One radio, and a second shared by everyone in range.
One radio and one antenna, and a second cut the same way by every device in range.
Where the second goes.
There is one radio and one antenna in the device, so the second is cut into dwells and each dwell gets one frequency. GPS gives the top of the second to the microsecond, and every skyBlip, every skyPost and every FLARM-generation device in range cuts the same second the same way.
| After the top of the second | Tuned to | What is on it |
|---|---|---|
| 0 to 200 ms | 868.400 MHz | the tail of the dwell that opened at 800 ms in the previous second: FLARM-generation traffic is still transmitting here, so the device is still listening. Receive only |
| 205 to 395 ms | 869.525 MHz | the skyPost uplink: the ADS-B traffic the ground station received and the weather it holds, at 200 kbit/s. Receive only, always |
| 400 to 800 ms | 868.200 MHz | direct traffic on the lower channel, ADS-L and FLARM-generation together |
| 800 to 1200 ms | 868.400 MHz | the same on the upper channel, running 200 ms into the next second |
Visiting both 868 channels is not a refinement. ADS-L alternates channels from one transmission to the next, and a FLARM-generation device transmits on the lower channel in the first half of the second and the upper in the second half. A receiver that camped on one of them would hear half of its neighbours and never know which half.
On those two channels the device hears two systems in one dwell. ADS-L and the FLARM-generation wire both hide their frames behind a 32-bit sync word, and the two words agree on their first eighteen chips: the receiver arms on the chips they share, and the software names the system from the chips that follow. The few milliseconds either side of the 869.525 MHz dwell are the margin a band change needs, because the two bands are two modulations and not two frequencies: 100 kbit/s Manchester FSK below, 200 kbit/s GMSK above.
Transmission is the narrow part. The device speaks on the 868 channels only, between 450 and 1000 ms, in whichever of the two channels this second belongs to, and what goes out is always ADS-L: the FLARM-generation format is received, never transmitted. 869.525 MHz is the ground station’s to talk on and ours to listen to, so nothing this device does ever keys there. The instant of the burst inside the slot is drawn from the device address and the current second, redrawn every second, which is what keeps two aircraft that collided once from colliding again.
- Four dwells a second, aligned on GPS to the microsecond
- 868.200 and 868.400 MHz, 400 ms each
- 869.525 MHz, 190 ms, listening for the ground
- One sync window, two systems decoded
- A burst of about 5 ms, at an instant nothing on air can move
- 1% of the hour allowed, half of it used