GELE Geodesy — Satellite Geodesy and GNSSStudy Notes

Global Navigation Satellite Systems operate on the principle of **trilateration**—determining a point's position by measuring its distances to multiple known reference points (satellites). In GNSS, a receiver calculates ranges (pseudoranges) to satellites of known position, then solves a system of equations to find the receiver's coordinates and correct its clock bias. **The Positioning Equation:** A GNSS receiver measures the time $\Delta t$ that a signal takes to travel from a satellite to the receiver. The pseudorange is: $$\rho = c \cdot \Delta t$$ where $c = 299,792,458$ m/s is the speed of light in vacuum. Each pseudorange equation relates the receiver's unknown position $(X, Y, Z)$ and clock bias $\Delta t_{rx}$ (expressed in distance units) to a known satellite position $(X_s, Y_s, Z_s)$: $$\rho_i = \sqrt{(X - X_i)^2 + (Y - Y_i)^2 + (Z - Z_i)^2} + c \cdot \Delta t_{rx}$$ where $i$ denotes the $i$-th satellite. **Why Four Satellites Minimum?** A GNSS position fix requires solving for **four unknowns**: three spatial coordinates $(X, Y, Z)$ and one temporal unknown (receiver clock bias $\Delta t_{rx}$). Therefore, a minimum of **four satellites** is necessary for a complete 3-D solution. Three satellites would give only three equations, leaving the clock bias uncorrected and introducing a large systematic error. **Geocentric Reference Frame:** GNSS operates inherently in an **Earth-Centered, Earth-Fixed (ECEF)** geocentric Cartesian coordinate system. GPS broadcasts coordinates in the **WGS84** datum (specifically, GPS uses WGS84(G1954) and later realizations). Positions computed from GNSS observations are in this frame and must be **transformed to the local datum** (PRS92 in the Philippines) for cadastral surveys and mapping, as mandated by RA 4374 and professional standards.

GNSS receivers measure two primary observables from satellite signals: **pseudorange** (code) and **carrier phase**. Each has distinct accuracy characteristics, applications, and processing requirements. **Pseudorange (Code) Observable:** The pseudorange is derived by measuring the time it takes for a satellite's transmitted code (e.g., the C/A code on GPS) to reach the receiver. The measured time, multiplied by the speed of light, gives the pseudorange. Pseudorange is called "pseudo" because it includes the receiver's clock error. Accuracy: **0.5–3.0 m**, depending on atmospheric conditions and receiver quality. Advantages: Unambiguous, rapid acquisition, used for positioning and velocity. Disadvantages: Lower precision than carrier phase; sensitive to ionospheric and tropospheric delays. **Carrier Phase Observable:** The receiver measures the phase of the carrier wave (e.g., L1 at 1575.42 MHz for GPS). The observable is the fractional number of wavelengths between the satellite transmitter and receiver. The wavelength is: $$\lambda = \frac{c}{f}$$ For GPS L1: $\lambda_{L1} = \frac{299,792,458}{1,575,420,000} \approx 0.190$ m (19.0 cm). Carrier phase measures: $$\phi = \frac{\rho}{\lambda} + N$$ where $N$ is the **integer ambiguity** (the number of complete wavelengths between satellite and receiver, unknown at initialization). Accuracy: **0.5–5.0 mm** (after fixing integer ambiguities). Advantages: Very precise; provides sub-centimetre positioning when ambiguities resolved. Disadvantages: Ambiguous; requires initialization or kinematic motion; susceptible to cycle slips (momentary loss of lock). **Multi-Frequency GNSS:** Modern GNSS satellites (GPS III, Galileo, BeiDou, modernized GLONASS) transmit multiple frequencies (e.g., GPS L1, L2, L5). Multi-frequency observations allow receivers to: - Mitigate ionospheric refraction (by forming ionosphere-free linear combinations) - Resolve integer ambiguities more rapidly (ambiguity resolution) - Improve positioning accuracy and reliability **Dual-frequency receivers** are standard in professional surveying; single-frequency receivers are acceptable for many applications but suffer greater ionospheric errors.

