REPORT 685 PROTECTION CRITERIA AND SHARING CONSIDERATIONS RELATING TO DEEP-SPACE RESEARCH (Question 1-1/2) (1978) 1. Introduction This Report discusses the sharing of frequencies in the range 1 to 20 GHz between deep-space research stations and stations of other services. Deep-space earth and space station protection criteria are discussed. Potential interference is considered, and conclusions are drawn about the feasibility of sharing. The 1 to 20 GHz range includes current allocations applicable to deep-space research and some of the frequencies desired for future operational use. Preferred frequencies are given in [CCIR 1974-78a]. United States earth and space station characteristics for deep-space research are given in [CCIR 1974-78b]. 2. Deep-space earth station factors pertinent to sharing The principal deep-space earth station parameters which pertain to interference and sharing are transmitter power, antenna gain and pointing, receiver sensitivity (including noise temperature) and bandwidth. Typical values of these parameters are given in [CCIR 1974-78bJ. This section of the Report considers some aspects of antenna pointing, and develops protection criteria for receivers at deep-space earth stations. The section finishes with remarks about co-ordination. Considerations of transmitter power are given in a later section. 2.1 Intersections of satellite orbits and antenna beams from deep-space earth stations The locations of the US deep-space earth stations are given in [CCIR 1974-78bJ. The stations are spaced approximately equally in longitude (120° apart) with two stations in the northern hemisphere, and the third in the southern hemisphere. Spacecraft in deep space currently remain in or near the plane of the ecliptic, which is tilted at 23.5° from the Earth’s equatorial plane. The daily rotation of the Earth causes the antenna beam of at least one earth station to intersect the equatorial plane and hence the geostationary satellite orbit when tracking a given spacecraft. The earth station may be subjected to interference from satellites within the antenna beam, and may cause interference to those satellites. Satellites that are not geostationary can pass through one or more deep-space tracking beams each day. Details of visibility statistics and in-beam duration times for satellites in low altitude orbits are contained in Report 684. In the future the United States plans to deliver deep-space probes into orbits out of the plane of the ecliptic. These missions will also result in some earth station tracking beams passing through the orbits of both geostationary and non-geostationary satellites. 2.2 Susceptibility of deep-space earth station receivers to interference A deep-space telecommunication system is typically a phase-sensitive system. The earth station receiver utilizes phase-locked loops for carrier tracking and data recovery. CW or noiselike interference in these loops can result either in degradation or loss of tracking and data. Report 544 contains information on the effects of interference in phase-locked loops. Report 545 presents information on the degradation of telemetering performance due to interference. 2.2.1 CW signal interference 2.2.1.1 Receiver capture The changing Doppler shift of a received desired signal can cause the receiver pass band io move past a fixed frequency unwanted CW signal. Depending upon rate of motion and the amplitude of the unwanted signal, the receiver may lock to the interfering signal if R, the ratio of the CW signal power to the desired signal power, satisfies the relation: where d/'is the rate of frequency change in Hz/s and fn is the loop natural frequency in Hz. An interfering CW signal that is 10 dB above a strong desired signal and is moving through the receiver passband at 100 Hz/s would cause the receiver to lock to the interfering signal. At a lower rate of movement, the required interfering signal level is proportionately lower until at a signal-to-interference ratio greater than one, the interfering signal will no longer capture the receiver, even if the movement rate is zero. Undesired CW signals not strong enough to cause receiver capture may cause interference to carrier tracking.
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