SEA AREA 1 and Radio Range Calculation

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
jimh
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SEA AREA 1 and Radio Range Calculation

Postby jimh » Mon Jun 03, 2019 11:46 pm

I begin this topic by reprising an article I wrote earlier and posted in the old forum in February 2015, over four years ago:

On January 20, 2015 the Coast Guard of the USA formally declared its SOLAS (Safety of Life at Sea) compliant Sea Area A1 designation. According to SOLAS regulations, the definition of Sea Area A1 is:

Sea Area A1 means an area within the radiotelephone coverage of at least one VHF Coast Station in which continuous Digital Selective Calling (DSC) alerting is available, as may be defined by a Contracting Government.


The completion of the coastal portion of the RESCUE 21 project has enabled the USA to be able to comply with this SOLAS definition and declare a Sea Area A1. The defined Sea Area A1 has some exclusions:

The Coast Guard is declaring Sea Area A1 in certain areas off the coast of the United States based upon the performance of the Coast Guard's Rescue 21 System, and in accordance with applicable provisions of the International Convention for the Safety of Life at Sea, 1974 (SOLAS). The Coast Guard defines Sea Area A1 as those areas where more than ninety percent of the area within 20 nautical miles seaward of the territorial baseline along the East, West and Gulf Coasts of the United States, excluding Alaska, and including Hawaii, Puerto Rico, Guam, the Virgin Islands of the United States, and the Northern Mariana Islands of Saipan, Tinian, and Rota, is within coverage of Coast Guard very high frequency (VHF) Coast Stations that provide both a continuous watch for Digital Selective Calling (DSC) distress alerts on Channel 70 and a capability to respond to distress alerts.


Now I will expand a bit on this prior mention of Sea Area 1. First I will excerpt the definition without so many exemptions appended to it:

The Coast Guard defines Sea Area A1 as those areas where more than ninety percent of the area within 20 nautical miles seaward of the territorial baseline along the East, West and Gulf Coasts of the United States...is within coverage of Coast Guard very high frequency (VHF) Coast Stations....


The full announcement then comments:

Based upon the demonstrated coverage and performance of the Rescue 21 System, and upon the applicable requirements of SOLAS, the Coast Guard is declaring as Sea Area A1 those areas within 20 nautical miles seaward of the territorial baseline along the East, West, and Gulf coasts of the United States...


Complete details from the Federal Register are found at:

https://www.federalregister.gov/articles/2015/01/20/2015-00798/declaration-of-sea-area-a1

jimh
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Re: SEA AREA A1 and Radio Range Calculation

Postby jimh » Tue Jun 04, 2019 12:54 am

The basis for the radio coverage that must be provided in a declaration of SEA AREA A1 is defined by the International Maritime Organization in IMO resolution A801(19).

Here is the definition as cited in an IMO document:

Sea area A1 is that sea area which is within a circle of radius A nautical miles over which the radio
propagation path lies substantially over water. The radius A is equal to the transmission distance between a ship's VHF antenna at a height of 4 m above sea level and the antenna of the VHF coast station which lies at the centre of the circle.


The definition above relies on knowing a particular radius value, A. The formula to calculate radius A is provided:

Determination of radius A
The following formula should be used to calculate the range A in nautical miles:

A = 2.5 (√H (in metres) + √ h (in metres))

H is the height of the coast station VHF receiving antenna and h is the height of the ship's transmitting antenna which is assumed to be 4 m.

The formula given above applies to line-of-sight cases but is not considered adequate for cases where both [antennas] are at a low level.


The IMO also provides two pre-calculated values for radius A when H = 50-meters or H = 100-meters, with h=4-meters. Again, H is the shore staton antenna height and h is the ship station antenna height:

    For h = 4-meters
    If H=50-meters then A=23-nautical miles [22.67]
    If H=100-meters then A=30-nautical miles

Thus when the USCG declares that their SEA AREA A1 is that area in which 90-percent lies within 20-nautical miles of the coast of the USA, they must be figuring that radius A is 20-nautical miles.

I thought it would be interesting to modify the formula to find the radius M in statute mile from the heights, F and f, in feet, so I made the necessary unit conversions. Here is my formula based on the IMO version for different units:

    The following formula may be used to calculate the range M in statute miles:

    M = 1.588 (√F (in feet) + √ f (in feet)

    F is the height of the coast station VHF receiving antenna in feet and f is the height of the ship's transmitting antenna in feet.

We can then use this formula to predict radio range according to the IMO. For example a boat with antenna at height f=13.12-feet (i.e. 4-meters) and a shore station with antenna at height F=164-feet (i.e. 50-meters) the radius to seaward M will be

    M = 26.08 statute miles (i.e., 22.67 nautical miles or just what the original IMO formula predicted)

Although the IMO cautions this won't be entirely applicable for lower antennas heights, let's calculate a radius M using more typical values for H, h = 6-feet (i.e. two small boats}:

    M= 1.588 (√6 + √6 )
    M = 7.78 statute miles

This sounds like a reasonable range for two boats with each having their antenna at 6-feet above the sea.

jimh
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Re: SEA AREA 1 and Radio Range Calculation

Postby jimh » Tue Jun 04, 2019 9:18 am

Since the IMO document mentions the term "line of sight", I thought it would be interesting to compare their calculation of a radio range to a calculation for the distance to the optical horizon based on the height of the eye of the observer.

The IMO formula suggests that if there were only one height H in meters available, the distance to the horizon A in nautical miles then would be

    (1) A = 2.5 × √H

and the equivalent formula for distance to horizon M in statute miles for a height F in feet would then be

    (2) M = 1.588 × √F

Since the term "line of sight" was used, this could be considered an optical horizon. In another article ("Radio Horizon"), the distance d in statute miles to the optical horizon for observer at height h in feet was already discussed, and according to no less an authority than Nathanial Bowditch was found to be

    (3) d = 1.345 × √h

In that same article it was demonstrated that a factor of 4/3 is often applied to the earth radius to compensate for the higher refraction of radio waves, which produces a formula for the radio horizon to be

    (4) d = 1.414 × √h

Comparing formula (2) and (3) shows the IMO calculation for "line of sight" horizon is more generous than Bowditch's formula for the optical horizon, and this recognizes that radio waves will bend over the horizon more than light waves. Comparing formula (2) and (4) shows the IMO calculation is even more generous than the nominal radio horizon formula based on 4.3-Earth effect. This generous allowance provided in the IMO formula may account for their caution about using their formula to calculate radio ranges when values of height are low. As mentioned in a separate article ("VHF Radio Propagation Over Water"), the effect of antenna height is quite spectacular at decreasing path loss, and since the IMO formula is designed for antennas of significant height (12 to 100 meters), its more generous allowance for the effect of height is probably justified.