Entry probes communication basics

The frequencies used for space communication lie in bands coordinated by the ITU (the International Telecommunication Union, a branch of the United Nations). The designation of the wavebands generally derives from radar development in World War 2 in the UK and Germany (Table 10.1).

Frequencies below about 100 MHz cannot be used for space communication as the Earth's ionosphere absorbs or reflects the radiation. Caution must similarly be exercised in choosing frequencies for planetary missions. For example, although Mars' ionosphere has a lower density and thus allows lower frequencies through, frequencies of 50 MHz and lower may be unusable depending on solar activity and time of day. At Jupiter, the synchrotron radiation from its Van Allen belts provides a significant background radiation, while ammonia contributes absorption in the atmosphere at specific wavelengths.

The efficiency of a data transmission link is defined by the RF energy required to transmit one bit of information at a certain probability of error. On a probe, it is controlled by the RF power of the transmitter, antenna gain in the direction of the

Figure 10.2. One-way DTE communication system.

Ground station

Ground station

Figure 10.2. One-way DTE communication system.

receiving station (either on Earth or on the orbiter relay) and the telemetry coding scheme.

The accuracy of trajectory measurements, primarily Doppler velocity and VLBI (Very-Long Baseline Interferometry) position measurements, is controlled by the frequency stability of the signal radiated from the probe. To the first order the Doppler shift measurements fD determine the line-of-site velocity VD as

fo fo where c is the velocity of light, f0 the frequency of the signal radiated from the probe and fr the frequency of the signal received on the other end of the link (Earth or relay orbiter). Unknown bias, drift or short-term instability of the probe's signal frequency -f will result in a corresponding error in the velocity -VD = c-f /fo. For example, a relative frequency instability of 10" 9 will produce a velocity error 0.3 m s"x.

Any probe communication system consists of a set of basic elements: a transmitter that includes an exciter and power amplifier, an oscillator, a receiver, a coder, an antenna. The one-way DTE link consists of an oscillator, a transmitter and an antenna (Figure 10.2). An ultra-stable oscillator (USO) generates the reference signal for the transmitter, data from the probe transmits directly to Earth. The accuracy of Doppler velocity measurements is governed by the bias and stability of the USO (Table 10.2).

Accurate velocity measurements require the use of USOs operating during high deceleration loads (up to 500 g for Venus entry), and rapid change of the temperature inside the probes. Earlier Venera probes had oscillators with temperature instability up to 10"6, short-term instability ~10"10, and required extensive test calibrations to extract the Doppler data, while the more advanced rubidium USO on Huygens had a stability of ~10"12 (averaged over 100 s) (Bird et al., 1997, 2002).

The main parameters of a transmitter are its output RF power and its efficiency, defined as the ratio of the RF power to the DC power consumption of the transmitter.

Table 10.2. Radial velocity error due to oscillator-frequency instability

Relative frequency

10~6

1O-7

1O-8

10-9

io-10

1O-11

instability Sf/fO

Radial velocity error, m s-1

300

BO

B

0.3

0.03

O.OOB

Figure 10.3. Two-way DTE communication system.

Addition of a command receiver will build the simplest two-way link that will allow control of the probe by commands from Earth and the upload of data for software modification. Improvements in trajectory measurements would require the use of a more complicated and expensive transponder (Figure 10.3). A transponder is the receiver-exciter that locks its frequency to the frequency of the received signal and multiplies it to form the transmitter frequency. Noise temperature and capabilities of signal tracking (bandwidth of acquisition and lock of signal, tracked rate of change of signal frequency) are the main parameters of the transponder.

The frequency stability in the two-way link is determined mostly by the stability of the reference oscillator on the ground station and propagation effects. A ground-reference oscillator can be much more stable than the probe oscillator. The transponder may also be used for range measurements that can improve knowledge of a planet's ephemeredes and rotation state (i.e. using the lander as a tracking beacon on the planet), as well as the location of the probe.

The antenna is the element that defines the spatial distribution of transmitted or received power; it can be the bulkiest element of the probe telecom system. Very often the antenna defines the capability of the link.

