els of sensors are used for redundancy. During optimum operating conditions, several components of each level of redundancy will be used. The primary, secondary, and emergency sensor arrays rely on a radar imaging/altimeter system, several sets of accelerometers and gyros, a transponder system, a close proximity altitude detection device, and the communications system. The communications system is only used as a sensor for emergency situations.
The onboard navigation computer will be a fault tolerant advanced computer that will be capable of out-performing today's most advanced Cray computer. The rapid pace of computer and software development has shown that a system of this type will not only be possible, but will have little mass and power consumption. The navigation computer will be responsible for monitoring the output and status of each sensor, monitoring the status of and providing input for each of the control devices, and providing an interface between the two.
The lander will utilize three control techniques: momentum exchange devices, small directional thrusters, and gimbaled/throttled main engines. While some redundancy exists using all three systems, the optimum operating conditions will use each technique where best suited.
The control of the unloader will be primarily automated with a remote control system as a backup. The unloader will have optical sensors which will inform the unloader's onboard computer of obstacles. The computer will then instruct the wheel motors to make the required adjustments. The unloader will be in constant communication with the lander, in case it becomes necessary to employ the back up remote control system.
The communications systems provide three basic functions: telemetry, command, and tracking. There are three areas of communications that will be performed by the lunar lander system: Lander/Earth, Lander/OTV, and Lander/Unloader.
S-band (2.3 GHz) will be used for direct communications between the lander and earth. The antenna on the lander will be a parabolic dish with steering capabilities similar to that on the Apollo spacecraft. The Apollo pointing system is more than sufficient for the communication link with earth.
It is also recommended that a communications satellite for Earth link capabilities be placed in a halo orbit about the L2 Lagrangian point. The satellite would allow transmissions to be made between the lander and earth when the lander is on the far side of the moon.
Communications between the OTV and the lander will be done with a VHF system. This system will be necessary for docking procedures. Once the lander is docked with the OTV a data feed umbilical will be connected to the lander by means of a manipulator arm on the OTV.
Communications between the lander and unloader will be done using a UHF system. The UHF receivers and transmitters are small, lightweight, and require little power. The UHF antennas are also small.
The energy for the power system is provided by the heat generated during engine cool down cycles. A power conversion loop transforms the heat into electrical energy, which is then stored in rechargeable Na-S batteries on the lander and the unloader. The conversion loop also serves to cool down the nuclear motors and keep the batteries at a higher operating temperature.
Two sets of batteries provide 11 kWh of power on both the lander and the unloader. The power for the unloader allows it to carry the payload 5 km at a speed of 2.5 km/hr. In the event that the unloader remains on the surface for an extended period, two solar arrays totalling 20 sq. m. are mounted on the unloader. These GaAs/Ge arrays are able to recharge the batteries fully in about one solar day. The solar arrays are also used to heat the unloader's batteries during payload transport.
Thermal control will be done using several methods. The first method will employ the use of a cryogenic refrigeration system that will be powered by the power generation loop. The second method will employ the use of 2 1/2" of multi-layer insulation on the propellant tanks and other areas that 