The first DSN complex was constructed near the Jet Propulsion Laboratory (JPL), in Goldstone, California. It was here that the first large (26 m) DSN antenna, DSS-11 “Pioneer” (named after, unsurprisingly, the early Pioneer program) was constructed.
The first overseas DSN location was in Woomera, Australia, about 350 km north of Adelaide. DSS-41, another 26 m antenna, was constructed there in 1960. The second overseas DSN complex was built near Johannesburg, South Africa. The 26 m antenna of DSS-26 was constructed there in 1961. Together, Goldstone, Woomera and Johannesburg made it possible to initiate and maintain continuous communication with distant spacecraft at any time of the day9, marking the beginning of the DSN.
Today, three locations – Goldstone, California; Madrid, Spain; and Canberra, Australia – form the backbone of the DSN. Presently, each of the three DSN complexes hosts several tracking stations with different capabilities and antennas of different sizes [152, 241].
The primary purpose of the DSN is to maintain two-way communication with distant spacecraft. The DSN can send, or uplink, command instructions and data, and it can receive, or downlink, engineering telemetry and scientific observations from instruments on board these spacecraft.
The DSN can also be used for precision radio science measurements, including measurements of a signal’s frequency and timing observations. These capabilities allow the DSN to perform, for instance, Doppler measurements of line-of-sight velocity, ranging measurements to suitably equipped spacecraft, occultation experiments when a spacecraft flies behind a planetary body, and planetary radar observations .
The antennas of the DSN are capable of bidirectional communication using a variety of frequencies in the L-band (1 – 2 GHz), S-band (2 – 4 GHz), X-band (8 – 12 GHz) and, more recently, K-band (12 – 40 GHz). Early spacecraft used the L-band for communication  but within a few years, S-band replaced L-band as the preferred frequency band. The Pioneer 10 and 11 spacecraft used S-band transmitters and receivers. X-band and, more recently, K-band is used on more modern spacecraft.
In addition to the permanent DSN complexes that presently exist at Goldstone, California, Madrid, Spain, and Canberra, Australia, and the now defunct complexes in Woomera, Australia and Johannesburg, South Africa, occasionally, non-DSN facilities (e.g., the Parkes radio observatory in Australia) were also utilized for communication and navigation. During the long lifetime of the Pioneer project, nearly all the large antennas of DSN tracking stations and also some non-DSN facilities participated in the tracking of the Pioneer 10 and 11 spacecraft at one time or another.
The capabilities of the DSN evolved over the years. By the time of the launch of Pioneer 10, the DSN was a mature network comprising a number of 26 m tracking stations at four locations around the globe, a new 64 m tracking station in operation at Goldstone, California, and two more 64 m tracking stations under construction in Australia and Spain. The Goldstone facility was connected to the then new Space Flight Operations Facility (SFOF) built at the JPL via a pair of 16.2 kbps communication links (Figure 3.1), allowing for the real-time monitoring of spacecraft telemetry .
The most important characteristics of a DSN tracking station can be described using parameters such as antenna size, antenna (mechanical) stability, receiver sensitivity, and oscillator stability. These characteristics determine the accuracy with which the DSN can perform radio science investigations and, in particular with respect to the Pioneer 10 and 11 Doppler frequency measurements.
Large, precision-steerable parabolic dish antennas are the most recognizable feature of a DSN tracking station. In 1972, several 26 m antennas were in existence at the Goldstone, Madrid, Johannesburg and Woomera facilities. Additionally, the 64 m antenna at Goldstone was already operational, while two 64 m antennas at Madrid and Woomera were under construction (Table 3.1).
|Station||Location||Size||Pioneer support function|
|DSS-11 “Pioneer”||Goldstone||26 m||Cruise|
|DSS-12 “Echo”||Goldstone||26 m||Cruise|
|DSS-14 “Mars”||Goldstone||64 m||Mission enhancement and|
|DSS-43*||“Ballima”||64 m||Mission enhancement and|
|DSS-51||Johannesburg||26 m||Launch and cruise|
|DSS-63*||Robledo||64 m||Mission enhancement and|
*After July 1973.
To appreciate the impressive performance of the DSN in support of the Pioneer 10 and 11 missions in deep space, one needs to be able to evaluate the factors that contribute to the sensitivity of an antenna. The maximum strength of a signal received by a tracking station or spacecraft is a function of antenna area and distance between the transmitting and receiving stations. The gain of a parabolic antenna of diameter at wavelength is calculated as. Later, with the installation of new receivers, for an antenna at 60° elevation, the system noise was reduced to 12.9 K . The receiver bandwidth in 1971 was 12 Hz, reduced to 3 Hz for the Block IV receivers and then eventually to as low as 0.1 Hz for the Block V receivers (details are in Section 3.1.3).
Antenna sensitivity is measured by its signal-to-noise ratio, relating the power of the received signal to the noise power of the receiver. The strength of the received signal can be calculated as :
To calculate the actual signal-to-noise ratio, one must also take into account additional losses. The effective area of an antenna may be less than the area of the dish proper, for instance due to obstructions in front of the antenna surface (e.g., struts, assemblies, other structural elements). For a 64 m DSN antenna, these losses amount to 2.7 dB , whereas for the 2.74 m parabolic dish antenna of Pioneer 10 and 11, these losses are about 3.7 dB.
Further (circuit, modulation, pointing) losses must also be considered. For the downlink from Pioneer 10 and 11 to the ground, the sum total of these losses is about 10.4 dB.
