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Introduction to ∆DOR
T. Morley OPS/GFI
OPS-G Forum: ∆DOR System
22 April 2005

Contents
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❒ What is ∆DOR?
❒ Why ∆DOR?
❒ Why ESA ∆DOR now?
❒ ∆DOR accuracy.
OPS/G Forum: ∆DOR System T. Morley OPS/GFI
22-04-05, Slide 2 / 11

What is ∆DOR ?
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❒ ∆DOR stands for Delta Differential One-way Range.
❒ ∆DOR is a radiometric (tracking) data type that
complements conventional Doppler and range
measurements for deep space missions;
- main use is during heliocentric cruise at large geocentric distance.
❒ ∆DOR is a kind of Very Long Baseline Interferometry
(VLBI) for spacecraft, with an extra-galactic radio
source (quasar) used for calibration.
OPS/G Forum: ∆DOR System T. Morley OPS/GFI
22-04-05, Slide 3 / 11

Differential One-way Range Measurements
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Measure differential one-way delay of spacecraft signal.
Measure differential one-way delay of near-by quasar signal.
Subtract differences - cancels errors, e.g. media, clocks, instrumentation..
OPS/G Forum: ∆DOR System T. Morley OPS/GFI
22-04-05, Slide 4 / 11

∆DOR Features
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❒ Observables are the differential delay (τ) for s/c and quasar:
- “delta” conversion to ∆DOR is done within the orbit determination.
❒ Basic equations:
cτ c
sin θ = ---- ,
- δθ = --------------- δτ
-
B B cos θ
- angular error inversely proportional to length of baseline!
- long baseline means low elevation at mutual station visibility.
❒ Quasar direction known precisely (best < 2 nanoradians error).
❒ Quasar chosen so that direction close to spacecraft (< 15 ° ):
- similar spacecraft and quasar signal paths through atmosphere.
❒ Same frequency channels recorded for spacecraft and quasar:
- no frequency-dependent differential effects.
OPS/G Forum: ∆DOR System T. Morley OPS/GFI
22-04-05, Slide 5 / 11

Why ∆DOR ? (1)
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❒ Range and Doppler are line-of-sight measurements:
- range gives accurate information on only 1 component of position;
- Doppler gives accurate information on only 1 component of velocity;
- direction information is obtained only indirectly thanks to Earth
rotation but the accuracy is limited even after many passes;
- poor accuracy in declination when s/c near the equator;
- (in planetary orbit, fast-changing dynamics gives direction).
❒ A single ∆DOR data point gives one direction angle:
- nearly instantaneous measure (less than 1 hour);
- very accurate measurement;
- need ∆DOR data points from 2 baselines for full direction.
OPS/G Forum: ∆DOR System T. Morley OPS/GFI
22-04-05, Slide 6 / 11

Why ∆DOR ? (2)
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❒ Improves orbit determination (OD) accuracy:
- saves fuel !
❒ Independent confirmation of conventional OD solutions:
- improves robustness !
❒ Needed when critical reliance on navigation accuracy:
- planetary swing-by (except Earth), e.g. Rosetta/Mars, Bepi Colombo;
- planetary orbit insertion, e.g. MEX, VEX, Bepi Colombo;
- landing on a planet, e.g. Beagle 2.
❒ Fast recovery of orbit knowledge after unknown ∆V:
- e.g. MEX or VEX safe mode entry with unbalanced thrusters.
OPS/G Forum: ∆DOR System T. Morley OPS/GFI
22-04-05, Slide 7 / 11

Why ESA ∆DOR now ?
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❒ 2 ESA Deep Space antennas will be operational in 2005.
❒ To meet needs of current and future deep space
missions without being dependent upon NASA/DSN:
- MEX: used DSN ∆DOR measurements;
- VEX: both ESA and DSN ∆DOR being planned;
- Rosetta: Mars swing-by in February 2007;
- Bepi Colombo: navigation absolutely needs ∆DOR.
❒ Complementary to DSN ∆DOR (and other VLBI systems):
- Cebreros - New Norcia is a new baseline orientation;
- future use of 1 ESA + 1 DSN station could be foreseen;
- possibilities with other stations, e.g. 70 m. antenna at Evpatoria.
OPS/G Forum: ∆DOR System T. Morley OPS/GFI
22-04-05, Slide 8 / 11

Station Baselines and Visibility for ∆DOR
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80. NASA/DSN ESA
70.
60.
50.
40. MADRID
GOLDSTONE CEBREROS
30.
20.
10.
0.
-10.
-20.
-30.
NEW NORCIA
-40. CANBERRA
YELLOW REGIONS OF SUBSATELLITE POINT SHOW
-50.
MUTUAL VISIBILITY ABOVE 10 DEGREES ELEVATION
-60.
-70.
-80.
140. 160. 180. 200. 220. 240. 260. 280. 300. 320. 340. 360. 380. 400. 420. 440. 460. 480. 500.
0. 20. 40. 60. 80. 100. 120. 140.
OPS/G Forum: ∆DOR System T. Morley OPS/GFI
22-04-05, Slide 9 / 11

∆DOR Accuracy
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❒ ESA system initial target accuracy of 1 nanosecond for τ:
- for 10 000 km baseline, angular accuracy = 30 nanoradians;
- 30 nanoradians at 1 AU (150 million km) = 4.5 km.
❒ Comparison with MEX NASA/DSN ∆DOR:
- expected accuracy before launch of ~0.5 ns;
- assumed RMS error during cruise was usually 0.25 ns;
- overall RMS error value from navigation analysis ~0.17 ns;
- less than 1 km for MEX arrival at Mars.
OPS/G Forum: ∆DOR System T. Morley OPS/GFI
22-04-05, Slide 10 / 11

MEX B-plane estimates on 23 December 2003
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3-SIGMA ERROR ELLIPSES
1200
DOPPLER ONLY
263.0 X 66.5 KM
1100
DOPPLER + RANGE
87.9 X 5.6 KM
B-R - KM
1000
TARGET
900
DOPPLER, RANGE + DELTA-DOR
800 12.8 X 1.9 KM
7600 7700 7800 7900
B-T (PROJECTION OF MARS EQUATOR) - KM
OPS/G Forum: ∆DOR System T. Morley OPS/GFI
22-04-05, Slide 11 / 11