# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Test flight with A-7 bubble sextant to investigate accuracy on Noonan's navigation on Earhart flight**

**From:**Gary LaPook

**Date:**2011 May 5, 02:54 -0700

(I am also attaching a PDF copy of this post to preserve formatting and special characters.)

Last Sunday, May 1, 2011, my wife and I and another pilot and his wife flew out to Catalina

Island for the buffalo burgers that they have at the airport restaurant. (There is a large herd of

buffalos living on Catalina and they are routinely turned into buffalo burgers.) .) We landed at the

“airport in the sky” which is 1600 feet above the sea and is on top of a mesa with drop offs at

each end of the 3,000 foot long runway. If you have seen the 1994 movie staring Diane Keaton

“Amelia Earhart: The Final Flight” then you have seen this airport since it was used as the stand-

in for the Lae airport.

Attached as Chart 1 is the aeronautical chart showing the course from the Camarillo airport to the

Catalina airport. The true course is 142̊ and the distance is 59 nautical miles.

Since I would have another pilot along to handle the controls, I decided to take this opportunity

to take some sun sights with my A-7 bubble sextant to investigate how difficult it would have

been for Fred Noonan to take similar sights as they approached Howland Island on July 2, 1937.

Because the random movements of the plane in flight makes the bubble (being used for the

establishment of a level reference point inside the sextant) move around, the measurement of the

height of the sun (or star) varies around the correct value. Because of this, a flight navigator must

take a series of sights and then average them to find an accurate height of the sun, measured in

degrees and minutes of arc (1/60th of a degree) and Noonan had to do this by hand. I have taken

other sights in flight with my A-7 sextant but I have used the averager incorporated in this

improved model of the Pioneer instrument being used by Noonan that didn’t have an averager.

For photos of this bubble sextant see:

https://sites.google.com/site/fredienoonan/topics/pionneer-octant

The way these bubble sextants are used is that you look though the eyepiece and you see the

circular bubble in the center of the field of view and you also see the sun in the field of view.

You then turn an adjustment knob that moves the altitude scale and makes the image of the sun

move up and down. You adjust the knob until the sun is level and next to the bubble and then

your read the altitude off the scale. This is a little more difficult than this sounds because the

bubble is in constant motion due to the random accelerations of the plane. The averager on the A-

7 is a simple device that holds a pencil. When you have brought the sun level with the bubble

you push a lever and the pencil makes a mark on the altitude adjustment knob. By lining the

pencil up with the mark on the adjustment knob you will automatically be lining the scale up

with the index showing the altitude of the sun at the time you pressed the lever. You take a series

of shots, pressing the lever each time you line the sun up with the bubble. After you have taken

enough shots you then look at the series of pencil marks on the knob, estimate the center

(median) of the marks and line up this center with the pencil and then read the scale which

represents the average, or median, of all the shots. Similar averagers were used on many WW 2

sextants including the A-10 and the A-12. Noonan, however, would have had to line up the sun

with the bubble, read the scale, write down the reading and then take another sight. To get a

reliable average you want to take about 10 shots so this can take some time. When using the

averager it takes me about two minutes to take ten shots. Using the sextant for manual averaging

my procedure was to line up the sun with the bubble, look at my watch and write down the

starting time of the series of shots. I then read the altitude off of the sextant scale and wrote it

down. I then took eight more shots, reading out the altitudes and writing them down but without

the need to note the time for these eight shots. On the tenth shot I used the same procedure as for

the first shot and the noted time became the ending time of the series of shots. I then averaged the

starting and the ending time to find the average time of the observations, and this was the normal

procedure for determining the time of the observation. I found out on this flight that it took me a

few seconds more or less than three minutes to take the ten shots without the use of the averager.

