Antennas

Receivers
Title	Antennas
Author(s)	GMV
Level	Advanced
Year of Publication	2011

From a user prespective, antennas are the main interface between the GNSS Space Segment (the satellite constellations) and the User Segment (GNSS receivers), as they are responsible for capturing the L-band signals transmitted from space. Antennas requirements can range from gain vs. azimuth and elevation, mutipath and interference rejection, or stability of the electrical phase center, to size, shape and environmental constraints. Due to the characteristics of GNSS systems, receiver antennas are typicaly Right-Hand Circularly Polarized (RHCP) antennas, and the spacial reception pattern is near hemispherical: this configuration enables user reception of satellite signals in any azimuthal direction, and from zenith to horizon, thus maximizing satellite visibility.

Antenna types

Different applications may require different types of antenna design, technology and location. In fact, as show in Figure 1, antennas can range from large roof-mounted antennas (e.g. for a static open sky view) to embedded antennas in receiver boards (e.g. for mobile handheld devices).

The most common types in terms of implementation are:

Figure 1: Examples of GNSS antenna types: rooftop Dome (left), embedded Patch (top-right), Choke Ring design (bottom-right).

Figure 2: View of the antennas at Galileo Control Centre in Fucino, Italy (Contents ESA).

• Patch Antennas: Patch antennas are the most common antenna type, as they are used in handheld mobile devices such as portable navigation devices and smartphones. The antenna is buit as a low-profile thin microstrip (i.e. patch), suitable for small form-factor devices.

• Dome Antennas: As the name suggests, dome antennas are housed in a hemispherical-like dome, and are seen in several roof-mounted installations.

• Helical Antennas: Helical antennas are typicaly built as a conducting wire around a cilindrical core, in a helix form, down to a ground plane. In GNSS receivers, the helix antennas can be used as directional antennas, with a radiation pattern along the axis of the antenna.

• Choke Ring Antennas: A choke ring antenna design consists in sorrounding a central antenna with several concentric conductive rings, enclosed in a protective dome. Its design is notable for the ability to reject multipath and low elevation signals (including reflections on the ground).

• Phased-Array Antennas: Phased-array antennas are mostly used for military or high-end applications, and consist of numerous antenna elements arranged in an array pattern. Each signal from an antenna in the array can be processed by introducing dynamic phase shifts, and when the outputs of the array are summed, the effective radiation pattern can be maximized in some directions and nulled in other (e.g. to insert nulls in the antenna pattern to adjust to jamming threats).

• Parabolic Antennas: Resembling popular TV and communications satellite dish antennas, parabolic antennas use a parabolic reflecting surface to direct the radio waves. The parabolic antenna is highly directive, so RF signals comming from the satellites can be received from one particular direction only. Its directive build structure and high gain make this type of antenna suitable for Ground Segment stations, such as monitoring or uplink stations, as shown in Figure 2.

An alterantive distinction of GNSS antennas, in terms of application, is:

• Geodetic Antennas: Survey and geodetic applications typicaly require fixed high-accuracy receivers and antennas, so it is common for multi-frequency / multi-constelaltion choke-ring antennas to be used for best performance.

• Rover Antennas: Rover antennas are used in land survey, forestry, construction, and other portable or mobile applications, and represent a good trade-off between accuracy and portability. Due to the need for mobility, the antenna is usually mounted on a handheld pole, stand or tripod: nevertheless, due to size and weight constraints, choke ring antennas are not a good solution for rover applications, so a high-end alternative must be found to ensure the necessary accuracy.

• Handheld Receiver Antennas: These antennas are found in most current smatphones and portable navigation devices, and are chosen for size, cost and power consumption constraints. Usualy single-frequency antennas, they are available in a range of implementations such as helical or patch antennas, both passive and active (see section below for details).

Characteristics

Antennas used in GNSS receivers are targeted at capturing L-band signals originating from satellite constellations, so care must be taken when matching design specifications in terms of signal bandwidth, center frequency, desired response, transfer function, multipath mitigation, and electromechanical factors.

Radiation Pattern

Figure 3: Example of a patch antenna pattern.

GNSS signals use Right Hand Circular Polarization (RHCP), but antennas are not perfect, so a RHCP antenna will pick up some Left Hand Circular Polarization (LHCP) energy as well. In radiation terms, RHCP is referred to as the co-polar component, and LHCP is referred as the cross-polar component. The typical radiation pattern coverage is around 160º, resulting in low gain for signals comming in at low elevation angles, as shown in Figure 3 for a patch antenna. This pattern aims at maximizing the gain at zenith, decreasing to near unity at an elevation angle of around 10º-15º, and to negative gains below the elevation threshold (sometimes refered to as elevation mask). Real-world GNSS antennas experience a gain roll-off of 10 to 20 dB from broadside (90º elevation) to the horizon (0º elevation). At zenith, the ratio of the vertical electric field to the horizontal electric field response is near unity, and this ratio is referred to as the axial ratio (AR), defined as:

[math]\displaystyle{ AR(\theta) = 20.log \left ( \frac{E_{RHCP}(\theta) + E_{LHCP}(\theta)}{E_{RHCP}(\theta) - E_{LHCP}(\theta)} \right ) }[/math]

AR increases toward the lower elevations: AR is less than 3 to 6 dB at 10° elevation for a high-performance antenna.

