Landing Radar and Rendezvous Radar: The LM's Two Eyes
How the Lunar Module used two separate radar systems to solve two different problems—finding the ground during descent and finding the Command Module in orbit
The Lunar Module carried two completely independent radar systems that served completely different purposes. The Landing Radar, mounted on the bottom of the descent stage, pointed at the Moon and measured altitude and velocity relative to the surface during the powered descent. The Rendezvous Radar, mounted on the top of the ascent stage, pointed at the sky and tracked the Command Module’s transponder for rendezvous navigation. The two systems shared no components, no antennas, no processing electronics. They operated in different frequency bands, used different measurement techniques, and were built by different contractors. But both were essential—the Landing Radar for getting down safely and the Rendezvous Radar for getting back.
The Landing Radar: Altitude and Velocity from a Moving Platform
The Landing Radar was built by Ryan Aeronautical Company in San Diego and was one of the most technically demanding radar systems of its era. It had to measure the LM’s altitude above the lunar surface and its velocity relative to that surface, continuously, while the vehicle was decelerating from over 5,000 feet per second to zero, at altitudes ranging from 50,000 feet down to a few feet.
The system used a four-beam antenna array mounted on the descent stage, aimed generally downward. The antenna operated at microwave frequencies (approximately 10.51 GHz for the altitude beam and 9.58 GHz for the velocity beams) and was mechanically tilted during the descent to account for the changing LM attitude—the antenna needed to point at the surface regardless of the vehicle’s pitch angle.
Altitude measurement: One beam measured altitude using a frequency-modulated continuous wave (FMCW) technique. The radar transmitted a signal whose frequency swept linearly over a defined range. The return echo from the lunar surface arrived with a time delay proportional to the altitude. The difference in frequency between the transmitted signal and the received echo—the “beat frequency”—was directly proportional to the round-trip travel time and therefore the altitude. The system measured altitude from approximately 40,000 feet down to about 3 feet with an accuracy of about ±2% or ±2 feet, whichever was greater.
Velocity measurement: Three beams measured the LM’s velocity relative to the surface using the Doppler effect. Each beam was aimed in a different direction relative to the vehicle—forward-left, forward-right, and aft—and measured the Doppler frequency shift of the reflected signal. The three Doppler measurements were processed to extract the three components of velocity: forward, lateral, and vertical. The velocity measurement accuracy was approximately ±1 foot per second.
The four-beam geometry was chosen because three Doppler measurements plus one altitude measurement provided a complete description of the LM’s state relative to the surface. The processing electronics—analog signal processing circuits that extracted the beat frequencies and Doppler shifts—converted the radar measurements into digital words that the AGC could read through its input channels.
Incorporating Radar Data: When to Trust the Beam
The AGC maintained the LM’s state vector primarily through inertial navigation—integrating accelerometer data over time to track position and velocity. The IMU was accurate but subject to drift and to errors in the initial state vector. The Landing Radar provided independent measurements that could correct these errors, but incorporating radar data into the navigation solution was not a simple matter of replacing the IMU values with radar values.
The Luminary software used a data weighting algorithm that blended the radar measurements with the IMU-derived state estimate. The algorithm computed the residual—the difference between the radar-measured value and the predicted value from the state vector—and applied a gain factor to determine how much to adjust the state vector. If the residual was small, the radar and IMU agreed and only a minor correction was needed. If the residual was large, either the state vector had significant error (and the correction should be large) or the radar measurement was bad (and the correction should be rejected).
The crew controlled the radar data acceptance process. The AGC displayed the radar altitude and velocity data on the DSKY along with the residuals. If the residuals were within acceptable limits—indicating that the radar data was consistent with the current trajectory estimate—the crew accepted the update. If the residuals were large and inconsistent with the expected trajectory, the crew could reject the radar data to prevent it from corrupting the navigation solution.
The Landing Radar was not available during the early part of the braking phase because the LM was oriented nearly horizontal with the engine pointed forward, and the radar antenna, which pointed generally downward, was aimed at the lunar horizon rather than the surface directly below. As the LM pitched over during the braking phase, the antenna’s beam eventually swept to point at the surface. The altitude at which usable radar data first became available—called “radar altitude acceptance”—was typically around 25,000 to 35,000 feet, depending on the pitch angle profile.
Before radar data was available, the AGC relied entirely on inertial navigation for the descent solution. Any error in the initial state vector (uploaded from Mission Control before PDI) would propagate uncorrected during this phase. The radar data, when it arrived, provided the first independent check on the trajectory since the descent began. On several missions, the radar data showed that the IMU-derived altitude differed from the actual altitude by hundreds of feet—a discrepancy that the navigation filter corrected over the next several radar updates.
Apollo 14’s Landing Radar Anomaly
The Landing Radar’s most dramatic in-flight moment occurred on Apollo 14. During the powered descent, the radar failed to acquire the lunar surface—the antenna was not seeing usable return signals. Without radar data, the AGC would continue the descent on inertial navigation alone, but the crew and Mission Control would have no independent altitude verification. The mission rules specified that if the radar hadn’t acquired by a certain altitude, the descent would be aborted.
