Radio Metrics --Tools and Techniques

In addition to being able to exchange forward and return link data with an exploring spacecraft, it is equally important that we understand where the spacecraft is and where it is going. The radio signal exchanged with the spacecraft conveys some of the necessary information about the position and motion of that spacecraft, which can be extracted and refined by appropriate processing and analysis. Since its inception, the DSN Advanced Systems Program has worked to develop effective instrumentation, observing strategies, and analysis techniques that enable the DSN to provide an increasingly capable radio location service to distant spacecraft.

Conventional Doppler and Range

If the Earth and the spacecraft were standing still, the time taken for a radio signal to travel from the Earth to the spacecraft and back would be a measurement of the distance between them. This is referred to as the round-trip light time (RTLT). However, since the Earth and the spacecraft are both in motion, the RTLT contains both position and velocity information, which can be disentangled through multiple measurements and suitable analysis. The precision at which such measurements can be obtained is limited by the precision at which one can attach a time-tag marker to the radio signals.

Precise measurements of changes to this light time are far easier to obtain via observing the Doppler effect resulting from the relative motions. Such measurements are mechanized via the phase-locked loops in both spacecraft and ground receivers--using the spacecraft's replica of the forward link residual carrier signal to generate the return link signal, and counting the local replica of the return link residual carrier against the original carrier for the forward link signal. The raw precision of these measurements is comparable to the wavelength of the residual carrier signal, i.e., a few centimeters for an X-band signal (8 GHz). Numerous interesting error sources tend to corrupt the accuracy of the measurement and the inferred position and velocity of the spacecraft, and have provided significant technical challenge for work under the Advanced Systems Program.

The observed Doppler contains numerous distinct components, including the very significant rotation of the Earth. As the Earth turns, the position of any specific site on the surface describes a circle, centered at the spin axis of the Earth, falling in a plane defined by the latitude of that site. The resultant Doppler component varies in a diurnal fashion, with a sinusoidal variation, which is at its maximum positive value when the spacecraft is first observable over the eastern horizon, and its corresponding negative value as it approaches the western horizon. A full-pass Doppler observation from horizon to horizon can be analyzed to extract the apparent spacecraft position in the sky, although the determination is somewhat weak near the equatorial plane. Direct measurements of the RTLT are useful for resolving this difficulty.

Three distinct generations of instruments designed to measure the RTLT were developed by the Advanced Systems Program and used in an ad hoc fashion for spacecraft support before a hybrid version was designed and implemented around the DSN. The third instrument designed, the Mu-II Ranging Machine, was used with the Viking landers in a celestial mechanics experiment, which provided the most precise test to date of the general theory of relativity.

These devices function by imposing an additional "ranging" modulation signal on the forward link, which is copied on the spacecraft (within the limits imposed by noise) and then imposed on the return link. The ranging signal is actually a very long period-coded sequence that provides the effect of a discrete time tag. The bandwidth of the signal is on the order of 1 MHz, giving the measurement a raw precision of a few hundred meters, which is resolvable with care to a few meters. Among other features, the Mu-II Ranging Machine included the first demonstrated application of the digital detection techniques that would figure strongly in future developments for the DSN.

Timing Standards

Whether for Doppler or range, the measurement unit for the radio metric observations derives from the wavelength of the transmitted signal. Uncertainties or errors in knowledge of that wavelength are equivalent to errors in the derived spacecraft position. The need for accurate radio metrics has motivated the DSN Advanced Systems Program to develop some of the most precise, most stable frequency standards in the world. While the current suite of hydrogen maser frequency standards in the DSN field sites was built outside of JPL, the design is the end product of a long collaboration in technology development, with research units being built at JPL under the DSN Advanced Systems Program and elsewhere.

Continued research under the Advanced Systems Program for improved frequency standards has resulted in the development of a new linear ion trap (LIT) that offers improved long-term stability of a few parts in 1016 as well as simpler and easier maintenance than that required by the hydrogen masers. Work is currently under way to implement the LIT standard in the field in the DSN, while research efforts continue for improvements that can be transferred to field operation in the future.

Earth Rotation and Propagation Media

The transformation from a stream of Doppler (and range) data into the apparent position of a spacecraft in flight defines that position relative to the position and attitude of the rotating Earth. The Earth, however, is not a perfectly rigid body with constant rotation, but contains fluid components as well, which slosh about and induce variations in rotation of perhaps a few milliseconds per day. Calibration of the Earth's attitude is necessary so that the spacecraft's position in inertial space can be determined, which is a necessary factor in navigating it toward a target planet. Such calibration is available via the world's optical observatories, and with greater precision via radio techniques, which will be discussed further in the paragraphs titled "VLBI and Radio Astronomy" and "Global Positioning System."

Material in the signal path between the Earth and the spacecraft affects the accuracy with which the Doppler and range can be determined. The charged ions in the tenuous plasma spreading out from the Sun, known as the solar wind, will bend and delay the radio signal. Likewise, the charged ions in the Earth's own ionosphere and the water vapor and other gasses of the denser lower atmosphere will bend and delay the radio signal. All of these factors are highly variable because of other factors, such as intensity of solar activity, season, time of day, and weather. All factors must be calibrated, modeled, or measured to achieve the needed accuracy; over the years, the DSN Advanced Systems Program has devised an increasingly accurate series of tools and techniques for these calibrations.

Radio Science

Radio science is the term used to describe the scientific information obtained from the intervening pathway between the Earth and a spacecraft by the use of radio links. The effects of the solar wind on the radio signal path interfere with our efforts to determine the location of the spacecraft, but if the relative motions of the Earth and spacecraft are modeled and removed from the radio metric data, much of what remains is information about the solar wind and, thus, about the Sun itself. Other interfering factors can be similarly of scientific interest to other investigators.

