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Coverage of L5 single frequency user with barometric altimeter aiding in CONUS is 97.69% with VAL=50m, HAL=40m.
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This paper investigates the vertical guidance performance of a multiple frequency WAAS receiver (L1, L2, and L5) in the presence of inclement weather and radio frequency interference (RFI). There are several ways to take advantage of the multiple frequencies. For example, one can calculate ionosphere delay in the airplane. This would replace the gr...
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... Depending on whether the aircraft is above or below transition altitude, pilots need to adjust the knob on the altimeter in order to choose the suitable reference pressure level (i.e. altimeter setting) [3]. The barometric altitude is referenced to the International Standard Atmosphere (ISA) (i.e. ...
In Air Traffic Control (ATC), aircraft altitude data is used to keep an aircraft within a specified minimum distance vertically from other aircraft, terrain and obstacles to reduce the risk of collision. Two types of altitude data are downlinked by radar; actual flight level (Mode C) and selected altitude (Mode S). Flight level indicates pressure altitude, also known as barometric altitude used by controllers for aircraft vertical separation. ‘Selected altitude’ presents intent only, and hence cannot be used for separation purposes. The emergence of Global Navigation Satellite Systems (GNSSs) has enabled geometric altitude on board and to the controllers via the Automatic Dependent Surveillance-Broadcast (ADS-B) system. In addition, ADS-B provides quality indicator parameters for both geometric and barometric altitudes. Availability of this information will enhance Air Traffic Management (ATM) safety. For example, incidents due to Altimetry System Error (ASE) may potentially be avoided with this information. This work investigates the use and availability of these parameters and studies the characteristics of geometric and barometric data and other data that complement the use of these altitude data in the ADS-B messages. Findings show that only 8·7% of the altitude deviation is < 245 feet (which is a requirement of the International Civil Aviation Organization (ICAO) to operate in Reduced Vertical Separation Minimum (RVSM) airspace). This work provides an alert/guidance for future ground or airborne applications that may utilise geometric/barometric altitude data from ADS-B, to include safety barriers that can be found or analysed from the ADS-B messages itself to ensure ATM safety.
... Depending on whether the aircraft is above or below transition altitude, pilots need to adjust the knob on the altimeter in order to choose the suitable reference pressure level (i.e. altimeter setting) [3]. The barometric altitude is referenced to the International Standard Atmosphere (ISA) (i.e. ...
Altitude data has long been deployed for aircraft vertical
navigation (pilot) and separation (Air Traffic Control
(ATC)). Two types of altitude data are down-linked by
radar; actual flight level (Mode C) and selected altitude
(level). Selected altitude presents intent, and therefore
cannot be used for separation purposes. Mode C indicates
aircraft pressure altitude (barometric altitude) used by
ATC for aircraft vertical separation. Emergence of satellite
technology; Global Satellite Navigation System (GNSS)
has introduced geometric altitude in the cockpit via GNSS
receiver and to the ATC via Automatic Dependent
Surveillance Broadcast (ADS-B) system broadcast.
Literatures to date have identified many advantages of
geometric altitude over barometric altitude. However, till
today, the barometric altitude is still the only altitude data
used for aircraft navigation and separation. This paper
analyzes characteristics of geometric altitude data in the
ADS-B messages. It then measures deviation of the
geometric altitude from the barometric altitude data.
Finally it identifies and discusses potential factors that
may influence the variations. Findings showed obvious
variation between the altitudes during different phases of
flight. The barometric altitude displayed higher readings
than geometric height especially while the aircraft is
cruising. The discrepancies between the two altitudes were
increasing during the climbing phase and decreasing
during the descend phase. It is also found that the absolute
difference between geometric height and barometric
altitude ranges from 25 feet – 1325 feet with an average of
569 feet. Various statistical methods are used to analyze
the sample data collected from ADS-B ground stations and
aircraft avionics and make model information from airline.
... te and offer higher availability. [2] showed the simulation results of the CONUS (CONterminous US) coverage of the LPV [1] precision approach services for a dual-frequency user. While experiencing the RFI (Radio Frequency Interference), a dual-frequency user might lose all but one GPS frequency, which introduces the single-frequency GPS user cases. [3] showed the simulation results of the CONUS coverage of the LPV precision approach services for a single-frequency user. While comparing the results of [3] with [2], the CONUS coverage of LPV precision approach services for a single frequency user is less than the coverage for a dual-frequency user. Therefore, the objective of this paper ...
... [3] showed the simulation results of the CONUS coverage of the LPV precision approach services for a single-frequency user. While comparing the results of [3] with [2], the CONUS coverage of LPV precision approach services for a single frequency user is less than the coverage for a dual-frequency user. Therefore, the objective of this paper is to investigate techniques which can sustain dual-frequency performance while descending into the RFI field. ...
... The nominal UIRE σ is 0.32m [2]. An L5 single-frequency user has LPV precision approach services available 99.9% of time over 49.25% CONUS [3]. In this situation, the nominal UIRE σ is 6m at the coast, and 3.5m in the center. ...
