posted 02-09-2013 09:11 AM ET (US)        

http://www.navcen.uscg.gov/?pageName=mtNds

posted 02-09-2013 02:45 PM ET (US)            

Rescue 21: Western Rivers

Along the Mississippi River and the Ohio River Valley, the Coast Guard will deploy a modified version of the Rescue 21 system, to provide a more cost-effective and realistic solution tailored for the unique operating environment along these busy inland waterways. Due to the geographic and operational nature of the western rivers region, the modified system design will not include direction-finding or digital selective calling. The modified system comprises new remote radio control console systems and new VHF FM radios for the three sectors of the western rivers region.

Why this project?

Rescue 21 replaces the National Distress and Response System (NDRS), which has been in use since the 1970s. Rescue 21 Western Rivers will enable the Coast Guard to continuously monitor digital selective calling and more accurately identify the location of callers in distress.

How is the project implemented?

The Coast Guard will implement Rescue 21 in the western rivers region in-house. For Sectors Upper and Lower Mississippi River, and Sector Ohio River Valley, the Coast Guard will use a modified version of Rescue 21 to recapitalize legacy NDRS equipment. The process of recapitalization will include 49 existing sites along these heartland waterways where the Coast Guard will replace legacy VHF radios and antennas, as well as adding a second VHF channel capability to allow two simultaneous calls.

posted 02-10-2013 09:22 AM ET (US)            

The general behavior of path loss as a function of distance is characterized by a term in the form of

n x log(d)

where n is some coefficient and d is the distance. For free space the relationship is usually described as

loss = 20 x log(d)

If we increase power in a circuit, we add gain. This means we can tolerate more path loss. In the case above, going to 25-watts from 1-watt adds 14-dB of gain to the radio circuit. We could estimate the increase in distance by solving the above relationship

14 = 20 x log(d)

d = 10^(0.7) = 5

If the path were in free space, no intervening Earth or terrain, an increase of 14-dB gain in the circuit would permit the distance to increase by five.

Unfortunately, over real Earth and terrain, path loss increases at a faster rate with distance than in free space. There are many ways to model or predict path loss. A simple model is to assume that path loss behaves according to

loss = 40 x log(d)

If we use that approximation, we can find the increase in distance resulting from the addition of 14-dB of gain as

14 = 40 x log(d)

d = 10^(0.35) = 2.2

This is the estimate I mentioned earlier. Adding 14-dB to the circuit may produce an increase in range by a factor of about 2.

In the case above, the gain is only added in one direction, that is, the transmitter power of one station was increased to 25-watts from 1-watt. There was no increase in the gain in the circuit in the other direction. However, it is likely that the other station in the circuit was already transmitting at 25-watts, and the circuit was being limited by the lower transmitter power of the 1-watt station. We can assume that the path would remain a two-way path.

For more detail on path loss calculations, see my article

Estimating Path Loss on Marine Non-Line-of-Sight Paths
http://continuouswave.com/whaler/reference/pathLoss.html

The coverage predictions shown in the United States Coast Guard website on the RESCUE 21 system are noted as being based on the Longley-Rice model for prediction of propation over irregular terrain. For more on that model see

http://en.wikipedia.org/wiki/Longley–Rice_model

I am not familiar with the method, but it appears to be a standard method often used in predicting propagation at VHF.

There is an on-line calculator that uses the Longley-Rice model. It might be interesting to explore a few paths using this model to see what sort of coverage might occur. Of course, the details of the USCG RESCUE 21 site would have to be known. Details such as antenna gain, height of antenna, and accurate location of antenna would be necessary to have to use the calculator. See

posted 02-10-2013 09:49 AM ET (US)            

Your VHF radio is intended mainly for short range communications, generally 5-10 miles, and at least 20 miles to a USCG station. To communicate at longer ranges, you will normally need a satellite telephone or an MF/HF marine radiotelephone.

posted 02-10-2013 02:44 PM ET (US)            

http://www.ccg-gcc.gc.ca/folios/00027/images/gmdss-canada.gif
Sea Area 1 is denoted by "A1" and is only in the ocean and coastal areas, not in the Great Lakes.

See: http://www.ccg-gcc.gc.ca/eng/CCG/SAR_Gmdss

posted 02-10-2013 11:59 PM ET (US)            

quote:

---

Declaration of Sea Area A1.

With the approaching completion of the Rescue 21 Project, the Coast Guard plans to declare Sea Area A1 operational, probably by the end of the year [i.e., 2012]. Sea Area A1 is created when there is a continuous shore watch on the DSC Calling and Distress channel 70. Declaration of Sea Area A1 will not include Alaska but will include the Continental U.S. coasts, the Great Lakes, Guam, Hawaii, Puerto Rico, and the U.S. Virgin Islands. According to provisions of the FCC Rules, a declaration of Sea Area A1 will require that all vessels mandatorily equipped with VHF, upgrade to VHF-DSC within one year.

