1. Mars Data Workshop Mars Express Radio Science MaRS
Introduction
Experiment, Operations, Observations
Martin Pätzold, Silvia Tellmann
Rheinisches Institut für Umweltforschung RIU
Abt. Planetare Forschung
Universität zu Köln
ESAC 9th June 2008

2. 4 Years in Cruise 4.5 Years in Orbit 2 Years in Orbit
Mars Express
Rosetta
Venus Express
4 Years in Cruise
4.5 Years in Orbit
2 Years in Orbit
9. November 2005
14. April 2006
Launch: June 2003
Arrival: 25. December 2003
3. March 2004

3. Radio Science Experiments at RIU, Köln
Radio Science Experiments on interplanetary spacecraft
Radio Science Experiments at RIU, Köln
35-m New Norcia Antenna
Rosetta
Mars Express
Venus Express
Mars
Venus
comets

4. What is Radio Science? Radio Science is interested in:
Small changes in frequency (phase), signal power,
polarisation of a radio carrier signal, transmitted by a
spacecraft and received at a ground station on Earth

5. What is Radio Science? ... You may conclude on...
Was ist Radio Science?
What is Radio Science?
... You may conclude on...
... the media which the radio signal propagated through
... the perturbing forces acting on the spacecraft.

6. In the Beginning.... the basic idea
Was ist Radio Science?
In the Beginning.... the basic idea
1962 studies about radio propagation done at JPL
1962 Review on Space Research, Iowa City
Von Eshleman, Stanford University
Bistatic Radar Astronomy
„one man´s noise is the other man´s data“

7. „Bistatic Radar Astronomy“
Was ist Radio Science?
„Bistatic Radar Astronomy“
Interplanetary spacecraft
ground station antenna
Idea:
ground station transmist radio signal
-> received by s/c
-> processed and analyzed on board
(radio spectrometer)
-> s/c transmits results via TM to Earth

8. realize.... Plan B oscillators, radio spectrometers with
Was ist Radio Science?
realize.... Plan B
oscillators, radio spectrometers with
power and mass optimized for s/c
were not available
Plan B:
s/c transmits radio signal,
G/S transmits and records,
process and analyze data at home
advantage:
use the onboard radio subsystem
no dedicated hardware
(well... Ok... later USOs)
problem: low RF power and SNR

9. well... back to... Plan A Pluto Kuiper – Belt (PKB) „New Horizons“
Radio Science Experiment (REX):
powerful uplink radio signal (X-Band)
signal processing on board
radio spectrometer, USO 1kg, 1W
results via TM to Earth
cooperation with Prof. G.L. Tyler, Stanford
University (Co-I contribution RIU Cologne)
launch 2006; Pluto flyby 2014
Science objectives PKB Radio Science:
Density, pressure, temperature of the neutral and ionized atmosphere of Pluto (Charon?)
Gravity field Pluto and Charon; separation of masses, bulk density

10. RF Functional Block Diagram
Mars Express

11. RF Functional Block Diagram
Venus Express
Rosetta
USO

12. Spacecraft High Gain Antennas
Rosetta
Mars Express
Venus Express

13. S/C High Gain Antennas (HGA)
Rosetta
Mars Express
Venus Express
diameter 2,20m ,70m ,30 m
X-band (8,4 GHz)
gain dBi dBi dBi
RF Power 20 Watt Watt Watt
EIRP dBm dBm dBm

15. ESA´s New Norcia 35-m ground station

16. NASA Deep Space Network
34 m BWG cluster, Goldstone
70 m
26 m
34 m HEF

17. Radio Science: observed parameters
What is Radio Science?
Radio Science: observed parameters
radio carrier frequency shifts:
change of the relative speed between the ground station on Earth and the spacecraft (Doppler effect)
propagation of the radio signal in dispersive media
Change of the signal power by
absorption in media
scattering (particles and surfaces)
Change of polarisation by
reflection at surfaces (Brewster angle)
Faraday-Rotation in ionized media with external magnetic field

18. Frequency bands Telekomm. Sat. GPS ASTRA TV terrestrisch
UKW (WDR Köln)

19. Radio Link Modes
Mars Express
Venus Express

20. Why two downlink frequencies?
contributions to the received carrier frequency:

21. Why two downlink frequencies?
contributions to the received carrier frequency:
transmitted carrier frequency: X-band 8400 MHz
S-band 2300 MHz

22. Why two downlink frequencies?
contributions to the received carrier frequency:
thermal noise; s/c radio subsystem
G/S equipment
sf ~ order of mHz
transmitted carrier frequency: X-band 8400 MHz
S-band 2300 MHz

23. Why two downlink frequencies?
contributions to the received carrier frequency:
Doppler frequency term due to relative motion between s/c and G/S:
vr ~ 10 km/s: Doppler shift 280 X-band
Doppler shift 77 S-band

24. Why two downlink frequencies?
contributions to the received carrier frequency:
changes in Doppler frequency due to gravitational and
non-gravitational perturbing forces acting on the s/c
Doppler frequency term due to relative motion between s/c and G/S:
vr ~ 10 km/s: Doppler shift 280 X-band
Doppler shift 77 S-band

25. Why two downlink frequencies?
contributions to the received carrier frequency:
„plasma term“ due to the propagation of the radio signal through
ionized media (solar wind, ionosphere)
depends on the change of electron density along the ray path
depends inversely on carrier frequency
looks like noise => „plasma noise term“
sf ~ order of mHz to Hz

