8
LA010065D
© 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.
than four hours old, hence, invalid.
2.2.3 Cold start
A cold start acquisition state results when position
and/or time are unknown and unavailable, either of
which results in an unreliable satellite visibility list.
Almanac information stored in nonvolatile memory
in the receiver is used to identify previously healthy
satellites.
2.3 Navigation modes.
The Jupiter receiver supports two types of
navigation mode operations: Three-Dimensional
(3D) and Two-Dimensional (2D).
2.3.1 Three-dimensional (3D) navigation
The receiver defaults to 3D navigation whenever
at least four GPS satellites are being tracked. In
3D navigation, the receiver computes latitude,
longitude, altitude, and time information from
satellite measurements. Accuracies that can be
obtained in 3D navigation are shown in table 2-2.
2.3.2 Two-dimensional (2D) navigation
When only three GPS satellite signals are
available, a fixed value of altitude can be used
to produce a navigation solution. The Jupiter
receiver enters the 2D navigation mode from
3D navigation by using a fixed value of altitude,
either as determined during prior navigation, or as
provided by the OEM or zero. In 2D navigation, the
navigational accuracy is primarily determined by
the relationship of the fixed value of altitude to the
true altitude of the antenna.
If the fixed value is correct, the horizontal
accuracies shown in table 2-2 are approached.
Otherwise, the horizontal accuracies degrade
as a function of the error in the fixed altitude. In
addition, due to the presence of only three satellite
signals, time accuracy degrades and the computed
position can be expected to show considerable
effects of noise, multipath, and partial blockages.
2.0 Technical description
2.1 General information
The Jupiter 12 re3.3 to +5.0 V primary DC
input power. The receiver can operate from either
an active or passive GPS antenna, supplied by the
OEM, to receive L-band GPS carrier signals.
2.2 Satellite acquisition
As the receiver determines its position by ranging
signals from three or more GPS satellites orbiting
the Earth, its antenna must have a good view of
the sky. This is usually not a problem when the
receiver is used outdoors in the open, but when
used indoors, or inside an automobile, the antenna
should be positioned to allow clear view of the sky.
To establish an initial navigation fix, the receiver
requires three satellites in track and an entered
or remembered altitude. If satellite signals are
blocked, the time for the receiver to receive those
signals and determine its position will be longer.
If less than three satellites are being tracked,
signal blockage may result in a failure to navigate.
The Jupiter 12 GPS receiver supports three
types of satellite signal acquisition (see table 2-1)
depending on the availability of critical data.
2.2.1 Hot start
A hot start occurs when the receiver has been
reset during navigation. Most recent position and
time are valid in memory. Ephemerides of visible
satellites are in SRAM (valid ephemerides are less
than four hours old).
2.2.2 Warm start
A warm start typically results from user supplied
position and time initialisation, or from position
data stored in memory and time from the Real-
Time Clock (RTC) maintained by backup power.
Table 2-1 shows the required accuracy of
initialisation data. Satellite ephemerides, are more
Satellite
acquisition
state
Time to first fix
(seconds)
Initial error uncertainties (Note 1)
typical
90% probable
position (km)
velocity (m/s)
time (min)
Hot start
24
30
100
75
5
Warm start
42
66
100
75
5
Cold start
60
180
N/A (Note 2)
Times are for a receiver operating at 25°C with no satellite signal blockage.
Note 1: required accuracy of data used for initialised start.
Note 2: initial error uncertainties do not apply to cold start.
Table 2-1 Jupiter receiver signal acquisition
