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REFRACTING SUBMARINE RADIO SIGNALS OFF THE IONOSPHERE

by Naval Radioman Donald Courcy

Half a century ago.... in the 1960s

 

Back in my days in the Navy, half a century ago, there were no satellite communications available to us. Submarines were at the mercy of the ionosphere for ship-to-shore and long distance radio communications. Sky waves generated on the HF radio band were either refracted by the ionosphere and bounced back to earth, or they went through the ionosphere and were lost in outer space. Bouncing radio signals off the ionosphere was the only way to communicate with far away shore stations.

 

It's hard to imagine that solar activities on the surface of the sun, 150 million kilometres away from the earth, had such an impact on long distance radio communications from submarines. Solar activities affected the layers of the ionosphere so different frequencies were refracted at different times of the day, and each day was different. The challenge was to pick the right frequencies at the right time, an easier job if you are onboard a surface ship but not so easy of a job if you are onboard a submarine which spends most of the time underwater without access to the HF band.

 

The Radioman on a submarine had to guess the conditions of the ionosphere before going up to periscope depth. He knew he was right or wrong only when the radio mast was raised above the surface. If he was wrong, delays occurred while he looked for other freqencies to be used. While this was happening, the submarine was vulnerable to detection because the radio mast and at least one periscope were raised above the surface.

 

It was not a problem when the submarine came up up to periscope depth for other operational reasons or for snorkeling. During those times, the Radioman did not have to rush. But if copying a submarine schedule and/or sending a check report were the only reasons to go up to periscope depth, there was no time to be wasted.

 

The subject of this article is to talk about the challenges of using the ionosphere for long distance communication from a submarine's perspective. However, let's first review the electromagnetic spectrum and see which radio bands were used by the Oberon submarines in the 1960s.

 

ELF - Extremely Low Frequencies - 3 Hz to 300 Hz

Not used by Canadian submarines in the 1960s

 

ELF radio waves can travel thousands of kilometers around the earth and can penetrate water to a depth of a few hundred meters. ELF transmit antennas are extremely large because the wavelength itself measures almost 10,000 kilometers !!! The smallest ELF transmit antenna that I know measures 54 kilometers in length and is not very efficient because of its "small" size which is a compromise. For obvious reasons, ELF transmit antennas cannot be installed on ships and submarines. It is however possible to install ELF receive antennas onboard so ELF reception is possible at sea.

 

Only a few characters can be sent per minute on ELF because the bandwidth is extremely narrow.  Although entire messages cannot be sent, ELF is very useful for submarines deep in the ocean. The band can be used to send  a few coded characters to submarines operating at depth of a few hundred meters. One example would be a short coded message of a few letters ordering the submarine to come up to periscope depth for priority traffic.

 

ELF was not used by Canadian submarines in the 1960s.

 

Voice Frequencies - 300 Hz to 3 kHz

Used by Canadian submarines in the 1960s

 

Voice frequencies or acoustic waves were used by sonar and by the Underwater Telephone also known as Gertrude.

 

VLF - Very Low Frequencies - 3 kHz to 30 kHz

Used by Canadian submarines in the 1960s

 

VLF radio frequencies were available to submarines for radio reception above the surface and below the surface down to a shallow depth of about 20 meters. VLF was limited to slow morse code or slow data because the bandwidth is very narrow. It is impossible for a submarine to transmit on VLF because the size of a VLF transmit antenna is too large to be fitted onboard ( more than one kilometre across !!! ). So VLF was only useful to receive short and concise messages and could not not be used for ship-to-shore communication. There were three VLF receiving antennas used by the Oberon submarines. The VLF loop inside the fin; the buoyant disposible VLF wire which could be floated to the surface; and the VLF retractable buoy which could be deployed above the maximum VLF reception depth while the submarine remained below the maximum VLF reception depth.

 

LF - Low Frequencies - 30 kHz to 300 kHz

Used by Canadian submarines in the 1960s

 

LF radio frequencies were available to submarines for radio reception at periscope depth or on the surface when the main radio mast was raised. LF was the lowest band which can was used to receive the naval broadcast. LF was used to receive the naval broadcast when the submarine was within a few hundred miles of the shore station. LF radio signals are propagated mostly on ground waves following the curb of the earth so the strength of the signals was dependent on the distance from the shore station to the submarine, and on the level of the transmitter radiated power. The main radio mast was required to receive on LF.

