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.