Satellites in HEO constellations orbit the Earth in planes that are inclined nominally 63,4° against the equatorial plane.This is necessary in order to keep the apogees in the most northern (southern) positions within their elliptical orbits.Typically HEO orbital periods are between 8 and 24 hours. HEO satellites are normally active only about their apogees where they appear nearly stationary to an earth observer for about eight hours, and then have to hand over to a following satellite.The satellites belonging to one particular system appear in time shift in the same celestial region. In the HEO track is sketched in profile showing at every point the true distance to the Earth's surface. In this specially depicted case, the orbital period is 12 hours and the satellites appear
alternatively at the opposite sides of the rotating globe. Therefore the illustrated HEO track reaches a maximum height at both ends above the geographical latitude of 63,4° North. At both upper ends (solid line), the satellite payloads are active. The dotted line constitutes the part where the satellite payloads are (typically) switched off. For comparison, see figure 2, where two HEO loops are indicated corresponding
to the two ends in profile in figure 1.Under the above conditions, the HEO apogee (maximum height above the Earth's surface) can be up to 42 000 km.However the maximum range to the Earth's surface is in the order of 47 000 km resulting in a maximum propagation delay of the order of 310 ms. HEO satellites reach high relative speeds during their active phase (order of magnitude:2 km/s), so that the Doppler shift (1.3 x 10-5 of radio frequency and bit rate) cannot be neglected: the radio frequency
shift is mainly due to the microwave feeder link and is of the order of 50 kHz for C-band feeder links. The satellite motion is mainly radial relative to the user community, so that common compensation of the Doppler main component is feasible.Irrespective of any user roaming, HEO systems require handover from the descending to the ascending satellite typically every eight hours. Depending on the specific system design, the distance to the two satellites at handover could be significant and a jump in path length cannot be excluded. However, a large Doppler jump will always happen.Within Europe HEO satellites can appear near the zenith. Therefore the user can work under vertical line-of-sight condition for most of the time, with blockage only being experienced in tunnels or under bridges, trees, etc. However vertical propagation is not very good within multi-storey buildings and hence paging, alerting, etc. may not be satisfactory.
Because vertical propagation can be in principle multipath-free, high data rate services are possible for outdoor operation.A number of HEO orbits have been studied extensively and given names such as "Molnya", "Tundra", and "Loopus".
alternatively at the opposite sides of the rotating globe. Therefore the illustrated HEO track reaches a maximum height at both ends above the geographical latitude of 63,4° North. At both upper ends (solid line), the satellite payloads are active. The dotted line constitutes the part where the satellite payloads are (typically) switched off. For comparison, see figure 2, where two HEO loops are indicated corresponding
to the two ends in profile in figure 1.Under the above conditions, the HEO apogee (maximum height above the Earth's surface) can be up to 42 000 km.However the maximum range to the Earth's surface is in the order of 47 000 km resulting in a maximum propagation delay of the order of 310 ms. HEO satellites reach high relative speeds during their active phase (order of magnitude:2 km/s), so that the Doppler shift (1.3 x 10-5 of radio frequency and bit rate) cannot be neglected: the radio frequency
shift is mainly due to the microwave feeder link and is of the order of 50 kHz for C-band feeder links. The satellite motion is mainly radial relative to the user community, so that common compensation of the Doppler main component is feasible.Irrespective of any user roaming, HEO systems require handover from the descending to the ascending satellite typically every eight hours. Depending on the specific system design, the distance to the two satellites at handover could be significant and a jump in path length cannot be excluded. However, a large Doppler jump will always happen.Within Europe HEO satellites can appear near the zenith. Therefore the user can work under vertical line-of-sight condition for most of the time, with blockage only being experienced in tunnels or under bridges, trees, etc. However vertical propagation is not very good within multi-storey buildings and hence paging, alerting, etc. may not be satisfactory.
Because vertical propagation can be in principle multipath-free, high data rate services are possible for outdoor operation.A number of HEO orbits have been studied extensively and given names such as "Molnya", "Tundra", and "Loopus".
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