3.2 Acquisition of radiometric Doppler data

Radiometric observations are performed by the DSN stations located at several DSN sites around the world [235Jump To The Next Citation Point240Jump To The Next Citation Point]. Routine radiometric tracking and navigation of the Pioneer 10 and 11 spacecraft was performed using Doppler observations. The Doppler extraction process is schematically depicted in Figure 3.3View Image. A reference signal of a known frequency (usually a frequency close to that of the original transmission) is mixed with the received signal. The resulting beat frequency is than measured by a frequency counter. The count over a given time interval is recorded as the Doppler observation.
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Figure 3.3: The Doppler extraction process. Adapted from [374Jump To The Next Citation Point].

Receiving stations of the DSN are equipped with ultra-stable oscillators, allowing very precise measurements of the frequency of a received signal. One way to accomplish this measurement is by comparing the frequency of the received signal against a reference signal of known frequency, and count the number of cycles of the resulting beat frequency for a set period of time. This Doppler count is then stored in a file for later analysis.

The operating principles of the DSN evolved over time, and modern receivers may not utilize a beat frequency. However, all Pioneer data was stored using a format that incorporates an actual or simulated reference frequency.

Below we discuss the way Doppler observables are formed at the radio-science subsystem of the DSN. This description applies primarily to Block III and Block IV receivers, which were used for Pioneer radiometric observations throughout most of the Pioneer 10 and 11 missions.

3.2.1 DSN frequency and timing system14

Present day radiometric tracking of spacecraft requires highly accurate timing and frequency standards at tracking stations. For a two-way Doppler experiment, for instance, that involves transmitting a signal to a distant spacecraft and then receiving a response perhaps several hours later, long-term stability of tracking station oscillators is essential. Furthermore, if more than one tracking station is involved in an observation (e.g., “three-way” Doppler in which case one station is used to uplink a signal to the spacecraft, and another station receives the spacecraft’s response), frequency and clock synchronization between the participating tracking stations must be very precise [38Jump To The Next Citation Point].

Originally, the DSN used crystal oscillators. In the 1960s, these were replaced by rubidium or cesium oscillators that offered much improved frequency stability, and made precision radio-tracking of distant spacecraft possible. Rubidium and cesium oscillators offered a frequency stability of one part in ∼ 1012 and one part in ∼ 1013, respectively [241Jump To The Next Citation Point], over typical Doppler counting intervals of 102 – 103 seconds. (This is in agreement with the expected Allan deviation for the S-band signals.)

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Figure 3.4: Block diagram of the DSN baseline configuration as used for radio Doppler tracking of the Pioneer 10 and 11 spacecraft. Adapted from [27Jump To The Next Citation Point101]. (IF stands for Intermediate Frequency.)

A further improvement occurred in the 1970s, when the rubidium and cesium oscillators were in turn replaced by hydrogen masers, which offered another order of magnitude improvement in frequency stability [241Jump To The Next Citation Point]. Today, the DSN’s frequency and timing system (FTS) is the source for the high accuracy mentioned above (see Figure 3.4View Image). At its center is a hydrogen maser that produces a precise and stable reference frequency. These devices have Allan deviations (see Section 5.3.6) of approximately 3 × 10–15 to 1 × 10–15 for integration times of 102 to 103 seconds, respectively [38374Jump To The Next Citation Point]. These masers are good enough that the quality of Doppler-measurement data is limited by thermal or plasma noise, and not by the inherent instability of the frequency references.

However, for two-way Doppler analysis, the relevant quantity is the stability of the station’s frequency standard during the round-trip light travel time. During the 1980s, hydrogen maser frequency standards were required to have Allan deviations of less than 10–14 over time scales of 3 – 12 hours [80123]. This corresponds to a Doppler frequency noise of 2 × 10–5 Hz.

For three-way Doppler analysis, where the transmitting and receiving stations are different, the frequency offset between stations is the relevant quantity. During the 1980s, station frequencies were controlled to be the same, to a fractional error of 10–12 [308356]. This corresponds to a Doppler frequency bias of up to 2 × 10–3 Hz between station pairs. Station keepers did maintain frequency offset knowledge to a tighter level, but for the most part this knowledge is not available to analysts today.

Three-way Doppler analysis is weakly sensitive to clock offsets between stations. By 1968, the operational technique for time synchronization was the “Moon Bounce Time Synch” technique, in which a precision-timed X-band signal from DSS-13 (which served as master timekeeper) was transmitted to overseas stations by way of a lunar reflection, achieving a clock accuracy of 5 μs between stations [241]. Such timing offsets would produce Doppler errors of less than 10–6 Hz. Later, DSN stations were synchronized utilizing the Global Positioning Satellite (GPS) network, achieving a synchronization accuracy of 1 μs or better [374Jump To The Next Citation Point].

