Durgadas S. Bagri

Angular Position Determination of Spacecraft by Radio Interferometry

This paper describes a variety of interferometric techniques that may be used for measuring the angular location of a spacecraft with respect to natural celestial radio sources or another spacecraft. The differential propagation time-delay techniques largely cancel the common error sources and normally achieve low angular coordinate errors. Currently, the accuracy of the techniques are in the 1-2 nrad range for observations with a duration of one hour and 3-6deg of mean angular separation between the spacecraft and the reference sources at X-band frequencies. There are various possible ways to implement the differential angular measurements depending upon the determination of the phase cycle ambiguities associated with the differential propagation time delays of remote sources. There are methods that utilize a sufficiently large range of observing frequencies and others that rely upon the spatial arrangement of the receiving system and the rotation of the terrestrial platform. We summarize the methodologies and the advantages and disadvantages of the various techniques.

Proposed Array-Based Deep Space Network for NASA

The current assets of the deep space network (DSN) of the National Aeronautics and Space Administration (NASA), especially the 70-m antennas, are aging and becoming less reliable. Furthermore, they are expensive to operate and difficult to upgrade for operation at Ka-band (321 GHz is shorthand for the allocated 31.8-32.3 GHz. GHz). Replacing them with comparable monolithic large antennas would be expensive. On the other hand, implementation of similar high-sensitivity assets can be achieved economically using an array-based architecture, where sensitivity is measured by G/T, the ratio of antenna gain to system temperature. An array-based architecture would also provide flexibility in operations and allow for easy addition of more G/T whenever required. Therefore, an array-based plan of the next-generation DSN for NASA has been proposed. The DSN array would provide more flexible downlink capability compared to the current DSN for robust telemetry, tracking and command services to the space missions of NASA and its international partners in a cost-effective way. Instead of using the array as an element of the DSN and relying on the existing concept of operation, we explore a broader departure in establishing a more modern concept of operations to reduce the operations costs. This paper presents the array-based architecture for the next-generation DSN. It includes system block diagram, operations philosophy, users view of operations, operations management, and logistics like maintenance philosophy and anomaly analysis and reporting. To develop the various required technologies and understand the logistics of building the array-based low-cost system, a breadboard array of three antennas has been built. This paper briefly describes the breadboard array system and its performance.

Availability of Calibration Sources for Measuring Spacecraft Angular Position with Sub-Nanoradian Accuracy

Precision measurements are now capable of determining the angular position of spacecrafts with accuracies of 2-5 nanoradians. To achieve this level of precision, compact radio sources with flux density of at least a few hundred milli-Jansky (at 8.4 GHz) are used for calibration purposes. Further improvements in position measurement accuracy may be possible with use of appropriate calibrators near the direction of the spacecrafts even if the calibrators are much weaker (a few milli-Jansky) in flux density. In this paper we discuss the calibrator flux density required to achieve sub-nanoradian astrometric accuracy and attempt to estimate the density of suitable calibrators, using existing source count surveys. We point out, however, that the fraction of these sources that are suitable for use as calibrators is not well understood and requires further study.

Accurate spacecraft angular position from DSN VLBI phases using X-band telemetry or DOR tones

At present spacecraft angular position with Deep Space Network (DSN) is determined using group delay estimates from very long baseline interferometer (VLBI) phase measurements employing differential one way ranging (DOR) tones. As an alternative to this approach, we propose estimating position of a spacecraft to half a fringe cycle accuracy using time variations between measured and calculated phases as the Earth rotates using DSN VLBI baseline(s). Combining fringe location of the target with the phase allows high accuracy for spacecraft angular position estimate. This can be achieved using telemetry signals of at least 4–8 MSamples/sec data rate or DOR tones.

Pros and cons of using arrays of small antennas versus large single dish antennas for Deep Space Network

This paper briefly describes pros and cons of using arrays of small antennas instead of large single dish antennas for spacecraft telemetry, command, and tracking (TT&C) - communications and navigation (C&N) - and science support that the Deep Space Network (DSN) normally provides. It considers functionality and performance aspects, mainly for TT&C, though it also considers science. It only briefly comments on the cost aspects that seem to favor arrays of small antennas over large single antennas, at least for receiving (downlinks).

