During the active night, most of the satellites' signals experienced scintillation. Ionospheric irregularities modulated the C/No, leading to incursions in excess of 20 dB–Hz at times. The enhanced rapidity of the signals fluctuations could have led to navigation bit errors; cycle slipping, or loss of tracking. Invariably, even when lock is maintained, too rapid fluctuations on the C/No of satellites could impact negatively on the navigation solutions [Carrano et al., 2005]. Since the lost satellite signals cannot be used for position calculation, the geometry of the useable satellite constellation is degraded during periods of outages. Dilution of precision (DOP) quantifies the influence of receiver-satellite geometry on GPS positioning accuracy. The best DOP is obtained when the satellites in view of the receiver are evenly distributed in the sky. For instance, in a case of four satellites in view of the receiver, the best DOP will be obtained if three of the satellites are equilaterally distributed over the sky, and the fourth satellite is located directly overhead in the centroid of the equilateral triangle, collectively leading to tetrahedron geometry by their line of sights (LOS) to the receiver [Kaplan, 1996; Misra and Enge, 2001].
 Typically, HDOP values are between 1.0 and 2.0 [Kaplan, 1996]. VDOP values are larger than HDOP values, indicating that vertical position errors are larger than the horizontal position errors. We suffer this effect because all the satellites from which we obtain signals are above the receiver. The horizontal coordinates do not suffer a similar fate as we usually receive signals from all sides. On the less active night, the receiver showed good tracking capability, as it consistently maintained lock on six to eight satellites, and the GDOP and PDOP values were generally less than 2.2 with no single record of an outage (Figures 3a–3e). In contrast, on the active night, between 2200 and 2300 UT, the number of satellites that the receiver maintained lock on often reduced to four and in some cases less than four, leading to the observed six navigation outages (Figures 4a–4e).
 Conventionally, integrity approach was based upon a notion that the actual error distributions would be close to Gaussian, and that their convolutions would result to positioning errors that are also close to Gaussian, although, there might be deviations from the ideal behavior [Walter et al., 2010]. Integrity analyses are cumbersome and laborious. Walter et al. [2009, 2010] provided detailed explanations of integrity analyses, especially from the standpoint of WAAS. As of date, aviation integrity capabilities are designed for SBAS operations. For single frequency SBAS, Radio Technical Commission for Aeronautics (RTCA)  defines
where Kv is the Gaussian tail (Kv ≡ 5.33) and σuis the cover-bound of true 1σ value of the vertical position error. KH is the Gaussian tail (KH ≡ 5.73) and σmajoris the cover-bound of true 1σ value of the horizontal position error.
4.1. Statistics of Signal Fading
 On the less active night, four of the satellites namely PRNs 7, 8, 24 and 28, at elevation angles (el. angles) of 48°, 71°, 19° and 33° respectively were observed to maintain perfect tracking, as their C/No fluctuated slowly within the threshold bounds in the order of increasing elevation angles, while the other three; PRNs 4, 26 and 27 at elevation angles of 55°, 12° and 42° respectively, experienced frequent fading (Figure 5a). However, since four of the satellites maintained tracking consistently at good geometry, navigation solution was continuously achieved. On the active night, signals from PRNs 8 and 28 experienced no scintillations as they were observed to fluctuate slowly along the 45 dB-Hz and 40 dB-Hz baselines respectively. The signals of other satellites that were in view of the receiver were significantly impacted. Overall, six navigation outages were observed during the active night. The first, which was the worst during the active night in terms of duration (50 s), occurred between 22.102 and 22.117 UT (Figure 5b). During this period, PRNs 8 (el. angle: 67°) and 28 (el. angle: 34°) maintained good tracking, while PRNs 7 (el. angle: 45°) and 27 (el. angle: 31°) were observed to fluctuate into and out of fades. The two satellites' signals fluctuated simultaneously into fade at about 22.117 UT. Prior to this time, PRN 7 signal was lost at two instances; 22.105 and 22.112 UT respectively. It is important to note that after a signal that is lost to fades recovers, it usually takes few seconds for the pseudo-range data from such satellite to be included in the navigation solution estimation. PRN 7 signal was lost at two instances, concurrently as PRN 27 signal had just recovered from fades, perhaps, still waiting for a clean bill of health for its pseudo-range data to be included in calculating the navigation solution. This could have impaired the number of satellites readily available to the receiver to calculate the navigation solution. PRN 4 (el. angle: 61°) showed up briefly at 22.116 UT only to re-appear at 22.12 UT and maintain lock to 22.15 UT. PRNs 24 (el. angle: 24°) and 26 (el. angle: 14°) only showed up briefly. The inability of the receiver to track up to four satellites with good geometry within this time period (22.102–22.117 UT) led to the observed 50 s navigation outage. After this outage, there was a continuity of service for about 14 min before a second outage which lasted for 5 s, followed by other four outages that lasted for 2, 3, 1 and 7 s respectively.
