Basics of Space Flight Section 3

Chapter 18. Deep Space Network     CONTINUED

  On this page we'll follow a typical communications link from a spacecraft into the DSS antenna, through the DSN Downlink Tracking & Telemetry subsystem (DTT), and on to JPL. Also, command data is traced from JPL to the DSCC and out the DSS antenna toward the spacecraft via the DSN Uplink Tracking & Command subsystem. The first diagram shows equipment located within a DSS. The second diagram shows equipment located within the SPC.

Data Flow at the DSCC

  • Downlink RF enters the DSS antenna reflector shown in black in the diagram, and proceeds down the green line, which represents waveguides. Along this initial path is where the five reflectors of a BWG would be located, directing the RF into the basement where the rest of the equipment is UWV S/S located. With other DSSs, all the equipment in the first diagram is located in the feedhorn and just below the reflector, where it all moves with the reflector as it tracks the spacecraft.

    The blue arrow on the left indicates antenna control signals going to the DSS antenna equipment from the DTT in the SPC. All the other components in this diagram belong to the Microwave subsystem (UWV).

  • The green line comes to a dichroic plate, also called a dichroic mirror, which is perforated with an array of precision holes. RF at one frequency, for example X-band, passes through the plate's holes (because the wavelength is small enough), to the gold colored path below. RF of another frequency, for example S-band, finds the holes too small to pass its longer wavelength, and reflects off to follow waveguides in another path colored blue. Some DSSs can also select Ka-band or other bands of RF using dichroic plates. The desired polarization is then selected using filters. This might be right-hand circular polarization, RCP, or left-hand, LCP, or none.
  • The RF of each band, S and X in this example, goes to a low-noise amplifier, LNA. The LNA used depends on what is installed at a particular DSS and the needs of the user. It may be a cryogenically cooled amplifier called a maser (an acronym from "Microwave Amplification by Stimulated Emission of Radiation"), or it may be a solid-state device called a high-electron-mobility transistor, HEMT. The function of the LNA is to amplify a band of RF while introducing an absolute minimum amount of noise. DSN's masers are cooled with liquid helium to keep RF noise down. An amplified band of RF leaves the LNA and is directed toward the receivers, which are shown in the next diagram. Before leaving the DSS, the downlink RF is converted to a lower frequency signal known as the Intermediate Frequency (IF) signal. The IF signal can be carried more conveniently, by coaxial cable or fiber optics for example, than the RF signal which typically needs waveguides.
  • Before getting to the next diagram, notice the red line labelled "From Exciter" coming up to the klystron on the right. This represents the uplink signal to be amplified by the klystron, which is a microwave power amplifier vacuum tube. The signal is generated by the exciter (part of the receiver) based on a reference frequency provided by FTS, and other inputs to be discussed later. Polarization of the klystron's output is selected by a filter to match the spacecraft's receiver. The klystron's output illuminates the DSS's antenna reflector so it can be seen by the spacecraft. Most klystrons have to be actively cooled by refrigerated water or other means. The klystron, its high-voltage-DC, high-current power supply, and its cooling apparatus are collectively known as the Transmitter subsystem, TXR, or XMTR.
  • Looking at the lower diagram, which represents equipment in the SPC, the IF signal from the LNA in the DSS enters at the top. If two or more LNAs are operating, the path would be multiple. Depending on operations, the LNA may feed either a closed-loop receiver, an open-loop receiver, or both. The switches in the diagram show there's an operational choice. An open-loop receiver is used for radio science, and also for VLBI.

    The open-loop receivers select a band of frequencies to amplify for further processing and storage by RS or VLBI equipment. VLBI equipment typically outputs data to tapes that are delivered to a correlator at a different location. Radio DSCC S/S science equipment, controlled remotely from JPL, will typically output its high-volume data online for transmission to JPL via the GCF, indicated by the block at the bottom of the diagram, or by other carriers.

