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How the Mars Microrover Radio Works [Image]


Let's take a look at how the Microrover Telecommunications Hardware works

These pages are designed to show the operation and hardware details of the four main components that make up the Microrover Telecommunications system.

In this page we will describe:

  • The Principles of Operation
  • The Electrical and Mechanical Specifications

    The four components of the telecommunications system are:

    The Sojourner Rover UHF Radio Modem
    The Lander LMRE (Lander Mounted Rover Equipment) UHF Radio Modem
    The Sojourner Rover UHF Antenna
    The Pathfinder Lander LMRE UHF Antenna

    Additionally we have information on the:

    Sojourner Rover Antenna Patterns
    Lander Mounted UHF Antenna Patterns
    UHF Telecommunication System Link Analysis
    Radio Modem Bit Error Rate (BER) Measured Data

  • The Radio Modem that is inside the Microrover. . . .

    Principles of Operation

    These radio modems operate, in many ways, just as walkie-talkies do. However, instead of sending and receiving voice, these radios send and receive data in the form of digital symbols. The radio modems transmit short bursts of data symbols, termed "packets", each consisting of 2000 eight-bit bytes. On Mars, the data packets will transfer rover camera images and engineering telemetry detailing the operational status of the rover, as well as commands from Earth. Like walkie-talkies, the radio modem can either talk or listen at any given time, with the direction of the flow of information between the rover and the lander being controlled by the rover radio modem using a communication protocol called "half-duplex operation". In other words, the rover itself starts the telecommunication sessions with the lander, so most of the time the LMRE radio is in the receive mode.

    There are two main parts to these radio modems: the digital portion on one printed wiring board, and the analog portion on a separate circuit board. The digital board acts as an interface between the analog board and the computer inside the Sojourner rover (or the computer inside the Pathfinder lander). This digital board processes the data to be sent and received, and directs the communication protocol, that is, when to talk and when to listen. The analog board, when transmitting data, turns on its 459.7 MHz UHF transmitter and sends out modulated radio waves which correspond to the digital information formatted by the digital board. During receive, the analog board is tuned to radio waves that are the same 459.7 MHz frequency. It amplifies and filters them, and extracts, in a process called demodulation, the digital symbols in such a way that the digital board can input each information bit within a packet as it is received.

    The rover radio modem also has a 0.5 W heater attached to its metal frame. This heater is used to raise the rover radio modem's temperature in the early hours of the Martian morning in preparation for the first telecommunication session of the day. This heater was added to the rover radio modem because its crystal oscillator (and that of the LMRE radio modem as well) is not temperature-compensated, allowing the transmit and receive frequency of the radio modem to change with temperature. As the radio modem temperature gets warmer, the transmit and receive frequencies increase; as the temperature gets colder, the frequencies decrease. The maximum permissible frequency shift is on the order of 5 kHz. Testing has shown that when the lander radio runs at about 0C, the fewest communication transmission errors occur when a temperature difference of 20C or less is maintained between the rover and lander radio modems. This will be accomplished in part by monitoring the engineering telemetry and issuing commands from Earth to control power to the rover radio modem heater. Typically the lander battery temperature and therefore LMRE modem temperature, will be between 20C and 30C for daily operations, so with the rover modem temperature running between 25C and 40C we can maintain a temperature difference of less than 20C. This will be accomplished in part by monitoring the engineering telemetry and issuing commands from Earth to control power to the rover radio modem heater.

    Specifications

  • Mass: 105.9 grams
  • Dimensions: 8.13 cm (3.2") length by 6.35 cm (2.5") width by 2.3 cm (0.9") height
  • RF Connector Type: Coaxial SMA
  • DC Connector Type: 9 pin Micro-D (signal and power)
  • DC Bus Voltage: +9 Volts, Regulated
  • DC Bus Current: 28 mA Standby; 35 mA Receive; 170 mA Transmit
  • Operating Voltage: +7.5 Volts
  • DC Power: 1.7 W (includes +9V regulator efficiency)
  • RF Center Frequency: 459.7 MHz
  • RF Channel Bandwidth: 25 KHz
  • RF Signal Modulation: DGMSK (Differential Gaussian Minimum Shift Keying), basically FM modulation
  • Handshaking: Half Duplex (Simplex)
  • RF Transmit Power: 100 mW
  • Computer Interface: RS232 converted to TTL levels
  • Maximum Data Rate: 9600 BPS (Bits Per Second) Asynchronous; Effective :2400 BPS
  • Temperature Range: -30C to +40C (operational), -55C to +60C (storage)

    Flight Rover Radio Modem Hardware Images

    Opened up view of Rover Radio Modem


    The LMRE Radio Modem that is inside the Surface Lander. . . .

