Delta-Differential One-Way Ranging (or Delta-DOR, Δ-DOR for short) is an interplanetary radio-tracking and navigation technique.
How it is done
Radio signals from the spacecraft are received by two widely separated deep-space ground stations on earth and the difference in the times of signal arrival is precisely measured (and used to calculate a bearing). This is corrected using information about the current delays due to Earth’s atmosphere, obtained by simultaneously tracking (from each ground location) radio signals from a quasar (within 10 degrees of the same direction).
NASA uses its Deep Space Network to pick up signals from the spacecraft that tell navigators where the spacecraft are located. This navigation service is called "tracking coverage" and it includes Doppler, ranging and delta differential one-way ranging, or "Delta DOR." 
Missions that used it
- Golombek, M. P. et al. (15 July 2004). "Delta Differential One-way Ranging" (PDF). DSMS Telecommunications Link Design Handbook.
- Quasar to guide ESA's Mars landing. July 2016
- "Delta DOR". ESA. 28 September 2007.
- "Delta Differential One-way Ranging Design Handbook" (PDF). NASA Deep Space Network. July 15, 2004.
- Maddè Roberto, Morley Trevor, Abelló Ricard et al. (November 2006). "Delta-DOR A New Technique for ESA's Deep Space Navigation" (PDF). ESA Bulletin 2006,.
- "The European ΔDOR System" (PDF). Luciano Iess, Dipartimento di Ingegneria Meccanica e Aerospaziale Sapienza Universita’ di Roma, Italy.
- "Recommendation for Space Data System Practices. DELTA-DIFFERENTIAL ONE WAY RANGING (DELTA-DOR) OPERATIONSю RECOMMENDED PRACTICE. CCSDS 506.0-M-1. MAGENTA BOOK" (PDF). Consultative Committee for Space Data Systems (CCSDS). April 2011.
- "The XF type correlator for Delta-DOR in deep space navigation". Chinese Academy of Sciences. 1 September 2014.
- "Orbit determination of Chang'E-3 and positioning of the lander and the rover". Chinese Science Bulletin. 1 September 2014.
- ISRO, Indian Space Research Organisation (2015). Rao, P. V. Manoranjan, ed. From Fishing Hamlet to Red Planet: India's Space Journey (EPUB). Noida, Uttar Pardesh, India: Harper Collins Publishers India. ISBN 9789351776895.
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