In CAE OAA Book11-Radio Navigation Chapter-18 Page-318, under GPS Error, "The optimum position for the antenna is on top of the fuselage close to the aircraft's centre of gravity".


In the FCOM of A320, GPS(MMR) Antenna are situated in the front of the fuselage


My questions are:

  1. Why are all GPS Antenna situated on top of fuselage ?. (Is it because of satellite reception)
  2. Why is the GPS (MMR) antenna situated in the front top of the fuselage ?.
  3. Why GPS antenna should be close to aircraft's CG ?.
  4. Why are there 2 GPS Antenna ?. (Is it because of redundancy or because faster lock-in?.)
  • 2
    $\begingroup$ This is pure speculation, but maybe it's because CG is basically the "centre" of the aircraft; on large aircraft, the extremities might be quite far away from the CG, and placing the antenna there might reduce the accuracy (e.g. if the antenna was placed at the wingtip, the location it would show might not give a good picture of the "average" location of the aircraft). $\endgroup$ Dec 20, 2022 at 10:32
  • $\begingroup$ @AdityaSharma To avoid speculations, i have attached image from an official book. If it is given, there will be reason behind it. $\endgroup$ Dec 20, 2022 at 10:49
  • $\begingroup$ Surely there must be a good reason; I was just sharing my thoughts. Good day. $\endgroup$ Dec 20, 2022 at 11:10
  • 3
    $\begingroup$ Not a full answer, but consider that when a long aircraft changes pitch attitude, the nose and tail will move by several meters in opposite directions. Placing the antenna near the CG mitigates perceived altitude changes due to this effect, especially during altitude-sensitive phases of flight, like on an RNP approach. $\endgroup$
    – Tyzoid
    Dec 20, 2022 at 20:15
  • $\begingroup$ Is it not true that an aircraft has basically, five points of reference? I deviate from standard usage to call the points front and back; left, right and centre… assuming the dheight of the body is too slight to matter Left and right can't matter. Wingtip transponders gain what? That leaves - untechnically - front, back and centre and takes us back to Tyzoid $\endgroup$ Dec 21, 2022 at 13:13

3 Answers 3

  1. GPS signals need line of sight to the satellite, so positioning the antenna on top of the aircraft maximizes the satellites that can be seen and minimizes multipathing, and also gives some protection to ground based interference/jamming.

  2. Not listed in the CAE but there are some advantages in not having critical cables running half the length of the aircraft, so the location may be the one that got a cheap, simple and easy to install and test location for the antenna, and per below it may not make a difference for this particular nav system.

  3. Keeping the antenna near the C of G has two two effects, the first is that it means the position output is near the actual aircraft center, avoiding issues with systems having it off the side of the taxiway or showing altitude while still on the runway. There are also several effects the reduce the precision if there is rapid motion of the antenna during fixing, so a central location will get more accurate position. All of this can be 'solved' with software, but does make accurate GPS dependent on accurate pitch/roll information (and good software). Presumably in this case software was traded against the other effects of central location to produce the current placement.

  4. Two antennas would be for redundancy. While aircraft can operate without GPS, it generally increases crew workload which produces risk of some other problem causing a crash.

  • 4
    $\begingroup$ To be more specific about point 3, while an aircraft is pitching, rolling, or yawing, it more or less rotates about the CofG. So putting the antenna there keeps it moving with the path of the aircraft as a whole, not on a long level far from that. (The roll axis would be inside the fuselage, except at the nose/tail, so really it's pitch/yaw that you're fully avoiding by placing it at CofG.) $\endgroup$ Dec 21, 2022 at 3:14
  1. All GPS signals are line of sight and most receivers use all satellites that are visible and a minimum of 9 - 12 degrees above the horizon. So being on the top of the aircraft near the centerline is essential to providing the best reception.
  2. It's somewhat arbitrary as all antenna locations are a tradeoff. All the various antennae have to separated to avoid potential interference and parasitic effects. Cable lengths are an issue for two reasons. Signal attenuation and weight. Most of the antenna cables end up routed to the E/E bay aft of the nose gear well where the majority of the avionics are located (HF and radio altimeters are located in an aft bay.) I don't know why this diagram shows 4 GPS antennae, 2 MMR and 2 'GPS'. I only know of the GPS receivers in the MMR. Every aircraft will have some variation is locations. The biggest known interferer for GPS is the VHF com, so keeping them separated is essential. Forward locations will avoid any shadowing by the vertical stabilizer/rudder.
  3. The aircraft C.G. is the reference point for the navigation system. It's the point about which all aircraft rotation is measured. As the nature of the GPS solution is time difference of arrival of the signals from the satellites, the resolved position is that of the GPS antenna. The antenna location offset from the aircraft C.G. is loaded into each GPS(MMR) to allow the GPS to compensate for the offset. Additionally, the GPS data will be merged with the IRS data (which has its own compensation for installation location) in the FMS to obtain a hybrid GPS/IRS position. So ultimately any offset from the C.G. is accounted for. The only benefit of being close to the is to reduce 'noise' due to aircraft rotational motion (pitch/roll/yaw). Since it will always be present with even small offsets, there is an internal Kalman filter to reduce it to a minimum.
  4. Navigation is an essential function whose loss is considered Major or Hazardous depending on phase of flight. In either case it requires redundancy so there are two of everything - GPS(MMR), FMS, etc. There may be 2 or 3 IRS depending on the system architecture and type of flight controls. Antennae can be shared between systems if they are simple antenna and meet a minimum reliability. Because GPS antennae are complex, containing an amplifier and downconverter, each GPS receiver has a dedicated antenna.

Specific to question 2, that has not been addressed in other answers… The GPS MMR antennae receive GPS augmentation signals. In the case of WAAS, which is provided by stationary satellites, (SBAS), you want those on top. But MMR augmentation also includes ground-based signals, (see GBAS), which are transmitted from the airport. GBAS airports are rare today but are likely to become more common over time. So the logical location that is line-of-sight to both satellites and the runway while on final approach is above the windshields.

  • $\begingroup$ The GBAS augmentation is broadcast using VHF Data Broadcast (VDB) on LOC/VOR frequencies, which the GPS antennae are incapable of receiving. Since the MMR has a perfectly functional VHF receiver (traditionally for LOC), it repurposes this hardware with supplemental software to receive the VDB signals via the LOC antenna when the GBAS landing function is selected. $\endgroup$
    – Gerry
    Dec 22, 2022 at 15:23

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