Astronomy Principles and Practice Fourth Edition.pdf

Observational radio astronomy 427
via the formula
cos φ = cos2 α max + (R 2 − cos 2 α max ) 1/2 sin α max
. (24.6)
R
It will also be appreciated that for a satellite to be on the observer’s meridian (i.e. with azimuth
A = 180 ◦ ), its longitude value must match that of the observer’s longitude.
Thus, from measurements of the azimuth and altitude of a number of satellites, it is possible to
determine the latitude and longitude of the observing station. In order to achieve this with reasonable
accuracy, the measurements of the satellite positions should be made to an accuracy of about ±1/2 ◦ .
Table 24.1 gives a list of some of the geostationary satellites presently in orbit according to their
longitudes about the Greenwich meridian, with a guide as to their relative signal strengths.
Table 24.1. Longitudes of geostationary communication satellites visible from Glasgow (λ s − ve is E, λ s + ve
is W).
Name Strength λ s Name Strength λ s
Eutelsat II f1 ** −48·0 Intelsat 707 * +1·0
Eutelsat Sesat W4 ** −36·0 Telecom 2B * +5·0
Astra 2A II f1 **** −28·2 Telecom 2A ** +8·0
Kopernikus 3 ** −23·5 Eutelsat IIf2 ** +12·5
Eutelsat II f3 ** −21·5 Telstar 12 *** +15·0
ASTRA **** −19·2 Intelsat 705 * +18·0
Eutelsat W2 ** −16·0 NSSK ** +21·5
Hot Bird *** −13·0 Intelsat 605 ** +27·5
Eutelsat II f4 ** −10·0 Hispasat ** +30·0
Eutelsat W3 ** −7·0 Intelsat 801 * +31·5
Sirius ** −5·0 Intelsat 601 * +34·5
Telecom 2C II f1 ** −3·0 Telstar 11 ** +37·5
Thor ** +0·8 Panamasat 3R ** +43·0
Having set up the simple radio telescope, commence by finding the brightest satellite in the sky—
ASTRA. It should be found at an azimuth of about 150 ◦ . Keep adjusting the sensitivity setting of the
power meter to read mid-scale and ‘home-in’ on the satellite position in both altitude and azimuth. It
should be possible to estimate the measured angles to about 0·2 ◦ . Note the two values.
Next, adjust the telescope to search for another satellite, Thor. This should be found at an azimuth
of about 150 ◦ . Again home-in on its position and record its altitude and azimuth.
The altitude of the celestial equator changes only slowly with azimuth at a station with high
latitude, so that the altitude of all the satellites should be fairly similar (within 10 ◦ ). Similarly, the
azimuthal angle between two satellites should be similar to their difference in longitude. Work your
way onto other satellites, noting down their positions. Try to find at least six satellites.
With the data, construct a graph of satellite altitude, a, against satellite azimuth, A. Draw a smooth
curve through the points and determine a value for α max corresponding to an azimuth of 180 ◦ . Estimate
how well the value has been determined. Together with the value for R as calculated in equation (24.5),
insert the value of α max in equation (24.6) to determine the latitude of the observing station. Make an
estimate for the uncertainty in the determination by trial adjustments of the value of α max according to
the estimate of its accuracy.
Plot also the satellite longitudes against the measured satellite azimuths. From this graph, estimate
the longitude a satellite would have if it were on the site’s meridian (i.e. if it had an azimuth of
180 ◦ ) and, hence, determine the longitude of the observing station. Again, make an estimate for the
uncertainty of this determination.