2.2.2. GNSS-R Instrument Dependencies

The GNSS-R observation distributions are highly dependent on multiple instrument configuration parameters. The number of viable science observations generated by a given instrument on a single satellite is constrained by two primary instrument capabilities: (a) the processing bandwidth of the instrument and the resulting number of available surface specular points it is capable of processing into science observations and (b) the surface area coverage of the nadir oriented GNSS-R science antennas which are required to capture the reflected signals with sufficient SNR to be usable. Other secondary spacecraft specific system requirements such as satellite down-link capability, attitude knowledge and other aspects are not analyzed here and assumed to be sufficient for the scenarios studied in this analysis.

In this analysis, the processing bandwidth of the GNSS-R instrument considers two cases: the existing on-orbit performance of the CYGNSS GNSS-R instrument which is capable of tracking 4 parallel reflections from GPS only [4], and a next Generation GNSS-R instrument (NGRx) which has shown in initial testing to be capable of tracking up to 16 parallel GNSS reflections from both GPS and Galileo [13]. It is possible that at some point in the future instruments will be capable of tracking more parallel reflections. However, we believe that 16 is a realistic estimate of the state-of-the-art GNSS-R instrument capability and grounds this analysis in existing hardware performance.

The surface coverage of the GNSS-R instrument antennas is also a key element in filtering the actual observations suitable for science applications. As expected, reflections captured in the main high gain lobes of the reflection antennas result in higher SNR observations and generally better retrievals. In this regard, we have extensive data based on the achieved performance of the CYGNSS constellation where it has been observed that ocean wind speed observations can be achieved within the mission error requirements at range corrected gain (RCG) levels of 15 and above [17]. This allows us to assess additional instrument antenna configurations with respect to this threshold and make realistic judgments as to what specular points will result in viable science observations and which specular points to omit due to low quality. In the subsequent analysis, we simulate both the actual CYGNSS antennas as well as theoretical enhanced antennas to assess the performance of different antenna gains and beam widths, as well as at alternative spacecraft altitudes to provide a more complete trade-space with respect to various spacecraft instrument configurations.
