GPS/GNSS Positioning Flashcards
PublicFlashcards covering GPS/GNSS concepts for land surveying and the CA PLS exam
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Cards (48)
What is the orbital altitude of GPS satellites?
Approximately 20,200 km (12,550 miles)
How many satellites are in the GPS constellation?
24-32 satellites in 6 orbital planes
What is the orbital period of a GPS satellite?
11 hours 58 minutes (sidereal half-day)
What is the minimum number of satellites needed for 3D positioning?
4 satellites (3 for position, 1 for time/clock correction)
What does GNSS stand for?
Global Navigation Satellite Systems - the umbrella term for all satellite navigation systems (GPS, GLONASS, Galileo, BeiDou)
What is the L1 carrier frequency?
1575.42 MHz with a wavelength of 19.03 cm
What is the L2 carrier frequency?
1227.60 MHz with a wavelength of 24.42 cm
What is the wavelength of the L1 carrier?
19.03 cm (approximately 19 cm)
What is the C/A code chip rate?
1.023 MHz with a code wavelength of approximately 293 meters
What is the precision of carrier phase measurements?
1-2 mm (but includes unknown integer ambiguity)
What is the precision of C/A code pseudorange?
0.3-3 meters
What is the largest error source for single-frequency GPS?
Ionospheric delay (2-50 meters, can exceed 100m during solar storms)
How is ionospheric delay mitigated in survey GPS?
Dual-frequency observations allow computation of ionosphere-free combination, or differential GPS eliminates common delay
What is the typical zenith tropospheric delay?
2.3-2.5 meters (dry component ~2.3m, wet component 0.1-0.4m)
What is multipath error?
Error caused by GPS signals reflecting off surfaces before reaching the antenna, up to 5 cm for carrier phase
What is PDOP?
Position Dilution of Precision - a measure of how satellite geometry affects position accuracy; lower is better
What PDOP value is considered good for surveying?
PDOP < 4 is generally acceptable; 1-2 is excellent
What datum does GPS natively produce coordinates in?
WGS84 (World Geodetic System 1984)
What is California's legal horizontal datum?
NAD83 (specifically NAD83(2011) epoch 2010.0)
What is California's legal vertical datum?
NAVD88 (North American Vertical Datum of 1988)
What is ellipsoidal height?
Height above the reference ellipsoid - what GPS directly measures
What is orthometric height?
Height above the geoid (mean sea level) - what leveling measures and is shown on topo maps
What is the geoid height equation?
h = H + N (ellipsoidal height = orthometric height + geoid height)
How many State Plane zones does California have?
6 zones, all using Lambert Conformal Conic projection
What is the typical accuracy of static GPS surveying?
Horizontal: 3-5 mm + 0.5-1 ppm; Vertical: 5-10 mm + 1-2 ppm
What is the typical accuracy of RTK surveying?
Horizontal: 10-20 mm + 1 ppm; Vertical: 20-30 mm + 2 ppm
What does RTK stand for?
Real-Time Kinematic
What is the difference between fixed and float solutions?
Fixed: integer ambiguities resolved (1-3 cm accuracy); Float: ambiguities estimated as real numbers (10-50 cm accuracy)
What is PPP?
Precise Point Positioning - single receiver technique using precise orbit/clock products instead of differential corrections
What is Network RTK?
RTK using corrections from a network of reference stations (VRS, MAC, or FKP), providing better error modeling over larger areas
What is the integer ambiguity in GPS?
The unknown number of whole carrier wavelengths between satellite and receiver at start of tracking
What is a cycle slip?
Loss of continuous carrier phase tracking, causing a new integer ambiguity to be introduced
What is the ratio test in ambiguity resolution?
Comparison of best vs second-best integer candidate; ratio > 3 generally indicates reliable fix
What is the wide-lane combination wavelength?
86.2 cm (L1-L2 difference) - longer wavelength makes ambiguity resolution easier
What is OTF initialization?
On-The-Fly initialization - resolving integer ambiguities while the receiver is moving
What is the most common GPS survey blunder?
Incorrect antenna height measurement
What is the recommended minimum elevation mask for GPS surveying?
15 degrees above horizon
How do you convert slant height to vertical height?
V = √(S² - R²) where V = vertical, S = slant height, R = antenna radius
What should you check when setting up RTK base station?
Verify the entered coordinates match the actual control point coordinates exactly
What is the approximate rate of Pacific Plate motion in California?
40-50 mm/year to the northwest relative to NAD83
What geoid model should be used for California?
GEOID18 (current NGS model)
What is OPUS?
Online Positioning User Service - NGS free service for processing static GPS data
What new GPS signal is designed for safety-of-life applications?
L5 (1176.45 MHz)
What are the new horizontal datums planned for NSRS modernization?
NATRF2022 (North American plate), PTRF2022 (Pacific plate), and others, targeting 2025-2026
What will replace NAVD88?
NAPGD2022 (North American-Pacific Geopotential Datum 2022) - a geoid-based vertical datum
What is a CORS?
Continuously Operating Reference Station - permanent GNSS receiver providing data for positioning
What is the advantage of dual-frequency GPS receivers?
Enables ionospheric correction, wide-lane ambiguity resolution, and faster/more reliable integer fixing
What SNR value indicates good GPS signal tracking?
45-50 dB-Hz; minimum acceptable is typically 30-35 dB-Hz