Welcome to the McGill Radar Observatory!
I will be your tour guide. Please follow me for a virtual visit of our main radar facility. Together, we will basically follow the travels of a radar pulse from its transmission to its processing.
Our main radar, inaugurated in 1968, does the weather surveillance for the Montreal area. What this means is that it is used 24 hours a day by Environment Canada to monitor the weather in a radius of 250 km around Montreal. At the same time, we, at McGill University, use it for scientific research.
To start with, we will climb up the tower to see the first key element of the radar, the transmitter. It is located 25 m up, next to where you can see the big air conditioning unit hanging out one of the windows two floors below the concrete platform.
I hope you are not too afraid of heights! Actually, it is not that bad, but it is worse on the way down. On the way up, it is simply a good exercise…
We made it to the radar transmitter. Raman, one of our engineers, is tuning it. Because the space is limited, two wide-angle photos were stitched and corrected for deformation, resulting in Raman’s head and body to expand. Sorry Raman!
A weak microwave signal is first shaped to a short transmit pulse lasting one micro-second by the boards above Raman’s hands, the modulator. That pulse is then amplified to more than 600 kilo-Watt of power in a few steps that culminates in a big microwave amplifier tube, known as a klystron, located inside the light blue canister on the right. The transmitter fires these high power pulses up to 1200 times per second. The radar pulse then travels upward towards the antenna inside hollow metal tubing called waveguides.
One floor up. This messy plumbing does two things: First, it splits the transmit pulse traveling in a waveguide from the transmitter (1) into two pulses travelling in two waveguide channels (2a and 2b) that ultimately go up (5) towards the antenna. Second, for each channel, it also acts as a traffic cop, thanks to circulators (3). These circulators direct the transmitted pulse coming from (2a) to (3) towards (4) and (5), and direct the signal received by the radar from (5) and (4) to (3) towards (6).
We have two transmit and two receive signals because the radar transmits and receives microwaves at horizontal and at vertical polarization. The horizontally polarized signal is more sensitive to the horizontal dimensions of objects while the vertically polarized signal is more sensitive to the vertical dimensions of objects. By transmitting pulses and receiving signals at these two polarizations, we can detect whether targets are mostly spherical, or are much larger in the horizontal than in the vertical; this is a useful piece of information to help identify them.
We will now follow the transmit pulse upwards of (5). We will come back to the receive signal beyond (6) on our way down.
Finally, we are on the antenna platform, inside the radome (the big white golf ball on the first picture). Raman is pushing the 9 m antenna. No, that is not how it normally works: we are doing maintenance on the system, and it is the only time I am allowed up here to take pictures! Normally, the antenna rotates clockwise, making one rotation every 10 s. As it does these rotations, it also gradually points higher; this way, the radar makes measurements at 24 elevation angles from near the horizon to more than 34 degrees. Then, the antenna comes back down, and this cycle, known as a volume scan, is repeated every five minutes.
You may be able to see the two waveguides that came from one floor below poke through the antenna pedestal near the top left corner of the picture. The two waveguides then run behind the antenna until they are next to a gray box, where they turn through a hole in the antenna (the gray box is an old shorter-wavelength transmitter we used in the past to look at snow).
Above is a picture of the antenna viewed from the side. The waveguides follow a strut from the bottom of the picture towards the left, where we can see the feed of the antenna. There is a larger picture of the feed on the left. The feed is the longish tube that ends with a small plastic dome. The feed “shines” the transmit pulse on the 9 m dish, and the dish focuses the energy in one direction, here towards the left. The radar pulse then goes through the radome and out (the radome is designed to be transparent to microwaves, and is used to protect the antenna against the elements).
The radar pulse travels outside until it hits an object, what radar people call a target. That object can be anything: the ground, an airplane, a bird, an insect, or something more interesting for us: a raindrop, a snowflake, a hailstone... A small fraction of the pulse, what we call an echo, travels back towards the radar and comes inside the radome. It is focussed by the dish back towards the feed, and gets back in the waveguides in which it travels towards the receiver. We can know where the target is from the direction the antenna is pointing and from the time it took to get back an echo after we transmitted the radar pulse.
The optional part of the tour: the catwalk around the radome. I say it is optional because it is not essential to understand how radars work; it is also where most of the people afraid of heights refuse to go.
Two pictures: one looking out, one looking down towards the entrance of the tower. The reason radars are on towers is so that they have an unblocked view all around.
OK, I admit it: the two pictures were not taken on the same day. But I liked too much the picture through the catwalk. Let us go back down one floor.
We are back on the floor where the transmit pulse got split in two. The other thing that happened there was that the received signal, the echo, was redirected by the circulator in another direction, towards the radar receiver, shown on this picture. What come back to the radar are echoes with very low power, sometimes less than 0.00000000000001 Watt, and we can detect them. These very weak signals at horizontal and vertical polarization are brought to the receiver by the cooper-colored coaxial cables. The task of the receiver is to first amplify these signals, and then convert them to a lower frequency so they can be analyzed by a specialized computer, the radar signal processor.
But because the radar signal processor is in the other building, we must go back down. So does the amplified signal and information on antenna position, inside the black and colored cables around which the staircase circles.
No amount of creative photography could make the radar signal processor look visually appealing: it is just an ordinary looking computer, with no special display. So I chose not to show it. What it does is to convert the signals coming from the receiver into numbers, and these numbers into basic quantities such as radar reflectivity (how strong the echo is) and radial velocity (how quickly are the targets approaching or moving away from the radar). We then store that information for future reference on DVDs. They are not very visually appealing either, but if you insist, this is how 10 years of data from our radar look like.
These raw quantities are then further processed using other computers with specialized software, called radar product generators, that transforms them into weather information used in a variety of applications: weather surveillance, agriculture, aviation, hydrology, etc. Below is Aldo, one of our researchers, studying a storm using our radar product generator, RAPID. It is these systems that produce, among other things, the radar images you see on this website.
This ends our virtual visit. I hope you enjoyed it!