    Principle of Operation

    The operation of the LMRE (Lander Mounted Rover Equipment) UHF radio in the Surface Lander is identical to the one in the rover except that it is meant to be powered using a +28 volt source. Therefore it has an extra LMRE electronics board attached to it. Also, the LMRE radio is attached to the Lander battery case and covered by a silvered thermal blanket, and does not get that cold at night because internal heaters are used to maintain the battery temperature. This is an advantage over the rover radio which will need to use a heater in the early morning hours to raise it to a warmer temperature before we start communicating. You'll also notice that the LMRE modem has two DC connectors, one is for power and one is for the signals. In most cases telecommunications and other flight hardware use separate connectors for power and signals to help reduce the effects of noise in the signal lines.

    Specifications

  • Mass: 265.2 grams
  • Dimensions: 10.6 cm (4.2") length by 7.1 mm (2.8") width by 5.3 mm (2.1) height
  • RF Connector Type: Coaxial SMA
  • DC Connector Types: 9 pin micro-D (signal) 15 pin micro-D (power)
  • DC Bus Voltage: +28 Volts, Regulated
  • DC Bus Current: 28 mA Standby; 35 mA Receive; 170 mA Transmit
  • DC Power: 1.5 Watts (not including +28V DC converter)
  • RF Center Frequency: 459.7 MHz
  • RF Channel Bandwidth: 25 KHz
  • RF Signal Modulation: DGMSK (Differential Gaussian Minimum Shift Keying), basically FM modulation
  • Handshaking: Half Duplex (Simplex)
  • RF Transmit Power: 100 mW
  • Computer Interface: RS232 converted to TTL levels
  • Maximum Data Rate: 9600 BPS (Bits Per Second) Asynchronous; Effective :2400 BPS
  • Temperature Range: -30C to +40C (operational), -55C to +60C (storage)

    LMRE Radio Hardware Images

    Completed LMRE Radios
    LMRE Radio Bottom View
    LMRE Radio Modem Outside View
    LMRE Radio DC Converter Board Top View
    LMRE Radio DC Converter Board Bottom View
    LMRE Radio RF Board Outside View
    LMRE Radio RF Board Inside View
    LMRE Radio Digital Board Inside View
    LMRE Radio Digital Board Outside View


    The Antenna that is on the Microrover. . . .

    Principle of operation

    The main function of an antenna is to aid in the effective transmission of radio waves through space. The Microrover's and Lander's UHF antennas work very much like the antennas on walkie talkies, or on car radios. This type of antenna is referred to as a "monopole" antenna. There are other type of antennas, for example the round parabolic dish antennas for satellite cable TV's or the Yagi style TV antennas seen on many houses. But these antennas are much too bulky for this application. A monopole antenna has a single (mono) element which is used to transmit the electromagnetic signal. The radio signal enters the antenna through a coaxial connector located at the bottom, travels through a short section of balanced coaxial line and is radiated by the monopole. The coaxial line is balanced by the use of a 1/4 wave choke. The UHF radio signal, like all transverse electromagnetic radiation, travels at the speed of light (2.997925 x108 meters per second). The rover antenna is on a mast which deploys when the rover stands up for the first time. Once deployed, it latches into place vertically and remains that way for the duration of the mission. The height of the rover antenna when it is deployed is about 83 cm.

    Rover Antenna Specifications

  • Overall Length: 45.0 cm (includes support tube)
  • Materials: Fiberglass tube, Aluminum Tube, Teflon supports, coaxial cable
  • RF Connector Type: Coaxial SMA
  • RF Center Frequency: 459.7 MHz
  • RF Bandwidth: 700 KHz for < 2:1 VSWR
  • RF Gain: 1.4 dBiv
  • Free Space Match: 1.09:1 VSWR at center frequency

    Rover Antenna Patterns

    Why is it important to know what the antenna patterns look like?

