Mars Microrover Telecommunications
Frequently Asked Questions

(Text only version)

Detailed answers to selected Frequently Asked Questions about the Mars Rover and Lander

Microrover Mission Milestones
Rover Telecom Lessons Learned
What is Sojourner doing now?
What happened to the Rover Telecom System on Sol 1 & 2?
How do the radio modem telecom protocols work?
How does the lander telecom system communicate with Earth?
How far is the communications range of the rover?
What is the communications delay between Earth and Mars?
Why are the rover batteries not rechargeable?
How long can the rover keep functioning on Mars?
Can I get schematics and engineering drawings of the Microrover?
Where can I download Rover and Lander Images?
Does the Lander or Rover have a microphone for recording sounds?
Will the Rover be coming back to Earth?
Will the Rover collect soil samples?
What kind of computer is in the Rover and Lander?
Where can I get a pair of 3D glasses?

To get to the Live From Mars Frequently Asked Questions site click HERE. This site has dozens of answers to FAQ's about Mars and Mars missions. Also, to hear the daily report on the Mars Pathfinder Mission from North America call 1-800-391-6654 and press 3. Other mission reports are also available at that number.

Microrover Mission Milestones

For those of you who like trivia and factoids, here is a table listing some of the Sojourner microrover's accomplishments. Yes, there is some relative uncertainty in a few of these values. In particular, the odometry values have some statistical error due to gyro drift, accelerometer noise and rover driver heading and position corrections.

Property
Quantity
Notes
Total Sols in Operation 83 Number of Sols the Rover was Known to Function on Mars
Total Hours of Operation 2,023.08 Number of Hours the Rover was Known to Function on Mars
Total Sols of Battery Use 58 Number of Sols the Rover Batteries Provided Power
Last Signal Received by the Rover September 27, 1997 10:17 a.m. PST Sol 83 14:04:34 MLST (Local Mars Time)
Coldest External Rover Temperature xxx °C Measured at Rover Wheels on Sol
Coldest Internal Rover Temperature xxx °C Measured inside WEB on Sol
Warmest External Rover Temperature xxx °C Measured at rover wheels on Sol
Warmest Internal Rover Temperature xxx °C Measured inside WEB on Sol
Left Front Camera xxx Number of B&W images taken
Right Front Camera xxx Number of B&W images taken
Rear Camera xxx Number of Color Images Taken
Alpha Proton X-Ray Spectrometer xxx Number of APXS Soil Spectra Taken
Alpha Proton X-Ray Spectrometer xxx Number of APXS Rock Spectra Taken
Total Amount of Rover Data Sent 245.2825 Mega Bits sent to Earth
Most Bits Sent in one Sol 15.2803 Mega Bits sent to Earth on Sol 5
Average Bits Sent Per Sol 2.9200 Mega Bits sent to Earth over 84 Sols
Farthest Radial Distance 12.336 Meters Rover Distance from the Lander Origin on Sol 74, based on Odometry
Longest Distance Driven in one Sol 7.769 Meters Longest Traverse made by the Rover on Sol 32, based on Odometry
Total Distance Driven 101.756 Meters Based on Rover Odometer Readings
Total Number of Rover 360° Rotations 23.398 As Derived from Accumulated Turning Angle Data

Under Construction

Rover Telecom Lessons Learned

Coming Soon!

What is Sojourner doing now?

On September 26, 1997 we had our last communications session with the rover; that was Sol 83. During normal operations, each command sequence that is uplinked to the rover contains what we call a "runout" sequence. It is basically six days of stationary rover activity consisting mainly of automatic health checks, and MAE or APXS experiments. Sol 81 was the last day that a new sequence was uplinked to the rover, so on Sol 83 the rover was already two days into its "runout" sequence. There are no driving commands performed during the "runout" sequence so the rover just sits and waits. On Sol 87, the sixth day of the "runout" sequence, the rover automatically went into "contingency mode" operation. Contingency mode is implemented under the assumption that the rover has gone out of communications range with the lander by either driving too far away, into a trench or behind a big rock. To help regain communications, when activated, the contingency sequence commands the rover to drive to the origin or center of the lander (Sagan Memorial Station), in effect, go find papa! If during its trek back to the lander, communications is restored, then a new set of commands will be received by the rover ending its contingency operations. If communications is not restored then the rover will try to get there, autonomously on its own. The center of the lander is by design inaccessible to the rover, it can't physically get there. For that reason a 3 meter radius virtual barrier or wall was programmed in the rover software to prevent it from accidently driving into the lander. This 3 meter stayout zone can be reprogrammed by ground controllers if necessary.

