Software Defined Radios
The SCaN Testbed consists of three reconfigurable and reprogrammable Software Defined Radio (SDR) transceivers/transponders:
JPL SDR: S-Band and L-Band (GPS)
GD SDR: S-Band
Harris SDR: Ka-Band
The SCaN Testbed orbits the earth in low orbit, pointing to a series of NASA Space Network (SN) TDRSS satellites in geosynchronous orbits and NASA Near Earth Network (NEN) stations, as well as experimenter-provided facilities. The three SDRs will provide:
S-band (duplex) microwave Radio Frequency (RF) links directly with the ground, (also referred to as the Near Earth Network (NEN)),
S-band (duplex) microwave RF links with the Tracking and Data Relay Satellite System (TDRSS), (also referred to as the Space Network (SN)),
Ka-Band (duplex) with TDRSS,
L-Band (receive-only) with the Global Positioning Satellite System (GPSS).
Software Defined Radio Software
Each SDR has an Operating Environment (OE) which provides a software infrastructure (including an operating system), command processing, interact with hardware, and configure the SDR. All three OEs comply with the STRS Standard. SDR must run waveforms which implement the capability of the radio and generate the RF signal that will be transmitted. The OE does not actually generate or receive signals or perform communication functions. That is done by loadable waveforms which use the resources provided by the hardware platform and OE to communicate, network, or keep time (or anything else the experimenter wishes to do).
Radio Frequency (RF) Subsystem
The RF subsystem enables the SDRs to transmit/receive RF signals from the SN and NEN, and receive GPS signals, through one of five antennas (3 fixed, 2 movable).
The Radio Frequency (RF) Subsystem is comprised of:
Traveling Wave Tube Amplifier (TWTA)
three Coaxial Transfer Switches
transmission lines to interconnect the RF Subsystem components with the SDRs.
The RF Subsystem radiates RF signals intended for the Tracking and Data Relay Satellite (TDRS) and the ground and receives RF signals from the TDRS, the ground, and the GPS system. The architecture of the SCAN Testbed permits, and there is a requirement for, the ability of sending RF signals from two separate SDRs to two antennas simultaneously. The ability to send RF signals from two separate SDRs to the same antenna is not supported by the architecture and cannot happen due to switch positions required.
RF Subsystem Interfaces
The RF Subsystem interfaces with the Avionics Subsystem, the Flight Enclosure, the Antenna Pointing Subsystem, and the three SDRs. The Mission Operations tab further details RF communication paths.
Antenna Pointing System (APS)
The Antenna Pointing System (APS) allows the Ka-Band High Gain Antenna (HGA) and S-Band Medium Gain Antenna(MGA) to be moved to track TDRSS (or other experimenter selected targets). The antenna pointing may be done in either open loop or closed loop mode. In the former, the antennas are pointed according to a pre-computed track profile. In closed loop mode, the tracking algorithm uses signal strength information from the Ka-band radio to point the Ka-band HGA more accurately to the Ka-band source. The ISS is sufficiently large and flexible that open loop pointing of the Ka-band antenna may have pointing errors reducing the maximum data rate that can be carried. The gimbaled antennas are locked for launch and deployed on-orbit.
Figure 8. SCaN Testbed Antenna Pointing System (on ExPA)
As shown in Figure 8, the APS consists of:
|Gimbal Control Electronics (GCE)||Receives position and rate commands through a 1553 interface with the Flight System avionics system. The GCE provides position information to the Flight System avionics system through the same 1553 interface.|
|Integrated Gimbals Assembly (IGA)||Consists of rotary coupled azimuth and elevation actuators, rotary joints for Ka-Band and S-Band, support bracket with connectors, and antenna arm with connectors and harness. Each APS actuator has a stepper motor and harmonic gear. The antenna gimbals rotary joints enable transmission and reception of radio frequency (RF) signals throughout the range of motion of the IGA.|
|Interfaces||The APS interfaces with the Avionics Subsystem via MIL STD 1553B and 28VDC and 120 VDC power, Mechanical Subsystem Flight Enclosure, and RF Subsystem S-Band and Ka-Band waveguides as shown below.