Matthew J. Reinhart

A two-way noncoherent ranging technique for deep space missions

The Johns Hopkins University Applied Physics Laboratory (APL) has adopted an integrated electronics module (IEM) approach for many of its spacecraft programs. As a result APL has developed a relatively simple X-band transceiver that allows the telecommunications subsystem to be manufactured on plug-in cards that fit into the IEM. An issue with the transceiver approach is that the downlink frequency is not related to the uplink frequency. The noncoherent relationship between the uplink and downlink signals has implications in both Doppler tracking and ranging. APL has developed a method for performing highly precise noncoherent Doppler tracking (J.R. Jensen and R.S. Bokulic, IEEE Trans. Aerospace and Electronic Sys., vol. 35, no. 3, pp. 963-973, 1999). This paper addresses a technique for performing accurate ranging with a noncoherent system. Comet Nucleus Tour (CONTOUR) is the first deep space mission to employ a transceiver and rely on the noncoherent ranging technique. Analysis and testing of the technique implemented for the CONTOUR mission is presented. Tests of the noncoherent ranging technique using the CONTOUR communications hardware at the Deep Space Networks (DSN) Development and Test Facility (DTF21) verified that the technique will provide ranging measurements that meet its navigation requirements.

An advanced synthesized ultra-stable oscillator for spacecraft applications

Current ultra-stable oscillator (USO) technology relies on highly precise quartz resonators that are selected based on the desired output frequency and stability. These constraints on the crystal specifications significantly increase the lead time and expense of each USO. Recent research and development efforts in USOs by The Johns Hopkins University Applied Physics Laboratory (JHU/APL) have focused on a frequency synthesized USO based on a standardized, fixed-frequency resonator. The result of these efforts is a synthesized USO that will provide a frequency reference for transponders and other on-board users on future space missions. The frequency reference is stable enough for radio-science and navigation applications (Allan deviation <1.5 /spl times/ 10/sup -13/ at /spl tau/ = 10 s), and is electronically adjustable to cover the entire deep-space communications band. This frequency agility allows in flight re-assignment of the transponder frequencies. The synthesized USO offers low mass and DC power consumption yet maintains world-class noise performance and frequency stability performance.

The CONTOUR radio communications system

This paper provides a detailed description of the radio communications system developed for the Comet Nucleus Tour (CONTOUR) Project. The communications system embodies a delicate balance of minimizing cost while providing the high performance needed to support a deep-space science mission. CONTOUR employs a transceiver-based X-band system instead of traditional deep-space transponders. For navigation, we have a conventional ranging channel and employ a novel Doppler frequency measurement technique. A reference oscillator with low phase noise is included to allow narrow bandwidth downlink carrier tracking at the ground stations. The antenna system is a combination of high- and low-gain antennas to support high-data-rate science returns and low-data-rate emergency operations. As CONTOUR is spin stabilized for most of the mission, including emergency operations, all antennas have been designed to provide continuous coverage around 360/spl deg/ of spacecraft rotation.

Autonomous characterization of clock drift for timescale improvement at the AHU/APL time and frequency laboratory

We have reported on continuous improvements in the capability of our time and frequency laboratory. A substantial portion of our progress in capability was achieved through the incorporation of new clock hardware, improvement in GPS time recovery and coordination of our clocks into the computation of TAI. We have discussed our ensemble of hydrogen maser and cesium beam atomic clocks into a timescale that enables UTC (APL) to be steered within plusmn 30 nanoseconds per month of UTC. The propagation of the APL timescale is based on a modified version of the Percival method, requiring regular characterization of each clocks frequency rate and drift. Here, we discuss our results in an autonomous characterization of the individual clocks contributing to the APL timescale. This improvement in our operation has minimized the need for routine operator timescale maintenance and realizes the advantages in clock estimation using frequency, described by J.A. Barnes and D.W. Allan (1985). We discuss how our approach at characterizing the nonlinear drift observed in our hydrogen masers has aided our attempt to discipline the long term frequency drift behavior of quartz ultra-stable oscillators in the space environment. As in previous reports, we present updated laboratory performance in the form of UTC-UTC(APL)