Anasphere FAQ


Q: What are the legal issues surrounding the type of transmitter used on the AnaSonde-E and -3M models?

A: This question pertains to the AnaSonde-E and AnaSonde-3M only. Our other sensor systems, including SmartTether and LiteTether, never require an amateur radio license to operate.

Small radiosondes, like our AnaSonde-E and -3M, use small radio transmitters operating at nominally 433.92 MHz. Devices using this frequency can fall under FCC regulations (US Title 47, Code of Federal Regulations) at two points. One is unlicensed operation under FCC Part 15 regulations. The other is licensed operation under FCC Part 97 regulations (Amateur Radio).

The following discussion is intended to summarize the regulations and their practical interpretation as they pertain to the AnaSonde and other related devices (such as CricketSats popularized by student ballooning programs) which use similar transmitters.

Unlicensed operation:

With respect to unlicensed operation, 47CFR15.231 applies, and more specifically paragraph (e). Paragraph (a) is narrowly defined and excludes operation of devices such as the AnaSonde. Paragraph (e) allows anything, including the AnaSonde. Both of these paragraphs address the use of periodic, pulsed operation. However, for AnaSonde operation, the more restrictive field strength limits found under paragraph (e) apply.

It is easiest to translate these field strength numbers into a corresponding transmitter power by referring to an application note presently available from Lemos International, a distributor for Radiometrix, a company which makes transmitters of the same general type and power as used in the AnaSonde and similar devices. Here is the link:

Looking at their table, we find that the FCC field strength limit for 433.92 MHz corresponds to a transmitter power of approximately -22 dBm. Notice that the rest of the world (Europe, at least) allows much more power - up to +10 dBm - at this frequency. That's why these modules are so common and cheap around the world. Unfortunately, this has led to their widespread availability in the U.S. where they far exceed unlicensed transmission power limits.

For reference, the transmitter used in the AnaSonde has a nominal maximum power of +10 dBm. Again referring to the Radiometrix application note, they note that for use in the U.S. transmitters such as these would require about 35 dB of attenuation to be legal. Antenna efficiencies and supply voltages may contribute a few dB of reduction, but are not going to be anywhere near even 15 dB let alone the required 35 dB!

And, if one were to attenuate their transmitter to this level to make it legal as an unlicensed transmitter under Part 15, the range would be so very short as to be useless except as an across-the-room transmission.

Licensed operation:

The 433.92 MHz frequency also falls within the U.S. Amateur Radio bands. In this case, a licensed amateur radio operator may transmit on this frequency at powers up to 1500 watts - so the power limit is no longer an issue.

However, the transmitter must broadcast a station identification (generally, the call sign of its operator) at least once every 10 minutes per 47CFR97.119. This rule also specifies the formats in which identification may be transmitted.

This is the approach Anasphere has taken with the AnaSonde.

Q: What are the regulations controlling the operation of the various flight platforms (free balloons, tethered blimps, and kites) used to fly Anasphere's sensors (AnaSondes, the dropsonde system, SmartTetherTM, etc.)?

A: Anyone flying sensors on balloons, blimps, or kites should be very familiar with Part 101 of the Federal Aviation Regulations. The following discussion is primarily intended to help guide your attention to relevant items in the regulations – be sure to read the regulations for yourself!

Free Balloons:

In brief, when flying free balloons with AnaSondes or the dropsonde system, it is usually advisable to try to keep the weight of your payload below 12 pounds total, with no more than 6 pounds per package. Also, be sure that the lines supporting your payload will break under a 50 pound load or less (many people in the ballooning world factor in the fact that knots usually reduce line strength by a significant percentage). It is possible, in this way, to be exempt from any other notice requirements or other regulations (with some exceptions as noted under the "If you're going to fly exempt" section below).

Many university scientific ballooning groups also give the FAA notice of planned flights – this is usually a courtesy since the payloads are technically exempt from Part 101 due to low weights (see the preceding paragraph). It is certainly good practice to still give notice, because the same procedures will apply should you move up to larger and heavier payloads.

Blimps and Kites:

The tethered platforms that are big enough to carry a useful payload are generally – but not always – large enough to fall under the regulations. Small ones (blimps with gas capacity under 115 cubic feet, or kites under 5 pounds) will be exempt.

If your platform is subject to regulations, here is what to expect. First, you can fly up to 150 feet with no notice or markers, as long as you’re not close (within 5 miles) to an airport. To go up to 500 feet, you need to give notice to the FAA, and need markers on your tether line every 50 feet starting at 150 feet above the surface.

If you want to go above 500 feet, it can be done in some cases but with significant extra effort. To do this, you will require a waiver from the FAA. This will require working with your local FSDO (Flight Standards District Office) which should have a good understanding of your local airspace. Don't expect waivers near busy airports, though. Another route to working above 500 feet is if you can gain access to restricted airspace, which is usually paired with some sort of military area. "Restricted" is the specific term you are looking for. If you can establish a working relationship with the military people in control of a restricted airspace area (and this can be done!), suddenly you will find yourself with a lot more altitude room to work with...and most likely no marking requirements either. The catch, of course, is whether or not you can achieve your measurement objectives in that specific geographic area.

With regard to markers, we usually use the common one-inch plastic survey tape found at hardware stores. We cut 12 foot lengths of it, and tie them in their middle to the line (be sure to pull the knot tight so the streamer doesn’t slip).

Talking to the FAA:

Part 101 outlines what information you need to provide the FAA when you need to (or desire to) give notice of your operation. You will typically call 1-800-WXBRIEF, a phone number which will connect you with the nearest FAA flight service station. You will tell them you need to file a NOTAM (Notice to Airmen), and give the information outlined in Part 101.

One thing which occasionally catches the non-pilot population by surprise is when the person at the other end wants to know where the operation is relative to the nearest VOR. This is aviation navigation terminology. VORs are radio navigation beacons scattered around the country, and pilots often reference them for position. You can probably find a tutorial online, or else go to your local airport and find a pilot or flight instructor to talk to. They can quickly show you how that method of position location works (and, while you're there, you can pick up a "sectional" map of your area which will show these stations). Speaking of sectionals, they show restricted airspace too – which could be handy if you want to try to establish a working relationship to fly in such airspace.

If you're going to fly "exempt":

If you are flying below the various limits enumerated in Part 101, you may be exempt from various notice, reporting, and marking requirements – but do not forget that you are still obligated, under this part, to not operate your system in such a way that it presents a hazard.