RESCUE 21 Sites in the Great Lakes

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
jimh
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RESCUE 21 Sites in the Great Lakes

Postby jimh » Sat Mar 12, 2016 1:29 pm

A new article on RESCUE 21 sites is now available in the REFERENCE section. See

http://continuouswave.com/whaler/refere ... tions.html

This article discusses the new Coast Guard 21st-Century radio system known as Rescue 21. It documents my visits to five local installations and shows detailed photographs of the antennas, towers, and facilities. Unfortunately, I was not able to gain access to the receiver shacks, so I can't show you the actual gear in use, but I have drawn some quite reasonable inferences about the equipment based on the antennas and feedlines.

I have also created (in collaboration with another radio enthusiast) a Google Earth mapping of almost all Rescue 21 sites in the Great Lakes of Superior, Michigan, Huron, and most of Erie.

jimh
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Re: RESCUE 21 Sites in the Great Lakes

Postby jimh » Sun Mar 13, 2016 1:03 pm

A corollary topic to RESCUE 21 facilities is the accuracy of the direction-finding antennas and receivers. The accuracy will depend on the range from the transmitter to the receiver. The bearing to a transmitter comes from an advanced design digital direction finding antenna and receiver system. The accuracy of the system is rated to be 1-degree-RMS. The farther out on the bearing line from the receiver site, the more a 1-degree error increases, so the possible distance in error varies with range.

The antenna is a digital direction finding antenna and the bearing to the transmitter is determined by digital processing of the signals from multiple antennas in the antenna array. The antennas are mechanically fixed and are steered electrically. The digital signal processors work in parallel. Also, the accuracy of the antenna bearing varies with the mounting position of the antenna on the tower. Not every site has its direction finding antenna mounting in the best accuracy position. Mounting the antenna array atop the tower is believed to give better performance than side-mounting on the tower, but due to use of leased tower space, many sites employ side-mounted DF antennas.

A position is determined by getting two bearings on the transmitter from two receiver sites. The location of the transmitter relative to the two receiving sites also affects the accuracy. If the two bearing lines cross at 90-degrees, the position accuracy is best.

If a transmitter is 20-miles from the RESCUE 21 receiving location, a 1-degree error could mean an error of position of the transmitter of 2 x sin(.5) x 5280 x 20 = 1,840-feet.

If the other RESCUE 21 receiving site is farther away, say 30-miles, their 1-degree error could mean an error of position of the transmitter of 2 x sin(.5) x 5280 x 30 = 2,765-feet

If the bearings cross at a right angle, they make a box of uncertainty with 508,659-ft-sq, or an area of about 0.18-sq-mile.

The minimum signal duration for a bearing is very short, on the order of milliseconds. Of course, this depends if the receiver was already tuned to the transmitter's frequency or if it was in scanning mode, and on the direction finding mode being used. (There are several modes available. The USCG practice is to leave the primary array always tuned to 156.800-MHz or Channel-16, the distress hailing channel.) Other factors affecting accuracy are the signal strength, the nature of the path, the propagation conditions, and so on. Receiver sensitivity is rated to be very good, so bearings can be taken on signals that are quite weak (such as a signal from a 1-Watt transmitter whose antenna elevation is 2-meters and whose range to the site is 20-miles ).

jimh
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Re: RESCUE 21 Sites in the Great Lakes

Postby jimh » Wed Apr 06, 2016 5:31 pm

I was reading the quoted text in my article about the distance of coastline covered by RESCUE 21, which says the sysem "is currently active along approximately 42,000 miles of coastline." I began to wonder if I had made an error in quoting that passage. I checked again and found there was no error. The original source says 42,000 miles of coastline is covered. This got me wondering how many miles of coastline there are in the USA. I found that according to NOAA there are 94,471 miles of coastline in the USA. NOAA comments about this:

As there is no reference that designates one specific shoreline as the "legal" shoreline, numbers for the length of the U.S. shoreline can vary depending on how the shoreline is defined.

The NOAA shoreline length calculation of 95,471 miles was determined by hand in 1939-40 with a recording instrument on the largest-scale charts and maps available at that time. Shorelines of outer coast, offshore islands, sounds, bays, rivers, and creeks were included to the head of the tidewater or to a point where tidal waters narrow to a width of 100 feet. For the Great Lakes, the shoreline lengths were measured in 1970 by the International Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data.

The total length of tidal shoreline includes measurements of the coastal states as well as the outlying U.S. territories and possessions.


Cf.: http://oceanservice.noaa.gov/facts/shorelength.html

jimh
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Re: RESCUE 21 Sites in the Great Lakes

Postby jimh » Mon May 02, 2016 4:26 pm

It will be interesting to compare the accuracy of a position fix determined by radio-direction-finding methods receiving a transmission from a vessel with the accuracy of a position fix that could be discovered from the digital selective calling distress alert call transmitted from a vessel. As we saw above, at a typical distance from the radio-direction-finding array, and with the typical performance of the array receivers, the radio-directing-finder fix could be about 2,000 to perhaps 3,000-feet in error when determining the position of the vessel that is transmitting.