When the lander problems began, the rover was at a rock named Chimp, where it had performed an APXS measurement on Sol 81 and 82. Chimp is about 9.3 meters radially from the center of the lander. The drive back to the lander could have taken several different paths. One possibility is that the rover drove in an arcing turn toward the lander as shown HERE. Note, the forward ramp is inside the 3 meter stayout zone. In another possibility, the rover would have executed a left turn and made a beeline to the lander. This path would have possibly taken it between the rocks named Hassock and Wedge. However, the rover uses its hazard avoidance to drive around rocks, so if it encountered any nearer rocks, particularly Ender, first it would have tried to drive to either side of them depending on its assessment of the hazards. If it got past those obstacles (located about 4.5 meters from the lander) without any severe driving (articulation) errors it could have arrived at the lander in the vicinity of either the forward ramp or near Torres rock. If it made it to the 3 meter virtual wall, it would have stopped and then tried to drive around the wall like any other hazard. But since it's onboard software won't let it go beyond the 3 meters, the rover will begin to drive around the lander following an arcing circular path. Since the rover has inherent drift in its gyros and somewhat noisy accelerometers, its autonomous driving may incur heading errors. Depending on how long the rover is driving, the accumulated heading errors may cause it to spiral toward or away from the lander, possibly getting it into trouble. In it's driving around the Lander, Sojourner could have accidently driven up onto a rock and received a traverse error that would have stopped it. If that has happened then the rover will be parked in that one spot, never to move again on her own. Without any hard data, these scenarios are mere speculation based on how we know the rover operates. In reality we just don't know and may never know where Sojourner is and what she's doing right now.

What happened to the Rover Telecom System on Sol 1 & 2?

Before we discuss the assessment of the rover telecommunications problems encountered on Sol 1 and Sol 2, we want to preface this FAQ by pointing out the simple nature of the telecommunications hardware in use. These are commercial 'off-the-shelf' radios that have been upgraded to the best of our abilities with available time and funding to meet the flight requirements. These radios have only temperature monitoring telemetry of internal hardware conditions, so it is difficult to truly understand how well they are functioning in this new environment. For that reason any conclusion of what happened on those two days, based on such meager measured data, is still somewhat speculative. Nonetheless, a best estimate of what we feel happened will be presented. Note however, that the Motorola radio modems *DID NOT* suffer a hardware failure at any time, and given the current circumstances are working as expected; nothing had to be fixed, just understood. It is unfortunate that the July 5 press briefing was not as revealing as it could have been. At that time the the most accurate statement that could have been made was: "we don't know why rover telecommunications has degraded, and the rover telecommunications team is in the process of making an assessment". Any statements made about possible causes were personal speculations by project representatives based upon sketchy information and were expressed as such. Unfortunately, the (sometimes) interrogative nature of press briefings tend to pressure the person being interviewed to confess an answer, even though there isn't one. Not to fault the media, but sometimes the 'possible answers' get transformed into 'the answer' and published that way. That is just the nature of the beast! Hopefully, this FAQ will set the record straight and help those interested understand what happened, how it was overcome and how we are proceeding now.

Sol 1 Rover Telecom Scenario:

The rover woke up via lander reed relay at 06:59:13 TLST as expected and immediately produced erroneous level 2 health check telemetry caused by a known +12V regulator (A/D converter) problem not related to the radio modem. The first level 2 health check data was unusable and a subsequent commanded level 3 health check (cmd 1034) was performed (291 bytes) at 07:35 TLST. Telemetry from the rover health check indicated that the telecom system was working nominally with a modem operating temperature of -4.0 degree C. At approximately 7:40 TLST the lander LGA downlink session had ended. It was noticed from 7:38 to 9:21 TLST that the LMRE (Lander Mounted Rover Equipment) link quality, which is a measure of how well it is receiving good data frames (1 complete frame = 6 byte ACK + 250 bytes of data) from the rover, dropped to 44% and continued to degrade to 14%. After that point no more good data was received from the rover for the remainder of the sequence. A large number of garbled frames (ones containing CRC errors), however, continued to be received by the LMRE modem which indicated that the rover modem was still transmitting, and had not completely failed. During this period when rover communications degraded, the lander switched from the low gain antenna to the high gain antenna after doing a sun-search with the IMP camera. Also, the lander +Y petal was elevated to 45°, an air-bag retraction sequence was run, and the petal was returned to a horizontal position. All during this time the rover was in the stowed configuration on the petal with its antenna down. Lastly, it was decided not to deploy both rover ramps that day and wait until Sol 2 to perform the rover release, standup and egress down the rear ramp.

Sol 1 Considerations:
The following have been considered and discounted as communications problems:

  • A lander AIM software problem interfacing with the LMRE modem.
  • A lander Bus voltage problem with the LMRE DC-DC converter board.
  • An interference condition (modem front-end overload) between the lander microwave transmissions and the rover UHF communications.
  • Rover or lander UART crystal frequency drift.
  • Rover CPU crystal oscillator failure.
  • LMRE voltage regulator failure.
  • LMRE low power latchup condition.
  • A failure of the HOLD_COM command sequence in the rover sequence.
  • Increased Solar X-ray and Proton Flux activity which would induce an SEU event.
  • An SEU event in either the LMRE modem or Rover modem. No events were detected or recorded.
  • A disconnection of the SMA RF connector to the LMRE antenna. (visually intact)

    The following are the most likely contributors to the communication degradation problem:

  • A 24C temperature difference between the LMRE radio and the Rover radio causing shifts in the Rover TX and LMRE RX frequencies. The LO crystals' nominal center frequency are temperature dependent.
  • The Rover antenna was in the stowed configuration creating a cross-polarized condition between the two UHF monopole antennas. Based on measured data with a rover model, this results in at least 10 dB of attenuation in the UHF RF link. Also while stowed the rover antenna is in close proximity to the solar panel which significantly de-tunes the antenna. Furthermore, the LMRE antenna in this configuration is blocked to the line-of-sight of the rover by the lander LGA.
  • The rover location on the +Y petal is in a region of significant RF scattering and multi-path caused by the lander structure.
  • The environment close to the lander was somewhat noisy electrically during this time, with stepper motors, actuators and electronics causing a general increase in the noise floor in the vicinity.
  • Analysis of the garbled frames received by the LMRE radio indicated a large number of CRC (Cyclic Redundancy Character) errors and short frame errors (truncated frames) which implies that the telecom link BER performance was degraded.

