Lunar Prospector Tester Simulates Moon and Sun
A test system built from off-the-shelf components tests the spacecraft from assembly to launch.
Martin Rowe, Technical Editor -- Test & Measurement World, 8/1/1997
The Lunar Prospector is the product of a NASA program for "faster, better, cheaper" spacecraft development. The faster comes from the spacecraft's relatively short development cycle (30 months), and the cheaper comes from its $63 million cost.The "faster, better, cheaper" approach to spacecraft development includes the Lunar Prospector's test system. To meet the cost and time demands of the Lunar Prospector project, Lockheed Martin Missiles and Space (Sunnyvale, CA) needed a test system built from off-the-shelf components. The test system also had to be flexible: It not only had to test the Lunar Prospector's systems from final assembly through launch, but it also had to be reconfigurable for use on other projects once the Lunar Prospector leaves the ground.
Lockheed Martin contracted with Hewlett-Packard (Mt. View, CA) to develop a test system. The test system had to test each subsystem and also simulate the signals that the Lunar Prospector will require during space flight. The test system, called Lunar Prospector Electrical Test System (LPETS), has a dual role. Besides testing the spacecraft, the LPETS will supply power and monitor the Lunar Prospector until the moment of launch. During launch, the LPETS will reside in a utility room located under the launch pad at Cape Canaveral, FL. (For a description of the LPETS' components, see " What's in the Test System?". )
To the Moon
The Lunar Prospector will carry instruments that will measure the moon's magnetic field, radon outgassing, gravity fields, and chemical elements. One goal of the mission is to determine if frozen water exists on the moon.
Following the launch, the Lunar Prospector will orbit the earth before heading to the moon. After separating from the Translunar Injection Stage booster rocket, the spacecraft will receive commands from earth. Those commands control functions such as the firing of thrusters and the deployment of the three booms that hold scientific instruments. There are 60 commands required to fly the spacecraft.
The Lunar Prospector is a remote-controlled spacecraft. It has no computers that make decisions on spacecraft positioning, scientific experiments, or any other operation. All command decisions are made on earth. Lunar Prospector sends all data to mission control at Ames Research Center (Mt. View, CA), where all decisions are made about controlling the spacecraft. Processors are used only for interpreting commands from the earth and for creating serial bit streams (telemetry) for transmission of the scientific instruments' readings.
Because all spacecraft decisions are made on earth, mission control must know the spacecraft's position at all times to determine if the Lunar Prospector is on course. Computers on the ground calculate the spacecraft's position based on sensor readings that indicate the Lunar Prospector's position relative to the earth (in earth orbit), the moon (lunar orbit), and the sun (throughout the flight). The spacecraft, therefore, requires sensors that detect the earth, moon, and sun relative to the spacecraft. These sensors are mounted at the base of the Lunar Prospector's antenna cone.
The Sun Sensor is a light-sensitive device that detects when it's pointed at the sun. As the spacecraft rotates, the sun sensor will pass the sun. When the sun is in sight, the sensor's voltage changes from low to high.
The Earth/Moon Sensor is an infrared temperature detector that detects temperature differences between space and the earth or between space and the moon. Even while over the moon's dark side, the sensor will detect enough temperature difference to create a pulse when it's pointed at the lunar surface rather than into space.
Computers on earth look for the transitions between space and the moon or space and the sun and then calculate the spacecraft's position. Should those computers conclude that the Lunar Prospector requires a course correction, they can send the appropriate commands to fire the six hydrazine-powered thrusters that adjust the spacecraft's position while in orbit.
Simulate Sun and Moon
To simulate the outputs of the Sun and Earth/Moon Sensors during functional testing, the LPETS uses two arbitrary waveform generators (AWGs). Lockheed Martin uses software-selectable risetimes and falltimes, pulse amplitude, and duty cycle because those times will vary when the Lunar Prospector is in space.
Besides simulating sensor outputs, the LPETS must supply power to the Lunar Prospector and simulate the voltages produced by the spacecraft's solar panels. There are three solar panels that form a circumference around the Lunar Prospector. When the Lunar Prospector is in sunlight, the solar-array panels convert sunlight to electricity that powers the spacecraft and recharges the battery. The battery will provide enough power to keep the Lunar Prospector's systems operating while behind the moon. The battery provides a nominal 28 VDC.
