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On May 13, 2004 at approximately 19:00Z (1:00pm MDT, noon PDT) a number of NEXRAD radar anomalies were observed at the Riverton WY, Billings MT, Missoula MT, Spokane WA, and Pendleton OR sites. The event also appears to have been detected to a lesser extent by the Boise ID and Glasgow MT sites.

The event seems to have consisted of an object which created electromagnetic interference traveling northwest from northwestern Wyoming to north central Idaho. It appears this was an object as opposed to a meteorological event due to the "spoking" observed which is consistent with aerial traffic (such as was observed with the Reno anomalies). Aircraft are not generally detected by NEXRAD, but NEXRAD "spoking" anomalies do occur if an aircraft "illuminates" the NEXRAD site with its own radar or with another electromagnetic source that is operating at approximately the same frequency as the nexrad site (3GHz).

Since the event was detected by multiple radar sites at the same time, a relatively accurate position for the object at each point in time can be obtained by triangulating from two or more sites. This is accomplished in the images below by overlaying image from one radar on top of the image of another radar that detected the event at approximately the same time (within a minute or two). In each case, the image shown is the overlayed/triangulated version while the links are provided to the source images for each radar site, the date/time of that site's image, and the angle at which the event was recorded by that site.

#1: 18:45Z Billings 18:46Z: 172-176 (174) Riverton 18:44Z: 1-2 (1) Object At: 44.23076N, 108.44863W	#2: 18:56Z Billings 18:56Z: 274 Missoula 18:55Z: 110-111 (110) Object At: 45.90601N, 109.80705W	#3: 19:06Z Billings 19:06Z: 283-284 (283) Missoula 19:05Z: 112-114 (113) Object At: 46.27504N, 111.46954W
#4: 19:26Z Missoula 19:24Z: Saturation not overlayed Spokane 19:28Z: 105-110 degrees Object At: 47.0411N, 113.986W	#5: 19:35Z Missoula 19:32Z: 274-277 (275) Spokane 19:33Z: 105-112 (108) Pendleton 19:35Z: 50-51 (50) Object At: 47.22696N, 116.09931W Mis/Pend: 47.15305N, 116.23861W Pend/Spo: 47.30088N, 115.96002W Mis/Spo: 47.10422N, 115.14391W	#6: 19:38Z Missoula 19:38Z: 280-282 (281) Spokane 19:39Z: 106-119 (112) Object At: 47.33130N, 116.37736W

A couple of notes regarding the above triangulation...

First, at point #4 at 19:26 we assume the object is over Missoula because the spoking from Spokane is consistent with Missoula while the imagery from Missoula shows complete saturation (as seen to the right). While we cannot be completely sure, it seems reasonable to believe that the object was overhead at Missoula such that the electromagnetic radiation was detected in all directions. Prior and subsequent images show spoking in directions consistent with the object in those directions so it seems logical to conclude that the object was over Missoula and when the Missoula imagery showed the complete saturation.

Second, at point #5 we have a total of three sites that presented the anomaly so we have three possible locations for the object depending on which set of sites we use to triangulate. The Missoula/Spokane triangulation calculated a point about 40 miles east of the triangulation calculated by Pendleton/Spokane and Pendleton/Missoula. This is not surprising due to the fact that the relative angles between Missoula and Spokane are very small--the object is almost directly between them so using the bearings from each of them to determine the location of the object is not going to be very accurate. Therefore, we discard the Missoula/Spokane triangulation and average the Pendleton/Spokane and Pendleton/Missoula triangulation (which were accurate within 16 miles) to determine our point location for #5.

Analysis of object location, speed, and direction

According to the object locations calculated by the triangulations above, we are able to conclude the following:

From Point	To Point	Distance	Time	Speed	Bearing
1: 44.23076N, 108.44863W @ 18:45	2: 45.90601N, 109.80705W @ 18:56	133 mi	11 mins	725 mph (Mach 1.01)	300
2: 45.90601N, 109.80705W @ 18:56	3: 46.27504N, 111.46954W @ 19:06	83.6 mi	10 mins	502 mph	282
3: 46.27504N, 111.46954W @ 19:06	4: 47.0411N, 113.986W @ 19:26	130 mi	20 mins	390 mph	286
4: 47.0411N, 113.986W @ 19:26	5: 47.22696N, 116.09931W @ 19:35	100 mi	9 mins	666 mph	275
5: 47.22696N, 116.09931W @ 19:35	6: 47.33130N, 116.37736W @ 19:38	14.8 mi	3 mins	296 mph	290

The object was generally traveling west northwest with an trip bearing of 291 degrees. The first leg of its trip was 300 degrees after which the average bearing was 282. It appears that the object may have broken the speed of sound on the first leg of its trip, slowed down as it passed over Missoula, and then may have sped up. However, the speed of 666 mph on the second to last leg may not be accurate since it involves point #5 for which we triangulated 3 different positions. Nevertheless, the 666 mph figure seems to be quite reasonable since if we use the Spokane/Missoula triangulation we actually end up with a speed of around 1000 mph for the last leg.

Analyis

It seems quite certain that an object radiating electromagnetic energy traveled from approximately point 1 to point 6 covering a total of 461 miles in 53 minutes for an average speed of 522 mph although the initial speed of the object was significantly higher than the average. It appears quite probable that the object was traveling initially at a relatively high speed (725 mph) and slowed down as it approached and passed over Missoula. It then appears to have accelerated as it left the Missoula area.

It most probably was a military aircraft. It is interesting, though, that it was illuminating radars all the way and that there don't appear to be any significant military installations along the travel path. It is not uncommon for military aircraft to cause this kind of interference with nearby NEXRAD radars due to the electromagnetic energy transmitted by the plane (probably its onboard radar), but it is most often seen in specific areas such as central Nevada at Fallon airbase. It is also normally sporadic, appearing on some (but not all) imagery from nearby radars. This is due to the fact that it will only appear on the nearby radar(s) if the plane's radar happens to be aimed at the NEXRAD site at the moment the NEXRAD is doing a scan in that direction. This is normally not common except at sites like Fallon where you may have dozens of aircraft flying in many directions at the same time which increases the chances that at least one of them will illuminate the radar site. Any single aircraft normally has a relatively low probability of illuminating a NEXRAD site in a way that will create this kind of spoking interference--the aircraft's radar would have to be actively transmitting in the direction of the NEXRAD site at the same time the NEXRAD is "listening" in the direction of the aircraft.

This event was unique in that the object covered a large distance at a relatively high speed outside of restricted military airspace and the object was detected quite consistently by all affected NEXRAD sites. This suggests the electromagnetic energy emitted by the object was omnidirectional and probably constant which is not typical of radar. If the EM was not constant it would be unlikely that so many NEXRADs would be able to pick up the object. Likewise, if the EM was not omnidirectional it is unlikely that NEXRAD sites would pick up the object once it passed since the EM would then be aimed away from the NEXRAD site.

Conclusion

It appears that an object traveled at a relatively high rate of speed from northwestern Wyoming to north central Idaho. The object appears to have emitted constant omnidirectional electromagnetic energy at a frequency that was detectable by NEXRAD sites along the path. The fact that the EM energy appears to have been omnidirectional and constant is not consistent with typical aircraft radar.