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Microrover Radio Modem

(Text only version)


Microrover Telecommunications

Greetings,

First, if you are not yet familiar with the Microrover and the Mars Pathfinder mission we suggest that you visit the Introduction to the Microrover page. There you will get a brief overview of the Mars Pathfinder Mission, the Lander and Microrover. After you have looked over the introduction, come back here.

These pages are organized such that after you have read the Introductions page and this Rover Telecom page, you can then click each section of Mars to learn something new. These pages will be frequently updated as new information, pictures and data from the mission come in.

What is Microrover Telecommunications?

The Microrover telecommunications system is a two-way wireless UHF (Ultra High Frequency) radio link between the Lander and the Rover. The radio link is used to send commands from Earth to the Rover and receive images and data from the Rover. Because the Microrover radio has a signal range similar to a walkie-talkie, we cannot communicate directly to the Rover from Earth. All Rover communications is done with the aid of the Lander communications interface. Click here to see a diagram showing the Lander and Rover radio links.

What makes up the Microrover Telecommunications System?

The telecommunications system is composed of two UHF radios and two UHF whip antennas. The Microrover radio is located inside the Rover WEB (Warm Electronics Box) where it is protected from the extreme cold of the Martian environment. The radio is connected to the Microrover antenna using a short piece of coaxial cable that passes through the wall of the WEB. The radios that are used in the Microrover telecommunications system were purchased from Motorola's Paging Products Division. Several components that were designed and used in these radios were made by a company named DataRadio. These are off-the-shelf commercial radio modem's (modulator+demodulator) that were modified to meet the communication needs of the Microrover mission. The antennas were designed and built by our Telecom team here at JPL.

Before we purchased a large number of Motorola RNet 9600 radio modems, we did a study to see if it was better to 'make' the radios or to 'buy' them ourselves. The main factors under consideration in the 'make' or 'buy' trade-off study are, in order of priority: funding level (we knew how much money we could spend), schedule of hardware delivery (we knew when we had to deliver the radios) and environmental specifications (we knew the operating environment of space and the Martian surface). It was known based upon a Mars Lander-Rover radio link analysis performed in 1989 that a Radio Frequency (RF) range of 100 to 450 MHz (1Mega Hertz = 1 million cycles per second) should be used for surface-to-surface communications. Consequently an industry search was made for sources of wireless radio modems and antennas in this frequency range.


Below we will look at the history of the decisions and progress made in delivering the Rover telecommunications system as well as answers to frequently asked questions.

A bit of History....

  • In July 1992, JPL section 336 and 339 engineering personnel started looking to see who sold wireless UHF radios and antennas. Preliminary investigations started with the identifications of 11 Modem and 6 antenna companies (both commercial and military) were identified and a careful comparison was made of the specifications of these components. The goal was to see if we could purchase a radio modem or an antenna that was as close to the desired mission specifications as possible. This way we would not have to spend so much effort to rework the components to make them meet the flight requirements. The vendor survey was completed in August and the Motorola RNET 9600 Radio Modem was selected as the best candidate. Subsequently, in September, five radio units were ordered, three for evaluation by the telecommunications hardware/systems group and two for the Rover Control Systems group. In October, five UHF quarter-wave monopole antennas were purchased and received from Motorola. The radio modems were received in November along with PC based TelNet software from Metric Systems Corp. and RNETix MS-DOS drivers from COMtrix Systems, Inc. Lastly, a laboratory was set up at JPL building 161 to test the hardware and software interfaces.

  • In 1993 we started an extensive study which was carried out through 1993. We needed to understand several important issues:

    The main factors of considerations were:

    With the Motorola radio chosen as the candidate for the 'buy' side, we looked at how we could successfully 'make' our own version of a UHF radio. We determined that JPL Rover telecommunications spending levels would not permit the 'make' option (we could not afford to make it ourselves). It was adopted then that we would procure these best available commercial radios and space qualify them ourselves. The terms 'best' and 'space qualify' for this project, mean delivering a set of radios which meet:

    We projected that, with some effort, we could meet these requirements using the commercial Motorola radios. A second layer trade-off study was later conducted, to determine that following the philosophy of space qualifying the best available commercial radios was still the right approach. This second trade-off study compared the Motorola commercial radios with military standard radios, using the same factors of considerations. We found that we could still maintain schedule within cost, using the commercial radios. Though military radios would be more mechanically robust and require almost no additional re-work, unfortunately, they were also bigger, heavier, and consumed more power. So the decision remained to buy the Motorola radio modems.

