CERTIFICATION PLAN  FOR THE LOWFLYING
AIRCRAFT MONITORING SYSTEM

synopsis:
An airplane's altitude can be easily, accurately, and economically calculated from the ground by using two cameras. This can be done by the optimum positioning of the cameras to reduce errors and by using valid simplification for calculations.

Overview:

The monitoring system uses a basic triangle to determine the altitude of an aircraft. Two cameras (camcorders) are placed on opposite sides of the airplane's approach to form the base of the triangle. The airplane creates the third point of the triangle. By knowing the elevation angle from each of the sites to the aircraft you can determine the height of the aircraft. Using a camera's picture to measure angles is well documented. By measuring the distance (inches) between two objects on a picture you can determine the angular (degrees) distance between the two objects. I will call the factor to convert from inches to degrees the "number of degrees per inches" factor.

The U.S. Naval Observatory's Almanac data for the sun elevation angle is used to calculate the "number of degrees per inch" factor for the monitoring system. By knowing the time and location of an observation of the sun, you can determine how many degrees the sun is above the horizon by using the Almanac's data. The "number of degrees per inch" factor is calculated by dividing the number of degrees the sun is above the horizon by the number of inches the sun is above the horizon on the TV screen. You may then determine the number of degrees any object is above the horizon by measuring its distance (in inches) above the horizon on the TV screen and then multiplying this distance by the "number of degrees per inch" factor.

An Example of Calculating the "Number of Degrees Per Inch" Factor:
The below example shows the calculations for a monitoring system with a 13" TV screen.

In Camarillo, CA on April 14, 2001 at 8:30:30 PST the sun is videotaped. It is near the top of the TV screen. The position of the sun on the TV screen is measured to be 8 9/16 inches (8.5625 inches) up from the artificial horizon mark that is near the bottom of the screen.

The sun's elevation (altitude) angle above the horizon for that time (8:30:30 PST) is looked up in the Almanac (see the table below). Since the table only has the sun's elevation angle for each minute and the observed time is halfway between 8:30:00 and 8:31:00, the average value 37.0 degrees ( (36.9+37.1)/2) is used.

Astronomical Applications Dept., U.S. Naval Observatory Almanac Data
CAMARILLO, CALIFORNIA W119 02, N34 14
(See http://riemann.usno.navy.mil/AA/)

The "number of degrees per inch"= 37.0 degrees/8.5626 inches= 4.321168 degrees/inch.

An Example of Finding the Altitude of an Airplane:
At observation site "A" (See Overview Diagram) an airplane on the TV screen is measured to be 4 inches above the artificial horizon. This airplane's angular distance above the horizon would be equal to 4 inches X 4.321168 degrees/inch=17.284 degrees.

At observation site "B" an airplane on the TV screen is measured to be 3 inches above the artificial horizon. This airplane's angular distance above the horizon would be equal to 3 inches X 4.321168 degrees/inch=12.964 degrees.

Observation sites "A" and "B" were determined to be 4000 feet apart from each other by using the GPS receiver to determine their locations and then calculating the distance between the two locations. The MSL altitude of the sites is determined to be 1000 feet by locating the sites position on the USGS quadrangle map which has the MSL altitude contour lines on it.

Aircraft Altitude AGL
= (Sin(17.284)*Sin(12.964)*4000 feet)/Sin(180 -17.284 -12.964)
= ( .297 * .224 * 4000 feet)/Sin (149.196)
= (266.112 feet)/(.512)
= 519.75 feet AGL

Aircraft altitude MSL
= Site's MSL Altitude + Aircraft's AGL altitude 
=1000 feet +519.75 feet
=1519.75 feet

The "number of degrees per inch" is an average value. It is used to make a straight line approximation of the actual degree value. You can check to see how accurate the straight line approximation "number of degrees per inch" is by measuring the sun's elevation angle and comparing it to the Naval Observatory's almanac data. I have compared the straight line approximation elevation angles for the camcorder used in this procedure against the sun's actual elevation angle. The results are summarized in the chart below.

The max error caused by using the straight line approximation method is 1/10 of a degree. If both observation sites had a +1/10 degree error to the aircraft in the above example the calculated altitude would 533.07 feet versus the 519.75 feet from the above example. The straight line approximation would induce a maximum error of +/- 13.32 feet in calculating the altitude of the aircraft.

Introduction:
The Low Flying Aircraft Monitoring System requires three sources of data to determine the altitude of an aircraft. They are the Mean Seal Level (MSL) altitudes of the monitoring sites, the distance between the two monitoring sites, and the elevation angles from the monitoring sites to the aircraft.

The MSL altitude of the monitoring sites is determined by using one of two different government sources. One source of altitude data is the United States Geological Survey (USGS) quadrangle maps. These USGS maps have the Mean Sea Level (MSL) elevation contour lines on it. If you know your GPS location you can determine your MSL elevation. The second source of MSL altitude data is the County Bureau of Horizontal measurements. Benchmarks throughout the county mark what the MSL elevation is at its location. Because both of these sources of MSL altitude data are accepted industry standards neither one will be verified in this procedure.