    The protocol at the data-link layer used on the UHF link between Sojourner and the Pathfinder lander is of the acknowledge/negative-acknowledge (ACK/NAK) type. There is no forward error correction channel coding whatsoever. The response of ACK or NAK is dictated by a cyclic redundancy character (CRC) check computed over the contents of each transmission frame. A frame generating a NAK can be retried up to three times before the software errors-out and skips to the next frame in the transmission queue. ACK/NAK protocols are very simple to implement and permit confidence in the fidelity of the data transfer when signaled by the ACK confirmation, but they are extremely fragile data-link layer implementations: that is, the throughput of the link can collapse catastrophically for small incremental changes in the bit error rate (BER) of the physical layer. Click on this graph to see how quickly the probability of receiving different size data frames can change with just a small change in BER performance.

    The maximum frame size used on the UHF link between the Sojourner rover and the Pathfinder lander is 256 bytes, and examination of the graph shows a rapid collapse of communications throughput for bit error rates exceeding approximately 10-4. Note also that short frames (those of length less than 256 bytes) can get through with high probability even when those of maximal length are likely to fail. It was this phenomenon that was frequently observed on Sol 1 and 2: short frames got across; fully stuffed frames did not. Clearly the communications problem was not one of hardware failure or even of hardware intermittency, it was mainly an increase in bit error rate that was coupled with an extremely unforgiving data-link layer protocol.

    Sol 2 Rover Telecom Scenario:

    During the night of Sol 1 the lander experienced a software reset which, it was concluded, did not affect the quality of the lander to rover UHF link. Also, it has been concluded that switching between both LMRE modem DC-DC converters had no corrective effect. It is likely that because of configuration differences with the location of the HGA and IMP that scattering/multipath conditions at the beginning of Sol 2 had changed. The changes may have made the communications environment favorable enough to allow reception to occur.

    The rover woke up nominally via alarm clock on Sol 2 at 07:01:53 TLST with no A/D problems. The small packet size command sequences for Sol 2 were received normally by the rover. This indicated that the rover was able to receive data without any problems and it then began transmitting data to the lander. Its first level 2 health check showed that its modem temperature was at -30C, much colder than the day before because no modem heating was performed. The rover was still stowed on the petal with its antenna down. Data was apparently buffered on the rover overnight when comm with the lander was lost. A total of 31,491 bytes of sequence data were received. This coincides with the amount of EEPROM available for buffering data (32K). All indications are that the rover continued to perform runout or commanded sequence but had no place to buffer the additional data. The rover UNSTOW (cmd 2520) command executed nominally at 11:47:55 TLST. At that time the rover UHF antenna was deployed and a co-polarization condition existed between the rover and lander antennas. LMRE Link quality was marginal, and remained at about 40% until rover egress. During the second downlink pass no rover data was received because a mistake in the lander sequence turned off the LMRE radio from 8:01 and 11:29 TLST. This was reflected in the rover telemetry as an increase in timeout errors as the rover attempted to communicate with the lander which was not receiving. In downlink session 3 the rover was executing the remainder of its final Sol 2 sequence which included the petal egress move down the rear ramp.

    The traverse down the rear ramp is executed with two 1000mm MOVE commands. After confirmation of the first MOVE command, which would have placed the rover half way down the ramp at 14:40 TLST, communication with the rover ceased. Rover telemetry at that moment showed the modem temperature to be +30C. Lander data indicates that the rover continued trying to communicate with the lander for another 16 minutes (trying every 10 seconds to send the second 3628 byte MOVE command and other commands) until the rover executed a SHUTDOWN command at which point the lander no longer heard from the rover. Lander data also indicates that at that time the LMRE link quality had degraded to 27%. With 100% link quality, it should have taken just 12 seconds to receive the second move command. At a lower link quality it would have taken significantly longer to receive. Being unable to send the MOVE command data, the rover buffered that data to memory. From examination of the rover sequence (50250V), after the rover performed the second MOVE command, it was commanded to HOLD_COMM and deploy the APXS onto the soil. After that was done, HOLD_COMM was removed. Note, it is a requirement that rover communication be suspended during APXS activities because a +9V DC converter is switched from the modem to the APXS (both can't operate at the same time). With 6 wheels on soil and in a slightly different location after the final MOVE, the rover then successfully transmitted some of its buffered telemetry data to the lander, including a rover image taken of the front ramp. The rover performed an auto shutdown at 19:08 TLST and remained silent (no communication) for approximately 18 hours and took two APXS spectra of the soil.

    Sol 2 Considerations:
    The following have been considered and discounted as communications problems:

  • A thermal problem with either the lander or rover radio.
  • A relative angular condition between antennas that would have caused cross polarization problems.
  • Rover or lander command sequence execution. There were no commands that would have suspended communications during the final move down the ramp.
  • An SEU latchup condition in either the LMRE or Rover radio modem. No events were detected or recorded.

