Navman LA000508 - Manual (Page 8)

LA000507G © 2006 Navman New Zealand. All rights reserved. Proprietary information and specifications subject to change without notice.
3.6 Differential aiding
3.6.1 Differential GPS (DGPS)
DGPS specification improves the Jupiter 20 horizontal position accuracy to <4 m 2 dRMS.
3.6.2 Satellite Based Augmentation Systems (SBAS) including WAAS and EGNOS
SBAS improves horizontal position accuracy by correcting GPS signal errors caused by
ionospheric disturbances, timing and satellite orbit errors. The Jupiter 20 is capable of
receiving WAAS and EGNOS differential corrections. Both SBAS and DGPS should improve
position accuracy. However, other factors can affect accuracy, such as GDOP (Geometric
Dilution of Precision), multipath, distance from DGPS reference station and latency of
corrections. Note also that XTrac does not support differential aiding.
3.7 Navigation modes
The Jupiter 20 GPS receiver supports 3D (three‑dimensional) and 2D (two‑dimensional) modes
of navigation.
3D navigation: The receiver defaults to 3D navigation when at least four GPS satellites are being
tracked. In 3D navigation, the receiver computes latitude, longitude, altitude, and time from
satellite measurements.
2D navigation: When less than four GPS satellite signals are available, or when a fixed altitude
value can be used to produce an acceptable navigation solution, the receiver will enter 2D
navigation mode using a fixed value of altitude determined by the host. Forced operation in 2D
mode can be commanded by the host.
In 2D navigation, the navigational accuracy is primarily determined by the relationship of the
fixed altitude value to the true altitude of the antenna. If the fixed value is correct, the specified
horizontal accuracies apply. Otherwise, the horizontal accuracies will degrade as a function of
the error in the fixed altitude.
3.8 Core processor performance
The standard Jupiter 20 with GSW2 software runs at a CPU clock speed of 12.28 MHz. Using
XTrac software (Jupiter 20S), the clock speed increases to 24.5 MHz. An SDK (Software
Development Kit) is available from SiRF to customise the Jupiter 20 firmware. Using the SiRF
SDK the clock speed can be increased up to 49 MHz.
The processing power used by the navigation software is shown in Table 3‑2.
Parameter J20/J20D J20S
typical performance 2‑3 MIPS 4‑5 MIPS
peak performance 6‑7 MIPS 8‑9 MIPS
Table 3-2: Software processing performance
3.9 Sensitivity
The GPS receiver performance of the Jupiter 20 is shown in Table 3‑3.
Parameter J20/J20D J20S
acquisition sensitivity –135 dBm 33 dBHz –135 dBm 33 dBHz
navigation sensitivity –141 dBm 28 dBHz –152 dBm 17 dBHz
tracking sensitivity –143 dBm 26 dBHz –154 dBm 15 dBHz
Table 3-3: GPS receiver performance