Estimates of signal strength
Signal Strength Quantifiers
Mike Everette, October 22, 2001, Forum.
As for reported signal strengths in terms like "S-5" this really does not have any relationship to the meter reading. In fact a signal reading high on the S-meter scale can still be covered up in strong noise.
The operator in those days used a scale of 1 to 5.
- S-1 means "signal very weak, in the noise, barely intelligible if readable at all."
- S-2 means "weak signal, still in noise, but readable, maybe with difficulty."
- S-3 means "signal stronger, noise, but readable."
- S-4 means "fairly strong signal, some noise, very readable."
- S-5 means "very strong, clear signal."
This is SUBJECTIVE. There is a LOT of room for interpretation. An operator's ears might tell him/her that a signal is S-1 if any appreciable amount of noise is heard. Or, that a signal is NOT S-5 if there is ANY NOISE AT ALL heard. It's a judgment call. Different operators hearing the same signal may disagree.
Estimates of signal strength on the final flight
In the report submitted by Air Corps Lt. Daniel Cooper on July 27, 1937, there is a summary of the strength of transmissions that the Itasca received from Earhart on the morning of July 2. All times are local.
|Time||Quality||S-code||Highlight of transmission||0345||very faint||S1||"Will listen on hour and half on 3105..."||0445||faint||S2||(not confirmed by log)||0600||fair||S3||"Two hundred miles out..." (actually rec'd at 06:15)||0646||good||S4||"About 100 miles out.."||0741||very loud||S5||"We must be on you but can not see you..."||0750||very loud||S5||"We are listening(?) but can not hear you..."||0758||very loud||S5||"We rec'd your signals but unable to get a minimum."||0843||very loud||S5||"We are on the line 157 337"|
"The radio operator reported that from 0741 on, her signal strength was at a maximum and judging from her volume, she was practically over Howland. All this seems to indicate that she passed close to Howland, probably within 50 miles" (Cooper).
This sounds like a steadily strengthening signal over a period of five hours.