    The following are the most likely contributors to the communication degradation problem:

  • Scattering and multipath conditions caused by close proximity to the lander +Y petal and rear rover metallic ramp.
  • Degraded BER performance which delayed lander receipt of the second MOVE command that was fairly large, 3628 bytes, in size.

    Again, based on these two key factors listed above and the sensitivity of the rover protocol layer to BER we feel that it was an individual factor or combination of these factors which resulted in the degradation and unexpected loss of radio communication during the remainder of the Sol 2 sequence.

    Sol 3 Rover Telecom Scenario:

    The rover woke up nominally on Sol 3 using solar power at 05:27 TLST. The command sequence for Sol 3 was queued on time and sent to the rover. The rover radio modem was much warmer (-15C) at wake up on Sol 3 than Sol 2 because of the execution of a modem heating command. The two overnight Sol 2 APXS spectra were received by the rover, as well as the remainder of the final Sol 2 telemetry. The Sol 3 sequence performed very well , with the rover positioning its APXS on the rock named 'Barnacle Bill' for the night. LMRE Link quality steadily improved throughout the day and into the night, with the improvement in link quality being related to the rover moving out of the region of scattering/multipath by the lander, and its modem operating temperature being above +25C. The only problem relating to telecom was 15 lost packets of rear image data with all other data successfully sent.

    Conclusions and Operational Recommendations for the Mission:

    Initially, all rover telecom problems were caused by a combination of environmental and configuration conditions. At no time were there any electrical, mechanical or software interface failures with either the LMRE radio modem or Rover radio modem. LMRE modem UART interface resets occur periodically, and are a normal part of operation on that side of the interface. Rover modem resets occur when the rover performs a SHUTDOWN or any APXS related activities in its sequence, which requires a HOLD_COMM command. Note, the only factor which has not been conclusively ruled out is the possibility of the loss of hermeticity in one of the four crystal local oscillator cans used in the modems. The crystals utilize a solder perform hermetic seal between the base and body of the can, in a 1 atmosphere inert gas. A failure of the hermetic seal would definitely shift the crystal's frequency (on Mars) because of the change in pressure inside the can. According to the manufacturer, a 5 PPM change in the resonant frequency of the crystal equates to about a 2 KHz frequency shift of the output frequency. But, based on the recent operation of the telecom system, and a comparison of pre-delivery thermal test data performed at JPL and thermal data from Mars, the probability of loss of hermeticity is low. If it has occurred, the consensus is that the LMRE receive LO crystal may be the culprit. To thoroughly understand how a loss of hermeticity would shift the operating frequencies of the radios, an experiment could be performed on a group of modems at different pressure levels, and with the loss of hermeticity invoked in combinations of the crystals. This experiment would be time consuming to perform and give insight into only one problem associated with the function of the radio modems.

    It should be noted that the aging process of the LO crystal has been examined carefully. Pre-launch lab measurements of three spare radio units showed that the aging rate is consistent with expectation for that design, namely a negative drift in the output frequency that is logarithmic in time; linear on a log-scale of time.

    For present and future Sojourner rover telecommunications activities, here are the recommendations for hardware usage and when planning operations sequences:

  • Perform 10 to 15 minutes of rover modem heating and 5 minutes of reheating (if needed) after each rover morning wakeup. The goal is to have the rover modem above -10C for early morning communications.
  • Keep the rover in line-of-sight of the lander at all times. This includes not driving the rover down into trenches or behind large rocks like "Yogi Bear".
  • Be aware of where the lander and rover null zones are located when planning traverses.
  • Know the location of the HGA and its configuration relative to the LMRE antenna, in particular, when the flops in the clock (elevation) angle are to occur.
  • When it is desired to transmit large volumes of data, like images, endeavor to begin this activity when the rover modem is above +25C. Based on accumulated Sols experience, the telecommunications system link quality is best when the rover modem temperature is between +30C and +40C, and when the LMRE modem is at +20C or above. This may involve doing a short duration modem heat just before the taking and transmission of multiple rover images.
  • Have a lander software mechanism ready to implement that can detect when the telecommunications link is choking (i.e., the of a receipt large number of NAK'ed frames) and force it to free up the link by sending ACK's back to the rover after 2 NAK's occur. This will improve telecom throughput at the expense of losing some data.
  • Have a lander software mechanism ready to implement which can buffer garbled frames in lander memory for later downlink and reconstruction on the ground, allowing the recovery of lost image packets.
  • Have a series of rover sequence commands which will buffer rover images into RAM for later transmission if needed.

    Most of the these recommendations have already been communicated to the Mars Pathfinder project and implemented as part of rover operations. They have proved to work leading to significant improvements in the functioning of the Sojourner telecommunications subsystem.

    How do the radio modem telecom protocols work?

