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Thesis - Leigh Moody.pdf - Bad Request - Cranfield University

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Chapter 3 / Sensors / NAVSTAR GPS<br />

_ _<br />

The 4 th satellite is chosen such that its (Gi) minimises the following<br />

parameter from the remaining visible satellites,<br />

3.11.8 Sensor Limitations<br />

ϕ<br />

2<br />

( U1<br />

• Gi<br />

) ⋅ U1<br />

+ ( U 2 • Gi<br />

) ⋅ U2<br />

+ ( U3<br />

• Gi<br />

) U3<br />

: =<br />

⋅<br />

3.11-19<br />

Equation 3.11-71<br />

Provision of GPS measurements at the sensor output interface depends on:<br />

• User measurement selection ( MS_MSAV2 bit 22/25 cleared )<br />

• Receiver tracking limitations ( MS_GP_ER bit 24 is set )<br />

When active, if any of these limitations are violated, bits 24-25 in the<br />

measurement ready flag MS_MSRY2 are cleared. Failure to provide<br />

measurements at the user’s request is indicated by setting bit 0 of<br />

MS_BA_VF. The effect of receiver tracking failures is dealt with in §3.11.7.<br />

If any of these thresholds are exceeded for any satellite bit 2 of MS_BA_VF<br />

is set and the effected satellites are treated as “invisible”.<br />

The following limitations are not user selectable. If there are insufficient<br />

satellites for triangulation bit 3 of MS_BA_VF is set. When the satellites are<br />

near to coplanar with a GDOP > 1000 triangulation is not performed and<br />

bit 4 is set.<br />

3.11.9 Output Interface<br />

The GPS measurement vector passes through a signed digital interface at a<br />

frequency (GPfO) of 1 Hz (clock 23).<br />

GP<br />

Z ~<br />

: =<br />

ϕ<br />

TD<br />

( ( ( ( Z ) ) ) ) ~<br />

ϕ ϕ ϕ<br />

Q<br />

LIM<br />

ZOH<br />

Equation 3.11-72<br />

The longitude and latitude pass through a 32 bit interface with range limits<br />

of ± 180° and ± 90° respectively, subject to a 1/50 cycle transmission delay.<br />

The geodetic height and velocity are limited to ± 300 m and ± 350 m/s by a<br />

16 bit interface with the same cyclic delay.

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