|
Information from a spacecraft is transmitted using a technique
similar to a fax machine or modem. From this signal detailed
images of distant planets are formed. Spacecraft typically transmit
at a power equivalent to the light in a refrigerator (approx.
20 watts). Over millions of kilometres this signal decreases
in strength and by the time it reaches Earth, is one billionth
that of the signal that your television requires to form an image.
In order to be useable, this extremely weak signal needs to be
greatly amplified.
There are two main types of
amplifier used by the site, MASER and HEMT. The Microwave Amplification
by Stimulated Emission of Radiation (MASER) was the original
amplifier used by the DSN. The instrument operates at -269°C
using a synthetic ruby rod to minimise intrinsic noise. The High
Electron Mobility Transistor (HEMT), is used as an alternative
to the MASER, also operating at very low temperatures (-259°C)
to reduce intrinsic noise. Conventional amplifiers generate noise
which would be stronger than the signal being received and are
thus unsuited to deep space reception.
The output from the amplifier
is passed through a receiver and into a demodulator. This removes
the information from the carrier frequency and converts it to
a digital data stream. This stream is then passed onto a data
processor where it is formatted for immediate transmission to
JPL via optic fibre link.
Small errors in timing on board spacecraft can result in large
errors in position. To ensure a high level of accuracy, the complex
uses highly stable frequency and timing standards. All frequencies
and time codes used within the DSN are derived from the online
standard in the ‘frequency standards room’. This is
a rigidly controlled environment with regards to temperature
and magnetic fields.
The suite of standards comprises
two Hydrogen MASERs and two atomic Cesium beam standards. Only
one standard is online at any time, the other three are backups.
Three identical electronic
master clocks operate on a ‘triple redundant, majority voting
system’. The output of all three clocks is monitored continuously
and the ‘epoch time’ is derived from the two clocks
whose outputs are closest to each other. The Hydrogen MASER uses
the natural frequency of atomic Hydrogen (1.4 GHz) to produce
ultra stable output frequencies. The MASERs drift by less than
1 second in every 30 million years. |