    The rover and lander UHF radios communicate using a relatively simple protocol. The rover is the master of the link and controls the initiation of all communication sessions. The session types are:
  • Heartbeat
  • Time Request
  • Command Request
  • Telemetry
    The function of the lander radio is to upload fairly small command sequences to the rover and receive data and telemetry downloads from it. Data transfer is performed in sessions whereby individual frames are exchanged in a well controlled manner. A frame is a group of byte-aligned data that is transmitted during keying of the RF modem. The maximum length of a data frame is 256 bytes and many, like a 250 byte telemetry frame, are shorter.
    A complete frame consists of:
  • A 6 byte acknowledge (ACK) frame, which contains a Sync code, a frame ID (FID), a frame number (FNUM) and a CRC value.
  • Followed by the 256 bytes of data, for a total of 262 bytes.
    Commands, data (like images and APXS spectra) and telemetry are organized into packets. Some packets are small and some are large depending on what they contain. By definition, the largest number of bytes that a packet can have is 2000. Also by definition, a session contains 8 frames. The packets are further divided up into frames for transmission. For example, a 600 byte packet is composed of a minimum of 3 frames (two 256 byte frames and an 88 byte frame). All communication sessions begin with a Start frame that is sent by the rover, followed by an ACK (acknowledge) from the lander. Heartbeats are very small and done most frequently to assure that the rover has not driven out of communication range. Time Requests are used to resynchronize the rover clock with the lander clock. Command Requests are done when the rover wants an upload of a new command sequence stored on the lander. Telemetry is rover engineering or science information that needs to be sent back to Earth. If for some reason there is a problem with radio transmission and the lander receives a bad ACK frame, then it will send the rover an NAK (No Acknowledge) response. The rover will try to resend frames up to 3 times. If it can't get the frame across then it goes on to the next frame. Dropped frames do indicate incomplete packets. Images are particularly sensitive to lost frames, because they cause missing strips or sections in the image.

    These radio modems use the standard RTS/CTS handshake method, similar to modems used in personal computers. The standard interface on these radios is RS232, however, the RS232 converter chip on the digital board has been replaced with jumpers so that 0-5 v TTL levels could be used with the rover and lander computers. The radios are hardwired for 9600 bps asynchronous communication with 8 data bits and 1 stop bit.

    How does the lander telecom system communicate with Earth?

    The microwave data link from the Mars Pathfinder lander to earth is handled by the Deep Space Network, in many ways a more remarkable link than the lander to rover link described in our web page. Information on the DSN can be found     HERE.
    Basically the DSN consists of a collection of 34 and 70-meter radio telescopes equipped with cryogenically cooled low-noise amplifiers (approx. 20 degrees Kelvin noise temp). They operate in two microwave frequency bands, S-band (2.3 GHz) and X-band (7.2 GHz TX uplink , 8.4 GHz RX downlink). For detailed information on the technical specifications of the DSN antennas click HERE. Mars Pathfinder uses a 12 Watt RF output X-band SSPA (Solid State Power Amplifier) transmitter to downlink data back to Earth. The X-SSPA was designed, built and tested here at JPL. The Pathfinder lander has two different ways to communicate with Earth. One way is with its High Gain Antenna (HGA) and another is with its Low Gain Antenna (LGA). Both antennas are capable of receiving the 7.2 GHz uplink signal and transmitting the 8.4 GHz downlink signal. When the LGA antenna is used, its maximum data rate is 600 bps. The LGA is a choked circular waveguide design having about an 70 degree 3 dB beamwidth pattern with about 6 dBic of peak boresite gain at 7.2 and 8.4 GHz. When the HGA is in use, the maximum data rate, based upon the performance of the Pathfinder RFS (Radio Frequency Subsystem), also built at JPL, is 11.06 Kbps. Actually the DSN could support an 11.06 Kbps link, but our ground data system would be overwhelmed at that rate (there's an irony there somewhere!) The noise levels received along with the Pathfinder data signals are fairly high, but with the carefully designed front end of the DSN Block V receiver and the extremely high gain of our large dish antennas, we can obtain carrier-to-noise ratios of about 20 to 40 dB-Hz (ballpark). At Mars distances (depending on spacecraft configuration), the signals can easily be tracked by phase-lock loops with a 1 Hz loop bandwidth. Uplinks to the Pathfinder lander are sent via high power transmitters (up to 20 KW) through the same high gain 34 or 70m antennas. Typically though we run Pathfinder high gain downlinks at 8.250 Kbps. The HGA on the lander was provided by Ball Aerospace and is a printed dipole array design utilizing a meanderline RCP (Right Circularly Polarized) polarizer. It is about 11" in diameter and weighs 1.2 Kg. It has about 20.4 dBic boresite gain at 7.2 GHz and 25 dBic boresite gain at 8.4 GHz. In all, the telecommunications systems used for deep space are large, complex and expensive to build, maintain and operate. The radio signal transmitted from Mars to Earth is so weak by the time it gets here it is only 7.9x10-19 watts. That is much too weak for the average home satellite dish and receiver to detect and discriminate from random background noise. So all of you folks out there with satellite dishes can relax, because this channel is one you'll have a tough time receiving. For now, just sit back and enjoy the wealth of new images and data from Mars, brought to you by NASA-JPL and paid for by your tax dollar contributions.

    How far is the communications range of the rover?