The beamwidth of the antenna should enclose the solid angle that encompasses the direction to the receiving station in the probe-fixed co-ordinate system. During entry and descent the attitude of the probe with respect to the receiving station is subject to rapid and large variations in all directions. Omnidirectional or low-gain antennas are used commonly for communication at these phases.

After landing, the direction to the receiving station in the probe-fixed co-ordinate system is still a priori unknown but it changes slowly (at least in the case of landers or LTA probes). With appropriate pointing it enables the use of high-gain antennas that provide significantly higher transmission data rates.

Antenna size controls other parameters. The beamwidth of the directed antenna depends on the antenna size and wavelength. For high-gain parabolic or flat array antennas the beamwidth measured at the —3 dB level can be estimated as

where , is the wavelength and D the diameter of the antenna.

Antenna gain is the ratio of power transmitted by the antenna in a given direction to the power transmitted by an ideal isotropic antenna. The maximum gain G is

where S = kA, the effective area of the antenna, A is the geometric area and k is a geometric efficiency factor, ~0.5-0.6. The expression for beamwidth becomes

NB: G is a dimensionless number, not in dB. Examples for high-gain antennas are given in Table 10.3.

Low-gain antennas and antennas for longer wavelengths are more often characterized by their gain. Antenna patterns for many omnidirectional antennas are well known. For low-gain antennas radiating primarily along their axis the previous formula can be used to estimate beamwidth. For short wavelengths and high directivity, parabolic ('dish') antennas are typically used on spacecraft, with the dish usually fed with a horn antenna at a Cassegrain focus (much like an optical telescope), or sometimes for structural reasons in an offset position. The Viking lander had an S-band dish. In general, landers and especially probes and rovers, because of their variable attitude, cannot point high-gain narrow-beam antennas, and so low- or medium-gain antennas are used, and packaging constraints make dishes unattractive even when high gain is required. Flat plate

Table 10.3. Beamwidth and gain for high-gain antennas in the X-band (, = 3.6 cm) and in the Ka-band (, = 0.9 cm)

Antenna diameter (m)

0.2

0.4

0.7

1.0

2.0

0.4

1.0

Wavelength (cm)

3.6

3.6

3.6

3.6

3.6

0.9

0.9

Beamwidth (°)

13

6.5

3.6

2.6

1.3

1.6

0.63

Gain (dB)

22

28

33

36

42

40

48

phased arrays are common - for example the X-band flat-plate array on Pathfinder had an on-axis gain of some 24 dBi.

Low-gain antennas include canted turnstile (crossed dipole), helical, patch and dipole antennas, with various gain, polarization and structural characteristics. Helical antennas are suited for circularly polarized signals (the Huygens probe used quadrifilar helix antennas for its two telemetry links, separated both in frequency - 2040 and 2097 MHz - and in polarization).

The antennas used on the Pioneer Venus probes operating at 2.3 GHz used a crossed dipole fed with a quadrature hybrid. The antenna elements were made of steel, and were covered in a PTFE radome (electrically transparent) to prevent heat and corrosion damage during entry and descent. The antenna gain was about 2 dB out to 60° off-axis, falling to 0 dB at the horizontal (Hanson, 1978). All Venera/VeGa landers and VeGa balloons used helical antennas.

The effective area of an antenna expressed via gain is

It is important to note that with a given gain the effective area is proportional to the square of the wavelength. The lower frequency (and longer wavelength) low-gain antennas have greater effective area than high-frequency (and shorter wavelength) antennas with the same gain and beamwidth. The effective area of a low-gain antenna for several values of gain is shown in Figure 10.4.

The gain of the transmitting antenna is the primary parameter for transmission of signals. For reception, effective area is paramount. Since gain is inversely

ta io

ta io

0.1 0 Wavelength (m)

0.01

0.1 0 Wavelength (m)

Figure 10.4. Effective area of low-gain antennas with gains —10, 0 and 10 dB.

proportional to the square of the wavelength for a given antenna size, a shorter wavelength would yield greater gain and thus a higher data rate for the same size of receiving antenna.

0 0

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