The ROSA Receiver

The ASI is very active in the field of GPS Radio Occultation (hereafter GPS RO) since several years. The milestones of such developments were the manufacturing of a LAGRANGE GPS receiver devoted to LEO-POD and navigation applications flown onboard the Argentinian SAC-C mission, the development and the manufacturing of the proto flight model receiver ROSA and, finally, the accommodation of ROSA on OCEANSAT-2. The ROSA development is currently carried out by THALES-Alenia. In Fig. 1 the receiver with the antennas is sketched.

Thus GPS signals are acquired through multiple antennas: A zenith antenna is used for navigation purpose and for Precise Orbit Determination (POD), two sounding antennas, pointing toward velocity and anti-velocity satellite vectors are used for Earth limb rising-setting occultation observations for scientific purposes. The

Fig. 1 The receiver is shown on the left hand side. The antenna devoted to radio occultation to be placed on the spacecraft facing the velocity and the anti-velocity directions is sketched right top, while the figure right bottom shows the hemispheric antenna devoted to navigation

Navigation module is able to track up to 8 satellites on both L1 and L2 frequencies while the radio occultation module is able to track simultaneously up to 3 occulting satellites. Anyway, the main specifications of the instrument are summarized in Table 1. The performance of the instrument shall guarantee the capability to retrieve atmospheric profiles with a vertical resolution of «0.3 km in the lower troposphere. One only of the two RO antennas can be installed on OCEANSAT-2. It will point along the velocity direction. Thus the receiver will catch only RO of rising GPS satellites (the angular velocity of LEO (Low Earth Orbit) satellites indeed is faster than GPS satellites) and the number of RO events is halved as well. In Fig. 2 the global distribution of the RO events is shown.

Table 1 Requirements and specifications, which ROSA is supposed to fulfill

Requirement

Impact on the instrument configuration

To perform accurate positioning

Hemispherical coverage antenna: ±75° around zenith

To maximize the number of

Both GPS and GLONASS tracking, rising and setting.

occultations per satellite

Antennae for occultation: ±45° fore and aft.

16 dual-frequency channels, arbitrary allocations to

GNSS satellites.

Sounding penetration: down to

Antenna with high gain and low tracking threshold.

surface

High sampling rate and open loop mode for sounding the

lower troposphere.

High performance codeless tracking scheme in the

receiver signal processor.

To limit resources

Weight < 25 kg

Volume < 20 L + antennae

Power < 60 W

Fig. 2 The global distribution of RO events with an antenna pointing in the velocity direction

The main problem to face are the disturbances induced by the other instruments on the RO observations. Such disturbances can have an impact on the measurements of the phase and amplitudes of the GPS signal caught by the antenna. In Fig. 3 the shape of the satellite and the position of the RO antenna are depicted. A severe disturbance is the multipath mainly due to the scatterometer antenna and the solar panels.

Thus a simulation has been performed just to understand the impact of multipath on the phase and amplitude measurements of the GPS signal. The simulation has been performed by rotating in turn the panel and the scatterometer antenna fixing

Fig. 3 The arrangement of the antenna on the spacecraft and the other payloads

-0.3 -0.2 -0.1 0 0.1 0.2 0.3 -0.3 -0.2 -0.1 0 0.1 0.2 0.3

Fig. 4 The figures are the multipath diagrams of two different configurations of the GPS antenna. In (a) the antenna is tilted 15° in azimuth (i.e., the angle between the velocity vector and the antenna pointing direction) but not in (b). The horizontal axis is the azimuth position of the transmitted source (GPS); while the vertical is its elevation. In both selected cases the attitudes of the scatterometer antenna and the solar panel are the same

-0.3 -0.2 -0.1 0 0.1 0.2 0.3 -0.3 -0.2 -0.1 0 0.1 0.2 0.3

Fig. 4 The figures are the multipath diagrams of two different configurations of the GPS antenna. In (a) the antenna is tilted 15° in azimuth (i.e., the angle between the velocity vector and the antenna pointing direction) but not in (b). The horizontal axis is the azimuth position of the transmitted source (GPS); while the vertical is its elevation. In both selected cases the attitudes of the scatterometer antenna and the solar panel are the same them in 4 positions (16 different configurations) both with GPS antenna placed as it is shown in Fig. 3 or tilted 15° (32 diagrams). In Fig. 4 some diagrams are shown presenting a selection of the most favorable positions of the GPS antenna.

From Fig. 4 the azimuth tilting of about 15° of the antenna minimizes in a no-negligible way the multipath effects of the satellite, which induces an uncertainty of up to 12°. The drawback of the proposed solution is the reduction of the number of occultations that will be observed, but only few tens of events are missed. Therefore we deem as the best solution the antenna position tilted 15° in azimuth with respect to the velocity direction.

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