  • In 1994 after careful consideration of all the factors and after conducting a series of engineering evaluation tests on the Motorola radio modems, the final decision was to buy commercial radio modems and to design & build the antennas at JPL. We proceeded with purchasing the commercial radios, developed a procedure to modify them to suit this mission and performed many engineering tests. We also started the design of the UHF antennas.

  • In 1995 the modifications and tests of the radio modems continued. Many long weeks were spent testing and screening the lot of 30 radios we purchased, looking for at least 8 which could be used as flight and flight spare candidates. Also, to complete the empirical designs of the antennas, we did numerous tests at the JPL outdoor Mesa antenna test facilities.

  • In 1996 we completed a series of final qualification tests. Flight worthy radio modems and antennas were completed and installed in the Rover and Lander in preparation for launch in December 1996. A total of 5 Microrover style radios were built, one flight unit, one flight spare, one SIM (System Integration Model) unit, one SIM spare and one qualification unit. A total of 3 Lander LMRE (Lander Mounted Rover Equipment) style radios were completed, one flight, one flight spare and one qualification unit. One EM (Engineering Model) and one flight rover UHF antenna were built. One EM and one flight LMRE style antenna were built. All of the flight hardware was delivered to the project between January and March 1996.

    The journey from commercial-grade radio modems to space-flight ready hardware....

    As you'll recall, for this Microrover mission we decided to buy "off-the-shelf" commercial radio modems, then modify them to be space-flight ready. Our concerns were both electrical and mechanical.

    Electrically, we could only test these radios to make sure they will work at the low temperatures that are required. On Mars, their real operating temperatures are expected to be -30C to +40C. Preliminary tests show that these radios will operate at the low temperatures, with some performance degradation. So we proceeded to address the mechanical issues.

    Mechanically, we had to make these radios more rugged, to survive the shock and vibration associated with launch and the harsh landing onto the Martian surface. Further, we had to replace parts that are not suitable for space/vacuum operations. Our philosophy was to modify as little as necessary. With a team of packaging experts, we considered carefully the risks and benefits of these modifications.

    Our decision was to replace the plastic connectors with discrete wires, replace the commercial bracket of the radios with a more rugged stainless steel one, replaced the commercial outer metallic box with a wrap made out of layers of fiber glass tape and aluminum tape, and replace the commercial BNC RF connector with an SMA connector. These modifications were done on eight radios. Remember we needed a minimum of eight radios for: Microrover Flight radio and its spare, a Lander radio and its spare, two radios for qualification environmental tests, a Rover SIM and its spare. The SIM Rover is named Marie Currie and is an exact replica of Sojourner. We keep Marie Currie here on Earth for mission operations simulations in the Mars Pathfinder testbed. We did a series of preliminary electrical screening tests to select the eight top-performing radios, to be modified into space-flight hardware. We simply did not see the need to modify anymore radios, especially when these modifications were costing both time and money, both of which we did not have plenty of.

    The Environmental Tests (Thermal, Shock & Vibration): Procedures and Results....

    As we were deciding what sort of modifications we needed to make on the radio modems, to make them space-flight ready, we worked in parallel on the environmental test procedures.

    We understood the expected temperature environment, the shock and vibration environment in which these radio modems will have to survive and operate. For temperature, the radios have to survive -55C to +60C, while they will have to operate from -30C to +40C. As for shock and vibration, the radio modems have to survive the launch vibrations and the shock of landing on the Martian surface equivalent to an impact at 40 mph; they are not required to operate during launch and landing.

    Once these radios were modified to be space-flight ready, we put them through a series of environmental tests. We first put them through shock and vibration tests, by mounting the radio modems on a 'shaker table' which shook them at the necessary vibration levels. Then we placed the radios inside thermal chambers, which cooled and heated them. By first shaking the radios, and then thermal cycling them, we more closely emulated the sequence of events they will see during the mission - launch, landing and then Martian surface operations. We expected these radios to pass the environmental tests and they did.

    What about the outer space radiation environment?

