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A radar reflector is not required if your balloon launch is “exempt” under FAR Part 101 Subpart A, which is discussed in details in the FAR Part 101 chapter. However, even if a radar reflector is not required, it is good practice to make and use one. It will not add a lot of weight to your craft, but it will make the entire project a more well-engineered system.
Radar reflectors can be purchased off the shelf, though they are usually intended for small watercraft (like canoes) so that they show up on the radars of larger ships to avoid collisions. Interestingly, this type of big-boat/little-boat relationship is very similar to that experienced with aircraft and high altitude balloons. Large aircraft have radar to navigate and identify airborne objects, both natural and unnatural. Very small weather balloons (those flying “exempt”) pose little threat to larger aircraft (based on the statistically likelihood of collision and the physics of an airplane designed to keep people safe in a wide range of conditions) but colliding with objects is not something they want to do. Plus, it would follow that if you put money and effort into building a homemade spacecraft then you would like to get it back too.
Large aircraft send out information about what they are doing with Automatic Dependent Surveillance – Broadcast (ADS-B), but a balloon is not complex or large enough to do this easily (there is an ongoing effort to change this). So, the best we can do is reflect radar signals, so that aircraft can still see your nearspacecraft.
Weight is not very important on a boat, but durability is, so the commercially available radar reflectors tend to be made out of sheets of metal. For a high altitude balloon launch, the requirements are almost perfectly flipped, because every ounce of additional weight requires more lifting gas (more expensive) and can eventually lead to needing a larger balloon (more expensive again). At the same time, the environmental conditions that our radar reflector are not very harsh or long-lasting, unlike on a boat. Therefore, our radar reflectors can be almost disposable and easily made at home. This method will be great because the radar reflectors will be cheaper (nice!) and much lighter (very nice!).
Instructables user “edjez” has some great tutorials on how to make a light radar reflectors at home. I recommend making the “Lightweight Radar Reflector” with cardboard or you can use a similar method with CDs. The instructions have been re-created below, with permission.
To start, get three square pieces of cardboard with side lengths (all the same) of 6”-12”. Cut the cardboard in the locations that you see the 2-dimensional graphic below. Make sure that the slots you cut (thick colored lines in the graphic below) are only as wide as the cardboard thickness plus the thickness of the tape, or a little bit less; it is better to have it tight than loose so the friction will work with you to keep the shape you set. Once the panels are cut, you can affix a thin aluminum layer either as aluminum tape or with plain aluminum foil sheets (as used for cooking) and some crafty taping. The three pieces of the reflector will need to be taped together anyway, and having the entire surface of the aluminum bonded will be much more robust than affixing with tape by hand, so I recommend just using the aluminum tape.
Assemble the pieces. Some bending will be required but it should be no problem for the cardboard. Use the end of a table or a box to square up each of the panels by making sure the angles are as close as you can reasonably get to 90º.
I recommend stringing your cord through the radar reflector top and bottom and then securing it with one knot above and one below to prevent it from sliding along your rope in either direction.
One can see that, regardless of the orientation, the radar reflector will send back at least some radiation. This is illustrated in 2D below, but still holds in 3D.
Version Two Below
A radar reflector is not required as long as your High Altitude Balloon (HAB) is below the limits set in FAR Part 101 Subpart A (in technical speak, it means your launch is ‘exempt’). However, even if you are not required to use a radar reflector it is good practice and good fun to make one. The reflector won’t add much weight to your craft, but it will be a more well-engineered system. If you are flying non-exempt, then you do need to put a radar reflector on your craft and this is the place to learn how!
Typical radar reflectors that you can buy are often made for small watercraft (like canoes) so that they will show up on the radars of larger ships so everyone can avoid a collision. Interestingly, this type of “big- boat/little-boat” interaction is very similar to what we will deal with in the aviation world.
A typical naval radar reflector made from aluminum.
Large aircraft have radar systems and ADS-B (explained at the end of this post) to stay safe in the air. Radar looks for other airborne objects (both natural and unnatural) so that pilots can make easy early avoidance maneuvers. Very small weather balloons are not a significant threat to large aircraft but I’m sure they would prefer to avoid you, and I’m sure you would like to get your carefully designed craft back too.
An example of an aircraft radar system that would be way too much for small balloon payload. The system shown here is for a private jet. Source: Dtom (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons
The Wikipedia page on Corner Reflectors has additional information and graphics.
Weight on a boat is not as important as durability so the radar reflectors are made out of sheets of metal. For the relatively light use of the radar reflectors on HABs we are fine using radar reflectors that are less durable because they will be exposed to far less severe weather and other stresses. This method will be great because the radar reflectors will be cheaper (nice!) and much lighter (very nice!).
Instructables user ‘edjez’ has some great tutorials on how to make lightweight radar reflectors at home. I recommend the “Lightweight Radar Reflector” which is re-created and expanded on below.
To start, get three square pieces of cardboard with side lengths of 6”-12” (you pick the size but all pieces must be the same size). You can then cut slots in the cardboard in the locations that you see below. Make sure that the slots you cut are only as wide as the cardboard thickness plus the thickness of the aluminum tape (details below), or a little bit less since it is better to have it snug so the friction will work with you when you are putting the pieces in place.
Cut slots in the locations marked by thick lines. Slots should be the thickness of the cardboard plus the thickness of the reflective tape, or a little less for a snug fit. Dashed lines are only for reference to mark the middle of the square in each dimension.
Once the panels are cut you can affix a thin aluminum layer as aluminum tape. If you insist on doing it the hard way you can use plain aluminum foil sheets (like you use for cooking) and some tricky taping. The three panels of the reflector will need to be taped anyway and affixing a new surface completely through adhesive will also make it stronger.
An example of the aluminum tape used for the radar reflector.
One the panels have slots and are covered with aluminum it is time to start putting them together. I think you will find the pictures below quite sufficient and self-explanatory. You want every angle to be as close to 90o as possible and you can use the corner of a table to help you achieve this.
Connect the first and second panel by combining them at matching notches.
Adding the third panel takes some bending but the cardboard should have no problem with this.
I recommend stringing your cord through the radar reflector and securing it with at least one knot above and one below to prevent it from sliding along your rope in either direction. Make sure you do this at some point below the parachute so you don’t lose it once the balloon bursts.
You can see that regardless of the orientation the radar reflector will send back at least some radiation. This will hold up in 3 dimensions as well but is far more difficult to illustrate! See Wikipedia page on Corner Reflectors for a decent 3D visualization.
Large aircraft also transmit information about location, airspeed, and related information for themselves and other nearby aircraft using Automatic Dependent Surveillance – Broadcast, or “ADS-B,” so that they show up on air traffic control displays and other aircraft show up on their own displays. A balloon is not complex enough or large enough to do easily carry an ADS-B system so the best we can do is have radar signals reflected so that aircraft can still see the craft.
Copyright 2013–2021 Bryan Costanza