Multipath

Signals sent from GNSS satellites will directly arrive at the antenna but they also may be reflected off the ground, buildings or other obstacles, arriving at the antenna multiple times and delayed in time. This phenomenon is called multipath, and since it degrades position accuracy, it should be avoided or reduced. Multipath signals can basically come from three directions: the ground, hitting the back of the antenna, the ground or an object, hitting the antenna at a low elevation, or an object, hitting the antenna at a high elevation. In specular reflections, the reflected signal changes its polarization with respect to the original one: in other words, a RHCP signal will be reflected as a LHCP signal. The multipath susceptibility of an antenna can then be quantified, with respect to the antenna’s radiation pattern characteristics, by the multipath ratio (MPR). The multipath ratio is computed as the ratio between the RHCP gain at the satellite signal impinging angle [math]\displaystyle{ \theta }[/math] with respect to the sum of the RHCP and LHCP gains at the multipath signal impinging angle (180º - [math]\displaystyle{ \theta }[/math] for ground reflections; [math]\displaystyle{ \theta }[/math] for vertical object reflections):

[math]\displaystyle{ MPR_{ground} = \frac{E_{RHCP}(\theta)}{E_{RHCP}(180\deg - \theta) + E_{LHCP}(180\deg - \theta)} }[/math]

[math]\displaystyle{ MPR_{vertical} = \frac{E_{RHCP}(\theta)}{E_{RHCP}(\theta) + E_{LHCP}(\theta)} }[/math]

Gain roll-off towards the lower elevations helps to suppress multipath signals: however, a good AR is always a must, since gain roll-off alone is insufficient.

Phase Centre

Another important design factor for antennas is the phase centre. In fact, when a receiver determines a position solution, it is actualy estimating the position of the electrical phase centre]] of the antenna, where the signals are captured, regardless of the distance from the antenna the receiver device actualy is. This electrical phase centre should not be mistaken with the physical phase centre: although the latter can be observed and is a physical constant, the first may vary, depending on the direction of arrival of the received signal. See the articles on antenna phase centre and receiver antenna phase centre for more details.

Antenna Power

In general, GNSS antennas don't need any electrical power to operate, and are refered to as passive antennas. Typical scenarios for passive antennas are mobile devices, where power consumption is a critical issue. However, passive antennas must connect to the receiver's front end with very low losses, since there is no amplification mechanism involved at signal reception. In applications in which the antenna can be away from the receiver (e.g. rooftop antennas), signal amplification can be implemented in the antenna itself, and for that fact they are called active antennas. Their purpose is to amplify the received signal to compensate for potential losses in long coaxial cables, at the cost of a higher power consumption (power is usually delivered from the front end through the coaxial cable itself). In this approach, a Low-Noise Amplifier (LNA) is usualy added to the antenna setup, in order to ensure a low noise figure in the signals to process. An example of active antenna use is for indoor reradiating purposes, where an active antenna inside a building reradiates the signals comming from another antenna, placed outside the building.

Signal Bandwidth

Besides electrical considerations, GNSS antennas must also take into account the signal structure in terms of RF spectrum and bandwidth, and should be selected not only in accordance with the front end specifications, but also with the envisaged usage and application. New constellations and modulations brind different spectrum allocations and bandwiths (see each constellation's articles for specifications on spectrum allocation and bandwidth). As an example, Table 1 ilustrates GPS L1 and Galileo E1 bands (which share the same center frequency), and shows several parameters related to the bandwidths of incoming signals.

Table 1: GPS and Galileo L1/E1 civil signal bandwidth characteristics.

	GPS L1 C/A (BPSK)	Galileo E1 B/C (BOC)	Galileo E1 B/C (CBOC)
Chip Rate [Mcps]	1.023	1.023	1.023 / 6.138
Primary Code Length [chips]	1023	4092	4092
Primary Code length [ms]	1	4	4
Symbol Rate [sps]	50	250	250
Centre Frequency [MHz]	1575.42
Receiver Reference Bandwidth [MHz]	4.092	8.184	24.552

Typically, the bandwidth of a GPS patch or helix antenna ranges from 1% to 2% of the center frequency. Two percent bandwidths for L1, L2, and L5 center frequencies are 31.5 MHz, 24.6 MHz, and 23.5 MHz, respectively. GPS receivers that track P(Y) code on both L1 and L2 need to accommodate on the order of 20.46-MHz bandwidths on both frequencies. If the set only tracks C/A code or L1C on L1, the antenna (and receiver) need to accommodate bandwidths of approximately 2.046 and 4.092 MHz, respectively. It should be noted that the receiver’s antenna/front-end bandwidth is directly proportional to the accuracy required for the specific application of the receiver. That is, the more frequency content of the received satellite signal that is processed, the better the accuracy performance will be. For example, a survey receiver antenna/front end will most likely be designed to pass the full 20.46 MHz of the P(Y) code. Whereas, a low-cost hiking receiver designed for C/A code may only have a front-end bandwidth of 1.7 MHz instead of the full 2.046 MHz. (Further elaboration on bandwidth and accuracy performance is contained in Chapter 5.) For example, those utilizing L1 C/A code and L2C will need a dual-band antenna. (Dual frequency measurements enable determination of the ionospheric delay and provide robustness to interference. Ionospheric delay determination and compensation are discussed in Chapter 7.) SOL signal users that require operation in the ARNS bands will need antennas to receive C/A code on L1 and the L5 signal on L5.

Front end connection

Table 2: Coaxial cables...

Related articles

• Generic Receiver Description
• Receiver Types
• System Design Details