    It is desirable to have an antenna radiation pattern shaped to match its particular application. Satellite dishes are designed to look at a particular location in space and therefore need to have narrow and directive radiation patterns. The rover and LMRE antennas do not need to look up into space, but rather need to look horizontally in 360 degrees. An ideal monopole has a 360 degree radiation pattern that is donut shaped. It is not meant to look straight up, so has poor reception (gain) in that direction. Certain metallic or rocky structures and ground reflections near the monopole antenna will distort its radiation pattern and cause holes or null zones to form. In these null zones the signal can drop significantly causing poor reception. It is important to know where the rover is relative to the lander when these null zones exist, for if two nulls happened to get pointed at each other, there may be no radio reception at all! So knowing the shapes of the antenna patterns can help us predict where weak signal areas may be and work to avoid them. Avoiding these problems may be a simple matter of turning the rover to a certain angle.

    These antenna patterns were taken on the JPL Mesa antenna range using a static model rover. A flight-like rover antenna was mounted to the rover mast and placed a height of 83 cm from the ground. A radio modem operating in CW (Continuous Wave) mode was used to transmit a 459.7 MHz, 100 mW signal from the rover to a receiving antenna attached to a spectrum analyzer receiver. The receive antenna was a flight-like LMRE antenna mounted to the receiver at a height of 80 cm. You'll notice that the antenna pattern taken at a distance of 2 meters looks quite irregular. This is due to near-field distortion and scattering of the RF energy. Farther away, beyond 3 meters, the rover antenna is in the far-field and the true shape of the antenna's radiation pattern becomes more visible.

    Rover Antenna Patterns

    Rover Antenna Pattern at 2 Meters
    Rover Antenna Pattern at 3 Meters
    Rover Antenna Pattern at 5 Meters
    Rover Antenna Pattern at 7 Meters
    Rover Antenna Pattern at 10 Meters
    Rover Antenna Patterns from 2-10 Meters

    Rover Model Antenna Test Range Images

    Rover Model Antenna Image


    The Antenna that is on the Surface Lander. . . .


    Principle of Operation

    The LMRE antenna works very similar to that of the rover antenna. The one exception is that it is not a deployable antenna. Its position is fixed on the support structure next to the LGA antenna.

    LMRE Antenna Specifications

  • Overall Length: 33.6 cm
  • Materials: Fiberglass tube, Aluminum Tube, Teflon supports
  • RF Connector Type: Coaxial SMA
  • RF Center Frequency: 459.7 MHz
  • RF Bandwidth: 16 MHz for < 2:1 VSWR
  • RF Gain: 1.4 dBiv
  • Free Space Match: 1.25:1 VSWR at center frequency

    Flight LMRE UHF Antenna Images

    Flight LMRE Antenna on Lander at Kennedy Space Center SAEF-2 Facility
    Flight LMRE Antenna at Kennedy Space Center SAEF-2 Facility



    Lander Mounted UHF Antenna Patterns

    These antenna patterns were taken on the JPL Mesa antenna range using a static lander model. A flight-like LMRE antenna was mounted to the LGA (Low Gain Antenna) mast and placed a height of 83 cm from the ground. A radio modem operating in CW (Continuous Wave) mode was used to transmit a 459.7 MHz, 100 mW signal from the LMRE antenna to a receiving antenna attached to a spectrum analyzer receiver. The receive antenna was a flight-like rover antenna set to a height of 80 cm and connected to the receiver via a coaxial cable. You'll notice that the antenna pattern taken at a distance of 3 meters looks quite irregular. In particular, at 10 and 330 there are noticeable null zones. This is due primarily to scattering and out-of-phase reflections of the RF energy from the metallic components (e.g. Low Gain Antenna, High Gain Antenna, IMP mast, solar panels) of the lander structure. Farther away, beyond 5 meters, the LMRE antenna is away from this near-field scattering and the shape of the antenna's radiation pattern becomes better defined.

  • LMRE UHF Antenna Patterns

    LMRE UHF Antenna Pattern at 3 Meters
    LMRE UHF Antenna Pattern at 5 Meters
    LMRE UHF Antenna Pattern at 7 Meters
    LMRE UHF Antenna Pattern at 10 Meters
    LMRE UHF Antenna Patterns from 3-10 Meters

    Lander Model Antenna Test Range Images

    View of Lander Model from 0 degree angle
    View of Lander Model from 180 degree angle
    Close up of top of Lander Model


    Rover Telecommunications Link Analysis

    The performance of any radio frequency telecommunications system depends on numerous link parameters. These parameters include things like antennas, transmitters, modulation techniques, coding schemes, carrier performance, ranging performance, noise immunity, etc. All of these various parameters can improve the overall communication efficiency in their own way. When it comes to designing the entire communications system, communications engineers put all the components or subsystems together and determine the system's performance capability. Below we list some of the common link parameters that characterize telecommunications performance:

    Transmitter parameters:

  • RF power to antenna, dBm*
  • Transmitter power, dBm
  • Transmitter circuit and cable loss, units of dB**
  • Antenna circuit loss, units of dB
  • Antenna gain, units of dBi***
  • Cone angle, units of degrees
  • Pointing loss, units of dB
    Path Parameters:
  • Space loss, units of dB
  • Frequency of operation, units of MHz
  • Communications Range, units of meters
  • Atmospheric attenuation, units of dB
  • Weather Conditions; pressure, density, relative humidity
    Receiver Parameters:
  • Polarization loss, units of dB
  • Antenna gain, units of dBi
  • Receive circuit and cable loss, units of dB
  • Pointing losses, units of dB
  • Noise spectral density, units of dBm/Hz
  • Total system noise temperature, units of Kelvins
  • 2-sided threshold loop noise bandwidth, units of dB per Hz
    Power Parameters:
  • Received power, units of dBm
  • Received PT/N0, units of dB-Hz
  • Carrier suppression by ranging channel, units of dB
  • Carrier suppression by telemetry rood, units of dB
  • Carrier power/total power, units of dB
  • Received carrier power, units of dBm
  • Carrier margin, units of dB
    Data Channel Performance:
  • Data bit rate, units of dB
  • Data power/total power, units of dB
  • Data power to receiver, units of dBm
  • System losses, units of dB
  • ST/N0 to receiver, units of dB
  • Threshold ST/N0, units of dB
  • Performance margin, units of dB
  • Array performance increase, units of dB
  • Arrayed performance margin, units of dB
    (ref: JPL 82-76; Deep Space Telecommunications Systems Engineering; p. 6-21)
    *dBm = decibels relative to 1 mW, **dB = decibels, ***dBi = decibels relative to an isotropic radiator

    For the rover UHF telecommunications link analysis we found it was not necessary to consider all of the link parameters listed above, but only a subset. Here is a simplified representative Lander-to-Rover UHF Communications Link Analysis:

  • Worst-case Lander transmitter output power: PTX = +19 dBm
  • Transmitter-to-antenna cabling loss: LTX Cabling = -1.0 dB
  • Average antenna gain relative to an isotropic radiator: GTX Antenna = +1.4 dB
  • Worst-case Lander antenna azimuth pattern null depth: LTX Antenna Pattern = -15 dB
  • Effective (worst-case) Lander isotropic radiated power: LEIRP = +4.4 dBm
  • Required Rover receiver input signal strength for allowed bit-error-rate, given a worst-case radio-link transceiver temperature differential between the Rover and the Lander: PRX = -90 dBm
  • Receiver-to-antenna cabling loss: LRX Cabling = -1.0 dB
  • Average antenna gain relative to an isotropic radiator: GRX Antenna = +1.4 dB
  • Worst-case Rover antenna azimuth pattern null depth: LRX Antenna Pattern = -6.0 dB
  • Allowance for Lander-to-Rover polarization mismatch: LPolarization = -3.0 dB
  • Rayleigh fading margin for <= 10-4 probability of outage: LRayleigh = -40 dB
  • Permissible free-space path loss: LFree Space = -45.8 dB
  • Radial range at given free-space path loss: DMaximum Range =10.12 meters

    P = Power; G = Gain; L = Loss; TX = Transmit; RX = Receive

    The results of this link analysis show the worst case condition and its outcome. Even at the worst case, a 10 meter distant link can still be maintained with a Bit Error Rate of <= 10-4. If we decide to traverse beyond 10 meters there is a very good probablility of having acceptable BER performance and maintaining a good lander-rover telecommunications link.

    Bit Error Rate (BER) Measured Data

    The following table lists the BER of the UHF link between the two flight radios under different temperature and signal attenuation conditions. Click HERE to see a plot of these BER data curves. The radio modems were tested with TTC-6000 BER Communications Analyzers, which transmitted a pseudo-random 65535 bit long pattern through one radio that was received by the other. To get a good sample, a total of 2.3 mega-bits of data was transferred during each 4 minute test. Two mega-bits was also chosen because that is about the size of a typical uncompressed rover image.

