How do they do it!
Here is a quick review of a previous blog and then I will give some detailsed design information right after that.
NASA COBALT team and the Masten Xodiac team with Xodiac in the background (Image courtesy of NASA)
There are so many places in our solar system in which man wants and needs to explore. Most of these are not realistically reachable by self-guided, robotic lander spacecraft because of technology gaps in current landing systems. To remedy this, NASA created The CoOperative Blending of Autonomous Landing Technologies (COBALT) project, which is conducted by NASA's Space Technology Mission Directorate (STMD) and the Human Exploration and Operations Mission Directorate.
To do this, the team will test two new landing sensor technologies that may give the highest-precision navigation solution ever tested for NASA space landing applications.
These technologies are a Navigation Doppler Lidar (NDL), that is capable of giving ultra-precise velocity and line-of-sight range measurements, and the Lander Vision System (LVS), that will enable terrain relative navigation. These will be integrated and flight tested aboard a rocket-powered vertical takeoff, vertical landing (VTVL) platform. The platform, is called Xodiac and was developed by Masten Space Systems in Mojave, California.
The Navigation Doppler LIDAR System (NDL) (Image courtesy of NASA)
COBALT hardware. (Image courtesy of NASA)
Xodiac vehicle (Image courtesy of Masten Space Systems).
An internal view of COBALT and the Xodiac vehicle (Image courtesy of NASA)
Here is how it works
The system design, for the Doppler LIDAR on COBALT, uses an optical homodyne configuration. This is a low-power, single frequency LASER operating at 1.55 micron as the master oscillator.
The LASER output is modulated via the waveform in Figure 6.
The LASER frequency is linearly modulated to create a sawtooth waveform. The reflected waveform from the target is time-delayed and Doppler shifted. The lower trace in this figure is the difference frequency obtained by homodyning the LASER and the returned beams which contain range and velocity data. (Image courtesy of Reference 1)
The LIDAR actually transmits three laser beams separated by 120o from each other in azimuth and are pointed 22.5o from the nadir. The three Line-of-Sight (LOS) measurements are combined in order to determine the three components of the vehicle velocity vector as well as measure the altitude accurately and altitude relative to the local ground.
The precision of this Doppler LIDAR system measurement is achieved via the spectral linewidth of the LASER, the linearity of the modulation waveform, the signal-to-noise ratio (SNR), and the signal processor frequency resolution. See Figure 7.
The Doppler LIDAR system architecture showing three transmit beams and their corresponding receivers providing LOS velocity and range measurements in three directions. (Image courtesy of Reference 1)
The signal processing digitizes and processes the three optical receivers in real-time to produce a vector velocity, altitude, and attitude data at 30 Hz. See Figure 8.
The real-time signal processing block diagram. Shown here is an FPGA board with FFTs, a stacked Analog-to-Digital Converter (ADC) daughter-board, a single-board computer (SBC) and an analog and digital Data Acquisition (DAQ) board. (Image courtesy of Reference 1)
In the first flight campaign, NASA will endeavor to complete the integration, flight testing and performance analysis of the COBALT payload. This is just a passive test, in which COBALT will be only collecting data, while the Xodiac vehicle will rely on its GPS for active navigation.
In a follow-up flight campaign this summer, COBALT will become the active navigation system for Xodiac, and the vehicle will use GPS solely as a safety monitor and backup system.
Stay tuned for my upcoming in-depth technical article on EDN regarding this technology and how sequential data fusion of Global Navigation Satellite Systems (GNSS) and dopplers with map-based vision systems increase accuracy and repeatability of positioning data that can be applied to automobiles.
1 Doppler LIDAR Sensor for Precision Landing on the Moon and Mars, F. Amzajerdian, L. Petway, G. Hines, B. Barnes, NASA; D. Pierrottet, G. Lockard, Coherent Applications, Inc., 2012