 Generally, over the entire campaign period, PRN 27 signals scintillated with characteristic prolonged fading duration (∼2.2 s at times). PRNs 4, 7, 13, 24 and 26 signals on the other hands were observed to scintillate with characteristic rapidity and brief fading durations (0.1–0.4 s) that resulted into intermittent loss of lock. Two satellites; PRNs 1 and 28 recorded the least percentage occurrences of fades at each C/No threshold, while PRNs 4, 7, 13, 24, 26 and 27 recorded higher percentage occurrences of fades (Figure 6b). Although, it is important to note that PRN 1 satellite appeared only at three nights of the campaign. At 15 dB-Hz threshold and above, PRNs 8 and 28 recorded percentage occurrences of fades within the range 33.3–53.3% and 20–33.3% respectively. PRN 1 satellite recorded percentage occurrences of fades generally lower than 15% at all threshold level. All other satellites, apart from PRNs 1, 8 and 28, recorded percentage occurrences of fades within the range 33.3–86.6% at 15 dB-Hz threshold and above. At 10 dB-Hz threshold, PRNs 1, 8 and 28 recorded 6.6% occurrences of fades each, PRNs 4 and 24 recorded 13.3% each, while PRNs 7, 11, and 13 recorded 20% each. Finally, PRNs 26 and 27 recorded 26.6% occurrences of fades each at 10 dB-Hz threshold.
 Humphreys et al.  and Carrano and Groves  stressed the importance of signal decorrelation time as a parameter that could quantify signal fading rates. The fading rate is related to the satellite's effective scan velocity with respect to the magnetic field and plasma drift as the satellite traverses ionospheric irregularities. The satellite's effective scan velocity is defined as the rate with which the radio LOS cuts across contours of equal correlation in the ionosphere [Fremouw, 1980]. It accounts for the effect that the anisotropy of irregularities has on the conversion of spatial structures to temporal fluctuations. The zonal drift was measured during the campaign by the spaced receiver technique at the UHF frequency (250 MHz). Carrano and Groves gave detailed explanation of the spaced receiver technique. The zonal drift velocity was 135 m/s at 22:00 UT during the active night. Furthermore, on the active night, the average effective velocities of the scintillating satellites during the observed outages are PRN (eff. vel.): 4 (146 m/s), 7 (88 m/s) and 27 (26 m/s). PRNs 24 and 26 showed brief or discontinuous appearances, while PRNs 8 and 28 were quiescent during the period; hence, there is no meaningful correlation between their effective velocities and the fading rates of their signals. The effective velocity was computed in the post-processing, using the locations and velocities of the GPS satellites–determined from GPS ephemeris and the technique described inRino . The average effective velocity was thereafter calculated by taking the average of the computed effective velocity data of each satellite during the active night.
