Delta DOR method
The Delta-DOR method or Delta-DOR for short (Delta Differential One-way Ranging), also written as ΔDOR , for interplanetary navigation is based on a simple but effective idea in two steps in which a triangulation and a measurement error are determined with which the triangulation can be corrected. Delta-DOR is currently the most precise method for position measurement. Precise data is particularly important when pivoting spacecraft into orbit, as well as when positioning a lander or a rover.
Triangulation
Two widely spaced receiving antennas or radio telescopes with known positions down to the millimeter range simultaneously track a space probe . The stations send signals, these are received by the spacecraft and sent back immediately. The time difference between the moment at which a signal is sent and the moment at which the signal arrives at the two stations is determined. In principle, this corresponds to the difference of a two-way transit time measurement , from which a distance and a position can be triangulated . The distance to the spacecraft is determined by measuring the time it takes for a radio signal to travel to the spacecraft and back to Earth. The Doppler shift of the signal, in turn, supplements this measurement and also gives the radial velocity along the line of sight.
Theoretically, this difference only depends on the positions of the two antennas and the observed spaceship, but there are a number of systematic and unsystematic interfering influences on the measurement results.
- The radio waves must pass through the ionosphere and the troposphere , where their speed by free electrons and the air density is reduced
- The wave fronts are disturbed by the plasma of the solar wind
- The clocks of the base stations are not exactly synchronized
- The length of the baseline is only known within a certain tolerance. This systematic disturbance can be minimized by determining the position of the radio telescope more precisely to within a few millimeters using VLBI and other methods.
The real speed in space is required to control the space probe, the runtime measurement alone is not precise enough and contains a measurement error.
Determination of the measurement error
The difference to the correct running time can for the most part be calculated using the differential one-way range (DOR). To do this, a quasar is used e.g. B. from the ICRF3 catalog in the direction of the space probe (within 10 ° of the probe position) used for correction. The position of the quasar is known very precisely from previous measurements using VLBI with an accuracy of 30 µas ( micro-arcseconds ). The measurement of the quasar is subject to the same disturbances as the measurement of the probe. Since the position of the quasar is known before the measurement, the difference to the measurement, the delta , can be determined in this way. The time difference measured by the radio signal of the quasar (the delta ) is now offset against the transit time of the signal from the space probe and thus results in a much more accurate position. The Delta-DOR method is so accurate because the error in measuring the position of the space probe can be corrected by the similar error in measuring the position of the quasar.
The probe's transmitters operate in a relatively narrow band, while quasars emit radio waves over a wide range of the spectrum. The results can be improved by using the broadest possible frequency bands or multiple bands. Quasars do not send any defined signals, so that you record a few minutes from both stations and look for small interferences in the signal, the differences in runtime of which can be evaluated, a method that is also used at VLBI to determine runtime. The reliability can also be improved by measuring multiple quasars.
Normally, at least three measurements are carried out in one measurement cycle, either quasar-probe-quasar or probe-quasar-probe, but longer sequences of alternating measurements can also take place. Usually three measuring cycles are made at a certain time interval. A particularly reliable measurement can be achieved if at the same time a measurement with a baseline offset by approximately 90 ° is carried out by means of further radio telescopes.
The method was used with the NASA space probes Phoenix and the two Mars Exploration Rovers and with the ESA probes Venus Express and Rosetta .
disadvantage
The use of the Delta-DOR method also has some disadvantages that limit the applicability of the method. In practice, Delta-DOR measurements are limited to the particularly critical phases of the mission and are replaced in the remaining times by the simpler and less precise transit time and Doppler measurements.
- Two antennas must always be used for the measurement
- The measurement is more complex than measurement methods with only one antenna
- The measurement can only take place in an area that can be observed by both antennas at the same time
- With two given antennas, a measurement can only be made at a specific time each day
- A measurement takes a certain amount of time, plus the signal transit times
- Telemetry data from the spacecraft cannot normally be collected during measurements
ESA Delta-DOR
NASA has been using the Delta-DOR method for a long time. ESA has only been technically capable of its own measurements since the first two deep-space antennas were installed in New Norcia (DSA 1) in 2002 and Cebreros (DSA 2) in 2005. Previously, you had to rely on support from NASA's DSN in critical phases even for pure ESA missions . ESA used the process for the first time at Mars Express . ESA then used this method independently in operation with the Venus Express Mission and was able to compare its own measurements with those of the DSN and verify the high quality of the results.
Delta-Dor in exchange between space organizations according to CCSDS
With the use of Delta-Dor at ESA, there was a need for the first time to think about the exchange of data. ESA uses different receivers than NASA, which deliver their own data formats. ESA developed software that allowed the exchange of Delta Dor data between NASA, ESA and JAXA .
In the meantime, the CDSN can also carry out independent Delta-DOR measurements. There is an increasing need for delta DOR measurements between antennas of different space agencies for mutual benefit. Many missions are already operated jointly by several agencies. There is a Delta-DOR Working Group of the CCSDS that sets certain standards, e.g. B. Data formats for exchange. This enables the Delta-DOR method between antennas from different networks and organizations.
literature
- Francesco Barbaglio: Precise Angle and Range Measurements: Advanced Systems for Deep Space Missions . 2012 ( uniroma1.it [PDF]).
Individual evidence
- ↑ esa: About delta DOR . In: European Space Agency . ( esa.int [accessed December 3, 2017]).
- ↑ Delta DOR. Retrieved December 3, 2017 (UK English).
- Jump up ↑ Mars Express: Rifle help with Phoenix landing. ESA, August 26, 2006, accessed September 28, 2008 .
- ↑ Golombek, MP, et al .: Delta Differential One-way Ranging . In: DSMS Telecommunications Link Design Handbook . July 15, 2004 ( Delta Differential One-way Ranging ( Memento of February 18, 2013 in the Internet Archive ) [PDF; accessed September 28, 2008]). Delta Differential One-way Ranging ( Memento of the original from February 18, 2013 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Madde Roberto, Morley Trevor, Abelló Ricard, et al .: Delta-DOR A New Technique for ESA's Deep Space Navigation . In: ESA Bulletin 2006 . November 2006 ( esa.int [PDF; accessed September 28, 2008]).
- ↑ M. Mercolino, R. Maddè, L. Iess, M. Lanucara, P. Tortora, A. Ardito, G. Rapino: Results and future applications of the ESA Delta-DOR; 4th ESA International Workshop on Tracking, Telemetry and Command Systems for Space Applications; TTC 2007 11-14 September 2007 at ESOC, Darmstadt, Germany . Ed .: ESA.
- ^ Delta-DOR - Technical Characteristics and Performance; Green Book . 2013 ( ccsds.org [PDF]).
- ↑ Delta-Differential One Way Ranging (Delta-DOR) Operations; Magenta Book . 2018 ( ccsds.org [PDF]).
