INFLUENCE OF THE DOPPLER EFFECT IN COMMUNICATION LINK BETWEEN ESMO AND ESEO SATELLITES AND USAGE OF THE DOPPLER RADAR

IN ESMO MISSION

Bartosz Idźkowski

Abstrakt: This paper shows the technical issues relating to the simulations of the communication microwave link between the ESMO (European Student Moon Orbiter) or the ESEO (European Student Earth Orbiter) minispacecraft to Earth. The paper presents the application of the Doppler pulse radar onboard the ESMO for the precise navigation above the Moon surface.

1. Introduction The aim of this paper is to consider the use of a satellite radio link ESMO - Ground Station (GS) and

ESMO-ESEO-GS and also the advantages of using the Doppler radar working in X band onboard the Moon orbiter.

The ESMO project is one of the international SSETI projects (Student Space Exploration and Technology Initiative) supervised by ESA (European Space Agency). A number of channels and channel bandwidth (in the X band) have been derived with a turn around ratio 765/900 [1] and by specifying the channel bandwidth:

For the uplink (telecommand link) and downlink (telemetry link) the following bandwidths have been assumed [1] :

50 kHz (BPSK/QPSK modulated signal with 8 kHz subcarrier), 100 kHz (modulated signal with 16 kHz subcarrier).

When the space between the channels is equal to the channel bandwidth, the partitioning of the frequency subrange into channels is as shown in Fig. 1 and Fig. 2, for uplink and downlink respectively.

Fig.1.Adopted channel allocation for the uplink in the X band for the ESMO, where the space between the channels is equal to the

single channel bandwidth (without the consideration of the Doppler)

Fig.2. Adopted channel allocation for the downlink in the X band for the ESMO, where the space between the channels is equal to

the single channel bandwidth (without the consideration of the Doppler)

The safeguards between the channels can be narrower, when more information is needed to be sent but

this creates the hazard of channel interference and the strict control of instant channel bandwidth must be ensured. The other primary effect narrowing the safeguards is the unavoidable frequency shift caused by the

.71919007658460

900765

,8460

MHzff

MHzf

downup

down

=⋅=⋅=

=

Doppler effect. In this case when more information is required to be sent, compensation in the transmitter should be considered. This kind of solution depends on the level of the Doppler shift, how much information is being sent and the number of channels in the bandwidth.

For the ESEO mission the number of channels and the channel bandwidth (in the S band) has been derived with a turn around ratio 221/240 and by specifying the channel bandwidth:

MHzff

MHzf

downup

down

25,20442402212210

240221

,2210

=⋅=⋅=

=

For the uplink (telecommand link) and downlink (telemetry link) the following bandwidths have been assumed: 72 kHz (modulated signal PSK/PM with 16 kHz subcarrier), 192 kHz (fast downlink modulated signal BPSK with 16 kHz), 168 kHz (slow downlink modulated signal PSK/PM with 38,4 kHz).

The allocation of the channels in a slow downlink (9600 b/s) and a fast uplink and downlink (128 kb/s) is shown in Fig.3 , Fig.4 and Fig.5.

Fig.3. Adopted channel allocation for the uplink in the S band for the ESEO, where the space between the channels is equal to the

single channel bandwidth (without the consideration of the Doppler)

Fig.4. Adopted channel allocation for the fast downlink in the S band for the ESEO, where the space between the channels is

equal to the single channel bandwidth (without the consideration of the Doppler)

Fig.5. Adopted channel allocation for the slow downlink in the S band for the ESEO, where the space between the channels is equal to the single channel bandwidth (without the consideration of the Doppler)

2. Doppler effect in communication between ESMO and ESEO

There are a few reliable approaches considered for maintaining the ESMO radio link to Earth. The most sophisticated scenario is when the communication between the Moon orbiter is maintained via the Earth orbiting satellite (Fig.6). The communication links between the satellites can operate in several different bandwidths.

EARTH

MOON

ESMO

ESEO

GS uplink/downlink uplink/downlink

Fig.6. The most advanced set up for the ESMO-ESEO-GS communication link in the case when no dirrect link from ESMO to GS is available.

