Telemetry

Telemetry

An introduction to RF telemetry systems, by Gale Allen (pdf link, 11 pages).

To summarize, with the same type of antenna, the higher the frequency the lower the range. We should prefer a 500MHz band to a 2.4GHz for example. However directive antennas with higher gain are more practical in higher frequencies because the wavelength is shorter and antennas are sized to the wavelength. They may also be cheaper. With a high gain antenna, a higher frequency link can reach the same range as a lower frequency link with a unity gain antenna.

A more complete reading is the Telemetry Systems Radio Frequency Handbook, US military document, 2008 (pdf link, 133 pages).

There are some license-free radio frequency bands available, the ISM bands (Industrial Scientific and Medical) [1]:

• In the EU: 433MHz – up to 10mW power, 868MHz: multiple channels with power output up to 500mW. 2.4GHz with outputs up to 10mW.
• In the US: 433MHz up to 1mW output, 315MHz up to 10mW output, 915MHz up to 500mW (with restrictions on protocol – spread spectrum).

The ~900MHz band seems more promising since it allows for relatively high power without requiring a license, and the bandwidth will be more than enough if no video relay is considered on the telemetry link. Here is an example of RF module of 500mW on 869MHz ($100), here a telemetry module of 1W on 902-928 MHz ($90).

dBm to Watt conversion table

Weather balloons are launched very often (more than 850 twice a day around the world) and the most used product is the Radiosonde Vaisala RS92 and variants. Their RF output is 200mW for the 1680MHz version, and at least 40mW for the 403MHz version. Amateur radio operators have reported catching signals from those radiosondes several hundreds of kilometres away, so we definitely don't need more than the allowed 500mW ISM RF power. That may however require a high quality reception station with high gain antennas and low-noise amplifiers.

Copenhagen Suborbitals has an open source approach to rocketry too, and the Sapphire Telemetry System is avaiable on GitHub. They use two 1 Watt links, in bands above 2GHz.

Amateur satellites have to declare their orbit and frequencies to the International Telecommunication Union (ITU). This can be done for free now.

List of emission modules available on the 869MHz ISM band, 500mW power

• TIMWO HP868, also known as ARF7581AA, 1-page PDF link
• ARF35 (IP65 package), price 650 EUR.
• ARF27, price 67 EUR TX, 30 EUR RX.
• ARF 29, price 115 EUR.
• FC-RF209
• [2]
• [3]
• Yishi YS-C30L
• Mellow ml808
• XBee-PRO 868HP, announced for 80km with RF line of sight, but limited to 315mW
• Radiometrix TX3H transmitter, requires coding circuitry, 450mW

Reception equipment for the 869MHz band

Three kinds of choices are offered to us for reception:

• the reception module matching the emission module, some of them are indeed developed and sold together; the advantage is that we know what is the sensitivity of the receptor and we know that it will operate without issue on the same band,
• an amateur radio equipment,
• a software defined radio equipment (SDR), like the populars FunCube Dongle Pro+, bladeRF and the hackRF. SDR allows a large range of frequencies to be received and kind of encoding to be decoded. All the work and control is done by a computer, contrary to amateur radio equipment that does it in hardware. SDR interfaces are generally USB dongles on which an antenna is plugged.

In any case, a high gain directive antenna operating in the 869MHz band will be required to pickup the signal that far away, or even send some data upstream. It will need to be directed towards, which can prove difficult when objects are behind clouds or in a not well known orbit.

We may need a low noise amplifier too, depending on the chosen reception equipment.

Flying object tracking

It may not be easy to track a flying object with a directional antenna, even inside the atmosphere. If it passes behind clouds for example, you lose the ability to track it visually and it may be complicated to find it again later. In the case of a high altitude balloon with clear sky, that can be done easily if winds don't push it hundreds of miles away. Otherwise, it may get behind mountains and the line of sight can be lost if it's not high enough in altitude.

There is no magical solution. What is generally done is that the aircraft provides its position through telemetry, which is then used to refine the pointing of the tracking antenna. If it's lost at some point, a wider beam antenna is used, like a patch antenna to try to get a position information. However since these antennas have a lower gain, they may not be able to catch the data correctly, but they can still provide a cone of plausible localization.