Navman 11 User Manual

Page 61

used to identify previously healthy satellites and

to generate “working” visible satellite lists, while

frequency standard data minimises satellite

acquisition uncertainties.

4.4.4 Frozen start
This state is entered if there are no valid data

sources available (SRAM, RTC, EEPROM). This

is considered to be a recovery mode because

EEPROM should always contain valid information.

An “out-of-the-box” board or a unit that has not

operated for a signiﬁcant amount of time (months)

may approximate this state because the data in

EEPROM may be valid but expired or partially

complete.

4.5 Satellite management

This section describes the satellite management

functions of the Jupiter family of GPS receivers.

4.5.1 Visible list generation.
A list of satellites visible to the receiver antenna

is maintained whenever possible. A satellite is

considered visible if its elevation in the sky is

known to be above the horizon, if its almanac and

ephemeris data indicate it is healthy, and if it has

not been excluded by manual candidate satellite

speciﬁcation. Note that although a satellite is

visible, its measurement is only available for use if

the satellite is above the elevation mask angle.

The receiver’s channel resources are directed

toward acquiring only those satellites which appear

in this list except when the receiver is in cold start

mode. Satellites within the list are ordered from

highest to lowest elevation which, for sequential

acquisition, also dictates the order in which

acquisition attempts are made.

Receiver position and current time are required

to compute satellite positions from orbital data.

If position and/ or time is not considered to be

well known (i.e. their expected errors are large),

then the list is extended below the horizon

and is ﬁlled to the maximum of 12 satellites. If

DGPS corrections are available, the satellites

represented in the corrections are used to set the

list membership instead, since they also represent

satellites visible to a nearby transmitting DGPS

base station.

New visible satellite lists are generated by

events that could cause a change in satellite list

membership or could indicate a signiﬁcant change

in a satellite position relative to the antenna. These

events include receipt of an elevation mask angle

or candidate satellite speciﬁcation command,

downloading of a new satellite almanac, and

changes in satellite health status reﬂected in new

almanac or ephemeris data.

In the case where DGPS corrections are used to

establish list membership, a change in the set of

satellites reﬂected in the corrections also causes a

new list to be generated. During initial acquisition,

a new list is generated when the receiver makes

step adjustments to position and time. In the

absence of these events, the visible satellite list is

updated every 30 seconds. The visible satellite list

is output in the Visible Satellites message (binary

Message 1003)

4.5.1.1 Dilution Of Precision (DOP)

Geometric Dilution of Precision (GDOP) is a

measure of the quality of a satellite constellation

geometry. GDOP reﬂects the inﬂuence of satellite

geometry on the accuracy of user position and

time estimates. The best geometry is that which

produces the lowest GDOP value. GDOP acts as a

multiplier of the error in position and time estimates

due to other sources.

GDOP is a composite measure. It can be

separated into:

• Position Dilution of Precision (PDOP)
PDOP reﬂects the effects of geometry on

three-dimensional position estimates
• Time Dilution of Precision (TDOP)
TDOP reﬂects geometric effects on time

estimates. The relationship can be expressed

as:

GDOP =

In turn, PDOP can be separated into horizontal

and vertical components:

• Horizontal Dilution of Precision (HDOP)

• Vertical Dilution of Precision (VDOP)

These components represent the effects of

satellite geometry on two-dimensional horizontal

position and on vertical position (altitude)

estimates, respectively.

This relationship can be expressed as:

PDOP =

The receiver computes the best available

GDOP and each of its components in the Visible

Satellites message (binary Message 1003). The

best available GDOP is that associated with

the satellite constellation consisting of all visible

satellites above the mask angle (satellites whose

measurements may be used).

At least four satellites are required to estimate

position and time, and therefore to compute a

(PDOP)

+ (TDOP)

(HDOP)

+ (VDOP)

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