    Modem
    Transmitter
    Temperature
    (C)
    Frequency
    (MHz)
    Modem
    Receiver
    Temperature
    (C)
    LO Frequency*
    (MHz)
    Attenuation
    (dB)
    Bit Error Rate
    (BER)
    LANDER SN001 20 459.697567 ROVER SN001 20 414.696925 26 1.00E-07
    LANDER SN001 20 459.697567 ROVER SN001 20 414.696925 56 1.00E-07
    LANDER SN001 0 459.697858 ROVER SN001 0 414.698450 26 1.00E-07
    LANDER SN001 0 459.697858 ROVER SN001 0 414.698450 56 1.00E-07
    LANDER SN001 0 459.697858 ROVER SN001 -30 414.696008 26 1.00E-07
    LANDER SN001 0 459.697858 ROVER SN001 -30 414.696008 56 1.00E-07
    LANDER SN001 10 459.697367 ROVER SN001 -30 414.696008 26 1.00E-07
    LANDER SN001 10 459.697367 ROVER SN001 -30 414.696008 56 1.00E-07
    LANDER SN001 30 459.696350 ROVER SN001 40 414.694275 26 1.00E-07
    LANDER SN001 30 459.696350 ROVER SN001 40 414.694275 56 1.00E-07
    ROVER SN001 20 459.696375 LANDER SN001 20 414.697567 26 1.00E-07
    ROVER SN001 20 459.696375 LANDER SN001 20 414.697567 56 1.00E-07
    ROVER SN001 -30 459.692950 LANDER SN001 0 414.699683 26 1.00E+0
    ROVER SN001 -30 459.692950 LANDER SN001 0 414.699683 40 3.53E-01
    ROVER SN001 -30 459.692950 LANDER SN001 0 414.699683 56 1.50E-01
    ROVER SN001 -30 459.692950 LANDER SN001 0 414.699683 70 9.07E-02
    ROVER SN001 -30 459.692950 LANDER SN001 0 414.699683 90 1.09E-01
    ROVER SN001 -30 459.692950 LANDER SN001 10 414.699050 26 1.00E+0
    ROVER SN001 -30 459.692950 LANDER SN001 10 414.699050 56 4.15E-02
    ROVER SN001 -25 459.693275 LANDER SN001 0 414.699683 26 1.16E-02
    ROVER SN001 -25 459.693275 LANDER SN001 0 414.699683 40 1.25E-02
    ROVER SN001 -25 459.693275 LANDER SN001 0 414.699683 56 1.02E-02
    ROVER SN001 -25 459.693275 LANDER SN001 0 414.699683 70 1.44E-02
    ROVER SN001 -25 459.693275 LANDER SN001 0 414.699683 90 3.62E-02
    ROVER SN001 -20 459.694500 LANDER SN001 0 414.699683 26 7.00E-04
    ROVER SN001 -20 459.694500 LANDER SN001 0 414.699683 40 1.08E-03
    ROVER SN001 -20 459.694500 LANDER SN001 0 414.699683 56 3.93E-04
    ROVER SN001 -20 459.694500 LANDER SN001 0 414.699683 70 7.69E-04
    ROVER SN001 -20 459.694500 LANDER SN001 0 414.699683 90 3.47E-03
    ROVER SN001 -20 459.694500 LANDER SN001 0 414.699683 110 3.97E-03
    ROVER SN001 0 459.696550 LANDER SN001 0 414.699683 26 1.00E-07
    ROVER SN001 0 459.696550 LANDER SN001 0 414.699683 56 1.00E-07
    ROVER SN001 40 459.694942 LANDER SN001 30 414.696233 26 8.88E-05
    ROVER SN001 40 459.694942 LANDER SN001 30 414.696233 56 1.00E-07

    * The Receive Local Oscillator (LO) Frequency = TX Frequency - 45 MHz

    This concludes the overview of the Mars Microrover Radio and Antenna hardware, for a detailed look at Rover Telecom experiments and mission operations click HERE.


    Telecom Team 1-5 [Image]

    Do you have any Questions or Comments relating to Rover Telecom?
    Send them to: rover-telecom@jpl.nasa.gov :-{)
    We do not promise a prompt reply, but will endeavor to answer all email. Please be patient.
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    Contact information on this page is as it appeared during the mission.

    Direct all current requests to: marsoutreach@jpl.nasa.gov


    All information on this site, including text and images describing the Rover is copyright 1997, Jet Propulsion Laboratory, California Institute of Technology and the National Aeronautics and Space Administration.

    This page was last updated Friday October 3, 1997.
    Web Author: Scot Stride, NASA-JPL, Telecommunications Hardware Section 336

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