The signal-to-noise ratio also determines the bit error rate at various bit rates through the equation:
The discussion above allows one to present a typical downlink communications power budget for Pioneer 10, which is given in Table 3.2 (adapted from ).
|2.75 m antenna gain||+32.7||dB||+32.7||dB|
|64 m atenna gain||+61.0||dB||+61.0||dB|
|Received signal level||–156.2||dBm||–176.7||dBm|
|Less: S/N Ratio||–17.1||dB||–6.9||dB|
|Noise level at receiver||–173.3||dBm||–184.1||dBm|
Using the facilities in existence in 1973, the DSN would have been able to track Pioneer 10 and 11 up to a geocentric distance of 22 AU, but not beyond. However, due to numerous improvements of the DSN it was in fact possible to track Pioneer 10 all the way to over 83 AU from the Earth. Not the least of these improvements was an increase in antenna size, when the DSN’s 64 m antennas were enlarged to 70 m, and many of the 26 m antennas (notably, DSS-12, DSS-42, and DSS-61 from Table 3.1) were enlarged to 34 m.
In addition to the stations used initially (see Table 3.1) for communication with Pioneer 10 and 11, over the years many other stations were utilized, which are listed in Table 3.3.
|DSS-13 “Venus”||Goldstone||26/34 m|
|DSS-15 “Uranus”||Goldstone||34 m|
|DSS-16 “Apollo”||Goldstone||26 m|
|DSS-44||Honeysuckle Creek||26 m|
In order to perform precision tracking, the location of these radio stations must be known to high accuracy in the same coordinate frame (e.g., a solar system barycentric frame) in which spacecraft orbits are calculated.
This task is accomplished in two stages. First, station locations are given relative to a a geocentric coordinate system, such as the International Terrestrial Reference Frame (ITRF). Second, a conversion from the geocentric coordinate system to the appropriate solar system barycentric reference frame is performed.
For operating DSN stations, station location information is readily available, e.g., from NASA’s Navigation and Ancillary Information Facility (NAIF10). Such information usually consists of station coordinates at a given epoch, and station drift (e.g., as a result of continental drift.) Higher precision station data (e.g., taking into account the effects of tide) is also available, but such precision is not required for the tracking of Pioneer 10 and 11.
Station data is harder to come by for stations that have been decommissioned or moved. Some decommissioned stations are listed in Table 3.411.
Antenna size may have been the visually most apparent change at a DSN complex, but it was upgrades of DSN receiver hardware that resulted in a really significant improvement in a tracking station’s capabilities.
Receivers of the DSN have been improved on a continuous basis during the lifetime of Pioneer 10 and 11. These improvements were a result of other mission requirements, but the Pioneer project benefited: far beyond the predicted range of 22 AU, it was possible to maintain communication with Pioneer 10 when it was at an incredible 83 AU from the Sun. (The same DSN complex is now used to communicate with Voyager I spacecraft from heliocentric distances of over 110 AU.)
The receiver can contribute to a tracking station’s sensitivity in three different ways. First, the bandwidth of the receiver (loop bandwidth) can be reduced, increasing the signal-to-noise ratio. Two additional improvements, lowering the system noise temperature and eliminating other sources of signal attenuation, not only increase the signal-to-noise ratio but also decrease the bit error rate.
The “Block III” DSN receivers in use at the time of the launch of Pioneer 10 and 11 had an S-band system noise temperature of 28 K, and a receiver loop bandwidth of 12 Hz. These receivers were replaced in 1983 – 1985 by “Block IV” receivers in which the S-band system noise was reduced to 14.5 K in receive only mode, and the receiver loop bandwidth was reduced to 3 Hz. Further improvements came with the all-digital “Block V” receivers (also known as the Advanced Receivers ) installed in the early 1990s that had, under ideal circumstances, an S-band system noise of only 12.9 K, and a receiver loop bandwidth of 0.1 Hz.
Together with the enlarged 70 m antenna, these improved receivers made it possible to receive the signal of Pioneer 10 at 83 AU with a bit error rate of 1%. This error rate was reduced further by the convolutional code in use by Pioneer 10 and 11, which amounted to a 3.8 dB improvement in the signal-to-noise ratio, resulting in an error rate of about one in 104 bits.
During planetary encounters, the rapidly changing velocity of the spacecraft can result in a Doppler shift of its received frequency. During Pioneer 11’s close encounter with Jupiter, this rate could be as high as . Such a rapid change in received frequency can exceed the capabilities of the DSN closed loop receivers to remain “in lock”. To maintain continuous communication with such spacecraft, a “ramping” technique was implemented that allowed the tuned frequency of the DSN receiver to follow closely the predicted frequency of the spacecraft’s transmission . This ramping technique was used successfully during the Pioneer 10 and 11 planetary encounters. Later it became routine operating procedure for the DSN. (As late as Pioneer 11’s 1979 encounter with Saturn, ramp frequencies at DSS-62 were tuned manually by relays of operators, as an automatic tuning capability was not yet available ).
To maintain continuous communication with distant spacecraft, the DSN must be able to transmit a signal to the spacecraft. The 70 m stations of the DSN are equipped with transmitters with a maximum transmitting power of 400 kW in the S-band12. This is sufficient power to maintain continuous communication with distant spacecraft even when the spacecraft are not oriented favorably relative to the Earth, and must use a low-gain omnidirectional antenna to receive ground commands.
Just like its receivers, the DSN’s transmitters are also capable of ramping. Ramped transmissions are necessary during planetary encounters in order to ensure that the spacecraft’s closed loop receiver remains in lock even as its line-of-sight velocity relative to the Earth is changing rapidly13.
Results of Pioneer radio observations were packaged in data files (see Figure 3.5). Initially, these data files were transcribed to magnetic tape and delivered physically to the project site where they were processed. The present-day DSN uses electronic ground communication networks for this purpose.
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