The location of the two monitoring sites is determined by using a Global Positioning Satellite (GPS) receiver. If you know the two locations you may calculate the distance between them. The Garmin 12XL GPS is specified to determine its position on the earth's surface within +/- 6-feet when its location averaging feature is used. This source of location data is an industry standard and it will not be verified in this procedure.

This leaves only the third source of input, elevation angle, data to be verified. A camcorder's videotape of the aircraft will be used to measure the elevation angle from the monitoring site to the aircraft. Using an optical lens to measure angles is a well established science. This certification procedure will verify how well this system's camcorder measures the elevation angle from the monitoring site to the aircraft. I'm proposing to test the camcorder angle measuring ability like you would test a sextant. I've run many sets of these tests already (See http://lowflying.com/charts.htm). The videotapes of these previous tests are available for review. This certification test plan describes the procedure on how to verify the camcorder's angle measuring ability.

Certification Test Procedures:
There will be two different parties used during this certification procedure. One is the certification test conductor (Al DeLorey) and the other is the certification observer (to be named). The certification will be done using the data recorded on the videotape. This videotape will be generated by the test conductor. The observer will review the data on the tape to certify the monitoring system's performance.

The time and place of the testing shall be agreed upon by both parties. The certification observer is welcome to witness all testing done to generate the certification videotape. The test conductor will supply the place and equipment required to do the testing and certification.

The certification observer shall receive the following items:

1. Videotape of the sun ascending with the camcorder's date/time stamp on the videotape

2. Hand notes taken during the testing

3. A generated (paper) angle measurement template for the TV screen

4. U.S. Naval Observatory's data for the specific time and location of the test

Definitions and Concepts:

Digitized Video Picture--
Each frame of the videotape can be made into a digitized picture by using a computer and a video capture card. The digitized picture is made up of individual dots called pixels. The pictures are saved away in 640X480 or 1500X1125 pixel format. There are many advantages of using a digitized picture to calculate the altitude of an airplane. There are no parallax viewing errors and the distances on the picture can be more accurately measured by using pixels as a measurement of distance. The disadvantage of using digitized video pictures is that it takes more time and equipment to digitize the videotape.

Field of View--
When you talk about a field of view you have to identify the device that you are using to do the viewing. What you view on the TV screen is less than what you view through the camcorder's viewfinder. What you view on the camcorder's viewfinder is less than what you see when you view the digitized (un-cropped) picture on a computer.

Artificial Horizon--
The U.S. Naval Observatory's sun's angular data is referenced from standing at the shoreline and viewing to the ocean's horizon. Since our observation to the sun is not over the ocean we have to create our own "artificial horizon" reference. This is done by setting up a front sight (front target) and a rear sight that are level with each other. The fact that we are not at sea level does not significantly affect the data's angular accuracy because the sun is so far (93,000,000 miles) away. The error caused by not being at sea level is less than 1 billionth of a degree.

Straight Line Approximation--
A straight-line approximation occurs when an average value is used to approximate a more complicated formula. The difference of using a straight-line approximation instead of the more complicated lens formula for this monitoring system was less than 1/10 of a degree.

Radial Distortion--
The camera's lens is designed to have different magnifications as you move out along a radius from the center of the lens. This is intentional and it is called "radial distortion." This distortion varies by the radius distance raised to the 3, 5, and 7 power. The camcorder's (Panasonic PV-D209) lens magnification is 3% greater near the edge of the lens when compared to the center of the lens. This means that if 1 degree is equal to 1 inch distance (on the picture) near the center of the lens then 1 degree is equal to 1.03 inches distance (on the picture) near the edge of the lens. (See gordian.com/users/jonboy/thesis/node34.html#1429)

Parallax Viewing Error--
A parallax viewing error occurs when your eye is not in the correct location when you are measuring something. An example of a parallax viewing error would be when you are going 50 MPH, and you view the speedometer on your car from the wrong angle. If you view the speedometer too far from the left then the observed speed will appear higher than it actually is, and if you view the speedometer too far from the right then the observed speed will appear to be lower than it actually is. When using the monitor's paper degree template on the TV screen your eye should be level with the object being read. This is important because the paper template is a fraction of an inch away from the actual image on the TV screen because of the thickness of the glass of the TV tube.

I can provide a videotape to be used as a training device to reduce parallax viewing error. You can practice by viewing the videotape on a TV screen and then overlaying the video template picture with the paper degree template. By moving your viewing angel up and down you will see how the observed value will change. The correct reading occurs when your eye is level with the object being read.

Considerations When Videotaping--
It is important to have the camera as level as possible. The bottom of the viewfinder's frame should be aligned with the rear sight's straight edge bubble level. (See lowflying.com/Photos/sun.jpg). Because the camcorder's lowest magnification has a slight fisheye effect the straight edge level will appear to bend up at the ends. When aligning the camcorder you will have to make both ends of the straight edge equal height with each other in reference to the bottom of the camcorder's viewfinder.

While videotaping the sun its location should be kept near to an imaginary vertical line that passes through the center of the lens. The "number of degree per inch" factor was calculated for this section of the lens.

