Critical for serious FPV Racers is the racing transponder from Seriously Pro Racing.

As you fly around the track the transponder continually transmits your unique code via IR. As your aircraft passes the start/finish timing gate the track-side receiver records your lap-time.

Features:
Built-in 5v regulator (BEC)
Dual IR LEDs
iLAP Compatible
Highspeed transmission to eliminate interference
7-Digit code
Flexible mounting
Can be powered using 2S-5S Lipo OR power from existing 5V supply.
Ideal for racing clubs 

Specs:
Dimensions: 36 x 13.5mm with 2 x 30.5mm mounting hole tabs.
Weight: 3 grams
Power: 5V regulator, 1 AMP
LED: Power/Activity
Pins/Functions:
1x 2pin though-holes for pin headers for battery
1x 2pin though-holes for pin headers for 5V in/out
1x 4pin bottom mounted, JST-SH socket for SWD debugging
1x 4pin bottom mounted, JST-SH socket for UART
2x 2pin through-holes for IR LEDs 

Includes:
1 x SPRacingTBEC Race Transponder Board
Pre-programmed unique 7-digit code