  • RF from the LNA can also go to the closed-loop receiver. DSN uses its highly advanced receiver known as the Block-V receiver (V is the Roman numeral five), BVR. The BVR is part of the Receiver & Ranging Processor (RRP) in one of several Downlink Channels (DC) in the DTT subsystem. In the BVR a single frequency, the spacecraft's downlink, is selected and amplified. If there are any subcarriers carrying telemetry or ranging data, they are detected here, and if symbols are present on the carrier or subcarriers, they are recovered within the BVR for decoding and further processing.
  • At the discretion of the Ace, a program known as conscan may be invoked. Conscan, which stands for conical scanning, observes the closed-loop receiver's signal strength and adjusts the antenna pointing via the DTT. The antenna constantly moves in small, tightening circles as it optimizes its pointing. Conscan must be disabled when the spacecraft's signal changes or disappears. It is not desirable to conscan during VLBI or RS operations due to the variations it induces in signal level. RS does, however, have a feature called monopulse to optimize Ka-band reception. Monopulse creates records of the adjustments it induces that can be accounted for in data analysis.
  • In the RRP, the downlink's Doppler shift is measured and compared with the predicted Doppler shift. The difference is called the Doppler residuals. If there are ranging symbols on the downlink, they are processed within this subsystem as well. The ranging and Doppler data is passed to the navigators at JPL via the GCF.
  • If there are telemetry symbols present on the downlink, they are further processed within the DC. First, if applicable to the particular spacecraft, they are Viterbi-decoded to recover data bits from the convolutionally coded symbol stream. The assembly that does this is the maximum-likelihood convolutional decoder, MCD. If any other coding, such as Reed-Solomon is present, it can be decoded here or at JPL. The bits are then grouped into the same packages, called transfer frames, that the spacecraft had grouped them prior to downlink. Most newer spacecraft comply with the CCSDS standards for grouping bits into packets and transfer frames. The TLM data is then sent to JPL via the GCF.
  • Command data intended to be sent to the spacecraft comes from JPL via the GCF as indicated on the right side of the diagram. The Uplink Tracking & Command subsystem (UPL) processes the data and sends the bits, on a subcarrier if applicable, to the exciter. Also, and not shown in the diagram, is a response from the UPL to JPL. The response, also designated CMD data, includes information identifying the CMD bits that have left the antenna, and reports on UPL states and operations.
  • If operations call for placing ranging symbols and/or a ranging subcarrier on the uplink, the UPL generates and passes these signals to the exciter.
  • The exciter then creates a complete uplink signal with any appropriate command subcarrier, ranging subcarrier, and/or direct data modulation. It sends this signal to the transmitter, which will amplify it enough for the spacecraft to receive it across vast reaches of space.
  • The GCF, indicated by the lower block, uses a reliable network service, RNS, to deliver data to a central data recorder, CDR, at JPL. RNS, using the TCP/IP communications protocol, automatically replays any data that may have gotten dropped during its trip to JPL. The result, given enough time to identify and process the replays, is 100% error-free data transmission.
Some DSN subsystems are not represented in the simplified diagrams above. Frequency & Timing, for example, has inputs to all the subsystems shown. MON data is also collected from all the subsystems and sent to JPL via the GCF.

Colorful Equipment


In operations, subsystems and assemblies are called "green" when they have been operating nominally for a period of time including at least one previous tracking pass. Anything inoperable is designated "red" equipment. And anything that has been repaired and is returning to use during the present tracking pass is designated "orange."

Data at JPL


All the data streams, TLM, MON, TRK, RS, CMD, are processed and/or stored at JPL by the Deep Space Mission System, DSMS. The DSMS uses advanced software and high-performance workstations to process and route the data, to broadcast data in real time, to distribute data, to display data, to store data in online repositories for later query by users, and to archive data on permanent media.

 




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  SECTION I  
ENVIRONMENT
1 The Solar System
2 Reference Systems
3 Gravity & Mechanics
4 Trajectories
5 Planetary Orbits
6 Electromagnetics

SECTION II
FLIGHT PROJECTS
7 Mission Inception
8 Experiments
9 S/C Classification
10 Telecommunications
11 Onboard Systems
12 Science Instruments
13 Navigation
SECTION III
FLIGHT OPERATIONS
14 Launch
15 Cruise
16 Encounter
17 Extended Operations
18 Deep Space Network