    Certain flight components are susceptible to the damaging effects of radiation. There are different types of radiation, some more harmful than others. Spacecrafts need to function in the presence of ultraviolet radiation. This is the type of radiation which we on Earth are protected from by the ozone layer. The ozone layer is about 30 Km above earth's surface and protects us from the damaging effects of ultraviolet radiation by absorbing solar wavelengths between 2,000 and 3,000 Angstroms (1 Angstrom = 10-8centimeters). The entire Mars Pathfinder spacecraft in the cruise and landed configurations was tested in a 25' space simulator where it was subjected to high intensity light twice as bright as the sun. The Martian atmosphere is very thin which does not protect the planets surface from ultraviolet radiation, but the Mars Pathfinder and Microrover hardware have been designed, built and tested to survive this radiation for an extended time period.

    The sun gives off other types of radiation which is potentially damaging to spacecraft. Solar flares and prominences erupting from the suns surface eject a stream of high energy ionizing radiation and sub-atomic particles into space. In addition, cosmic rays originating outside the solar system add ultra-high energy ionizing radiation to the mix. These ions can strike certain electronic circuitry on the spacecraft and cause any number of Single Event Phenomena (SEP) to occur. These range from soft errors, hard errors, latchup, burnout and transients. Fortunately, the probability of such particles affecting the rover telecommunications hardware is small because the sun is expected to be at a solar minimum during the mission. But, nevertheless, the experimental radios used during screening were taken through extensive radiation testing at Brookhaven National Laboratories on Long Island using their Van De Graaff accelerator. We learned from these tests what can happen to the radios if highly energetic ions hit them. It did not permanently damage them, but caused a condition called 'Single Event Latchup' or just 'latchup' to occur. Latchup is a condition in which certain transistor junctions in VLSI (Very Large Scale Integrated) and ASIC (Application Specific Integrated Circuits) chips short to ground, thereby drawing very high current (specifically, an abnormal low impedance, high current state induced in a parasitic P-N-P-N structure of a bulk CMOS IC). The latchup current in the radios was high, but limited by the presence of an on-board voltage regulator. It was found that the radio modems can recover from this latchup condition, with no damage, by simply turning them off and then on again (this is known as power-cycling). There are software algorithms in the rover and hardware timing circuitry in the lander which detect this condition and automatically power cycle the radio to provide this protection and restore it to a normal working condition.

    Modem Radiation Test Setup 1 at Brookhaven
    Modem Radiation Test Setup 2 at Brookhaven


    What about Electrical Interference from the Lander and its Telecom System?

    The telecommunications system on the Lander uses X-band microwave uplink and downlink signals. These microwave signals occupy two narrow bands that are about 15.5 times higher in frequency than the UHF band used by the rover. There is no way that the lander signals can be received by the rover and vice-versa. But to confirm this we had to run a series of EMC (ElectroMagnetic Compatibility) tests. These tests were performed at three different locations:

  • At the JPL EMC laboratory with the lander RFS (Radio Frequency Subsystem) and a set of non-flight radio modems,
    Test Image 1 in JPL EMC Chamber
    Test Image 2 in JPL EMC Chamber

  • At the Mesa antenna range with the Lander High Gain Antenna (HGA), 12 Watt microwave transmitter and a set of radio modems,
    EMC Test Image 1 in Mesa Anechoic Chamber
    EMC Test Image 2 in Mesa Anechoic Chamber

  • At the SAEF-2 facility at KSC using the fully functional Lander and Rover Telecom systems,
    EMC Test Image 1 at KSC SAEF-2 facility
    EMC Test Image 2 at KSC SAEF-2 facility
    EMC Test Image 3 at KSC SAEF-2 facility
    EMC Test Image 4 at KSC SAEF-2 facility
    EMC Test Image 5 at KSC SAEF-2 facility

    These tests confirmed that the effects of interference by the lander and rover telecommunications hardware on each other were within acceptable limits and not a problem at all!


    This concludes the overview of the Mars Microrover Telecommunications System, for a detailed look at the Radio Modem and Antenna hardware click HERE.


    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.

    All information on this site, including text and images describing the Rover is copyright 1997, 1998 Jet Propulsion Laboratory, California Institute of Technology and the National Aeronautics and Space Administration.

    This page was last updated Thursday September 3, 1998.
    Web Author: Scot Stride, NASA-JPL, Telecommunications Hardware Section 336

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