Over the last two decades, ocean altimetry using signals of opportunity (SoOp) has been demonstrated using transmissions from the Global Navigation Satellite System (GNSS). Recently, techniques first developed for GNSS have been expanded to digital communication signals with the promise that the wider bandwidth and higher power would enable sea surface height (SSH) retrievals at a scientifically useful precision.

Analysis has shown that utilizing a 400 MHz direct broadcast SoOp, a SSH precision of 5.3 cm height precision can be achieved from a typical altimetry orbit of 1380 km and 4.1 cm can be achieved from a 500 km orbit. An error model first developed for GNSS-R was used to make these predictions after being validated by tower-based experiments. In addition to the tracking error model, a mission error budget must incorporate uncertainty in both transmitter and receiver orbits, as well as propagation effects of the ionosphere and troposphere. Transmitter orbit knowledge is the one part of this error budget that is not present in conventional altimetry. By definition, SoOp sources are not under control of the science mission, and orbit determination must be performed on these non-cooperative transmitters independently of the receiver. We will present an assessment of the impact of transmitter orbit knowledge on the altimetry error budget and a review of preliminary approaches for passive orbit determination.

Requirements for a high-gain (>20 dB) antenna and the more complex and variable bistatic scattering geometry impose some antenna design challenges as well. This is particularly true in using constellations of small satellites. We will present results from coverage simulations which can be used to define working antenna requirements.