In some situations, the signal path passes close by a planet or other object, and the signal itself is bent, delayed, obscured, or reflected by that object and its surrounding atmosphere. These situations provide a unique opportunity for us to extract information from the signal about object size, atmospheric density profiles, and other factors not otherwise observable. Algorithms and other tools devised to help calibrate and remove interfering signatures from radio metric data for use in locating a spacecraft often become part of the process for extracting scientific information from the same radio metric data stream. The precision frequency standards, low-noise amplifiers, and other elements of the DSN derived from the Advanced Systems Program are key factors in the ability to extract this information with a scientifically interesting accuracy. And, on the occasion of some unique events, the engineering models developed by the Advanced Systems Program will be placed into the field for ad hoc support of the metric data gathering, perhaps in parallel with operational instrumentation.

The effects of gravity can also be observed by means of the radio link. Several situations are of interest. If the spacecraft is passing by or in orbit about an object that has a lumpy uneven density, that unevenness will cause a variation in the spacecraft's pathway that will be observable via the radio metric data. If the radio signal passes near a massive object such as the Sun, the radio signal's path will be bent by the intense gravity field, according to the theories of general relativity. And in concept, gravitational waves (a yet-to-be-observed aspect of gravity field theory) should be observable in the Doppler data from a distant spacecraft. All of these possibilities depend upon the stability of the DSN's precision frequency standards for the data to be scientifically interesting.

VLBI and Radio Astronomy

The technical excellence of the current DSN is at least in part a result of a long and fruitful collaboration with an active radio astronomy community at the California Institute of Technology (Caltech) and elsewhere. Many distant stars, galaxies, and quasars are detectable by the DSN at radio frequencies. The furthest of these are virtually motionless and can be viewed as a fixed-coordinate system to which spacecraft and other observations can be referenced. Observations relative to this coordinate set help to reduce the distorting effects of intervening material in the radio signal path and uncertainties in the exact rotational attitude of the Earth during spacecraft observations.

Little precise information can be extracted by observing these objects one at a time and from a single site, but concurrent observation at a pair of sites will determine the relative position of the two sites referenced to the distant object. The observing technique is known as very long baseline interferometry (VLBI) and was developed by the research of many contributors, including substantial work by the DSN's Advanced Systems Program. If three sites are used in VLBI pairs and multiple objects are observed, the positional attitude of the Earth and the relative positions of the observed objects can be determined. If one of the observed is a spacecraft transmitting a suitable signal, its position and velocity in the sky can be very accurately defined. A demonstration of this technique via the Advanced Systems Program led to operational use for spacecraft such as Voyager and Magellan.

VLBI can also be used in conjunction with conventional radio metric data types to provide the calibration for the positional attitude of the Earth. Such observations can be made without interfering with spacecraft communication, except for the time utilization of the DSN antennas. In addition to determining the attitude of the Earth, the observations measure the relative behavior of the frequency standards at the widely separated DSN sites, and thus help to maintain their precision performance. Again, demonstration of this capability via the Advanced Systems Program led to routine operational use in the DSN.

Design and development of the DSN equipment and software needed for VLBI signal acquisition and signal processing (correlation) was carried out in a collaboration involving the Advanced Systems Program, the operational DSN, and the Caltech radio astronomy community. Tools needed to produce VLBI metric observations for the DSN were essentially the same as those for interferometric radio astronomy. Caltech was funded by the National Science Foundation for this activity, and both Caltech and the DSN shared in the efforts of the design, while obtaining products that were substantially better than any that they could have obtained independently.

Another area of common interest between the DSN and the radio astronomy community is that of precision wideband spectral analysis. Development efforts of the Advanced Systems Program produced spectral analysis tools that have been employed by the DSN in spacecraft emergency situations and in examining the DSN's radio interference environment, and have served as pre-prototype models for equipment for the DSN. Demonstration of the technical feasibility of the very wide-band spectral analysis and preliminary observations by a megachannel spectrum analyzer fielded by the Advanced Systems Program helped establish the sky survey planned as part of the former SETI (Search for Extraterrestrial Intelligence) Program.

Another technique (one similar to the use of VLBI for a radio metric reference) is used if two spacecraft are flown to the same target; the second can be observed relative to the first, providing better target-relative guidance once the first has arrived at the target. Techniques for acquiring and analyzing such observations have been devised by the Advanced Systems Program.

Global Positioning System

The Global Positioning System (GPS) is a constellation of Earth-orbiting satellites designed (initially) to provide for military navigation on the Earth's surface. As has been shown by research under the Advanced Systems Program, these satellites provide an excellent tool to calibrate and assist in the radio metric observation of distant spacecraft. GPS satellites fly above the Earth's atmosphere and ionosphere in well-defined orbits, so that their signals can be used to measure the delay through these media in a number of directions. With suitable modeling and analysis, these measurements can be used to develop the atmospheric and ionospheric calibrations for the radio path to a distant spacecraft. Research by the Advanced Systems Program is continuing on this process.

Additionally, since the GPS satellites are in free orbit about the Earth, their positions are defined relative to the center of mass of the Earth, and not its surface. They thus provide another method to observe the uneven rotation of the Earth. This method can supplement or in part replace the VLBI technique currently used. Position of spacecraft in Earth orbit can also be determined relative to the GPS satellites as long as that spacecraft carries a receiver for the GPS signals. The potential of this technique was initially demonstrated by the Advanced Systems Program. GPS was subsequently used by the TOPEX/POSEIDON Project for precise orbit determination and a consequent enhancement of its scientific return.