Ranging between unsynchronized communication terminals

A system for determining a range between two unsynchronized communications terminals comprising:

• at least one data processor;
• and
• memory comprising instructions which, when executed by the at least one data processor, result in operations comprising:
• transmitting, by a first terminal, a range request to a second terminal over a line-of-sight communications channel, wherein the first and second terminals are independently in motion with respect to each other and have unsynchronized timing domains, wherein the terminals are satellites;
• storing, by the first terminal, a first timestamp in memory corresponding to a time at which the range request message was transmitted;
• receiving, by the first terminal from the second terminal, a range response to the range request, the range response including a residence time that characterizes an amount of time the second terminal required to send the range response after receiving the range request;
• storing, by the first terminal, a second timestamp in memory corresponding to a time at which the range response was received;
• calculating a roundtrip time for the range request and range response based on the second timestamp minus the first timestamp and the residence time;
• calculating a distance between the first terminal and the second terminal based on the roundtrip time;
• repeatedly measuring a doppler effect through a periodic sequence of range measurements;
• and
• using the doppler effect measurements to compute a corrected range.

A system comprising:

• a first communications terminal in communication with
• a second communications terminal over a line-of-sight communications channel, wherein the first and second communications terminals are independently in motion with respect to each other and have unsynchronized timing domains, and the terminals are satellites;
• wherein the first communications terminal:
• transmits a range request to the second terminal;
• stores a first timestamp in memory corresponding to a time at which the range request message was transmitted;
• receives, from the second communications terminal, a range response to the range request, the range response including a residence time that characterizes an amount of time the second communications terminal required to send the range response after receiving the range request;
• stores a second timestamp in memory corresponding to a time at which the range response was received;
• calculates a roundtrip time for the range request and range response based on the second timestamp minus the first timestamp and the residence time;
• and
• calculates a distance between the first communications terminal and the second communications terminal based on the roundtrip time;
• repeatedly measures a doppler effect through a periodic sequence of range measurements;
• and
• uses the doppler effect measurements to compute a corrected range.