Although the Lunar Prospector's nominal operating voltage is 28 VDC, the LPETS needs 60-V power supplies because the LPETS supplies power to the Lunar Prospector while on the launch pad prior to launch. Cables between the power supplies and the Lunar Prospector can reach 1000 ft. HP engineers had to provide enough voltage to compensate for losses in the cables. A 60-V supply provides what NASA calls external power, which charges the batteries prior to launch and supplies power for the systems because the solar panels are inoperative until the Lunar Prospector is in orbit.
The test system also has to simulate power waveforms—the outputs of the spacecraft's solar-array panels. That's difficult because the spacecraft has three booms that hold the scientific instruments, and the booms create shadows in front of the solar panels as they spin. Each panel consists of 11 strings of cells composed of approximately 2500 solar cells. The test system can simulate waveforms from solar cells based on the angle of the sun and the shadows. Because the cells of each string are wired in series, all cells in that string must be in sunlight for the array to produce a voltage. The strings themselves are wired in parallel.
When the Lunar Prospector is in sunlight, up to one half of the solar panel's surface will be in sunlight. When no shadows block the light, the solar panels' output is at 100% of capacity. But because a string needs only one cell in the shadows to cut off the entire string, the total output can drop to a worst-case 65% of total when a boom eclipses the sun.
The Lunar Prospector should spend one year in a polar orbit searching for water on the moon. During that time, the spacecraft will be in full sunlight. Later in the mission, NASA may change the orbit so the Lunar Prospector spends half of its time behind the moon, out of sunlight.
Solar Panel Simulation
The test system contains three solar-panel simulators that engineers can program to simulate all power waveforms that are possible in space. When in the moon's shadow, the Lunar Prospector's solar panels' output drops to 0%, and the spacecraft runs on battery power.
The LPETS has a power supply for testing the propellant isolation valves, which control the thrusters. The valves are operated by nominal 28-V pulses. Ted Marcopulos, the LPETS system manager at HP, explained that the LPETS uses electronic loads, rather than switches, to pulse the power to the relays. "The problem with switches," says Marcopulos, "is that they bounce. You can also get arcing, which can cause EMI that can upset the spacecraft's systems." To simulate the control signals for the relays, the LPETS uses an electronic load in series with the power supply.
The LPETS has a 64-channel analog-to-digital converter (ADC) that monitors sensor outputs of voltage, current, temperature, and resistance. The ADC monitors current by measuring voltage across shunt resistors and temperature from thermistors. Pressure transducers produce a voltage proportional to pressure.
Logging the data from the measurement sensors and simulating the position sensors' outputs requires precise timing. For example, the triggers on the two AWGs must be synchronized so the simulated sensor outputs will appear as they will in space. In addition, the simulation of shadows as the booms pass over the solar strings requires time synchronization.
The time-and-frequency processor receives its initial time from the LPETS' PC's clock. The PC's clock may not keep accurate absolute time, but that's not important. What's important in testing the Lunar Prospector is that the LPETS be synchronized to other instruments.
Synchronization is important because the LPETS logs all activity on the Lunar Prospector and reports to engineers each second. That activity includes logging and time stamping of all communications between the Lunar Prospector and a telemetry/command (TLM/CMD) system located in a communications test van parked outside the building where the Lunar Prospector is tested. The TLM/CMD system must be time synchronized to the LPETS' computer. Another time/processor card in the TLM/CMD computer receives an IRIG-B signal from the time/frequency card in the LPETS, which keeps the two computers synchronized. With the two computers in sync, engineers can time correlate all measurements to command and telemetry signals.
When You Build Just One
Testing a one-of-a-kind spacecraft differs from testing products that you'll build in quantity. You get only one chance to make the product work. "When you test a spacecraft, you must simulate all the conditions of space," says Thomas Dougherty, Lunar Prospector project manager at Lockheed Martin. "We tested the Lunar Prospector under high vacuum and at temperatures from -30°C to -50°C. We ran tests on the spacecraft first in the assembly area, then placed it in a thermal-vacuum chamber and next into an acoustic chamber for vibration testing. We also tested the spacecraft in the assembly area after each environmental test to be sure that the harsh environments didn't affect the systems."
Lockheed Martin's engineers began using the LPETS in March, after the Lunar Prospector was fully assembled. The "faster, better, cheaper" concept required the engineers to assemble the entire spacecraft before performing any tests. Each subcontractor that built the electronic systems was required to deliver tested subsystems to Lockheed Martin. Once the spacecraft was fully assembled, Lockheed Martin's engineers started the functional tests.
