Tracking

Somewhat distinct from the function of receiving data from a planetary probe is determining its location. Although the same radio signal is usually used, the hardware is often different.

Location may be determined by a variety of means; it is important to understand what is meant by location - all locations are relative. On Mars, where comprehensive high-resolution mapping datasets exist, correlation of local images with orbital data is often used. On the other hand, precise Doppler and/or range measurements give a better location with respect to the Earth.

Table 10.4. Examples of probe link budget calculation

Parameter

Units

DTE S-band link

UHF relay link

Link

Frequency

MHz

2300

400

Wavelength

m

0.1304

0.75

Range

m

1.05 x 1011

2.5 x 106

Space losses

dB

-260.1

-172.4

Venusian atmospheric

dB

-0.1

0.0

absorption loss at 50 km

Venusian atmospheric

dB

-0.1

-0.1

refraction loss

Probe

TX power

W

5.0

5.0

Cable losses

dB

-0.3

-0.3

TX antenna gain

dB

0.0

0.0

Pointing loss

dB

0.0

0.0

TX antenna ellipticity

dB

2.0

2.0

EIRP

dBW

6.7

6.7

RX station

Receiving antenna diameter

M

64.0

0.9751

Antenna gain

dB

61.2

10.0

Beamwidth

deg.

0.14

53.84

Pointing losses

dB

-0.1

-0.3

Polarization losses

dB

-0.2

-0.2

System temperature

K

25.0

100.0

Venus temperature contribution

K

1.3

500.0

Total system temperature

K

26.3

600.0

Received power

dBW

-192.7

-156.3

Noise spectral density

dBW Hz"1

-214.4

-200.8

Ps /Noise

dBHz

21.7

44.5

Modulation index

degrees

60.0

60.0

Carrier losses

dB

-6.0

-6.0

Data losses

dB

-1.2

-1.2

Carrier power/no

dBHz

15.7

38.5

Loop bandwidth

Hz

1.0

200.0

SNR in bandwidth

dB

15.7

15.5

Processing losses

dB

-1.0

-2.0

Data power/noise

dBHz

19.5

41.3

Bit rate

bps

10

1500

Eb /noise

dB

9.5

9.5

Threshold (7,|) code

dB

4.5

4.5

Data margin

dB

5.0

5.0

1 Equivalent diameter.

1 Equivalent diameter.

Doppler measurements require a stable or known transmitter frequency (using an ultrastable oscillator or transponder) on the probe, and a receiver with a much more stable reference on Earth. This allows the line-of-sight velocity to be determined. Doppler navigation is also possible using the frequency history of the signal received from an orbiting transmitter, which depends on the altitude and

Table 10.5. Main characteristics of probe links used on several planetary landers and entry probes

Tx

Link

Frequency

power

Antenna

Craft

type

(MHz)

(W)

type

Modulation

Bit rate (bps)

Venera 4-8

1-way

922

20

Hemispherical

FSK

1

probes

DTE

spiral;

bifilar

conical on

Venera 8

Pioneer Venus

1-way

2294

10

Crossed-

?

16 to 64

Small Probes

DTE

dipole

Pioneer Venus

1-way

2294

40

Crossed-

?

128 to 256

Large Probe

DTE

dipole

(data)

VeGa AZ

1-way

1667.8

4.5

Helicone

?

1 to 4

DTE

Galileo probe

Relay

1387.0 and

2 x 23

Crossed-

?

128

1387.1

dipole

2x80961

Huygens

Relay

2040 and

2 x 10

Helicones

BPSK

2097

Ranger 3, 4, 5

DTE

960

0.05

Crossed-

PM

analogue

landers

dipole

Luna 9, 13

2-way

183.538

?

4 blades/

PM

?

DTE

petals

Mars 6, 7

1-way

122.8;

?

4 blades/

?

256?

Landers

Relay

138.62 x 30

petals

Mars 96

Relay

437.100

>1

Quadrupolar

Manchester

8

small

and

stations

401.5275

Mars

2-way

8420

12

30 cm

BPSK

4740

Pathfinder

DTE

diameter

Beagle 2

2-way

401.6 and

5

patch

?

2000-128 000

relay

437.1

and

2000-8000

MER

2-way

?

?

UHF antenna

?

128 000

UHF

relay

2-way

?

?

High or

?

>11000

DTE

low-gain

(HGA)

antennas

Surveyor 1-7

2-way

2295 and

10 or

Planar array

?

?

DTE

2113

0.1

HGA or two

conical omni-

directional

antennas

Lunokhod 1,2

2-way

922.76

?

?

?

?

DTE

Luna 16,20,24

2-way

922.764

?

?

?

?

DTE

Table 10.5. (Cont.)

Tx

Link

Frequency

power

Antenna

Craft

type

(MHz)

(W)

type

Modulation

Bit rate (bps)

Venera 9-14

1-way

122.8;

Quadrifilar

PSK

256 to

landers

relay

138.62 x 30

helical

3072

VeGa lander

1-way Relay

122.8; 138.62 x 30

Quadrifilar helical

PSK

3072

Viking lander

2-way DTE

2294

HGA

?

500

2-way

381

30

8-element

?

4000 and

UHF

crossed

16 000

relay

dipole

MPL/Phoenix

2-way DTE

X-band

?

Parabolic dish

?

2100-12600

2-way

UHF

?

Spiral UHF

?

128 000

UHF

antenna

relay

Mars 96

Relay

UHF

5

Spiral UHF

?

8000

penetrators

antenna

DS-2 Mars

1-way

UHF

?

Whip antenna

?

7000

microprobes

relay

'whiskers'

Lunar-A

2-way

400

?

?

?

256-1024

penetrators

relay

Phobos DAS

2-way DTE

1672

?

Spiral

?

4-20

Phobos

Relay

?

?

?

?

?

PROP-F

Philae

2-way relay

2208; 2033

1

Patch

?

16,000

MINERVA

2-way relay

?

?

?

?

9600

1 In the event, data from only one of the two channels was received correctly on board Cassini.

1 In the event, data from only one of the two channels was received correctly on board Cassini.

horizontal miss distance. Simultaneous measurements in DTE and orbiter relay link would provide two components of the probe velocity. A third component (vertical) can be measured independently using either altimetry or pressure/ temperature measurements. One-way Doppler measurements were the main source of data on planetary winds in all Venera, Mars 6 and Huygens probes.

An entirely different approach relies on the combination of data from several telescopes. The different phase of the signals as received on Earth allows the direction to the transmitter to be very precisely established. This VLBI technique was applied to the VeGa balloons, and also the Huygens probe.

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