GNSS measurements are affected by numerous error sources, both systematic and random. Understanding and mitigation of these errors is critical for accurate surveying. **Satellite Clock and Ephemeris Errors:** Satellite clocks are precise atomic clocks (cesium or rubidium standards), but small timing errors occur. GPS satellites transmit their clock bias correction via the navigation message, reducing clock error to ~1–2 m. Ephemeris errors (errors in the satellite's transmitted orbit) introduce ~1–2 m uncertainty in satellite position. **Ionospheric Delay:** The ionosphere is a layer of ionized gas in the upper atmosphere (altitude ~50–2000 km). Free electrons delay electromagnetic wave propagation. The pseudorange is delayed; the carrier phase is advanced. The delay depends on the **Total Electron Content (TEC)** along the signal path and the frequency: $$\Delta \rho_{iono} \approx \frac{40.3}{f^2} \cdot \text{TEC}$$ For a vertical signal path under quiet ionospheric conditions, delay ranges from ~2–5 m at solar minimum to 20–50 m at solar maximum. Under severe ionospheric storms (common near the geomagnetic equator, where the Philippines is located), delays can exceed 100 m. **Mitigation:** - Dual-frequency observations allow forming the ionosphere-free linear combination, removing ~99% of ionospheric delay - Single-frequency users experience significantly greater errors, especially near the geomagnetic equator - Differential GNSS (DGPS) can reduce ionospheric error via common-mode cancellation **Tropospheric Delay:** The troposphere (neutral atmosphere, 0–12 km altitude) delays GNSS signals through refraction caused by water vapour, dry gas, and hydrostatic pressure. Unlike ionospheric delay, tropospheric delay is independent of frequency, so it cannot be eliminated via dual-frequency combinations. $$\Delta \rho_{tropo} \approx 2.3 \text{ m (zenith)} \text{ to } >10 \text{ m (low elevation)}$$ Tropospheric delay depends on: - Height and pressure (hydrostatic component) - Temperature and humidity (wet component) **Mitigation:** - Use tropospheric models (Hopfield, Saastamoinen) with surface meteorological data - Estimate tropospheric parameters from GNSS data itself (post-processing) - Differential GNSS reduces tropospheric error through correlation over short baselines (<10 km) **Multipath:** Signals reflect off nearby objects (buildings, terrain, water surfaces) before reaching the antenna, creating multiple signal paths. The receiver measures a combination of direct and reflected signals, causing range errors of 1–10 m for code, 1–100 mm for carrier phase. **Mitigation:** - Use a choke-ring antenna (suppresses reflected signals) - Careful survey site selection (avoid reflective surfaces) - Signal-tracking algorithms (correlator spacing) **Receiver Noise:** Random measurement error in the receiver electronics and antenna. Typically: - Pseudorange noise: ±0.5–2.0 m (1σ) - Carrier phase noise: ±1–5 mm (1σ) **Signal Blockage and Availability:** Signals are blocked by buildings, dense vegetation, tunnels, and other obstructions. In urban canyons or dense forest, satellite availability may be insufficient for positioning. Receiver must track minimum four satellites for 3-D fix; more satellites improve accuracy and reliability.

The geometric distribution of satellites relative to the receiver significantly affects positioning accuracy. The quality of this geometry is quantified by the **Dilution of Precision (DOP)**—a dimensionless parameter that scales the measurement error to the resulting position error. **DOP Concept:** The position error depends on two factors: 1. **Measurement error (σ):** the accuracy of individual pseudorange or phase measurements 2. **Geometric dilution:** how well the satellites surround the receiver The fundamental relationship is: $$\text{Position Error} \approx \text{DOP} \times \text{Measurement Error}$$ DOP is derived from the geometry matrix of satellite positions. A well-distributed constellation of satellites (spread across the sky) gives low DOP (good geometry); satellites clustered in one direction give high DOP (poor geometry). **Types of DOP:** **PDOP (Position Dilution of Precision):** $$\text{PDOP} = \sqrt{\sigma_X^2 + \sigma_Y^2 + \sigma_Z^2 + \sigma_t^2}$$ Combines horizontal and vertical position errors plus time error. Typical range: 2–10. **HDOP (Horizontal Dilution of Precision):** $$\text{HDOP} = \sqrt{\sigma_X^2 + \sigma_Y^2}$$ Represents horizontal position error only. For cadastral surveys, HDOP is often the critical parameter. **VDOP (Vertical Dilution of Precision):** $$\text{VDOP} = \sigma_Z$$ Represents vertical (elevation) error. **TDOP (Time Dilution of Precision):** $$\text{TDOP} = \sigma_t$$ Represents receiver clock bias error (time). **Geometric Interpretation:** Conceptually, DOP relates to the size and shape of the error ellipsoid formed by satellite geometry. If satellites form a narrow cone above the receiver (poor vertical geometry, high VDOP), height errors are large. If satellites are spread uniformly around the sky, errors in all directions are small (low PDOP). **DOP Values and Interpretation:** | DOP Value | Rating | Notes | |-----------|----------------|-------| | < 2 | Excellent | Ideal geometric distribution | | 2–5 | Good | Suitable for most surveying | | 5–10 | Moderate | Acceptable but less precise | | 10–20 | Poor | Significantly degraded accuracy | | > 20 | Very Poor | Avoid positioning if possible | **Practical Strategy:** A surveyor should check the predicted DOP before fieldwork using GNSS planning software (e.g., based on broadcast ephemerides). For cadastral surveys requiring ±0.05 m horizontal accuracy, aiming for **HDOP < 3** is a reasonable guideline. RTK surveys can tolerate slightly higher DOP because carrier-phase measurements are more precise than pseudorange.