26. Why two downlink frequencies?
contributions to the received carrier frequency:
bending of the ray path in media:
depends on carrier frequency in the ionosphere
independent on frequency in the neutral atmosphere
Ionosphere: Dfbend,max ~ 1 Hz
Atmosphere: Dfbend,max ~ -10 Hz (Mars); Hz (Venus)

27. Why two downlink frequencies?
contributions to the received carrier frequency:
bending of the ray path in the Earth´s troposphere and ionosphere:
a constant or slight trend for the duration of observation
will be corrected by models (GPS, Klobuchar)
bending of the ray path in media:
depends on carrier frequency in the ionosphere
independent on frequency in the neutral atmosphere
Ionosphere: Dfbend,max ~ 1 Hz
Atmosphere: Dfbend,max ~ -10 Hz (Mars); Hz (Venus)

28. Why two downlink frequencies?
contributions to the received carrier frequency:
there are contributions which are
proportional to carrier frequency
invers proportional to carrier frequency

29. Why two downlink frequencies?
contributions to the received carrier frequency:
there are contributions which are
proportional to carrier frequency
invers proportional to carrier frequency
a matter of fact:
(it is not only designed like this, it is also a good idea)

30. Why two downlink frequencies?
contributions to the received carrier frequency:
compute: fS,rec – 3/11 fX,rec (differential Doppler)
and apply baseline fit

31. Why two downlink frequencies?
contributions to the received carrier frequency:
compute: fS,rec – 3/11 fX,rec (differential Doppler)
and apply baseline fit
The differential Doppler is the frequency shift due to
the propagation of the radio wave in a plasma relative
to a one-way downlink at S-band

32. single frequency experiment feasible?
Is the experiment feasible at a single downlink frequency?
In principle, yes, but...
for gravity: at the cost of higher noise caused by the plasma
Higher plasma noise at S-band and lower Doppler SNR
Lower plasma noise at X-band and higher Doppler SNR compared to S-band
for the ionosphere: at the cost of a potential (small) bias in the electron density; refractivity and SNR stronger at S-band than at X-band
for the atmosphere: no effect (independent of frequency)

33. comparison differential Doppler at opposition & conjunction
DOY 338, 2005
DOY 243, 2004

34. comparison differential Doppler at opposition & conjunction
solar opposition: rms ~ 4.8 mHz
DOY 338, 2005
solar conjunction: rms ~ 58 mHz
DOY 243, 2004

35. Earth occultation What is an Earth occultation?
As seen from the Earth, the s/c is disappearing behind the planetary disk and reappears at opposite hemisphere or opposite limb
Occurs at specific constellations between Earth & Mars location and orbit plane orientation
Occurs in „seasons“
The radio signals propagate through the ionosphere and the atmosphere before the s/c is occulted
The radio link is interrrupted during the occultation proper
Mars Express can only observe the INGRESS into occultation

36. bending of the radio wave
Earth occultation
Ionosphere
refractive index < 1
Neutral atmosphere
refractive index > 1
Change of the propagation path! a a
neutral atmosphere
Mars
ionosphere
a: bending angle

37. Data Pipeline Observables: Frequency Range Signal power Polarization
Data types:
Closed-loop
Open-loop
DSN 35m, 70m
ESA NNO 35m
ESOC, Darmstadt, GE
JPL, Pasadena, CA
IGM, Cologne, GE
Stanford U, CA
NNO data processing, archiving
DSN data processing

38. Data processing – the road to useful data
Level 1a: raw data from the ground stations
Binary or ASCII, in specific format
Translation from L1a -> L1b
Level 1b: extracted data from Level 1a
ASCII, in defined format by radio science group

39. Data processing – the road to useful data
Level 1a: raw data from the ground stations
Binary or ASCII, in specific format
Translation from L1a -> L1b
Level 1b: extracted data from Level 1a
ASCII, in defined format by radio science group
Reconstructed orbit, frequency prediction
from UniBw Munich

40. frequency prediction by Radio Science Simulator
Based on flown orbit => reconstructed orbit file
Compute expected received frequency at the ground station feed:
considering all motions: Earth, Mars, spacecraft, Earth rotation, plate tectonics
Consider relativistic frequency shifts
Correct for light times
Assume that Mars (Venus) has no atmosphere
Compute frequency residuals (received frequency minus predicted frequency)
Frequency residuals are precise up to 50 mHz (worst case); to be compared with 8400 MHz or 2300 MHz

41. Frequency residuals
DOY 354, 2005

42. Frequency residuals DOY 354, 2005 start of obs. occultation
lost radio link
receiver noise

43. Frequency residuals DOY 354, 2005 bias of 29 mHz
difference between observation and prediction
1-sigma standard deviation 32 mHz

44. Frequency residuals

45. Frequency residuals
2 minutes

46. Frequency residuals

47. Frequency residuals
main peak M2 M1
neutral
atmosphere
ionopause

48. Occultation observed by a 70-m station
same scale as before....
1-sigma standard deviation: 3 mHz

49. Data processing – the road to useful data
Level 1a: raw data from the ground stations
Binary or ASCII, in specific format
Translation from L1a -> L1b
Level 1b: extracted data from Level 1a
ASCII, in defined format by radio science group
Reconstructed orbit, frequency prediction
from UniBw Munich
Level 2: frequency, Doppler residuals, differential Doppler,
range, signal power; each frequency
ASCII, timely ordered, calibrated; in defined format
to be archived