    The rover telecom system was functionally tested (BER --> Bit Error Rate tests, etc.) in the Arroyo (dry stream bed) next to JPL up to a distance of 250 meters. It performed quite well under environmental conditions typical of a warm (35C) August day. Another less controlled test (done about a year earlier) of the telecom system was made at a farther distance and under more extreme conditions. A stationary radio modem, antenna and computer interface was set up outside on the Mesa antenna range. Another modem and laptop computer were placed on the bed of a truck, with someone monitoring its operation. The truck was driven to different locations on the Mesa as well as down the winding paved road to the main part of the lab. The distance down to the lab was well over 700 meters and given the surrounding buildings and other structures, had quite an abundance of multipath reflections. The radio modems performed well under those conditions and did not lose communications. Both of these tests were performed under warmer conditions which is where these radios like to be operated. Many other tests of these radios were conducted under controlled thermal and signal attenuation conditions. We found that under certain conditions, there can be a degradation in the quality of the communications link. In particular at the lowest acceptable operating temperature of -30C, the Bit Error Rate (BER), due to an operating frequency shift may under certain conditions cause a communications blackout problem. Given the terrain we landed on it is difficult to predict just how far we can go with the rover. However, if the rover is kept in the line of sight of the lander and the radios are kept at a warmer operating temperature, the range of the rover telecommunications system should be at least 700 meters. The real constraint on how far we can drive is based upon the stereo imaging range of the lander IMP camera. Beyond about 10 meters, the IMP camera resolution may not be able to provide good enough stereo coverage of a particular location to assist the rover navigation team in driving the rover. This would put additional weight on the rover to get its navigation information from its own stereo cameras. This is not impossible, but certainly more tricky to plan and implement. If this is undertaken, the rover traverses would most likely be on the order of 2 meters at a time, because that is about the distance a ray can be projected by the rover cameras. However, given the banquet of interesting geologic formations near the lander, the scientists are content, for now, to remain in the immediate vicinity of the lander. If we do venture out farther, we will most likely travel to a location that is higher in elevation than the lander and in line-of-sight. The hill beyond Desert Princess is a prime area for this type of venture.

    What is the communications delay between Earth and Mars?

    We communicate with the Pathfinder lander using radio waves, which travel at a speed of 2.9979245x108meters per second. During the July 4 landing Mars and Earth were 192 million Km apart. At that distance it took 10 minutes and 39 seconds for the radio signal to travel in one direction. Because of their orbits, Mars and Earth are moving farther apart. As of November 6 Mars is approximately 291 million Km from Earth. At that distance it now takes 16 minutes and 10 seconds for the signals to travel in one direction. On May 13, 1998 Earth and Mars will be in conjunction (opposite sides of the sun) at a distance of 2.49 AU (1 AU is defined as 1.4956x1011 meters). At that distance it will take 20 minutes and 42 seconds for a radio signal to reach Mars. On June 22, 1998 Earth and Mars will be their farthest apart at 2.52 AU. At that distance it will take 20 minutes and 57 seconds for a radio signal from Earth to reach mars. It is these time delays which makes it impossible to communicate with and control the rover in real time.

    Why are the rover batteries not rechargeable?

    The primary source of power for the rover is its 0.22-square meter (1.9-sq-ft) solar panel, and not its batteries. The solar panel is comprised of 13 strings of 18 GaAs (Gallium Arsenide) cells. The power output of the panel at noon on Mars is approximately 16 W (@ 17V). Power from the solar panel alone is sufficient to operate the rover for several hours during the day. In fact the rover was designed primarily to run on solar alone with the batteries providing backup power when needed. The LiSOCL2batteries are arranged in 3 strings of diode-isolated D-cells. They are placed in 3 tubes located at the rear inside the Warm Electronics Box (WEB) next to the RHU's (Radioisotope Heating Units). These batteries can provide 150 W-hours of energy and supply 8-11 volts of unregulated power to the different converters and regulators attached to the core power bus. There is also a tiny Lithium battery located inside the APXS sensor; it is also not rechargeable. When the power subsystem for the rover was being designed, mass and size requirements for the battery pack were already defined, based upon available space inside the WEB. It *WAS NOT* a requirement that the batteries be rechargeable. An industry search was made for 'class-D flight' batteries which would meet these requirements, as well as energy output. Lithium Thionyl Chloride battery technology at the time had some flight heritage (space shuttle) and could provide a solution for these needs, so the choice was made to use these cells which met the requirements. Further, rechargeable batteries, like NiCads require a trickle charge to keep them alive. During cruise the rover solar cells are in the dark and useless for charging, also the rover had no power supply conection to the lander. About 1/3 of the battery power is budgeted for nominal rover operations during the primary mission of 7 Sols. At this time the primary mission objectives have been met and we are now into the 'extended' rover mission, which is baselined at 30 Sols. Battery usage during this time will be to support ongoing rover activities in a frugal manner, for once the batteries are depleted, no more nighttime activities such as APXS measurements and overnight health checks can be performed as the rover will be completely shutdown. After that all APXS activities will be shifted to daytime operations. As of August 30, the rover batteries became totally depleted, so from that point forward we are on a 'solar-only' mission.

    How long can the rover keep functioning on Mars?