This paper investigates techniques to sustain dualfrequency ionosphere performance when a dual-frequency airborne user loses all but one GPS frequency while descending into the radio frequency interference (RFI) field. In this paper, we are particularly interested in the case where the user transitions from L1-L5 to having L5only. That is because the uncertainty of the L5-only ionospheric delay estimation is larger than the L1-only ionospheric delay estimation. An L1-L5 dual-frequency user has LPV (HAL = 40m, VAL = 50m) [1] precision approach services available 99.9 % of time over 100 % CONUS, with a nominal σUIRE of 0.32m [2]. An L5 single-frequency user has LPV precision approach services available 99.9 % of time over
One of the goals of SBAS (Space Based Augmentation System) is to correct the ionospheric delay which contributes the largest and most unpredictable error to GPS users’ range measurements. SBAS works well under the quite ionosphere and the moderate ionospheric storm. However, SBAS is susceptible to local ionospheric disturbances and severe ionospheric storms. This will be more challenging task for an SBAS in Asia Pacific region to provide a good ionosphere model, because most of the Asia Pacific Region is in the low latitude region in comparison with the U.S., and the ionospheric scintillations and the severe ionospheric storms happen more often in the low altitude regions. Therefore, this paper will investigate and evaluate the SBAS ionosphere model developed in the U.S. for this region. To that end, this paper uses the Southeast Asia GPS Networks in Scripps Orbit and Permanent Array Center (SOPAC) database. This paper first selects few stations from SOPAC to act as the SBAS reference stations to collect dual-frequency GPS measurements, and then these measurements will be used to generate the ionospheric measurements in the Southeast Asia. These ionospheric measurements include the GPS satellite hardware group delays (TGD) and the GPS receiver Inter-Frequency Bias (IFB). Thus, this paper uses a software calibration method to calibrate the hardware group delays to generate the clean ionospheric delay measurements. This paper then use these clean ionospheric delay measurements to develop the standard SBAS thin-shell ionospheric delay model. The developed SBAS ionospheric model will be evaluated by the dual-frequency GPS measurements collected in the same region. This paper will also change some of the parameters used in the SBAS ionospheric model to gain possible improvement in the availability of SBAS in Asia Pacific region.
As we all know, GPS cannot totally meet the need of safety-of-life applications in civilian aviation especially the vertical accuracy and integrity. It is widely accepted that integrating GPS with barometric altimeter (Baro) can improve geometry and consequently provide performance enhancement especially in the vertical direction. This paper presents a novel data fusion algorithm of Baro and GPS based on delta-altitude. This algorithm removes some of the conservatism built into the prior positioning algorithm of Baro/GPS integrated system. This paper focuses on analysis of accuracy and integrity performance of Baro/GPS integrated system. The results of simulation show that this algorithm can substantially improves navigation accuracy performance of the integrated system, especially in the vertical direction. Meanwhile, it indicates that great enhancement is achieved in integrity performance.
This research investigates the performance of an airborne GPS receiver
using differential corrections and associated error bounds from the WAAS
when three civil GPS signals become available. There are three ways to
take advantage of the multiple frequencies. First, one can measure
ionospheric delay directly in the airplane. This would replace the grid
of ionosphere delay corrections currently broadcast by the WAAS. This
direct use of multiple frequencies would be more accurate, and offer
higher availability. Second, one can use the additional GPS frequencies
to mitigate unintentional radio frequency interference (RFI). Even if
two frequencies are lost, the user could revert to the WAAS grid. Third,
one can take advantage of stronger civil signal power of the modernized
GPS to acquire a low elevation satellite before using it for the
position solution. Earlier acquisition would allow for longer
carrier-aided smoothing of multipath. This research evaluates the
performance of a multiple-frequency GPS landing system that depends on
the number of available GPS frequencies and includes the following
scenarios: Case 1. All three GPS frequencies are available, Case 2. Two
of three GPS frequencies are available, Case 3. One of three GPS
frequencies is available. This research also presents a solution to
sustain multiple frequency performance when an aircraft descends into an
RFI field and loses all but one of the frequencies. There are three
available techniques. First, one can use the code-carrier divergence to
continue ionospheric delay estimation. Second, one can use the WAAS
ionospheric threat model to bound the error. Third, one can use the
maximum ionospheric delay gradient model to bound the ionospheric delay
during the ionosphere storm period. These three techniques all provide
the ability to continue operation for more than 10 minutes after the
onset of RFI. This research provides the first three-frequency GPS/WAAS
LPV coverage predictions for CONUS. The current L1-only WAAS user has
LPV precision approach services available 99.9% of the time over 97.46%
of CONUS, although this may be reduced during ionosphere storms. After
the GPS and WAAS modernizations, an L1-L2-L5 three-frequency user, an
L1-L2 dual-frequency user, and an L1-L5 dual-frequency user all have LPV
precision approach services available 99.9% of time over 100% of CONUS
even during ionosphere storms.
Techniques available to sustain dual-frequency ionosphere performance when a dual-frequency airborne Global Positioning System (GPS)/wide area augmentation system (WAAS) user loses all but one GPS frequency while descending into radio frequency interference (RFI) are investigated. We are particularly interested in the case where the user transitions from L1L5 to having L5-only since the uncertainty of the L5-only ionospheric delay estimation is larger than the case for L1-only. The goal is to provide the techniques necessary for single-frequency users to sustain a performance similar to those of dual-frequency users. The proposed techniques are 1) the code and carrier divergence technique, 2) the WAAS ionosphere threat model technique, and 3) the maximum ionospheric delay gradient model technique. The results show that all three proposed techniques provide good ionospheric delay estimation for the full duration of approach.
This paper develops an empirical confidence bound for barometric altimeter altitude errors and shows how this bound may improve the performance of GPS-based approach and landing systems. This empirical bound is developed using historical meteorological data collected at a set of geographically diverse locations over a thirty year period. The confidence bound developed is shown to provide a Gaussian overbound on altimeter altitude errors in standard atmospheric conditions between a 10-5 and 10-6 confidence level. This confidence bound is integrated into the standard methodology for analyzing the performance of GPS-based landing systems and the results of a performance trade study using the confidence bound are presented. The results show that incorporating the empirical barometric altimeter confidence bound provides an increase in the coterminous United States (CONUS) service volume for lateral precision with vertical guidance (LPV) type approaches. While this increase is approximately 2% for an L1 single-frequency GPS user, it jumps to roughly 40% for an L5 single-frequency user.