---

I have not been able to find any official notice that the USCG has declared a GMDSS Sea Area 1 to date. Note that it is anticipated the Great Lakes will be in the designated Sea Area 1. Also note that recreational vessels are voluntarily equipped and are not subject to the mandatory upgrade to VHF-DSC capability.

posted 04-24-2013 09:03 PM ET (US)            

It may be interesting to estimate the increase in the coverage that could be obtained if the vessel station were using more than 1-Watt of power. Let us assume that a vessel may use a standard 25-Watt radio, that the radio delivers an actual 20-Watts to the antenna, and that the antenna has 3-dB of gain. This would increase the transmitted signal from the vessel to 40-Watts from the 1-Watt signal of the RESCUE 21 coverage prediction. Now we can apply some reasonable adjustment in the coverage range based on the higher power of the vessel transmitted signal.

Compared to a 1-Watt signal, a 40-Watt signal will be an increase of 16-dB. With 16-dB great signal, we can extend the path a distance that represents a loss of 16-dB, and the received signal should be the same. The problem is determining what added path length can be achieved with an increase in signal of 16-dB.

The model for path loss in free space represent the least possible loss, or the greatest possible increase in path length that could occur. The free-space model calculates path loss in dB as a function of distance by the relationship

dB = 20 log (distance)

By rearranging this we can find the distance as a function of change in gain, according to

distance = 10^(dB/20)

Solving this for 16-dB we find the distance (ratio) is going to be 6.3. This means that if the RESCUE 21 has sufficient coverage for a 1-Watt signal at 20-miles, it has coverage for a 40-Watt signal to over 125-miles. This seems very optimistic, and is probably too optimistic. As I mentioned, the free-space model is the absolute best outcome for increase in distance.

A more conservative model of path loss calculates path loss in dB as a function of distance by the relationship

dB = 40 log (distance)

This is a model that is often used for predicting path loss over real terrain. If we use this model, we can calculate the added path length for a 16-dB increase in signal. This model predicts the coverage will increase to a range of 50-miles from 20-miles when the transmitted signal increased 16-dB. This seems much more likely to be obtained in the real world.

The RESCUE 21 coverage prediction is also rather stringent in its demand to be able to detect a 1-second transmission. In radio propagation there is often a continual change in atmospheric conditions, resulting in the signal level on a path varying up and down. The RESCUE 21 model appears to account for this by imposing the requirement that the signal be detected in any one-second interval. In a real world situation, a signal transmitted for a longer duration is more likely to be detected. I believe that this will also tend to increase the range of RESCUE 21.

In summary, the RESCUE 21 system seems to have been designed to provide at least 20-mile coverage offshore in circumstances that are rather minimal for the transmitting station: only 1-watt of power is used, the antenna is only 2-meters high, and the duration of the transmission is only one second. Even with those limitations, the coverage maps show RESCUE 21 often exceeds the 20-mile range of coverage in many sectors. If the transmitted signal is increased to 40-watts of power, the coverage should increase. With a 40-Watt signal, it appears that a range of 50-miles for RESCUE 21 may be possible.

The RESCUE 21 system coverage maps show that some sectors have extremely good coverage, and within the restrictions specified (1-Watt, 2-Meter, 1-Second signal) the coverage maps show a range of more than 20-miles. Below is a segment of RESCUE 21 stations on Lake Michigan. The station at Bucks Corner provides coverage to 45-miles, and the station at Fayette provides coverage to 42-miles.

posted 05-01-2013 08:50 AM ET (US)            

"Charts showing predicted areas of VHF coverage from shore can be downloaded in .jpg format. As indicated on the charts, coverage plots assume a mobile transmitter power of 1 watt with an antenna two meters above water level."

A further notation says:

posted 05-01-2013 09:34 AM ET (US)            

http://lrcov.crc.ca/main/index.php

I have calculated the coverage for a vertically polarized antenna of gain 5dB, transmitter power 25-watts, with an antenna height of 4-meters (about 15-feet) located in Lake Superior and Southeast of Isle Royale. The calculator generated the coverage map shown below. This one is for a receive antenna with only 1.5-meter height above ground.

The light blue area is a field stength of 10 to 30 dBuV/Meter, which is probably on the edge of communication quality signal. The blue area is 30 to 40 dBuV/Meter, and the dark blue greater than 40 dBuV/Meter, or very strong signals.

The model predicts coverage with good signals to the higher parts of the land on Isle Royale and the Keweenaw Peninsula. It also predicts some coverage beyond those areas due to the scattering of the signal on the higher land.

Here is the plot:

posted 07-02-2013 09:47 PM ET (US)            

http://www.ccg-gcc.gc.ca/folios/00026/docs/part-4ae-2008-eng.pdf

posted 07-03-2013 09:55 AM ET (US)            

http://continuouswave.com/whaler/reference/graphics/RadioCoverageMaps/ VHF_CoverageMapGreatLakes.jpg