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The anomalies at Billings were quickly apparent. They had the appearance of numerous "spokes" projecting out from the radar site, either to the south or to the northwest.

I named these anomalies the "Riverton" and the "Great Falls" anomalies based on the source of the interference as was eventually determined in the study.

CONSULTATION WITH NOAA RADAR OPERATOR

Upon observing the spokes first-hand at Billings, I submitted a question to the "Contact Us" address at NOAA. My inquiry was forwarded to the person responsible for the Billings NEXRAD site. I received a response within several hours and had a follow-up conversation over the course of several emails regarding additional questions I had on the topic (included below).

From: Billings MT NEXRAD Operator (???@noaa.gov)
To: letxa2000
Date: Fri, 23 Jan 2004 18:07:11 -0600
Subject: Spokes Question

Thanks for your question concerning the WSR-88D imagery from Billings, MT. We recently sent a team of engineers and technicians from our organization to Billings to determine the cause and resolution plan. This pattern is caused by interference from radars transmitting near the same frequency. We are taking action to change transmit frequencies in the area in order to avoid reoccurrence of the anomalous patterns.

Thank you for your interest in WSR-88D data and the quality of the data. Please contact me if you have additional questions.

From: Billings NEXRAD Operator (???@noaa.gov)
To: letxa2000
Date: Sat, 24 Jan 2004 11:28:15 -0600
Subject: Re: Spokes Question

The nearby WSR-88D systems are the source of the interference. We have a narrowband of frequencies assigned. The WSR-88D transmitted energy is spread over nearby frequencies, though the center of the transmitted energy is at the assigned frequency. The WSR- 88D receiver at a site is tuned to the frequency of the transmitter at its own site. Under certain atmospheric conditions and if the antennas of adjoining radars happen to be pointing at each other, it is possible for the sensitive receiver at one site to detect some of the energy from another radar even though they are tuned to different frequencies.

From: Billings NEXRAD Operator (???@noaa.gov)
To: letxa2000
Date: Sat, 24 Jan 2004 17:37:21 -0600
Subject: Re: Spokes Question

The Riverton and Great Falls WSR-88Ds are the sources of energy causing the interference at Billings.

I then asked him why, if the energy sources are Riverton and Great Falls, they don't appear as single "beams" coming from Riverton and Great Falls (as is the case in Reno NV with the San Juaquin station to the south, see below). He replied:

From: Billings NEXRAD Operator (???@noaa.gov)
To: letxa2000
Date: Mon, 26 Jan 2004 15:53:45 -0600
Subject: Re: Spokes Question Again

Multiple spokes occur because the beam from the other radars is very wide at long range and there are reflections of energy. The antennas of both radars are turning. This is a case of "ducting" where the radar beam is bent downward rather than propagating on a normal path of higher altitude. Not all interference is the same.

At this point I thanked him for his time and left it at that since I considered that he had adequately answered all my questions.

RIVERTON ANOMALY

The Riverton Anomaly presented itself in 30 (1.45%) of the images on 10 (71%) of the days. A full catalog of the Riverton Anomaly is available here. The composite image to the right is a combination of all the anomaly images copy/pasted into a single composite image which allows the angle of all the spokes in all the anomalies to be observed and measured in a single image. The huge "yellow mass" that fills a large portion of the composite is due to the fact that many of the images had widespread cloud coverage and the composite image combines all of them together.

Riverton Composite

If one analyzes the "spoking" at Billings, one finds that the spokes occur at the same radials (angles) each time. Any given spoke may or may not appear in a spoking event, but when they do, they appear consistently at the same angles. This is consistent with interference from Riverton or Great Falls reflecting off fixed targets--probably mountains. Since the mountains don't move, the angles hitting Billings are always the same, although the radials don't [i]always[/i] appear since the interference doesn't always necessarily bounce off those mountains with sufficient intensity given changing atmospheric conditions. Hence on some occasions only a few spokes will be observed while in other events a large set of spokes will appear.

