ETSO-C144#1 Airborne Global Positioning System Antenna
Deviate from §2.2.2.1 “Preamplifier Gain and Noise Figure” of RTCA/DO-228.
Requirement:
RTCA/DO-228 specifies:
2.2.2.1 Preamplifier Gain and Noise Figure
The preamplifier shall provide additional gain of 26.5dB, minimum, to that specified in Section 2.2.1.4, with a maximum 4dB noise figure, including all circuitry in the integrated antenna/preamplifier. This additional gain is applicable to points A and B in figure 2-1, and does not include cabling between the preamplifier and the GNSS receiver at point C.
Industry:
Some receivers specify to use a maximum antenna gain of less than 26.5 dB. To allow the development of a GPS antenna to be usable for such receivers a deviation is requested to allow a 16 dB +/- 1dB antenna gain instead of the requested 26.5 dB. The use of the required gain of 26.5 dB would lead to problems by combining the antenna with GPS receivers having a lower maximum gain specification.
An equivalent level of safety is provided by limiting the use of the antenna having such deviation to installations together with GPS receivers being compatible to such gain.
EASA:
CS-ETSO C-129a (ED-72A paragraph 3.1.1) assumes 29.5 ± 3 dB for the preamplifier gain. Therefore, 26.5 dB minimum is a compatible value with ETSO C-144 (DO-228). For the cable loos ED-72A assumes a value between -3.0 dB to -13 dB.
EASA has no indication from its baseline of GPS receivers and GPS antenna CS-ETSO’d qualified equipment that there is a general interoperability issue between CS-ETSO C-129a and ETSO C-144. Moreover, ETSO C-144 envisages flexibility for the maximum preamplifier gain and the complete gain (preamplifier and antenna).
Nevertheless there may exist dedicated installation combinations where a lower antenna gain can be sufficient. This may lead to circumstances where a lower preamplifier gain can be accepted for a dedicated installation.
An active antenna will amplify noise as well as the signal. Therefore, the potential solution to use an attenuation device or additional cable loss to lower the signal to a level acceptable for the receiver after having it amplified is not a good technical solution either. Consequently we accept such a large pre-amplifier gain deviation for such combination with GPS-receivers being compatible for such signal. The limitation in using the antenna has to be clearly stated in the installation manual and the DDP. Further the installation manual shall contain a clear advice that the installed system performance has to be assessed in respect to signal reception.