For information on the operation of the A-7 sextant averager see:

http://www.fer3.com/arc/m2.aspx?i=116307

Although you can take any number of shots, it makes it a lot easier to find the average if you take

exactly ten shots because it is much easier to divide the total of the readings by ten than by any

other number, just by moving the decimal point. Since we don’t normally deal with sexigesimal

numbers (base 60) we must use a special method to find the average of measurements taken in

degrees and minutes. To find the average you add up the minutes and degrees in separate

columns. Divide the sum of the degrees by the number of shots, simply moving the decimal point

if you have taken ten shots. You now have degrees and decimal degrees after the decimal point.

To convert the decimal degree part to minutes just remember that 0.1 degree is 6 minutes and

0.01 degree is 0.6 minutes. For example, say the total degrees of ten observations is 674 and the

total of the minutes is 162. Move the decimal point of 674 total giving you 67.4. then multiply

the “4" by 6 to convert to minutes making this part of the average 67̊ 24'. Next divide the sum

of the minutes, again just moving the decimal point if you have taken ten shots, and this gives

you 16.2' which you then add to the minutes found in the first step so the average then becomes

67̊ 24' plus 16.2' which equals 67̊ 40.2'. It takes about a minute and a half to average ten sextant

shots using this method. (If you were using a calculator you would have to convert all the

measurements to decimal degrees, add them up, divide by the number of shots and then convert

the result back to degrees and minutes format. It is easier doing it by hand. Noonan didn’t have a

calculator so he had no choice.)

When trying to find a destination using the “celestial landfall” procedure being used by Noonan

you don’t actually have to plot the lines of positions that you could derive from the sextant shots

as you would for normal position finding since you are only interested in how far you are from

the destination, which is the same distance that you are from the LOP running through the

destination. In the normal process of celestial navigation the navigator picks a nearby spot on

earth (which he calls the “assumed position” or A.P.) and by using various trig tables, computes

the altitude of the sun that he would have measured if he had actually been at the chosen A.P. He

then compares this “computed altitude” (Hc) with what he actually measured with his sextant. If

he happened to be at the A.P. then the measured altitude would be the same as the Hc. If it is

not, then he subtracts the smaller from the larger and the difference in minutes of arc equals the

number of nautical miles that he is away from the A.P. If he measured a higher altitude then he

knows he is closer to the sun than the A.P. is and if his measured altitude is lower he knows he is

farther away from the sun. Since the important piece of information to have is the Hc at the

destination, the normal procedure when accomplishing this type of “landfall” is to use the

destination as the A.P. for the computations. Since, as part of the computation process, the

navigator also computes the direction, or azimuth to the sun, the navigator then knows in which

direction he is located in relationship to the A.P. and he also knows how many miles he is off by

the difference between his measured altitude and the Hc. In Noonan’s case he was approaching

Howland Island from the southwest and he had computed that the azimuth of the sun, which

remained for about an hour after sunrise at Howland, as 67̊. So he would know if the altitude he

measured was less than what would have been measured at Howland at the same time of day that

he is till somewhere to the southwest of Howland. If he measured a higher altitude then he would

have known that they had overshot the island.

For more information about the landfall procedure see Flight Navigation (1938) written by

Noonan’s friend P.V.H Weems:

https://sites.google.com/site/fredienoonan/resources/weems/weems-394-395.JPG?attredirects=0

and

https://sites.google.com/site/fredienoonan/resources/weems/weems-396-397.JPG?attredirects=0

For other standard navigation manuals’ treatment of this process see:

https://sites.google.com/site/fredienoonan/topics/landfall-procedure

and:

https://sites.google.com/site/fredienoonan/discussions/navigation-to-howland-island

Using the trig tables that Noonan was using ( Navigation Tables for Mariners and Aviators, U.S.

government publication number H.O. 208, see:

https://sites.google.com/site/fredienoonan/resources/dreisonstock-h-o-208) for doing the

computations, it takes about four minutes to compute the altitude and azimuth for each sun

observation. This also requires concentration so it would be possible to make math errors,

especially late in the flight as they approached Howland after nineteen hours in flight. Because of

these considerations it was common practice, before takeoff, to estimate the time of arrival and to

do all the necessary computations on the ground before even getting into the aircraft. Once the

time of takeoff was determined the navigator or ground support personnel would do the

computations for the estimated time of arrival (ETA) and for a couple of hours before and after.