Sometimes the automatic light contrast feature of the camcorder will make the camcorder too sensitive to the light of the sun when viewed through the welders shade. As strange as it might seem when videotaping the sun, the ambient light around the camcorder was sometimes too dark. This occurs because the #14 welders shade is dark, and the dark shade is mostly what is in the camcorder's field of view. The camcorder's enclosure (Seelowflying.com/images/Do%20it%20yourself%20640/temp_20%20640.jpg)   built to hold the welders shade always has the camcorder in a shadow. The camcorder's extra sensitivity will sometimes make the sun appear larger than it actually is. The sun should be about .53 degrees in diameter. To counter the dark ambient light effect I draped a thin white cloth around the camcorder's enclosure and put a white matting around the welder's shade to get more ambient light.

The certification videotape will be made by the test conductor (Al DeLorey):

1. The tape will be recorded on a VHS videotape in the 6-hour mode. It is quicker to find a specific time on a tape when it's recorded in the 6-hour mode.

2. The artificial horizon's front and rear aligned sights must be visible near the bottom of the TV screen. This will require a TV set to be connected to the camcorder while the camcorder is being aligned to the artificial horizon. There is a possibility that the front and rear sights that are visible on the camcorder's viewfinder would not show up on the TV screen. The could happen because TV's field of view is less than the camcorder's

3. The camcorder's date/time will be synchronized with the GPS receivers satellite transmitted time. The camcorder's date/time will be set to PST since the Almanac's time is in PST. A one-second accuracy is sufficient.

4. The front target sight will be 114.6 inches in front of the camera lens. This would mean that every inch on the target is ½ degree angular distance near the artificial horizon.

5. The sun's center will be used as the location of the sun. The U.S. Naval Observatory's angular data is to the center of the sun. The sun's diameter is about .53 degrees.

6.The camcorder's zoom lens will be set to the lowest magnification.

7. A paper printout of the U.S. Naval Observatory's data of the angular distance from the horizon to the sun will be provided for each minute of the test.

Data analysis of the videotape is to be done by the certification observer (To Be Named):

A pass/fail criteria of +/- 1 degree will be used for this certification. A 1 degree error would result in a 37.2 foot error when the aircraft is 2000 feet away. Since this system's intention is to identify airplanes flying 200 feet below the glide slope, +/- 1 degree is a reasonable criteria. Previous tests show that the system's error is close to .2 degrees.

The test conductor will set up the necessary equipment for the test observer to do the analysis. The test observer may request the test conductor to perform any or all of the following steps while the test observer monitors:

1. The videotape will be fast-forwarded until the sun's position is near the top of the TV screen. The distance in inches on the TV screen from the artificial horizon to the center of the sun will be measured and hand recorded. The videotape's date/time stamp on the screen will also be hand recorded.

2. The U.S. Naval Observatory's angular distance for the above observation's date and time for the sun will be hand recorded.

3. The straight-line approximation of degrees/inch will be calculated by dividing the hand recorded degrees by the hand recorded distance from the above hand recorded data.

4. The test conductor will make up a paper template with degrees on it using this calculated "number of degrees per inch" factor.

5. The tape will be rewound. Then the tape will be played forward until the sun is closest to each one of the nearest degree marks. The videotape's time stamp will be hand recorded for each one of these events.

6. The U.S. Naval Observatory's angular distance data to the sun will be hand recorded for the videotapes observation time. Since the Naval Observatory's data is available only for each minute and camcorder data is available for each second, the two nearest minutes of the Naval Observatory's data will be hand recorded.

7. An analysis will be made of the error between the paper degree template and the U.S. Naval Observatory's data.

Required Certification Test Equipment:
All test equipment to be provided and set up by Al DeLorey

1. Fairly sunny morning since no data can be gathered while the sun is blocked by clouds.
2. Naval Observatory Almanac data for the sun (http://aa.usno.navy.mil/)
3. Video tape recorder, Sharp model VC-H800
4. VHS Video Tape, 2-hour Standard Play
5. Camcorder, Panasonic model PV D209
6. AC to DC adapter for the Panasonic camcorder
7. TV, Sharp 13" model 13H-M100
8. 2 Tripods
9. Enclosure for camcorder with  # 14 welders shade, used to videotape the sun
10 3'X5" cloth cover for back of enclosure to block out reflected light
11. Set of spring loaded clips used to hold items onto the enclosure
12. Electronic water level. Zircon Electalevel (TM) and 25 feet of tubing
13. Straight edge level, Bubble Stick by Empire
14. GPS Receiver, Garmin 12XL
15. 120 volt A/C extension cord, 50 foot
16. Video cables to connect camcorder/VCR/TV
17.120 volt A/C power source, (Minimum of 100 watts)
18. 8.5"  X 11" horizontal line paper to help reduce parallax errors
19. 8.5" X 11" front "Artificial Horizon" target paper with 1 in. marking 
20. Roll of 2" wide duct tape
21. Roll of 1" wide blue masking tape  

This certification procedure will not be final until all responsible participants agree that this procedure obtains its objectives. There may be multiple reviews, tests and edits before it becomes final.