Doppler Compensation

The relative motion between a satellite and an observer causes a frequency shift known as the Doppler effect. Doppler is well-understood and can be corrected automatically by satellite tracking software

Modern amateur satellite transceivers such as the Yaesu FT-736R (shown below) have Computer Aided Transceiver (CAT) interface ports for automatic computer control.  You can download source code for commanding the Yaesu FT-736R and FT-847 transceiver CAT ports.

Yaesu FT-736R

The Yaesu FT-736R CAT interface port uses Transistor-Transistor Logic (TTL) which operates at 0V, 5V levels for "0" (low) and "1" (high) logic states. Computer RS-232 serial (COM) ports operate at -10V, +10V levels for low and high logic states. You therefore need an RS-232/TTL converter to control this radio.

Yaesu sells a converting interface for about $100 retail.  You can build a converter for about $10. You need the following parts:

  • (1) DB25 Female 25 pin connector and shell
  • (1) 6 Pin DIN plug
  • (2) 2N2222A NPN transistors
  • (3) 10K resistors
  • (1) 11K resistor
  • (1) 4.3K resistor

This circuit can be built within the shell of the DB25 connector.  Use the schematic diagram below, provided by Eduardo Jacob EA2BAJ, as a your guide for making the connections.


Pin 2 (TXD) is connected to 10K resistor R1. The other end of R1 is connected to the base of transistor Q1.  The emitter of Q1 goes to ground.

  • Pin 3 (RXD) is connected to the collector of transistor Q2 and to one end of 10K resistor R2.  The other end of R2 is connected to one end of 11K resistor R4 and 4.3K resistor R5.  The emitter of Q2 and the free end of R4 both go to ground.
  • Pin 4 (RTS) is connected directly to Pin 5 (CTS).
  • Pin 7 (SG) is connected to ground and to Pin 1 of the CAT interface DIN plug.

Make the following connections to the CAT interface 6-pin DIN plug:

  • Pin 2 (SO) to one end of 10K resistor R3.  The other end of R3 is connected to the base of transistor Q2.
  • Pin 3 (SI) to the collector of transistor Q1.
  • Pin 6 (+13.8V) to the free end of 4.3K resistor R5.

Programmers writing code to communicate with the FT-736R CAT port need to set up the serial port at 4800 baud, 8 data bits, 2 stop bits, and no parity. You must disable the flow control since the FT-736R will not do any "handshaking." All commands are 5-byte sequences with a 50 to 200 ms delay between the bytes. Refer to the FT-736R operating manual for specific  character sequences for the instruction codes.


Satellites move fast enough that you need to correct the Doppler frequency shift on on the signals by offsets in radio tuning

 Tracking software can compute the amount of Doppler shift on a signal and correct it by automatically re-tuning the radio through a COM port