Combined GMSK Modulation and PN Ranging for Communications and Navigation

Renewed interest in human exploration of space suggests a future need for high throughput earth-space links. Anticipating crowding within spectrum used for deep- space communications, particularly at X-band, NASA is investigating the use of bandwidth-efficient modulation (BEM). Requirements for trajectory/orbit determination and vehicle guidance will persist, however, implying that signals for radiometric tracking must coexist with communications within the available spectrum. Because navigation signals in current use may be insufficiently compatible with BEM, the question arises as to how to combine ranging and BEM communications. This paper presents one waveform approach potentially applicable to the space-to-earth return link problem. GMSK modulation is combined with a pseudonoise (PN) ranging signal on a subcarrier. Waveform parameters are adjusted to achieve: (1) a constant-envelope waveform; (2) adequate separation between the communications and ranging components; (3) high data rates (~ 6 Msps); and (4) conformance to international standards such as Space Frequency Coordination Group (SFCG). We define the waveform, present its power spectrum, exhibit receiver structures for simultaneous GMSK data demodulation and PN tracking for accurate time-of-arrival estimation, and show relationships among the waveform parameters that must hold to satisfy the demands of applicable standards.