(Earhart sent a radiogram the day before they took off telling her husband that they would take

off at 9:30 a.m. on July 2, 1937. They actually took off at 10:00 a.m. Noonan had all the

information he needed to do all the computations the day before takeoff so would not have

needed to do them in flight. If he didn’t do the computations the day before he could have done

them during the early part of the flight before becoming fatigued. Doing the computations at the

same time for the entire period of possible arrival allows you to do them in production line

fashion, cutting the individual computation times about in half to about two minutes per data

point computation.)

Since, at the time of doing this precomputation process, the navigator would not know the exact

time that he would be taking his sextant shots, it would not be possible to do the computations

for those exact times. The navigator dealt with this problem by doing the computations at

intervals (usually every twenty minutes) and plotting the computed altitudes and azimuths on a

graph against time. Then, when the navigator had taken his sight, he simply looked at the graph

for the time of the shot to determine the Hc, instantly comparing it with the measured altitude,

and he then instantly knew how much farther he had to go to get to his destination or on to the

LOP that ran through the destination. No heavy computations needed to be done, just the

averaging of the shots and the subtraction of the measured altitude from the graphed Hc values.

For make more information available on the making these graphs of precomputed altitudes, I

have posted excerpts from four contemporary flight navigation manuals as well as a newspaper

article written by Noonan at:

https://sites.google.com/site/fredienoonan/topics/precomputed-altitude-curves

Based on my sextant shots on the way to Catalina on Sunday I now know that the observations

take between 2 minutes 47 seconds and 3 minutes 9 seconds and the manual averaging about

another minute and a half. Then comparing the average of the sextant shots with the graph takes

less than another minute so it would have been very easy for Noonan to take a sun sight every six

or seven minutes as he approached Howland to determine how much further they had to go and

to confirm that they were staying on the LOP once they had intercepted it.

Since I wanted to do the whole process on the flight to Catalina, I did the precomputations the

night before and drew the computed altitude curve on a graph that I took with me in the plane.

Attached as graph 1 is the small scale graph showing the altitudes and azimuths computed for the

Catalina airport for May 1, 2011 for the period of 10:30 a.m. to 12:00 noon. On the scale of this

overall graph, each division represents 12' of altitude change so would not allow me to read out

the computed altitudes to the necessary level of precision. I made this graph only to show the

overall process.

Attached as graph 2 is the actual graph that I prepared for taking on the flight. On this graph I

broke up the altitude curve into sections so that now the divisions represent only 2' of altitude so

it is easy to read out the computed altitudes to a precision of 1', equal to a one nautical mile

accuracy on the ground.

I had only enough time to take four observations on this short flight.

Time: 10:58:07 11:09:23 11:16:09 11:24:37

The computed altitude: 58̊ 46' 60̊ 47' 61̊ 57' 63̊ 19'

The measured altitude: 58̊ 02' 60̊ 10' 61̊ 38' 63̊ 09'

Dtance to destination: 44 NM 37 NM 19 NM 10 NM

Azimuth: 118̊ 123̊ 125̊ 128̊

Graph 3 shows the final graph with my sextant observations plotted on the graph and the

computed altitudes marked on the graphed altitude curves. The differences between the

computed altitudes and the plotted measured altitudes determines the distance remaining to the

airport on Catalina. Graph 4 is the same as graph 3 but with colored flags pointing out the

markings of the computed altitudes (red flags), measured altitudes (green flags.)

I also made a ruler so I could just measure the distances between the computed altitude and the

measured altitude so I didn’t even have to do the subtraction. On Graph 5 I have placed the ruler

(in pink) next to the plotted altitudes for the 10:58:07 observation. I lined up the “Hc” index with

the computed altitude and read out the distance to destination at the plotted observed altitude, 44

NM.