Beyond simply accepting the comments of the Billings radar operator, I chose to look at a map of Montana and Wyoming to see if I could find the possible sources of refelction. Since the spokes always appeared in the same radials it seemed logical I could follow those radials until it hit some target tall enough to be a probable source of the spoke. This was, in fact, the case.

Using both a road map and, in some cases, an aeronautical map, I traced each radial out from Billings until I found what appeared to be a probable source for the reflection. In the list below, the number indicates the angle of the spoke in degrees (0=North, 90=East, 180=South, 270=West). This is followed by the probable source of the reflection, followed by the height of the mountain in question.

Spoking from Riverton WY (south of Billings):

298: Little Belt Mountains just north of Martinsdale, 8255'
279: Sacagawea Peak north of Bozeman in Bridger Range, 9665'
229: Colter Pass at border of Wyoming and Montana, 8000'
206: Heart Mountain
188-203: Francis Peak southwest of Cody, 13153'
175-184: Direct from Riverton
165-171: Guffy Peak northeast of Riverton 8046'
161: Mountain southwest of Barnum WY in Big Horn Mountains, 9109'
138-145: Dome Peak and other peaks in Big Horn Mountains southwest of Sheridan, 10828'
131: Pumpkin Buttes southeast of Buffalo WY 6216'
120-126: Black Hills SD, Inyan Kara Mountain approx 6368'

GREAT FALLS ANOMALY

The Great Falls Anomaly presented itself in 21 (1.01%) of the images on 8 (57%) of the days. A full catalog of the Great Falls Anomaly is available here. The composite image to the right is a combination of all the anomaly images copy/pasted into a single composite image which allows the angle of all the spokes in all the anomalies to be observed and measured in a single image.

Great Falls Composite

In addition to the Riverton Anomaly, another set of anomalies was determined to be originating from the Great Falls NEXRAD site. These anomalies were observed to have spokes at the following radials:

0-4: Hills south of Ft. Belknap Indian Reserve near Zortman MT, 5760'
349: Mountain near Lloyd MT, 6000'
337-344: Porphry Peak, 6069'
332-336: Big Snowy Mountains just south of Lewiston, 8720'
321-328: West side of Big Snowy mountains just east of Garneill, 8213'
305-311: Big Baldy Moutain southeast of Monarch, 9175'
297-301: Little Belt Mountains just north of Martinsdale, 8255'
285: Conical Peak in Crazy Mountains north of Livingston, 10,731'
274: Sacagawea Peak north of Bozeman in Bridger Range, 9665'
260: Mountain south of Big Timber, 9316'
238: Granite Peak, Highest point in Montana, 12,799'
214: Beartooth Pass just inside of Wyoming, 10,947'
208: Heart Mountain WY north of Cody WY, 8123'

CONCLUSION

In addition to the above, it is worth noting that not once did all of the above spokes appear at the same time. It was either a set of "Great Falls spokes" or a set of "Riverton spokes." That is to say, you never see a spoke directly to the north (0-4 degrees) at the same time you see a spoke to the southeast from the Black Hills (120-126 degrees). That would only happen if there was simultaneous interference from both Great Falls and Riverton which is unlikely and, in fact, was never observed.

However, considering that the Riverton anomaly occurs in 1.45% of the images and the Great Falls amomly occurs in 1.01% of the images, it can be calculated that a case of both occuring simultaneously in the same image should occur approximately 0.0145 * 0.0101 = 0.015% of the time, or one out of every 6828 images. Considering there are about 148 images per day it would not be surprising to see a single image contain both anomalies once every 46 days (6828 / 148 = 48).

Further, it should be noticed that the above spokes all come in the direction of mountains. There were never any spokes observed to the northeast since there are no mountains in that direction for Great Falls or Riverton to reflect off of. Additionally, it is not impossible that spokes would be observed at slightly different radials than those mentioned above since the mountains cited do not stand by themselves but exist in mountain ranges. What's important to note is that every single spoke ocurred in the direction of a mountain and no spoke ocurred in directions where there are no mountains.

All anomalies observed at the Billings NEXRAD site were consistent with known interference from neighboring NEXRAD sites and the angles of the interference were consistent with reflections off of tall mountains in Montana and Wyoming.