Few radio parameters can be as confusing as modulation and over-the-air speed, not only for the novice user, but also for the experienced. Here are a few comments that hopefully help to shine some light on the subject:
Although Port Speed and over-the-air speed have some relation to each other, they are totally separate settings.
While the Radio Port speed and GPS receiver 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. It’s actually 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, PacCrest FST at 12.5kHz,
the speed over the air is fixed at 9600bps.
Whereas if you are using the following modulations:
– PacCrest GMSK, or Trimtalk 450S at 12.5 kHz, the speed over the air is fixed at 4800bps.
If a half-speed modulation type such as GMSK is used, the Base radio Transmitter is on-the-air for twice the length of time respect to a faster modulation. This means higher battery consumption and more overheating 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.
The most common modulation today is 4FSK (4 Frequency Shift Keys). 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 required by the FCC. This means that the TX will constantly “shift” to 4 frequency points, up to 6.25 kHz away from its central frequency, both to the right and to the left in the frequency spectrum. A rough example: the Tx “shifts” from its central 450.000000 MHz to -> 450.006250 MHz and then back and forth from and to -> 449.9993750 MHz. Each “shift” is interpreted as a data 1 or zero and the whole NMEA string of the GPS is recomposed in the rover.
Some equipment offer even higher over-the-air speed by using more complex types of modulation (ex. 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.