Standard point positioning (using pseudoranges from a single receiver) achieves 1–5 m accuracy due to residual atmospheric and orbital errors. To improve accuracy for surveying and engineering applications, **differential** methods leverage a reference station on a known point to compute and broadcast corrections. **Differential GNSS (DGPS) Principle:** A DGPS base station sits on a control point of known position. It observes the same satellites as the rover (mobile user) and computes the **pseudorange correction** for each satellite: $$\Delta \rho_i = (\rho_i^{measured} - \rho_i^{true})_{base}$$ where $\rho_i^{true}$ is the geometric range computed from the base's known position and the satellite's position. The rover applies this correction (assumed common over short baseline < 10–20 km): $$\rho_i^{corrected} = \rho_i^{measured}_{rover} - \Delta \rho_i$$ This removes most systematic errors (clock, orbit, ionosphere over short baseline). Typical DGPS accuracy: **0.5–2.0 m**. **Modes of DGPS:** 1. **Post-Processed DGPS:** Rover records pseudoranges; base's corrections are downloaded later; suitable for mapping and non-real-time surveys. 2. **Real-Time DGPS:** Base transmits corrections via radio, cellular, or satellite link; rover applies corrections in real-time; suitable for navigation. **Real-Time Kinematic (RTK):** RTK extends differential positioning to **carrier phase**, achieving centimetre-level accuracy in real-time. The key innovation is **integer ambiguity resolution**—the receiver must determine the integer number of carrier wavelengths between base and rover. **RTK Processing Steps:** 1. **Initialization:** Both base and rover track common satellites and accumulate carrier-phase data. 2. **Ambiguity Resolution:** Using the fixed-base-to-rover baseline geometry (from preliminary pseudorange positioning) and multiple frequencies/epochs, the integer ambiguities are resolved using algorithms (e.g., LAMBDA, integer least squares). 3. **Kinematic Positioning:** Once ambiguities are fixed, the carrier-phase observations provide centimetre-level real-time positioning for the rover. **RTK Accuracy and Requirements:** - **Horizontal accuracy:** ±2–5 cm + 1 ppm of baseline length (typical) - **Vertical accuracy:** ±3–8 cm + 1 ppm of baseline length - **Baseline length:** Typically < 10–20 km; longer baselines require more processing time and higher-quality receivers - **Convergence time:** 10–300 seconds from power-on, depending on geometry and algorithm - **Atmospheric stability:** Ionosphere and troposphere must be reasonably coherent over the baseline **Communication Link:** RTK requires continuous two-way communication between base and rover: - **Radio link:** UHF or VHF radio (0–5 km range typical) - **Cellular (NTRIP):** Internet-based GNSS corrections via NTRIP protocol; allows longer baselines (up to ~100 km with proper setup) - **Network RTK (nRTK):** Multiple reference stations provide interpolated corrections; regional coverage; used in cadastral surveys in the Philippines via DOST-maintained networks **Professional Standards in the Philippines:** Under **RA 4374 (Cadastral Survey Law)** and **PD 1529 (Subdivision Law)**, cadastral surveys require monumentation and positional accuracy of ±0.05 m (5 cm) horizontal and ±0.10 m (10 cm) vertical for boundaries. **RTK GNSS is now the standard method** for control densification and boundary delineation, provided: - Base station coordinates are established via static GNSS or conventional control network tie - Observations are dual-frequency - Ambiguity resolution is verified and documented - Results are transformed to PRS92 datum - Professional reports include DOP, skyplot, and convergence statistics

