Types 1, 2, and 3 were derived from
Instrument Landing System (ILS) requirements and are
intended to support Category I, II and IIIa, and IIIb
operations, respectively.
The development of the infrastructure within the aviation
community to widely field systems compliant with the
standards is well underway. Certification and approvals
for use of GNSS/GBAS equipment on aircraft is one area
being addressed. Historically, models of navigation
system errors (NSE) based on the requirements for the
NAVAID have been used in the aircraft certification
process in the context of Monte Carlo simulation trials. In
addition to NSE, the simulations exercise the range of
conditions an aircraft may see during service, including
the variation of atmospheric effects (wind speed and
direction, turbulence level, etc.) and aircraft configuration
(weight, center of gravity location, flap settings, etc.).
Samples of aircraft landing performance for Monte Carlo
trials are included.

Tim Murphy

Papers

LAAS Signal-in-Space Integrity Monitoring Description and Verification Plan

Validation of the Airframe Multipath Error Allocation for Local Area Differential GPS

Method and apparatus for detecting anomalies in landing systems utilizing a global navigation satellite system

Program for the Investigation of Airborne Multipath Errors

A GBAS Landing System Model for CAT I