 

MF - Medium Frequencies - 300 kHz to 3 MHz

Used by Canadian submarines in the 1960s

 

MF radio frequencies were available to submarines for radio transmission and reception at periscope depth or on the surface when the main radio mast was raised. MF radio waves are propagated on ground waves and direct waves. As the upper part of the MF band approaches the HF band, some sky waves begin to form under certain conditions. The MF band was used mostly for short distance ship-to-ship communications while at sea when the VHF and UHF transmitters were busy and not available. The main radio mast was required to receive and transmit on MF. When on the surface or in port, a portable whip on the bridge could also be used for MF communication. As an example, communication with port authorities was done on MF when entering a port (2182 kHz).

 

HF - High Frequencies - 3 MHz to 30 MHz

Used by Canadian submarines in the 1960s

 

The HF radio band generates sky waves which can be refracted by the ionosphere to provide long distance and worldwide communications. It will be covered further down this page.

 

VHF - Very High Frequencies - 30 MHz to 300 MHz

&

UHF - Ultra High Frequencies - 300 MHz to 3 GHz

Used by Canadian submarines in the 1960s

 

The VHF and UHF radio bands were used for line-of-sight communications because the radio waves at those frequencies travelled mostly in a straight line and eventually end up in outer space without being refracted by the ionosphere. Because the earth is round, those radio waves are used only for short distance communication between ships, submarines and aircrafts at sea as well as with shore stations within line of sight. VHF and the lower part of UHF were used mostly for voice communications. The upper part of the UHF band, above 1 Ghz, was used mostly for radar. There were two VHF and UHF antennas onboard for voice communication. One antenna on top of the main radio mast and one antenna on top of the ECM mast. Either of these masts had to be raised above the surface to communicate on VHF or UHF.

 

SHF - Super High Frequencies - 3 GHz to 30 GHz

Used by Canadian submarines in the 1960s

 

The SHF band was mostly used for radar and includes the C, X and S radar bands. In the 1960s, the main radar on the "O" submarines operated in the X band. Reception was also possible in the other radar bands for Electronic Counter Measures (ECM) through the ECM mast. These microwaves have other uses today, including satellite radio communications but we didn't have that luxury back in the 1960s.

 

EHF - Extra High Frequencies - 30 GHz to 300 GHz

Not used by Canadian submarines in the 1960s

 

The EHF band is at the top edge of radio waves, approaching the infrared portion of the electromagnetic spectrum. Compared to lower bands, EHF radio waves have high atmospheric attenuation; they are absorbed by the gases in the atmosphere. Therefore, they have a short range and can only be used for communication over about a kilometer. The EHF band is commonly used in radio astronomy and remote sensing. I do not know if EHF is used by today's submarines but it was not used back in the 1960s.

 

 

HF - High Frequencies - 3 MHz to 30 MHz

Used by Canadian submarines in the 1960s

 

Back to the sky waves of the HF radio band and the ionosphere.

 

In the 1960s, the HF radio band was the primary band for ship-to-shore and worldwide radio communication because it generated sky waves which could be refracted by the ionosphere. This refraction made it possible for the signals to skip many times between the ionosphere and the earth as it traveled around the earth. If the ionospheric conditions were good, a signal could travel more than half way around the earth with only a few watts of radiated power. The sky waves however were at the mercy of the constantly changing ionospheric conditions and did not cover all of the earth all of the time. Even when a wave was refracted, there were many blank spots or skip zones where reception was degraded or not possible. The challenge was to find the optimal usable frequencies at a specific time of the day to be refracted and bent by the ionosphere in order to hit a desired  location on the earth.

 

Another factor affecting the ionosphere was solar activity and the number of sunspots on the surface of the sun. Sunspots are correlated with solar activity and solar activity affects the layers of the ionosphere. So how the ionosphere refracts and bends radio signals is not the same from year to year.

 

Solar activity varies on a 11 year cycle. Charged particles from the sun streaming by Earth affect the ability of the ionosphere to refract radio signals back to earth. Although radio did not yet exist in the 18th century, measurement of solar activity began in 1755 with Cycle 1. Fast forward to my days in the Navy. Cycle 20 lasted 11.4 years. It began in October 1964..... six months before I joined the Navy and ended in March 1976..... 3 years after I left the Navy. The month of highest solar activity during Cycle 20 was November 1968. Coincidentally, this is the month when I reported onboard HMCS Onondaga after graduating from Submarine Basic Training.

 

From the time when I reported onboard my first submarine in 1968  to the time when I left the Navy in 1973, solar activity was on the decline, affecting the density of the ionospheric layers from year to year.

 

HF rado propagation around the world is affected by radiation from the sun which affects the layers of the ionosphere, so different frequencies work better at different times of the day. The sky waves are either refracted by the ionosphere or go through the ionosphere to outer space

 

Sky waves are affected by the daily changes of the layers in the ionosphere; the disappearing of the D layer at night, the F layer becoming the F1 and F2 layers during the day and the E layer becoming weaker at night.