This method of communication will be used only if there is no possibility of contacting the GS directly (for instance when the GS is on a non-visible side of Earth according to ESMO). Extending this approach, we can make ESEO comunicate with other satellites until it is possible to contact the GS. A generic scheme of the communication link between several satellites is shown in Fig.7.

Fig.7. Generic scheme of communication link between k satelittes.

fo , fj – carrier frequences, Si , Sj – satelites number i, j, kj - satelite mikser frequency \ carrier frequency. In the communication link between the satellites ESEO and ESMO according to relevant orbit parameters the magnitude of the Doppler shift has been computed and illustrated in Fig.8. This kind of plot is influenced by several causes such as the relevant velocity of the satellites, different orbits etc.

-30

-20

-10

0

10

20

30

40

50

60

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Time of visibility in minutes

Dop

pler

freq

uenc

y [k

Hz]

Fig.8. Doppler frequency correction in the communication link between ESMO and ESEO.

We can see two stages of the Doppler shift’s influence on the CW. In the first stage (first minutes of the flight), the Doppler frequency changes rapidly and in an unpredictable manner. When ESMO reaches the final orbit, variations of the Doppler frequency become more predictable and regular, making the tracing of the frequency shift easier to accomplish (including advance corrections). 3. The onboard Doppler radar

One of the technical issues of the Doppler radar is that it can measure the distance from the Moon

surface to the orbiter and the velocity of the orbiter. This is an important navigational and scientific instrument. This solution will allow a quick correction of the orbiter’s trajectory from the GS after measuring the alttitude and proper telemetry messaging from ESMO to the GS. The maximum radar range is:

f0 f0 syntezis modulation

Si-Sj channel

mixer

Sjkj

fj

Si

( ) ( )LnDkT

AGtPR

xs

rtftm 2

23

02

4πθσ

= , (*)

The maximum emitted power from ESMO (Pt) is in the range of 5W and the antenna gain Gt (12 dBi). The maximum range of the radar, calculated with (*) is mR =2823 km. The frequency tracing in the Doppler radar can be used, which will allow the calculation of the center frequency of the signal influenced by the Doppler.

Frequency tracing systems give information on three frequencies, which are interpreted as velocity vectors that give information about the combined velocity. These vectors are being converted to the pulses with a proper length and further to the binary representation. If the perturbation of the satelite orbit will occurs the number of impulses will be different compared to the case when no perturbation happens. This will allow the detection of flight track discrepancies and the correction of the satellite’s trajectory (Fig.9).

Fig.9. ESMO perturbated orbit correction to the operational orbit using the Doppler radar.

The relative velocity in different time spots can be calculated from the equation:

(**)

Di,j – location of the satellite in time spots tj and ti.

4. Summary This paper shows the value of the communication link between several satellites especially with ESEO

and ESMO. This kind of approach will enable continous contact with the Moon orbiter via satellites constellations using different bandwidths. Also, the onboard Doppler radar provides achievements such as range and velocity measurements, which allows for the control of the flight trajectory more securely (not only using telemetry messages to the GS, but directly to the onboard computer).

5. References [1] EUROPEAN COOPERATION FOR SPACE STANDARIZATION, Radio frequency and modulation, ESA-ESTEC 2003

[2] CONSULTATIVE COMMITTEE FOR SPACE DATA SYSTEMS, Radio frequency and modulation systems. Part 1:

Earth station and spacecraft, CCSDS 2003

[3] Skolnik Merrill , RADAR HANDBOOK, John Wiely, second edition 1990 [4] Idzkowski Bartosz and Kalarus Adam “ Kompensacja efektu Dopplera na orbitach okołoziemskich i okołoksiężycowych” M.Sc Thesis , Wrocław University of Technology, Wroclaw 2005

α'

Moon

α

Correct orbit

X1

X2

D1

D2 X3

D3 Perturbated

Orbit

,)(

ij

xij

r ttDD

v−

−=

α