On chart 2 I have plotted the LOPs through the Catalina airport at times 10:58 and 11:24. During

this time the azimuth to the sun changed from 118̊ to 128̊, a change of 10̊ making the resulting

LOPs (that are at right angles to the azimuths) change from 028̊ /208̊ to 038̊ /218̊. The LOPs

for the two intermediate times fall between the two plotted LOPs.

Doing some dead reckoning, we can see that we moved toward the destination from 44 NM at

10:58 to 10 NM at 11:24, 34 NM in 26 minutes making our ground speed 78 knots. At this speed

we will cover the remaining 10 NM to the airport (or LOP) in 7 and a half more minutes making

the ETA 11:32. Looking at our graph we see that the azimuth of the sun at 11:32 is 131̊ so the

resulting LOP runs 041̊/221̊ and I have drawn this LOP on chart 3. To accomplish the landfall

procedure we do not actually need to plot these LOPs, we can just continue until our measured

altitude is equal to the computed altitude at the destination, look at the graph to find the azimuth

of the sun at that time and then turn 90̊ away from that azimuth which then puts us on the LOP.

We would know which way to turn because we would have aimed off to one side of the

destination.

Although not necessary for the landfall procedure, I have plotted the LOPs derived from my four

observations on chart 5 to look at the effect of normal observational errors on the calculated

ground speed. If you measure the distances between consecutive LOPs and divide by the time

periods you will find wildly varying ground speeds. This is the result of normal observational

inaccuracies, times determined only to the whole minute. and distances to the whole mile all

combined with very short time intervals. Using the spacing between the first and last LOPs, 34

NM and 26 minutes, produces a more accurate ground speed of 78 knots due to the longer base

line and time interval. The plotted spacing between the first and second LOP is 7 NM, the time

interval was 11 minutes so the calculated ground speed was only 38 knots. Doing the same

computation between the second and third LOP we determine a ground speed of 154 K, and

between the third and fourth LOP the ground speed was 90 K.

Since this is the third millennium, I could determine the actual errors in the sextant observations.

I had with me a GPS unit that records its position every five seconds. I downloaded my actual

GPS positions for the times of the sextant observations. I then did the normal calculations to

determine the altitudes that should have been measured at those positions and times. By

comparing what I should have measured with what I actually measured I determined the errors in

each of the observations. The first observation was 3' too high; the second, 2' too low; the third

observation, 6' too high and the fourth was 1' too high. These are pretty good results. Allowing

for these errors, which resulted in incorrect spacing of the LOPs, we can determine more accurate

ground speeds of 65 K, 86 K and 105 K with the overall ground speed being 83 K. Still some

variations due to measuring the spacing only to the whole mile and the time to the whole minute

and the spacing and time intervals are still short. Using the more accurate ground speed of 83 K

and the correct distance from the 11:24 observation of 11 NM would provide an ETA of 11:32,

the same as the less accurate ground speed previously determined.

So my conclusions are:

1. The pioneer type of sextant used by Noonan provided sufficient accuracy to find Howland

Island.

2. Taking 10 sights for hand averaging takes about 3 minutes.

3. Computing the average by hand takes less than 2 minutes.

4. Comparing the observed altitude with a precomputed altitude on a graph takes less than 1

minute.

5. That Noonan could have taken sights every 6 or 7 minutes to find and track the LOP through

Howland (assuming that he had prepared a precomputed altitude curve graph early in the flight or

the night before. This is a reasonable assumption as it was a published and well known

procedure.)

6. If Noonan had not precomputed the altitude curve then he would have been able to take a

sight, determine the average altitude and then calculate the computed altitude and distance to go

in about 10 to 12 minutes.

6. An interval of about 30 minutes allows finding a sufficiently accurate ground speed for

intercepting an LOP and completing a landfall.

----------------------------------------------------------------

NavList message boards and member settings: www.fer3.com/NavList

Members may optionally receive posts by email.

To cancel email delivery, send a message to NoMail[at]fer3.com

----------------------------------------------------------------