BACKGROUND

Telecommunications connectivity via the Internet, cellular data networks and other systems is available in many parts of the world. However, there are many locations where such connectivity is unavailable, unreliable or subject to outages from natural disasters. Some systems are able to provide network access to remote locations or to locations with limited networking infrastructure via satellites or other high altitude platforms (HAPs) that are located in the stratosphere. HAPs may communicate with each other and with ground-based networking equipment and mobile devices to provide telecommunications connectivity, for instance according to the Long-Term Evolution (LTE) standard.

A HAP may include a communication platform with multiple sectors that provide Internet service to different areas on the ground via respective beams. The narrower the beam that is generated, the better the gain of the antenna and the better the link budget. A desired coverage area may include a number of beams, where each beam corresponds to a different sector from a client device perspective.

BRIEF SUMMARY

As a given HAP provides service to a selected location, it may need to maneuver or take other corrective action to remain “on station”. Depending on the type of HAP and its corrective action, the beams it emits may move (e.g., rotate) relative to the ground at a rapid rate. This movement can require repeated handovers or other adjustments for the sectors of the communication platform, which can increase processing requirements, create communication delays and cause battery drain for the HAP and client devices. Some handover problems may be addressed via digital beam forming (DBF). However, DBF can be expensive in terms of power and equipment weight because a large number of antenna elements may be required, in which more elements may be necessary at higher frequencies. Further, if parabolic reflector antennas are used, the scan angle would be limited to a few degrees, thus making it difficult or impossible to track the HAP's movement.

During station keeping, the sectors of the communication platform can rotate at a rapid rate relative to the ground, for instance as the HAP moves in a circular or other pattern in the sky. In one example, a user equipment (UE) or other client device may encounter a new beam every 10-20 seconds. This can cause the UE to perform repeated re-selections while in idle mode, or multiple handovers while in a connected mode. This can significantly degrade battery life of the UE, and adversely impact throughput of the telecommunication system.

In view of this, according to aspects of the technology, specialized antenna structures are employed on HAPs that station keep over a region of interest. These structures avoid the constraints of digital beam forming, and provide a robust architecture for mobile HAPs, such as those that operate in the stratosphere.

In one aspect, an antenna system for a high-altitude platform (HAP) is provided to move in a pattern in the stratosphere to provide communication services to a region of interest, the antenna system configured to compensate for movement of the HAP in the stratosphere relative to the region of interest. The antenna system comprises a controller, an antenna array switching circuit configured to receive communication signals from the controller associated with a plurality of beams, and a plurality of antennas operatively coupled to the antenna array switching circuit, each of the antennas being configured to transmit beams that provide communication services to a selected portion of the region of interest. The controller is configured to determine that the HAP has moved in the stratosphere relative to the region of interest, determine, based on the relative movement, that a first one of the plurality of antennas is no longer able to cover the selected portion of the region of interest, and cause the antenna array switching circuit to adjust the beams transmitted by the antennas by switching to a second one of the plurality of antennas to cover the selected portion of the region of interest, such that the communication services provided to the region of interest are not interrupted due to the movement of the HAP.

The plurality of antennas may form either a phased antenna array or a non-phased array. The plurality of antennas may be cylindrical antennas. Each of the cylindrical antennas includes a two-element fed reflector having two feeds placed along an axis of a semi-cylindrical reflector, each of the feeds having a phase that is variable so as to scan in azimuth or elevation. A center beam and any exterior beams surrounding the center beam may be configured to cover the region of interest having a diameter of at least 40 km. The beams include at least a set of exterior beams disposed around a center beam. The set of exterior beams may comprise at least 6 beams. The antenna array switching circuit may comprise a field-programmable gate array (FPGA) configured to route feed signals to the plurality of antennas. The plurality of antennas may be torus antennas. Feeds to the antennas may be switched to compensate for the movement of the HAP. Each of the feeds to the antennas may be selected based on at least one of elevation or azimuth. A HAP may be configured to comprise the antenna system and operate in the a propulsion system operatively coupled to the antenna system. The propulsion system may be configured to adjust a position of the HAP in the stratosphere relative to the region of interest.

In another aspect, a method of controlling a HAP to move in a pattern in the stratosphere to provide coverage to a region of interest and compensate for movement of the HAP relative to the region of interest The method comprises monitoring, by a controller, movement of the HAP in the stratosphere, determining, by the controller, that the HAP has moved in the stratosphere relative to the region of interest, determining, by the controller based on the relative movement, that a first one of a plurality of antennas in the HAP is no longer able to cover a selected portion of the region of interest, and cause, by the controller, an antenna array switching circuit in the HAP to adjust beams transmitted by the antennas by switching to a second one of the plurality of antennas to cover the selected portion of the region of interest, such that the communication services provided to the region of interest are not interrupted due to the movement of the HAP.