They began by applying power to the Lunar Prospector's subsystems one at a time and measuring each subsystem's power consumption. That procedure let the engineers verify that the cables and harnesses were correctly built and installed.
Next, the engineers tested each subsystem individually—first under simulated launch conditions, then under simulated in-flight conditions. During the simulated launch conditions, they measured the power consumption of each subsystem and simulated the control commands that the Lunar Prospector will receive while in flight. For example, they verified that when given the command to fire a thruster, the Lunar Prospector carried out that command. The engineers also verified that the Lunar Prospector was sending telemetry. "The first procedure for ringing out the system and performing a functional test took about a month," said Dougherty. "Now, we can perform a functional test in just 24 hours."
Engineers also had to simulate the signals that the Lunar Prospector will use in space. These tests included verifying that the Lunar Prospector operated correctly with varying amounts of power from the solar arrays, taking the shadows into account.
Next, the engineers simulated the motion of the Lunar Prospector relative to the earth, moon, and sun. Here's where engineers used the AWGs to simulate the outputs of the Sun and Earth/Moon Sensors. As part of the functional test, the Lunar Prospector had to properly transmit telemetry representing the sensors' outputs.
Environmental Tests
Following the month-long test, the Lunar Prospector went into a thermal vacuum chamber for approximately one week with power applied to all systems. Only when the chamber's atmospheric pressure reached 10–7 torr could engineers perform tests on the scientific instruments that the Lunar Prospector would carry. The instruments had to be tested in a chamber because the high voltages (400 V) they require would have created arcing in air.
With the Lunar Prospector in the chamber, engineers performed thermal testing. Here, the chamber's temperature cycled from –30°C to –50°C, cycling twice from cold to hot.
The thermal vacuum tests required approximately one week and ended on May 3. Following the tests, Lockheed Martin engineers removed the Lunar Prospector from the chamber and repeated all tests to verify that the environmental extremes had not damaged any of the systems or degraded their performance. According to Kim Foster, Lunar Prospector test manager, thermal vacuum tests will uncover any weak solder joints.
Next came the vibration tests. Although the engineers needed only one day to perform the tests, they needed a week to set them up. The engineers installed the solar panels and placed the Lunar Prospector in an acoustic chamber and subjected the spacecraft to acoustic vibration. They subjected the Lunar Prospector to amplitudes of 146 dB for one minute at a time. That amplitude simulates the vibrations the Lunar Prospector will have to withstand during launch. No electrical tests were run during acoustic testing, for all of the spacecraft's systems are powered down during this phase of the flight.
During this summer, the Lunar Prospector is scheduled to undergo an end-to-end functional test. In this test, engineers will operate the spacecraft as though it were on its mission. The LPETS will have to simulate the outputs of the Sun Sensor and the Earth/Moon Sensor as well as the voltages from the solar arrays.
Once the tests at Lockheed Martin are completed, the Lunar Prospector and the LPETS will be shipped to Cape Canaveral. Upon arrival at the Kennedy Space Center, the spacecraft will undergo another functional test. About two weeks prior to launch, the Lunar Prospector will be attached to its booster rocket. At that time, the solar-array simulators will no longer be required, and the LPETS will take the role of supporting the launch.
During the launch preparation, the LPETS will both support and monitor the Lunar Prospector. The power supplies will provide power to the spacecraft and power to charge the batteries. Another power supply will provide power for sensors that measure temperature and pressure.
Engineers at the launch site will operate the tester from a remote location where they will monitor temperature and pressure while the LPETS is in the utility room under the launch pad. The LPETS will monitor more than 100 signals and provide engineers with updates on those measurements once per second. Alarms in the LPETS software will warn engineers of any out-of-tolerance measurements up until the moment of launch.
Once the Lunar Prospector is launched, though, the test system's life will not be over. Engineers will eventually begin reconfiguring its modules for testing another satellite.
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Lunar Prospector Web Sites Lunar Prospector home page, West Coast: Lunar Prospector East Coast mirror sites: For details about the spacecraft and instruments, see:
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A 4x16 matrix module switches the command/telemetry signals from a VMEbus computer to either an RS-422 transmitter or to an RF transponder for communicating to the Lunar Prospector.