RF Protocol

The Davis system uses frequency-hopped spread spectrum for its transmission. That might sound scary, but it isn't. In a nutshell, here is how it works

• the ISS transmits a packet at one frequency, goes into a low power mode, and then wakes up and transmits its next packet a short time later.
• the ISS is set to one of eight different IDs. All this does is change the timing between the packets.
• The sequence of frequencies is always the same for a given setup, but the number of frequencies varies according to the region in which the console was sold. For example, North American consoles use a set of 51 frequencies in the 915 MHz band, whereas consoles sold in Europe use only four frequencies in the 868 MHz band.

The VP2 ISS uses a CC1020 chip to transmit the data in Gaussian Frequency Shift Keying (G-FSK) format. That also is not as scary as it sounds. Basically, the carrier is modulated by one of two tones representing a 0 bit or a 1 bit. The CC1020 is transmit-only, whereas the CC1021 in the VP2 Console can either transmit or receive. Davis did this to allow the console to act as a repeater that can relay data from one console to another.

The Vantage Vue uses a newer variant of this chip that offloads a lot of the packet processing from the console's processor while maintaining compatibility between the older system and the newer one. This allows a VP2 console to receive transmissions from a Vantage Vue.

Packets are sent from the ISS every 2.5 seconds for an ISS set to a transmit ID of zero. The rate gets slower as the transmit ID increases by 1/16 of a second for every station ID number e.g. ID 1 transmits at an interval of 2.5625 seconds (ref: Davis Serial Protocol document). The data rate is 19.2 kbps and is transmitted from the ISS with the least significant bit first. More here.

TODO: Add references in to the low level parts of the RF transmission