GNSS satellites broadcast their ephemerides (positions) and operate natively in a **geocentric (Earth-centered) reference frame**. Results from GNSS processing are inherently in this frame, not in the local geographic datum used for cadastral surveys and mapping. Understanding reference frame transformations is essential for converting GNSS results to usable coordinates. **WGS84 and ITRF:** The **World Geodetic System 1984 (WGS84)** is the reference frame defined by GPS. It is an **Earth-Centered, Earth-Fixed (ECEF)** Cartesian frame with origin at Earth's center, Z-axis aligned with the conventional terrestrial pole, and X-axis passing through the prime meridian. In practice, GPS satellites transmit ephemerides in a specific **WGS84 realization** (e.g., WGS84(G1954), WGS84(G1674)). Modern scientific users work with the **International Terrestrial Reference Frame (ITRF)** realizations (e.g., ITRF2014, ITRF2020), which are more accurate and regularly updated. ITRF and WGS84 differ by ~1–2 m in coordinate origin, but are functionally equivalent for most geodetic purposes. **Philippine Reference System (PRS92):** The **Philippine Reference System of 1992 (PRS92)** is the official local datum for the Philippines, defined by: - **Reference ellipsoid:** WGS84 (semimajor axis a = 6,378,137 m; inverse flattening 1/f = 298.257223563) - **Datum origin:** Assumed at a monument near Kalayaan, Laguna Province - **Coordinates:** Expressed in ellipsoidal (latitude φ, longitude λ, height h) or projected (Cartesian X, Y) in the Philippine Transverse Mercator (PTM or sometimes called Philippine Polyconic Coordinate System—PPCS) projection PRS92 is maintained by the **National Mapping and Resource Information Authority (NAMRIA)** and is the legal datum for all cadastral surveys, mapping, and engineering projects in the Philippines under RA 4374, RA 8560, and related laws. **Transformation from WGS84/ITRF to PRS92:** Since both WGS84 and PRS92 use the same ellipsoid (WGS84), the transformation is primarily a **datum shift** (translation and rotation in 3-D space) plus conversion between coordinate systems (Cartesian ECEF to geodetic φ,λ,h, then to projected X,Y). Steps: 1. **ECEF Cartesian (X,Y,Z) WGS84 → Geodetic (φ,λ,h) WGS84:** Use iterative algorithms (e.g., Heikkinen's method) to convert from Cartesian to geodetic. 2. **Geodetic WGS84 → Geodetic PRS92:** Apply a datum shift. The most common transformation is a **7-parameter Helmert transformation**: - 3 translations (ΔX, ΔY, ΔZ) in Cartesian space - 3 rotations (Rx, Ry, Rz) small rotation angles - 1 scale factor (s) For WGS84 to PRS92, the shift parameters are approximately: - ΔX ≈ −127.5 m, ΔY ≈ −67.0 m, ΔZ ≈ −47.5 m (typical values; varies by location) - Rotations and scale factor are small 3. **Geodetic PRS92 → Projected PTM (X_PTM, Y_PTM):** Use Transverse Mercator projection (similar to UTM, but specific zones for Philippines). PTM uses 3° wide zones (Zone 1, 2, 3, ..., 5) with central meridians at 121°, 124°, 127°, 130°, 133° E. **Practical Transformation Tools:** Professional geodetic software (e.g., Leica Infinity, Trimble Business Center) and NAMRIA-published transformation grids handle the conversion automatically. Surveyors must specify: - Input datum (WGS84, ITRF) - Output datum (PRS92) - Projection zone (PTM zone for the survey area) **Legal Requirements:** Under **RA 4374** and **PD 1529**, all cadastral surveys and subdivision plans must be: - Tied to the existing geodetic control network (maintained by NAMRIA) - Expressed in PRS92 datum and PTM projection - Monuments and boundaries described in the local coordinate system Reports and plans that reference WGS84/ITRF coordinates alone are **not legally accepted** for cadastral purposes. Transformation to PRS92 is mandatory. **Quality of Transformation:** Transformation accuracy depends on: - Quality of WGS84 coordinates (from GNSS) - Applicability of transformation parameters (localized vs. national) - Whether using a single 7-parameter transformation or a refined grid Typical accuracy: ±0.05–0.10 m with modern transformation grids. For boundary surveys, this is sufficient. **Temporal Stability:** Coordinates in a geocentric frame (WGS84/ITRF) change with time due to plate tectonics and Earth orientation variations. For Philippine surveys, continental drift (Philippine Plate motion) causes coordinate changes of ~8 cm/year horizontally. Reference frame realizations are updated every 4–5 years. For legal cadastral work, NAMRIA specifies the valid epoch (year) of coordinates; surveys more than a few years old may require re-observation or coordinate adjustment.