 

In the illustration above, the green lines indicates radio signals transmitted on different frequencies from a shore station to ships and aircraft at sea. Some signals are refracted by the ionosphere but at different angles based on the frequencies used. One signal can be received by the aircraft on the left and three signals can be received by the aircraft on the right. Two signals from the shore station also go through the ionosphere and are lost in outer space.

 

The red lines indicate three radio signals transmitted back to the shore station from the aircraft on the right.  The three signals are on different frequencies so they separate after hitting the ionosphere because they are refracted at different angles. As a result, the three signals which originated from the same location are received at different locations on shore.

 

The ship is located in the skip zone of all those frequencies and will not hear any of the signals from the shore station. For the ship to communicate with the shore station, he will have to find usable frequencies up and down the HF band that will make it possible for the signals to be received at the desired shore station for that time of the day.  A good way to do this is to monitor the multiple HF frequencies of the naval broadcast originating from the shore station. If reception of the naval broadcast is good on a certain frequency, it is likely that ship-to-shore frequencies will also work well in reverse in that part of the HF band.

 

So refracting radio signals off the ionosphere was quite a challenge, especially when time was of the essence because the Captain wanted to return below periscope depth as quickly as possible. It was sometimes unnerving for the Radioman when delays occur in sending priority radio traffic to the shore station while the Captain was breathing down his neck. If the ionospheric conditions were causing delays, the Radioman felt responsible for causing the submarine to remain longer at periscope depth, increasing the chance to be detected by the enemy.

 

Surface ships were able to track the conditions of the ionosphere at all times and in real time because they were able to listen to the entire HF band without disruption. They used multiple receivers tuned to the naval broadcast on 4, 6, 8, 12, 16, 22 and 25 MHz. Frequency discriminators constantly measured the conditions and selected the receiver with the best signal. As ionospheric conditions changed throughout the day, the frequency discriminators automatically switched between receivers to maintain the best signal for copying the naval broadcast. When it was time for ship-to-shore communications, the Radioman on a surface ship knew right away which part of the HF band to use.

 

This method of monitoring the conditions of the ionosphere was impossible for submarines below. Before going up to periscope depth, books of ionospheric predictions had to be used to determine as best as possible which part of the HF band to use at a specific time to reach CFH on Canada's east coast or CKN on Canada's west coast. If a Check Report or priority traffic needed to be sent, or if the submarine schedule had to be copied, there was very little time to assess the condition of the ionosphere once the radio mast was raised above the surface. Receivers and transmitters had to be tuned in advance to the right frequencies, based on predictions, so the radio call could begin immediately as soon as the radio mast was raised.

 

There was only one problem.  Ionospheric predictions were not always accurate, especially in the middle of the ocean away from Canada where the ionosphere had been measured to create the predictions. When it became obvious that you were using the wrong part of the HF band, there was no time to waste. The Radioman had to use his past experience and intuition to quickly find a usable frequency before everyone onboard thought he was a complete clut.

 

When making the initial call, the Radioman crossed his finger that an immediate response would be heard. Time was of the essence at periscope depth and it was important to be on the right frequency for a speedy transmission and acknowledgment. If nothing was heard on the first call, adrelanine started to kick in. Sometimes, the Captain was breathing down the Radioman's neck. What should the Radioman do ? Remain in the same section of the HF band and try the naval station on Canada's other coast. Try a NATO station or a Commonwealth station like Gibraltar in the same section of the HF band; or continue the attempts to establish contact with the naval station originally selected by going up or down the HF band. Frequencies were available in the 4 MHz, 6 MHz, 8 MHz, 12 MHz, 16 MHz, 22 MHz and 25 MHz sections of the HF band. When closer to Canada, frequencies were also available in the LF and MF bands. VLF was out because it was only available for reception and not for transmission.

 

Sending the Check Report and copying the submarine schedule on the naval broadcast are two examples of when time was of the essence when dealing with the ionosphere. At other times, radio traffic was handled during the long periods when the submarine was at periscope depth to snorkel and recharge the batteries. During those times, or when on the surface, Radiomen had all the time in the world to deal with the ionosphere. There was no hurry. Submarines were in fact operating like surface ships when it comes to dealing with the ionosphere. Snorkeling periods normally lasted long enough for Radiomen to track the conditions of the ionosphere in real time and to be ready for ship-to-shore traffic.

 

Those were the days....

 

Donald Courcy

24 May 2017

 

 

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