The flight is expected to take between 32 and 38 hours.  We look at weather forecasts for the north Atlantic and for Ireland, hoping to find a period where there will be a favorable net tailwind and reasonable landing conditions.  We all know that weather can be quite changeable, and often is different from forecasts.

So what if the plane arrives before dawn, because the tailwind is larger than expected?  What if there is fog, a low ceiling, or rain at the landing site and the airplane can't be seen in the sky at its cruising altitude?

This is where the short-range telemetry will help immensely.  We are fortunate to have some amateur radio operators in Ireland willing to help with the recording of telemetry data.  These hams, including Ronan Coyne and Tom Frawley, will help us at the landing site.  Using a Yagi directional antenna with a custom receiver we provide, they should be able to pick up signals from the plane perhaps 25 miles or more before it reaches Mannin Beach.  They should be able to hear the radio signals long before the plane can be seen, or its engine heard.

The hams will be able to view the data on a laptop computer fed by the telemetry receiver.  We're currently testing a program they will use to display three numbers which will tell observers on the ground exactly where to look in the sky for the airplane.  (We could have installed the "plane retriever" used successfully by Maynard Hill and his colleagues several years ago, but that would have added weight to the plane.)

Plane Finder

 When TAM telemetry is received on the ground, it provides data about the airplane’s position and altitude.  Since we know the location and altitude of the ground station, we can use a computer to tell ground observers where to look for the plane.

We compute three numbers:

Azimuth:            The direction (in degrees from true north) in which the ground observer should look for the plane.  This number will be about 275 as the plane approaches Mannin Beach.  270 degrees is due west.

Elevation           The angle (in degrees) above the horizon where the observer should look for the plane.  This number will be very small, becoming a few degrees at the plane rises above the horizon.  The elevation would be 90 degrees if the plane is directly overhead.

Distance:          The approximate distance (in km/meters) from the observer to the plane.

The distance at which the airplane the airplane can first be seen depends on the observer, weather conditions, lighting, and the attitude of the plane from the observer’s viewpoint.  Lighting, of course, depends on the relative positions of the sun, the plane, and the observer.  In our experience, an observer without binoculars can generally track the plane to a distance beyond a kilometer as long as one doesn’t lose sight of the plane.  At this distance, the plane may appear to vanish as it turns (the wing is seen end-on, making it harder to see), and it may be difficult to reacquire the plane until it gets closer.

POSTSCRIPT (Added Oct. 12, 2003)

This program was used at the landing on August 11, 2003, and worked perfectly.  The first data were acquired at 125346 UTC as the plane came in along a heading of 91 degrees, at a speed of 81 km/hr (50 mph).  At that time, the plane was 12.3 kilometers (7.6 miles) from the eventual landing site.

The main reason the program was written was concern that visibility would be poor if TAM arrived under adverse conditions (fog, rain, night).  Fortunately, the weather was excellent.  Despite the availability of the Azimuth, Elevation, and Distance data, observers on the ground did not actually spot the airplane until it was almost directly overhead, and it was the children at the landing area who alerted the adults to the presence of the plane!