For precise cadastral and engineering control surveys, **static GNSS** (dual-frequency, carrier-phase positioning over fixed observation periods) provides the highest accuracy. Unlike RTK (real-time kinematic), static surveys involve post-processing of phase data and often incorporation of multiple baseline observations into a least-squares adjustment. **Static GNSS Procedure:** 1. **Reconnaissance and Planning:** Identify control monuments and new survey points; compute DOP for planned observation windows; verify ionospheric conditions (low solar activity preferred). 2. **Equipment Setup:** Dual-frequency receiver on stable tripod or antenna mount; minimize multipath (choke-ring antenna); record metadata (antenna height, instrument serial, exact time). 3. **Observation Sessions:** Continuous tracking of satellites for 20 minutes to several hours, depending on baseline length and desired accuracy: - **Short baseline (< 5 km):** 20–30 minutes sufficient - **Medium baseline (5–20 km):** 1–2 hours recommended - **Long baseline (> 20 km):** 2–4 hours or more; or overnight sessions 4. **Data Collection:** Both base and rover(s) record raw observations (pseudorange, carrier phase, Doppler) at 1–30 Hz sampling rate. 5. **Post-Processing:** Software (e.g., Trimble, Leica, Riegl baselines; or open-source RTKLIB, Gipsyc) processes observations: - Resolves integer ambiguities - Computes baseline vectors (ΔX, ΔY, ΔZ) between stations - Applies precise ephemerides (IGS products, typically available within days to weeks) - Corrects for relativistic effects, Earth orientation variation, and other systematic errors 6. **Network Adjustment:** If multiple baselines are observed forming a network (e.g., 5 stations with multiple inter-station observations), use least-squares adjustment to compute final coordinates and their uncertainties. This process distributes measurement noise and resolves any inconsistencies. **Accuracy and Precision:** Static GNSS accuracy depends on several factors: - **Baseline length:** Longer baselines accumulate ionospheric and tropospheric errors. Typical accuracy improves with geometry: - **Short baseline (< 5 km):** ±(0.005 m + 0.1 ppm × baseline) - **Medium baseline (5–20 km):** ±(0.01 m + 0.5 ppm × baseline) - **Long baseline (> 100 km):** ±(0.02 m + 1 ppm × baseline) or worse without careful modeling - **Observation duration:** Longer sessions improve ambiguity resolution and error averaging. - **Satellite geometry (DOP):** Lower DOP (< 5) is preferable. - **Atmospheric quality:** Low ionospheric activity (Kp index < 5) and stable troposphere (no rain, low humidity) improve precision. **Network Adjustment (Least-Squares):** When multiple baselines are observed, the adjustment process is: $$\mathbf{A} \mathbf{x} = \mathbf{l}$$ where: - **A:** Design matrix relating observed baselines to unknown coordinates - **x:** Vector of unknown coordinates (parameters to solve) - **l:** Vector of observed baseline vectors (observations) The least-squares solution minimizes residuals: $$\hat{\mathbf{x}} = (\mathbf{A}^T \mathbf{W} \mathbf{A})^{-1} \mathbf{A}^T \mathbf{W} \mathbf{l}$$ where **W** is the weight matrix (inverse of covariance; better-measured baselines have higher weight). Output: Final coordinates and confidence ellipses for each station, residual vectors, and goodness-of-fit statistics (χ² test). **Quality Control in Static GNSS:** - **Rinex Check:** Verify observations in standard RINEX format; check for data gaps, receiver resets, antenna type changes - **Ephemeris:** Use precise ephemerides (IGS, GFZ) rather than broadcast ephemerides; improves orbit accuracy to ~5 cm - **Ambiguity Resolution:** Verify that integer ambiguities are fixed (not float). Fixed ambiguities yield phase residuals < 5–10 mm; float ambiguities show residuals in cm to m range, indicating poor solution - **Closure Check:** In a network, compute closure (e.g., traverse around a loop: A→B→C→A should return to starting point within measurement uncertainty) - **Repeat Baselines:** If budget permits, re-observe baseline between two stations on different days or epochs; should agree within ±0.05 m **Professional Standards in the Philippines:** Under **RA 4374** and **RA 8560**, static GNSS surveys used for cadastral/engineering control densification must include: - Dual-frequency observations - Ambiguity resolution documentation - Transformation to PRS92 datum - Tie to existing NAMRIA-maintained monuments - Formal least-squares adjustment if multiple baselines - Uncertainty statements (confidence ellipses) for all coordinates - Complete observation log and metadata Many cadastral firms in Metro Manila and major cities now use static GNSS to establish local control networks every 2–3 km, densifying NAMRIA's primary control (typically 10–20 km spacing in remote areas).

