Scott Bryant

Range Measurement as Practiced in the Deep Space Network

Range measurements are used to improve the trajectory models of spacecraft tracked by the deep space network. The unique challenge of deep-space ranging is that the two-way delay is long, typically many minutes, and the signal-to-noise ratio is small. Accurate measurements are made under these circumstances by means of long correlations that incorporate Doppler rate-aiding. This processing is done with commercial digital signal processors, providing a flexibility in signal design that can accommodate both the traditional sequential ranging signal and pseudonoise range codes. Accurate range determination requires the calibration of the delay within the tracking station. Measurements with a standard deviation of 1 m have been made.

Operations comparison of deep space ranging types: Sequential tone vs. pseudo-noise

NASAs Deep Space Network currently has a ranging system that uses a sequence of square wave tones to determine spacecraft distance. This ranging system correlates tones received from the spacecraft with those it transmitted. The phase shift that maximizes the correlation value is then related to the round trip light time distance. A new ranging system is being developed that can be configured to use sequential square wave tones or to use repeating pseudonoise tones. This allows more flexibility and possibly better performance. The tradeoffs between the two types of ranging are presented. A detailed derivation of the ranging performance as a function of configuration, signal strength, and other variables is given. The ranging performance for the two ranging types is compared. A configuration for pseudo-noise ranging that provides better performance is provided. Operational issues and simplifications resulting from using pseudo-noise ranging are also discussed.

Lunar pole illumination and communications maps computed from GSSR elevation data

A Digital Elevation Model of the lunar south pole was produced using Goldstone Solar System RADAR (GSSR) data obtained in 2006.12 This model has 40-meter horizontal resolution and about 5-meter relative vertical accuracy [Ref 1]. This Digital Elevation Model was used to compute average solar illumination and Earth visibility with 100 km of the lunar south pole. The elevation data were converted into local terrain horizon masks, then converted into lunar-centric latitude and longitude coordinates. The horizon masks were compared to latitude, longitude regions bounding the maximum Sun and Earth motions relative to the moon. Estimates of Earth visibility were computed by integrating the area of the region bounding the Earths motion that was below the horizon mask. Solar illumination and other metrics were computed similarly. Proposed lunar south pole base sites were examined in detail, with the best site showing yearly solar power availability of 92% and Direct-To-Earth (DTE) communication availability of about 50%. Similar analysis of the lunar south pole used an older GSSR Digital Elevation Model with 600-meter horizontal resolution. The paper also explores using a heliostat to reduce the photovoltaic power system mass and complexity.

Using digital signal processor technology to simplify deep space ranging

Commercially available Digital Signal Processor (DSP) technology has enabled a modified design Deep Space Network (DSN) ranging system. The modified design eliminates custom correlator printed circuit boards in favor of Commercial Off The Shelf (COTS) DSPs. This reduces overall size, parts count, and complexity. Embedded DSP software provides the flexibility to handle pseudo-random noise, sequential squarewave, or other periodic ranging tones. The design implementation will also meet the Jet Propulsion Laboratory (JPL) requirements for both near-Earth and deep space ranging. JPLs Network Simplification Project will consolidate the DSN ground systems functions into less hardware and simplify network operations. This effort includes implementing the new spacecraft ranging design described here. This implementation will reduce the amount of ranging hardware (and thus maintenance) while increasing the operational flexibility. The new ranging implementation also provides a new DSN capability to support the new Spacecraft Transponding Modem (STM) being developed at JPL.

A radioisotope powered cryobot for penetrating the Europan ice shell

The Cryobot team at JPL has been working on the design of a Cryo-Hydro Integrated Robotic Penetrator System (CHIRPS), which can be used to penetrate the Mars North Polar Cap or the thick sheet ice surrounding Jupiter’s moon, Europa. The science for either one of these missions is compelling. For both Mars and Europa the major scientific interest is to reach regions where there is a reservoir of water that may yield signs of past or extant life. Additionally, a Mars polar cap penetration would help us understand both climatic and depositional histories for perhaps as far back as 20 million years. Similarly, penetration of the Europa ice sheet would allow scientists to unravel the mysteries surrounding the thick ice crust, its chemical composition, and subsurface ocean properties. Extreme mass and power constraints make deep drilling/coring impractical. The best way to explore either one of these environments is a cryobot mole penetrator vehicle, which carries a suite of instruments suitable for sampling an...

Future capabilities for the Deep Space Network

This paper will look at three new capabilities that are in different stages of development. First, turbo decoding, which provides improved telemetry performance for data rates up to about 1 Mbps, will be discussed. Next, pseudo-noise ranging will be presented. Pseudo-noise ranging has several advantages over the current sequential ranging, anmely easier operations, improved performance, and the capability to be used in a regenerative implementation on a spacecraft. Finally, Low Density Parity Check decoding will be discussed. LDPC codes can provide performance that matches or slightly exceed turbo codes, but are designed for use in the 10 Mbps range.

Testing theories of relativity with ranging via dual Radio links

The BepiColombo mission to Mercury has Radio Science objectives that explore planetary gravity and interior structure as well as tests of aspects of the general theory of relativity. Both classes of science objectives rely on high precision Doppler and ranging at dual wavelengths, X-band and Ka-band. In particular, the relativistic tests require ranging between a ground station and the spacecraft to 15 cm (+/− 5 cm), which is approximately one order of magnitude better than current capabilities at X-band. A new scheme has been developed to reach this accuracy via three techniques: (1) utilizing the simultaneous propagation of two links modulated with ranging codes in order to calibrate the dispersive media, (2) recording the uplink reference tone in the downlink data stream to provide accurate calibration, and (3) employing open-loop receivers with user-developed post-processing algorithms to produce the final data products. This paper12 will present the proposed instrumentation being prototyped, the encountered challenges and their solutions, as well as the expected results and their implications to the scientific investigations of BepiColombo and future missions.