• Should we 'make' the radio modems and antennas ourselves at JPL, or should we 'buy' them from the outside supplier who can build them to meet our requirements?
• If we buy the radio modems and the antennas, should we buy commercial grade or military grade?
• If we buy the commercial grade, can we reliably fly them to Mars?
• If we buy a military grade, can we carry the heavier weight and provide the larger power supply needs?
• What kinds of modifications and tests do we need to perform on the hardware to prove their reliability?
• If we make the radios and the antennas, will we have enough time and enough money?
• What communication frequency should we choose?

The main factors of considerations were:

• Funding Level ---
  NASA Space Sciences is challenging itself to develop low cost missions series, starting with the Discovery class missions, therefore, we had only a limited amount of funds available.
• Schedule Constraint ---
  We had approximately 36 months to launch the Pathfinder mission, which gave us about 26 months to get the radio modems and antennas ready for launch.
• Harsh Mission Environmental Requirements ---
  The radio modems and antennas have to survive launch, the 7 month cruise to Mars and a rather hard landing onto the Martian surface at about 40 m.p.h. The surface temperature of Mars is very cold, -100C at night and 0C during the day. Also the radios have to survive outer space radiation environment.

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:

• The funding constraints.
• The short delivery schedule.
• The environmental requirements of shock, vibration and temperature.

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.

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 -30°C to +40°C. 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 -55°C to +60°C, 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:

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