Early GNSS surveying relied on GPS alone. Modern systems integrate signals from multiple constellations, dramatically improving positioning reliability, accuracy, and speed. **GPS (Global Positioning System, USA):** - **Satellites:** 31 in operation (nominal constellation: 24) - **Frequencies:** L1 (1575.42 MHz), L2 (1227.60 MHz), L5 (1176.45 MHz) - **Status:** Mature, worldwide coverage, dense user base - **Accuracy:** ±1–3 m (code), mm-level (phase, post-processed) **GLONASS (Russia):** - **Satellites:** 24 in orbit - **Frequencies:** FDMA modulation (each satellite uses slightly different frequency), recently transitioned to CDMA (Code Division Multiple Access, same frequency) - **Advantage:** Complements GPS geometry, especially at high latitudes - **Accuracy:** Similar to GPS - **Note:** GLONASS frequencies near L2; some receiver filtering required to avoid interference **Galileo (European Union):** - **Satellites:** 30 constellation (full deployment ~2024) - **Frequencies:** E1 (1575.42 MHz, same as GPS L1), E5a (1176.45 MHz, same as GPS L5), E5b (1207.14 MHz), E6 (1278.75 MHz) - **Advantage:** High precision, open signal access (unlike GPS), rapid ambiguity resolution - **Status:** Still expanding; superior accuracy as constellation matures **BeiDou (China):** - **Satellites:** 35+ constellation (geostationary + inclined MEO + IGSO) - **Frequencies:** B1 (1561.1 MHz), B3 (1268.52 MHz), B2a (1176.45 MHz) - **Advantage:** Strong coverage over Asia-Pacific region (including Philippines) - **Status:** Fully operational; increasingly integrated in commercial receivers **Multi-Constellation Benefits:** 1. **Increased Satellite Availability:** Whereas GPS alone in an urban canyon might have 5–6 satellites, a multi-constellation receiver in the same location may track 15–20 satellites, dramatically improving geometry (lower DOP). 2. **Rapid Ambiguity Resolution:** More satellites enable faster and more reliable integer ambiguity resolution in RTK, reducing time-to-fix from 45–60 s (GPS-only) to 5–15 s (GPS+GLONASS+Galileo). 3. **Robustness in Adverse Conditions:** Signal blockage, multipath, ionospheric disturbances affect each constellation differently; redundancy improves availability. 4. **Better Vertical Geometry:** Different orbital inclinations (e.g., GLONASS 65° vs. GPS 55°) complement each other, improving VDOP. **Practical Implication for Philippine Surveys:** The Philippines is near the geomagnetic equator, where ionospheric disturbances are frequent (especially during local sunset). GPS-only receivers may experience ambiguity resolution failures or even loss of lock during geomagnetic storms. Multi-constellation receivers with Galileo and BeiDou (strong over Asia) provide redundancy and improved resilience. **Receiver and Software Considerations:** - Modern professional receivers (Leica Zeno, Trimble R10, Septentrio, Javad) support GPS+GLONASS+Galileo+BeiDou. - Older single-constellation receivers (GPS-only) are obsolete for precise surveying. - Software must handle inter-constellation biases (different reference clocks, frequencies); most commercial packages address this automatically. **Standards and Testing:** Independently verified testing of multi-constellation performance is ongoing (e.g., by IGS). For cadastral surveys under RA 4374, surveyors should document which constellations were used and whether minimum constellation/geometry criteria were met.