In comparison, a vessel sending its position from a GNSS receiver via DSC could transmit a position fix that was within about 30-feet of its actual location, if the vessel is using a good GNSS receiver and a modern Class-D or better DSC radio. In this comparison, the DSC distress alert position is much more accurate than the radio-direction-finder position fix.

It should be noted that older DSC radios which were only qualified to the USA requirement of meeting RTCM SC-101 recommendations (which were very popular in the USA beginning in c.2001 and were still being sold as recently as 2011) would transmit a distress alert with a position that was truncated to whole minutes of latitude and longitude. A minute of latitude is equal to 6,080-feet and a minute of longitude is COS(latitude)×6080-feet. This suggests that a DSC distress alert from on older DSC radio is likely to not be as accurate as the radio-direction-finder fix from a RESCUE 21 station. This is a non-intuitive result. Most people think their GNSS or GPS receiver is very accurate, which it actually is, but they do not realize that an old DSC radio only transmits a truncated position fix that could be a mile off.

For more about the limitation of using an older DSC radio qualified only to RTCM SC-101 recommendation, see my article at

http://continuouswave.com/whaler/refere ... sTest.html

Of course, a further problem for DSC vessel position is the tendency for many of these older radios to not have been connected to a GNSS or GPS receiver, so they would not be transmitting any position. And, even worse, since the operational practice of the RESCUE 21 system is to not monitor the DSC channel, if a vessel makes only a DSC distress alert transmission without any position or with the RTCM SC-101 truncated position, the RESCUE 21 system will never have a chance to get a direction-finding fix on that transmission (since it wasn't tuned to that channel).

jimh
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Re: RESCUE 21 Sites in the Great Lakes

Postby jimh » Sun Jan 29, 2017 2:30 pm

I added details about RFF ARCADIA to the reference article.

In the Fall of 2016 I visited the RESCUE 21 site at RFF ARCADIA. It was a small detour on our drive that day, and I couldn't miss the opportunity to visit another site. There was nothing unusual about this installation with the one exception: it seemed to be the only tenant on this leased private-operator tower. Again, the main article on RESCUE 21 can be found at

http://continuouswave.com/whaler/reference/Rescue21Stations.html

jimh
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Re: RESCUE 21 Sites in the Great Lakes

Postby jimh » Mon Aug 07, 2017 10:12 am

I have updated the reference article on RESCUE 21 sites with some interesting information about the equipment used in the system.

jimh
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Re: a visit to RFF EMPIRE

Postby jimh » Sun Nov 05, 2017 5:17 pm

Last week we made a road trip to the RESCUE 21 site near Glen Arbor known as RFF EMPIRE. This USCG installation is located atop a big hill, about 3.7-miles inland from Lake Michigan. The hill has a very commanding view in all directions. There is a scenic lookout drive just on the west side of the highway that offers a spectacular panorama of the terrain with a line of sight of at least 50 miles to some distant hilltops--one of the best views in northwest Michigan you can find.

Unfortunately, we could not see any sign of the USCG tower from the highway. We did park the car and make an exploratory hike trying to find an access road up the big hill to the top. And we found the road, but several hundred yards up the hill we encountered a big gate with plenty of signage prohibiting entry and thus ended our exploration of RFF EMPIRE. I am sure the site is right up that access road, but it is a long walk up a big hill to reach it on foot, at least 0.6-mile, and I do tend to obey posted signs forbidding entry.

viewToLakeMichiganFromEMPIREsite.jpg
From the scenic turnout on the highway, we could get this view to the southwest and to Lake Michigan. The horizon line on the lake should be about 27-miles out.
viewToLakeMichiganFromEMPIREsite.jpg (58.47 KiB) Viewed 12395 times

In the view above, the scenic turnout is at elevation 1,100-feet-ASL. The lake level is about 580-feet-ASL, so we are about 520-feet above the lake. The visual horizon with height-of-eye of 520-feet should be more than 27-miles.

According to the FCC tower database the site of RFF EMPIRE tower is actually at a little lower elevation, about 1,030-feet at the tower base, but the tower adds 230-feet. Assuming the RDF array is at the top, the array would be 1,260-feet-ASL. Assuming a boater with a radio antenna at 6-feet high (or 586-feet-ASL), the radio horizon for that path would be about 40-miles. This is about double the system requirement of a 20-mile radio horizon.

As often seen, the USCG is leasing this tower space; the tower owner is the State of Michigan. I found the tower data in the FCC ASR database by using a location search with arguments 44-49-14.0 N 085-59-26.0 W.