Radar systems will be used for tracking distances between two objects several times in the Reference Mission. The three cases considered here are terminal descent, the ascent stage lunar transfer vehicle docking, and the final LEO rendezvous with the Shuttle (or whatever vehicle we use to get the crew back on the ground). Although for descent, the radar is used primarily for checking the distance to the lunar surface, in the case of rendezvous the radar system plays is also used to find the LTV in the first place. Not considered are laser altimeters, since they are still somewhat novel, and there's a lot to be said for using a well-understood and tested, if older, technology (as long as the mass and power requirements are reasonable)
The requirements of descent and ascent are fairly distinct and so it is tempting to have two disparate systems rather than a common design. Here are some fairly preliminary specifications:
|Range resolution:||Of order 5m|
|Range rate res:||Of order 0.25m/s|
|Range limit:||Of order 10 km|
|Mass constraints:||Get it to LLO|
|Power constraints:||Must run on Spacehab power for ~30min.|
Inertial measuring systems can provide data from LLO to first-contact for the radar. The pointing requirements are minimal, as long as the descent stage has inertial rate sensors (start trackers or any other optical system just don't have the precision, accuracy, and speed). The ground-track can be identified with confidence. So, given that the descent stage will need attitude control to better than 100 mrad if the burns are reasonably accurate, then an antenna with a reasonable beamwidth (a few degrees) will need only to be fixed with respect to the vehicle for the ground-spot to be accurately identified.
With no pointing mechanism for descent, the requirements then are similar to an upgraded traffic-cop radar gun that puts out a watt or so of RF at a few GHz and draws 10-30W total for under 10 kg including the dish, structure, harness, etc. The Apollo module altimeter weighed 16 kg and drew 130 W, but RF amp efficiencies and masses have improved vastly since then.
This equipment is bolted onto the structure of the ascent stage, and provides range-rate and angular fix info to the onboard Attitude and Control Systems system which drives the single hypergolic engine.
|Range resolution:||Of order 10m|
|Rate resolution:||Of order 0.25 m/s|
|Angular res:||Relatively fine, of order 20 mradian (<1 degree)|
|Range limit:||Of order 10km|
|Mass constraints:||Tight, from LLO to surface and back|
|Power constraints:||Tight, ~20min operations|
Either the radar system actively scans the region (and relies on the orbiting Lunar Transfer Vehicle just being a "dumb" target) or the return craft can have a little 1 W transponder onboard which would make rendezvous a much easier operation by increasing the signal to noise ratio. This means less mass and less power will be required for the same first-contact range.
Unless the LTV can download range and bearing info to the rising ascent stage, the ascent stage will need a steerable antenna (probably articulated) with which it tracks the LTV. Since the descent and ascent requirements are so similar, it is tempting to use the ascent stage's radar system for the descent. It would mean that the antenna on the ascent stage will have a locked look-down position for the descent, and a feed into the descent stage's Command Data Management Unit (which is harder then it sounds)
After the LTV returns from Lunar Orbit, it performs a braking burn to enter Low Earth Orbit. From here is must rendezvous with the Space Shuttle, although this is far less of a problem than the other two radar applications. The Space Shuttle has a radar system and amenities (such as fuel to spare) for rendezvous, so it is likely that the Shuttle will be the only active vehicle for the LEO rendezvous operation.
STS has the IRACS onboard (Integrated Radar and Communications Subsystem). Built by Hughes, this kit gives a 16 km range with 3m range accuracy, and 30cm/s range rate accuracy (at 3 sigma). Its pointing info is just as good, giving 8 milliradians, and angle rates of 0.14 milliradians. It all works at 13-14 GHz, and can operate in two modes, passive, and active. Passive relies on the target being dumb metal and gives the above 16 km range, if the target carries a transponder then this can go up to several hundred km.
The Lunar Transfer Vehicle should host a 0.1W output homing beacon, weighing approximately 2kg and drawing 10W.
During descent the ascent stage's radar is fixed with the hab's axis and acts as a plane altimeter. This would weigh 10-30kg and ~30W total power (2-5W output). The weight will be for redundancy, as this is a mission-critical component for both the descent and ascent stage.
Once the habitat module has been lowered, the ascent stage is left on trickle charge, the data ring link is disconnected (an ethernet-type connection at most) and the dish mechanism is unlocked to give it its tens of degrees of freedom in alt (az control is unneccesary if it's a co-planar ascent, so that's one degree of freedom less and a few kilograms of motors and gears that we don't need).
Most of the ascent stage specifications are based on (with a 10-20% decrease in mass and power in most cases) a rendezvous radar proposed by TRW for an OTV.