Few radio parameters can be as confusing as modulation and over-the-air speed, for users at all experience levels. Here we hope to briefly shine some light on the subject:
Although serial port speed and over-the-air speed have some relation to each other, they are totally separate settings.
While the radio’s serial port speed and the GPS receiver’s port speed must match exactly (for example GPS = 9600 bps & radio = 9600bps) to allow the normal transfer of data over the connecting cable, the speed over-the-air cannot be set by simply changing the port speed. Over the air speed is a fixed value which depends on both bandwidth and modulation.
In the US, the most common bandwidth today is 12.5Khz. So if your radio uses any of the following modulations—Satel 4FSK, PacCrest 4FSK (Transparent EOT/4FSK), PacCrest FST at 12.5Khz — the speed over the air is fixed at 9600bps. Whereas if you are using the following modulations — PacCrest GMSK (Transparent EOT/GMSK), or Trimtalk 450S at 12.5 Khz— the speed over the air is fixed at 4800bps.
If a low speed modulation type such as GMSK is used, the base radio transmitter is on-the-air for a longer time than a faster modulation. This means higher battery consumption and more heating of the radio itself.
Modulation can be an even more complex subject and it has a major influence on both the speed over the air and the actual range of the radio equipment. Here is a quick and simplified explanation:
The most common modulation used today in UHF radios for RTK and other precision positioning applications is 4FSK (4-level Frequency Shift Keying). This means that the radio transmitter (base) transforms the NMEA data from the GPS into radio frequency variations that shift to 4 different points around its central working frequency. All the points must be contained within the 12.5Khz channel (up 11.25kHz usage is actually allowed by the FCC). This means that the transmitted frequency will constantly “shift” to 4 frequency points, both above and below the center frequency. As an example, the radio could represent different data patterns by transmitting at 2.5 and 5 kHz both above and below your central frequency. Each distinct frequency is interpreted as a binary pair (00, 01, 10, or 11) and the thusly the whole NMEA string of the GPS can be recomposed in the rover.
Some equipment offer even higher over-the-air speed by using more complex types of modulation (such as 8FSK, 16FSK). Some even use 32QAM, where not only frequency “shifts” but also “phase” variations are measured. Satel offers an internal OEM module, already in production and used by major GPS manufacturers, with 8FSK and 16FSK modulation and over-the-air speed up to 14400bps. This becomes useful when several constellations are received (for example GPS + GLONASS), which cause the radio to transmit more data. In order to be able to contain all data within 1 Hz (once a second) a higher over-the-air speed is desirable. The downside of higher modulation is that the receiver must work harder to detect very small variations, thus a stronger radio signal is required to counterbalance the environmental noise present in the air. Ultimately this means a decreased working range.
When considering over-the-air speed and higher modulation, factors such as battery consumption, radio heating, radio signal strength and desired working range should be reviewed. While higher modulation is generally ideal, the benefits may not be as impactful in areas of higher environmental noise. This trade-off may make a difference when analyzing your equipment and application.