Here at the Center for Earth and Planetary Studies (CEPS), radar instruments play an important role in our study of Earth’s nearest neighbors, such as the Moon, Venus, and Mars. Radar can provide a range of information regarding the materials that make up the surface of a planet and offer a unique perspective on the underlying structure. Being geoscientists, this last point is really important as it allows us to understand how the surfaces of planets build up through time.
So how do we collect radar data from such distant targets? At CEPS, we use a combination of giant radio telescope facilities, such as the Arecibo Observatory in Puerto Rico and radars that are currently in orbit around Mars, including the SHAllow RADar (SHARAD) on the Mars Reconnaissance Orbiter. Much of our work is devoted to processing the radar data returned so that we can make sense of the geologic units we are targeting. To get the most out of our research it is important to have a fundamental understanding of the hardware that makes up a radar instrument. Naturally there is no better way to achieve this than to roll up your sleeves and build one yourself! Here we describe the first-hand experience of two members of the CEPS team in assembling radars.
Lets begin 30 years ago, with early ground-penetrating radar work being carried out at Texas A&M University. CEPS geophysicist Bruce Campbell was an undergraduate working on the application of radar to support mining safety.
I was working on a research project with Dr. Robert Unterberger, who was one of the early scientists to apply radar to ground-penetrating studies. The major application was to salt mines, where mining companies had a strong interest in finding cracks or water ahead of their excavation. He had already built several generations of such radar systems (“Alpha” through “Foxtrot”) with his graduate students, and I was given the chance to assemble a seventh, or “Golf” system. This was built from a military-surplus aircraft radar altimeter that operated in the L-band range, or a wavelength of about 24 cm (9 inches).
Most of the work involved finding the right places in the circuits to tap the outgoing and reflected radar signals, and to bypass those parts of the electronics that adjusted the receiver for reflections that the system expected only from a distant target (the ground below the plane). Recording the results at this stage was a matter of using a Polaroid camera to photograph an oscilloscope screen that showed the echoes along a plot of time delay; some of the other systems used digital sampling. Once it was ready, we took it on the roof of the Halbouty Building with a pair of feed-horns borrowed from Johnson Space Center. The radar successfully bounced its signal off the water tower a couple of miles across town.
Getting it to work in a mine was less successful, but the trip was worth it. We took “Golf” and one of the other systems to a potash mine in Esterhazy, Saskatchewan. The air temperature was well below zero, but 1,090 meters (3,600 feet) down in the mine it was 68 degrees year round. There was water (a lot of it) coming in through the mine roof, and the radars were to locate the cracks through which it entered. It was a difficult setting, and “Golf” did not see anything through the mine walls. I graduated not long after that, so I don’t know what became of the radar system. Dr. Unterberger passed away just this year at the age of 94. Building the “Golf” radar was a great experience that helped to motivate my interest in radar and my eventual participation in planetary radar mapping. One of these planetary radars, the SHARAD instrument, can see down three kilometers into the polar caps of Mars – ground-penetrating radar has come a long way in 30 years!
Despite the varied application of radar systems, from subsurface probing to air traffic control to — in our case — studies such as understanding the volcanic history of the Moon, all radars are comprised of essentially the same basic components. Of course the advances in electronics since the inception of radar during World War II has meant that the various radar subsystems have become more compact and economical over the years. In fact, one of the fundamental components, analog to digital converters that, as the name suggests, enable you to process a digitized version of the signal returned from a radar, are actually present in your typical laptop computer. This piece of electronics forms part of the microphone input, allowing you to be heard when you make online calls. Taking advantage of this, MIT has recently developed a short course centered on building a radar system that can be fed into this laptop input. CEPS planetary geologist, Gareth Morgan, attended the course over the summer and brought back the radar he built to be used as an educational tool at the Museum.
Having an opportunity to assemble my own instrument was a really enjoyable experience, especially given that SHARAD, the radar I probably use the most in my research, is typically 225 million kilometers (140 million miles) away! Looking at the photograph above, you will see the most distinctive features of the instrument are the two paint cans bolted to the side of the board. These tins make up the transmitting and receiving antennas, and despite being designed to carry paint, they do a very effective job. As the antennas came largely ready made, the majority of my time was spent assembling the circuit board (the red board in the photo) responsible for generating the radar signal and tuning it to produce the desired output. To maximize the range resolution and thus get the most out of the instrument, the radar is designed to transmit a linear frequency modulated waveform (also known as a ‘chirp’), rather than a single frequency. Needless to say, this requires careful tuning.
The end result was a radar system that is quite sophisticated in that it can perform a range of tasks:
- Measure the speed of moving objects using their Doppler shift, in effect acting like a police speed gun. The plot shows my changing speed as I run away, slow down, turn around, and run back to the radar –I am no Olympic sprinter!
- Measure the time delay of returned radar signals bounced off a target so that its range (or distance) can be measured.
- Produce synthetic-aperture radar images similar to the ones we make of the Moon and Venus using the Arecibo Observatory.
Actually, the central wavelength of the MIT radar is 12.6 cm (5 inches), which is the same as what we use for some of our lunar mapping work. However, due to the power supply being provided from eight AA batteries, the radar only has a maximum effective range of around 1 km (0.6 miles), so we won’t be using it for our Moon work anytime soon. Nevertheless, the instrument will certainly provide a lot of fun demonstrating the principles of radar in educational outreach activities for years to come.