Precision Spacecraft Tracking Using In-Beam Phase Referencing

The deep space network (DSN) Array of the future provides an intriguing possibility of using the techniques of in-beam phase referencing to determine the angular position of spacecraft with accuracy at the level of 0.1 nano-radian (nrad). In this paper, we discuss the prospects for carrying out such measurements at both 8.4 GHz (X-band) and 32 GHz (Ka-band). Our study suggests that at X-band in-beam calibration may be available as an astrometric tool over 20-30 percent of the sky. The prospects at Ka-band, on the other hand, are not very hopeful. We point out that these estimates depend strongly on the number density of compact sources at the 1-mJy level. We also present the results from our recent VLBA (very long baseline array) observations at X-band, which has determined the compact source density at the 10-mJy level and discuss future observations to probe weaker source population. Finally, we discuss issues related to phase cycle ambiguity resolution and potential techniques to resolve them.

Operation's concept for array-based deep space network

The array-based deep space network (DSN-Array) will be a part of more than 103 times increase in the downlink/telemetry capability of the deep space network (DSN). The key function of the DSN-array is to provide cost-effective, robust telemetry, tracking and command (TT&C) services to the space missions of NASA and its international partners. It provides an expanded approach to the use of an array-based system. Instead of using the array as an element in the existing DSN, relying to a large extent on the DSN infrastructure, we explore a broader departure from the current DSN, using fewer elements of the existing DSN, and establishing a more modern concept of operations. This paper gives architecture and operations philosophy of DSN-array. It also describes customers view of operations, operations management and logistics, and maintenance philosophy, anomaly analysis and reporting

Frequency Synthesis Approach to Determine Spacecraft Angular Position with Sub-nanoradian Accuracy

This paper describes a possible approach to measuring angular position of a spacecraft with reference to a nearby calibration source (quasar) with an accuracy of a few tenths of a nanoradian using a very long baseline interferometer that measures the interferometer phase with a modest accuracy. It employs (1) radio frequency (RF) phase delay to determine the spacecraft position with a high precision, and (2) multiple group delay measurements using either frequency tones or telemetry signals at different frequency spacing to resolve ambiguity of the fringe (cycle) containing the direction of the spacecraft.

Calibration of Antennas During Construction or Expansion of Radio Arrays

The calibration of radio antenna performance is a well understood process, especially for single large-diameter antennas. However, the measurement of efficiency, optimal focus, pointing corrections, system temperature, and other parameters as functions of observing frequency, antenna elevation and azimuth, temperature, and wind velocity can be time-consuming. For any future arrays consisting of very large numbers of small antennas, it will be necessary to minimize the time spent calibrating each antenna. This paper considers ways to speed up the calibration of antennas being added to an existing array by taking advantage of interferometer measurements and script-based automation.

Same Beam Tracking with the Proposed DSN Array Using Calibration Signal from Multiple Sources

The accuracy of tracking measurements using very long baseline interferometer (VLBI) is seriously limited due to angular and temporal separation between the calibration and spacecraft measurements. Same beam interferometry, where the calibration source is in the same primary beam as the spacecraft being observed eliminates temporal affects and considerably reduces calibration errors due to reduced angular separation between the calibrator and the spacecraft which minimizes errors due to propagation variations and uncertainty in geometry between source and baseline. With the current measurement methods this requires having a suitable calibration source of enough signal strength within the primary beam of the antennas used for the inteferometry measurements. Normally such sources are not available because of a low number density of suitable strong calibration sources. However there are many more weaker calibration sources and it may be possible to use combined signal from a number of weaker sources within the primary beam of the antennas if a single strong source is not available within the primary beam at any time. The probability of having enough combined total signal increases considerably if we accept using multiple calibration sources. Expected accuracy of a tracking measurement using signal from multiple calibration sources should be similar to the accuracy expected with a single calibration source within the beam, if the total combined signal from the individual weaker sources is similar to the desired single source. An important point that is worth noting here is that if we want to take advantage of utilizing the same beam interferometry for accurate spacecraft tracking most of the times, then the diameter of the antennas of the proposed array-based deep space network (DSN Array) should be limited to as small a value as practical to give a large field of view. This would enable having suitable calibration source(s) within the primary beam of the antennas. Considering practical aspects it appears that the diameter for the proposed DSN array antenna should be limited to a maximum of about 12 m.