 As shown in Figures 7a–7b, among all the satellites which were scintillating at the time (PRNs 4, 7, and 27), only PRN 27 had an IPP with a significant component in the direction of the zonal drift (i.e., the magnetic east direction). When the effective scan velocity is computed, the zonal drift velocity is subtracted from the IPP velocity. Hence, the effective scan velocity for PRN 27 becomes comparatively less than those of the others. Given the inverse dependency of signals decorrelation time on the effective satellite's velocity, and thus, the direct dependency of the fading rate on the effective satellite's velocity, it became apparent why PRN 4 showed characteristic rapid fading, followed by PRN 7, while PRN 27 showed relatively slow fading rate (large fading duration). Consequently, the chances of losing lock on PRN 4 by the receiver were comparatively higher than those of the other two scintillating satellites. Generally, the satellite elevation angle, as well as the scan velocity of the IPP of the satellite with respect to magnetic field and plasma drifts influences tracking sustainability at a given strength of ionospheric perturbations.
 During the active night, the statistics of the observed deep fades for different scintillating satellites shows that PRNs 8 and 28 did not experience fades, while PRNs 4 and 7 signals experienced frequent fades, with occasional loss of locks. PRNs 13, 24 and 26 signals generally faded below the lower bound of the tracking lock threshold (less than 30 dB-Hz baseline); although PRN 13 showed brief appearance and incursion into fades. PRN 27 signals maintained lock at some instances, but with repeated incursions into fades, thereby dwelling comparatively longer in fades before recovery, with occasional loss of lock. As shown inFigure 8a, the fading duration is less than 1 s most of the time for PRNs 4, 7, 24 and 26, with denser samples distribution within 0.1–0.4 s, during the active night. For PRN 27, a contrary distribution was observed. The observed fades were characterized with longer durations (∼2.2 s at times), with a distribution that peaks at 1.5 s.
 Figure 8b shows signals fading durations over the entire period of the campaign. The fading duration was binned into 0.2 s interval. Similarly, PRNs 1, 4, 7, 11, 13, 24, 26 and 28 showed denser samples distribution within 0.1–0.4 s, while PRN 27, showed a distribution that peaks at the 1.6 s bin. The distributions of PRNs 1 and 28 were generally low over the campaign period. Generally, rapid fades with shorter durations like those displayed by PRNs 4, 7, 11, 13, 24 and 26 could cause high phase dynamics which could further lead to cycle slips, loss of lock or complete outages [Humphreys et al., 2010a, 2010b]. Increase in the strength of the ionospheric turbulence could increase the rapidity of the fades of signals that traverse the ionosphere, resulting in abrupt phase transitions that could further lead to overbearing impacts on the receiver's phase lock loop (PLL) [Strangeways, 2009; Strangeways et al., 2011]. As reported by Humphreys et al. [2010a], prolonged fading like the one experienced by PRN 27, is usually accompanied by phase dynamics that are comparatively slow, thereby allowing broadband measurement noise to dominate.
 Adapting the signals to possible bandwidth enhancements may assist in improving GPS receivers' carrier loop tracking performance, although the major drawback of this option is that wider bandwidth tracking loops incur more noise. Therefore, it is important to carefully consider the trade-off between bandwidth enhancements and simultaneous increase in the receiver's noise level. As suggested byGanguly et al. , an adaptive carrier tracking loop for GPS receivers may be reasonable. The loop could operate by using only the PLL portion with a tight bandwidth. When phase and amplitude scintillations are detected, loop bandwidth will be adaptively increased to accommodate new signal conditions. On the other hand, when a deep amplitude fade occurs, there is no signal to track, and as such, only the PLL will lose lock. At this point, the frequency lock loop (FLL) portion of the loop will be activated in order to facilitate the re-acquisition of the signal as it recovers from fade.
 Furthermore, international cooperation on multiconstellation operations will increase the number of satellites in the sky, and this will increase the chances of always tracking more than four satellites by the receiver, even during disturbed ionospheric conditions. Although, we also have a caveat on this option, as we have no control over which portion of the sky that would be covered by scintillation and to what extent. In other words, scintillation being a nature-made phenomenon may cover LOS of most of the satellites if their tracking locations are in the region of scintillation in the sky. Moreover, SBAS satellites are geostationary satellites. This implies that they have a permanent IPP, and if they are covered by scintillation patches, until the patches move away, they will continue to suffer fading because they (geostationary satellites) are not moving [Bandyopadhayay et al., 1997]. Applications of other augmentations such as GBAS and ABAS will be of significant advantage in enhancing aviation safety under these conditions.