The tracking of Sputnik

Sputnik 1 was famous for its 'bleep', produced by a powerful radio transmitter operating at 20.005 MHz and 40.002 MHz, and powered by chemical batteries. Indeed this was virtually the only instrumentation of Sputnik 1, and very effective it was in announcing the new era of space flight.

These strong signals gave radio scientists the chance to take the lead in tracking the satellite. The first and most obvious technique is just to listen in and measure the change in frequency due to the Doppler effect as the satellite crosses the sky. When a satellite rises above the horizon, the signals come in at a frequency about 1 kHz higher than the standard, then there is a rapid decrease to the standard as the satellite passes closest, and a further decrease to 1 kHz below the standard as it recedes into the distance. It is like standing on a railway station as a whistling train passes through.

Recording the variation of frequency with time, as shown in Fig. 2.2, easily tells you two things. You can find the time of closest approach to the receiving station, when the frequency is at the standard value; and you can also estimate the satellite's distance of closest approach as it passes by - the closer the approach, the more rapid the change in frequency. By making such measurements at two or more stations at least 100 km apart, you can determine the time, height and track of the satellite. For Sputnik 1 the results were not expected to be very accurate, because the radio signals were of a frequency low enough to suffer appreciable distortion by the ionosphere, so much so that the simple picture of Fig. 2.2 was rarely recorded. Much more accurate Doppler tracking is possible at higher frequencies and later formed the basis of the US Navy's Navigation Satellite System.

A second method of radio tracking is the interferometer, a rather long name for a quite simple arrangement. Basically, you need to have two aerials at the same height above the ground, separated by a known

Fig. 2.2. The change in the frequency of radio signals when a satellite passes, as a result of the Doppler effect. The nearer the satellite passes, the more quickly the change occurs: so, from the observed values (the line of crosses), the passing distance can be estimated (here 160 miles). Reproduced from Satellites and Scientific Research (1960).

Fig. 2.2. The change in the frequency of radio signals when a satellite passes, as a result of the Doppler effect. The nearer the satellite passes, the more quickly the change occurs: so, from the observed values (the line of crosses), the passing distance can be estimated (here 160 miles). Reproduced from Satellites and Scientific Research (1960).

two aerials is equivalent to 2 wavelengths (left), the satellite is at an elevation of 60°. When it is 1 wavelength (right), the elevation is 75° (or 75.52°, to be precise).

distance, perhaps 4 wavelengths as in Fig. 2.3. Then the signals from the aerials are brought together and compared. Whenever the signals are the same, 'in phase', the distance of the satellite from the two aerials must differ by an exact number of wavelengths. If the difference is 2 wavelengths, as on the left in Fig. 2.3, the satellite is at an elevation of 60°; if it is 1 wavelength, the angle is near 15°. As the satellite passes across the sky it produces a series of peaks and zeros which give a record of the elevation angle in the plane of the aerials - assume they are on an east-west line. A second pair of aerials on a north-south line gives the elevation angle in the perpendicular plane. These 'direction cosines' in two planes fully define the direction of the satellite. In practice, the zero signals give more accurate results than the peaks, but there is no need to go into further detail. In essence, the radio interferometer tells you the direction of the satellite at each moment as it goes across the sky.

In 1957 the RAE had a large Radio Department, where many of the scientists were eager to be involved in measuring the signals from Sputnik 1. To start with, they wanted to see if they could track it, but for them it was also a heaven-sent beacon for probing the ionosphere. They were keen to establish its orbit so that they would know where their new beacon was at any time. Within a few days of the launch the Radio Department set up an interferometer at Lasham airfield, a quiet site 12 miles southwest of Farnborough. The signals used were those at 40 MHz (7.5 m wavelength). There were two pairs of dipole aerials at right angles, the aerials in each pair being about 30 m apart, or 4 wavelengths. This hastily-constructed instrument worked very well and often detected the satellite on transits when it rose only a few degrees above the horizon. On some nights up to eight transits were recorded. The angular accuracy was estimated to be about 0.1°. A rather similar interferometer was constructed by scientists at the Mullard Radio Astronomy Observatory at Cambridge.

Doppler measurements were also made by the RAE and many other organizations, and gave further information on the height and track of Sputnik 1.

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