Integrity Parameters

In order to enable arriving aircraft to bound their residual position errors for the GBAS corrected measurements, the ground station transmits several integrity parameters relating to different error sources. The following subsections describe these parameters, their role and their application.

2.2.2.3.1 P-values

The P-values are the ephemeris decorrelation parameters transmitted in the Type 1 message. They are applied to bound the effect of a potential ephemeris error that has a different effect on a user at a cer- tain distance from the ground station. They “shall characterize the impact of residual ephemeris errors

due to spatial decorrelation” [36] for all corrections broadcast in the Type 1 message. The P-value is defined as the minimum detectable error (MDE) of the ephemeris monitor implemented in the GBAS ground station divided by the range between the GBAS reference point and the satellite. The MDE depends on the type of ephemeris monitor implemented and for many implementations also on the baseline length and orientation between the receivers. Another algorithm is based on checking the difference between the last validated ephemeris (up to 24h old) and the current ephemeris of a satellite and monitor the change [40]. For current implementations of GBAS this monitoring is considered sufficient, however the P-values obtained in this way may be too large to expand the current service volume for future operations like continuous descent operations where also sufficient reliability has to be ensured at larger distances from the airport.

2.2.2.3.2 B-values

The B-values are an “estimate of the error contribution from” [36] each reference receiver to the cor- rections provided to the aircraft. They are computed for each satellite iand reference receiver j as

,100 ,100 1 ( , ) ( ) ( , ) ( ) 1 _i tx sca k S k j B i j PRC i PRC i k M i ∈ ≠ = − −

∑

(6)

where M i( ) denotes the number of reference receivers which were used to generate the correction for satellite i and S_i is the set of reference receivers tracking satellite i. Before a correction for a specif- ic satellite is calculated the corresponding B-values are compared to a monitoring threshold to identify potentially faulty measurements from one reference receiver and exclude the corresponding data from the calculation of corrections.

2.2.2.3.3 Sigma PR Ground

, pr gnd

σ is the standard deviation of a zero-mean normal distribution associated with the residual un- certainty in the GBAS corrections. It “shall account for all equipment and environmental effects, in- cluding the received signal power, the local interference environment, and any transient error in smoothing filter output, relative to steady-state, caused by ionospheric divergence” [36]. The estimate of σ_{pr gnd,} for a ground station can be obtained by either observing code minus carrier measurements or by a B-value assessment. The later method is usually the preferred one since in the code minus car- rier observations the ionospheric error has to be estimated to separate it from the noise and multipath effect. For the ionospheric estimate dual frequency measurements are necessary which are typically not available in GAST C and GAST D ground stations. The estimate takes into account at least one day of recorded measurement data from B-values associated with validated corrections. For each ref- erence receiver only one sample every 200 seconds is used to ensure that the data is uncorrelated. The remaining B-values are then sorted according to the elevation of the corresponding satellite into bins. For each such bin the mean and standard deviation

σ

_B are calculated. The mean is expected to be close to zero and the root mean square (RMS) for each reference receiver j is then calculated as

_ , , 1 ( 1) pr gnd j B j N RMS M N

σ

+ = − (7)

where Mis the number of reference receivers and

N

is the average number of satellites which were tracked at the epoch the B-value was evaluated. This process yields one curve for each reference re- ceiver. The broadcast σ_{pr gnd,} for each elevation bin is taken as the maximum value of the individual estimates and is transmitted in the Type 1 message. Due to different signals on the different frequen- cies and constellations it will be necessary do define this parameter for each signal and possibly also for each constellation individually.

2.2.2.3.4 Tropospheric parameters

For enabling the aircraft to calculate a differential tropospheric correction and bound the residual trop- ospheric error there are three parameters provided by the ground station as part of the Type 2 message. The first one is the refractivity index NR of the atmosphere at the GBAS location. It consists of a dry

and a wet component and can be calculated based on locally measured weather data (temperature, air pressure and humidity) [41]. As the variations of NR are small it can be assessed and used as a yearly

average [42]. To account for the uncertainty introduced by considering such a long time span a refrac- tivity uncertainty σn is calculated by assessing a daily value and compare it to the yearly average. This uncertainty is then expressed as a standard deviation of a normal distribution and provided to the air- craft for inclusion in the error bounding process in the on-board system [41]. A third parameter which is provided by the ground station is the tropospheric scale height h0. It is also calculated based on the

earth surface height at the GBAS location and based on the dry and wet components of the refractivity indexNR [41].

2.2.2.3.5 Sigma Vertical Iono Gradient

The parameter providing information about the residual uncertainty associated with nominal iono- spheric impact in vertical direction is described by the value σvig which is transmitted in the Type 2 message. It is a parameter which is contained in the ionospheric threat model of the region where the GBAS is located. Every national regulator has to ensure that a valid and representative threat model for the ionospheric impact is developed. This is usually done by evaluating data collected over one solar cycle from a network of reference receivers in a representative region. Ionospheric threat models have been determined e.g. for the contiguous United States (CONUS) [43], Germany [44], South Ko- rea [45]. Based on the elevation of a satellite the vertical uncertainty is then mapped into a slant uncer- tainty corresponding to an overbound of the expected error on the pseudorange measurement.

In document Total System Performance of GBAS-based Automatic Landings (Page 37-39)