    This question is difficult to answer for there are many unknowns to consider. The rover hardware was designed to last at least seven days which it has done. Even when the rover batteries become depleted, we can still function during the daytime. After that, if the solar panel fails or becomes covered with dust, the rover will not have enough power to operate. A sudden dust storm could cut short the rovers life by obscuring the sun and covering its solar panel with dust. Also, during the winter months on Mars, it will be much colder and the sun will be lower in the Martian sky. During that season the lander may stop functioning because it can't generate enough solar power to keep running some of its mission critical hardware, like the telecommunications system. If the batteries on the lander do become depleted and it is decided to not keep recharging them, then we can run a lander 'solar-only' mission. If that were to happen then operation on Mars would be limited to between 4 and 6 hours a day. Furthermore, the orbital positions of Earth and Mars are changing and moving the planets farther apart. On May 13, 1998 Earth and Mars will be in conjunction (opposite sides of the sun) at a distance of 2.49 AU (1 AU is defined as 1.4956x1011 meters). This distance is not as much of a problem as having the sun in the way, for it produces a lot of radio interference making communication almost impossible. Indeed, for distances of less than 10 solar radii around the sun, the thermal noise contribution is quite severe. From and engineering perspective, the biggest enemy of the lander and rover are the extreme temperature swings that occur on a daily basis. These temperature cycles produce mechanical fatigue on the exposed components which can lead to a failure. For now it is safe to say that the rover will function as long as the lander does, for once it falls silent there will be no more communicating with the rover.

    Footnote: As of November 4, 1997 the Mars Pathfinder mission has entered the Contingency Mission Phase. No data has been received from the lander or rover since September 26 and no communications at all since October 7. In all probability the lander is too cold to operate its systems and has fallen silent.

    Can I get schematics and engineering drawings of the Microrover?

    Unfortunately, the electrical schematics, mechanical drawings and blueprints of the rover are not available for public distribution; they are intended for JPL internal use only. Companies or Universities that provided hardware to the mission hold the rights to their drawings and it is forbidden by law for JPL to release that information to the general public without receiving written permission by the manufacturer. Information about certain specifications or operating conditions for the hardware can be released to the general public after such information has been cleared for release by JPL document control. The intent of not releasing all this information into the public domain is to protect the vested interests of NASA, JPL and their contractors. However, some technical information has been made available to the public, and inquiries about JPL technical documents may be sent to Elizabeth Moorthy of the JPL Archives and Records Group at the address archives@jpl.nasa.gov. You may also write or call:
    Archives and Records Group
    Mail Stop 512-110
    Jet Propulsion Laboratory
    4800 Oak Grove Drive
    Pasadena, CA 91109-8099
    Phone (818) 397-7952
    FAX (818) 397-7121

    Where can I download Rover and Lander images?

    There are several web sites that have been established for the dissemination of Mars Pathfinder images into the public domain. Here are ones that are known, and as new sites are found, they will be posted here as well.

    Mars Pathfinder Mission Images - Click on the Mars Sol day to see selected images from that day.
    Arizona State IMP Camera Images - Click on the Image Archive link to see Pathfinder Images.
    NASA Planetary Photo Journal - Click on the planet Mars then select either the Mars Pathfinder Lander or Rover image database.
    Rover Telecom Pictures site - Good grab bag of images of the Rover hardware, video clips and selected images from the IMP and Rover camera.
    Mars Pathfinder News and Information site - Another source of selected Lander and Rover Images from the mission.
    Rover Animated GIFs - Great place to see animated GIFs of the Rover taken by the IMP camera.
    Mars Pathfinder Image site - A clickable directory listing of Lander and Rover Images

    Does the Lander or the Rover have a microphone for recording sounds?

    The answer is no. For this mission, it was not one of the scientific objectives to study atmospheric acoustics on Mars. Perhaps future missions that are sent to study the Martian atmosphere will have instruments for measuring sound. If such instruments were used, what are the things we could listen for?

  • The wheel motor sounds from the rover as it drives on the Martian soil.
  • Martian winds rushing by the rocks (eolian sounds).
  • Possible lightning strikes or discharges in the thin atmosphere.
  • Explosions from meteor impacts.
  • Eruption sounds from volcanic activity.
  • Atmospheric displacements due to Marsquakes.
  • Background acoustic noises caused by infrasonics.

    The atmospheric pressure on Mars is between 6 and 10 Torr (units of Torr are mm Hg @ 0C and 1 Torr = 1.33X10-3 bar), compared to 735.56 Torr (at sea level) for atmospheric pressure on Earth. Here are two equations that are frequently used to calculate sound levels. The first equation is for acoustic pressure levels and is equal to 20 log10 (P/Pref) in decibels, where P is the pressure in the medium and is a function of the density of that medium, and Pref = 2X10-10 bar is the equilibrium pressure. The equation for sound intensity levels is 10 log10 (I/I0) also in decibels, where I0 = 10-16 watt per square centimeter. Pref and I0 are reference pressure and intensity values which both correspond to near absolute silence (no pressure changes). Both equations are logarithmic and this is convenient because the sensitivity of the human ear is roughly logarithmic. One thing to note is that on Earth, the pressure changes that create sounds are very small compared to the overall atmospheric pressure.
    Here are the A-weighted RMS (root-mean-square) pressure levels of a few common sounds:

  • 120 dB - Jet aircraft
  • 100 dB - Electric table saw
  • 80 dB - Automobile interior while moving
  • 60 dB - Normal conversation in home or office
  • 40 dB - Quiet room in the home
  • 20 dB - Recording studio