The problem was traced to a circuit breaker configuration—the radar’s antenna assembly was not receiving proper excitation because of a circuit state that hadn’t been anticipated in the procedures. Mission Control rapidly developed a workaround: the crew recycled the landing radar circuit breaker, which reset the antenna electronics. The radar acquired the surface, and the descent continued.
The fix was identified and radioed to the crew with minutes to spare before the abort decision altitude. The incident demonstrated both the vulnerability of having a single landing radar system (there was no backup) and the value of having engineers on the ground who understood the radar’s internal architecture well enough to diagnose a fault in real time and talk the crew through a repair.
The Rendezvous Radar: Finding the Mothership
The Rendezvous Radar was built by RCA and served a fundamentally different purpose: tracking the Command Module for rendezvous navigation after the LM left the lunar surface. The radar operated as a transponder-interrogation system—the LM transmitted a coded signal, and a transponder on the CM received the signal, amplified it, and retransmitted it back. The LM radar measured the round-trip time delay (for range), the Doppler shift (for range rate), and the antenna pointing angles (for line-of-sight direction).
The system operated at C-band frequencies (approximately 9.83 GHz) and could acquire and track the CM’s transponder at ranges up to approximately 400 nautical miles. The antenna was a parabolic dish approximately 24 inches in diameter, mounted on a two-axis gimbal on top of the LM’s ascent stage. The gimbal allowed the antenna to track the CM across a wide field of regard as the two vehicles moved relative to each other.
The Rendezvous Radar provided four measurements to the AGC:
- Range: Distance to the CM, measured by the round-trip signal travel time, accurate to approximately ±50 feet at typical rendezvous distances
- Range rate: Closing or opening velocity, measured by Doppler shift, accurate to approximately ±1 foot per second
- Shaft angle: The azimuth angle of the antenna relative to the LM body, giving the left-right direction to the CM
- Trunnion angle: The elevation angle of the antenna, giving the up-down direction to the CM
These four measurements—range, range rate, and two angles—provided a complete description of the CM’s position and velocity relative to the LM, which the AGC’s P20 (Rendezvous Navigation) program used to maintain and refine the CM’s state vector.
The Antenna Gimbal Problem: How the Rendezvous Radar Almost Stopped Apollo 11
The Rendezvous Radar’s antenna gimbal had two operating modes: “LM mode” (automatic tracking, where the radar controlled the antenna to keep it pointed at the CM) and “SLEW mode” (manual, where the antenna was pointed by AGC command or crew input). During powered descent, the Rendezvous Radar was powered on and the antenna was positioned in a standby configuration, ready for immediate use if the crew had to abort and rendezvous with the CM.
The problem that caused the Apollo 11 1202 and 1201 alarms was related to the Rendezvous Radar’s coupling data unit (CDU)—the electronics that reported the antenna gimbal angles to the AGC. In the configuration used during Apollo 11’s descent, the CDU was generating counter-increment interrupts to the AGC at a rate that depended on the relative phasing of two AC reference signals: one from the LM’s primary power system and one from the radar’s own reference signal.
When these two references happened to be out of phase (which occurred randomly depending on the exact moment the systems powered up), the CDU generated interrupts at a high rate—approximately 6,400 per second. Each interrupt stole a small amount of AGC processing time. The cumulative load—roughly 15% of the processor’s capacity—pushed the Executive past its limits during the computation-heavy powered descent phase, triggering the overflow alarms.
The phasing issue was a subtle hardware interaction that hadn’t been identified during integrated testing because it was timing-dependent and didn’t occur on every power-up sequence. After Apollo 11, the problem was analyzed and a software fix was incorporated into subsequent Luminary revisions that prevented the radar CDU from stealing cycles during powered descent unless the radar was actively being used for tracking.
Two Radars, Two Lifelines
The Landing Radar and Rendezvous Radar represented two of the highest-risk single-point systems on the LM. Neither had a backup. If the Landing Radar failed completely, the crew would have no independent altitude or velocity data during the final descent—the mission rules called for an abort if the radar didn’t acquire by a specified altitude. If the Rendezvous Radar failed, the LM would have to rely entirely on the CM’s sextant tracking and ground-based solutions for rendezvous navigation—technically feasible but operationally degraded and higher-risk.
The decision not to carry backup radar systems was driven entirely by weight. Each radar system—antenna, gimbal, electronics, cabling—weighed over 60 pounds. A backup Landing Radar would have added roughly 70 pounds to the descent stage, and a backup Rendezvous Radar would have added similar weight to the ascent stage. In a vehicle where the SWIP program was fighting for every ounce, redundant radar systems were a luxury the LM couldn’t afford.
The risk was managed through quality. Ryan and RCA built the radar systems to specifications that demanded extraordinary reliability—extensive environmental testing, material screening, and component-level burn-in testing that weeded out infant mortality failures. The systems were tested under vibration, thermal cycling, vacuum, and electromagnetic interference conditions that exceeded the expected flight environment. The flight units were the survivors of a manufacturing and testing process designed to ensure that only the most reliable hardware made it to the spacecraft.
Both radar systems worked on every Apollo landing mission. The Landing Radar acquired the surface and provided accurate data for all six lunar landings. The Rendezvous Radar tracked the CM and supported the rendezvous on every ascent. The single-string design—one radar, no backup—held because the engineering behind each system was thorough enough that the backup was never needed.