Gregory W. Heckler

A GPS Receiver for High-Altitude Satellite Navigation

Although GPS has found wide application for precision spacecraft navigation and formation flying applications in low Earth orbit (LEO), its application to geosynchronous (GEO) and other high-altitude missions has been limited to an experimental role because of the sparsity and weakness of the GPS signals present there. To fill this gap, NASA Goddard Space Flight Center (GSFC) has developed a new space-borne GPS receiver called Navigator that can operate effectively in the full range of Earth orbiting missions from LEO to GEO and beyond. Navigator employs special signal processing algorithms in radiation-hardened hardware that enable very fast signal acquisition capabilities and, more importantly, greatly improved sensitivity (a 10-dB improvement over previous space-based GPS receivers). Because of these unique capabilities, Navigator has generated a large amount of interest in the spacecraft navigation community. The first flight version of the receiver has been integrated into a relative-navigation experiment on the Shuttle-based Hubble Space Telescope Servicing Mission 4, due to launch in 2009. Navigator will be also serving as a critical navigation sensor on NASAs Magnetospheric Multiscale mission, which is one of NASAs first high-altitude formation-flying missions, NASAs Global Precipitation Measurement mission, and the Air Force Research Labs Plug-and-Play spacecraft. Finally, key aspects of the Navigator design are being integrated into a GPS receiver being developed for NASAs Orion Crew Exploration Vehicle.

Space Mobile Network: A near Earth communications and navigation architecture

This paper shares key findings of NASAs Earth Regime Network Evolution Study (ERNESt) team resulting from its 18-month effort to define a wholly new architecture-level paradigm for the exploitation of space by civil space and commercial sector organizations. Since the launch of Sputnik in October 1957 spaceflight missions have remained highly scripted activities from launch through disposal. The utilization of computer technology has enabled dramatic increases in mission complexity; but, the underlying premise that the diverse actions necessary to meet mission goals requires minute-by-minute scripting, defined weeks in advance of execution, for the life of the mission has remained. This archetype was appropriate for a “new frontier” but now risks overtly constraining the potential market-based opportunities for the innovation considered necessary to efficiently address the complexities associated with meeting communications and navigation requirements projected to be characteristics of the next era of space exploration: a growing number of missions in simultaneous execution, increased variance of mission types and growth in location/orbital regime diversity. The resulting ERNESt architectural cornerstone - the Space Mobile Network (SMN) - was envisioned as critical to creating an environment essential to meeting these future challenges in political, programmatic, technological and budgetary terms. The SMN incorporates technologies such as: Disruption Tolerant Networking (DTN) and optical communications, as well as new operations concepts such as User Initiated Services (UIS) to provide user services analogous to todays terrestrial mobile network user. Results developed in collaboration with NASAs Space Communications and Navigation (SCaN) Division and field centers are reported on. Findings have been validated via briefings to external focus groups and initial ground-based demonstrations. The SMN opens new niches for exploitation by the marketplace of mission planners and service providers.

Commissioning of NASA's 3rd Generation Tracking and Data Relay Satellites (TDRS KLM)

In the summer of 2017, the third and final spacecraft of the 3rd generation of the Tracking and Data Relay Satellites (TDRS) launched aboard an Atlas V rocket from Complex 41 on the Eastern Test Range. Finishing final testing and integration in the first quarter of 2018, the TDRS-M communication and navigation satellite completes a constellation that began service in the early 1980s. The 3rd generation of spacecraft, TDRS-K, L, and M, not only provided beneficial systems engineering lessons in handling anomalous Radio Frequency and Doppler interference as well as integrating new spacecraft into an aging ground support infrastructure, but also supplies NASA with a valuable test bed for new operational concepts and technologies useful in defining the future architecture of the NASA Space Network. This paper presents an overview of the TDRS-K, L, and M missions, including transfer orbit, Level 5 bus and payload testing, and finally NASA-led Level 6 testing, which includes active TDRS System (TDRSS) users. Highlights include relevant testing results, commissioning challenges, and lessons learned. The final discussion includes a brief overview of future NASA communication and navigation technologies and network architectures.

Payload Performance of Third Generation TDRS and Future Services

NASA has accepted two of the 3 generation Tracking and Data Relay Satellites, TDRS K, L, and M, designed and built by Boeing Defense, Space & Security (DSS). TDRS K, L, and M provide S-band Multiple Access (MA) service and S-band, Kuband and Ka-band Single Access (SA) services to near Earth orbiting satellites. The TDRS KLM satellites offer improved services relative to the 1 generation TDRS spacecraft, such as: an enhanced MA service featuring increased EIRPs and G/T; and Ka-band SA capability which provides a 225 and 650 MHz return service (customer-to-TDRS direction) bandwidth and a 50 MHz forward service (TDRS-tocustomer direction) bandwidth. MA services are provided through a 15 element forward phased array that forms up to two beams with onboard active beamforming and a 32 element return phased array supported by ground-based beamforming. SA services are provided through two 4.6m tri-band reflector antennas which support program track pointing and autotrack pointing. Prior to NASA’s acceptance of the satellites, payload on-orbit testing was performed on each satellite to determine on-orbit compliance with design requirements. Performance parameters evaluated include: EIRP, G/T, antenna gain patterns, SA antenna autotrack performance, and radiometric tracking performance. On-orbit antenna calibration and pointing optimization was also performed on the MA and SA antennas including 24 hour duration tests to characterize and calibrate out diurnal effects. Bit-Error-Rate (BER) tests were performed to evaluate the end-to-end link BER performance of service through a TDRS K and L spacecraft. The TDRS M is planned to be launched in August 2017. This paper summarizes the results of the TDRS KL communications payload on-orbit performance verification and end-toend service characterization and compares the results with the performance of the 2 generation TDRS J. The paper also provides a high-level overview of an optical communications application that will augment the data rates supported by the Space Network.