    Even the thin atmosphere on Mars supports the propagation of sound waves. But because of the lower pressure, the same sounds would have to be more intense (displace more atmosphere) for a typical microphone or human ear to hear them at the same level as here on Earth. For instance, given the same distance between the sound source and destination (say 10 feet), the reduction of pressure by a factor of 10 would reduce the sound pressure level by 20 dB. The reduction of pressure by a factor of 100 would reduce the sound pressure level by 40 dB. The pressure on Mars (at ground level) is approximately 100 times less than the atmospheric pressure on Earth (at sea level).
    Using the same reference pressure level of 2X10-10 bar, here are the equivalent Martian sound levels for the same common sounds on Earth:

  • 80 dB - Jet aircraft
  • 60 dB - Electric table saw
  • 40 dB - Automobile interior while moving
  • 20 dB - Normal conversation in home or office
  • 0 dB - Quiet room in the home
  • -20 dB - Recording studio

    As you can see, for the same given distance, the sound of an electric table saw on Mars would be perceived as loud as a normal conversation here on Earth. The design of a dynamic microphone or some sort of acoustic sensor to function in the harsh Martian environment could definitely be accomplished, as could the design of an audio frequency amplifier with 40 dB more sensitivity to the lower sound levels. So it is entirely possible to measure and record sound on Mars, but the scientific return is probably not high enough to justify flying the extra equipment weight.

    Will the Rover be coming back to Earth?

    Unfortunately, the rover does not come home unless someone goes to get it. On Apollo 12 the Lunar Module (LM) landed within 200 meters of the Surveyor III spacecraft, astronauts Pete Conrad and Alan Bean did an investigation of the craft's landing site and removed its camera and other components for return to Earth for study. The earliest (unmanned) Mars return mission is planned to be launched in 2005, and will not be returning the rover but rather soil and rock samples collected for a 2008 sample return mission. In the distant future, we may send astronauts to Mars for a scientific return mission. It is possible they may visit the Mars Pathfinder landing site and retrieve Sojourner and bring it back with them. If that is done, it will be studied in detail back on Earth, and take its place beside a similar rover at a National museum as a monument to past engineering and scientific achievements.

    Will the Rover collect soil samples?

    No, this rover cannot collect soil for return to Earth. It does engineering mobility experiments on the soil and determines the rock/soil composition with a deployable spectrometer (APXS). Rovers in the year 2001 and 2003 are planned to navigate the Mars terrain for several kilometers and operate semi-autonomously for as long as a year, analyzing rocks and soil and collecting samples along the way. In 2005 it is envisioned that a sample return vehicle (with a small short range rover) will land close to either the 2001 or 2003 rover with its bounty of collected samples. The small rover will go get the samples held by the bigger rover and deliver it to the return vehicle. If all goes well we could have those samples safely delivered back to Earth in hermetically sealed containers by 2008 or early 2009. Wow!

    What kind of computer is in the Rover and Lander?

    The rover computer board uses an Intel 80C85 CPU that has been radiation hardened for class - S space flight applications. The 80C85 uses a multiplexed address/data bus with an 8-bit address bus and an 8-bit data bus. It runs on between 3 to 6 volts and has been designed to have a low power consumption. The rover has a total of 672 KBytes of memory, 160 Kbytes of which are non-volatile EEPROM. Of its total non-volatile memory, 48 KBytes is radiation hardened to protect it from SEU (Single Event Upset) latchups or damage. Not suprisingly, the code for the computer OS (Operating System) is stored in this area of memory. There is no hard disk in the rover for storing images or data, which are typically stored in a local buffer before being transmitted to the lander for downlink back to Earth. The CPU runs at a mind blowing speed of 2 MHz (0.1 MIPS - Million Instructions Per Second), which is significantly slower that many personal computers in use today. The source programs for the rover computer were written primarily in ANSI C and Assembler. C and Assembler for the Intel CPU is fairly easy to program and gives very good access to the low level instructions of the CPU. The rover computer is not as powerful as you might think, but it can easily get the job done of executing command sequences and controlling the rover.

    The lander computer on the other hand has much more computing horsepower. Its 32-bit RISC CPU and architecture is the derviative of a commercially available IBM 6000 computer. It executes at about 20 MIPS. The lander also does not have a hard disk, but has a rather large 128 MB of DRAM where data and images can be buffered for transmission back to Earth. Like the rover, it also uses radiation hardened components on its computer board.

    Where can I get a pair of 3D glasses?

    Sometimes novelty or electronics hobby stores will carry 3D glasses for viewing red/blue anaglyph images. Sometimes they come with certain software programs. But if you can't find them locally, you can buy them from certain on-line companies. Reel 3-D Enterprises Inc. is a source for high quality 3D glasses and other products. Click     HERE to visit their web site. Another place you can go is Deep Vision 3D, they will give you a pair of 3D glasses is you send them a SASL letter with $2 to cover shipping. To find other companies that sell 3D glasses, do a web search on "3D glasses". The standard anayglyph has the left part of the image registered and colored red and the right part of the image is the green-blue part . When you wear your glasses, make sure the left eye has the red filter and the right eye the blue.


    Under Construction

    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, Jet Propulsion Laboratory, California Institute of Technology and the National Aeronautics and Space Administration.

    This page was last updated Tuesday January 20, 1998.
    Web Author: Scot Stride, NASA-JPL, Telecommunications Hardware Section 336

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