WAAS and the Ionosphere – A Historical Perspective: Monitoring Storms

Process flow diagram showing how WAAS monitors ionospheric disturbances: a) measurements are recorded at WAAS reference stations (WRSs); b) measurements are converted to estimates of vertical delay; c) an ionospheric grid delay (IGD) is computed for each ionospheric grid point (IGP); d) the irregularity metric is evaluated at each IGP; e) the ionospheric perturbation metric (IPM) for the entire coverage region is evaluated; f) if the Moderate Storm Detector has not tripped, the grid ionospheric vertical error (GIVE) at each IGP is evaluated using the quiet-time branch of the ionospheric threat model; g) otherwise, if the Extreme Storm Detector has not tripped, the GIVE at each IGP is evaluated using the disturbed-time branch of the ionospheric threat model; h) otherwise, each GIVE is set to its maximum possible value GIVE_max; and i) the IGDs and GIVEs are broadcast to WAAS users.

WAAS IGP masks and pierce point filter lines (PPFL): (a) IOC (July 10, 2003, WAAS IGP mask 002, no PPFL); (b) LPV Release 6/7 (September 27, 2007, WAAS IGP mask 003, PPFL in brown); (c) LPV Release 8/9.2 (September 22, 2008, WAAS IGP mask 004, PPFL in blue); for WFO Release 3A (October 20, 2011), the IGP enclosed by the red square was added (WAAS IGP mask 005), and the PPFL was extended 20˚ eastward; (d) WFO Release 3A1 (January 20, 2012, WAAS IGP mask 006, PPFL in amber). Note: WAAS IGP mask 007 for Release 51-CY18 (September 11-14, 2018) is identical to mask 006 in terms of IGP locations, but GIVE floor values at [15˚N 100˚W] and [15˚N 105˚W] change from 15 m to 3 m. Black squares identify IGPs near Hawaii where, due to the isolation of the WRS in Honolulu from other WAAS WRSs, GIVEs are always set to “not monitored.”

The distribution of IPPs for equivalent vertical delays observed at 20:57:30 UTC on June 1, 2013. The black dashed line encloses the IPPs included in the fit centered on the IGP at [35˚N, 125˚W] identified by the x. The red arrow points from the IGP to the centroid of the IPPs included in the fit. The fit domain radius R_fit is 1,589 km and the relative centroid metric (i.e., the length R_centroid of the centroid vector divided by the fit radius R_fit) is 0.58. Magenta open circles indicate the positions of GPS satellites projected onto the ionospheric shell. Open brown squares identify receiver locations.

Median value of 100 largest absolute vertical delay residuals for 16 storms in solar cycle 24, no data deprivation. Brown lines are contours of constant geomagnetic latitude.

The value of the irregularity metric of each IGP at 23:00 UTC for (a) a weak storm on July 26, 2004; (b) a moderate storm on October 24, 2011; and (c) an extreme storm on April 6, 2000. The peak value is indicated by the magenta rectangle in each plot.

The magnitude of the D_st index (nT) from October 27, 2003 through November 2, 2003. [Available at: World Data Center for Geomagnetism, Kyoto, Final D_st index (1957 – 2014), http://wdc.kugi.kyoto-u.ac.jp/dstdir/]

The equivalent vertical delay (in meters) for observations over the conterminous United States (CONUS) at 03:10:30 UTC, October 31, 2003, as sampled by (a) the CORS network and (b) the WAAS network (when WAAS included only 25 stations)

The equivalent vertical delay (in meters) for observations of the CORS network over the conterminous United States (CONUS): (a) April 7,2000, 04:09:00 UTC; (b) July 16, 2000, 04:43:30; (c) April 1, 2001, 04:07:00; (d) October 30, 2003, 03:10:30 UTC; and (e) November 21, 2003, 03:00:00 UTC. The epoch chosen for each figure is roughly 4½ hours after sunset in Miami, Florida.

Schematic diagram of the ionospheric perturbation metric (IPM) as a function of time for an ionospheric storm that trips the Extreme Storm Detector (ESD). The dashed lines represent detector thresholds as they have been defined since WFO Release 3A: the ESD onset and recovery thresholds (T_ESD,trip and T_ESD,recovery) are in red; the Moderate Storm Detector (MSD) onset threshold (T_MSD,trip) is in magenta (in practice, the MSD recovery threshold T_MSD,recovery is set equal to T_MSD,trip); and the irregularity detector trip threshold (T_irreg,trip) is in blue. The ESD enters the storm Onset Confirmation State when it crosses T_ESD,trip. The extreme storm is confirmed when the IPM has remained above T_ESD,trip for a time duration of t_ESD,confirm. The ESD enters the Recovery Confirmation State when it crosses T_ESD,recovery and resumes its normal state after it has remained below T_ESD,trip for a time duration of t_ESD,recovery.

Ionospheric perturbation metric (IPM) as a function of time for the moderate storm of October 24, 2011. The area of the region in red defines the ionospheric disturbance index (IDI) used to rank the magnitude of the storm. The dashed lines represent detector thresholds as they have been defined since WFO Release 3A: the ESD onset and recovery thresholds (T_ESD,trip and T_ESD,recovery) are in red; the moderate storm detector onset threshold (T_MSD,trip) is in magenta; and the irregularity detector trip threshold (T_irreg,trip) is in blue.

Ionospheric perturbation metric (IPM) as a function of time and interpolated map of vertical delay (in m) at the peak of the IPM for the five extreme storms of solar cycle 23: (a) IPM for April 6–7, 2000; (b) vertical delay map for April 6–7, 2000; (c) IPM for July 15–16, 2000; (d) vertical delay map for July 15–16, 2000; (e) IPM for March 31, 2001; (f) vertical delay map for March 31, 2001; (g) IPM for October 29–31, 2003; (h) vertical delay map for October 29–31, 2003; (i) IPM for November 20–21, 2003; and (j) vertical delay map for November 20–21, 2003. Note that the length of each x-axis in the plots of IPM vs. time varies from storm to storm.

The number of minutes that the MSD is in a tripped state for the data interval January 6, 2011, to November 13, 2013