3.2.2 The digitally controlled oscillator and exciter

Using the highly stable output from the FTS, the digitally controlled oscillator (DCO), through digitally controlled frequency multipliers, generates the Track Synthesizer Frequency (TSF) of ∼ 22 MHz. This is then sent to the Exciter Assembly. The Exciter Assembly multiplies the TSF by 96 to produce the S-band carrier signal at ∼ 2.2 GHz. The signal power is amplified by Traveling Wave Tubes (TWT) for transmission. If ranging data are required, the Exciter Assembly adds the ranging modulation to the carrier. The DSN tracking system has undergone many upgrades during the 34 years of tracking Pioneer 10. During this period internal frequencies have changed (see Section 3.1.1).

This S-band frequency is sent to the antenna where it is amplified and transmitted to the spacecraft. The onboard receiver tracks the up-link carrier using a phase lock loop. To ensure that the reception signal does not interfere with the transmission, the spacecraft (e.g., Pioneer) has a turnaround transponder with a ratio of 240/221 in the S-band. The spacecraft transmitter’s local oscillator is phase locked to the up-link carrier. It multiplies the received frequency by the above ratio and then re-transmits the signal to Earth.

3.2.3 Receiver and Doppler extractor

When the signal reaches the ground, the receiver locks on to the signal and tunes the Voltage Control Oscillator (VCO) to null out the phase error. The signal is sent to the Doppler Extractor. At the Doppler Extractor the current transmitter signal from the Exciter is multiplied by 240/221 (or 880/241 in the X-band) and a bias of 1 MHz for S-band or 5 MHz for X-band is added to the Doppler. The Doppler data is no longer modulated at S-band but has been reduced as a consequence of the bias to an intermediate frequency of 1 or 5 MHz.

The transmitter frequency of the DSN is a function of time, due to ramping and other scheduled frequency changes. When a two-way or three-way (see Section 3.2.5) Doppler measurement is performed, it is necessary to know the precise frequency at which the uplink signal was transmitted. This, in turn, requires knowledge of the exact light travel time to and from the spacecraft, which is available only when the position of the spacecraft is determined with precision. For this reason, DSN transmitter frequencies are recorded separately so that they can be accounted for in the orbit determination programs that we discuss in Section 5.

3.2.4 Metric data assembly

The intermediate frequency (IF) of 1 or 5 MHz with a Doppler modulation is sent to the Metric Data Assembly (MDA). The MDA consists of computers and Doppler counters where continuous count Doppler data are generated. From the FTS a 10 pulse per second signal is also sent to the MDA for timing. At the MDA, the IF and the resulting Doppler pulses are counted at a rate of 10 pulses per second. At each tenth of a second, the number of Doppler pulses is recorded. A second counter begins at the instant the first counter stops. The result is continuously-counted Doppler data. (The Doppler data is a biased Doppler of 1 MHz, the bias later being removed by the analyst to obtain the true Doppler counts.) The range data (if present) together with the Doppler data are sent separately to the Ranging Demodulation Assembly. The accompanying Doppler data is used to “rate-aid” (i.e., to “freeze” the range signal) for demodulation and cross correlation.

3.2.5 Radiometric Doppler data

Doppler data is the measure of the cumulative number of cycles of a spacecraft’s carrier frequency received during a user-specified count interval. The exact precision to which these measurements can be carried out is a function of the received signal strength and station electronics, but it is a small fraction of a cycle. Raw Doppler data is generated at the tracking station and delivered via a DSN interface to customers.

When the measured signal originates on the spacecraft, the resulting Doppler data is called one-way or F1 data. In order for such data to be useful for precision navigation, the spacecraft must be equipped with a precision oscillator on board. The Pioneer 10 and 11 spacecraft had no such oscillator. Therefore, even though a notable amount of F1 Doppler data was collected from these spacecraft, these data are not usable for precision orbit determination.

An alternative to one-way Doppler involves a signal generated by a transmitter on the Earth, which is received and then returned by the spacecraft. The Pioneer 10 and 11 spacecraft were equipped with a radio communication subsystem that had the capability to operate in “coherent” mode, a mode of operation in which the return signal from the spacecraft is phase-locked to a signal received by the spacecraft from the Earth. In this mode of operation, the precision of the frequency measurement is not limited by the stability of equipment on board the spacecraft, only by the frequency stability of ground-based DSN stations.

When the signal is transmitted from, and received by, the same station, the measurement is referred to as a two-way or F2 Doppler measurement; if the transmitting and receiving stations differ, the measurement is a three-way (F3) Doppler measurement.

Knowing the frequency of a signal received at a precise time at a precisely known location is only half the story in the case of two-way or three-way Doppler data: information must also be known about the time and location of transmission and the frequency of the transmitted signal.

The frequency of the transmitter, or the frequency of the receiver’s reference oscillator may not be fixed. In order to achieve better quality communication with spacecraft the velocity of which varies with respect to ground stations, a technique called ramping has been implemented at the DSN. When a frequency is ramped, it is varied linearly starting with a known initial frequency, at a known rate of frequency change over unit time.

Thus, a Doppler data point is completely characterized by the following:

and, for two- and three-way signals only,

Below we discuss the radiometric Doppler data formats that were used to support navigation of the Pioneer 10 and 11 spacecraft.

3.2.6 Pioneer Doppler data formats

The Pioneer radiometric data was received by the DSN in “closed-loop” mode, i.e., it was tracked with phase lock loop hardware. (“Open loop” data is recorded to tape but not tracked by phase lock loop hardware.) There are basically two types of data: Doppler (frequency) and range (time of flight), recorded at the tracking sites of the DSN as a function of UT Ground Received Time [103]. During their missions, the raw radiometric tracking data from Pioneer 10 and 11 were received originally in the form of Intermediate Data Record (IDR) tapes, which were then processed into special binary files called Archival Tracking Data Files (ATDF, format TRK-2-25 [95]), containing Doppler data from the standard DSN tracking receivers (Figure 3.5View Image)15. Note that the “closed-loop” data constitutes the ATDFs that were used in [27Jump To The Next Citation Point]. Figure 3.5View Image shows a typical tracking configuration for a Pioneer-class mission and corresponding data format flow.

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Figure 3.5: Typical tracking configuration for a Pioneer-class mission and corresponding data format flow [397Jump To The Next Citation Point].

ATDFs are files of radiometric data produced by the Network Operations Control Center (NOCC) Navigation Subsystem (NAV) (see Figure 3.4View Image). They are derived from Intermediate Data Records by NAV and contain all radiometric measurements received from the DSN station including signal levels (AGC = automatic gain control in dB), antenna pointing angles, frequency (often referred to simply as “Doppler”), range, and residuals. Each ATDF consists of all tracking data types used to navigate a particular spacecraft (Pioneers had only Doppler data type) and typically include Doppler, range and angular types (in S-, X-, and L-frequency bands), differenced range versus integrated Doppler, programmed frequency data, pseudo-residuals, and validation data. (Unfortunately there was no range capability implemented on Pioneer 10 and 11 [24Jump To The Next Citation Point27Jump To The Next Citation Point]. Early in the mission, JPL successfully simulated range data using the “ramped-range” technique [182Jump To The Next Citation Point]. In this method, the transmitter frequency is ramped up and down with a known pattern. One round trip light time later, this same pattern appears at the DSN receiver, and can be used to solve for the round trip light travel time. Later in the mission, this technique became unusable because the carrier tracking loop bandwidth required to detect the carrier became too narrow to track the ramped frequency changes.) Also, ATDFs contain data for a single spacecraft, for one or more ground receiving stations, and for one or more tracking passes or days.

After a standard processing at the Radio Metric Data Conditioning group (RMDC) of JPL’s Navigation and Mission Design Section, ATDFs are transformed into Orbit Data Files (ODF, format TRK-2-18 [96Jump To The Next Citation Point]). A program called STRIPPER is used to produce the ODFs that are, at this point, the main product that is distributed to the end users for their orbit determination needs (see more discussion on the conversion process in [27Jump To The Next Citation Point]). At JPL, after yet additional processing, these ODFs are used to produce sequentially formatted input/output files in NAVIO format that is used by navigators while working with the JPL Orbit Determination Program. (Note that the NAVIO input/output file format is used only at JPL; other orbit determination programs convert ODFs to their particular formats.)

Each ODF physical record is 2016 32-bit words in length and consists of 224 9-word logical records per data block. The ODF records are arranged in a sequence that consists of one file label record, one file identifier logical record, orbit data logical records in time order, ramp data logical records in time order, clock offset data logical records in time order, data summary logical records in time order and software/hardware end-of-file markers. Bit lengths of data fields are variable and cross word boundaries. An ODF usually contains most types of records, but may not have them all. The first record in each of the 7 primary groups is a header record; depending on the group, there may be from zero to many data records following each header. For further details, see Appendix B.

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